Ionic liquids: multifunctional agents of the polymer …theses.insa-lyon.fr/publication/2010ISAL0101/these.pdfIonic Liquids: Multifunctional agents of the polymer matrices Abstract

N° d’ordre 2010-ISAL-0101 Année 2010

Thèse

présentée devant

l’Institut National des Sciences Appliquées de Lyon

pour obtenir

le grade de Docteur

École Doctorale : Matériaux de Lyon

Spécialité : Matériaux Polymères et Composites

par

Sébastien LIVI

----------------------

IONIC LIQUIDS MULTIFUNCTIONAL AGENTS OF THE POLYMER MATRICES

---------

Soutenue le 02 décembre 2010 devant la Commission d’Examen :

JURY

DUCHET-RUMEAU Jannick Professeur (INSA Lyon) – Directrice de thèse GALY Jocelyne DR CNRS (INSA Lyon) – Examinateur GANTILLON Barbara Dr (Société TEFAL) – Examinateur GERARD Jean-François Professeur (INSA Lyon) – Co-directeur de thèse PHAM Thi Nhàn Maître de Conférences (Université de Caen) – Examinateur PLUMMER John Christopher Professeur (Ecole Polytechnique Fédérale de Lausanne) – Rapporteur SEGUELA Roland DR CNRS (Université de Lille 1) – Rapporteur

Ionic Liquids : Multifunctional agents of the polymer matrices

II

INSA de Lyon – Liste des Ecoles Doctorales

III

INSA Direction de la Recherche – Ecoles Doctorales – Quadriennal 2007-2010

SIGLE ECOLE DOCTORALE NOM ET COORDONNEES DU RESPONSABLE

CHIMIE

CHIMIE DE LYON http://sakura.cpe.fr/ED206

M. Jean Marc LANCELIN

Insa : R. GOURDON

M. Jean Marc LANCELIN Université Claude Bernard Lyon 1 Bât CPE 43 bd du 11 novembre 1918 69622 VILLEURBANNE Cedex Tél : 04.72.43 13 95 Fax : [email protected]

E.E.A.

ELECTRONIQUE, ELECTROTECHNIQUE, AUTOMATIQUE http://www.insa-lyon.fr/eea M. Alain NICOLAS

Insa : C. PLOSSU [email protected] Secrétariat : M. LABOUNE AM. 64.43 – Fax : 64.54

M. Alain NICOLAS Ecole Centrale de Lyon Bâtiment H9 36 avenue Guy de Collongue 69134 ECULLY Tél : 04.72.18 60 97 Fax : 04 78 43 37 17 [email protected] Secrétariat : M.C. HAVGOUDOUKIAN

E2M2

EVOLUTION, ECOSYSTEME, MICROBIOLOGIE, MODELISATION http://biomserv.univ-lyon1.fr/E2M2 M. Jean-Pierre FLANDROIS

Insa : H. CHARLES

M. Jean-Pierre FLANDROIS CNRS UMR 5558 Université Claude Bernard Lyon 1 Bât G. Mendel 43 bd du 11 novembre 1918 69622 VILLEURBANNE Cédex Tél : 04.26 23 59 50 Fax 04 26 23 59 49 06 07 53 89 13 [email protected]

EDISS

INTERDISCIPLINAIRE SCIENCES-SANTE Sec : Safia Boudjema M. Didier REVEL Insa : M. LAGARDE

M. Didier REVEL Hôpital Cardiologique de Lyon Bâtiment Central 28 Avenue Doyen Lépine 69500 BRON Tél : 04.72.68 49 09 Fax :04 72 35 49 16 [email protected]

INFOMATHS

INFORMATIQUE ET MATHEMATIQUES http://infomaths.univ-lyon1.fr M. Alain MILLE

Secrétariat : C. DAYEYAN

M. Alain MILLE Université Claude Bernard Lyon 1 LIRIS - INFOMATHS Bâtiment Nautibus 43 bd du 11 novembre 1918 69622 VILLEURBANNE Cedex Tél : 04.72. 44 82 94 Fax 04 72 43 13 10 [email protected] - [email protected]

Matériaux

MATERIAUX DE LYON M. Jean Marc PELLETIER

Secrétariat : C. BERNAVON 83.85

M. Jean Marc PELLETIER INSA de Lyon MATEIS Bâtiment Blaise Pascal 7 avenue Jean Capelle 69621 VILLEURBANNE Cédex Tél : 04.72.43 83 18 Fax 04 72 43 85 28 [email protected]

MEGA

MECANIQUE, ENERGETIQUE, GENIE CIVIL, ACOUSTIQUE M. Jean Louis GUYADER

Secrétariat : M. LABOUNE PM : 71.70 –Fax : 87.12

M. Jean Louis GUYADER INSA de Lyon Laboratoire de Vibrations et Acoustique Bâtiment Antoine de Saint Exupéry 25 bis avenue Jean Capelle 69621 VILLEURBANNE Cedex Tél :04.72.18.71.70 Fax : 04 72 43 72 37

[email protected]

ScSo

ScSo*

M. OBADIA Lionel

Insa : J.Y. TOUSSAINT

M. OBADIA Lionel Université Lyon 2 86 rue Pasteur 69365 LYON Cedex 07 Tél : 04.78.69.72.76 Fax : 04.37.28.04.48 [email protected]

*ScSo : Histoire, Geographie, Aménagement, Urbanisme, Archéologie, Science politique, Sociologie, Anthropologie


IV

Remerciements

V

Remerciements


VI

VII

«En science, la phrase la plus excitante que l’on peut entendre,

celle qui annonce des nouvelles découvertes,

ce n’est pas Eureka, mais c’est drôle.»

Isaac Asimov


VIII

Résumés

IX

Résumé : Une excellente stabilité thermique, une faible pression de vapeur saturante, une

ininflammabilité, une bonne conductivité ionique ainsi que les différentes combinaisons cations/anions possibles font des liquides ioniques l'objet d'un engouement grandissant de la Recherche. De part ces avantages, les LI se présentent comme une nouvelle voie dans le domaine des polymères, et en particulier dans le milieu des nanocomposites où leur utilisation est essentiellement limitée à la fonction de surfactant des silicates lamellaires. Néanmoins, avant de pouvoir prétendre à un statut d'alternative, il est nécessaire de mettre en évidence les effets bénéfiques de leur utilisation sur les propriétés finales des matériaux polymères.

Dans un premier temps, l’objectif de ce travail a été de synthétiser des liquides ioniques différents par la nature de leur cation et anion mais tous porteurs de longues chaînes alkyles afin de permettre une meilleure compatibilité avec la matrice. Ensuite, les excellentes propriétés intrinsèques des liquides ioniques ont motivé leur utilisation comme agents structurants dans une dispersion aqueuse fluorée. Ainsi, leur rôle d’agents ioniques sur la morphologie, les propriétés physiques, thermiques et mécaniques a été étudié. Dans une seconde partie, les liquides ioniques ont été utilisés comme agents intercalants des silicates lamellaires puis confrontés aux surfactants conventionnels dans le but de préparer des argiles thermiquement stables pour la préparation de nanocomposites thermoplastiques/argiles.

Dans une dernière partie, une faible quantité de ces argiles organiquement modifiées ont été introduites par intercalation à l'état fondu dans deux matrices différentes afin de mettre en évidence les effets de ces nouveaux agents interfaciaux sur les propriétés finales du matériau.

Mots-clefs : Liquides ioniques ; Nanocomposites ; Silicates lamellaires ; Agents structurants ; CO2

supercritique.


X

Ionic Liquids: Multifunctional agents of the polymer matrices Abstract : An excellent thermal stability, a low saturated vapor pressure, a no flammability, a

good ionic conductivity and the different cations / anions combinations possible of ionic liquids are currently the focus of the research. Because of its various benefits, they are as a new alternative in the polymer science, and particularly in the field of the nanocomposites where their use is currently limited to the function of surfactant for the layered silicates. However, before claiming the status of an alternative, it is necessary to highlight the benefits of their use on the final properties of polymer materials.

Initially, the objective of this work was to synthesize different ionic liquids by the nature of cation and anion, but all bearing with long alkyl chains to allow greater compatibility with the matrix. Then, the excellent intrinsic properties of ionic liquids have motivated their use as structuring agents in a fluorinated aqueous dispersion. Thus, their role in ionic agents on the morphology, physical, thermal and mechanical properties was studied. In a second part, ionic liquids have been used as agents intercalating layered silicates and then confronted with conventional surfactants in order to prepare thermally stable clays for the preparation of nanocomposite thermoplastic / clay.

In the last section, a small amount of organically modified clays were introduced by melt intercalation in two different matrices in order to highlight the effects of these new interfacial agents on the final properties of the material.

Keywords : Ionic Liquids ; Nanocomposites ; Layered silicates ; Building blocks ; Supercritical

CO2.

Sommaire

XI

Sommaire :

Pages

INTRODUCTION GENERALE ____________________________________________________ 1

RESUME ETENDU ____________________________________________________________ 3

Chapter I Ionic liquids: State of the art _______________________________________ 27

I.1 Ionic liquids ______________________________________________________________ 29 I.1.1 Origin of ionic liquids _________________________________________________________ 29 I.1.2 Properties of ionic liquids ______________________________________________________ 30 I.1.3 Structure and synthesis of ionic liquids ____________________________________________ 30

I.1.3.1 Effect of cation _____________________________________________________________ 31 I.1.3.2 Effect of anion _____________________________________________________________ 32 I.1.3.3 Synthesis of ionic liquids _____________________________________________________ 33

I.1.4 Applications of ionic liquids ____________________________________________________ 36 I.1.4.1 New alternative to conventional solvents _________________________________________ 36 I.1.4.2 Electrochemistry ____________________________________________________________ 36 I.1.4.3 Homogeneous and heterogeneous catalysis _______________________________________ 36 I.1.4.4 Metal ion capture ___________________________________________________________ 37 I.1.4.5 Chemistry in supercritical medium ______________________________________________ 37 I.1.4.6 Surfactants, plasticizers, and lubricants in polymer science ___________________________ 38

I.1.5 Main limitation of ionic liquids __________________________________________________ 39 I.1.6 Conclusion__________________________________________________________________ 39

I.2 Ionic liquids: A new way in the preparation of polymer/layered silicates nanocomposites ________________________________________________________________________ 40

I.2.1 Introduction _________________________________________________________________ 40 I.2.2 Ionic liquid-polymer interactions ________________________________________________ 41

I.2.2.1 Lubricants _________________________________________________________________ 41 I.2.2.2 Plasticizers ________________________________________________________________ 42 I.2.2.3 Polymer electrolytes _________________________________________________________ 43 I.2.2.4 Preparation of porous polymer _________________________________________________ 44 I.2.2.5 ILs supported on organic polymers ______________________________________________ 45 I.2.2.6 Preparation of supramolecular polymers based on ILs _______________________________ 45

I.2.3 Intercalating agents for layered silicates ___________________________________________ 47 I.2.3.1 Structure and properties of layered silicates _______________________________________ 47 I.2.3.2 Organic modification of layered silicates _________________________________________ 48 I.2.3.3 Conclusions ________________________________________________________________ 59

I.2.4 Polymer/layered silicates _______________________________________________________ 60 I.2.4.1 Preparation methods of PLS nanocomposites ______________________________________ 60 I.2.4.2 Characterization of PLS nanocomposites _________________________________________ 62 I.2.4.3 ILs treated-Layered silicates for polymer nanocomposites ____________________________ 63

I.2.5 Conclusions _________________________________________________________________ 68

Conclusions of chapter I ________________________________________________________ 69

References of chapter I _________________________________________________________ 70


XII

CHAPTER II POLYMER/IONIC LIQUID INTERACTIONS _____________________________ 75

II.1 New building blocks _____________________________________________________ 77 II.1.1 Introduction _________________________________________________________________ 77 II.1.2 Experimental ________________________________________________________________ 78

II.1.2.1 Materials __________________________________________________________________ 78 II.1.2.2 Processing and characterization of the IL/PTFE films _______________________________ 78 II.1.2.3 Synthesis of ionic liquids _____________________________________________________ 80

II.1.3 Morphology and mechanical performances of polymer/IL blends _______________________ 83 II.1.4 Conclusions _________________________________________________________________ 85

II.2 Nanostructuration of ionic liquids in fluorinated matrix: Influence on the mechanical properties ____________________________________________________________________ 86

II.2.1 Introduction _________________________________________________________________ 86 II.2.2 Results and discussion _________________________________________________________ 87

II.2.2.1 Effect of ionic liquids on the structuration of fluorinated polymer films _________________ 87 II.2.2.2 Effect of ionic liquids on the thermal properties of fluorinated polymer-based blends ______ 90 II.2.2.3 Effect of ionic liquids on the PTFE crystallinity ____________________________________ 91 II.2.2.4 Effect of ionic liquids on the mechanical properties of fluorinated polymer ______________ 93

II.2.3 Conclusions _________________________________________________________________ 99

Conclusions of chapter II ______________________________________________________ 100

References of chapter II _______________________________________________________ 101

Chapter III IONIC LIQUIDS AS NEWS INTERCALATING AGENTS FOR LAYERED SILICATES _ 103

III.1 A comparative study on ionic liquids used as surfactants: Effect on thermal and mechanical properties of high-density polyethylene nanocomposites __________________ 105

III.1.1 Introduction _____________________________________________________________ 105 III.1.2 Experimental ____________________________________________________________ 107

III.1.2.1 Materials _________________________________________________________________ 107 III.1.2.2 Synthesis of phosphonium and imidazolium salts _________________________________ 107 III.1.2.3 Organic modification of montmorillonite ________________________________________ 108 III.1.2.4 Processing and characterization of the HDPE/clay nanocomposites ___________________ 110

III.1.3 Results and discussion _____________________________________________________ 111 III.1.3.1 Characterization of modified montmorillonites ___________________________________ 111 III.1.3.2 Thermal stability of modified montmorillonites ___________________________________ 114 III.1.3.3 Structural analysis by WAXD _________________________________________________ 116 III.1.3.4 Surface energies of modified montmorillonites ___________________________________ 118 III.1.3.5 Influence of ionic liquid content _______________________________________________ 118

III.1.4 HDPE/clay nanocomposites _________________________________________________ 120 III.1.4.1 Thermal properties of nanocomposites __________________________________________ 120 III.1.4.2 Mechanical properties of nanocomposites _______________________________________ 121 III.1.4.3 Morphology of nanocomposites _______________________________________________ 122

III.1.5 Conclusions _____________________________________________________________ 123

Sommaire

XIII

III.2 Supercritical CO2-Ionic Liquid Mixtures For Modification of Organoclays ______ 124 III.2.1 Introduction _____________________________________________________________ 124 III.2.2 Experimental ____________________________________________________________ 125

III.2.2.1 Organic modification _______________________________________________________ 125 III.2.3 Results and discussion _____________________________________________________ 127

III.2.3.1 Effect of supercritical carbon dioxide as a exchange solvent on the thermal degradation of the modified MMT ___________________________________________________________________ 127 III.2.3.2 Structural analysis __________________________________________________________ 133 III.2.3.3 Surface energies ___________________________________________________________ 136

III.2.4 Conclusions _____________________________________________________________ 136

Conclusions of chapter III _____________________________________________________ 137

References of chapter III ______________________________________________________ 138

Chapter IV POLYMER/LAYERED SILICATES NANOCOMPOSITES _____________________ 141

IV.1 Synthesis of new surfactants: Effect of the ionic liquids on the thermal stability and the mechanical properties of high density polyethylene nanocomposites _______________ 143

IV.1.1 Introduction _____________________________________________________________ 143 IV.1.2 Experimental ____________________________________________________________ 144

IV.1.2.1 Materials _________________________________________________________________ 144 IV.1.2.2 Processing and characterization of HDPE/clay nanocomposites ______________________ 145 IV.1.2.3 Synthesis of imidazolium and phosphonium salts _________________________________ 146 IV.1.2.4 Organic modification of montmorillonite ________________________________________ 149

IV.1.3 Results and discussion _____________________________________________________ 150 IV.1.3.1 Thermal stability of ionic liquids ______________________________________________ 151 IV.1.3.2 Thermal stability of ionic liquid modified montmorillonites _________________________ 153 IV.1.3.3 Surface Energy of ionic liquid modified montmorillonites ___________________________ 154 IV.1.3.4 PE/modified-montmorillonites nanocomposites ___________________________________ 155

IV.1.4 Conclusions _____________________________________________________________ 160

IV.2 Ionic Liquids as Interfacial Agents in PVDF-based nanocomposites ____________ 161 IV.2.1 Introduction _____________________________________________________________ 161 IV.2.2 Experimental ____________________________________________________________ 162

IV.2.2.1 Materials _________________________________________________________________ 162 IV.2.2.2 Synthesis of ionic liquids ____________________________________________________ 162 IV.2.2.3 Organic modification _______________________________________________________ 163

IV.2.3 Results and discussion _____________________________________________________ 164 IV.2.3.1 Characterization of ILs exchanged montmorillonites _______________________________ 164 IV.2.3.2 Effect of interfacial interactions on the material physical properties ___________________ 168

IV.2.4 Conclusions _____________________________________________________________ 175

Conclusions of chapter IV _____________________________________________________ 176

References of chapter IV ______________________________________________________ 177

CONCLUSION GENERALE ____________________________________________________ 179


XIV

Abréviations et symboles

XV

Abréviations et symboles 1) Nomenclature

ILs : Ionic Liquids RTILs : Room Temperature Ionic Liquids EtNH3NO3 : Ethylammonium nitrate BF4

- : Tetrafluoroborate PF6

- : Hexafluorophosphate CF3SO3

- : Trifluoromethanesulfonate N(SO2CF3)2 : Bistrifluoromethanesulfonylimide HF : Hydrofluoric acid PLLA : Poly-L-lactide PMMA : Poly-(methylmethacrylate) PVC : Poly-(vinylchloride) PA6 : Polyamide PLA : Polylactide PEO : Poly(ethylene oxyde) PAN : Poly(acrylonitrile) PVDF : Poly(vinylidene fluoride) PVDF (HFP) : Poly(vinylidene fluoride-hexafluoropropylene) MMA : Methylmethacrylate PVA : Poly(vinyl) alcohol PE : Polyethylene PP : Polypropylene PET : Polyethylene terephtalate PEEK : Polyether ether ketone ABS : Acrylonitrile-butadiene-styrene PS : Polystyrene DOP : Dioctyl phtalate γ-APS : Aminopropyltriethoxysilane MMT : Montmorillonite PLS : Polymer Layered Silicates CEC : Cation Exchange Capacity ScCO2 : Supercritical Carbon Dioxide 2) Structural Characterization

TGA : Thermogravimetric Analysis DSC : Differential Scanning Calorimetry WAXD : Wide Angle X-ray Diffraction SAXS : Small Angle X-ray Scattering XPS: X-ray Photoelectron Spectroscopy TEM : Transmission Electron Microscopy DMTA : Dynamic Mechanical Thermal Analysis Tm : Melting Temperature (°C) Tc : Crystallization Temperature (°C) ∆Hm : Melting Enthalpy (J/g) Xc : Crystallinity percentage E’ : Storage Modulus (MPa) E : Young’s Modulus (MPa) tanδ : E’’/E’ (sans unité) 2θ : Diffraction angle (°)


XVI

Introduction générale

Page 1

INTRODUCTION GENERALE

Dans la science des matériaux, un des objectifs de la recherche est de développer des

matériaux polymères à haute performance. Afin d'y parvenir, l'incorporation de charges de

dimensions nanométriques dans une matrice polymère a été une voie privilégiée. Ces agents

nanométriques de différente nature : copolymères à blocs, composés du carbone (graphène,

nanotubes de carbone), oxydes métalliques (alumine, titane, zircone, silice) ou silicates

lamellaires (montmorillonite, mica) sont couramment utilisés. En raison de leur faible coût,

d'une surface spécifique (400 à 700m2/g) et d'un facteur de forme (100 à 1000) considérables,

les silicates lamellaires sont les nanocharges les plus communément utilisées dans

l'élaboration des nanocomposites. Néanmoins, la différence de polarité entre les argiles

(hydrophiles) et les polymères (hydrophobes) mène à de mauvaises interactions entre les

charges et la matrice avec comme conséquences une mauvaise dispersion des nanoargiles

dans la matrice accompagnée de propriétés finales diminuées. Il est alors nécessaire de

modifier la surface des silicates lamellaires afin d'améliorer la compatibilité vis-à-vis de la

matrice polymère. Le greffage d'organosilanes est un traitement de surface rendu possible par

la présence de groupements hydroxyle sur les bords des feuillets mais le traitement de surface

le plus répandu reste l'échange cationique qui consiste à remplacer les cations compensateurs

situés entre les feuillets d'argiles par des cations organiques nommés agents intercalants, le

plus souvent des alkylammonium quaternaires. Les différentes méthodes de préparation des

nanocomposites à base de silicates lamellaires modifiées par des sels d'ammonium (voie

solvant, polymérisation in situ, intercalation à l'état fondu) ont été largement étudiées.

Récemment, de nouveaux composés organiques sont apparus comme une nouvelle

alternative aux sels d'ammonium conventionnels. Ces sels fondus appelés liquides ioniques

ont l'avantage d'être ininflammable, de posséder une meilleure stabilité thermique, une faible

pression de vapeur saturante et une bonne conductivité ionique. Toutefois, malgré ces

nombreux avantages, leur utilisation dans le domaine des nanocomposites reste encore

limitée.

L'objectif de ce travail est de synthétiser des liquides ioniques différents par la nature

de leur cation et anion mais tous porteurs de longues chaînes alkyles permettant la

compatibilisation avec la matrice. Le liquide ionique sera ajouté dans la matrice polymère soit

en tant qu’agent structurant soit en tant qu’agent compatibilisant de la charge lamellaire. Le

rôle du liquide ionique sur la morphologie de la matrice et les propriétés physiques,

thermiques et mécaniques, sera étudié et discuté.


Page 2

Le manuscrit est divisé en 4 chapitres. Le premier chapitre comporte l’étude

bibliographique menée en 2 parties. La première partie présente un état de l’art sur les

liquides ioniques et leurs diverses applications dans le domaine de la synthèse organique, la

catalyse, l'électrochimie, les fluides supercritiques, ou l'énergie. La deuxième partie porte sur

le volet nanocomposite et sur l'apport des liquides ioniques comme nouveaux agents

intercalants des charges lamellaires sur les propriétés finales du matériau.

Dans un deuxième chapitre, les excellentes propriétés intrinsèques des liquides

ioniques ont motivé leur utilisation comme agents structurants dans une dispersion aqueuse

fluorée. Ce deuxième chapitre a été consacré dans un premier temps à la synthèse des liquides

ioniques pyridinium, imidazolium et phosphonium et à la structuration de ces agents ioniques,

assimilée à celle observée pour les ionomères et dépendante de la nature chimique des cations

et des anions utilisés, dans une matrice polymère. Dans un deuxième temps, l'influence des

liquides ioniques sur les propriétés mécaniques a été caractérisée en régime statique et

dynamique. Les effets de la vitesse de déformation sur la morphologie des domaines ioniques

ont été étudiés par microscopie électronique à transmission et par la diffusion des rayons X

aux petits angles. La stabilité thermique et la cristallinité des films préparés ont également fait

l'objet de cette étude.

Le traitement de surface des silicates lamellaires par les liquides ioniques utilisés

comme agents intercalants est décrit dans le troisième chapitre. Dans une première partie, les

propriétés des liquides ioniques sont confrontées à celles des ammoniums quaternaires

conventionnels. L'importance du rôle des espèces physiquement adsorbées à la surface des

argiles sur les propriétés physico-chimiques des silicates lamellaires a aussi été étudiée. Dans

une seconde partie, une nouvelle méthode de préparation des argiles organiquement

modifiées, respectueuse de l'environnement et basée sur la combinaison du CO2 supercritique

et des liquides ioniques a été envisagée ainsi que les conséquences de cette association sur la

stabilité thermique et le taux d'intercalation des argiles.

La préparation de nanocomposites polymères/argiles thermiquement stables par

intercalation à l'état fondu a fait l’objet de ce quatrième et dernier chapitre. En effet,

l'influence de la nature chimique des agents intercalants sur la morphologie ainsi que sur les

propriétés physiques et mécaniques des nanocomposites a été étudié sur deux matrices

différentes: le polyéthylène haute densité (PEhd) et le polyfluorure de vinylidène (PVDF).

Résumé étendu

Page 3

RESUME ETENDU

Chapitre 1 : Les liquides ioniques

Au cours de ces dernières années, les liquides ioniques (LI) ont suscité un intérêt

grandissant dans la Recherche académique et industrielle. Les principales raisons de cet

engouement s’expliquent par leurs propriétés attractives telles que leur stabilité thermique,

leur faible pression de vapeur saturante, leur ininflammabilité, leur bonne conductivité et une

température de fusion inférieure à 100°C. Tous ces éléments font des LI d’excellents

candidats pour un grand nombre d'applications dans des domaines aussi variés que

l’électrochimie, la catalyse, les matériaux polymères, ou encore dans la fabrication de

nanomatériaux comme agents tensioactifs pour les silicates lamellaires et silice, les composés

du carbone ou les oxydes métalliques. Dans la partie bibliographique, nous rappellerons ce

qu’est réellement un liquide ionique et nous décrirons ses diverses utilisations.

• Structure des liquides ioniques

- Cations

Les LI sont des sels fondus composés d'un cation organique associé à un anion

organique ou inorganique et qui possèdent des températures de fusion inférieure à 100°C.

Les cations organiques les plus rencontrés sont les ammonium, imidazolium,

phosphonium, pyridinium, pyrazolium, thiazolium, oxazolium ou pyrolidium. Des

fonctionnalisations différentes (amine, acide, thiol, ester, acrylate, nitrile) permettent

d'augmenter la gamme de cations disponibles et ont une influence significative sur les

propriétés physico-chimiques des LI. Ainsi, nous savons qu'une augmentation de la longueur

de chaîne peut augmenter la température de fusion du sel qui définit la gamme d'utilisation

des LI [1]. Les différentes structures possibles sont résumées sur la Figure 1. Malgré ce large

choix, les cations les plus couramment utilisés restent les cations imidazolium associés à un

nombre infini d'anions.


Page 4

X-NN

R1 R3

R5 R4

R2

+

X-

N NR1R2

+X-N

N NR1

R4 R3

R2

+X-

Y N

R1 R2

R4

R3+

N

R

+X-

NR2R1

+ [PRxH(4-x)]+ [NRxH(4-x)]

+

pyrazolium triazolium oxazolinium (Y = O)thiazolium (Y = S)

phosphonium ammoniumpyrolidiumpyridinium

imidazolium

Figure 1 – Les différents cations qui composent le liquide ionique

- Anions

Il existe deux types d'anions que l'on retrouve régulièrement dans la littérature: les

anions hexafluorophosphate (PF6-), tétrafluoroborate (BF4

-) et trifluorométhanesulfonate

(CF3S03-) dit anions fluorés et les anions conventionnels comme le brome (Br-), le chlore

(Cl-), l'iode (I-) et le chloroaluminate (AlCl4-) pour ne citer que les plus connus. La nature de

l'anion utilisé joue un rôle déterminant sur les propriétés finales des liquides ioniques,

notamment, en ce qui concerne la stabilité thermique des sels. Par exemple, lorsqu'un sel

imidazolium est associé à un anion fluoré (BF4-), sa stabilité thermique est considérablement

améliorée comparée au même LI combiné à un anion bromure (Br-) [2]. Il en est de même sur

la solubilité des liquides ioniques, l'utilisation du 1-méthyl-2-butyl-imidazolium

tétrafluoroborate est soluble dans l'eau alors que le même cation avec l'anion PF6-est non

miscible à l'eau. Dans le domaine des électrolytes et des batteries de piles à combustible, les

anions les plus communément utilisés sont les anions fluorés (BF4-) et (PF6

-). Les différents

anions sont résumés dans le Tableau 1.

Tableau 1 – Les différents anions inorganiques ou organiques les plus souvent rencontrés Inorganic anions Organic anions

F-, Br

-, Cl

-, I

-

BF4-, PF6

-, SbF6

-, AsF6

-

NO3-, ClO4

-

CuCl2-, AuCl4

-, SnCl3

-

CH3CO2-, CH3SO4

-, C6H5SO3

-

CF3CO2-, C(CF3SO2)3

-

N(SO2CF3)2

CF3SO3-

Résumé étendu

Page 5

• Applications des liquides ioniques

- Catalyse

Leur capacité à dissoudre de nombreuses substances comme les catalyseurs ainsi que

leur immiscibilité avec les réactifs et les produits confèrent aux LI un net avantage, très utile

en catalyse homogène et hétérogène. On les retrouve ainsi dans plusieurs réactions: les

réactions de couplage Suzuki-Heck [3], d'oxydation [4], sulfonation [5], isomérisation où les

LI imidazolium et ammonium sont couramment utilisés.

- Elimination des métaux

Les liquides ioniques sont de plus en plus utilisés en remplacement des solvants

organiques traditionnels dans les procédés d'extraction des métaux, en particulier dans le

domaine des déchets nucléaires [6] et de la contamination de l'eau [7]. Par exemple, Les

liquides imidazolium, notamment associés aux anions fluorés PF6-, BF4

- sont utilisés dans

l'extraction des ions sodium, cesium, lithium ou potassium [8].

- Science des polymères

L'utilisation de solvants organiques dans la préparation d'électrolytes de polymères est

fréquente. Néanmoins, des problèmes de volatilité et d’inflammabilité sont générés lorsqu'il

est nécessaire de travailler sur des gammes de température élevées. Ces inconvénients ont

conduit les chercheurs à se tourner vers les liquides ioniques qui contrairement aux solvants

conventionnels possèdent une excellente stabilité thermique, une faible volatilité, une bonne

conductivité et sont ininflammables. Les liquides ioniques imidazolium et pyridinium associés

aux anions PF6-, BF4

-, CF3SO3- et N(CF3SO3)

2- [9-11] ont été largement étudiés. Ainsi, les LI

ont été associés aux polymères électrolytes soit directement par polymérisation à partir du LI

[12] soit par solubilisation du polymère électrolyte dans le LI [13].

Dans l'industrie, les LI sont également utilisés comme plastifiants. Dans ce domaine,

les LI sont de bons substituts des plastifiants traditionnels dans des polymères comme le

PLLA, le PMMA et le PVC [14]. Des études ont également été menées sur l'utilisation des

liquides ioniques à température ambiante (RTIL) et leur grande capacité à réduire le

frottement et l'usure des polymères contre les métaux [15].

Dans le domaine des nanocomposites polymères à base de silicates lamellaires

(montmorillonite), l'utilisation des liquides ioniques ammonium comme agent compatibilisant


Page 6

des charges a fait l’objet de nombreux travaux [16]. Toutefois, la faible stabilité thermique

des sels d'ammonium due à l'élimination d'Hoffmann limite grandement leur utilisation lors

de la mise en œuvre à haute température de nanocomposites polymères/argiles. Afin de

contourner cette limitation, l'utilisation de liquides ioniques thermostables basés sur les

cations pyridinium, imidazolium ou phosphonium a été envisagée [17]. Pourtant, l'usage de

ces agents intercalants reste limité en raison du coût très élevé des LI mais également du

manque de choix disponible.

Cette partie bibliographique vise à donner une simple description des LI et de leurs propriétés

attractives. L’utilisation des LI dans le domaine des nanomatériaux reste encore limitée

compte tenu de leur coût. Le grand nombre de combinaisons possibles entre cation, anion et

nature du ligand permet d’envisager par une synthèse à façon une large palette de LI

répondant à un grand nombre d’applications et offrant de nombreuses perspectives.

• Références

[1] C. Chiappe, D. Pieraccini, J. Phys. Org. Chem. (2005); 18:275–297. [2] W.H. Awad, J.W. Gilman, M. Nyden, R.H. Harris, T.E. Sutto, J. Callahan, P.C. Trulove, H.C. DeLong, D.M. Fox, Thermochimica Acta (2004); 409:3. [3] M.J. Earle, S.P. Katdare, World Patent WO 2002030862 (2002). [4] M.J. Earle, S.P. Katdare, World Patent WO 2002030865 (2002). [5] J.F. Brennecke, E.J. Maginn, AIChE J. (2001); 47:2384–2388. [6] R.A. Bartsch, S. Chun, S.V. Dzyuba, Ionic Liquids Industrial Applications for Green Chemistry, American Chemical Society, Washington, DC, (2002); 58–68. [7] S. Chun, S.V. Dzyuba, R.A. Bartsch, Anal. Chem. (2001); 73:3737–3741. [8] H. Luo, S. Dai, P.V. Bonnesen, A.C.I. Buchanan, J.D. Holbrey, N.J. Bridges, R.D. Rogers, Anal.Chem. (2004); 76:3078–3083. [9] Noda, A.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B (2001); 105:4603. [10] Sakaebe, H.; Matsumoto, H. Electrochem. Commun. (2003); 5:594. [11] Lewandowski, A.; Swiderska, A. Solid State Ionics (2004); 169:21. [12] Ohno, H. Electrochim. Acta (2001); 46:1407. [13] Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electroanal. Chem. (1998); 459:29. [14] M. Rahman and C. S. Brazel, Polym. Degrad. and Stab. (2006); 91:3371–3382. [15] J. Sanes, F. J. Carrión, A. E. Jiménez and M. D. Bermúdez, Wear (2007); 263:658–662. [16] H.L. Tyan, Y.C. Liu, K.H. Wei, Chem Mater. (1999); 11:1942. [17] V. Mittal, European Polymer Journal. (2007); 43:3727–3736.

Résumé étendu

Page 7

Chapitre 2 : Interactions LI/polymère

Dans le domaine des matériaux polymère, les liquides ioniques ont souvent été utilisés

en tant que solvant vert et conducteur dans les gels electrolytes ou en tant que surfactant pour

les charges lamellaires. Mais jusqu’à présent, aucun travail à notre connaissance ne mentionne

l’utilisation des LI en tant qu’agent structurant d’une matrice polymère.

Dans ce deuxième chapitre, nous avons cherché à étudier l’impact du LI, introduit en

tant qu’additif dans la matrice polymère sur la morphologie et les propriétés physiques et

thermomécaniques du polymère. Les effets induits par le LI peuvent être modulés par le large

choix de combinaisons possibles cations/anions. Dans ce travail, nous avons choisi

d’introduire le LI dans une suspension aqueuse fluorée composée de polytetrafluoroéthylène

(PTFE) stabilisée. Le cahier des charges est difficile car le PTFE présente déjà une excellente

stabilité thermique, une résistance élevée aux acides et aux bases et un faible coefficient de

frottement. Quelle sera la plus value apportée par le liquide ionique introduit à un faible taux

(1%) dans la matrice PTFE après filmification ?

• Morphologie des LI dans le PTFE

Dans ce chapitre, différents liquides ioniques ont été synthétisés à partir de cations

pyridinium, imidazolium ou phosphonium et associés soit à des anions halogénés de type

iodure (I-), bromure (Br-) ou hexafluorophosphate (PF6-). Le Tableau 2 présente un récapitulatif

des différents sels synthétisés au cours de cette étude.


Page 8

Tableau 2 – Structure chimique des LI synthétisés

Cation Anion Designation

N NH37C18 C18H37

I-

C18C18Im I-

PC18H37

I- Br- PF6

-

C18P I- C18P Br- C18P PF6

-

I-

C18Py I- N

C18H37

- Influence du cation

La microscopie électronique à transmission a révélé des morphologies différentes en

fonction de la nature du cation introduit dans la matrice fluorée. Avec seulement 1% de

liquide ionique, une structuration volumique apparaît dans la matrice. Le liquide ionique

imidazolium (C18C18Im I-) génère deux types de morphologies coexistantes: la première

correspondant à la formation d’agrégats de domaines ioniques tandis que la seconde est

semblable à une morphologie co-continue, similaire à celle du liquide ionique pyridinium

(C18Py I-). A l’opposé, dans le cas du LI phosphonium, une excellente dispersion est observée

avec une structuration à l’échelle du nanomètre. Les différentes structurations des LI dans la

matrice fluorée en fonction de la nature du cation sont représentées sur la Figure 2.

200 nm 200 nm 200 nm

Figure 2 – Influence du cation sur la structuration de la matrice PTFE (imidazolium, pyridinium, phosphonium)

Résumé étendu

Page 9

Malgré la présence des longues chaînes alkyle (18 carbones) comme ligands sur les

différents liquides ioniques qui doivent interagir favorablement avec la matrice hydrophobe

du polytétrafluoroéthylène, la miscibilité des LI dans la matrice polymère est médiocre ce qui

conduit à la création d’une morphologie de phases séparées. De telles morphologies ont déjà

été observées et sont comparables à celles des ionomères dans les mélanges de polymères. En

effet, il est bien connu que le regroupement de paires d'ions dans un milieu de faible constante

diélectrique est responsable de la formation de micro ou de nanostructures qui peuvent être

prédites théoriquement [4, 5]. Le principal paramètre contrôlant la micro séparation de phase

dans un milieu non-polaire sont les interactions dipôle-dipôle entre les paires ce qui induit la

formation d’agrégats ioniques [6, 7]. Dans ce travail, une analogie peut être faite avec la

formation des agrégats de LI, dépendante des interactions entre le polymère et les différentes

combinaisons possibles cations/anions.

- Influence de l’anion

La nature de l’anion a également une influence significative sur la morphologie finale.

La Figure 3 illustre le rôle de l’anion sur la structuration générée dans la matrice PTFE à partir

d’un liquide ionique phosphonium associé soit à un contre anion iodé ou bromé soit à un

fluoré. Les anions bromé et fluoré conduisent à une morphologie grossière composée

d’agrégats. Cette structuration à l’échelle du micron contraste avec la morphologie à l’échelle

nanométrique obtenue avec l’anion iodure.

200 nm200 nm200 nm

Figure 3 – Structuration de l’échelle du micron au nanomètre de mélanges PTFE/LI (1% LI en poids) (C18P I-, C18P Br-, C18P PF6

-)


Page 10

• Influence des liquides ioniques sur les propriétés mécaniques des films

PTFE/LI

- Influence du cation

Dans cette partie, les propriétés mécaniques des films structurés en relation avec

leur morphologie ont été également étudiées. Le comportement mécanique est très

dépendant de la nature chimique des liquides ioniques. En effet, l’addition d’1% en poids

de liquides ioniques à base de pyridinium et d’imidazolium associés à l’anion iodure

présentent des performances mécaniques similaires en traction uniaxiale, conformes à

leurs morphologies co-continues identiques. Si des augmentations de module de l’ordre de

38 et 41% sont obtenues respectivement pour les LI pyridinium et imidazolium, une légère

diminution de l’allongement à la rupture de 11% est observée. En revanche, pour le LI

phosphonium qui conduisait à une spectaculaire structuration à l’échelle nanométrique, le

compromis propriétés à rupture/rigidité était fortement amélioré puisque des

augmentations de la rigidité et de la déformation à la rupture sont obtenues avec des

hausses respectives de +160% et +190% comme le montre la Figure 4.

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500 600

Strain at break (%)

Str

es

s (

MP

a)

PTFE

PTFE C18P I-

Figure 4 – Effet du liquide ionique phosphonium (1% en poids) sur les propriétés mécaniques à une vitesse de

déformation de 0.004 s-1 à température ambiante

De meilleures interactions c'est-à-dire une forte cohésion des interfaces due aux

interactions ioniques semblent avoir lieu entre la phase LI phosphonium et la matrice

fluorée ce qui conduit à une augmentation du module PTFE/LI. La phase LI agit comme

un agent de renforcement en formant un réseau de forte cohésion s’apparentant à celui des

réseaux percolants de nanocharges (noir de carbone, silice) mais qui est également capable

d’accomoder des déformations extrêmement importantes (délai de la rupture à haute

déformation).

Résumé étendu

Page 11

- Influence de l’anion

Les propriétés mécaniques peuvent également être modulées par la nature

chimique de l’anion. Ainsi, nous avons observé que le LI phosphonium associé aux anions

bromure et hexafluorophosphate conduit à des augmentations du module de 84% à 115%,

respectivement, en référence au PTFE non chargé. A l’opposé du LI C18P I-, la mauvaise

distribution des agrégats de LI C18P Br- et C18P PF6- dans la matrice fluorée a comme

conséquence une diminution de l’allongement à la rupture comprise entre 22% et 84%

respectivement.

• Influence des vitesses de déformation sur le comportement mécanique

des films polymère

En raison de l’excellente dispersion du LI phosphonium associé au contreanion

iodé dans la matrice fluorée ainsi que du très bon compromis rigidité/plasticité obtenu,

nous avons décidé d’étudier l’effet de la vitesse de déformation sur la morphologie de la

phase LI après déformation ainsi que sur les propriétés mécaniques qui en découlent.

Une des premières observations que nous avons faites est que l’addition du LI

phosphonium dans le PTFE conduit à une cristallisation à très haute vitesse de

déformation (0.2 s-1) avec une augmentation de la cristallinité de 10% en comparaison à

celle de la matrice seule. Pour une même vitesse de déformation, au niveau des propriétés

mécaniques, seule une légère augmentation de la rigidité est obtenue (200 MPa pour le

PTFE/C18P I- vs 170MPa pour le PTFE) qui peut être attribuée à une compétition entre la

réorganisation de la phase LI dans la matrice et la cristallisation sous déformation.

Pour corroborer cette hypothèse, nous avons utilisé de nouveau la microscopie

électronique à transmission (MET) pour révéler les domaines LI dans la matrice sous

différentes vitesses de déformation (0, 0.004 s-1, 0.2 s-1) (Figure 5).

PTFE/C18P I-

Initial state 0.004 s-1 0.2 s-1PTFE/C18P I-

0.004 s-1 0.2 s-1

200 nm 200 nm200 nm

Figure 5 – Effet de la vitesse de déformation sur la morphologie du LI phosphonium


Page 12

Lorsque la vitesse de déformation augmente, la nanostructure co-continue du LI est

maintenue et orientée dans l’axe de la déformation ce qui signifie que le réarrangement

des domaines ioniques nécessite un temps de relaxation plus important que le temps

caractéristique du processus de déformation. Ce phénomène est très proche de celui

observé par Visser et al [8], qui a proposé un modèle pour les ionomères mettant en

évidence un réarrangement spatial des domaines ioniques au sein de la matrice polymère.

Les auteurs ont également souligné le rôle de la nature des paires ioniques sur la

mécanique et le comportement de déformation.

Pour la première fois, une structuration à l’échelle nanométrique des liquides ioniques

dans un film polymère a été mise en évidence. Nous avons également démontré que les effets

de la nature chimique du LI déterminés par le choix du cation organique : pyridinium,

imidazolium ou phosphonium aussi bien que le choix de l’anion (halogènes ou fluorés)

peuvent affecter la structuration et les propriétés physiques et mécaniques du polymère. En

effet, une combinaison cation/anion adéquate génère une flexibilité sans précédent ainsi

qu’une amélioration significative de la rigidité. Les LI offrent ainsi une nouvelle alternative

pour structurer à l’échelle nanométrique les matériaux polymères.

• Références

[1] M. P. Scott, M. Rahman and C. S. Brazel, Eur Polym J. (2003); 39:1947–1953. [2] F. Avalos, J. C. Ortiz, R. Zitzumbo, M. A. López-Manchado, R. Verdejo and M. Arroyo, App.Clay Sci.

(2009); 43:27–32 [3] Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electrochem. Soc. (1997); 144:L67. [4] A.R. Khokhlov, E.F. Dormidontova, Phys. Uspekhi (2005), 118, 73. [5] I.A. Nyrkova, A.R. Khokhlov, Y.Y. Kramarenko, Polym. Sci. USSR (1990), 32, 852. [6] A. Eisenberg, B. Hird, R.B. Moore, Macromolecules (1990), 23, 4098. [7] I.A. Nyrkova, A.R. Khokhlov, M. Doi, Macromolecules (1993), 26, 3601. [8] S.A. Visser, S.L. Cooper, Polymer (1992), 33, 4705-4710.

Résumé étendu

Page 13

Chapitre 3 : Utilisation des liquides ioniques comme agents intercalants de silicates lamellaires

Depuis les années 80 et depuis les premiers travaux réalisés par l’équipe de Toyota sur

les nanocomposites polyamide-montmorillonite (PA-MMT), le domaine des nanocomposites

à charges lamellaires est en plein essor. En effet, le challenge lié à ces nouveaux matériaux,

vise à améliorer les propriétés finales des matériaux, notamment les propriétés thermiques,

mécaniques et barrière [1] avec un très faible taux de charge inorganique. La clé du succès

réside dans le contrôle de la dispersion des feuillets individuels, décrit comme l’état

d’exfoliation. Mais le manque de compatibilité entre les argiles (hydrophiles) et les

polymères, le plus souvent hydrophobes rend difficile l’obtention de cet état d’exfoliation.

Pour contourner cette difficulté et améliorer la compatibilité entre les argiles et le polymère,

l'utilisation d'espèces organiques nommées agents intercalants ou surfactants, est nécessaire

afin de réduire l'énergie de surface des silicates lamellaires et augmenter les distances

interfoliaires de façon à promouvoir la dissociation des feuillets en vue d’obtenir un état de

dispersion exfolié, plus propice à l'amélioration des propriétés finales des nanocomposites [2,

3]. Jusqu’alors, les sels d’ammonium sont classiquement utilisés.

Toutefois, la faible stabilité thermique des ammoniums quaternaires, qui se dégradent

dès 180°C [4], limite considérablement leur utilisation pour l’élaboration de nanocomposites

polymères/argiles nécessitant des températures de mise en œuvre élevées. Les liquides

ioniques apparaissent alors comme une nouvelle alternative aux ammoniums conventionnels.

L’objectif de ce chapitre a donc été de préparer des argiles organiquement modifiées par des

cations organiques thermostables, en particulier imidazolium et phosphonium connus pour

leur excellente stabilité thermique, et fonctionnalisés par de longues chaînes alkyle afin de

diminuer l’énergie de surface des argiles. En raison d'une offre limitée de liquides ioniques

commerciaux à longues chaînes alkyle (> 14 carbones), nous avons été amenés à synthétiser

des LI à façon.


Page 14

• Nomenclature des montmorillonites modifiées

Dans une première partie, afin de démontrer la supériorité thermique des liquides

ioniques sur les sels d’ammonium, une étude comparative sur quatre montmorillonites

modifiées a été décrite. Tout d’abord, nous avons sélectionné deux montmorillonites

commerciales (Nanofil 15 et Nanofil 919) traitées par les agents intercalants diméthyl ditallow

ammonium (MMT-DMDT) et diméthyl benzyltallow ammonium (MMT-DMBT) et nous les

avons comparés aux montmorillonites modifiées par les LI N-octadécyl-N’-

octadécylimidazolium (MMT-I) et octadécyltriphenylphosphonium (MMT-P). La Figure 6

décrit les différents cations organiques qui ont été comparés.

N+

Tallow

CH3

TallowH3C

N+

H3C

H3C Tallow

N

N

C18H37

C18H37

I

P C18H37

I

Versus

Figure 6 – Ammonium vs imidazolium et phosphonium

- Stabilité thermique des MMT modifiées

D’après la littérature [5-7], lorsque l’on modifie la surface des argiles par des cations

organiques, deux types d’interactions interviennent:

- (I) Les interactions de Van der Waals correspondant aux espèces organiques

physiquement adsorbées sur la surface de l'argile.

- (II) Les interactions ioniques correspondant aux espèces réellement intercalées entre

les feuillets de la montmorillonite.

Afin d’identifier et de quantifier le taux d’intercalation des espèces adsorbées et

intercalées, des lavages successifs au méthanol ont été effectués. Ainsi, nous avons démontré

par analyse thermogravimétrique (ATG) que les MMT-I et MMT-P ont des températures de

dégradation correspondant aux espèces physisorbées comprises entre 320°C et 340°C

(évaporation des LI), respectivement comparées aux MMT-DMDT et MMT-DMBT qui se

dégradent à plus basse température 220°C et 270°C.

Concernant les espèces intercalées, les montmorillonites modifiées par les liquides

ioniques imidazolium et phosphonium présentent une meilleure stabilité thermique que les

DMDT P DMBT I

Résumé étendu

Page 15

montmorillonites traitées ammonium puisque les températures de dégradation des nouveaux

surfactants s’étendent sur une plage comprise entre 420-490°C (MMT-I) et 510°C (MMT-P)

contrairement aux ammonium qui commencent à se dégrader à partir de 340-440°C (MMT-

DMDT) et de 300-400°C (MMT-DMBT). La Figure 7 représente le comportement de

dégradation thermique des montmorillonites organiquement modifiées.

N

N

C18H37

C18H37

I

P C18H37

I

N+

H3C

H3C Tallow

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60

80

100

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)

0 100 200 300 400 500 600 700 800

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� MMT-DMBT–––––––� MMT-P– – – –� MMT-I––––– ·

Universal V4.2E TA Instruments

Figure 7 – Courbes ATG des montmorillonites modifiées (Vitesse de chauffe : 20°K.min-1)

- Analyse structurale et propriétés de surface des argiles modifiées

Ensuite, nous avons caractérisé par la diffraction des rayons (DRX) et par la méthode

de la goutte posée les distances inter feuillets ainsi que les énergies de surface qui résultent de

la modification organique des silicates lamellaires. Le Tableau 3 récapitule les différents

résultats obtenus.

Tableau 3 – Energie de surface et distances interfoliaires

Montmorillonite Distance interfoliaire

(nm)

γ total (mN.m-1)

MMT-Na+ 1,2 73 MMT-DMDT 3,0 40 MMT-DMBT 1,9 49

MMT-P 4,2 37 MMT-I 3,7 32

Polyethylene [8]

- 34


Page 16

Avant le traitement de surface, l'espace interfoliaire de la montmorillonite sodique est

égale à 1,2 nm. Après la procédure d’échange cationique, le LI imidazolium conduit à une

distance interfoliaire de 3,7 nm, caractéristique d’une conformation paraffinique en position

trans-trans des chaînes alkyle alors que le LI phosphonium implique une distance de 4,2 nm

due à l’encombrement stérique des trois fonctions benzyle et de la chaîne alkyle. Ces

distances interfoliaires sont à comparer à celles de 1,9 nm et 3,0 nm obtenues respectivement

pour les MMT-DMBT et MMT-DMDT. Concernant les énergies de surface, l’utilisation des

LI mène à des valeurs très proches de celles des polyoléfines ce qui nous laisse suggérer une

excellente compatibilité de ces charges vis à vis des matrices hydrophobes comme le

polyéthylène (PE) ou encore le polypropylène (PP) [9].

• Association CO2 supercritique/LI pour la modification organophile des montmorillonites

Dans une seconde partie, notre but a été de reproduire les mêmes modifications

chimiques de la montmorillonite par les liquides ioniques mais cette fois-ci en utilisant le CO2

supercritique (ScCO2) afin de permettre un traitement de surface des silicates lamellaires

respectueux de l’environnement, c'est-à-dire sans l’utilisation de solvants organiques. En

effet, les avantages du dioxyde de carbone en condition supercritique sont nombreux: grande

diffusivité similaire à un gaz, faible tension de surface, viscosité et densité similaire à celles

d'un liquide ce qui lui confère un pouvoir de solvabilité élevé ajustable par contrôle de la

pression [10]. L’échange cationique s’effectue dans des conditions de pression et de

température de (75 bar, 80°C) pour la MMT-I et (80 bar, 90°C) pour la MMT-P.

- Stabilité thermique des MMT traitées LI

Nous avons montré que la combinaison du CO2 supercritique, des liquides ioniques

imidazolium et phosphonium permet la modification des silicates lamellaires avec des

résultats similaires à un échange cationique standard. Néanmoins, la faible solubilité des LI

dans le CO2 supercritique nécessite l’utilisation d’un co-solvant [11]. Dans notre cas, le co-

solvant utilisé est l’eau. L’utilisation de la combinaison CO2 supercritique-LI-eau lors de la

modification des silicates lamellaires conduit à une meilleure stabilité thermique ainsi qu’à un

taux d’intercalation des cations organiques entre les feuillets d’argiles plus important.

Résumé étendu

Page 17

Ainsi, la dégradation thermique des espèces intercalées des montmorillonites

modifiées par les sels d’imidazolium et phosphonium en milieu supercritique est retardée. En

effet, des températures de dégradation de 540°C et 570°C sont obtenues contre des

températures de 420-490°C et 510°C pour les MMT-I et MMT-P étudiées dans la première

partie de ce chapitre. Les courbes ATG de la montmorillonite traitée par le LI imidazolium

par échange cationique classique et sous ScCO2-eau sont représentées sur la Figure 8.

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100

120

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Universal V4.2E TA Instruments Figure 8 – Stabilité thermique des MMT-I modifiées par échange standard (a, a’ dérivée de ∆m/m) et sous

ScCO2-eau (b, b’ dérivée de ∆m/m)

Ces résultats ont pu être expliqués par une diminution de la température de fusion des

LI lors de l’exposition au CO2 supercritique en présence d’eau due à la mise en place de

faibles interactions acide-base de Lewis entre la partie basique des LI et la partie acide du

CO2 [12].

En conclusion, nous avons clairement démontré la meilleure stabilité thermique des

silicates lamellaires modifiées par les LI imidazolium et phosphonium comparés aux argiles

traitées par les ammoniums quaternaires conventionnels et l’utilisation du CO2 supercritique

associée à l’eau et aux liquides ioniques permet un échange cationique propre avec une

augmentation significative de la stabilité thermique des argiles modifiées pour des

caractérisations structurales comparables.


Page 18

• Références [1] E. Jacquelot, E. Espuche, J.F. Gerard, J. Duchet, P. Mazabraud, J. Polym. Sci. Part B: Polym. Phys. 44 (2) (2006) 431. [2] S.Y. Lee, W.J. Cho, K.J. Kim, J.H. Ahn, M. Lee, J. Colloid Interface Sci. 284 (2) (2005) 667. [3] H. He, J. Duchet, J. Galy, J.F. Gerard, J. Colloid Interface Sci. 295 (2006) 202. [4] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Chem. Mater. 13 (9) (2001) 2979. [5] L. Le Pluart, J. Duchet, H. Sautereau, J.F. Gérard, J. Adhes. 78 (7) (2002) 645. [6] W. Xie, R. Xie, W.P. Pan, D. Hunter, B. Koene, L.S. Tan, R. Vaia, Chem. Mater. 14 (11) (2002) 4837. [7] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Thermochim. Acta 367– 368 (2001) 339. [8] S. Boucard, J. Duchet, J.F. Gérard, P. Prele, S. Gonzalez, Macromol. Symp. 194 (1) (2002) 241. [9] C.M. Hansen, A. Beerbower, Kirk-Othmer Encyclopedia of Chemical Technology, second ed., Interscience, New York, 1971. p. 889. [10] S.P. Nalawade, F. Picchioni, L.P.B.M. Jansen, Prog. Polym. Sci. 31 (2006) 19-43. [11] S. Keskin, D. Kayrak-Talay, U. Akman, Ö. Hortaçsu, J. of Sup. Fluids 2007, 43, 150-180. [12] A.M. Scurto, E. Newton, R.R. Weikel, L. Drauker, J.Hallett, C.L. Liotta, W. Leitner, C.A. Eckert, Ind. Eng. Chem. Res. 47 (2008) 493.

Résumé étendu

Page 19

Chapitre 4 : Nanocomposites polymères/silicates lamellaires

Dans ce dernier chapitre, nous avons testé les montmorillonites rendues

thermiquement stables en les incorporant à l’état fondu dans deux matrices différentes, le

polyéthylène haute densité (PEhd) et le polyfluorure de vinylidène (PVDF) lors de

l’élaboration de nanocomposites. L’influence du ligand, sa nature chimique, le rôle du cation,

imidazolium vs phosphonium et de l’anion (Br-, I-, PF6-) ont été étudiées sur les propriétés

thermiques, physiques et mécaniques ainsi que sur la morphologie des nanocomposites

résultants.

• Utilisation des liquides ioniques comme agents interfaciaux dans les nanocomposites PEhd

Dans une première partie, l’influence de la nature chimique des cations (imidazolium,

phosphonium) et des anions (Br-, I-, PF6-) sur la stabilité thermique des liquides ioniques eux-

mêmes aussi bien que sur les montmorillonites modifiées par les LI a fait l’objet de ce travail.

La Figure 9 résume les différents liquides ioniques synthétisés et utilisés dans la préparation de

nanocomposites PEhd/MMT.

N NH37C18 C18H37

N N

H37C18C22H45

BrIN N

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Br

3a 3b 3c

X = I

Br

PF6

1a

1b

1c

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X

Imidazolium 3a-3c

Phosphonium 1a-1c Figure 9 – Liquides ioniques imidazolium et phosphonium synthétisés


Page 20

- Stabilité thermique des liquides ioniques

D’après la littérature, les cations imidazolium et phosphonium sont connus pour

posséder une excellente stabilité thermique [1, 2]. Dans le cas des sels d’imidazolium,

l’influence de la longueur des chaînes alkyle (C18 ou C22) est négligeable et leurs températures

de dégradation, proches de 320°C, sont similaires. A l’opposé, la combinaison de l’anion

associé au cation organique joue un rôle important sur la stabilité thermique des liquides

ioniques [3]. En effet, l’utilisation de l’anion hexafluorophosphate (PF6-) combinée avec le

cation phosphonium provoque une augmentation de la température de dégradation de 140°C

par rapport aux LI phosphonium contenant les anions bromure (Br-) et iodure (I-) qui se

dégradent à des températures proches de 330°C. La Figure 10 met en évidence cette stabilité

thermique améliorée.

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Figure 10 – Effet de la nature chimique de l’anion associé à des LI

phosphonium : Evolution de la perte de poids en fonction de la température (TGA, DTG)

(●) C18P Br-, (■) C18P I-, (○) C18P PF6-

(Vitesse de montée:20°C/min, N2)

Résumé étendu

Page 21

- Caractérisation physico-chimiques des MMT traitées LI

Les montmorillonites modifiées par les LI imidazolium et phosphonium ont été

également caractérisées par analyse thermogravimétrique (ATG), diffraction des rayons X et

par la méthode de la goutte posée. Ainsi, au niveau du comportement thermique, on observe

que ni la longueur de la chaîne alkyle ni la nature de l’anion utilisé dans le cas des

imidazolium n’ont d’influence sur les températures de dégradation des charges lamellaires qui

sont identiques aux températures constatées dans le chapitre 3, c'est-à-dire des températures

de 320, 410 et 480°C. Il en est de même pour les MMT traitées par les LI phosphonium

combinés aux anions halogénés tandis que dans le cas du LI phosphonium associé à l’anion

hexafluorophosphate, seule une augmentation de la température de dégradation correspondant

aux espèces physiquement adsorbées est améliorée passant de 330°C à 410°C. Ce résultat

s’explique par la meilleure stabilité thermique intrinsèque du liquide ionique. Par contre, la

gamme de température correspondant aux espèces intercalées (400-500°C) entre les feuillets

d’argiles est conservée. Par diffraction des rayons X, le changement d’anion ou une

fonctionnalisation différente n’influe aucunement sur les distances inter feuillets obtenues. Au

contraire, la nature de l’anion joue un rôle important sur les énergies de surface des MMT

modifiées. Les distances interfoliaires et les énergies de surface sont résumées dans le Tableau

4.

Tableau 4 – Distances interfoliaires et énergies de surface des montmorillonites modifiées par les liquides ioniques

Montmorillonite Distances interfoliaires

(nm)

γ total (mN.m-1)

MMT-Na+ 1.2 73 MMT-P I- 4.2 37

MMT-P Br- 4.1 43 MMT-P PF6- 4.2 36

MMT-C18C18Im 3.7 32 MMT-C18C22Im 3.7 39 MMT-C22C22Im 3.8 33

Le choix du liquide ionique, en particulier de son cation et de son anion, induit des

MMT modifiées avec des propriétés spécifiques adaptées à la matrice polymère utilisée. Dans

ce cas de figure, la modification des silicates lamellaires par les LI imidazolium et

phosphonium génère une bonne affinité avec les polyoléfines [5].


Page 22

- Comportement mécanique et thermique des nanocomposites PE/MMT-

LI

En termes de propriétés thermiques et mécaniques des matériaux mis en œuvre par

intercalation à l’état fondu, des améliorations ont été observées. Tout d’abord au niveau de la

stabilité thermique, on constate que l’addition de seulement 1% en poids des montmorillonites

traitées par les liquides ioniques imidazolium et phosphonium dans la matrice polyéthylène,

conduit à une augmentation de 10 à 15°C de la température de dégradation des

nanocomposites PEhd/MMT modifiées. Sur la Figure 11, le comportement thermique des

nanocomposites PEhd à base des MMT traitées par les sels d’imidazolium est représenté.

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Universal V4.2E TA Instruments Figure 11 – Stabilité thermique des nanocomposites PE/MMT modifiées par les LI imidazolium :

(●) PE/MMT-C18C18Im, (■) PE/MMT-C22C22Im (♦) Neat PE, (○) PE/MMT-C18C22Im (Vitesse de montée:20°C/min, N2)

En comparaison du comportement mécanique de la matrice non chargée, l’ajout de

seulement 1% de charges organiquement modifiées dans le polyéthylène conduit à des

augmentations de la rigidité de 25 à plus de 40% dans le cas des LI imidazolium C18C18Im et

C22C22Im, respectivement et de +30% en ce qui concerne le LI phosphonium C18P I-.

En conclusion, nous avons démontré que l’utilisation des montmorillonites traitées par

les LI imidazolium et phosphonium permet une amélioration de la stabilité thermique et la

rigidité des nanocomposites PE/argiles.

Résumé étendu

Page 23

• Utilisation des liquides ioniques comme agents interfaciaux de nanocomposites PVDF

Dans une seconde partie, l’influence de la nature chimique des cations organiques

imidazolium et phosphonium sur la structure polymorphique du PVDF, les propriétés

mécaniques ainsi que sur la morphologie des nanocomposites a été étudiée. Le Tableau 5

résume les LI synthétisés et les montmorillonites modifiées (1% en poids) pour ce type de

nanocomposites à base de MMT.

Tableau 5 – Nomenclature des LI et MMT modifiées Nom

commercial

Références

Intercalant

Nanofil 757

MMT-Na+

MMT-I

MMT-P

PC18H37

I

MMT-IC12F

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I

C18H37

N N(CH2)2(CF2)9CF3

I

H3C

- Caractérisations thermiques des MMT-LI

La fonctionnalisation du cation imidazolium par une chaîne perfluorée n’améliore pas

la stabilité thermique des montmorillonites par rapport à celles modifiées par les LI

fonctionnalisés par les chaînes alkyles. Une diminution de la température de dégradation des

espèces physisorbées, attribuée à la volatilisation des courtes chaînes fluorées, de 330°C pour

les LI à longues chaînes alkyles à 280°C est obtenue. Pour les espèces intercalées, une

température de 460°C est alors observée. Les différentes températures déterminées par ATG

correspondant aux espèces physisorbées et intercalées sont rapportées dans le Tableau 6. On

notera cependant que ces 3 MMT conservent toutes un caractère thermostable amélioré par

rapport aux MMT modifiées par les ions ammonium.


Page 24

Tableau 6 – Nomenclature des LI et MMT modifiées Sample Température de

degradation des espèces

physisorbées (°C)

Température de degradation

des espèces intercalées

(°C) MMT-P 330 510 MMT-I 320 420/480

MMT-IC12F 280 460

- Morphologie des nanocomposites PVDF/MMT-LI

Lorsque les silicates lamellaires organiquement modifiées sont introduits dans la

matrice PVDF, des morphologies différentes ont été observées en fonction de la nature

chimique du LI. En effet, dans le cas des LI imidazolium et phosphonium à longues chaînes

alkyle, une structure intercalée des nanocomposites est constatée contrairement à la

montmorillonite traitée par le cation imidazolium fluoré qui semble plus apte, en partie par

son caractère hydrophobe à induire une meilleure dispersion des feuillets d’argiles dans la

matrice fluorée. Les différentes morphologies obtenues sont représentées dans la Figure 12.

1 µm 1 µm 1 µm Figure 12 – Morphologie des nanocomposites PVDF/MMT-I, PVDF/MMT-P, PVDF/MMT-IC12F

(Gauche à droite)

- Comportement mécanique des nanocomposites PVDF/MMT-LI

Les propriétés mécaniques sont dépendantes de la morphologie des nanocomposites.

Un effet plastifiant est obtenu avec une augmentation de la déformation à la rupture en

traction uniaxiale de 100% pour la MMT-I et de 700% pour la MMT-IC12F tandis qu’une

légère diminution du module de Young de l’ordre de 15% est constatée dans les deux cas. En

conséquence, l’état de dispersion le plus abouti conduit au meilleur comportement mécanique,

c'est-à-dire à une augmentation importante de la déformation suivie d’une légère diminution

de la rigidité.

Résumé étendu

Page 25

- Influence des MMT-LI sur la structure cristalline polymorphe du PVDF

En raison des nombreuses phases cristallines du PVDF, nous avons étudié par

diffraction des rayons X l’effet de ces agents interfaciaux utilisés comme surfactant des

silicates lamellaires sur la cristallinité de la matrice fluorée. D’après la littérature [7, 8], nous

savons que les phases les plus couramment rencontrées sont les phases α et β. Il est toutefois

possible de les différencier puisque les pics de diffraction caractéristiques de la forme α sont

localisés à 17.8, 18.4, 19.2°2θ et ceux correspondant à la formation de la phase β peuvent être

détectés à 20.7°2θ. La Figure 13 montre les diffractogrammes des nanocomposites

PVDF/MMT.

10 15 20 25 30

0100020003000400050006000

2θ

PVDF

0500

10001500200025003000

PVDF/MMT

0

500

1000

1500

2000

Inte

nsit

y (

u.a

)

PVDF/MMT-P

0500

10001500200025003000

PVDF/MMT-I

0

500

1000

1500

2000 PVDF/MMT-IC12F

Figure 13 – Diffractogrammes des nanocomposites (a) PVDF/MMT-IC12F, (b)

PVDF/MMT-I, (c) PVDF/MMT-P, (d) PVDF/MMT et (e) PVDF non chargé Nous avons mis en évidence que la nature chimique du cation organique joue un rôle

important sur la structure cristalline polymorphe du PVDF. L’utilisation du LI imidazolium

fonctionnalisé par la chaîne perfluorée ainsi que le LI phosphonium génère la formation de la

phase β, généralement obtenue sous une déformation mécanique ou l’application d’un champ

électrique [9].

a)

e)

d)

c)

b)


Page 26

Dans ce travail, nous avons démontré que la compatibilité entre la matrice polymère et

les silicates lamellaires liée à la nature du cation organique est le paramètre principal

contrôlant les interactions physico-chimiques au sein de la matrice et contribue aussi à une

meilleure distribution et dispersion des argiles dans la matrice polymère. Ainsi, nous avons

observé que l’addition de seulement 1% en poids de MMT modifiée par l’imidazolium fluoré

conduit à une exfoliation des feuillets dans le polymère résultant en une amélioration du

compromis rigidité/déformation. En outre, nous avons également mis en évidence que la

nature chimique des agents interfaciaux, en particulier les LI à base de phosphonium et

imidazolium fluoré, favorise la formation de la forme β. Cette observation peut ouvrir de

nouvelles perspectives dans le domaine des membranes pour piles à combustible.

• Références

[1] H.L. Ngo, K. Le Compte, L. Hargen, A.B. McEven Thermochim. Acta 97 (2000) 357-358 . [2] W.H. Awad, J.W. Gilman, M. Nyden, R.H. Harris, T.E. Sutto, J. Callahan, P.C. Trulove, H.C. DeLong, D.M. Fox, Thermochimica Acta 409 (2004) 3. [3] W.H. Awad, J.W. Gilman, M. Nyden, R.H. Harris, T.E. Sutto, J. Callahan, P.C. Trulove, H.C. DeLong, D.M. Fox, Thermochimica Acta 409 (2004) 3. [4] C. Byrne and T. McNally, Macromolecular Rapid Communications 2007, 28, 780-784. [5] S. Boucard, J. Duchet, J.F. Gérard, P. Prele, S. Gonzalez, Macromol. Symp. (2002) 194 (1):241. [6] H.A. Patel, R.S. Somani, H.C. Bajaj, R.V. Jasra, Applied Clay Sci. (2007) ; 35:194-200. [7] T. U. Patro, M. V. Mhalgi, D. V. Khakhar and A. Misra, Polymer (2008), 49, 3486-3499. [8] D.J. Lin, C.L. Chang, F.M. Huang, L.P. Cheng, Polymer 44 (2003) 413-422. [9] J. Scheinbeim, C. Nakafuku, B.A. Newman, K.D. Pae, J. Appl. Phys. 50 (1979) 4399-4405.

Chapter I: Ionic Liquids: State of the art

Page 27

Chapter I Ionic liquids: State of the art Since the first discovery of the nanocomposites described by the Toyota team in the

80s, the industrial and academic research have focused particularly on the processing of

nanocomposites based on lamellar silicates of nanometer size. The promise of these

nanocomposites lies in their multifunctionality, the possibility of realizing unique

combinations of properties unachievable with conventional materials. What are the reasons

that allow in reaching this promise? The first one is linked to the confinement of polymer

chains into nanofillers due to the small size of fillers compared to polymer chains

dimensions. Among these nanofillers, lamellar or platelet–like fillers are made of platelets

having different composition and spatial arrangement. The most widespread lamellar fillers

belong to the family of smectite that are montmorillonite. The dimensions of each platelet

typically range in the following scales: few nanometers (thickness), tens of nanometers

(width) and from tens of nanometers to few micrometers (length). Their reduced dimensions

are responsible for a high specific surface (from 400 up to 700 m2.g-1) and their particular

morphology confer them a high aspect ratio (from 100 up to 1000). The second one is the

large amount of interfacial zones, i.e the multiplication of surfaces and interphases that

implies a strong synergy between the polymer matrix and inorganic nanofillers and the third

one is the spatial structuration of nanofillers. The well known challenges to get

nanocomposites include control over the distribution in size and dispersion of the nanofillers.

The montmorillonites are expandable clays: their interlayer inorganic cations can be replaced

by other cations having a higher affinity for the fixed ionic sites on the platelets. One

important consequence of the charged nature of the clay surfaces is that they are generally

highly hydrophilic and therefore naturally incompatible with a wide range of polymer. A

prerequisite for successful formation of polymer-clay nanocomposites is therefore screening

of the clay polarity to make the clay “organophilic”. This can be readily achieved through ion-

exchange reactions which is the most used modification method to modify clay nature (from

hydrophilic to hydrophobic) to an extent which depends on the nature of the organic

molecule. This surface treatment generates organically-modified clays (or organoclays)

characterized by an improved compatibility with most of the polymers.

The innovative part of this work is the use of a new generation of surfactants that are

the ionic liquids based on the pyridinium, imidazolium, or phosphonium cations. In the first

part, an overview of ionic liquids will be made. Then, in a second step, the use and the

influence of different ionic liquids as surfactants for nanofillers, plasticizers in polymers and

reinforcing agents will be described.


Page 28

Pages

I.1 Ionic liquids ............................................................................................................................. 29 I.1.1 Origin of ionic liquids ................................................................................................................... 29 I.1.2 Properties of ionic liquids.............................................................................................................. 30 I.1.3 Structure and synthesis of ionic liquids ......................................................................................... 30

I.1.3.1 Effect of cation .......................................................................................................................... 31 I.1.3.2 Effect of anion .......................................................................................................................... 32 I.1.3.3 Synthesis of ionic liquids .......................................................................................................... 33

I.1.3.3.1 Imidazolium ionic liquids ............................................................................................... 33 I.1.3.3.2 Pyridinium ionic liquids .................................................................................................. 33 I.1.3.3.3 Phosphonium ionic liquids .............................................................................................. 34 I.1.3.3.4 Anionic exchange ............................................................................................................ 34

I.1.4 Applications of ionic liquids ......................................................................................................... 36 I.1.4.1 New alternative to conventional solvents ................................................................................. 36 I.1.4.2 Electrochemistry ....................................................................................................................... 36 I.1.4.3 Homogeneous and heterogeneous catalysis .............................................................................. 36 I.1.4.4 Metal ion capture ...................................................................................................................... 37 I.1.4.5 Chemistry in supercritical medium ........................................................................................... 37 I.1.4.6 Surfactants, plasticizers, and lubricants in polymer science ..................................................... 38

I.1.5 Main limitation of ionic liquids ..................................................................................................... 39 I.1.6 Conclusion..................................................................................................................................... 39

I.2 Ionic liquids: A new way in the preparation of polymer/layered silicates nanocomposites .................................................................................................................................................. 40

I.2.1 Introduction ................................................................................................................................... 40 I.2.2 Ionic liquid-polymer interactions .................................................................................................. 41

I.2.2.1 Lubricants ................................................................................................................................. 41 I.2.2.2 Plasticizers ................................................................................................................................ 42 I.2.2.3 Polymer electrolytes .................................................................................................................. 43 I.2.2.4 Preparation of porous polymer .................................................................................................. 44 I.2.2.5 ILs supported on organic polymers ........................................................................................... 45 I.2.2.6 Preparation of supramolecular polymers based on ILs ............................................................. 45

I.2.3 Intercalating agents for layered silicates ....................................................................................... 47 I.2.3.1 Structure and properties of layered silicates ............................................................................. 47 I.2.3.2 Organic modification of layered silicates ................................................................................. 48

I.2.3.2.1 Grafting of organosilanes ................................................................................................ 49 I.2.3.2.2 Cationic exchange ........................................................................................................... 50

I.2.3.3 Conclusions ............................................................................................................................... 59 I.2.4 Polymer/layered silicates ............................................................................................................... 60

I.2.4.1 Preparation methods of PLS nanocomposites ........................................................................... 60 I.2.4.1.1 Solution intercalation ...................................................................................................... 61 I.2.4.1.2 In situ intercalative polymerization ................................................................................. 61 I.2.4.1.3 Melt intercalation ............................................................................................................ 61

I.2.4.2 Characterization of PLS nanocomposites ................................................................................. 62 I.2.4.3 ILs treated-Layered silicates for polymer nanocomposites ....................................................... 63

I.2.4.3.1 Polystyrene/IL-modified clays nanocomposites ............................................................. 63 I.2.4.3.2 PVDF/IL-modified clays nanocomposites ...................................................................... 65 I.2.4.3.3 Polyolefins/IL modified clays nanocomposites .............................................................. 66 I.2.4.3.4 Polyester/IL modified clays nanocomposites .................................................................. 67 I.2.4.3.5 Polyamide/IL-modified clays nanocomposites ............................................................... 67 I.2.4.3.6 PVC/IL-modified clays nanocomposites ........................................................................ 68

I.2.5 Conclusions ................................................................................................................................... 68

Conclusions of chapter I ................................................................................................................. 69

References of chapter I ................................................................................................................... 70


Page 29

I.1 Ionic liquids Over the last few years, the ionic liquids (ILs) have been of a large interest both for

the academic and industrial fields, because they have been widely promoted as a green

solvent. Indeed, their unique properties, such as their chemical stability, thermal stability, low

saturated vapor pressure, non-flammability, and good ionic conductivity make them as ideal

candidates in a wide variety of applications in the chemical industry. Their application

domains concerned more recently polymer science. Indeed, they have been used mainly as

polymerization media in several types of polymerization processes to prepare functional

polymers [1]. ILs are also investigated as components of the polymeric matrices, lubricants,

plasticizers, or components in the class of polymer gels [2-4], as templates for porous

polymers [5]. ILs are found both in the energy field, specifically in the electrolyte batteries

and fuel cells since ILs are considered as novel electrolytes for electrochemical

polymerizations [6, 7]. In the manufacture of nanomaterials, ILs are used as surfactants for

different fillers such as lamellar silicates or metal oxides.

I.1.1 Origin of ionic liquids

In recent years, research on ionic liquids was intensified, even if these ones are known

since the early twentieth century. In fact, according to the literature [8], the first ionic liquid

displaying a melting temperature below 100°C has been described for the first time was an

ammonium salt, i.e ethylammonium nitrate (formula EtNH3NO3). But it's really in the 70s that

the first ionic liquids were synthesized and used in batteries for nuclear warhead [9]. Wilkes

and others scientists have continued to use ionic liquids as electrolytes in batteries and study

different properties [8-10]. Then, during the 80s, another class of ionic liquids has emerged:

the imidazolium salts. Chloroaluminate anions associated with these salts have prevented their

development, due to the high reactivity between these anions and water and in

air. Nevertheless, it is only in the early 90s, after the synthesis of imidazolium salts,

particularly the 1-butyl-3-methylimidazolium tetrafluoroborate (BF4-) and 1-butyl-3-

methylimidazolium hexafluorophosphate (PF6-) that were stable in water and in the air that the

interest of researchers has focused on ionic liquids. The most commonly ionic liquids used are

imidazolium and pyridinium salts containing BF4- and PF6

- anions [11-13].


Page 30

Indeed, research into ionic liquids is booming and they could be found in various

fields: homogeneous and heteregeneous catalysis, solvent replacement for the synthesis

applications and polymer science.

I.1.2 Properties of ionic liquids

Unlike conventional solvents based on organic molecules, ionic liquids are a

combination of cations and anions, respectively, positive and negative. Furthermore, ionic

liquids have melting temperatures below 100°C, and could be denoted as RTILS (Room

Temperature Ionic Liquids) since they are liquid at room temperature. This advantage offers

them many application areas. According to the literature, the relatively low melting

temperature ionic liquids are the resulting combination of large and asymmetric organic

cations and inorganic anions which leads to a decrease of the lattice energy. Today, they are

considered as ‘green’ solvents and environmental friendly components due to their

recyclability and their ability to be re-used several times. They have many advantages such as

excellent thermal stability, i.e about 300°C and even higher when using fluorinated anion, and

low saturated vapor pressure to prevent evaporation. They are also non inflammable and

polar. In addition, they have good thermal and electrical conductivities that makes them

excellent candidates elsewhere in the electrochemical environment. Finally, it is also possible

to synthesize ionic liquids for applications covered by varying combinations of anions and

cations. ILs could be also used as solvents for many substances such as proteins,

carbohydrates, polysaccharides, DNA, crude oil, and plastics materials.

I.1.3 Structure and synthesis of ionic liquids

The ionic liquids (ILs) can reduce the use of many organic solvents because of their

many qualities and take part in different types of synthesis. They are the most often made of

an organic cation associated with an organic or inorganic anion. These many combinations

cation / anion make ionic liquids tunable for a specific application.


Page 31

I.1.3.1 Effect of cation

There are several types of ILs as a function of the chemical nature of cation. The

cation is usually a bulk organic structure with low symmetry. The most of ionic liquids are

based on different organic cations such as: ammonium, imidazolium, phosphonium,

pyridinium, pyrazolium, thiazolium, oxazolium, or pyrolidium. In the litterature, many other

cations have been studied and functionalized by various groups: amine, acid, thiol, ester,

acrylate and nitrile. The structure of the frequently used organic cations is given in Figure I-1.

X-NN

R1 R3

R5 R4

R2

+

X-

N NR1R2

+X-N

N NR1

R4 R3

R2

+X-

Y N

R1 R2

R4

R3+

N

R

+X-

NR2R1

+ [PRxH(4-x)]+ [NRxH(4-x)]

+

pyrazolium triazolium oxazolinium (Y = O)thiazolium (Y = S)

phosphonium ammoniumpyrolidiumpyridinium

imidazolium

Figure I-1 – Different types of cations considered for ionic liquids

So far, the most commonly used cations in the literature are mainly based on

imidazolium cations associated with an infinite number of anions. The nature of the cation

and the chemical nature of ligand play a key role in the physico-chemical properties of salts.

For example, the length of alkyl chains and the symmetry of the molecule have a significant

influence on the melting temperature. Chiappe and Pieraccini [14] indicated that as the size

and asymmetry of the cation increase, the melting point increases. Further, an increase in the

branching on the alkyl chain increases the melting point. The melting point of ILs is a key

issue as it represents the lowest limit of the liquid state. Melting temperature and thermal

stability define the range of temperature for which ILs could be used as solvents.


Page 32

I.1.3.2 Effect of anion

The change of anion results in an increasing number of alternative ILs with various

properties. There are two types of anions that are found commonly in the literature: the

fluorinated anions, such as hexafluorophosphate (PF6-) et tetrafluoroborate (BF4

-)

trifluoromethanesulfonate (CF3S03-), and the conventional anions i.e bromide (Br-), chloride

(Cl-), iodide (I-) and chloroaluminate (AlCl4). The final properties of ionic liquids depend on

the nature of the anion used. Indeed, whatever the type of cation used, significant differences

are observed according to the associated anion. In some cases, anion is responsible for the

decrease or increase of melting temperature and thermal stability [15]. For example, the ionic

liquid, 1-methyl-2-butyl-imidazolium tetrafluoroborate, has a better thermal stability than the

same salt in the presence of a bromide anion. The chemical nature of anion also plays on the

solubility of ionic liquids. For example, 1-methyl-2-butyl-imidazolium tetrafluoroborate is

soluble in water whereas the same cation with the anion PF6- is totally immiscible in water.

The choice of the anion also has an influence on the viscosity and the density of molten salts.

BF4- and PF6

- anions are the most commonly used in numerous applications, more especially

in electrolytes and batteries. Despite some advantages, they have important limitations due to

the formation of HF when heated and placed in the presence of water. It is for these reasons

that researchers have focused on the use of other anions such as CF3SO3- and (CF3SO3)2N

- by

bonding fluorine to carbon to prepare a C-F bond inert to hydrolysis. But fluorinated anions

tend to be expensive and in response to cost and safety concerns, new ILs with non-

fluorinated anions have been prepared such as alkylsulfate that are considered as nontoxic and

biodegradable [16-18]. The structures of anions commonly used are gathered in Table I-1.

Table I-1 – Inorganic and organic anions.

Inorganic anions Organic anions

F-, Br

-, Cl

-, I

-

BF4-, PF6

-, SbF6

-, AsF6

-

NO3-, ClO4

-

CuCl2-, AuCl4

-, SnCl3

-

CH3CO2-, CH3SO4

-, C6H5SO3

-

CF3CO2-, C(CF3SO2)3

-

N(SO2CF3)2

CF3SO3-


Page 33

I.1.3.3 Synthesis of ionic liquids

As the number of cation/anion combinations is almost infinite, we have considered in

this bibliographical part a small part in restricting itself primarly to a general description of

the synthesis of imidazolium, pyridinium, and phosphonium salts. In general, the anion

exchange and the quaternization reaction by alkyl chains were studied. Many papers describe

the synthesis of ionic liquids [19, 20].

I.1.3.3.1 Imidazolium ionic liquids

Many methods for the synthesis of imidazolium ionic liquids with short or long alkyl

chains containing halogenated anions, tosylates, or triflates have been reported [21].

Imidazole undergoes a deprotonation step with sodium methoxide then reacts with a halide

anion functionalized with alkyl chains. The both reactions are performed in solvent medium

(acetonitrile, tetrahydrofurane, etc...) at reflux temperature and for reaction time from several

hours to several days, under inert atmosphere. The chemical nature of R1,2 group allows tuning

of ILs towards organic medium. For example, perfluorinated chains can be introduced. Figure

I-2 describes the general synthesis of imidazolium salts.

N

NH

N

N

N

N

R1

R1X N

N

R1

R2

X

R2X

- NaX

MeO- Na+

R1=R2 = alkyl chains X = Br-, Cl-, I- Figure I-2 – Synthesis of imidazolium salts

I.1.3.3.2 Pyridinium ionic liquids

Over the last thirty years, pyridinium ionic liquids have been widely used by

researchers, especially in catalytic processes, such as Diels Alder reactions [22] and Friedal-

Crafts alkylations [23]. The pyridinium salts containing the anions Cl-, Br-, I-, BF4-, PF6

-, and

N(SO2CF3)2 are the best known. Recently, these ionic liquids were re-considered with new

chiral pyridinium salts and nitrile functionalized ones. The reaction scheme shown in Figure

I-3 for pyridinium ionic liquids is similar to that of imidazolium IL.


Page 34

N N

R

X

X = Cl, Br, I

RX+Heat MY

-MX N

R

Y

R = alkyl chain

MY = NaBF4, HBF4, NaPF6, HPF6 Figure I-3 – Synthesis of pyridinium salts

I.1.3.3.3 Phosphonium ionic liquids

Compared to imidazolium, ammonium, and pyridinium ionic liquids, research on the

phosphonium ionic liquids is more limited and the fields of applications are less numerous.

Phosphonium cations commonly used are based on triphenyl-, trihexyl-, or tridecylphosphine

and the associated anions are most often halide anions Cl-, I-, Br-, C6H5SO3-, CF3SO3

- and

conventional fluoroanions BF4-, PF6

- and N(SO2CF3)2. However, the number of patents on

phosphonium ionic liquids is increasing which gives an evidence of an ever-growing interest

by the industry [24]. For example, the chemical reaction of ionic liquids based on

triphenylphosphonium is reported in Figure I-4.

X

P

Heat

RXP

R MY

-MX

Y

PR

X = Cl, Br, I MY = NaBF4, NaPF6, HPF6, HBF4 Figure I-4 – Synthesis of phosphonium salts

I.1.3.3.4 Anionic exchange

The anion exchange is also possible, by using Lewis acids or organic salts (sodium

salts) or by metathesis reactions [25]. Whatever the methods used, the reaction yields are

excellent. The main disadvantage of these methods is that it requires a purification step which

is important to remove all traces of impurities due to incomplete exchange. The anions

commonly introduced after exchange are the followings: NO3-, AlCl4

-, BF4-, PF6

-, CF3SO3-,

(CF3SO2)2-, or CF3CO2

-. Figure I-5 and Figure I-6 show the two routes to perform exchange

reaction either with Lewis acids or organic salts from an imidazolium salt as an example.


Page 35

N

N

R1

R2

X

X = Cl, Br, I

N

N

R1

R2

MXn+1MXn

M = Al, Cu, Fe, Zn, Sn

Figure I-5 – Example of anionic exchange with Lewis acids

N

N

R1

R2

X

X = Cl, Br, I

N

N

R1

R2

YMY

MY = NaBF4, NaPF6, LiNTf2

-MX

Figure I-6 – Example of anionic exchange with organic salts

More recently, new fluorinated anions with low melting temperatures, i.e.

fluorohydrogenates [26], perfluoroalkyltrifluoroborates [27], trifluorophosphates [28], and

perfluoroalkyl-β-diketonates [29] were introduced by anionic exchange. The use of these new

fluorinated anions and perfluorinated alkyl chains containing cations [30] has consequences

on the physico-chemical properties of ionic liquids such as viscosity, density, solubility,

melting temperatures, and conductivity. According to the literature, when using cations

functionalized with perfluorinated chains or fluorinated anion or combination of both, ionic

liquids with specific properties are obtained. Excellent thermal stability, chemical resistance

to acids and bases, or a total inertia with conventional organic solvents were achieved [31].

Until now, these ionic liquids have numerous applications in catalysis, as surfactants to

stabilize the perfluorocarbons dispersion in ionic liquid media [32].


Page 36

I.1.4 Applications of ionic liquids

The physico-chemical properties of ionic liquids are opening a broad field of

applications.

I.1.4.1 New alternative to conventional solvents

The different properties of ionic liquids as their non flammability, low saturation

vapor pressure and the fact that ILs are not explosive is a valable asset for substitution of

conventional organic solvents. Currently, their ability to dissolve a wide variety of organic

substances or solutes in liquid-liquid extractions is retained. According to the literature, ionic

liquids are used for the extraction of aromatic compounds or alcohols of different binary

mixtures [33, 34].

I.1.4.2 Electrochemistry

Imidazolium ionic liquids are commonly used as electrolyte for the electrodeposition

of metals on the surface of a conductive material. The advantage of such salts is that they are

not easily reduced allowing the reduction of many metals at room temperature. In addition,

the use of hydrophobic ionic liquid prevents infiltration of hydrogen gas and formation of

holes linked to the formation of hydrogen bubbles in the metal during the evaporation step

[35]. On the other hand, it is known that ammonium and imidazolium salts used in

electrochemical reactions allow a better diffusion of organic compounds than in conventional

organic solvents.

I.1.4.3 Homogeneous and heterogeneous catalysis

In literature, many articles deal with various applications of ionic liquids in catalytic

reactions [36, 37]. Indeed, this area represents a large part of the activity of these salts. The

first ionic liquids used in the alkylation reactions of Friedal-Craft reactions or Diels-Alder

reactions are pyridinium salts. The imidazolium and ammonium salts are used in the reactions

of oxidation [38], nitration [39], sulphonation [37], isomerization, hydroformilation, and the

coupling reactions of Suzuki and Heck [40]. For example, Hermann et al. [41] showed that

using an ionic liquid instead of a conventional solvent improves the yield of the Heck


Page 37

reaction. Xiao [42] showed an increase in selectivity when using ionic liquids. In addition, ILs

combine several advantages. In fact, their ability to dissolve most of catalysts and their

immiscibility with the reactants and products give them both the benefits of a liquid and a

solid necessary in homogeneous and heterogeneous catalysis.

I.1.4.4 Metal ion capture

Ionic liquids are increasingly used instead of conventional organic solvents in

extraction processes and metal removal [43, 44] and particularly in the field of nuclear waste.

Many studies have focused on the extraction of radioactive metals such as lanthanides and

actinides [45]. Others authors have studied the behavior of uranium in several ionic liquids.

Water contamination by metals led scientists to use ionic liquids to allow the removal of

contaminants. In most cases, the imidazolium ionic liquids are the most often used : Visser et

al. [46], Chun et al. [47], and Luo et al. [48] have studied the use of imidazolium IL

containing anion PF6- to extract Na+, Cs+, Li+ and K+. Other researchers have considered

imidazolium ionic liquid associated to a TF2N- anion to extract strontium [49].

I.1.4.5 Chemistry in supercritical medium

One of the major challenges of our time is to eliminate the conventional organic

solvents commonly used in chemical industry to develop a green chemistry. To achieve this

objective, more and more studies suggest a novel alternative to conventional solvents.

Supercritical carbon dioxide (ScCO2), water, and ionic liquids are expected media of this new

chemistry. In particular the combination of ionic liquids and supercritical CO2 was studied in

the literature: In fact, this combination allows to couple the unexpected properties of

supercritical CO2 such as a high diffusivity and viscosity (as a gas), a low surface tension,

density and solvency (as a liquid) at tunable by adjusting pressure. Supercritical carbon

dioxide offers an acceptable combination of pressure and temperature to achieve supercritical

conditions, with a temperature above 31°C and a pressure above 73 bar. Moreover, it is not

toxic and its cost is low. When combined with the low volatility and the high polarity of ionic

liquids compared to apolar and volatile supercritical CO2, an excellent and complementary

combination is obtained. For these reasons, many studies focus on the behavior and the

understanding of interactions between supercritical CO2 and ionic liquids in biphasic systems

[50, 51]. The most studied of ionic liquids in these studies is the imidazolium salt containing

the anion PF6- due to the solubility of fluorinated anion in ScCO2 [52, 53].


Page 38

I.1.4.6 Surfactants, plasticizers, and lubricants in polymer science

In the field of polymer nanocomposites based on inorganic fillers, especially layered

silicates (montmorillonite, mica, laponite), ionic liquids could play a key role. Indeed, a

number of current issues of nanocomposites is the polarity difference between the hydrophilic

clays and the hydrophobic polymer matrix [54]. To avoid this difficulty, it is necessary to

modify the clay surface to fit the interfacial interactions and expand their layered structure.

Currently, ammonium ionic liquids are the most commonly used as intercalating agents into

layered silicates. The use of these organic salts can increase the interlayer space and reduces

the surface energy of clay which makes them more compatible with the polymer matrix.

Many studies examine the use of alkylammonium ionic liquids as surfactants and their

beneficial contributions on the thermal, mechanical and barrier properties of final

nanocomposites [55, 56]. However, alkylammonium salts have a serious disadvantage: their

low thermal stability. Indeed, according to the literature, they begin to degrade from 180°C

[57] due to the Hofmann elimination (Figure I-7) which limits their use in the nanocomposites

processing at higher temperatures (especially when intercalation is done in molten state) [58].

C C

H

+NR3

∆C C + H+ + NR3

Figure I-7 – Hofmann elimination on the quaternary ammonium

Recently, new surfactants have emerged with the use of pyridinium, imidazolium, and

phosphonium salts that have a better thermal stability (at about 300°C) than ammonium ionic

liquids. Moreover, the use of ionic liquids have a positive impact on mechanical, thermal and

electrical conductivity properties of nanocomposites [59]. Despite these benefits, the use of

these salts is limited because of the cost of ionic liquids and the fact that the organic salts with

long alkyl chains necessary to increase interlayer distances are not commercially available.

In industry, ionic liquids are also used as plasticizers to ameliorate the processability,

the flexibility and the ductility of rigid polymers. In this field, ionic liquids are excellent

candidates to replace conventional plasticizers used in PLLA, PMMA, and PVC [60]. The

advantages of ionic liquids include an excellent thermal stability and a low volatility

compared to conventional plasticizers such as polyethylene glycol or phtalate. Studies have

also been conducted on the use of ionic liquids at room temperature (RTILS) and their ability

to lower friction and wear of polymers against steels [61].


Page 39

I.1.5 Main limitation of ionic liquids

Despite the many advantages and unique properties of ionic liquids as well as the

multiple combinations of cations and anions that can be proposed, the price of these new

materials is their main disadvantage. Today, ionic liquids per kilogram costs tens of thousands

more than common organic solvents such as acetone. However, this cost can be significantly

reduced [62]. In the case of imidazolium ionic liquids, Wagner et al. [63] have anticipated

price of around 50-100 euros per kilogram if the industries produce large quantities.

Another limit is that the synthesis of ionic liquids involves the use of conventional

organic solvents such as acetonitrile, toluene, THF whereas such molten salts are intended to

replace the conventional organic solvents. However, in next year’s, the emergence of new

synthesis methods for avoiding the use of conventional solvents will appear, such as

supercritical medium and microwave synthesis [64].

Another limit is the viscosity significantly higher than that of organic solvents. This

problem can be offset by changing the nature of the anion, increasing temperatures or using

the supercritical medium at very high pressure.

Despite these drawbacks, these challenges can be overcome and ionic liquids are

really promising in many areas, especially in the case of polymer processing.

I.1.6 Conclusion

This work aims to summarize the information present in the literature about ILs. A

brief description of many properties of ionic liquids as well as a overview of beneficial

contributions of the use of these salts in different fields of applications, including that of

polymers have been described in this first part. ILs are receiving more and more attention

every day both in academic research and commercial applications.

The use of ILs in polymer science has quickly advanced from use as solvents and has

become focused an using ionic liquid as functional additive to polymer chains or to hybrid

materials.


Page 40

I.2 Ionic liquids: A new way in the preparation of polymer/layered silicates nanocomposites

I.2.1 Introduction

Nanocomposites are the focus for materials engineers to overcome the limitations of

traditional micrometer-scale polymer composites. The field of nanocomposites involves the

study of multiphase materials where at least one of the constituent phases has one dimension

less than 100 nm. Although some composites nanofilled with carbon black or silica have been

used for a very long time, research and development of polymer layered silicate

nanocomposites (PLS) has greatly increased since the eighties. The promise of PLS lies in

their multifunctionality, the possibility of realizing unique combinations of properties

unachievable with traditional materials. What are the reasons that allow in reaching this

promise? The first one is linked to the confinement of polymer chains within nanofillers due

to the small size of fillers compared to polymer chains dimensions. The second one is the

large increase of interfacial zones which could lead to increase the polymer-inorganic surface

interactions and the third one is the spatial structuration of nanofillers. The well known

challenges to get high performance nanocomposites include control over the distribution in

size and dispersion of the nanofillers which are strongly dependent on tailoring the interfaces

between organic and inorganic phases. Lamellar silicates are generally highly hydrophilic

species and therefore incompatible with a wide range of polymer. A necessary prerequisite for

successful formation of polymer-clay nanocomposites is therefore alteration of the clay

polarity to make the clay “organophilic”. This can be readily achieved through ion-exchange

reactions which is the most used modification method to modify clay nature (from hydrophilic

to hydrophobic) to an extent which depends on the nature of the organic molecule. The

cationic exchange is usually carried out with alkylammonium ions [65] resulting of the

protonation of aliphatic or aromatic amines in an acidic medium. Their basic formula is CH3-

(CH2)n-NH3+ with n varying between 1 and 18. The length of the ammonium ions has a strong

effect on the resulting structure of nanocomposites. The amine functionality has also an

impact on the final properties of materials [66]. However, the great disadvantage of these

ammonium salts is their poor thermal stability as degradation starts from 180°C which is a


Page 41

serious limitation to the preparation of nanocomposites processed at high temperatures.

Recently, a new alternative can be proposed with Ionic Liquids (ILs) which are subject of

many research in various areas, due to their excellent thermal stability, their non-

flammability, a low saturated vapor pressure, and good thermal and electrical conductivities.

However, in the field of nanocomposites, their use is limited as intercalating agents of layered

silicates and very few studies have actually investigated the effects of ionic liquids on the

final properties of the material. In this second part of the state of the art, the use of ionic

liquids as surfactants, plasticizers and lubricants in the nanocomposite field is described.

I.2.2 Ionic liquid-polymer interactions

The tunability of Ionic Liquids (ILs) via the different cation / anion combinations is a

huge advantage to design new surfactants with specific physico chemical interactions towards

polymer medium. Indeed, it is possible to synthesize ILs with suitable functionalities (epoxy,

fluorinated groups, and alkyl groups). Despite this impressive capability, the use of ionic

liquids is not well developed in the field of nanocomposites compared to their applications as

lubricants, plasticizers, or electrolyte gels.

I.2.2.1 Lubricants

In the field of lubricants, imidazolium ionic liquids are commonly used in different

assemblies steel-steel [67-69], aluminum-steel [70-72], polymer-steel [73] or in coatings

based on nickel and chromium [74-75]. The advantage of ionic liquids, in addition to being

very good lubricants is that they can be used on a wide range of temperature due to their

excellent thermal stability. Yao et al. [76] have synthesized series of 1,3 dialkylimidazolium

with long alkyl chains to be used as lubricants for steel-steel joints in a temperature range

between 25 and 150°C. They concluded that the addition of the ionic liquids functionalized

with long alkyl chains generated a reduction of friction and anti-wear properties, mainly at

high temperatures. Jimenez et al. [77] studied the influence of the chemical nature of the

cation or anion. Three ionic liquids were compared to improve the wear behavior of steel-steel

contacts, i.e. imidazolium ionic liquids denoted 1-methyl-3-octylimidazolium

tetrafluoroborate (BF4-) and 1-methyl-3-hexylimidazolium hexafluorophosphate (PF6

-), versus

a quaternary ammonium containing halide anion (Cl-). Then, the ionic liquids have been


Page 42

compared to a conventional oil at room temperature. They showed that the use of mineral oil

or ammonium ionic liquids as lubrifiant on steel gives high friction coefficients and wear

rates. On the other side, ionic liquids based on imidazolium cation functionalized with

hexafluorophosphate and tetrafluoroborate show better lubricating performance than the

ammonium salt from 25 to 200°C which is due to higher thermal stability of ionic liquids.

Recently, ionic liquids have been introduced as an internal or external lubricant in

polymer-metal assemblies. Sanes et al. [73] studied the influence of the ionic liquid, 1-hexyl-

3-methylimidazolium hexafluorophosphate, on the tribological properties of polyamide PA6.

The addition of ionic liquid (3wt.%) in the polyamide matrix does not affect the storage and

loss moduli whereas the presence of imidazolium ionic liquid plays a decisive role on the

tribological properties of the polymer. Indeed, at values of -35 ° C and +67 ° C, the neat PA-6

matrix already displays a reduced friction but the PA-6 combined with 3wt.% of IL leads still

lower friction constant values over the temperature range. They explain these performances

by the formation of stable adsorbed layers of the highly polar ionic liquid molecules on the

steel surface.

I.2.2.2 Plasticizers

In recent years, ionic liquids have represented a new alternative as substitute for

traditional plasticizers for many polymers; such as polyvinylchloride (PVC) and

polymethylmethacrylate (PMMA). However, very few studies have investigated these new

plasticizers for economic reasons. In fact, the cost of ionic liquids is currently too high for a

large diffusion of these products in the industry. Sankri et al. [78] have prepared

thermoplastic starch by melt processing using 1-butyl-3-methylimidazolium chloride as

plasticizer. The authors compared the influence of imidazolium ionic liquid to glycerol

plasticizer on the mechanical properties of thermoplastic starch. The results are significant in

the case of imidazolium since a increase of the strain at break from 100 to 400% is observed.

They attributed this modulus decrease to a reduction of hydrogen bonds between starch

molecules. Scott et al. [79] used room temperature ionic liquids (RTILs) based on

imidazolium cations as plasticizers for poly(methylmethacrylate). Generally, dioctyl phthalate

(DOP) is used as plasticizer. Butylmethylimidazolium and hexylmethylimidazolium

hexafluorophosphate were considered as excellent plasticizers for PMMA because they

reduce the glass transition temperature while improving the thermal stability.


Page 43

Rahmann et al. [80] have used different ionic liquids varying the cation chemical

nature, imidazolium, phosphonium, or ammonium, as novel plasticizers for

poly(vinylchloride) matrix. A higher decrease in the glass transition temperature of PVC

compared to conventional plasticizers was obtained with an improved flexibility of material.

A better leaching and migration resistance than the conventional plasticizers has been shown

as well. Nevertheless, the behavior of ionic liquids and conventional plasticizers remain

identical when subjected under far-range UV exposure. Park et al. [81] have investigated the

influence of phosphonium ionic liquids on the degradation of polylactic acid (PLA). In this

work, two phosphonium ionic liquids containing decanoate and tetrafluoroborate as anion

have been studied. Initially, they found that phosphonium ionic liquids were well dispersed

and were partially miscible in the PLA matrix. Then, they observed that the lubrication and

hydrolytic degradation were more pronounced when the ionic liquid based on decanoate anion

was used whereas the phosphonium ionic liquid with fluorinated anion led to an increase of

the thermal stability of PLA.

I.2.2.3 Polymer electrolytes

There are several types of polymer gel electrolytes that are used in various fields of

applications such as in secondary batteries, sensors, and various ionic devices [82, 83].

Typically, the preparation of polymer electrolytes from solutions requires polar organic

solvents and electrolyte salts in a polymer matrix. The term “gel electrolyte” is used to

describe these materials. In this case, the properties of solvents used such as viscosity and

dielectric constant as well as concentration of salts play a key role on the conductivity of the

electrolyte [84]. However, the use of organic solvents which are volatile generates

flammability problems when used at high temperatures.

These drawbacks have prompted the research to find a new alternative to conventional

organic solvents. For replacing these solvents, ionic liquids have recently been selected and

are of great interest in research on the electrochemical applications. These ionic liquids have

been chosen for their unique characteristics, including excellent thermal stability,

nonflammability, non volatibility, low melting temperature, and very high ionic conductivity

[85, 86].


Page 44

The most important and widely studied cations are the imidazolium and pyridinium

ionic liquids combined with anions such as PF6-, BF4

-, CF3SO3-, and N(CF3SO3)2

- [87-89]. In

the literature, different polymer electrolytes containing ionic liquids obtained by various

methods were described, i.e. polymerization of ionic liquids [90, 91], inclusion of different

polymers in ambient temperature ionic liquids [89, 92], and preparation of polymer gel with

hydrophilic and hydrophobic ionic liquids [93]. All research reported previously, have

demonstrated high conductivity suitable for applications.

Usually, the majority of polymers used in polymer electrolyte systems have been

based on high-molecular weight poly(ethylene oxide), PEO [83]. Recently, various polymer

as poly-(methylmethacrylate) PMMA, poly(acrylonitrile) PAN, poly(vinylidene fluoride)

PVDF, and poly(vinylidene fluoride-hexafluoropropylene) PVDF(HFP) have been studied

[94, 95]. For example, Susan et al. [96] have synthesized polymer gel by polymerization of

methyl methacrylate (MMA) in 1-ethyl-3-methylidazolium bis(trifluoromethylsulfonyl)imide

ionic liquid with a small amount of cross linker. They obtained ionic gels that exhibit ionic

conductivities at room temperature, high mechanical strength, transparency and flexibility

required for polymer electrolytes. For the preparation of electrolyte membrane with improved

mechanical properties and higher ionic conductivity in the temperature range from 20 to

140°C, Sing et al. [97] have used imidazolium ionic liquid, denoted 2,3-dimethyl-1-

octylimidazolium, with triflate anions into a PVDF(HFP) matrix.

In the field of polymer electrolytes, ionic liquids have shown their importance. All

these promising results as plasticizers, lubricants, or polymer electrolyte allows to expect new

insights in the field of nanocomposites.

I.2.2.4 Preparation of porous polymer

The use of ionic liquids in the preparation of porous polymer is a path that has also been

studied. Indeed, the elimination of ILs from a mixture of polymer is simple. Zhu et al. [98]

have prepared polyurea with pore sizes of 100-500 nm by the interfacial polymerization

between hexane and a series of imidazolium combined with hexafluorophosphate and

tetrafluoroborate anions. Other authors have prepared by in situ polymerization a series of

porous composites containing polymers and ionic liquids [99].


Page 45

I.2.2.5 ILs supported on organic polymers

The use of ionic liquids supported on hybrid materials is a new area of application of

ILs. Indeed, the idea to use recoverable and reusable catalysts is very important that it is

economically and environmentally interesting. According to the literature, the hybrid organic-

inorganic materials containing imidazolium based on silica are most often encountered [100].

However, some recent works mention the use of polystyrene to support ILS as shown in

Figure I-8 [101].

Figure I-8 – PSIL: Polymer supported imidazolium salts [101]

Chi et al. [101] studied the catalytic properties of this material for nucleophilic

fluorination and different substitutions. They have demonstrated that the longest linker

(dodecyl) associated with tetrafluoroborate counteranion (BF4-) leads to the best catalytic

activity unlike to PSIL associated with other anions [102]. Despite a slight development in

catalytic reactions [103], the synthesis of poly (ethylene) glycol (PEG) functionalized ionic

liquids showed interesting physical and chemical properties [104].

I.2.2.6 Preparation of supramolecular polymers based on ILs

The preparation of supramolecular structures is governed by hydrogen bonds [105],

host-guest interactions [106], metal-ligand coordination [107] as well as ionic interactions

[108]. These, last ones, the most frequently encountered in the field of electrolytes [109] have

been widely used to create chemical/physical cross-link in polymer matrix such as alginates

[110], halatopolymers [111] and a wide variety of ionomers [112].

Recently, Wathier and Grinstaff [113] suggested that ionic liquids may play an

important role in the formation of ionic networks based on coordinating ion pairs. Indeed, the

authors have synthesized an ionic liquid composed of a dication, tetraalkylphosphonium


Page 46

covalently linked and tetraanion ethylenediaminetetraacetate. They have demonstrated that

Coulomb interactions are governed by pairwise interactions between cation and anion and the

extended structure of the ionic liquid may lead to a supramolecular ionic network (Figure I-9).

+

Anion Cation Figure I-9 – Schematic diagram of a supramolecular ionic network

The authors have also suggested that the ramification of multivalent ions could lead to

networks while the presence of large ions would allow mobility which lead to an increase of

the toughness of the resulting materials.

The combination of the mechanical properties linked to ionomer with the homogeneity

and high charge densities typical of ionic liquids could lead to a new range of optimized

materials. However, these ionic materials are often subject to phase separations, long range

interactions that may hinder the structure-properties relationships.

Recently, ionic liquids have been used in the preparation of the self-assembly of

dendrons into supramolecular columns and sphere [115]. These supramolecular structures

contain the ionic liquid part segregated as a core leading thus ILs nanoreactors with the

intention of making reaction in confined ionic liquid geometries.

In summary, the diversity of ionic liquids opens a new path in the field of

supramolecular polymers and it is possible to envisage ionic materials with enhanced

properties as is the case for the ionomers.


Page 47

I.2.3 Intercalating agents for layered silicates

I.2.3.1 Structure and properties of layered silicates

For the preparation of PLS nanocomposites, the most commonly used are layered

silicates (montmorillonite, hectorite, saponite) which belong to phyllosilicates 2:1. Their

crystal lattice consists of layers made up of an octahedral sheet of either aluminum or

magnesium hydroxide confined between two tetrahedrally coordinated silicon atoms layers.

Details concerning the structure and chemistry for these layered silicates are provided in

Figure I-10.

Figure I-10 – Structure of 2:1 phyllosilicates

The layer thickness is about few nanometers, and the width and length dimensions of

these platelets vary from tens of nanometers to few micrometers, depending of the nature of

the layered silicate. Their nanoscale dimensions are responsible for a high specific surface

(from 400 up to 700 m2.g-1) and their particular morphology confers them a high aspect ratio

(from 100 up to 1000). The consequence of the stack of the sheets leads to a Van der Waals

gap between the layers denoted interlayer space or clay gallery. In fact, isomorphic

substitutions (Al3+ by Mg2+) create locally negative charges on the layer surface that are

compensated by positive ions (alkali or alkaline cations) localized inside the clay galleries.


Page 48

These exchangeable cations are easily replaced by other organic cations which

separates the platelets and whose dimension (basal spacing d001) depends on the nature of the

cations and the degree of clay hydration. At a larger scale, clay is formed of particles and

particle arrangements with multiple organizations both from the nanoscale to micrometer

scale. Silicate platelets represent MMT elementary particles: MMT primary particles include

5 to 10 platelets (size 8 to 10 nm) and MMT aggregates consist in several MMT primary

particles stacked together without any preferential orientation (size 0.1 to 10 µm), as shown in

Figure I-11.

400 à 700 nm

400 à 700 nm

Aggregates

Ø de 1 à 30 µµµµm

Primary particles

e= 5 à 10 nm

Layers

e= 1 nm

400 à 700 nm

400 à 700 nm

Ø de 1 à 30 µµµµm

e= 5 à 10 nm

e= 1 nm

Figure I-11 – Structure of 2:1 phyllosilicates

In addition, the layered silicates, for example the montmorillonite with chemical

formula Mx (Al4-xMgx)Si8O20(OH)4 have hydroxyl groups on the edges of clay layers (or M is the

monovalent cation and x is the degree of isomorphous substitution between 0.5 and 1.3).

These two particular characteristics of layered silicates, exchangeable cations and hydoxyl

groups, are key parameters for the modification and the preparation of PLS nanocomposites.

I.2.3.2 Organic modification of layered silicates

The low cost of layered silicates, especially montmorillonite, is the driving force for

their use in the field of nanocomposites. However, it appeared that this nanofiller is only

compatible with the hydrophilic polymers such as polyethylene oxide (PEO) [116] or

poly(vinyl)alcohol (PVA) [117]. Regarding hydrophobic polymers, an immiscibility,

analogous to polymer blends is observed which results from the poor interfacial interactions

between organic and inorganic compounds. Such a poor affinity leads to poor thermal and

mechanical properties. It is therefore necessary to modify the surface of clays. The commonly

used surface treatments involve cationic exchange protocol and/or grafting of organosilanes.


Page 49

I.2.3.2.1 Grafting of organosilanes

This surface modification method is widespread in the case of metal oxides and silica

[118, 119] which has a surface chemistry allowing condensation reactions on the OH groups

of the surface. Such a chemistry could be transposed to layered silicates which possess

hydroxyl groups available on the edges of layers and between galleries [120]. This process

requires the use of organosilanes deposited in solution or gas phase. According to the

literature, the nature of the clay and the solvent are key parameters [121, 122]. In fact, He et

al. [122] have shown that grafting of aminopropyltriethoxysilane (γ-APS) on two clays, a

pristine montmorillonite and a synthetic fluorohectorite generates differences especially in

terms of reactivity and functionality of silane groups (Figure I-12).

Figure I-12 – The hypothetical diagram for the intercalation and silylation of g-APS

into clay interlayer and the possible structural models for T1, T2, T3 units [122]

Then, Shanmugharaj et al. [123] proposed a model similar to that established by

Negrete-Herrera [124] which shows that the localization of the silane groups (on the edges of

the layers or inside the galleries) is dependent on type of solvent used during the grafting

reaction.


Page 50

I.2.3.2.2 Cationic exchange

• Definition

The intercalation of inorganic exchangeable cations of layered silicates by organic

cations in an aqueous medium has been shown for the first time by Giesekind [125] and

Hendricks [126]. The mineral host structure and the nature of the intercalating agents are two

important parameters determining the success of the cationic exchange.

• Influence of the mineral host structure and the compensating cation

This type of layered silicates is characterized by a specific property known as the

cation-exchange capacity (CEC), and generally expressed in milliequivalent per gram

(meq/g). The cationic exchange capacity corresponds at the substitution of the sodium or

calcium compensating cations initially present at the platelet surface by monovalent cations in

100 grams of clay. Generally, the typical value of the CEC is included between 60 and 200

meq/100g. Below 50 meq/100g, the CEC is too low to allow sufficient intercalation while at

200meq/100g, separation of layers is prevented by excessive interlayer attractive forces. The

hectorite and montmorillonite are the best compromise to get an efficient cationic exchange.

The nature of cation plays a crucial role on the silicate swelling. Indeed, smaller the

cation is, greater mobility is which facilitates the exchange process.

• Selection of the organic cation

For several years, many of organic salts have been used as modifiers agents of layered

silicates and have been studied for the preparation of polymer-clay nanocomposites. The

surfactants most commonly used are alkylammonium salts [127, 128]. Recently, nitrogen and

phosphorus compounds are emerging as the alkylpyridinium, alkylimidazolium or

alkylphosphonium. The role of these organic salts is a key issue. In fact, their uses reduce the

surface energy of clay and improve the characteristics of the polymer matrix [129]. Besides

the presence of long chains based on organic cations induces a larger interlayer spacing which

allows the diffusion of polymer chains between the clay layers and dissociate them [130,

131].


Page 51

Sometimes, the organic cations are functionalized in order to react with the polymer or

from the addition of monofunctional or multifunctional components of the same nature as

surfactant to achieve a better compatibilization between filler surface and the polymer

matrices. For example, Ladika et al. [132] have used a combination ammonium modified

montmorillonite/ammonium polymer with high molar weights (3000g.mol-1) to get to the

exfoliation of clay layers.

- Ammonium salts

Currently, an intensive research is done on the modification of layered silicates by

several varieties of modifiers associated with chloride and bromide anions [127, 128]. The

alkylammonium salts which are derived from the synthesis of complete alkylation of amine or

ammonia, are the most used ionic liquids to prepare organoclays. The main role of quaternary

alkylammonium is to lower the surface energy of the modified clays and to improve the basal

spacing. The chemical natures of cation, length of carbon chains, the size and the shape of the

polar head have major effects on the success of the cationic exchange.

In the case of polymer/clay nanocomposites, alkylammonium influences the affinity

between the polymer and the clay surface. For example, it is known that the use of clays

treated with dialkyl dimethylammonium halides, in particular with two tallow chains of about

18 carbon atoms, a surface energy similar to polyolefins such as polypropylene and

polyethylene is obtained [129] while for polar polymers as polyamide, alkylammonium

functionalized by a benzyl or hydroxy functions have been recommended for a better affinity.

The alkyl chain length is responsible for the increase in interlayer space required for the

intercalation of polymer chains. Zig et al. [66] showed that organoclay modification with

protonated primary amines gives a much better toughness/stiffness balance with respect to

those modified with protonated secondary and tertiary amines or quaternary ammonium

cations, respectively. Because of the non polar nature of their chain, they reduce the

electrostatic interactions between the silicate layers and lower the surface energy of the

layered silicates so that an optimal diffusion of the polymer during the exfoliation process can

be obtained to dissociate the assembled clay layers.


Page 52

The combination of a quaternary ammonium salt with another organic compound

(methacrylate, maleic anhydride) was also very investigated to modify clays [133]. For

example, Liu et al. [134] developed a new class of modified clays based on a combination of

conventional alkylammonium and epoxypropyl methacrylate resulting a larger basal spacing

(2.98 nm).

The amino acids having a primary amino group -(NH2) and an acidic carboxyl group

(COOH) have been also intercalated between the clay layers of montmorillonite and used in

the synthesis of polyamide 6-clay hybrids [135].

Despite the compatibility of MMT modified by long alkyl-chain quaternary

ammonium with hydrophobic polymers (PE, PP), the lower thermal stability of conventional

alkylammonium ions showing a onset decomposition temperature of approximately 180°C

severely limits their use in the preparation of PLS considering matrices processed at high

temperatures such as polyamide (PA), polyethylene terephthalate (PET), or polyether ether

ketone (PEEK) [136].

- Ionic liquids

Organosilicas, silicas, metal oxides, carbon nanotubes and recently layered silicates

for the preparation of polymer nanocomposites from thermoplastics processed at high

temperature or from thermosets with high cure temperatures have been studied [137]. Figure

I-13 summarizes the different ionic liquids that are possible to be used as intercalating agents

for layered silicates [138]. The most commonly used are organic cations containing nitrogen

with pyridinium and imidazolium ionic liquids [139]. Other chemical compounds, such as

pyridinium, quinolinium [140] or phosphonium [141] were also studied due to their excellent

thermal stability compared to ammonium compounds [142].


Page 53

Figure I-13 – Cations and anions commonly used for the formation of ionic liquids [143]

The use of pyridinium salts, as a surfactant for layered silicates was described from the

70s by Slade et al. [144] who described and used hexadecylpyridinium ionic liquid to modify

vermiculite for the adsorption of non polar molecules. Others studies have shown the

adsorption capacity of pollutants such as chlorobenzene or phenanthrene by

dodecylpyridinium-modified bentonite [145, 146]. In opposite, very few studies have

investigated the thermal stability of these clays modified with such modifiers agents and their

uses in the preparation of PLS nanocomposites. However, the importance of the chemical

nature of the anion on the thermal stability of the modified clays has been shown. In fact, Kim

et al. [147] have demonstrated that the combination of pyridinium cation with fluorinated

anion such as BF4- leads to an increase of the thermal stability of the treated clays.

The imidazolium ionic liquids, opposite to the pyridinium salts have been the subject

of several studies for the preparation of organically modified clays and compared to the

conventional quaternary ammonium treated ones [138, 148]. The research focused on the

thermal decomposition of imidazolium salts and the role of their chemical structure.

According to the literature [149, 150], at high temperature, the imidazole cation is resistant to


Page 54

ring fission during thermal rearrangements of 1-alkyl and 1-aryl-imidazoles. These later

results explain a better thermal stability of the imidazolium cation compared to the

ammonium salts.

The influence of the anion type on the thermal stability of the imidazolium ionic

liquids have been studied by Awad et al. [139] that have demonstrated that for the

hexafluorophosphate PF6-, tetrafluoroborate BF4

- and bis(trifluoromethylsulfonyl)imide

N(SO2CF3)2, an increase of 100°C in the degradation temperature is obtained compared to use

of halide salts, i.e. bromide and chloride. Table I-2 summarizes the different imidazolium ionic

liquid used [139].

Table I-2 – Akyl imidazolium molten salts with different alkyl groups and counter ions

R1 R2 R3 X-

Methyl Methyl Propyl Br

Cl

BF4

PF6

Isobutyl

Hexadecyl

Eicosyl

Ethylbenzene

For clays modified with imidazolium salts as surfactant, the literature demonstrates a

large increase of degradation temperature under nitrogen and oxidative atmosphere compared

to ammonium-treated montmorillonites. In addition, despite the intrinsic stability of

imidazolium ionic liquids induced by the substitution of halide anions by fluorinated ones, no

significant improvement in the thermal stability of the MMT treated by ionic liquids having

tetrafluoroborate or hexafluorophosphate anions have been demonstrated.

Because of the well-known properties of phosphorous compounds, such as flame

retardancy and heat stabilization, the use of phosphonium ionic liquid as intercalating agents

for layered silicates has been widely studied [151, 152].

According to the literature the large increase (about 50°C) of the degradation

temperature for phosphonium-modified clays compared to ammonium-treated ones, implies a

large advantage for the preparation of PLS in the melt blending [153]. In 2007, Patel et al

[154] have prepared a series of montmorillonites modified with different bromide quaternary

phosphonium salts, denoted methyl, ethyl, propyltriphenylphosphonium,


Page 55

tetraphenylphosphonium, tributyltetradecylphosphonium, showing a higher thermal stability

than the ammonium treated montmorillonites. In particular, a large increase up to 300-400°C

of the thermal stability of the tetrabutylphosphonium- and tetraphenylphosphonium-modified

montmorillonites was shown.

• Consequences of the cationic exchange on lamellar silicates

Generally, the cationic exchange takes place in water or a mixture water / alcohol.

Indeed, the use of ultra-pure water is preferred in order to facilitate the clay swelling and to

get rid of no desired cation. Sometimes it is necessary to use an organic solvent as co-solvent

to dissolve fully intercalating agents. First, the layered silicates are mixed and stirred

vigorously in the aqueous media. Then, the modifiers agents based on the cation exchanged

capacity of the lamellar silicates are previously dissolved in the organic solvent and are added

to the clay suspension. However, several parameters such as temperature, organic modifiers

concentration and reaction time are all factors ensuring the success of the cationic exchange.

Indeed, the cationic exchange involves several types of interactions: first, electrostatic

interactions but also weak intermolecular interactions between the clay surface and the

surfactant. The nature of the compensating cations and the steric hindrance of the intercalating

agents play a crucial role in the distribution of the organic species shared between: i) the

physically adsorbed species on the clay surface ii) the intercalated species in the clay galleries

from 30 up to 40 wt% [155, 156].

- Effect of the long alkyl chain length on the organoclays structure

To characterize the orientation of the organic chains in the layered silicates, several

characterization methods, essentially X-Ray diffraction (XRD) and infrared (FTIR) have been

used in most of the papers [157, 158].

Initially, Lagaly and Weiss [157] deduced for the first time the orientations of organic

chains. They have demonstrated that the surfactant chain length and the charge density of the

layered silicate play a key role in the different arrangements of organic salts between the clay

layers. Figure I-14 highlights the different conformations adopted by the organic ions.


Page 56

Figure I-14 – Orientations of alkylammonium ions in the galleries of layered silicates: (a)

monolayer, (b) bilayers, (c) pseudotrimolecular layers, (d, e) paraffin-type arrangements of

alkylammonium ions with different tilting angles of the alkyl chains [160].

As illustrated in Figure I-14, as the chain length is less than 12 carbons, organic cations

retains a planar configuration, monolayer or bilayer type with a basal spacings of 1.35 nm and

1.75 nm, respectively [159, 65]. In contrary, when chain length increases, a

pseudotrimolecular configuration with the shift of the alkyl chains is obtained and leads to an

increase of intergalleries space (2.2 nm) [1]. When the organic cation has a long alkyl chain

(eighteen carbons), paraffin-type structures are observed with a larger interlayer distance (3.1

nm) [160].

A more practical method based on FTIR and XRD experiments has been proposed by

Vaia et al. [158] to observe the interlayer structure and the phase structure of organic salt.

Following the frequency shifts of the asymmetric and symmetric vibrations of methylene

groups (CH2)n of the aliphatic chain as a function of chain length, temperature, or the

interlayer density, different degrees of order of intercalated cations are evidenced. They have

showed that the intercalated chains can vary from liquid-like to solid-like state, with the

liquid-like structure resulting when the interlayer packing density or the chain length

decreases as well as the temperature increases. Overall, a more ordered structure is obtained

in the presence of long alkyl chain (eighteen carbons) with a liquid crystal behavior while the

chains with twelve or six carbons have a liquid-like and gas liquid-like, respectively. The

alkyl chain structuration models proposed by Vaia et al. [158] are presented in Figure I-15.


Page 57

Figure I-15 – Alkyl chain aggregation model proposed by Vaia et al [158]: a) short chain lengths, lateral

monolayer; b) medium chain lengths, in plane disorder and interdigitation to form quasi bilayers; c) long

chains lengths, interlayer order increases leading to a liquid-crystalline polymer environment.

Other methods have also been used to study the orientations of alkyl chains of treated

layered silicates such as molecular dynamics simulations [161] or solid state NMR. Hackett et

al. [161] have demonstrated that when the length of alkyl chains increases for a constant

interlayer distance, the stacking of the alkyl chains leads to increase pressure on the galleries

resulting in increased interlayer distances. Thus, a basal spacing of 1.32 nm is obtained for a

monolayer while bilayer and trilayer arrangement lead to basal spacing of 1.8 and 2.3 nm,

respectively. Figure I-16 shows the different arrangement of modifiers agents.

Figure I-16 – Molecular modeling of surfactant configurations with different chain lengths

intercalated in clays considering different cation exchange capacity by a) monolayer (CEC = 0.9

meq/g); b) bilayer (CEC = 1.2 meq/g); c) trilayer (CEC = 1.5 meq/g) [161]


Page 58

- Effect of the temperature

The temperature at which the cationic exchange is carried out has a significant

influence on the arrangement of the surfactant between clay layers. Le Pluart et al. [155] have

clearly demonstrated the effects of temperature on the cationic exchange mechanisms. Indeed,

the montmorillonite treated with octadecylammonium as modifiers agents at 60°C displays

two different organizations on the X-Ray spectrum with two quite brad peaks at 3.3 nm and

2.0 nm, indicative of the coexistence of a paraffin-like and a pseudotrilayer structures. When

the cationic exchange is performed at 80°C, the spectra show an intense, thin and regular

diffraction peaks at 3.2 nm and 1.6 nm that suggest a long-range order of paraffinic structure.

- Effect of the cationic exchange capacity of the modifiers agents

The amount of organic ions to be introduced during the cationic exchange process is a

important parameter [162]. In fact, an excess of salt is necessary to complete the layers of ions

ionically bonded to the platelet surface. An excess of organic species corresponding to 2 times

of the CEC at 80°C in deionised water is the most common condition reported in the literature

[163, 164].

- Identification of the organic species

Thermogravimetric analysis (TGA) may complete modified clay characterizations by

investigating the degradation mechanisms and the identification of the interactions between

clay mineral and intercalating agents. According to the literature [165], two kinds of

interactions take place which can be evidenced from thermal degradation kinetics: i) first

degradations from 150°C to 300°C correspond to the organic species physically adsorbed on

the clay surface via Van der Waals interactions and ii) degradations from 330 to 550°C are

related to the intercalated species via ionic interactions in the clay galleries.


Page 59

I.2.3.3 Conclusions

It was demonstrated from the literature that the use of ionic liquids as intercalating

agents of layered silicates was developed since many years by using quaternary

alkylammonium. However, the use of these later ILs is limited by their low thermal stability

which implies to focus on more thermostable ionic liquids based on pyridinium, imidazolium

or phosphonium cations.

Nevertheless, does the addition of such highly-thermostable IL surfactants improve the

dispersion of nanofillers in polymer matrices as well as the final properties of resulting

nanocomposites?


Page 60

I.2.4 Polymer/layered silicates

In the last decade, the dispersion of nano-layered silicate in a polymer matrix was a

challenge and has opened up new interests in materials science. These organic-inorganic

hybrid materials have shown significant increases compared to the properties of conventional

composites or pure matrix. It has also been demonstrated that the degrees of dispersion of

nanofillers in a polymer matrix and the method of preparation play a key role on the final

properties of the materials. Another important objective of the research is to create

nanocomposites with enhanced properties for a very low filler amounts in the polymer matrix.

To achieve this, different modified-clays have been used [138, 166]. In fact, there are many

studies on the use of ammonium-treated layered silicates [166, 167] while in opposite, the use

of thermostable ionic liquids such as pyridinium, imidazolium or phosphonium is poorly

developed. However, their use into polystyrene [122], polyethylene [168], polypropylene

[169], poly(vinylidene fluoride) [170], and poly(ethylene terephtalate) matrices [171] have

been reported.

I.2.4.1 Preparation methods of PLS nanocomposites

The preparation of PLS nanocomposites is strongly dependent on matrix parameters

such as hydrophobicity, molar masses, presence of reactive groups as well as characteristics

of the silicates, such as chemical nature of the cation and anion of the intercalating agents.

Three main routes are well-known to incorporate the layered silicates into the polymer matrix

(Figure I-17).

In situ

polymerization

In situ

Melt

Intercalation

Solution Intercalation

Layered silicates

Monomer

Figure I-17 – Different routes for the preparation of the nanocomposites


Page 61

I.2.4.1.1 Solution intercalation

Solution intercalation requires working on a solvent system in which the polymer or

pre-polymer is firstly dissolved while modified-clays or pristine clays are dispersed in a

suitable solvent to allow the swelling of the clay and to facilitate the insertion of polymer

chains between clay layers. Then, the solvent is evaporated and to form the nanocomposites.

This procedure is used for water soluble polymers such as PEO, PAA or PVP [116, 117, 172].

However, this method is used to prepare epoxy-based nanocomposites [173].

I.2.4.1.2 In situ intercalative polymerization

In situ polymerization is the first method used by Toyota [174, 135]. In the presence of

an initiator, the monomer is introduced between clay layers which activate the

polymerization. A variety of polymer nanocomposites has been prepared using this method,

i.e. polyamide (PA) [175], polystyrene (PS) [176] or polyolefins (PP or PE)/layered silicates

[165]. However, poor compatibility between the monomer and modified clays may prevent

the use of this method.

I.2.4.1.3 Melt intercalation

The great advantage of the melt intercalation technique is that the use of solvent is not

required. In fact, layered silicates or modified layered silicates are mixed with the polymer

matrix in the molten state. The mixing methods are extrusion and/or injection molding and are

easily applicable in the industry. That is why, this method is the most commonly used. Then,

the polymer chains are then intercalated or exfoliated to form nanocomposites. A wide range

of PLS nanocomposites such as polyolefins [165], poly(vinylidene fluoride) [166] have been

prepared using this method.


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I.2.4.2 Characterization of PLS nanocomposites

To characterize the morphology of nanocomposites, two techniques are often

associated X-ray diffraction (XRD) and transmission electron microscopy (TEM). The X-ray

diffraction is the method commonly used to determine the structure of nanocomposites. This

technique allows the determination of the interlayer distance between clay layers with Bragg's

law (nλ = 2d.sin θ, where n is the diffraction order, λ the wavelength of the incident X-ray and

θ the diffraction angle). The type of structure is determined by the displacement of the

diffraction peak. In fact, if no displacement of diffraction peaks towards lower angles is

observed, an intercalated structure is preserved. In opposite, if the peak is shifted to smaller

angles or totally disappears, it can assume that the clay layers are well distributed in the

polymer matrix. However, if the interlayer distances are greater than 8.0 nm, which

corresponds to an angle < 1 degree, the X-ray diffraction is limited. Generally, an overview of

the layered silicates distribution in the polymer matrix is necessary. Transmission electronic

microscopy is a good alternative but the morphological observations are based on a small part

of the sample and it may not be representative of the sample. The three different types of

nanocomposites based on the WAXD patterns and TEM images combination are shown in

Figure I-18.

Figure I-18 – WAXD patterns and TEM images of three different types of nanocomposites [177]


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I.2.4.3 ILs treated-Layered silicates for polymer nanocomposites

I.2.4.3.1 Polystyrene/IL-modified clays nanocomposites

Blends for acrylonitrile-butadiene-styrene (ABS) and ammonium- or imidazolium-

modified layered silicates, with ratios from 5 to 15 wt.% have been prepared in boiling

acetone by ultrasonic processing to improve the efficiency of the mixture [178]. Although the

XRD spectra are similar for both blends ABS/ammonium- or imidazolium-treated

montmorillonites, a higher degree of spatial distribution with the presence of several single

delaminated layers for ABS/imidazolium modified montmorillonite were evidenced.

Regarding the mechanical properties analyzed by dynamical mechanical analysis (DMA), an

increase of the stiffness of 40% at 25°C was obtained for ABS/montmorillonite treated with

imidazolium salt. The authors have attributed this increase to the degree of dispersion of clay

layers in the polymer matrix and also to the excellent thermal stability of imidazolium cation

that is not affected by the hot pressing procedure at 200°C for 10 min compared to ammonium

modified clays which must have been degraded.

In another work, PS/imidazolium clays nanocomposites have been prepared using a

similar method, i.e. in solvent blending (chlorobenzene) [176]. For identical intercalating

agents i.e. imidazolium ionic liquid, the nature of the clays, montmorillonite and fluorinated

synthetic mica, as well as that the use or not of sonication have been studied on the degree of

the clay exfoliation. The results clearly showed that the nature of clay mineral and the mixing

method are two key parameters. In fact, without sonification, they have demonstrated that the

use of montmorillonite led to a better clay layers distribution in the polymer matrix compared

to fluorinated synthetic mica. These different results could be explained by the higher charge

density and higher aspect ratio of the clay mineral. However, the only way to achieve

nanocomposites with exfoliated morphology is to use sonification as dispersion process.

The same authors have developed a technique based on melt rheology to describe the

morphology and to differenciate the degree of exfoliation in the PS / modified clays

nanocomposites [179]. A better dispersion of nanoclays was evidenced when clays are treated

with imidazolium surfactant. A schematic representation of clay dispersion state is given in

Figure I-19 as a function of the surface treatment of the montmorillonite.


Page 64

Afterdispersion

Afterdispersion

Figure I-19 – Schematic representation of speculated clay dispersion mechanism into the PS matrix as a function of the surface treatment

(a) ammonium modified MMT (b) imidazolium treated MMT

However, the authors concluded that the correlation between the melt rheology data

and the degree of clay dispersion is very difficult and that further studies are needed. In

conclusion, these studies demonstrated that the nature of the surfactant, the clay mineral, and

the mixing method are three key parameters to control in the processing of PS based

nanocomposites with a very fine dispersion of nanofillers.

In the literature, other methods of preparation of the nanocomposites based on

polystyrene, by melt compounding [180] or in situ polymerization [181] have been used as

well as various modifiers such as pyridinium, quinolinium, or phosphonium ionic liquids

[140, 180] in order to study their effects on the thermal, physical and the mechanical

properties of the nanocomposites.

Recently, Bottino et al. [181] prepared PS/montmorillonites nanocomposites at 3wt.%

via in situ polymerization. The influence of the surfactant (ammonium or imidazolium) and

the effects of the alkyl chains length for the imidazolium salt (C12, C16, C18) have been studied

on the morphology and the photo-oxydation properties. TEM micrographs of

polystyrene/montmorillonite treated with an imidazolium salt with short alkyl chain (C12)

show an intercalated morphology with very few exfoliated layers in the polymer matrix

compared to imidazolium functionalized with long alkyl chains (C16 and C18) or a partially

or fully exfoliated morphology is obtained. On the other side, the effects of imidazolium

treated montmorillonite are identical to the ammonium treated montmorillonite ones. And the

photo-oxydation properties of the PS/modified layered silicates are degraded compared to the

neat PS.

a)

b)


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Poly(styrene-co-acrylonitrile) (SAN)/modified clays nanocomposites by melt

compounding have been prepared [180]. Chu et al. [180] demonstrated that the

functionalization of the surfactant such as ammonium, imidazolium, or phosphonium is very

important. In fact, they have concluded that ammonium carrier polar functions (OH) or

aromatic imidazolium and phosphonium carriers aromatic function are the most adapted ones

for SAN nanocomposite preparation with the better clay dispersion. They also showed that the

use of the ammonium surfactant functionalized with hydroxyl groups improves the thermal

and the flammability properties of the nanocomposites. These results highlighted the

importance of the organic cation, especially its functionalization (alkyl chains, hydroxyl

groups, epoxy, vinyl) as a function of the matrix considered.

I.2.4.3.2 PVDF/IL-modified clays nanocomposites

Poly(vinylidene fluoride) (PVDF) nanocomposites based on ammonium, pyridinium

or phosphonium modified montmorillonites have been prepared by melt intercalation [170].

The influence of the modified clays on the morphology as well as on the physical and

mechanical properties has been investigated. XRD and TEM analyses show different

morphologies as a function of the nature of the surfactant. Thus, an exfoliated structure is

obtained for ammonium-treated and pyridinium-treated montmorillonites while phosphonium

ionic liquid modified montmorillonite leads to a partially exfoliated nanocomposite

morphology. The mechanical properties of nanocomposites are superior to those of neat

PVDF. However, the influence of thermostable ionic liquids, pyridinium and phosphonium,

on mechanical properties is limited. In fact, the storage moduli of the neat PVDF and the

PVDF nanocomposites are the same. Only the elongation at break increases (+175%) with 5

wt% of pyridinium treated montmorillonite compared to +200% of strain at break obtained

for ammonium modified montmorillonite. The authors are also interested in the effects of

modified clays on the polymorphic structure of the PVDF mainly composed of the most

common α and β phases. They have noticed that the use of phosphonium clay increases the

melting and crystallization temperatures of the matrix. Moreover, phosphonium

montmorillonite is the most efficient nucleating agent and an excellent generator of β-phase,

that is the phase required for dielectrical applications.


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I.2.4.3.3 Polyolefins/IL modified clays nanocomposites

Polypropylene/imidazolium modified clays nanocomposites have been prepared using

melt compounding method [169]. Morphological analyses of the nanocomposites containing

3vol% of organically treated montmorillonite (OMMT) by transmission electronic

microscopy showed a mixed intercalated/exfoliated morphology with few tactoïds and a few

single clay layers. The tensile properties of both composites with different filler volume

fraction have been studied. The modulus increases of 35% for PP/OMMT containing 4 vol%

of fillers while a decrease of relative yield strain is observed whereas an improvement of the

relative oxygen permeation is noticed. TGA analyses shows that the addition of only 1 vol%

of imidazolium modified montmorillonite improves the thermal behavior but this

improvement is optimized for PP/OMMT with 4 vol% of fillers. In conclusion, Mittal has

shown that the use of imidazolium treated clay allows an increase of the thermal, barrier and

mechanical properties.

Other studies on polypropylene nanocomposites based on imidazolium-modified

montmorillonite have been reported [182, 173]. In fact, Ding et al. [173] have used

imidazolium salt as intercalating agents of montmorillonite for the preparation of

nanocomposites by in situ dissolution of isotactic polypropylene (xylene being required as

solvent). The influence of solvent (water vs xylene) during the montmorillonite modification

was also investigated. XRD and TEM analyses show a well exfoliated montmorillonite

platelets which are disordely dispersed in the PP matrix. The consequences of the addition of

OMMT is a significant increase of 166°C respect to virgin polypropylene when the treatment

surface of montmorillonite is carried out in xylene compared to + 62°C when the water is

used.

In addition, Mittal [169] showed that the use of trialkylimidazolium modified MMT

has a significant effect on the final properties of the PP/layered silicates nanocomposites. In

opposite, the remplacement of trialkylimidazolium by dialkylimidazolium cation in the

polyethylene matrix does not affect the thermal properties of the PE nanocomposites [164].

These results show that the use of ionic liquids as surfactant could be very promising.


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I.2.4.3.4 Polyester/IL modified clays nanocomposites

The preparation of the polyethylene terephthalate (PET)/IL-modified clays requires

working at high temperature. PLS nanocomposites with different ILs-modified

montmorillonites have been prepared by melt intercalation using a brabender internal mixer at

280°C [171, 183]. Thermally stable montmorillonites modified with pyridinium, quinolinium,

imidazolium and phosphonium surfactants were considered. In both cases, the thermal

decomposition of the intercalating agents, evidenced by the change of the color of the

nanocomposites has not modified the final properties.

Nanocomposite from Poly(ε-caprolactone) have been prepared by in situ

polymerization with dibutylin dimethoxide as an initiator [184]. The materials showed an

excellent dispersion of clay layers with a highly exfoliated nanocomposite morphology. The

authors have demonstrated that the use of imidazolium functionalized MMT with hydroxyl

function bearing a long flexible alkyl chain allowed the grafting of polymer chains on the

montmorillonite surface. This good affinity between the polymer and the modified clays leads

to a significant increase of storage moduli.

I.2.4.3.5 Polyamide/IL-modified clays nanocomposites

Polyamide (PA) nanocomposites based on imidazolium or phosphonium treated

montmorillonites have been produced by melt compounding [151, 175]. In the first study

[175], the use of imidazolium or ammonium with one long alkyl chain as surfactant of

montmorillonite leads to a poor dispersion of the organoclay in the matrix. This poor

distribution could be explained by the size and the geometrical structure of the organic cations

which screen the silicate surface, i.e. leading to a difference of polarity between polyamide

(polar) and treated clays (hydrophobic).

The use of phosphonium or ammonium with alkyl chains to treat clays in PA6 matrix,

leads to a mixed intercalated/exfoliated morphology in the both cases [151]. This

phenomenom can be explained by the preparation method of organically modified

montmorillonites which are modified under supercritical carbon dioxide. This method of

treatment of the organically modified layered silicates may have played a role on the polarity

of the clay which had the effect of improving the interactions between clay and polyamide

matrix. Moreover, the use of phosphorous compound-based MMT play a positive role on the

fire retardancy properties of the nanocomposites compared to ammonium compound. In

conclusion, the polarity of layered silicates is the key parameter.


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I.2.4.3.6 PVC/IL-modified clays nanocomposites

The influence of modified layered silicates on the thermal stability, especially the

thermal behavior of the nanocomposites has been also studied [185]. PVC nanocomposites

containing 2wt% of clays treated with ionic modifiers as imidazolium, ammonium or

phosphonium salts and non-ionic modifiers such as polypropylene glycol or glycerol

monostearate were compared. The non ionic compounds did not affect the thermal stability

whereas the use of cationic surfactants accelerates the dehydrochlorination of PVC in the

following order: phosphonium ion, imidazolium ion, alkyl ammonium ion, and ethoxy/alkyl

ammonium ion.

These results show the importance of the choice of the intercalant and the matrix. In

this case, no increase of the thermal stability is obtained but for the PVC plasticized with

phthalates, an improved thermal stability has been reported [186, 187]. Instead of using ionic

liquids as intercalating agents for lamellar silicates to improve the thermal and mechanical

properties of the PVC nanocomposites, it may be preferable to consider only the use of ionic

liquids as reinforcing and plasticizers agents [162].

I.2.5 Conclusions

Layered silicates are one-dimensional nanofillers less than 100 nm with a significant

aspect ratio and an high specific surface commonly used to overcome the limitations of

conventional polymer composites. However, pristine layered silicates are not compatible with

organic polymers, especially with hydrophobic polymers. This poor affinity between the clay

mineral and the polymer leads to composites resulting in poor properties. In order to improve

the properties, the distribution of layered silicates in polymer matrices and the interactions

between the surface of layered silicates and the polymer requires to be improved with organic

treatment of clays. The surface modification of layered silicates using modifying agents based

on organic cations, especially ammonium salts have widely been reported.

Nevertheless, a lower thermal stability of the quaternary alkylammonium salt due to

the Hofmann's elimination has a limiting effect on their use in the processing of the

thermoplastic and thermosetting nanocomposites requiring higher temperatures. Recently,

ionic liquids based on pyridinium, imidazolium, or phosphonium cations are emerging as a

new alternative for the preparation of thermally stable organically modified clays. The


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preparation, the characterization and the properties of different nanocomposites using these

new surfactants to modify the fillers have been discussed in this part. Thus, we have

demonstrated that the PLS nanocomposites with low ILs modified clays amounts (< 5 wt%)

have displayed properties far mostly superior to the neat polymer matrix ones.

Conclusions of chapter I Ionic liquids are organic salts with melting temperatures below 100°C and offer many

properties such as excellent thermal stability, negligible vapor pressure, non-flammable, high

ionic conductivity, tunable solubility for organic and inorganic molecules and multiple

combinations of cations/anions. These many distinct advantages of the ionic liquids are the

focus of academic and industrial research in various applications such as organic synthesis,

inorganic materials synthesis, electrochemistry, fuell cells, supercritical fluids, homogeneous

and heterogeneous catalysis and in the polymer science, particularly in polymer gel

electrolytes, lubricants and plasticizers. Despite their many advantages, due to their good

thermal stability, the use of ionic liquids in the field of the nanocomposites is mainly limited

to the role of modifiers agents for layered silicates. We have shown that the influence of ILs

modified clays on the PLS nanocomposites are mixed though. The true potential of ILs based

on the infinite cation/anion combinations is not really exploited. In fact, the reduced choice

and the high cost of the commercial ionic liquids functionalized with different groups (epoxy,

hydroxy, vinyl, and fluorinated chains) are the main causes of this limitation. To achieve

significantly optimized PLS nanocomposites, a lot of studies are needed to find the suitable

association between the specific IL and the matrix.


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Chapter II: Interactions Polymer/Ionic Liquids

Page 75

Chapter II POLYMER/IONIC LIQUID INTERACTIONS In the field of polymer materials, ionic liquids have often been used as a green solvent

and conductors in the gel electrolyte or as a surfactant to the layered silicates. So far, to our

knowledge, no work mentions the use of IL as structuring agent of a polymer matrix.

In this second chapter, we sought to investigate the impact of the IL, introduced as an additive

in the polymer matrix on the morphology, physical and thermo-mechanical properties of the

polymer. The effects generated by the ionic liquids can be modulated by the wide range of

possible combinations of cations / anions. In this work, we have chosen to introduce the IL in

fluorinated aqueous suspension comprising polytetrafluoroethylene (PTFE) stabilized. The

specification is difficult because the PTFE have excellent thermal stability, high resistance to

acids and bases and a low friction coefficient. What are the contributions of using ionic

liquids introduced at a low rate (1%wt) in the PTFE matrix after film formation?


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Pages

II.1 New building blocks ........................................................................................................... 77 II.1.1 Introduction ................................................................................................................................... 77 II.1.2 Experimental ................................................................................................................................. 78

II.1.2.1 Materials __________________________________________________________________ 78 II.1.2.2 Processing and characterization of the IL/PTFE films _______________________________ 78 II.1.2.3 Synthesis of ionic liquids _____________________________________________________ 80

II.1.2.3.1 Synthesis of phosphonium salt ....................................................................................... 80 II.1.2.3.2 Synthesis of imidazolium salt ........................................................................................ 82 II.1.2.3.3 Synthesis of pyridinium salt ........................................................................................... 82

II.1.3 Morphology and mechanical performances of polymer/IL blends ................................................ 83 II.1.4 Conclusions ................................................................................................................................... 85

II.2 Nanostructuration of ionic liquids in fluorinated matrix: Influence on the mechanical properties ......................................................................................................................................... 86

II.2.1 Introduction ................................................................................................................................... 86 II.2.2 Results and discussion ................................................................................................................... 87

II.2.2.1 Effect of ionic liquids on the structuration of fluorinated polymer films _________________ 87 II.2.2.1.1 Nanostructures in bulk .................................................... Error! Bookmark not defined. II.2.2.1.2 Surface analysis of fluorinated polymer/IL blends ........................................................ 89

II.2.2.2 Effect of ionic liquids on the thermal properties of fluorinated polymer-based blends ______ 90 II.2.2.3 Effect of ionic liquids on the PTFE crystallinity____________________________________ 91

II.2.2.3.1 Influence of organic cation ............................................................................................. 91 II.2.2.3.2 Influence of halide and fluorinated anions associated on phosphonium cation.............. 92

II.2.2.4 Effect of ionic liquids on the mechanical properties of fluorinated polymer ______________ 93 II.2.2.4.1 Dynamical mechanical analysis of fluorinated polymer-IL blends ................................ 93 II.2.2.4.2 Mechanical properties of fluorinated polymer modified using ILs. ..... Error! Bookmark

not defined. II.2.2.4.3 Effect of strain rate on the uniaxial tension behaviour of polymer/IL films .................. 96 II.2.2.4.4 Effect of ionic liquids on the morphology after deformation ......................................... 97

II.2.3 Conclusions ................................................................................................................................... 99

Conclusions of chapter II ............................................................................................................. 100

References of chapter II ............................................................................................................... 101


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II.1 New building blocks The use of the multiple combinations of ionic liquids (ILs) as functional building

blocks based on pyridinium, imidazolium and phosphonium cations to achieve materials

combining a structuration at nanoscale with the dramatic mechanical properties of the

resulting ionomers has been successfully demonstrated for the first time in a fluorinated

matrix.

II.1.1 Introduction The ability to create regularly shaped nanoscale objects which serve as the building

block is an extremely important goal in materials science. One of the key issues is to design

and create new polymer materials with unprecedented improvements in their physical

properties. Different self-assembly pathways are described to lead to hierarchical structures

formed from heterogeneous chemical species, like organic molecules, polymers, organic-

inorganic nanobuilding blocks. Nanostructured thermosets may be obtained by the self-

assembly of amphiphilic block copolymers in a reactive solvent and freezing of the resulting

morphologies by crosslinking reactions [1]. The introduction of fillers with nanometer-scale

dimensions into a polymer matrix is another route to prepare nanostructured polymers [2].

Ionic liquids (Ils), which are organic salts with a melting point below 100°C and have

been widely promoted as “green solvents” are attracting much attention of academic and

industrial research in many fields of chemistry and industry due to their chemical stability,

excellent thermal stability, inflammability, low vapor pressure and high ionic conductivity

properties. The ionic liquids could present an advantageous and promising way in a wide

variety of applications: Over the last few years, ILs have been popularly used as solvents for

organic synthesis, as well as homogeneous and heteregeneous catalysis, electrochemical

applications, electrolyte batteries, fuel cells, and also been used as media for polymerization

processes [3]. ILs are also used as surfactants for lamellar silicates. The conventional ILs

frequently used in the layered silicates-based nanocomposite are mainly quaternary

ammonium [4].


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If few studies indicate the use of thermostable ionic liquids based on imidazolium or

phosphonium ions for layered silicates intercalation [5-7], no work has been published yet on

the use of ILs as new building blocks. This study reports for the first time the achievement of

a nanoscale structuration from ILs into a polymer matrix. This structuration can be tuned by a

wide choice of cation-anion combinations including pyridinium imidazolium, and

phosphonium as cation associated to iodide, bromide, or fluorinated anions. The preparation

of IL nanostructured films from a fluorinated polymer solution could open many applications

in energy and materials fields by creating new polymers with interesting properties.

II.1.2 Experimental

II.1.2.1 Materials

All chemicals necessary to the synthesis of ionic liquids, i.e. triphenylphosphine

(95%), imidazole (99.5%), pyridine (99%), iodooctadecyl (95%), and solvents (toluene,

sodium methanoate, pentane, and acetonitrile) were supplied from Aldrich and used as

received. The polytetrafluoroethylene used in this study, called PTFE, is considered as an

aqueous dispersion of PTFE from Solvay. The composition is as follows: PTFE (60%wt),

water (32-33%wt), octylphenol polyethoxylates Triton® (7% wt), and ammonium

perfluorooctanoate (0.1%wt). The pH of aqueous dispersion is 10 and the PTFE particle

average size is about 220 nm.

II.1.2.2 Processing and characterization of the IL/PTFE films

The aqueous suspensions of PTFE containing 1%wt of pyridinium or imidazolium or

phosphonium ionic liquids were stirred using a Rayneri® disperser with a speed of 1500 rpm

for 15 minutes. Then the suspension was spread on stainless steel plates using a bar coater

equipped to get 50 µm-thick wet layer. A thermal treatment at 400°C for 10 minutes was

applied to obtain the final polymer film (thickness of the dried and sintered film: 20 µm).

Thermogravimetric analyses (TGA) performed on the ionic liquids and on PTFE/IL

nanocomposites were characterized using a Q500 thermogravimetric analyser (TA

instruments). The samples were heated to 700°C at a rate of 20 K.min-1 under nitrogen flow of

90mL/min.


Page 79

DSC measurements were performed ousing a Q20 (TA instruments) from -20°C to

400°C. The samples were kept for 3 min at 400°C to erase the thermal history before being

heated or cooled at a rate of 10 K.min-1 under nitrogen flow of 50mL/min. For calculations of

enthalpy of melting and crystallization, the enthalpy of reference considered for PTFE is 82

J/g [19-20].

Wide angle X-ray diffraction spectra (WAXD) were carried out using a Bruker D8

Advance X-ray diffractometer at the H. Longchambon diffractometry center (Université de

Lyon) at room temperature. A bent quartz monochromator was used to select the Cu Kα1

radiation (λ = 0.15406 nm) and run under operating conditions of 45 mA and 33kV in Bragg-

Brentano geometry. The angle range analyzed is included between 1 and 30°2θ.

Small Angle X-ray Scattering was carried out on the D2AM beamline at the European

synchrotron Radiation Facility (ESRF, Grenoble, France). The incident photon energy was 16

keV. A bidimensional detector (CCD camera from Ropper Scientific) was used to collect the

scattered radiation. The contribution of the empty cell was subtracted from the scattering

images of the studied samples.

X-ray Photoelectron Spectroscopy experiments were carried out in a KRATOS AXIS

Ultra DLD spectrometer using a hemispherical analyzer and working at a vacuum better than

10-9 mbar. All the data were acquired using monochromated Al Kα X-rays (1486.6 eV, 150

W), a pass energy of 40 eV, and a hybrid lens mode. The area analysed is 700µm x 300µm.

The peaks were referenced to the C-(C,F) components of the C1s band at 284.6 eV.

Transmission Electron Microscopy (TEM) was performed at the Center of

Microstructures (Université de Lyon) using a Philips CM 120 field emission scanning

electron microscope with an accelerating voltage of 80 kV. The samples were cut using an

ultramicrotome equipped with a diamond knife, to obtain 60 nm thick ultrathin sections.

Then, the sections were set on copper grids for observation.

Uniaxial Tensile tests were carried out using a MTS 2/M electromechanical testing

machine at 22±1°C and 50±5% relative humidity at crosshead speed of 0.004 and 0.2 s-1. A

cookie cutter was used to obtain dumbbell-shaped specimens.


Page 80

Dynamic Mechanical Thermal Analysis were carried out using a Rheometric Solid

Analyzer RSA II operating at ± 0.01% tensile strain and a frequency of 1Hz. The heating rate

was 3 °K.min-1 from -130°C to 320°C.

II.1.2.3 Synthesis of ionic liquids

In this paper, a general and simple method for the synthesis of a serie of organic halide

and fluorinated salts is reported based on

i) Iodide (I-), bromide (Br-) and hexafluorophosphate (PF6-) combined

phosphonium cations with one long alkyl chain (1a-1c) denoted C18P I-, C18P Br-,

C18P PF6-; respectively.

ii) Iodide associated imidazolium cation with two long alkyl chains

denoted C18C18Im I- (2)

iii) Iodide combined pyridinium salt denoted C18Py I- (3). The synthesis of

pyridinium, imidazolium, and phosphonium ionic liquids were presented in Figure

II-1.

PC18H37

N N N NH37C18

HN N

MeO Na C18H37I

- NaI

N NH37C18

C18H37 I

X

X = I , Br

C18H37X

P

2

1a, 1b

25°C

HPF6

PC18H37 PF6

1c

N N

C18H37I

C18H37I

C18H37

I

3

Figure II-1 – Synthesis and structure of ionic liquids

II.1.2.3.1 Synthesis of phosphonium salt


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Two different procedures were used for the synthesis of phosphonium ionic liquids

combined to halide (I-, Br-) or fluorinated (PF6-) anions. The first one is the synthesis of

halide salts C18P I- and C18P Br-: In a 100 mL flask was placed under a positive nitrogen

pressure, triphenylphosphine (1 equiv., 20 mmol, 5 g) and a solid of alkyl iodide and bromide

[1 equiv., 20 mmol i.e; 13 g for octadecyl iodide (C18H37I) and 11 g for octadecyl bromide]

were diluted in toluene (20mL). The stirred suspension was allowed to react for 24 h at

120°C, and a yellow precipitate was formed. The reaction mixture was then filtered, and

washed repeatedly with pentane. The synthesis of salts was confirmed by 1H NMR, and 13C

NMR spectroscopies.

Octadecyltriphenylphosphonium iodide (C18P I-) 1a. White solid, Yield = 90%. 1H

NMR (CDCl3): δ 0.8-0.90 (m, 3H, CH3); 1.10-1.35 (m, 28, CH2Me); 1.50-1.70 (m, 4H, PCH2(CH2)2); 3.50-3.70 (m, 2H, PCH2); 7.70-7.90 (m, 15H, H arom). 13C NMR (CDCl3): δ 14.00 (CH3); 22.67 (CH2Me); 23.2; 29.37-29.66; 30.24; 31.85 (PCH2); 118.45; 130.43; 133.70; 135.15 (P-Carom.).

Octadecyltriphenylphosphonium bromide (C18P Br-) 1b. White solid, Yield = 90%. 1H NMR (CDCl3): δ 0.8-0.85 (m, 3H, CH3); 1.05-1.30 (m, 28, CH2Me); 1.55-1.75 (m, 4H, PCH2(CH2)2); 3.50-3.65 (m, 2H, PCH2); 7.70-7.85 (m, 15H, H arom). 13C NMR (CDCl3): δ 14.00 (CH3); 22.52 (CH2Me); 23.0; 28.90-29.66; 30.20; 32.00 (PCH2); 118.85; 130.80; 133.50; 135.45 (P-Carom.).

Octadecyltriphenylphosphonium hexafluorophosphate (C18P PF6-) has been prepared

by anionic exchange according to the following procedure : In a 100 mL flask,

octadecyltriphenylphosphonium iodide (C18P I-) (5.000 g, 13.14 mmol, 1 equiv.) was

dissolved into dichloromethane (25 mL). The mixture was stirred for 30 min at room

temperature. A solution of hydrogen hexafluorophosphate (HPF6) (3.830 g, 26.28 mmol, 2

equiv.) diluted in water (25 mL) was stirred for 30 min and added to the

octadecyltriphenylphosphonium iodide solution. The stirred suspension was allowed to react

for 24 h at room temperature. The reaction mixture was then introduced in a separatory funnel

and the organic layer was washed repeatedly with distilled water (4x 25 mL). The mixture

was dried over anhydrous magnesium sulfate, concentrated under reduced pressure. The

solvent was removed by evaporation under vacuum.

Octadecyltriphenylphosphonium hexafluorophosphate (C18P PF6

-) 1c. White solid, Yield = 90%. 1H NMR (CDCl3): δ 0.8-0.90 (m, 3H, CH3); 1.10-1.35 (m, 28, CH2Me); 1.45-1.70 (m, 4H, PCH2(CH2)2); 3.40-3.55 (m, 2H, PCH2); 7.55-7.85 (m, 15H, H arom). 13C NMR (CDCl3): δ 14.00 (CH3); 22.35 (CH2Me); 23.5; 29.12-29.74; 30.35; 31.75 (PCH2); 118.75; 130.22; 133.50; 135.05 (P-Carom.).


Page 82

II.1.2.3.2 Synthesis of imidazolium salt

Three steps are necessary for the synthesis of N-Octadecyl-N'-octadecylimidazolium

iodide (C18C18Im I-). The first one is the deprotonation of the imidazole ring with a solution of

sodium methoxide prepared as follows : Sodium (1 equiv., 0.465 g, 20 mmol) is dissolved in

dry freshly distilled methanol (10 mL) in a septum sealed, 100 mL round-bottomed, three-

necked flask equipped with a condenser, a nitrogen inlet and a magnetic stirrer. Then,

Imidazole (1 equiv., 1.37 g, 20 mmol) and a small amount of acetonitrile (10 mL) were

introduced into the solution of sodium methoxide at room temperature. After 15 min, the

white suspension was formed and concentrated under reduced pressure. The dried white

powder was diluted in acetonitrile and a powder of octadecyl iodide (C18H37I) was added

under nitrogen atmosphere. The mixture was heated at 85°C during 24 hours. After the first

alkylation, the same procedure was used for the second alkylation. After cooling to room

temperature, the solvent was removed by evaporation under vacuum, and the beige solid

obtained was filtered, washed repeatedly with pentane and dried. The assignment of 13C, 1H

NMR resonance peaks are reported below.

Octadecyloctadecylimidazolium iodide (C18C18Im I-) 2. White powder, Yield =

97%. 1H NMR (CDCl3): δ 0.75-0.90 (m, 6H, 2CH3), 1.15-1.30 (m, 64H, 32 CH2], 1.80-1.90 (m, 2H, NCH2CH2), 4.30 (t, 2H, CH2N=), 7.45 (m, 1H, H arom), 7.65 (m, 1H, H arom), 9.15 [s (b), 1H, H arom]. 13C NMR (CDCl3): δ 14.10 (2CH3); 22.67 (2CH2Me); 26.23; 28.97; 29.35-29.69; 30.24; 31.91 (CH2); 50.10; (CH2N=); 50.32 (CH2N-); 121.69; 122.48 (=CN); 136.88 (N-C=N).

II.1.2.3.3 Synthesis of pyridinium salt

The general procedure for the synthesis of pyridinium iodide (C18Py I-) is the

following: In a 100 mL flask was placed under a nitrogen pressure, 10 mmol of alkyl iodide

[octadecyl iodide (C18H37I)] and distilled pyridine (1.5 equiv.). The stirred suspension was

allowed to react for 24 h at room temperature. A yellow precipitate was formed. The reaction

mixture was then filtered, washed repeatedly with pentane. Most of the solvent was removed

under vacuum. Pyridinium iodide was isolated after drying and fully characterized by 1H

NMR, 13C NMR spectroscopies.

Octadecylpyridinium iodide (C18Py I-) 3. White solid, Yield = 90%. 1H NMR

(CDCl3) δ: 0.84 (t, J = 7.3 Hz, 3H, CH3), 1.15-1.30 (m, 30H, 15 CH2), 1.90-2.02 (m, 2H, NCH2CH2), 4.90 (t, J = 7.5 Hz, 2H, NCH2), 8.05 (t, J = 7.2 Hz, 2H arom), 8.45 (t, J = 7.8 Hz, 1H, H arom), 9.35 (d, J = 5.4 Hz, 2H, H arom). 13C NMR (CDCl3) δ: 14.04 (CH3); 22.58; 25.91; 28.99; 29.25-29.60; 31.81-31.84 (CH2); 62.02 (CH2N=); 128.57 (C=C); 144.82; 145.50 (=CN).


Page 83

II.1.3 Morphology and mechanical performances of polymer/IL blends

To prepare new supramolecular ionic networks, various functional ionic components

have been introduced in very low quantities (1 wt%) in a fluorinated matrix. The influence of

the organic cation, i.e pyridinium, imidazolium, or phosphonium on the structuration of the

matrix, was studied. TEM micrographs reveal the presence of ILs into the fluorinated polymer

matrix due to the difference of electronic density between polymer and IL phases. Images

carried out with only 1 wt % of pyridinium, imidazolium, or phosphonium ionic liquids are

reported in Figure II-2 show different types of structuration which are tuned by the chemical

nature of cations.

(a) (b)

(c) (d)

200 nm

200 nm

200 nm

200 nm

Figure II-2 – TEM micrographs of the neat PTFE matrix and polymer/IL blends (a) PTFE; (b) PTFE/C18C18Im I-; (c)

PTFE/C18Py I-; (d) PTFE/C18P I-

By using C18C18Im I- as IL, two different structurations of ionic liquid are evidency

simultaneously: a co-continued morphology and many aggregates of ILs. A less achieved co-

continuous morphology is also obtained with C18Py I-. Only the phosphonium ionic liquid

leads to an excellent distribution and dispersion of IL since a structuration at nanoscale is

observed in the whole polymer.


Page 84

The chemical nature of the counteranion significantly influences the final morphology

as shown in Figure II-3 by using the same phosphonium cation combined with different anions,

i.e. : iodide (I-), fluoride (PF6-), and bromide (Br-).

200 nm 200 nm 200 nm

Figure II-3 – TEM micrographs of the nanocomposites (a) PTFE/C18P I-, (b) PTFE/C18P Br-, (c) PTFE/C18P PF6-

Even if both C18P I- and C18P Br- have an identical thermal stability (degradation at

330°C), the final morphology is quite different. A fine structuration at nanoscale is clearly

obtained with the iodide anion whereas with the bromide anion, a poor distribution of large

aggregates can be observed. A coarse morphology resulting from a good distribution of

aggregates is also obtained with the fluorinated anion. This final morphology can be

associated to the excellent thermal stability of hexafluorophosphate based on phosphonium [6,

7].

These nanoscale structurations have a dramatic impact on the mechanical behaviour of

polymer films as shown in Figure II-4. But the mechanical performance is very dependent on

the ionic liquids used, i.e. pyridinium, imidazolium, versus phosphonium. Better interactions

seem to take place between the phosphonium ionic liquid functionalized with long alkyl

chains and the fluorinated matrix.

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500 600

Strain at break (%)

Str

es

s (

MP

a)

PTFE

PTFE C18P I-

Figure II-4 – Effect of the phosphonium ionic liquid (1wt%) on the mechanical properties determined by

uniaxial tensile tests at room temperature and 0.004 s-1: (♦) PTFE; (■) PTFE/C18P I-


Page 85

Indeed, in the case of phosphonium ionic liquid containing iodide anion, an excellent

dispersion in the polymer film provides a significant effect on the mechanical properties of

the polymer as both stiffness and strain at break are improved compared to the neat PTFE.

This behaviour could be explained by the existence of the web structure of IL which sustains

load during mechanical strain.

II.1.4 Conclusions

In conclusion, we have demonstrated that the chemical nature of cations and

counteranions plays a key role on the dispersion and the structuration of ionic liquids in the

polymer matrix. As a consequence, the morphologies and the physical properties of PTFE/IL

blends can be tuned by a suitable combination of cation-anion IL. This first work highlights

the huge potential of these new building blocks in the polymer materials.


Page 86

II.2 Nanostructuration of ionic liquids in a fluorinated matrix: Influence on the mechanical properties

In this work, Ionic liquids (IL) have been used for their intrinsic and unique properties.

Pyridinium, imidazolium, and phosphonium ionic liquids have been synthesized and used as

new synthetic building blocks in a polymer matrix. From a fine tuning of the IL cation-anion

combination, the influence of the chemical nature of buildings blocks was investigated on the

morphology by transmission electronic microscopy (TEM) and small-angle X-Ray scattering

(SAXS). The physical and mechanical properties of material were discussed in relationship

with the structuration. An incredible increase of both flexibility and stiffness was achieved

from a nanoscale structuration resulting good interactions between ionic liquid and the

fluorinated matrix.

II.2.1 Introduction

In the world of nanotechnology, the goal is to create new materials with significantly

improved physical properties by designing structuration of material at nanoscale. For many

years, several approaches have been described for preparing new structures from organic

molecules, polymers, or organic-inorganic hybrids. One of the well- known approaches

consists in the design of nanostructured thermosets obtained by the use of block copolymers

with amphiphilic block [1-3]. Another way for the development of nanostructured polymers is

the introduction of inorganic nano-objects having nanometer-scale dimensions such as silica

[4], layered silicates [5-10], or carbon nanotubes [11-13]. In addition, ionic liquids (IL)

appear as a new alternative and are subject to the attention of research in many applications

due to their unique properties such as excellent thermal stability, non-flammability, low

saturated vapor pressure, good electrical, and thermal conductivity. In most cases, ionic

liquids are used as green solvents in organic synthesis [14] and synthesis of nanoparticles

[15], homogeneous and heterogeneous catalysis [16], in polymer science as plasticizers [17],

or lubricants [18]. They can also be used as surfactants in the lamellar silicate nanocomposites

and the most frequently used are the ammonium salts [19-21]. In recent years, pyridinium,

imidazolium, and phosphonium ionic liquids known for their excellent thermal stability are

increasingly used [22-23]. Moreover, the wide range proposed of ionic liquid based on many

combinations cation/anion offer new perspectives in polymer science, particularly in the field

of energy. However, from our knowledge, no study describes the use of ionic liquids in order


Page 87

to design nanostructured materials from a polymer matrix. This paper will describe the

generation of nanostructured phase based on ionic liquids of different chemical natures in a

fluorinated polymer matrix as well as the consequences of this nanostructuration on the

physical and mechanical properties.

II.2.2 Results and discussion

II.2.2.1 Effect of ionic liquids on the structuration of the fluorinated matrix

II.2.2.1.1 Analysis of morphology by transmission electronic microscopy (TEM)

Transmission electron microscopy is the suitable tool to reveal the existence of IL as a

separated phase into the fluorinated matrix according to the difference in electronic densities.

With only 1 wt % of ILs, a structuration is reached (Figure II-5). In this study, the chemical

nature of the cation and halide anions plays a key role on the different morphologies obtained.

The dispersion of imidazolium ionic liquid (C18C18Im I-) generates two types of

nanostructurations: The first one corresponds to the formation of aggregates of ionic clusters

while the second one is similar to a co-continuous morphology. A less achieved co-

continuous morphology is also obtained with pyridinium ionic liquids (C18Py I-). In the

opposite of phosphonium ionic liquid, an excellent dispersion is achieved since a structuration

at nanoscale is observed in the polymer matrix.

(a) (b)

(c) (d)

200 nm

200 nm

200 nm

200 nm

Figure II-5 – TEM micrographs of neat (a) PTFE and blends

(b) PTFE/C18C18Im I-, (c) PTFE/C18Py I-, (d) PTFE/C18P I-


Page 88

The chemical nature of the different counteranion halide (I-, Br-) or fluorinated (PF6

-)

associated to phosphonium cation significantly influences the final morphology as reported in

Figure II-6. A poor distribution of important aggregates can be observed with bromide anion

whereas a coarse morphology resulting from a good distribution of aggregates is also obtained

with the fluorinated anion. Both of these microscale structurations display a large contrast in

comparison to the nanoscale morphology reached with the iodide anion.

(a) (b) (c)

200 nm 200 nm 200 nm

Figure II-6 – TEM images of (a) PTFE/C18P I-, (b) PTFE/C18P Br-, (c) PTFE/C18P PF6-

Due to the hydrophobic nature of polytetrafluoroethylene and strong interactions

between ionic compounds in PTFE/IL blends, a phase-separated morphology is generated

spontaneously. Whatever the ionic liquid considered, their miscibility remains very poor even

with the presence of the octadecyl chain. Nevertheless, such morphologies could be compared

to the ones observed in ionomers for which ion-containing polymers provide a mean of

generation of various types of morphologies and subsequent properties especially in polymer

blends [29, 30]. In such materials, ionic species introduce specific interactions between

components and could lead to miscible or partially miscible blends of two immiscible

polymers from the control of the type of ionic acid group and/or conteranion. It is well known

that the clustering of ion pair in a low dielectric constant medium is responsible of different

nano- and microstructures which can be predicted theorically [31, 32]. The main parameter

controlling the microphase separation in a non-polar media are the dipole-dipole interactions

between pairs leading to formation of multiplet structures [33, 34], i.e. ionic aggregates. In

the present case, the same phenomena could be evoked with the formation of ionic liquid

aggregates which from different types of morphologies depending on the balance of the

interactions between polymer medium and anion-cation pairs.


Page 89

In conclusion, the ionic liquids have a similar behavior to ionomers which were well

described in the literature. As reported, ionic liquids form ionic clusters of varied

morphologies from nanoscale up to microscale, easily tuned by the wide variety of salts

achieved by a great number of combinations possible between cation and anion.

II.2.2.1.2 Surface analysis by X-ray Photoelectron Spectroscopy (XPS)

According to the chemical nature of the two components, i.e. PTFE and ILs, a surface

segregation could occur. In order to evaluate the interactions between ILs and fluorinated

matrix and a possible enrichment of one of the components at the surface of the films, X-ray

photoelectron spectrometry was used to analyze the surface of the neat PTFE and ILs/PTFE

films. The Table II-1 shows the percentage of C-F and C-C bonds detected on the sample

surface.

Table II-1 – Atomic percentage of species detected on the surface of fluorinated films

Atomic % C-F C-C PTFE 59 30

PTFE 1% C18P I-

97 3

PTFE 5% C18P I-

93 5

PTFE 1% C18C18Im I-

88 12

The neat PTFE composed only of (-CF2-CF2-) monomer units displays a ratio of two

between the C-F (60%) and C-C (30%) bonds. As the ionic liquid is added to the polymer

matrix, a significant decrease of C-C bonds in the presence of ionic liquids C18C18Im I-, C18P

I- is observed. With C18P I-, only 3% of C-C bonds were measured instead of 30% on neat

PTFE film which cannot be explained by a surface segregation. These results are an evidence

of the influence of ionic liquids on the chain scission inducing an increase of C-F bonds

during post-treatment at high temperature. Moreover, the migration of IL takes place in the

bulk of the material no trace of iodine, phosphorous, or nitrogen has been found at the surface

even at higher ILs contents, i.e. 5 wt%.


Page 90

II.2.2.2 Effect of ionic liquids on the thermal properties of the fluorinated matrix

The thermal behavior of the PTFE films containing 1 wt% of ionic liquids was

characterized by thermo-gravimetric analysis in order to study the effect of pyridinium,

imidazolium and phosphonium ions on the thermal stability of the blends. The

thermogravimetric results carried out on neat PTFE and on PTFE modified with pyridinium,

imidazolium or phosphonium salts are compared in Figure II-7.

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Universal V4.2E TA Instruments Figure II-7 – Evolution of weight loss as a function of temperature (TGA) and derivative of TGA curves (DTG)

of PTFE/C18Py I- (a, a’), PTFE/C18P I- (b, b’), PTFE/C18C18Im I- (c, c’) and PTFE unfilled (d, d’) (heating rate : 20 K.min-1; nitrogen atmosphere).

PTFE has already an excellent thermal stability with a degradation peak at about

580°C. The addition of pyridinium, imidazolium or phosphonium ionic liquids in the

fluorinated matrix does not improve the thermal properties of material. The thermal

decomposition of PTFE/phosphonium ions films is not also delayed whatever the chemical

nature of the anion used bromide (Br-), iodide (I-), or fluoride (PF6-) as shown in Figure II-8.


Page 91

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- (a, a’), Br- and I- (b, b’) and PTFE unfilled (c, c’) (heating rate : 20 K.min-1; nitrogen atmosphere).

In conclusion, the insertion of ionic liquid in PTFE at low IL content, i.e. 1%wt., does

not affect the thermal stability of the fluorinated matrix.

II.2.2.3 Effect of ionic liquids on the crystallinity of the fluorinated matrix

II.2.2.3.1 Influence of the organic cation

The effect of IL on the polymer physical characteristics such as crystallinity was

studied (the samples were analyzed at a heating and a cooling rate of 10K.min-1). Table II-2

gathers melting (Tm) and crystallisation (Tc) temperatures as well as the corresponding

enthalpies of melting, ∆Hm, and crystallization, ∆Hc, for the neat fluorinated matrix and with

the addition of 1%wt. of the different types of IL.

Table II-2 – Influence of organic cation on the physical properties of PTFE films

Samples Tm (°C)

Tc (°C)

∆Hm (J/g)

Xc (%)

PTFE 326 310 32 39 PTFE/C18P I- 328 307 28 34

PTFE/C18C18Im I- 328 307 29 35 PTFE/C18Py I- 329 308 36 44


Page 92

The IL incorporation into polymer matrix has a minor effect on the thermal transitions.

The melting temperatures of neat PTFE or PTFE modified with pyridinium, imidazolium, or

phosphonium ionic liquids are included between 326°C (PTFE) to 329°C (PTFE/C18Py I-).

Regarding the crystallization temperatures, no significant difference is observed since the

values are nearly in the same temperature range, i.e. from 307°C to 310°C. Only, the chemical

nature of organic cation induces differences in the crystallinity of the PTFE matrix.

Indeed, the addition of phosphonium and imidazolium ionic liquids results in a slight

decrease in crystallinity. This decrease could be attributed to the steric hindrance of cations

caused by the presence of two long alkyl chains on imidazolium ion and the presence of three

benzyl groups and a long alkyl chain on phosphonium ion. On the other side,

octadecylpyridinium iodide that is less sterically hindered has a light nucleating effect on the

crystallization of fluoropolymer with an increase of 5%.

II.2.2.3.2 Influence of the conteranion

The effect of halide (Br-, I-) and fluorinated (PF6

-) anions combined to phosphonium

cation on the crystallinity of polymer films was studied by DSC (Table II-3).

Table II-3 – Influence of anion on the physical properties of fluorinated films

Samples Tm (°C) Tc (°C) ∆Hm (J/g) Xc (%) PTFE 326 310 32 39

PTFE/C18P I- 328 307 28 34 PTFE/ C18P Br- 327 307 28 34 PTFE/ C18P PF6

- 329 308 21 38

The melting and crystallization temperatures are not affected by the nature of the

anion since the DSC measurements are the same whatever the anion used. In terms of the

melting enthalpies, the cation has a predominant effect on the halide or fluorinated anions.

This phenomenom could be explained by the fact that the fluorinated nature of the anion

improves the compatibility with the matrix and contributes to the matrix crystallization.


Page 93

II.2.2.4 Effect of ionic liquids on the mechanical properties of the fluorinated polymer

II.2.2.4.1 Dynamical mechanical analysis of fluorinated films

As reported in Figure II-9, the dynamic mechanical spectra of PTFE and PTFE/C18P I-

display the storage moduli E’ and the main relaxation peaks. These peaks were previously

assigned by McCrum to the long range motions i.e. α relaxation at 120°C, to the β relaxation

attributed to the combination of a transition in crystal form at 20°C and at – 97°C, the random

chain motions is attributed to the γ relaxation [35].

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δ (δ (δ (δ ( °° °°C

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PTFE

PTFE/C18P I-

Figure II-9 – Evolution of storage moduli E’ and main an secondary relaxations determined from tan δ spectra

recorded at 1 Hz on the neat PTFE and on the blend PTFE C18P I-

The thermomechanical properties can be significantly improved thanks to the high

thermal stability of ionic liquids. The storage moduli, E', obtained by DMA at different

temperatures are summarized in Table II-4.

Table II-4 – Dynamical mechanical analysis of IL-modified PTFE: Evolution of storage modulus at various

temperatures at 1Hz

Sample E’ (*106 MPa)

25°C

E’ (*106 MPa)

150°C

E’ (*106 MPa)

250°C PTFE 505 84 42

PTFE/C18P I- 405 139 82 PTFE/C18P Br- 425 138 66 PTFE/C18P PF6

- 547 193 99 PTFE/C18C18Im I- 594 136 68

PTFE/C18Py I- 522 141 57


Page 94

• Influence of the organic cation

The chemical nature of the organic cation plays a key role on the thermomechanical

properties of polytetrafluoroethylene. At 25°C, the film modified with the imidazolium ion

shows at room temperature the highest value in the elastic moduli E' with an increase of 17%

compared to unfilled film and 47% compared to the film filled with phosphonium ion. Then at

150°C, whatever the organic cation used, similar moduli but always higher than one measured

on PTFE are obtained. At higher temperatures like 250°C, the highest values in moduli are

measured on the PTFE films filled with the phosphonium ionic liquid because of its thermal

stability better than the imidazolium salt one.

• Influence of the conteranion

At room temperature, the addition of phosphonium ionic liquid associated to halide

anions, causes a decrease in modulus of about 20%. On the contrary, for PTFE/C18P PF6-, a

slight increase of modulus is observed. On the other side, as the temperature increases, the

moduli values for the PTFE/IL materials increase slightly compared to the ones of neat PTFE.

This improvement is more significant when the fluorinated anion is used. In fact, the

hexafluorophosphate anion contributes to better thermomechanical behaviour of IL-modified

PTFE. This phenomenom is even more pronounced at higher temperature because of the

better thermal stability of the fluorinated anion.

These changes of storage moduli can be explained by the fact that the ionic liquids

form in the PTFE medium a separated phase which displays a strong cohesion due to the ionic

dipole-dipole interactions. According to the temperature dependence of ionic interactions, the

multiplet aggregates which exist at low temperature could exist in a larger range of

temperature, i.e. to higher temperatures, before reaching the temperature at which ionic forces

become too weak to contribute to the stiffness of the material. This hypothesis could be

similar to the temperature dependence of storage modulus of ionomers with temperature [30].

II.2.2.4.2 High deformation mechanical analysis of fluorinated films

The mechanical properties determined on the fluorinated polymer films with 1 %wt of

ionic liquids are gathered in Table II-5.


Page 95

Table II-5 – Effect of ionic liquids on the tensile properties of fluorinated films

(1 wt %) (crosshead speed : 0.004 s-1).

Sample Young’s modulus (MPa)

Strain at break (%)

PTFE 65 180 PTFE/C18P I- 170 522

PTFE/C18P Br- 120 140 PTFE/C18P PF6

- 140 70 PTFE/C18C18Im I- 110 160

PTFE/C18Py I- 90 160

• Influence of the organic cation

The mechanical performance is very dependent on the chemical nature of ionic liquid

used. If 1 wt% of pyridinium and imidazolium ions within the polymer matrix leads to a

similar mechanical behaviour with an increase of modulus of 38% and 41% respectively and a

slight decrease of 11% for the strain at break, the phosphonium ionic liquid introduced in the

fluorinated matrix give an excellent stiffness/failure properties compromise. Indeed, an

increase of 160% of the stiffness and 190% for the strain at break are achieved. Better

interactions seem to take place between the phosphonium ionic liquid functionalized with

long alkyl chains and the fluorinated matrix. In fact, as mentioned before, the confined ionic

liquid phase has a strong cohesion due to the ionic interactions which can contribute to an

increase of the stiffness of the IL-modified PTFE, i.e. the ionic liquid phase acts a reinforcing

agent. As the interfacial interactions between ionic liquid phase and PTFE medium become

better, an efficient stress transfer at the interface could contribute to a higher Young’s

modulus.

• Influence of the conteranion

The mechanical properties can be also tailored from the chemical nature of the anion.

By using the phosphonium ionic liquid, differences are observed for Young’s modulus and

the strain at break as a function of the anion used. For the three anions, a strong increase of

modulus is obtained of 160% with C18P I-, 84% with C18P Br- and 115% with C18P PF6-. On

the other hand, for the strain at break, the phosphonium ionic liquid containing iodide anion

has a plasticizing effect with a large effect (190%) whereas for the C18P Br- and C18P PF6-, the

addition of these salts reduces the fracture behaviour of the fluoride matrix with a decrease of


Page 96

22% and 84% respectively. These results are consistent with the morphologies shown

previously. In the case of C18P I-, a very fine structuration is achieved in the fluoride matrix.

Whereas for C18P Br-, the poor distribution of the phosphonium ionic liquid in the polymer

explains the increase in stiffness due to the presence of aggregates and the decrease of the

strain at break. For the PTFE/C18P PF6-films, many well-dispersed ionic aggregates in the

matrix explain the increase in modulus and the higher decrease at the strain at break as

observed in ionomers [36].

II.2.2.4.3 Effect of strain rate on the fluorinated films

• Effect of ionic liquids on the crystallinity and the mechanical properties

Due to the fine dispersion of the phosphonium ionic liquid in the fluorinated film and

the excellent mechanical compromise achieved, the effects of strain rate were investigated on

the Young’s modulus and ultimate properties as well as on the morphology changes after

uniaxial stretching at a given strain. In fact, DSC measurements after deformation at different

strain rates are listed in Table II-6 and the mechanical properties are summarized in Table II-7.

Table II-6 – DSC data on fluorinated strained at different strain rate (5 and 250 mm/min)

Samples Tm (°C) Tc (°C) ∆Hm

(J/g) Xc (%)

PTFE without strain

326 310 32 39

PTFE 0.004 s-1 327 309 32 39 PTFE 0.2 s-1

329 308 40 49

PTFE C18P I-

without strain 328 307 28 34

PTFE C18P I- 0.004 s-1

328 307 42 51

PTFE C18P I- 0.2 s-1

329 308 49 60


Page 97

Table II-7 – Effect of the strain rate on tensile properties of the polymer films

Sample Tensile modulus (MPa)

Strain at break (%)

PTFE 0.004 s-1 65 180 PTFE 0.2 s-1 170 450

PTFE/C 18 P I - 0.004 s-1 170 522 PTFE/C 18 P I - 0.2 s-1 200 715

First of all, one can remember that the PTFE films are not oriented due to the

processing method, i.e. drying from a water-based solution followed by heating at 400°C. At

low strain rates, the melting enthalpy of the neat PTFE is unmodified, whereas as the

fluorinated film is strained at high strain rates (0.2 s-1), a crystallization under strain takes

place. Indeed, the high strain rate applied promotes the chain extension which leads to an

increase of crystallinity in the polymer. This increase in crystallinity ratio can be associated to

the increase of Young’s modulus from 65 at 170 MPa.

The addition of phosphonium ionic liquid in the matrix enhances the crystallization

under strain with an increase of 10% compared to the neat polymer film. This increase could

be due to a rearrangement of the ionic liquid phase in the polymer matrix. In fact, the ionic

interactions have a reversible character and the ionic liquid phase based on multiplet

aggregates could be reorganized continuously during strain. Regarding mechanical properties,

this slight increase in the modulus at high strain rate could reflect the effect of competition

between relaxation for the re-organization of ionic liquid phase and deformation.

II.2.2.4.4 Effect of ionic liquids on the morphology after deformation

For a better understanding of the material change during uniaxial tensile test, SAXS

analysis were performed on the PTFE films with and without IL after deformation reached at

different deformation rates as reported by Visser et al. for ionomers [37].

The SAXS images performed on unfilled PTFE and PTFE/C18P I- at different strain

rates, i.e. 0, 0.004, 0.2 s-1 are shown in Figure II-10.


Page 98

PTFE Initial State

PTFE/C 18P I--

Initial state 0.004 s-1

0.004 s-1 0.2 s-1

0.2 s-1

PTFE Initial State

PTFE/C 18P I--

Initial state

PTFE Initial State

PTFE/C 18P I--

Initial state 0.004 s-1

0.004 s-1 0.2 s-1

0.2 s-10.004 s-1

0.004 s-1 0.2 s-1

0.2 s-1

Figure II-10 – SAXS images of neat PTFE and PTFE/C18P I- under different strain rates

Without deformation, the isotropic character of the PTFE film due to the processing

method used is observed. As a strain rate (even low) is applied, a configuration with four leaf

clover is observed and in presence of ionic liquid, a different organization is noticed. Indeed,

without strain, the PTFE/C18P I- sample shows an isotropic behavior that is kept at low strain

rates. At a strain rate of 0.2 s-1, this phenomenon disappears in favor of an anisotropic

behavior oriented in six directions. As a consequence, the presence of ionic liquid as a

dispersed nanophase which is based on multiplet clusters could be used to modify the

deformation phenomena of PTFE due to the dynamic character of this phase under strain.

The transmission electron microscopy is used to evidence the morphology of the film.

Figure II-11 shows the different morphologies observed on the strain sample after uniaxial test

carried out at different strain rates.


Page 99

PTFE/C18P I-

Initial state 0.004 s-1 0.2 s-1PTFE/C18P I-

Initial state 0.004 s-1 0.2 s-1

200 nm 200 nm200 nm

Figure II-11 – TEM images of PTFE/C18P I- under different strain rates

After a low strain rate applied, TEM micrographs reveal that the IL nanodomains

which are initially organized as a co-continuous nanostructure (‘spider web’-type) collapse to

form large domains whereas for an higher strain rate, the IL co-continuous nanostructure is

kept and is oriented towards the axis of tension. This means that the relaxation of the IL

nanostructures has a relaxation time larger than the characteristic time of the deformation

process. This phenomenon could be very close to the ones observed by Visser and al [37] who

purposed for ionomers a model invoking ionic aggregate spatial rearrangement within the

polymer matrix. The authors also pointed out the role of the nature of ionic pairs on the

mechanical and deformation behaviour.

II.2.3 Conclusions

For the first time, ionic liquids were used like new building block to achieve a

structuration at nanoscale into a polymer film. We have clearly demonstrated that the effects

of the chemical nature of ionic liquid determined by a choice of the cation: pyridinium,

imidazolium versus phosphonium and the choice of the anion halide versus fluorinated play a

significant role on the structuration and the physical properties of the polymer. A suitable

combination between cation and anion leads to a nanoscale structuration of polymer with an

unprecedented flexibility and a stiffness dramatic improvement. These new building blocks

can be a new alternative in material field for various applications.


Page 100

Conclusions of chapter II In this section, we have shown that the introduction of a low rate (1 wt%) of ionic

liquids in the polymer matrix has a behavior similar to that of ionomers. In fact, different

structuring of ILs, dependent on the chemical nature of the cation (pyridinium, imidazolium,

phosphonium) and anion (halide, fluorinated) were highlighted. Thus co-continuous

morphologies are obtained for pyridinium and imidazolium salts while an excellent dispersion

of phosphonium ionic liquid is observed.

Then, the influence of ionic liquids on the mechanical properties has been studied both

in static and dynamic mechanical or significant increases in modulus are observed. The

perfect distribution of the phosphonium IL in polymer film also provides increases in strain at

break. However, at high strain rates, a decrease of the effect of the ionic liquid is observed in

favor of the crystallization under strain.


Page 101

References of chapter II [1] S. Maiez-Tribut, J.P. Pascault, E.R. Soulé, J. Borrajo, R.J.J. Williams, Macromolecules (2007); 40:1268–1273. [2] A. Vermogen, S. Boucard, J. Duchet, K. Masenelli-Varlot, P. Prele, R. Seguela, Macromolecules (2005); 38:9661–9669. [3] J.Lu, F. Yan, J. Texter, Progress in Polymer Science (2009); 34:431–448. [4] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, and R. Vaia, Chem. Mater (2001); 13 (9):2979–2990. [5] S. Livi, J. Duchet-Rumeau, T.-N. Pham and J.-F. Gérard, J.colloid Interf Sci (2010); 349:424–433. [6] C.Byrne and T. McNally, Macromolecular Rapid Comm. (2007); 28:780–784. [7] W. H. Awad, J. W. Gilman, M. Nyden, R. H. Harris, T. E. Sutto, J. Callahan, P. C. Trulove, H. C. DeLong and D. M. Fox, Thermochimica Acta (2004); 409:3–11. [8] Q. Guo, J. Liu, L. Chen and K. Wang, Polymer (2008); 49:1737–-1742. [9] X. Yang, F. Yi, Z. Xin and S. Zheng, Polymer (2009); 50:4089–4100. [10] Z. Xu and S. Zheng, Polymer (2007); 48:6134–6144. [11] R. Yokoyama, S. Suzuki, K. Shirai, T. Yamauchi, N. Tsubokawa and M. Tsuchimochi, European Polymer

Journal (2006); 42:3221–3229. [12] L. Priya and J. P. Jog, Journal of Applied Polymer Science (2003); 89:2036–2040. [13] S. Pavlidou and C. D. Papaspyrides, Progress in Polymer Science (2008); 33:1119–1198. [14] S. Sharma and S. Komarneni, Applied Clay Science (2009); 42:553–558. [15] K. Stoeffler, P. G. Lafleur and J. Denault, Polymer Engineering & Science (2008); 48:1449–1466. [16] W. S. Wang, H. S. Chen, Y. W. Wu, T. Y. Tsai and Y. W. Chen-Yang, Polymer (2008); 49:4826–4836. [17] H.-K. Fu, C.-F. Huang, J.-M. Huang and F.-C. Chang, Polymer (2008); 49:1305–1311. [18] L. Li, B. Li, M. A. Hood and C. Y. Li, Polymer (2009); 50:953–965. [19] Z. Spitalsky, D. Tasis, K. Papagelis and C. Galiotis, Progress in Polymer Science (2010); 35:357–401. [20] S. Bose, R. A. Khare and P. Moldenaers, Polymer (2010); 51:975–993. [21] H. Vallette, L. Ferron, G. Coquerel, A.-C. Gaumont and J.-C. Plaquevent, Tetrahedron Letters (2004); 45:1617–1619. [22] A. Safavi and S. Zeinali, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2010); 362:121–126. [23] L. Xu, G. Ou and Y. Yuan, Journal of Organometallic Chemistry (2008); 693:3000–3006. [24] M. Rahman and C. S. Brazel, Polymer Degradation and Stability (2006); 91:3371–3382. [25] K. Park, J. U. Ha and M. Xanthos, Polymer Engineering & Science (2010); 50:1105–1110. [26] A. Vazquez, M. López, G. Kortaberria, L. Martín and I. Mondragon, Applied Clay Science (2008); 41:24–36. [27] H. He, J. Duchet, J. Galy, J.F. Gerard, J.colloid Interf Sci (2006); 295:202. [28] W. Xie, R. Xie, W.-P. Pan, D. Hunter, B. Koene, L.-S. Tan and R. Vaia, Chem Mater (2002); 14:4837–4845. [29] C.G Bazuin, A. Eisenberg, Ind. Eng. Chem. Prod. Res. Dev (1981); 16:41. [30] I. Capek, Adv. Coll. Interface Sci. (2005); 118:73. [31] A.R. Khokhlov, E.F. Dormidontova, Phys. Uspekhi (2005); 118:73. [32] I.A. Nyrkova, A.R. Khokhlov, Y.Y. Kramarenko, Polym. Sci. USSR (1990); 32:852. [33] A. Eisenberg, B. Hird, R.B. Moore, Macromolecules (1990); 23:4098. [34] I.A. Nyrkova, A.R. Khokhlov, M. Doi, Macromolecules (1993); 26:3601. [35] McCrum NG. J Polym Sci (1959);34:355–69. [36] R.F. Storey, D.W. Baugh, Polymer (2000); 41 (9):3205. [37] S.A. Visser, S.L. Cooper, Polymer (1992), 33:4705–4710.


Page 102

Chapter III: Ionic Liquids as news intercalating agents for layered silicates

Page 103

Chapter III IONIC LIQUIDS AS NEWS INTERCALATING AGENTS FOR

LAYERED SILICATES Since the 80s and the early work done by Toyota on polyamide-montmorillonite

nanocomposites, the field of layered silicates nanocomposites is booming. In fact, with these

new materials, we seek to improve the thermal, mechanical or barrier properties with a very

low ratio of inorganic filler. The key parameter is the control of the distribution of individual

sheets, described as the state of exfoliation. Nevertheless, the lack of compatibility between

the hydrophilic clay and mostly hydrophobic polymers makes it difficult to obtain this state of

exfoliation. To circumvent this difficulty and improve the compatibility between clay and

polymer, the use of organic species denoted intercalating agents or surfactants, particularly

ammonium salts is necessary to reduce the surface energy and increase interlayer distances of

the layered silicates in order to promote the separation of layers to obtain an exfoliated

dispersion state more conducive to improving the final properties of nanocomposites.


Page 104

Pages

III.1 A comparative study on ionic liquids used as surfactants: Effect on thermal and mechanical properties of high-density polyethylene nanocomposites ...................................... 105

III.1.1 Introduction ............................................................................................................................ 105 III.1.2 Experimental .......................................................................................................................... 107

III.1.2.1 Materials ................................................................................................................................. 107 III.1.2.2 Synthesis of phosphonium and imidazolium salts .................................................................. 107

III.1.2.2.1 Synthesis of octadecyltriphenylphosphonium salt ...................................................... 107 III.1.2.2.2 Synthesis of N-octadecyl-N'-octadecylimidazolium salt ............................................. 108

III.1.2.3 Organic modification of montmorillonite ............................................................................... 108 III.1.2.4 Processing and characterization of the HDPE/clay nanocomposites ...................................... 110

III.1.3 Results and discussion ............................................................................................................ 111 III.1.3.1 Characterization of modified montmorillonites ...................................................................... 111

III.1.3.1.1 Identification of interactions and effect of the washing .............................................. 111 III.1.3.2 Thermal stability of modified montmorillonites ..................................................................... 114 III.1.3.3 Structural analysis by WAXD ................................................................................................. 116 III.1.3.4 Surface energies of modified montmorillonites ...................................................................... 118 III.1.3.5 Influence of ionic liquid content ............................................................................................. 118

III.1.4 HDPE/clay nanocomposites ................................................................................................... 120 III.1.4.1 Thermal properties of nanocomposites ................................................................................... 120 III.1.4.2 Mechanical properties of nanocomposites .............................................................................. 121 III.1.4.3 Morphology of nanocomposites .............................................................................................. 122

III.1.5 Conclusions ............................................................................................................................ 123

III.2 Supercritical CO2-Ionic Liquid Mixtures For Modification of Organoclays ............. 124 III.2.1 Introduction ............................................................................................................................ 124 III.2.2 Experimental .......................................................................................................................... 125

III.2.2.1 Organic modification .............................................................................................................. 125 III.2.3 Results and discussion ............................................................................................................ 127

III.2.3.1 Effect of supercritical carbon dioxide as a exchange solvent on the thermal degradation of the modified MMT ..................................................................................................................................... 127

III.2.3.1.1 Thermal stability of imidazolium-modified montmorillonite ..................................... 127 III.2.3.1.2 Thermal stability of phosphonium-modified montmorillonite .................................... 131

III.2.3.2 Structural analysis ................................................................................................................... 133 III.2.3.2.1 Imidazolium modified montmorillonite ...................................................................... 133 III.2.3.2.2 Phosphonium modified montmorillonite ..................................................................... 134

III.2.3.3 Surface energies ...................................................................................................................... 136 III.2.4 Conclusions ............................................................................................................................ 136

Conclusions of chapter III ............................................................................................................ 137

References of chapter III .............................................................................................................. 138


Page 105

III.1 A comparative study on ionic liquids used as surfactants: Effect on thermal and mechanical properties of high-density polyethylene nanocomposites

Dialkyl imidazolium and alkyl phosphonium salts were synthesized to be used as new

surfactants for cationic exchange of layered silicates, such as montmorillonite (MMT). The

synthesized phosphonium (PMMT) or imidazolium ion (I-MMT)-modified montmorillonites

display a dramatically improved thermal degradation with respect to commonly used

quaternary ammonium salts. This thermal degradation window can still be shifted toward

higher temperatures after washing of modified clays. Two kinds of organic species can be

identified onto clay: physically adsorbed species versus chemically adsorbed species. To

Evidence the impact of these thermally resistant ionic liquids, the modified montmorillonites

were introduced in a great commodity polymer, i.e., high-density polyethylene. Thermoplastic

nanocomposites with a very low amount of nanofillers were processed in melt by twin screw

extrusion. If the thermal stability of polyethylene is slightly increased with only 2 wt.% of

thermostable made clays, the stiffness–toughness compromise is well improved since a strong

increase in modulus is achieved with both thermostable clays without loss of fracture

properties. But these mechanical performances are mainly obtained with unwashed

thermostable clays because the physically adsorbed organic species onto clay surfaces behave

like a compatibilizer that helps both the dispersion into the PE matrix and improves the

clay/matrix interface quality.

III.1.1 Introduction Although the clays have been recognized for a long time, the attention of academic

and industrial researchers was recently focused on organically modified clays as nanoscale-

reinforcing agents for polymer materials [1–3]. Indeed, the insertion of these lamellar fillers in

polymers can have significant effects not only on the mechanical [4] and barrier performances

[5,6] but also on ablation and flammability resistances [7] due to nanometric dimensions and

high aspect ratios of layered silicates and also due to synergism between polymer and

inorganic nanofillers. Among layered silicates, montmorillonite (MMT) is commonly used

[8]. Nevertheless, to ensure a good compatibility between montmorillonite and polymer

during the processing of nanocomposites, a surface modification of pristine MMT is required.


Page 106

By exchanging sodium or calcium cations for organic cations, the surface energy of MMT

decreases and the basal spacing expands [9]. For such a purpose, cationic exchange using

alkylammonium salts is very often used [10,11].

Melt intercalation is by far the most promising method and is industrially preferred for

processing thermoplastic polymer (TP)-based nanocomposites as it can be performed without

use of solvents and by considering conventional tools for processing, i.e., the extrusion

process. However, the process involves working at high temperatures as melting of

conventional TPs requires temperatures above 190°C. Although the commonly used

ammonium salts have been gaining significant success in the processing of polymer/MMT

nanocomposites, a common shortcoming is their low thermal stability. The thermal

degradation of MMT modified by long-chain alkyl quaternary ammonium ions begins from

180°C as shown by TGA studies carried out by Xie et al. [12]. Indeed, the Hoffmann

degradation during processing described in the literature [13,14] could initiate/catalyze

polymer degradation and could affect the physical and mechanical properties of final

materials. To increase the thermal stability of organically modified clays, the use of more

thermally stable compounds, such as ionic liquids based on phosphonium and imidazolium

salts, can offer a new alternative to the ammonium salts [15,16]. These salts may be

considered as ionic liquids because their melting point is below 100°C, and their glass

transition temperature is around 10°C for phosphonium salt and 42°C for imidazolium salt.

Intensive thermal studies on imidazolium and phosphonium salts have shown a better

thermal stability than the alkyl ammonium cations [15,17,18]. Despite many benefits, few

studies using such cations for layered silicate intercalation have been reported in the literature

maybe because of the higher price of these surfactants compared to ammonium salts [19–21].

Moreover, commercially available thermally stable surfactants only incorporate short

aliphatic chains (up to C14).

In this work, efforts have been made to synthesize ionic liquids with long alkyl chains

based on imidazolium (two chains in C18) and on phosphonium salts with three benzyl

groups and only one long aliphatic chain (1 chain in C18). The long alkyl chains cause

expansion of the distance between the layers, and the aromatic groups help to generate a

better intercalation of the clay platelets because aromatic groups can be trapped in the

hexagonal cavities of layers. The thermal stability of the obtained organoclays was compared

with that of ammonium-modified clays. Each ammonium cation shows exactly the same

substituents as the imidazolium cation, either two linear octadecyl chains or an aromatic

group and an aliphatic chain for the phosphonium cation. Then, a high-density polyethylene


Page 107

(HDPE) was selected to be mixed with lamellar silicates modified with highly thermally

stable ionic liquids. Finally, the morphology and the thermal and mechanical properties of

nanocomposites processed with the thermally stable ionic liquids were evaluated and

compared with ammonium-modified clay-based nanocomposites [22,23].

III.1.2 Experimental

III.1.2.1 Materials

A sodic montmorillonite, denoted Nanofil 757 (MMT), i.e., an aluminosilicate with

intercalated sodium was chosen as pristine clay and was provided by Süd Chemie (Germany).

The Nanofil 757 has a cation exchange capacity of 95 meq/100 g and is described by the

following formula Na0,65[Al,Fe]4Si8O20(OH)4. Two commercial organically modified

montmorillonites (Nanofil 15 and Nanofil 919) were purchased from Süd Chemie to obtain

reference organophilic clays. Both commercial organoclays are modified with quaternary

ammonium ion carriers of either tallow chains (Nanofil 15) or aromatic groups and tallow

chains (Nanofil 919). Tallow chains have the following composition: 65% C18, 30% C16,

and 5% C14. All chemicals necessary for the synthesis of ionic liquids, i.e.,

triphenylphosphine (95%), imidazole (99.5%), iodooctadecyl (95%), and all the solvents

(toluene, sodium methanoate, pentane and acetonitrile) were supplied from Aldrich and used

as received.

The polyethylene used in this study, called HDPE, is a high-density polyethylene from

Basell, with the trade name Hostalen GF 4750, showing a melt flow index of 0.4.

III.1.2.2 Synthesis of phosphonium and imidazolium salts

III.1.2.2.1 Synthesis of octadecyltriphenylphosphonium salt

In a 100-mL flask was, placed under a positive nitrogen pressure, 1 eq of

triphenylphosphine (5 g) and 1 eq octadecyl iodide (7.3 g). The stirred suspensions were

allowed to react for 24 h at 120°C in toluene (20 mL), and a yellow precipitate was formed.

The reaction mixture was then filtered and washed repeatedly with pentane. Most of the

solvent was removed under vacuum. The synthesis of salts was confirmed by 13C NMR

spectroscopy collected on a Bruker AC 250 (250 MHz) spectrometer. The assignment of 13C

NMR resonance peaks is reported below. 13C NMR (CDCl3): d 14.00 (CH3); 22.67 (CH2Me); 23.2; 29.37–29.66; 30.24; 31.85

(P–CH2); 118.45; 130.43; 133.70; 135.15 (P–C).


Page 108

III.1.2.2.2 Synthesis of N-octadecyl-N'-octadecylimidazolium salt

A solution of sodium methoxide was prepared from 1 eq of sodium (0.465 g) using

dry, freshly distilled methyl alcohol (10 mL) in a sealed septum, 100 mL round-bottom, three-

necked flask equipped with a condenser, under nitrogen atmosphere and magnetic stirring.

Imidazole (1 eq, 1.37 g) diluted in acetonitrile (10 mL) was then added into the stirred

mixture of sodium methoxide previously cooled at room temperature. After 15 min, a white

precipitate was formed. The suspension was then concentrated under reduced pressure for 1 h.

The dried white powder was dissolved in acetonitrile, and a solution of octadecyl iodide (1 eq,

7.70 g) diluted in acetonitrile (10 mL) was then added under an inert atmosphere of nitrogen

at room temperature. The mixture was stirred for 1 h and then heated under reflux at 85 C for

about 24 h. A solution of octadecyl iodide (1 eq, 7.70 g) diluted in acetonitrile (10 mL) was

added to the mixture at room temperature. The stirred suspension was heated under reflux at

100 C for about 24 h leaving a brownish viscous oil in each case. After cooling to room

temperature, the solvent was removed by evaporation under vacuum, and the orange-colored

or beige solid was filtered, washed repeatedly with pentane, and dried. Purification of the

resulting imidazolium salts was accomplished by crystallization from ethyl

acetate/acetonitrile: 75/25 mixture. The assignment of 13C NMR resonance peaks is the

evidence of the success of the ionic liquid synthesis. 13C NMR (CDCl3): d 14.10 (2CH3); 22.67 (2CH2Me); 26.23; 28.97; 29.35–29.69;

30.24; 31.91 (CH2); 50.10; (CH2N); 50.32 (CH2N–); 121.69; 122.48 (CN); 136.88 (N–CN).

III.1.2.3 Organic modification of montmorillonite

The montmorillonite (2 g, 1.9 meq) was dispersed in 400 mL of deionized water. The

amount of surfactant added was about 2 CEC, based on the cation exchange capacity (CEC =

95 meq/100 g) of the MMT used [9]. This dispersion was mixed and stirred vigorously at

80°C for 6 h, followed by filtration and continuous washing at 80°C with deionized water

until no iodide ions were detected using an aqueous silver nitrate (AgNO3) solution. The

solvent was removed by evaporation under vacuum. The modified montmorillonite was then

dried for 12 h, at a suitable temperature (not greater than 80°C). The imidazolium,

phosphonium, and the quaternary ammonium ions used for the exchange reactions are

presented in Table III-1. The following abbreviations were used to design the different


Page 109

montmorillonites: MMT-Na+ means the pristine montmorillonite. A phosphonium

montmorillonite denoted MMT-P was obtained when octadecyltriphenylphosphonium iodide

was used like surfactant. An imidazolium montmorillonite denoted MMT-I was obtained

when the N-octadecyl-N'-octadecylimidazolium iodide was used as an intercalation agent.

Both commercial montmorillonites modified with ammonium ions are MMT-DMDT for the

montmorillonite carrying a dimethyl ditallow quaternary ammonium as cation and MMT-

DMBT for the montmorillonite modified by a dimethyl (benzylmethyl) tallow quaternary

ammonium.

Table III-1 – Designation of pristine, commercial and synthesized ionic liquid modified montmorillonite

(MMT)

Designation Intercalant Trade name MMT-Na+ - Nanofil 757

MMT-DMDT

Nanofil 15

MMT-DMBT

Nanofil 919

MMT-I

MMT-P

N+

Tallow

CH3

TallowH3C

N+

H3C

H3C Tallow

P C18H37

I

N

N

C18H37

C18H37

I


Page 110

III.1.2.4 Processing and characterization of the HDPE/clay nanocomposites

Nanocomposites were obtained by melt intercalation of modified montmorillonite into

a high-density polyethylene (2% by weight) using a twin screw DSM microcompounder. The

mixture was sheared for about 3 min with a 100 rpm speed at 190 C and injected in a 10-cm3

mold at 30 C to obtain dumbbell-shaped specimens. Different nanocomposite samples were

prepared by varying the surface treatment used to modify the montmorillonite and by

considering the nonwashed, exchanged MMT.

Thermogravimetric analyses (TGA) of organically modified clay and composites were

performed on a Q500 thermogravimetric analyzer (TA instruments). The samples were heated

from 30 to 800°C at a rate of 20 K min1 under nitrogen flow.

Surface energy of modified clays was determined with the sessile drop method on a

GBX goniometer. From contact angle measurements taken using water and diiodomethane as

test liquids on pressed modified clay disks, polar, and dispersive components of surface

energy were determined using the Owens–Wendt theory.

Bruker D8 Advance X-ray diffractometer at the H. Longchambon diffractometry

center. A bent quartz monochromator was used to select the Cu Ka1 radiation (k = 0.15406

nm) and run under operating conditions of 45 mA and 33 kV in Bragg–Brentano geometry.

The angle range scanned is 1–102h for the modified clays and 1–302h for the nanocomposite

materials.

Uniaxial tensile measurements were taken using a MTS 2/M electromechanical testing

system at 22 ± 1 C and 50 ± 5% relative humidity. Tensile tests were performed with a speed

of 10 mm min1.

The transmission electron microscopy (TEM) was carried out at the Center of

Microstructures (University of Lyon) on a Philips CM 120 field emission scanning electron

microscope with an accelerating voltage of 80 kV. The samples were cut using an

ultramicrotome equipped with a diamond knife, to obtain 60-nm-thick ultrathin sections.

Then, the sections were set on copper grids.


Page 111

III.1.3 Results and discussion

III.1.3.1 Characterization of modified montmorillonites

III.1.3.1.1 Identification of interactions and effect of the washing

According to the literature [9,12,17,24], as a layered clay is modified with an organic

cation, two kinds of interactions between organic cations and inorganic clay can take place:

(i) Van der Waals bonds as the organic species are physically adsorbed on the clay surface.

(ii) Ionic bonds as the species are intercalated in the montmorillonite galleries.

The effect of washing allows identification of the interaction intensity, chemical

versus physical, linking the organic species to the layered silicate surface. After checking the

solubility of ionic liquids in different solvents, methyl alcohol was chosen as solvent for

washing. The effect of the washing is clearly shown on TGA analysis reported in Figure III-1.

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0 200 400 600 800

Temperature (°C)

� MMT-I aw1� MMT-I bw� MMT-I aw2

Universal V4.2E TA Instruments Figure III-1 – Derivative of TGA curves (DTG) of the MMT-I bw (before washing)

and MMT-I aw (after washing) (heating rate : 20 K.min-1). Three peaks of degradation are observed on the derivative curve of the weight loss

(DTG curve) of imidazolium-modified montmorillonite, before washing (called MMT-I bw).

After two successive washings (called MMT-I aw 2) with methanol, the first degradation peak


Page 112

that corresponds to the species physically adsorbed on the montmorillonite surface almost

disappears for the benefit of the second peak. Unlike the first peak, the second increases

significantly. This can be explained by the gradual removal of salt excess on the clay surface

that creates a larger aperture of clay galleries allowing a higher quantity of the salt to be

intercalated between clay layers. Figure III-2 reports DTG curves obtained from the thermal

analysis realized on the phosphonium-treated montmorillonite before (bw) and after washing

(aw).

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0 200 400 600 800

Temperature (°C)

� MMT-P bw� MMT-P aw2� MMT-P aw1

Universal V4.2E TA Instruments Figure III-2 – Derivative of TGA curves (DTG) of the MMT-P bw (before

washing) and MMT-P aw (after washing) (heating rate: 20 K.min-1). The first weight loss still corresponds to a partial physisorption at the edges (bearing

polar SiOH groups) or on the external surface of the platelets since the peak decreases after

washing. The fact that it does not completely disappear means that a part of the ionic liquid is

well intercalated but in a peripheral position with respect to the clay gallery as reported by

Davis et al. [25] or is physisorbed from p–SiOH interactions at the edges of the lamellar

silicates. Such portion of surfactant cannot be washed away easily (since it underwent cationic

exchange), but it is not thermally stabilized by the presence of the inorganic silicate platelets

in a confined position. As a consequence, it degrades at the same temperature as the

physisorbed surfactant. On the other hand, the degradation which is evidenced at about 500°C

corresponds to the well-intercalated species between layers.


Page 113

The same observation is made on the MMT-DMBT and the MMT-DMDT. A single

washing with methyl alcohol is enough to eliminate the species physically adsorbed. Table

III-2 summarizes the relative mass losses of the physically adsorbed species and the

intercalated species for the modified montmorillonites before and after washing.

Table III-2 – Relative mass loss of physically adsorbed and intercalated species measured by TGA on the modified montmorillonites

Sample % physically adsorbed species

% intercalated species

MMT-P bw* 46 12 MMT-P aw* 9 18 MMT-I bw 31 18 MMT-I aw - 23

MMT-DMDT bw 2 25 MMT-DMDT aw - 25 MMT-DMBT bw 16 20 MMT-DMBT aw - 18

The weight percent of physisorbed species varies before washing and cannot be

measured after washing since it is negligible. On the other hand, the intercalated cation

amount is about 20 wt.% for all the organically modified clays.


Page 114

III.1.3.2 Thermal stability of modified montmorillonites

Thermogravimetric analysis (TGA) may complete modified clay characterizations by

investigating the degradation mechanisms and the effects of functionalization and washing on

the thermal stability. We have already reported that the first peak corresponding to the first

weight loss on the derivative curve of the weight loss as a function of temperature typically

indicates the presence of organics that must be simply physisorbed [9–24]. Such organic

fraction either did not undergo the cation exchange process or stayed unconfined. The second

weight loss corresponds to organics intercalated into clay galleries. Indeed, organics inside

clay galleries display higher temperatures of degradation. In this study, the montmorillonites

modified with distinct cations (ammonium versus phosphonium or ammonium versus

imidazolium) were compared.

First, the montmorillonite modified with the dimethyl (benzymethyl) tallow

quaternary ammonium was compared to one treated with the phosphonium salt since both

salts display alkyl chains and benzyl rings as ligands. Figure III-3 summarizes the data

extracted from these thermogravimetric (TGA) curves and their derivative (DTG).

0 100 200 300 400 500 600 700 800

60

65

70

75

80

85

90

95

100

105

% W

eig

ht

Temperature (°C)

MMT-DMBT

0 100 200 300 400 500 600 700 800

-0,025

-0,020

-0,015

-0,010

-0,005

0,000

0,005

0,010

Deri

v.W

eig

ht

(%°C

)

Temperature (°C)

220°C

300°C

390°C

430°C

MMT-DMBT

0 100 200 300 400 500 600 700 800

40

50

60

70

80

90

100

% W

eig

ht

Temperature (°C)

MMT-P

0 100 200 300 400 500 600 700 800

-0,08

-0,06

-0,04

-0,02

0,00

0,02

0,04

Deri

v.W

eig

ht

(%°C

)

Temperature (°C)

340°C

510°C

MMT-P

2b

Figure III-3 – TGA and DTG curves of the MMT-DMBT (a) and

MMT-P (b) (before washing) (heating rate: 20K.min-1).

On the DTG curves of MMT-DMBT (Figure III-3a), the first peak corresponding to the

degradation of the species physically adsorbed on the surface of clay starts at 220°C, whereas

the phosphonium-modifiedmontmorillonite is still thermally stable at this temperature.

Indeed, in Figure III-3b, the first peak of degradation of the physisorbed species is at 340°C, a

a)

b)


Page 115

temperature corresponding to the evaporation one of neat ionic liquid. In this case, a dramatic

difference of 120°C is observed. For the intercalated species in the montmorillonite galleries,

the same trend is observed. Whereas the degradation takes place at about 500°C for MMT-P,

the degradation of intercalated species is extended between 300 and 400°C for MMT-DMBT.

These results show clearly the better intrinsic thermal stability of phosphonium salt compared

with the ammonium salt [12], and the confinement has a similar effect on both salts, i.e., the

temperature shift from adsorbed state to confined state.

Second, the imidazolium montmorillonite and the montmorillonite bearing a dimethyl

ditallow quaternary ammonium as cation was also compared, as both have only two similar

long alkyl chains. Figure III-4 reports the TGA curves and their derivatives.

0 100 200 300 400 500 600 700 800

60

70

80

90

100

% W

eig

ht

Temperature (°C)

MMT-DMDT

0 100 200 300 400 500 600 700 800

-0,10

-0,08

-0,06

-0,04

-0,02

0,00D

eriv.W

eig

ht

(%°C

)

Temperature (°C)

MMT-DMDT

270°C

340°C

440°C

0 100 200 300 400 500 600 700 800

40

50

60

70

80

90

100

% W

eig

ht

Temperature (°C)

MMT-I

0 100 200 300 400 500 600 700 800

-0,10

-0,08

-0,06

-0,04

-0,02

0,00

Deri

v.W

eig

ht

(%°C

)

Temperature (°C)

MMT-I

320°C

420°C

490°C

Figure III-4 – TGA and DTG curves of the MMT-DMBT (a) and MMT-P

(b) (before washing) (heating rate: 20K.min-1). By considering the thermal analysis carried out on MMT-DMDT and on MMT-I, the

difference is less significant. In Figure III-4a, a shoulder is observed at 270°C, followed by the

degradation of species intercalated at 340 and then at 440°C. In the case of MMT-I (Figure

III-4b), a first clear peak attributed to physically adsorbed species at 320°C is observed,

followed by the degradation of the species intercalated at 420 and 490°C. The increase in the

degradation temperature is much lower between the MMT-I and the MMTDMDT (50°C for

physisorbed species and 80°C for intercalated ones) because both alkyl chains as ligands

display a lower intrinsic thermal stability than benzyl groups. However, keep in mind that the

imidazolium or phosphonium-modified montmorillonites have a much better thermal stability

than the ammonium-treated ones.

a)

b)


Page 116

III.1.3.3 Structural analysis by WAXD

The cationic exchange process is clearly detectable by X-ray diffraction as shown in

Figure III-5.

0 2 4 6 8 10

0

500

1000

1500

2000

2500

3000

3500

4000

Inte

nsity (

a.u

)

2θ

MMT-DMDT

d001

= 3.0 nm

0 2 4 6 8 10

0

200

400

600

800

1000

1200

Inte

nsity (

a.u

)

2θ

MMT-Id

001 = 3.7 nm

0 2 4 6 8 10

0

500

1000

1500

2000

2500

3000

3500

4000

Inte

nsity (

a.u

)

2θ

MMT-DMBT

d001

= 1.9 nm

0 2 4 6 8 10

400

450

500

550

600

650

700

750

800

850

Inte

nsity (

a.u

)

2θ

MMT-P

d001

= 4.2 nm

Figure III-5 – X-Ray diffraction spectra of ionic liquid modified MMT before washing:

(a) MMT-DMDT; (b) MMT-I; (c) MMT-DMBT; (d) MMT-P.

Before treatment, the basal spacing of the sodic montmorillonite is 1.2 nm, which

corresponds to the d-spacing of MMT-Na reported in the literature [9]. After organic

treatment in water by the imidazolium and phosphonium salts, the MMT-P displays a (0 0 1)

diffraction peak at 2.12h, corresponding to an interlayer distance of 4.2 nm. This value could

be explained by the swelling of layered silicates due to the steric volume occupied by the

three ring functions and the alkyl chain. For the MMT-I, the diffraction peak situated at 2.42h

is significant at a distance of 3.7 nm, a distance similar to one characteristic of a paraffinic

conformation with trans–trans positions of the alkyl chain. On the other hand, for the

montmorillonite modified with a ditallow quaternary ammonium, i.e., MMT-DMDT, the

lower intercalation distance of 3.0 nm is significant of a paraffinic chain tilted on the clay

surface. For the montmorillonite functionalized with a dimethyl benzyl tallow quaternary

ammonium, MMT-DMBT, the intercalation distance only of 1.9 nm is reduced by nearly half

because the organic chains adopt a pseudo trilayer conformation [26]. For the MMT-P and the

MMT-I, the spectra show intense, thin, and regular diffraction peaks that suggest a long-range

order.


Page 117

The effect of washing reported in Figure III-6 was studied by X-ray diffraction on

different montmorillonites.

0 2 4 6 8 10

0

500

1000

1500

2000

2500

3000

3500

4000

Inte

nsity (

a.u

)

2θ

MMT-DMDT awd

001 = 2.4 nm

0 2 4 6 8 10

0

2000

4000

6000

8000

Inte

nsity (

a.u

)

2θ

MMT-I aw

d001

= 2.7 nm

0 2 4 6 8 10

0

500

1000

1500

2000

2500

3000

3500

4000

Inte

nsity

(a.u

)

2θ

MMT-DMBT aw

d001

= 1.9 nm

0 2 4 6 8 10

0

500

1000

1500

2000

2500

3000

Inte

nsity

(a.u

)

2θ

MMT-P aw

d001

= 2.1 nm

Figure III-6 – X-Ray diffraction spectra of ionic liquid modified MMT after washing:

(a) MMT-DMDT; (b) MMT-I; (c) MMT-DMBT; (d) MMT-P.

The washing step leads to a shift of diffraction peaks toward higher angles. The

intercalation distance decreases from 3.0 to 2.4 nm for the MMT-DMDT, from 3.7 to 2.7 nm

for the MMT-I, and from 4.2 to 2.1 nm for the MMT-P after washing with methyl alcohol.

Only the MMT-DMBT does not have any change in gallery height after washing. Washing

with methyl alcohol causes a reorganization of chains that explains the decrease in distances.

But in all cases, washing performed on MMT-P, MMT-I and MMT-DMDT does not induce

any decrease in the intercalation distance below 2.0 nm. This observation is corroborated by

the no change in distance obtained for the MMT-DMBT. As a result, the cationic exchange

leads to a part of organic cations that is really intercalated between the sheets inducing about a

2.0-nm gallery height, while another part of organic cations causes the clay swelling only due

to steric volume of the organic ligands.


Page 118

III.1.3.4 Surface energies of modified montmorillonites

The contact angles and surface energy determined by the sessile drop method on

pressed powder are collected in Table III-3.

Table III-3 – Determination of polar and dispersive components of the surface energy on pristine and on exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders) Montmorillonite Θwater (°) ΘCH2I2 (°) γ polar

(mN.m-1) γ dispersive (mN.m-1)

γ total (mN.m-1)

MMT-Na+ 22.9 ±0.9 33.6 ±0.8 30 43 73 MMT-DMDT 72.8 ±0.3 55.4 ±0.6 9 31 40 MMT-DMBT 61.0 ±0.5 48.8 ±0.7 14 35 49

MMT-P 88.9 ±0.1 49.4 ±0.6 2 35 37 MMT-I 92.8 ±0.1 55.5 ±0.6 1 31 32

Polyethylene [28]

102.0 53.0 0.05 34.10 34.15

Both ionic liquids based on phosphonium and imidazolium salts make the

montmorillonite more hydrophobic with a surface energy similar to the surface energy of a

polyolefin [27]. The polar components are very low which evidenced that the hydroxyl groups

are well covered by the organic chains. The steric hindrance of imidazolium and

phosphonium ionic liquids causes an efficient screen of the hydrophilic surface of lamellar

silicates. A stronger hydrophobic character can be obtained with the synthesized ionic liquids

compared to the ammonium cations. Hence, an efficient compatibility must be generated

between ionic liquid-modified nanolayers and polyethylene matrix.

III.1.3.5 Influence of ionic liquid content

The washing effect shows that such a large amount of cationic liquid corresponding to

2 CEC used for cationic exchange is useless for clay treatment. As a result, the ionic liquid

amount can be reduced during cationic exchange. Instead of adding 2 CEC of surfactants,

only 0.5 CEC was added to perform the cationic exchange. The TGA analysis performed

before the washing step on phosphonium-modified montmorillonite with two different

surfactant amounts (2 CEC versus 0.5 CEC) is reported in Figure III-7.


Page 119

0 100 200 300 400 500 600 700 800

0

10

20

30

40

50

60

70

80

90

100

% w

eig

ht

Temperature (°C)

MMT-P bw2CEC

MMT-P bw 0,5CEC

Figure III-7 – Effect of the amount introduced of ionic liquid (2 CEC versus 0.5 CEC) used for cationic exchange on thermal stability measured by TGA

before the washing step (heating rate: 20K.min-1)

The similar thermal degradation for both exchange treatments highlights that a lower

amount of surfactant is sufficient to efficiently modify the clay and gives the clay a thermal

stability up to temperatures higher than 400 C. The X-ray diffraction is witnessed that the

amount of ionic liquid can be reduced since the same spectrum is obtained with a lower

surfactant content. The modified clay is characterized by the same intercalation distance as

shown in Figure III-8.

0 2 4 6 8 10

0

200

400

600

800

1000

1200

1400

1600

Inte

nsity (

a.u

)

2θ

MMT-P bw 2 CEC

MMT-P bw0.5 CEC

Figure III-8 – Effect of the amount introduced of ionic liquid (2 CEC versus 0.5 CEC) used for

cationic exchange on intercalation distance measured by WAXD before the washing step


Page 120

III.1.4 HDPE/clay nanocomposites

III.1.4.1 Thermal properties of nanocomposites

The thermal behavior of the composites containing 2 wt.% of modified clay was

characterized by thermogravimetric analysis in order to study the effect of imidazolium and

phosphonium-treated montmorillonites on the thermal stability of the nanocomposites. The

thermogravimetric results carried out on polyethylene alone and the polyethylene filled with

modified montmorillonites are shown in Figure III-9.

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0 100 200 300 400 500 600

Temperature (°C)

� PE-(MMT-I bw)� PE-(MMT-P bw)� PE� PE-MMT

Universal V4.2E TA Instruments Figure III-9 – TGA curves of nanocomposites based on polyethylene matrix filled with 2%wt. of sodic montmorillonite (PE-MMT) and of phosphonium (PE-(MMT-P bw)) or imidazolium (PE-(MMT-I bw)) modified montmorillonite (heating rate : 20 K.min-1).

The improvement in thermal stability is not tremendous. The chosen polyethylene is

already thermally very stable up to about 450°C, which corresponds to the temperature from

which starts the modified montmorillonite degradation. Moreover, the samples are analyzed

under inert gas. The addition of only 2 wt.% of the unwashed imidazolium-modified

montmorillonite in the polyethylene matrix does not improve the matrix thermal degradation.

With 2 wt.% of phosphonium-modified montmorillonite, the thermal degradation of the

polyethylene matrix can be only improved by 10°C. By introducing two times more

phosphonium-modified clay, i.e., 5 wt.%, delay in the thermal degradation is also doubled as

shown in Figure III-10.


Page 121

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400 450 500 550 600

Temperature (°C)

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Universal V4.2E TA Instruments Figure III-10 – TGA curves of nanocomposites based on polyethylene matrix filled with 2 and 5

wt% of phosphonium modified montmorillonite (PE-(MMT-P)) (heating rate : 20 K.min-1).

III.1.4.2 Mechanical properties of nanocomposites

Mechanical properties of the PE/clays nanocomposites containing 2 wt.% are detailed

in Table III-4.

Table III-4 – Effect of clay washing on tensile properties of the 2 CEC ionic liquid modified montmorillonite-high density polyethylene nanocomposites (2wt%) crosshead speed: 10mm.min-1

*bw: before washing aw : after washing

A strong increase in modulus is obtained when the polyethylene was prepared with

phosphonium or imidazolium-modified montmorillonite: an increase of 40% for the MMT-I

and 50% for the MMT-P. However, this stiffness is reduced when the nanocomposites are

prepared with washed clays or with 0.5 CEC-modified clay. In both cases, only an increase of

20% is observed for both organically modified clays. This decrease in modulus could be

associated with the removal of physically adsorbed species on the edges of the silicate layers


Strain at break (%)

Stress at break (Mpa)

PE 740 18 84 PE-MMT 720 19 71

PE-(MMT-P bw) 1100 17 97 PE-(MMT-P aw) 887 18 88 PE-(MMT-I bw) 1041 17 96 PE-(MMT-I aw) 883 19 87

PE-(MMT-I 0.5 CEC) 835 18 74


Page 122

by the successive washings or by the low amount of surfactant present. Indeed, the excess of

salts may help the compatibilization between the polyethylene and the modified

montmorillonites. About the strain at break, the modified clays do not reduce the fracture

behavior of the matrix since the values are found to be similar.

III.1.4.3 Morphology of nanocomposites

Transmission electron microscopy micrographs carried out on the nanocomposites

processed with 2 wt.% of phosphonium or imidazolium-modified clays and reported in Figure

III-11 show a varying dispersion state following whether the introduced clays are washed or

not.

a)

d)c)

b)a)

d)c)

b)

Figure III-11 – Influence of washing clay on TEM micrographs performed on 2

%wt ionic liquid treated montmorillonite-polyethylene nanocomposites: (a) PE-(I-MMT aw), (b) PE-(I-MMT bw), (c) PE-(P-MMT aw), (d) PE-(P-MMT bw)


Page 123

When PE/clay materials are prepared with unwashed modified clays, the morphology

is more uniform, and the TEM pictures reveal much less contrast because the nanolayers are

well dispersed in the form of isolated layers. On the other hand, when the nanocomposites are

processed with washed clays, the morphology is composed of small tactoids and even of a

few isolated aggregates. The excess of salts present before washing in the nanocomposites

promotes a better layer exfoliation. The physisorbed and unremoved salts behave as an

efficient compatibilizer to create an intimate contact between filler and matrix. This very good

level of dispersion was obtained without the use of large amounts (about 20 wt.%) of maleic

anhydride-grafted polyethylene usually used as compatibilizer in the polyolefin

nanocomposite processing [28] but has a similar limiting effect in reducing the bulk matrix

mechanical performances, particularly stiffness. In this work, a very good stiffness–toughness

compromise was obtained in combination with a very high dispersion level.

III.1.5 Conclusions In this work, ionic liquids based on phosphonium and imidazolium salts with long

alkyl chains were synthesized and used as surfactants to modify a lamellar silicate surface and

to help its intercalation into a nonpolar polymer matrix. The thermogravimetric analysis

performed on the imidazolium and phosphoniumtreated montmorillonites shows an important

improvement in thermal stability of the salts compared to the conventional ammonium

cationsdue to their intrinsic thermal stability. These new surfactants behave as conventional

organic cations and are easily swollen inducing a high d-spacing. The washing of clays after

amodification step is not really required for processing nanocomposites based on HDPE

matrix. Indeed, the physically adsorbed surfactants on the layer edges have a key role in the

preparation of nanocomposites as they act as a compatibilizing agent. Polyethylene

nanocomposites were prepared by a melt process with only 2 wt.% imidazolium or

phosphonium-modified montmorillonites. The stiffness of PE matrix is clearly increased

without reducing its fracture behavior. The use of unwashed clays is the key parameter to

achieve a very fine dispersion state and the optimized mechanical properties because the

physically adsorbed surfactants act as a compatibilizing agent in situ generated at the

nanolayer/matrix interface.


Page 124

III.2 Supercritical CO2-Ionic Liquid Mixtures For Modification of Organoclays

The use of supercritical CO2 as solvent in the modification of montmorillonite by

imidazolium and phosphonium ionic liquids bearing long alkyl chains (C18) known for their

excellent thermal stability is described. The objective is to combine the environmentally

friendly character of ionic liquids and supercritical carbon dioxide for the organophilic

treatment of lamellar silicates. Dialkyl imidazolium and alkyl phosphonium salts were

synthesized to be used as new surfactants for cationic exchange of layered silicates. Then, the

synthesized phosphonium (MMT-P) or imidazolium (MMT-I) modified montmorillonites,

cationically exchanged under supercritical carbon dioxide with or without co-solvent, have

been analyzed by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) and

compared to montmorillonites treated by conventional cationic exchange.

III.2.1 Introduction Since the 80s, the industrial and academic research has a growing interest in the design

of polymer / clay nanocomposites. According to the literature, the insertion of lamellar

silicates in a polymer matrix can have beneficial effects on the final properties of the

nanocomposite. The most often reported improvements are for thermo-mechanical behaviour

[4] and gas barrier properties [5-6] due to nanometer-range dimensions and high aspect ratio

of layered silicates leading to a large amount of interfacial zones. Natural clays, such as

montmorillonite (MMT) are frequently used for that purpose [29].

However, the addition of non-modified clay due to the poor interfacial interactions is

very limited. Indeed, it is necessary to modify the surface of the pristine MMT, mostly by

cationic exchange using ammonium salts [10-11] to improve the compatibility between the

polymer and lamellar fillers during the processing step of nanocomposites, i.e. to improve the

final dispersion state and to get a full/large development of the interfaces.

Ionic liquids are organic salts with melting points below 100°C, which are considered as

green solvents. The most commonly used are imidazolium [21], pyridinium [8] and

phosphonium [17] salts. They are used in several applications: chemical reaction medium, i.e.

as solvents, electrolytes in batteries, lubricants, plasticizers, and catalysts. Recently, ionic


Page 125

liquids were used as surfactants to replace the conventional alkyl ammonium salts that show a

lower thermal stability [15, 16, 18]. Despite many benefits, very few studies using such

cations for layered silicates intercalation have been reported in the literature may be due of the

actual higher cost of these surfactants compared to ammonium salts [19, 20, 21]. Moreover,

thermally stable and commercially available ionic liquids which could be considered as

surfactants only incorporate short aliphatic chains (up to C14).

In this work, ionic liquids bearing long alkyl chains based on imidazolium (two chains

in C18) and on phosphonium salts with three benzyl groups and only one long aliphatic chain

(1 chain in C18) were synthesized. As these ionic liquids used as clay intercalants, the long

alkyl chains and the aromatic groups could cause expansion of the distance between the layers

and could contribute to a better intercalation of the clay platelets. To perform an

environmentally friendly MMT surface treatment, properties of ionic liquids and supercritical

CO2 were combined. The use of CO2 in the supercritical state should make useless the

addition of any solvent for that the cationic exchange succeeds. The main interest of

supercritical CO2 is to design a clean surface treatment compared to the ones using

conventional solvents. In fact, carbon dioxide has a high diffusivity like a gas, low surface

tension (close to zero), viscosity and density like a liquid which gives it a high solvency

power tuneable by adjusting pressure [30]. In this work, the combination of supercritical CO2

and ionic liquids should lead to an environmentally benign cationic exchange process to

modify the MMT with imidazolium and phosphonium salts with long alkyl chains. The

objective of this work is to design a process without use of solvent and to identify the

supercritical CO2 exposure effects on the physico-chemical properties resulting of modified

clays.

III.2.2 Experimental

III.2.2.1 Organic modification

As supercritical carbon dioxide was used as solvent instead of water, the procedure

was the following one: 2g of untreated MMT and an excess of surfactant (2 CEC:

imidazolium and phosphonium ionic liquid) were placed into a 300 mL high pressure reactor.

Then, an initial loading of the autoclave at a pressure of 50 bars and at a temperature of 20°C

was made.


Page 126

The choice of temperature used was chosen based on the melting temperatures of the

imidazolium and phosphonium ionic liquid, i.e. 71 and 85°C, respectively. Initially, tests were

made at these melting temperatures without to give results because of their high viscosity.

Higher temperatures i.e. 80°C for the imidazolium ionic liquid and 90°C for the phosphonium

salt, were chosen to perform synthesis. Once the reactor temperature setpoint, the pressure

displayed was 70 bars for the imidazolium-modified montmorillonite and 80 bars for the

phosphonium-modified montmorillonite. After 6 hours of reaction and depressurization

different settings to minimize the losses of modified clays, the autoclave was depressurized at

a rate of 3.6 bar per second. A phosphonium-montmorillonite, denoted MMT-PCO2 and an

imidazolium-montmorillonite denoted MMT-ICO2, were obtained in the supercritical carbon

dioxide. When 10 mL of water was added as a co-solvent, the nomenclature is as follows for

MMT-P(CO2+Water) and MMT-I(CO2+Water).

The structure of synthesized phosphonium and imidazolium ionic liquids are described

in Table III-5.

Table III-5 – Designation of pristine and both synthesized ionic liquid modified montmorillonite (MMT)

Trade name

Intercalant Cationic Exchange process

Designation

Nanofil 757

MMT-Na+ MMT-Na+

N NH37C18 C18H37

Water

Supercritical CO2

Supercritical CO2

(+ 10% water)

MMT-I

MMT-ICO2

MMT-I(CO2+Water)

PC18H37

Water

Supercritical CO2

Supercritical CO2

(+ 10% water)

MMT-P

MMT-PCO2

MMT-P(CO2+Water)


Page 127

III.2.3 Results and discussion

III.2.3.1 Effect of supercritical carbon dioxide as a exchange solvent on the thermal degradation of the modified MMT

III.2.3.1.1 Thermal stability of imidazolium-modified montmorillonite

ThermoGravimetric Analysis (TGA) may help to identify intercalated as well as

physisorbed species of modified clay by investigating the degradation mechanisms and the

effects of functionalization on the thermal stability. Figure III-12 shows the evolution of the

weight loss as a function of temperature and the corresponding derivative curves performed

on imidazolium modified montmorillonites either in water at room temperature and pressure

or in supercritical carbon dioxide. The thermal degradation of imidazolium modified MMT

reveal three weight losses whatever the cationic exchange process used. According to the

literature [7, 9, 20, 22], as a layered clay is modified with an organic cation, two kinds of

interactions between organic cations and inorganic clay can take place: i) Van der Waals

bonds as the organic species are physically adsorbed on the clay surface and; ii) Ionic bonds

as the species display ionic interactions inside galleries in the clays.

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Page 128

The first weight loss corresponds to a partial physisorption on the edges (bearing polar

SiOH groups) or on the external surface of the platelets since the peak decreases after

washing. The fact that it does not completely disappear after intensive washing is associated

to a part of ionic liquid is well intercalated but in a peripheral position respect to the clay

gallery as reported by Davis et al. [25] or is physisorbed from π-SiOH interactions at the

edges of the lamellar silicates. Such part of surfactant can not be washed away easily (since it

underwent cationic exchange) but it is no thermally stabilized by the presence of the inorganic

silicate platelets in a confined position. As a consequence, it degrades at the same temperature

as the physisorbed surfactant. On the other side, the degradation which is evidenced in a

temperature range from 400 to 500°C corresponds to the well intercalated species. Indeed,

organics inside clay galleries display higher temperatures of degradation. As reported in

previous works [31].

Indeed, on the MMT-I DTG curves, the first peak at 340°C corresponds to physical

adsorption onto clay surface and the second and third peak at about 420°C and 480°C are

related to the imidazolium ionic liquid species really intercalated between clay layers. The

same signature on TGA curve is clear evidence that the cationic exchange of montmorillonite

with imidazolium ionic liquid is possible using only supercritical carbon dioxide as a solvent.

Up to now, only phosphonium ionic liquids with much shorter alkyl were considered in

ScCO2 to modify MMT but working at very high pressures while using a small amount of co-

solvent [32]. The influence of a co-solvent on the modification of lamellar silicates in

supercritical carbon dioxide was also considered. However, there is a significant drawback,

i.e. the formation of sticky powders due to the presence of ionic liquid excess adsorbed on the

surface of montmorillonite having a very high viscosity. As a consequence, the easiest

solution is to reduce the amount of ionic liquid introduced into the autoclave from the use of a

co-solvent as increasing pressure (75 bars) up to reduce the viscosity of ionic liquid required.

According to the literature [33], the solubility of ionic liquid in supercritical CO2

remains extremely low and it is necessary to use organic co-solvents. Wu et al. [34] studied

the effects of organic solvents as acetone or ethanol in ScCO2. Large increase of the solubility

of the ionic liquid in ScCO2 was reported. This phenomenon is explained by strong interaction

of solvent with the ionic liquid due mainly to their high polarity. For this knowledge, we

selected the most polar solvent, i.e. water, that has the huge advantage of being a green

solvent (such as supercritical CO2 and ionic liquids) to design an environmentally sustainable

cationic exchange process. However, compared to organic solvents, it has the disadvantage of


Page 129

being CO2-phobic but water could be used as a real co-solvent due to the fact that the

synthesized ionic liquids are soluble in water.

With water as co solvent, the thermal degradation of imidazolium modified

montmorillonite is quite different from one of the modified montmorillonite by conventional

cationic exchanges, i.e. in aqueous solution or ScCO2 medium process (Figure III-13).

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derivative of TGA curves (DTG) of the MMT-I bw (a, a’) and MMT-I (CO2 + Water) bw (b, b’) (heating rate : 20 K.min-1; nitrogen atmosphere).

The modified montmorillonite in supercritical carbon dioxide combined with co-

solvent shows both a much earlier degradation of physisorbed species (240°C versus 340°C)

but in the opposite a delayed degradation of intercalated species (540°C versus 420/490°C).

Washed with methyl alcohol that is the more suitable solvent of ionic liquids removes

the physically adsorbed species corresponding to the first degradation peak (Table III-6).


Page 130

Table III-6 – Relative mass loss of physically adsorbed and intercalated species determined by TGA (imidazolium modified montmorillonites either in water or ScCO2.

Sample Cationic Exchange process

Physically adsorbed

species (%)

Intercalated species

(%) MMT-I bw Water

31 18

MMT-I aw - 23 MMT-ICO2 bw Supercritical

CO2

27 22 MMT-ICO2 aw - 25

MMT-I(CO2+Water)

bw Supercritical

CO2

(+ 10% water)

47 18

MMT-I(CO2+Water)

aw 9

33

Table III-6 summarizes the relative mass losses of the physically adsorbed species and

the intercalated species for the imidazolium modified montmorillonites either after water

solution or supercritical CO2 medium processes before and after washing with or without co-

solvent. One can observe that before and after washing, the results for imidazolium-modified

montmorillonite (MMT-I) and imidazolium-treated montmorillonite under supercritical CO2

(MMT-I (CO2 + Water)) are similar. In the case of MMT-I (CO2 + Water), when comparing with the

imidazolium modified MMT before washing with a standard cationic exchange process, the

weight percent of physisorbed species is significantly higher (a difference of 16%). We found

almost the same difference (10% instead of 16%) for the intercalated species. Thus, the use of

the supercritical CO2 leads to an increase of the intercalated species ratio up to 33%. This

means that the combined effect of supercritical CO2 and the solubility of imidazolium salts in

water play an important role on the intercalation process.

In order to have a better understanding of the role of the various components,

imidazolium ionic liquids were introduced in the autoclave, heated at 80°C under pressure of

70 bars for 6 hours. The melting temperature of the imidazolium ionic liquid after synthesis

denoted as C18I and after exposure to supercritical CO2, denoted as C18ICO2 are reported in

Table III-7.


Page 131

Table III-7 – Effect of exposure to supercritical CO2 on the melting temperature of imidazolium ionic liquid at 80°C

Ionic liquid Exposure under ScCO2

Time (h)

Melting temperature

(°C) C18I 0 71

6 38 C18ICO2 24 33

After 6 hours in supercritical carbon dioxide, a strong decrease of the melting point is

observed. Which remains about the same as the exposure time increases, this effect could be

explained by the presence of ScCO2 which remains soluble in the ionic liquid. In conclusion,

there is a solubility limit of the ionic liquid in the CO2 phase.

According to the literature reported on the influence of supercritical CO2 on ionic

liquids [35, 36], this phenomenon was also observed for example by Kazarian et al. [37] who

reported that the melting point of imidazolium ionic liquids based on a C16 chain and a

fluoride anion was reduced from 25°C after a ScCO2 treatment under a pressure of 70 bars.

Later, another study on the phosphonium and ammonium salts [38] showed in the both cases

an important decrease of 100°C but at higher pressure of exposure (150 bars). Recently,

Scurto et al. [39] concluded that CO2 interacted with the ionic liquid due to the establishment

of weak Lewis acid-Lewis base interactions between basic moieties of the organic salt and the

acidic carbon of CO2.

In conclusion, the decrease of the melting temperature of the ionic liquid after

exposure to the the supercritical CO2 in the presence of water as a co-solvent in which ionic

liquid is soluble allows an optimal cationic exchange and a better intercalation of imidazolium

salt in the clay layer galleries.

III.2.3.1.2 Thermal stability of phosphonium-modified montmorillonite

The same approach was considered for the modification of montmorillonite with

phosphonium ions. The cationic exchange was performed in water under atmospheric

pressure but also in supercritical carbon dioxide. The TGA analysis performed on

phosphonium-modified MMT before washing with methyl alcohol are reported in Figure

III-14.


Page 132

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modified montmorillonite by TGA; derivative of the TGA curves MMT-P bw (a, a’) versus MMT-PCO2 bw (b, b’) (heating rate : 20 K.min-1, nitrogen atmosphere).

The same MMT modification realized under supercritical fluid shows the same TGA

analysis which evidences the success of the cationic exchange in the conditions.

As previously, we led the same experiment with the presence of co-solvent such as

water. Figure III-15 displays the TGA and DTG curves of phosphonium-modified

montmorillonites MMT-P and MMT-PCO2+ Water.

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With the presence of a co-solvent, a shift of intercalated species is observed (570°C

versus 510°C) whereas the physically adsorbed species are the same degradation behavior


Page 133

(320°C versus 330°C). The DSC data realized on phosphonium ionic liquid after synthesis

and after treatment with ScCO2 shows the same behavior in Table III-8.

Table III-8 – Effect of exposure to supercritical CO2 on the melting temperature of phosphonium ionic liquid at 90°C

Ionic liquid Exposure under ScCO2

Time (h)

Melting temperature

(°C) C18P 0 85

6 57 C18PCO2 24 55

The melting temperature of the phosphonium salt is significantly reduced after

treatment with supercritical carbon dioxide (28°C), in the same order of magnitude than for

imidazolium-based ionic liquid. We can expected that, according to the melting temperature

depression, the solubility of the two types of exchanged-montmorillonites ionic liquids in

ScCO2 are similar.

III.2.3.2 Structural analysis

III.2.3.2.1 Imidazolium modified montmorillonite

The effect of the cationic exchange process on the MMT intercalation was studied by X-

ray diffraction and reported in Figure III-16.

0 1 2 3 4 5 6 7 8 9 10

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measured by X-Ray diffraction spectra of phosphonium ionic liquid modified MMT: (a) MMT-I; (b) MMT-ICO2; (c) MMT-I(CO2 + Water)


Page 134

After organophilic treatment by a conventional cationic exchange, the MMT-I displays

a (001) diffraction peak at 2.3°2θ, corresponding to an interlayer distance of 3.7 nm, distance

similar to one characteristic of a paraffinic conformation with trans-trans positions of the alkyl

chain. The CO2 step leads to a shift of diffraction peak towards lower angles with an

interlayer distance of 4.1 nm, corresponding to diffraction peak at 2.1°2θ. The use of water as

a co-solvent leads to reduce the effects of swelling by ionic liquid which is solubilized in

water. The interlayer distance was found to be similar to one of the imidazolium-modified

montmorillonite performed by conventional cationic exchange.

III.2.3.2.2 Phosphonium modified montmorillonite

Figure III-17 shows the X-ray diffraction spectra performed on MMT-P, MMT-PCO2 and

MMT-P(CO2+water).

0 1 2 3 4 5 6 7 8 9 10

0

500

1000

1500

2000

2500

3000

Inte

nsity (

u.a

)

2θθθθ

(a)

(b)

(c)

(a)

(b)

(c)

Figure III-17 – Effect of the cationic exchange process on the interlayers distance

measured by X-Ray diffraction spectra of phosphonium ionic liquid modified MMT: (a) MMT-P; (b) MMT-P CO2; (c) MMT-P(CO2 + Water)

Before treatment, the basal spacing of the sodic montmorillonite is 1.2 nm, which

correspond to the d-spacing of MMT-Na+ reported in literature [7]. After organic treatment in

water by the phosphonium salt, the MMT-P displays a (001) diffraction peak at 2.1°2θ,

corresponding to an interlayer distance of 4.2 nm. This value could be explained by the

swelling of layered silicates due to steric volume of the three ring structure and the alkyl

chain. For the MMT-PCO2, the diffraction peak located at 1.8°2θ, i.e. indicating a d001 of 4.9

nm, a slight increase in the interlayer distance can be explained by the ionic liquid swelling


Page 135

under the effect of supercritical CO2. The diffraction spectrum displays also a small peak at

3.1°2θ, corresponding to physically phosphonium salt adsorbed on the surface of

montmorillonite. By using water as co-solvent, the diffraction peak is located at 2.0°2θ, i.e.

indicating a distance of 4.4 nm for MMT-P(CO2 + Water), close to the one of the phosphonium-

treated MMT by conventional cationic exchange. In all the cases, the spectra show intense,

thin, and regular diffraction peaks which suggest a modification on a long range order.

The resulting structure of the ionic liquid-modified montmorillonites, i.e. the d001

distances, as well as the amount of intercalated ionic liquids species can be explained from the

solubility parameters of the various components : ionic liquids, water, and supercritical CO2

the spreading coefficient. In fact, as a conventional route is used, i.e. water solution of ioni

liquid as exchange process medium, the intercalation of imidazolium or phosphonium alkyl-

modified species proceeds from the well-known exchange process described for quaternary

ammonium intercalants [40-41]. As supercritical CO2 is used as medium for intercalation, the

driving force is the spreading of ionic liquid species onto the MMT surface as the surface

polarity of montmorillonite matches the surface tension (or solubility parameter) of the ionic

liquid better than the ScCO2 medium. In the later case, i.e. involving water as co-solvent, as

this one is not soluble in ScCO2, the ionic liquids remain in the water phase leading to a

highly concentrated ionic liquids-water phase, which spreads onto the polar montmorillonite

surface. As a consequence, the process involving water as co-solvent is similar to the

conventional water-solution based protocol.

The solubility of ionic liquids in supercritical CO2 became of a charge interest as such

an understanding requires modelling approaches [42-44] which could be used for practical

purposes [45]. Melting temperature of the phosphonium salt is significantly reduced after

treatment with supercritical carbon dioxide (28°C), in the same order of magnitude than for

imidazolium-based ionic liquid. We can expected that, according to the melting temperature

depression, the solubility of the two types of exchanged-montmorillonites ionic liquids in

ScCO2 are similar.


Page 136

III.2.3.3 Surface energies


pressed powder are collected in Table III-9.

Table III-9 – Determination of polar and dispersive components of the surface energy on pristine and on exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders)

Montmorillonite Θwater (°) ΘCH2I2 (°) γ polar (mN.m-1)

γ dispersive (mN.m-1)

γ total (mN.m-1)

MMT-Na+ 22.9 ±0.9 33.6 ±0.8 30 43 73 MMT-P 88.9 ±0.1 49.4 ±0.6 2 35 37 MMT-I 92.8 ±0.1 55.5 ±0.6 1 31 32

MMT-P ScCO2 co 81.5 ±0.1 45.5 ±0.7 3.6 36.8 40.4 MMT-I ScCO2 co 125.2 ±0.5 41.9 ±0.7 3.6 38.5 42.1

Both ionic liquids based on phosphonium and imidazolium salts make the montmorillonite

more hydrophobic with a surface energy close to the surface energy of a polyolefin [28]. The

polar components are very low which is an evidence that the hydroxyl groups are well

covered by the organic species, especially C18 chains. The steric hindrance of imidazolium

and phosphonium ionic liquids causes an efficient screening of the hydrophilic surface of

lamellar silicates.

For imidazolium and phosphonium-modified montmorillonites under supercritical

carbon dioxide without co solvent, the values are identical. Whereas the montmorillonites

MMT-P ScCO2 co and MMT-P ScCO2 co, i.e. by using water as a co-solvent, the values of

surface energy are slightly higher.

III.2.4 Conclusions In this work, we demonstrated that it is possible to modify lamellar silicates by ionic

liquid phosphonium and imidazolium using solvents as water and supercritical CO2. The

resulting properties are improved thermal stability of intercalated species and better

intercalation between the layers of montmorillonite. This process can be improved and

requires many additional studies on the interactions between different components which are

considered. Nevertheless, this study highlights that several solvents, such as water, ionic

liquids, and supercritical CO2, which are among the most promising components of green

chemistry, could be used relevant surface treatment of layered minerals.


Page 137

Conclusions of chapter III

In Chapter III, the surface treatment of layered silicates by intercalating agents like

ammonium, imidazolium and phosphonium has been studied in two ways: the cation

exchange method involving the use of conventional organic solvents and a method

environmentally friendly using supercritical CO2 as solvent.

Firstly, better thermal stability of ILs on the conventional ammonium has been

demonstrated. Indeed, increases of 50°C to 120°C are observed for physisorbed and

intercalated species. In addition, the amount physically adsorbed on the surface of

montmorillonite is compatibilizing agent between the filler and the polymer which results in

thermal and mechanical properties increased.

In a second step, we showed that using supercritical CO2 associated with water as co-

solvent leads to a decrease of melting temperatures of ILs important which leads to a better

intercalation of imidazolium ILs and phosphonium between the clay layers with a sharp

increase of the degradation temperature.


Page 138

References of chapter III [1] E.P. Giannelis, Adv. Mater (1996); 8:29. [2] P.C. Le Baron, Z. Wang, T.J. Pinavaia, Appl. Clay Sci. (1999); 15:11. [3] Y.W. Mai, Z.Z. Yu, Polymer Nanocomposites, Woodhead, Cambridge, (2006). [4] L. Le Pluart, J. Duchet, H. Sautereau, Polymer 46 (2005); (26):12267. [5] E. Jacquelot, E. Espuche, J.F. Gerard, J. Duchet, P. Mazabraud, J. Polym. Sci. Part B: Polym. Phys. (2006); 44 (2):431. [6] M.A.Osman,V.Mittal,M.Morbidelli, U.W. Suter, Macromolecules (2003); 36:9851. [7] J.W. Gilman, C.L. Jackson, A.B. Morgan, R. Harris, E. Manias, E.P. Giannelis, Chem. Mater. (2000); 12:1866. [8] S.M. Auerbach, K.A. Carrado, P.K. Dutta, Handbook of Layered Materials, Taylor & Francis, New York, (2004). [9] L. Le Pluart, J. Duchet, H. Sautereau, J.F. Gérard, J. Adhes. (2002); 78 (7):645. [10] S.Y. Lee, W.J. Cho, K.J. Kim, J.H. Ahn, M. Lee, J.colloid Interf Sci. (2005); 284 (2):667. [11] H. He, J. Duchet, J. Galy, J.F. Gerard, J.colloid Interf Sci. (2006); 295:202. [12] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Chem. Mater. (2001); 13 (9):2979. [13] T.D. Fornes, P.J. Yoon, H. Keskkula, D.R. Paul, Polymer (2001); 42:9929. [14] J.W. Gilman, Appl. Clay Sci. (1999); 15:31. [15] W.H. Awad, J.W. Gilman, M. Nyden, R.H. Harris, T.E. Sutto, J. Callahan, P.C.Trulove, H.C. DeLong, D.M. Fox, Thermochim. Acta (2004); 409:3. [16] J. Zhu, A.B. Morgan, F.J. Lamelas, C.A. Wilkie, Chem. Mater. (2001); 13:3774. [17] W. Xie, R. Xie, W.P. Pan, D. Hunter, B. Koene, L.S. Tan, R. Vaia, Chem. Mater. (2002); 14 (11):4837. [18] H.L. Ngo, K. Le Compte, L. Hargen, A.B. McEven, Thermochim. Acta (2000); 97:357–358. [19] F.A. Bottino, E. Fabbri, I.L. Fragala, G. Malandrino, A. Orestano, F. Pilati, Macromol. Rapid Commun. (2003); 24:1079. [20] J.W. Gilman, W.H. Awad, R.D. Davis, J. Shields, R.H. Harris, C. Davis, Chem. Mater. (2002); 14:3776. [21] V. Mittal, Eur. Polym. J. (2007); 43:3727. [22] C. Lotti, C.S. Isaac, M.C. Branciforti, R. Alves, S. Liberman, R. Bretas, Eur. Polym. J. (2008); 44:1346. [23] S. Filippi, C. Marazzato, P. Magagnigni, A. Famulari, P.V. Arosio, S. Meille, Eur. Polym. J. (2008); 44:987. [24] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Thermochim. Acta (2001); 339:367–368. [25] R.D. Davis, J.W. Gilman, T.E. Sutto, Clay Clay Miner. (2004); 52 (2):171. [26] F. Bergaya, B.K.G. Theng, G. Lagaly, Handbook of Clay Science, first ed., Elsevier, (2006). [27] C.M. Hansen, A. Beerbower, Kirk-Othmer Encyclopedia of Chemical Technology, second ed., Interscience, New York, (1971). p. 889. [28] S. Boucard, J. Duchet, J.F. Gérard, P. Prele, S. Gonzalez, Macromol. Symp. (2002); 194 (1):241. [29] Chigwada, G., D. Wang, et al, Polym Degrad. and Stab. (2006); 91(4):848–855. [30] S.P. Nalawade, F. Picchioni, L.P.B.M. Jansen, Prog. Polym. Sci. (2006); 31 19–43. [31] S. Livi, J. Duchet-Rumeau, T.-N. Pham and J.-F. Gérard, J.colloid Interf Sci (2010); 349:424–433. [32] E. Naveau, C. Calberg, C. Detrembleur, S. Bourbigot, C. Jérôme, M. Alexandre, Polymer (2009); 50:1438–1446. [33] S. Keskin, D. Kayrak-Talay, U. Akman, Ö. Hortaçsu, J. of Supercritical Fluids (2007); 43:150–180. [34] W.Wu, J.Zhang, B.Han, J.Chen, Z.Liu, T.Jiang, J.He, W.Li, Chem.Comm. (2003); 1412–1413. [35] V. Najdanovic-Visak, A. Serbanovic, J.M.S.S. Esperança, H.J.R. Guedes, L.P.N. Rebelo, M.Nunes da Ponte, Chem.Phys.Chem. (2003); 4:520. [36] M. Roth, J. of Chromatography, A (2009); 1216:1861–1880. [37] S.G. Kazarian, N. Sakellarios, C.M. Gordon, Chem. Comm. (2002); 1314. [38] A.M. Scurto, W. Leitner, Chem. Commun. (2006); 3681. [39] A.M. Scurto, E. Newton, R.R. Weikel, L. Drauker, J.Hallett, C.L. Liotta, W. Leitner, C.A. Eckert, Ind. Eng. Chem. Res. (2008); 47:493. [40] C.B. Hedley, G. Yuan, B.K.G. Theng, Applied Clay Science. (2007); 35:180–188. [41] A. Vasquez, M. Lopez, G. Kortaberria, L. Martin, I.Mondragon, Applied Clay Science. (2008); 41:24–36. [42] X. Ji, H. Adidharma, Fluid Phase Equilibria. In press, Accepted Manuscript. [43] J. Kumelan, A. Perez-Salado Kamps, I. Urukova, D. Turna, J.Chem. Thermodynamics. (2005); 37 (6):595–602.


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[44] J.S. Torrecilla, J. Palomar, J. Garcia, E. Rojo, F.Rodriguez, Chemiometrics & Intelligent Laboratory

Systems. (2008); 93 (2):149–159. [45] M.G. Freire, C.M.S.S. Neves, S.P.M. Ventura, M.J Patras, I.M. Marrucho, J.Oliveira, J.A.P. Coutinho, Fluid Phase Equilibria, In press, accepted manuscrit.


Page 140

Chapter IV: Polymer/Layered silicates Nanocomposites

Page 141

Chapter IV POLYMER/LAYERED SILICATES NANOCOMPOSITES In this last chapter, we have used these montmorillonites thermally stable during the

preparation of nanocomposites by melt intercalation in two different matrices, high density

polyethylene (HDPE) and polyvinylidene fluoride (PVDF). The influence of ligand, the

variation of chain length, the functionalization of perfluorinated chains, the role of the

imidazolium versus phosphonium cation, and the anion (Br-, I-, PF6-) have been studied on

thermal, physical and mechanical properties as well as on the morphology of the resulting

nanocomposites.


Page 142

Pages

IV.1 Synthesis of new surfactants: Effect of the ionic liquids on the thermal stability and the mechanical properties of high density polyethylene nanocomposites ................................ 143

IV.1.1 Introduction ............................................................................................................................ 143 IV.1.2 Experimental .......................................................................................................................... 144

IV.1.2.1 Materials ................................................................................................................................. 144 IV.1.2.2 Processing and characterization of HDPE/clay nanocomposites ............................................ 145 IV.1.2.3 Synthesis of imidazolium and phosphonium salts .................................................................. 146

IV.1.2.3.1 Synthesis of octadecylphosphonium bromide and iodide 1a-1b ................................. 146 IV.1.2.3.2 Synthesis of octadecylphosphonium hexafluorophosphate 1c .................................... 147 IV.1.2.3.3 General procedure for the synthesis of N-alkyl-N’-alkyl imidazolium salts 3a-3c. .... 148

IV.1.2.4 Organic modification of montmorillonite ............................................................................... 149 IV.1.2.4.1 Preparation of phosphonium-MMT ............................................................................ 149 IV.1.2.4.2 Preparation of imidazolium-MMT .............................................................................. 150

IV.1.3 Results and discussion ............................................................................................................ 150 IV.1.3.1 Thermal stability of ionic liquids ............................................................................................ 151 IV.1.3.2 Thermal stability of ionic liquid modified montmorillonites .................................................. 153 IV.1.3.3 Surface Energy of ionic liquid modified montmorillonites ..................................................... 154 IV.1.3.4 PE/modified-montmorillonites nanocomposites ..................................................................... 155

IV.1.3.4.1 Morphology of the nanocomposites ........................................................................... 155 IV.1.3.4.2 Thermal stability of the nanocomposites .................................................................... 157 IV.1.3.4.3 Mechanical properties of the nanocomposites ............................................................ 158

IV.1.4 Conclusions ............................................................................................................................ 160

IV.2 Ionic Liquids as Interfacial Agents in PVDF-based nanocomposites .......................... 161 IV.2.1 Introduction ............................................................................................................................ 161 IV.2.2 Experimental .......................................................................................................................... 162

IV.2.2.1 Materials ................................................................................................................................. 162 IV.2.2.2 Synthesis of ionic liquids ........................................................................................................ 162

IV.2.2.2.1 Synthesis of ILs with long alkyl chains ...................................................................... 162 IV.2.2.2.2 Synthesis of imidazolium functionalized with perfluorinated chain ........................... 163

IV.2.2.3 Organic modification .............................................................................................................. 163 IV.2.3 Results and discussion ............................................................................................................ 164

IV.2.3.1 Characterization of ILs exchanged montmorillonites ............................................................. 164 IV.2.3.1.1 Thermal stability of ionic liquid-modified montmorillonites ..................................... 164 IV.2.3.1.2 Structural analysis of ionic liquid-modified montmorillonites ................................... 166 IV.2.3.1.3 Surface energy of ionic liquid-treated montmorillonites ............................................ 167

IV.2.3.2 Effect of interfacial interactions on the material physical properties ...................................... 168 IV.2.3.2.1 On the morphology of the PVDF nanocomposites ..................................................... 168 IV.2.3.2.2 Crystallinity of PVDF based nanocomposites ............................................................ 170 IV.2.3.2.3 Mechanical properties of PVDF based nanocomposites ............................................. 174

IV.2.4 Conclusions ............................................................................................................................ 175

Conclusions of chapter IV ............................................................................................................ 176

References of chapter IV .............................................................................................................. 177


Page 143

IV.1 Synthesis of new surfactants: Effect of the ionic liquids on the thermal stability and the mechanical properties of high density polyethylene nanocomposites

Ionic liquids based on alkyltriphenyl phosphonium and dialkyl imidazolium surfactant

salts with long alkyl chains have been synthesized and used as intercalating agents for the

preparation of highly thermally stable organophilic montmorillonites. These new surfactants

behave as conventional organic cations and are easily swollen inducing a high d-spacing.

Thermoplastic nanocomposites with a very low amount of nanofillers have been processed by

melt mixing using a twin screw extruder. The thermal stability of the phosphonium- (MMT-P)

or imidazolium- (MMT-I) modified montmorillonites has been enhanced by up to 100°C

compared with conventional quaternary ammonium cations, making melt mixing of such

modified nanoclays possible with high density polyethylene (HDPE) processed at high

temperature.

IV.1.1 Introduction Recent research dedicated to the introduction of layered silicate-(montmorillonite,

MMT) into polymer matrices demonstrates an increase of thermal stability [1], mechanical

properties [2-4], and reduced flammability [5-7] for numerous types of polymer-clay

nanocomposites. The lamellar and confined structure of inorganic layers in polymer matrix

and the nanoscale dimensions of particles could explain changes in polymer physics, such as

molecular mobility and thermal resistance. This later one is of course also associated to the

thermal stability of the surface modifiers, i.e. interfacial agents. In fact, increasing the thermal

stability of montmorillonite and resulting nanocomposites is a key issue in the developement

of polymer-clay nanocomposites at the industrial scale.

The limited thermal stability of the conventionally used alkylammonium cations [8]

intercalated into layered minerals and the degradation occurring during processing of some

thermoplastic polymers (for example polyolefins) in the presence of nanodispersed MMT

have motivated the development of improved organophilic treatments for layered silicates.

Such new organomodifiers should enable the preparation of polymer/layered silicate

nanocomposites based on polymers that require high melt-processing temperatures [9-10]


Page 144

and/or long residence times under high shear as well as for thermoset reactive systems which

are cured at high temperatures.

Ionic liquids (ILs) are organic salts with melting temperature below 100°C. They are

known for their excellent thermal stability, non-flammability, low vapor pressure and high

ionic conductivity. Their properties can be tuned by different associations of cations and

anions. The cation is usually a bulky organic structure with a low symmetry, such as

pyridinium, imidazolium, phosphonium, and ammonium ions. The current research mainly

focused on Room Temperature Ionic Liquids (RTILs) composed of asymmetric N-N

dialkylimidazolium cations bearing short alkyl chains such as butyl-methyl or ethyl-methyl

functionalization. The original part of this work results in the synthesis of ILs functionalized

with long alkyl chains suitable for improving the compatibility and the dispersion of

nanolayered silicates within polyolefin matrix, i.e. hydrophobic medium.

The aim of this study is to synthesize new thermally stable intercalating agents and to

find the most relevant combination of cation and anion to design an ionic liquid efficient to

get organophilic montmorillonites and to improve the physical properties of resulting

nanocomposite. The effect of the chemical nature of cation, imidazolium versus

phosphonium, the influence of the alkyl chain length, i.e. octadecyl versus docosyl, for

imidazolium salts and the effect of the chemical nature of anion, i.e. halide versus fluorinated

for phosphonium salts on the morphology, the thermal, and mechanical properties of high

density polyethylene-based nanocomposites will be investigated.

IV.1.2 Experimental

IV.1.2.1 Materials

A sodic montmorillonite, denoted Nanofil 757 (MMT), i.e. an aluminosilicate with

intercalated sodium was chosen as pristine clay. This one was provided by Süd Chemie Co.

The Nanofil 757 has a cation exchange capacity of 95 meq/100 g and is described by the

following formula Na0,65[Al,Fe]4Si8O20(OH)4. All chemicals necessary to the synthesis of

ionic liquids, i.e. triphenylphosphine (95%), imidazole (99.5%), 1-iodooctadecane (octadecyl

bromide 95%), 1-bromodocosane, (docosyl bromide 96%) and solvents (toluene, methanol,

pentane and acetonitrile) were supplied from Aldrich and used as received.

The polyethylene used in this study, denoted HDPE, is a high-density polyethylene

from Basell, with the trade name Hostalen GF 7260 showing a melt flow index of 0.4.


Page 145

IV.1.2.2 Processing and characterization of HDPE/clay nanocomposites

Nanocomposites based on PE/organically modified montmorillonites (1% by weight)

were prepared using a 15g-capacity DSM micro-extruder (Midi 2000 Heerlen, The

Netherlands) with co-rotating screws. The mixture was sheared for about 3 min with a 100

rpm speed at 190°C and injected in a 10cm3 mould at 30°C to obtain dumbbell-shaped

specimens.

Thermogravimetric analysis (TGA) of organically modified clay and nanocomposites

were performed on a Q500 thermogravimetric analyser (TA instruments). The samples were

heated from 30 to 800 °C at a rate of 20 K.min-1 under nitrogen flow.

Differential scanning calorimetry (DSC) analysis were performed on a Q20 (TA

instruments). The samples were kept for 1 min at 200°C to erase the thermal history before

being heated or cooled at a rate of 10 K.min-1 under nitrogen flow. The crystallinity rate was

determined considering ∆H100% to be 102.7 J/g [30].

Surface energy of modified clays was determined with the sessile drop method using a

GBX goniometer. From contact angle measurements performed with water and

diiodomethane as probe liquids on discs obtained from clay powders by pressing, polar and

dispersive components of surface energy were determined by using Owens-Wendt theory.

Transmission Electron microscopy (TEM) was used at the Center of Microstructures

(Université de Lyon) on a Philips CM 120 field emission electron microscope with an

accelerating voltage of 80 kV. The samples were cut using an ultramicrotome equipped with a

diamond knife to obtain 60 nm thick ultrathin sections. Then, the sections were set on copper

grids.

Uniaxial Tensile Tests were carried out on a MTS 2/M electromechanical testing

system at 22±1°C and 50±5% relative humidity at crosshead speed of 10 mm.min-1.


Page 146

IV.1.2.3 Synthesis of imidazolium and phosphonium salts

IV.1.2.3.1 Synthesis of octadecylphosphonium bromide and iodide 1a-1b

The studied ionic liquids are not commercially available and their synthesis was

reported using the same protocol as the one described by Livi et al. [11].

Octadecyltriphenylphosphonium salts have been synthesized according to Figure IV-1 with the

corresponding iodide and bromide anions. Octadecyl hexafluorophosphate is not

commercially available and has been prepared by anionic exchange from octadecyl iodide and

hydrogen hexafluorophosphate (HPF6).

PC18H37

X

X = I , Br

C18H37XP

1a, 1b

+

Toluene

120 °C 25°C

HPF6

PC18H37 PF6

1c Figure IV-1 – Synthesis of the phosphonium ionic liquid 1a-1b-1c

General method

In a 100 mL flask were placed under a positive nitrogen pressure, triphenylphosphine

and octadecyl iodide or octadecyl bromide. The stirred suspensions were allowed to react for

24 h at 120 °C in toluene (20 mL), a yellow precipitate was formed. The reaction mixture was

then filtered, washed repeatedly with pentane. Most of the solvent was removed under

vacuum and the product was dried to a constant weight to give a white solid. The structure of

these salts was confirmed by 13C NMR spectroscopy.

a. Octadecyltriphenylphosphonium iodide 1a

1a was obtained by reaction of triphenylphosphine (5 g, 1 equiv.) and octadecyl iodide

(7.70 g, 1 equiv.). Yield = 90 %, white powder.

13C NMR (CDCl3): δ 14.00 (CH3); 22.67 (CH2Me); 23.2; 29.37-29.66; 30.24; 31.85

(PCH2); 118.45; 130.43; 133.70; 135.15 (P-Carom.).


Page 147

b. Octadecyltriphenylphosphonium bromide 1b

1b was obtained by reaction of triphenylphosphine (5 g, 1 equiv.) and octadecyl

bromide ( 9.8 g, 1 equiv.). Yield = 88 %, brown powder.


(PCH2); 118.85; 130.80; 133.50; 135.45 (P-Carom.).

IV.1.2.3.2 Synthesis of octadecylphosphonium hexafluorophosphate 1c

In a 100 mL flask, octadecyl iodide (C18H37I) (5.0 g, 1 equiv.) was dissolved into

dichloromethane (25 mL). The mixture was stirred for 30 min at room temperature. A

solution of hydrogen hexafluorophosphate (HPF6) (3.8 g, 2 equiv.) diluted in water (25 mL)

was stirred for 30 min and added to the octadecyl iodide solution. The stirred suspension was

allowed to react for 24 h at room temperature. The reaction mixture was then introduced in a

separatory funnel and the organic layer was washed repeatedly with distilled water (4x 25

mL). The mixture was dried over anhydrous magnesium sulfate and concentrated under

reduced pressure. The solvent was removed by evaporation under vacuum and the product

was dried to a constant weight to give a white solid.


(PCH2); 118.75; 130.22; 133.50; 135.05 (P-Carom.).


Page 148

IV.1.2.3.3 General procedure for the synthesis of N-alkyl-N’-alkyl imidazolium salts 3a-3c.

Dialkylimidazolium cation-based ionic liquids have been easily prepared by alkylation

of the commercially available imidazole with an alkyl iodide to give respectively the

corresponding N-alkyl-N’-alkylimidazolium halide (Figure IV-2).

N N N NR

HN N

MeO Na RI

- NaI

N NR R' I

R'I

C18

C22 C22

R = C18 R' = C18

C22

2a 3a

2b

2c

3b

3c Figure IV-2 – Synthesis of the imidazolium ionic liquids 3a-3c

A solution of sodium methoxide was prepared from sodium (1 equiv.) in dry freshly

distilled methyl alcohol (10 mL) in a sealed septum, 100 mL round-bottomed, threenecked

flask equipped with a condenser, under nitrogen atmosphere and magnetic stirring. Imidazole

(1 equiv.) diluted in acetonitrile (10 mL) was then added into the stirred mixture of sodium

methoxide previously cooled at room temperature. After 15 min, a white precipitate was

formed. The suspension was then concentrated under reduced pressure for 1 h. The dried

white powder was dissolved in acetonitrile and a solution of alkyl halide RX (1 equiv.) diluted

in acetonitrile (10 mL) was then added under an inert atmosphere of nitrogen at room

temperature. The mixture was stirred for 1 h, then heated under reflux at 85 °C for about 24 h.

A solution of alkyl halide R’X (1 equiv.) diluted in acetonitrile (10 mL) was added to the

mixture at room temperature. The stirred suspension was heated under reflux at 100 °C for

about 24 h leaving a brownish viscous oil in each case. After cooling to room temperature, the

solvent was removed by evaporation under vacuum, the beige coloured powder was filtered,

washed repeatedly with pentane and dried. Purification of the resulting imidazolium salts was

accomplished by crystallization from ethyl acetate/acetonitrile: 75/25 mixture. After drying,

alkyl imidazolium salts 3a-3c were fully characterized by spectroscopy. The assignment of 13C NMR spectroscopy resonance peaks is an evidence of the success of the ionic liquid

synthesis.


Page 149

N-octadecyl-N’-octadecyl imidazolium iodide 3a, see Livi et al. [11].

N-octadecyl-N’-docosyl imidazolium bromide 3b

According to the general procedure, N-octadecyl-N’-docosylimidazolium bromide 3b

were carried out with Na (0.460 g, 20 mmol), imidazole (1.370 g, 20 mmol, 1 equiv.),

octadecyl iodide (7.700 g, 20 mmol, 1 equiv.) and docosyl bromide (7.790 g, 20 mmol, 1

equiv.). Evaporation of the solvent under vacuum furnished the product 3b (13.290 g) as a

beige colour powder. Yield = 94 %.

13C NMR (CDCl3) δ (ppm) 14.00 (2CH3); 20.00 (2CH2Me); 26.00; 29.00; 29.35-29.69

(CH2); 30.24; 31.91 (NCH2CH2); 50.10; (CH2N=); 50.32 (CH2N-); 121.70; 122.50 (=CN);

136.90 (N-C=N).

N-docosyl-N’-docosylimidazolium bromide 3c

According to the general procedure, N-octadecyl-N’-docosylimidazolium bromide 3c

were carried out with Na (0.920 g, 40 mmol), imidazole (2.740 g, 40 mmol, 1 equiv) and

docosyl bromide (15.580 g, 40 mmol, 1 equiv.). Evaporation of the solvent under vacuum

furnished the product 3c (19.200g) as a beige colour powder. Yield = 63 %.

13C NMR (CDCl3) δ (ppm) 13.40 (2CH3); 19.40 (2CH2Me); 26.00; 29.00; 29.35-29.69

(CH2); 30.24; 31.90 (NCH2CH2); 50.10; (CH2N); 50.32 (CH2N=); 121.70; 122.50 (=CN);

136.90 (N-C=N).

IV.1.2.4 Organic modification of montmorillonite

IV.1.2.4.1 Preparation of phosphonium-MMT

After dispersion of montmorillonite Nanofil 757 (10 g) in deionized water (1 L),

phosphonium salts (9.1 mequiv., as 0.01 M solution) were slowly added per gram of

montmorillonite under continuous stirring at 80 °C during 10 h. The products were washed free

from halide ions tested using AgNO3 solution, dried at 35°C followed by over night drying at

110°C and then pulverized to pass through 300 mesh sieve. The phosphonium-MMT was

stored in a dessicator before analysis.


Page 150

IV.1.2.4.2 Preparation of imidazolium-MMT

An excess of the salts (2 equiv.) based on the cation exchanged capacity (CEC = 95

mequiv./100 g) of the montmorillonite Nanofil 757 was used [13]. Montmorillonite was dried

at 110°C for 48 h and washed with distilled water (10 wt%) with vigorous stirring for 24 h to

cause the delamination of the montmorillonite. An amount of obtained imidazolium salts (2.66

g) and the montmorillonite Nanofil 757 (2 g, 1.9 x10-3 equiv.) was dispersed in 400 mL of

deionised water. This dispersion was mixed and stirred vigorously at 80°C for 17 h, followed

by filtration and repeated washing with organic solvent (acetonitrile at room temperature or hot

ethanol) to remove some eventually remaining imidazolium salts, then with deionised water

until no halide ions were detected using an aqueous silver nitrate (AgNO3) solution. The

solvent was removed by evaporation under vaccuum. Modified montmorillonite was then dried

for one night, at a suitable temperature (not higher than 80°C) which prevents from any

degradation of alkyl ammonium montmorillonite by Hofmann elimination mechanism [14-15].

The imidazolium-MMT was stored in a dessicator before analysis.

IV.1.3 Results and discussion

We report herein, a general and simple method for the synthesis of a series of organic

halide salts based on octadecyltriphenylphosphonium salts 1a-1c and N-alkyl-N’-alkyl

imidazolium salts 3a-3c was developed in (Figure IV-3). These phosphonium and imidazolium

salts with long alkyl chains are used as new surfactants in place of conventional ammonium

salts commonly used to get organophilic lamellar silicate and to assist their intercalation into

non polar polymer matrices. By tuning both the chemical nature of cation and anion, the better

cation-anion combination is analyzed to get thermal stability of modified montmorillonites

and consequently of the resulting nanocomposites.

N NH37C18 C18H37

N N

H37C18C22H45

BrIN N

C22H45 C22H45

Br

3a 3b 3c

X = I

Br

PF6

1a

1b

1c

PC18H37

X

Imidazolium 3a-3c

Phosphonium 1a-1c Figure IV-3 – Structure of the phosphonium and imidazolium ionic liquids.


Page 151

IV.1.3.1 Thermal stability of ionic liquids

Because of their aromatic structure, the imidazolium cations are known to display a better

thermal stability than the ammonium cations [15]. Thermogravimetric measurements were

carried out on the three types of imidazolium ionic liquids with halide anions and long alkyl

chains. The thermal decomposition of imidazolium salts C18C18Im, C18C22Im, and C22C22Im is

described in Figure IV-4.

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Figure IV-4 – Effect of the alkyl chain length on the imidazolium ionic liquids: evolution of the weight loss as a function of temperature (TGA, DTG) of the (●) C18C22Im, (■) C22C22Im, (♦) C18C18Im

(heating rate : 20 K.min-1, under nitrogen atmosphere)

The imidazolium cation with C18-C18 alkyl ligands is thermally stable up to 320°C.

Nevertheless, a slight decrease in thermal stability of imidazolium ionic liquid is shown when

the alkyl chain length is increased. With C22-C22 alkyl chains, the weight loss takes as soon as

the thermal analysis starts even if the maximal degradation temperature is slightly higher.

Under oxidative atmosphere, Awad et al [16] also concluded that the thermal stability

decreased as the organic content of the molten salt increased. According to the literature [9],

at high temperature, i.e. 600°C, the imidazole ring is thermally resistant during the thermal

rearrangements of dialkylimidazolium ions which explains that the imidazolium ionic liquids

are not fully degraded.


Page 152

The chemical nature of cation, imidazolium versus phosphonium, has no really an effect

on the thermal stability. With the iodide anion, the ionic liquids based on phosphonium cation

display a degradation at the same temperature as one determined for imidazolium cation, i.e.

about 320°C, as reported in Figure IV-5.

The effect of the chemical nature of the anion, halide anions (I-), (Br-) versus fluorinated

anion (PF6-), on the thermal stability of the phosphonium ionic liquids C18P is presented in

Figure IV-5.

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Figure IV-5 – Effect of the anion chemical nature associated to phosphonium ionic liquids: evolution of the weight loss as a function of temperature (TGA, DTG) of the

(●) C18P Br-, (■) C18P I-, (♦) C18P PF6-

(heating rate : 20 K.min-1, under nitrogen atmosphere).

The combination of the anion associated with the organic cation has a significant role on

the thermal stability of ionic liquid. Indeed, the use of hexafluorophosphate anion (PF6-)

combined with the phosphonium cation provides an increase of temperature about one

hundred and forty degree celsius compared to the halide salts C18P I- and C18P Br- that are

degraded at the same temperature, about 320°C. This use of fluorinated anion in order to

enhance the thermal stability of the ionic liquids is not new. Awad et al [9] had demonstrated

that the use of anion type PF6- and BF4

- associated to imidazolium salt increased the thermal

stability of one hundred degrees. Regarding the phosphonium ionic liquid, the literature [17,

18] has already reported the lower thermal stability of ionic liquid associated to bromide

anion compared to one obtained in presence of tetrafluoroborate anion.


Page 153

IV.1.3.2 Thermal stability of ionic liquid modified montmorillonites

After cationic exchange with the phosphonium or imidazolium ionic liquids, two

organophilic montmorillonites were prepared with varying alkyl chain length and different

counteranions. From TGA analysis, two organic species populations have been identified the

physisorbed species onto the montmorillonite surface and the intercalated species between the

montmorillonite galleries and have already been described in a previous work [11] on the

phosphonium-modified montmorillonite (MMT-P I-) and the imidazolium-treated

montmorillonite (MMT-C18C18Im). The thermal degradation of montmorillonites modified by

the imidazolium ionic liquids is reported in Figure IV-6 as a function of the ligand alkyl chain

length and the halide anion nature associated.

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Figure IV-6 – Effect of the alkyl chain length and the halide anion associated to the imidazolium cation : evolution of the weight loss as a function of temperature (TGA, DTG) of the

(●) MMT-C18C22Im, (■) MMT-C18C18Im (♦) MMT-C22C22Im (heating rate : 20 K.min-1, under nitrogen atmosphere).

Neither the alkyl chain length (C18 versus C22) or the chemical nature of the halide anion

(bromide or iodide) has an influence on the thermal decomposition of modified clays. Indeed,

the same thermal degradation process takes place going through three decomposition steps

occurring at about 310, 410 and 480°C.

However, it was previously shown that the chemical nature of the anion played an

important role on the intrinsic thermal stability of phosphonium ionic liquids.


Page 154

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Figure IV-7 – Effect of the anion chemical nature associated to phosphonium cation: evolution of the weight loss as a function of temperature (TGA, DTG) of the

(●) MMT-P Br-, (■) MMT-P PF6- (♦) MMT-P I-

(heating rate : 20 K.min-1, nitrogen atmosphere).

Regarding the TGA analysis reported in Figure IV-7 on the phosphonium-modified

montmorillonites with bromide (Br-) and iodide (I-) conteranions, the same degradation is

observed with two main degradation peaks. The first weight loss at 330°C still corresponds to

a partial physisorption on the external surface of the platelets. On the other hand, the

degradation which is evidenced at about 500°C corresponds to the well intercalated species

between clay layers. In the case of MMT-P PF6-, the use of hexafluorophosphate anion causes

a shift measured at 80°C at higher temperatures (410°C instead of 330°C) of the degradation

peak corresponding to physisorbed species.

In conclusion, the montmorillonites modified with the imidazolium and phosphonium

ionic liquids display all both an excellent thermal stability up to nearly 500°C if the

physisorbed species are removed by washing which is a huge advantage for the processing of

polymer/clay nanocomposites at high temperatures.

IV.1.3.3 Surface Energy of ionic liquid modified montmorillonites

To evaluate the interactions able to be generated by the modified montmorillonite

towards the polymer matrix as a function of the alkyl chain length and of the anion nature, the

contact angles and surface energies were determined by the sessile drop method on pressed

powder and the values are collected in (Table IV-1).


Page 155

Table IV-1 – Determination of polar and dispersive components of the surface energy on pristine and on exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders) Montmorillonite Θwater (°) ΘCH2I2 (°) γ polar

(mN.m-1) γ dispersive (mN.m-1)

γ total (mN.m-1)

MMT-Na+ 22.9 ±0.9 33.6 ±0.8 30 43 73 MMT-P I- 88.9 ±0.1 49.4 ±0.6 2 35 37

MMT-P Br- 75.8 ±0.2 45.1 ±0.7 6 37 43 MMT-P PF6

- 92.2 ±0.1 48.6 ±0.7 1 35 36 MMT-C18C18Im 92.8 ±0.1 55.5 ±0.6 1 31 32 MMT-C18C22Im 80.1 ±0.2 50.8 ±0.6 5 34 39 MMT-C22C22Im 97.8 ±0.1 53.8 ±0.6 1 32 33

The both ionic liquids based on phosphonium and imidazolium salts make the

montmorillonite more hydrophobic with a surface energy similar to a polyolefin one [19]. The

organic chain length has indeed a strong effect on the surface energy. The steric hindrance of

imidazolium and phosphonium ionic liquids due to the presence of long alkyl chains causes

an important decrease of the energy surface of layered silicates. The coverage of the

hydrophilic surface by ionic liquids explains the extremely low values of polar components.

Longer the alkyl chain is, more hydrophobic the montmorillonite is. The most hydrophobic

surface is achieved with the imidazolium cation functionalized with both alkyl chains in C18

and C22. A symmetric configuration seems to be more suitable than an asymmetric one that

leads to a less hydrophobic surface. (surface energy of 39 mJ.m-² instead of 32 mJ.m-²). The

anion has also a significant influence on the surface energy. For the same cation,

phosphonium, the bromide anion leads to the highest surface energy whereas the polar

component is much lower with the iodide and fluorinated anions. In conclusion, the use of

ionic liquids as intercalating agents in the modified montmorillonites should generate a good

affinity with the polyethylene matrix and the surface properties of montmorillonite can be

tuned by the relevant choice of ionic liquid.

IV.1.3.4 PE/modified-montmorillonites nanocomposites

IV.1.3.4.1 Morphology of the nanocomposites

The distribution and dispersion of lamellar silicates in polyethylene matrix were

analyzed by transmission electron microscopy on the nanocomposites processed with 1 wt%

of imidazolium and phosphonium -treated montmorillonites. Micrographs are reported in

Figure IV-8 and Figure IV-9.


Page 156

• Effect of length alkyl chains

200 nm 200 nm200 nm

Figure IV-8 – TEM micrographs of PE-MMT nanocomposites: (a) PE/MMT-C18C18Im, (b) PE/MMT-C18C22Im, (c) MMT-C22C22Im

The imidazolium-treated montmorillonites are usually well distributed in the form of

isolated layers, despite the presence of small tactoïds, i.e. stacked layers. However,

differences exist depending on the alkyl chain length attached to imidazole ring. In fact,

MMT-C22C22Im is the modified montmorillonite which displays the better dispersion in the

high density polyethylene matrix. This result can be explained by the long alkyl chains (C22)

which has encouraged the hydrophobicity of the montmorillonite promising a better

compatibility between polymer matrix and clay surface.

• Effect of the chemical nature of anion

200 nm 200 nm200 nm

Figure IV-9 – TEM micrographs of PE-MMT nanocomposites: (a) PE/MMT-P I-, (b) PE/MMT-P Br-, (c) PE/MMT-P PF6

-

Previously, it was shown that the chemical nature of anion associated to the

phosphonium cation affected the surface energy of phosphonium-treated montmorillonites.


Page 157

The bromide anion led to a less hydrophobic montmorillonite. The final morphology of

nanocomposites processed with phosphonium modified montmorillonites depends on the

chemical nature of the anion as well. In fact, the presence of bromide anion induces a poorer

distribution of modified-montmorillonite and leads to the presence of microscale aggregates

in the high density polyethylene. The steric hindrance related to anions must play a role in the

coverage of hydroxyl groups present on the clay surface which results in a higher polar

component in the case of the halide anion (Br-).

IV.1.3.4.2 Thermal stability of the nanocomposites

Thermogravimetric analysis traces and their first derivative curve (DTG) of

imidazolium-treated montmorillonites nanocomposites are given in Figure IV-10.

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(heating rate : 20 K.min-1, under nitrogen atmosphere).

A very low amount (only 1 wt%) of modified-montmorillonites with imidazolium

ionic liquids is enough to increase the thermal stability of the polyethylene matrix. Indeed, the

thermal decomposition is delayed of 10°C by the addition of imidazolium ionic liquid for the

MMT-C18C18Im and MMT-C18C22Im. A more significant improvement (+ 15°C) is observed

with imidazolium ionic liquid C22C22Im. These results are promising and can be enhanced by

using a larger amount of modified clay for designing PE-nanoclay nanocomposites.


Page 158

The thermal degradation of nanocomposites processed with phosphonium-treated

montmorillonites is presented in Figure IV-11.

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Figure IV-11 – Evolution of weight loss as a function of temperature (TGA, DTG) of the (●) PE/MMT-P PF6

-, (■) PE/ MMT-P Br-, (○) MMT-P I-, (♦) Neat PE (heating rate : 20 K.min-1, under nitrogen atmosphere).

The addition of only 1 weight percent of phosphonium-treated montmorillonite also

induces a 10°C increase of the thermal stability of polyethylene. On the other side, the

chemical nature of the anion has no significant effect on the thermal properties of the

nanocomposite unlike to the effects observed on the modified montmorillonite, in particular

with hexafluorophosphate anion (PF6-). In fact, the thermal degradation of polyethylene filled

with MMT-P Br-, MMT-P I- or MMT-P PF6- is very similar: An increase of 10°C is observed

whatever the anion used.

IV.1.3.4.3 Mechanical properties of the nanocomposites

The uniaxial tensile properties were determined to evaluate the impact of this very

small modified montmorillonite amount on the mechanical behaviour of high density

polyethylene. The moduli and the fracture properties are reported in Table IV-2.


Page 159

Table IV-2 – Tensile properties of the ionic liquid-modified montmorillonites/high density polyethylene nanocomposites at room temperature (10 mm.min-1)


Strain at break (%)

Stress at break (MPa)

PE 480 18 46 PE-MMT 460 17 41

PE/MMT-C18C18Im 600 17 48 PE/MMT-C18C22Im 510 18 45 PE/MMT-C22C22Im 660 18 50

PE/MMT-P I- 620 16 50 PE/MMT-P Br- 476 15 42 PE/MMT-P PF6

- 548 17 48

The addition of only 1 wt% ionic liquid modified montmorillonite in the high density

polyethylene leads to an increase of the modulus without reducing the strain at break. This

result is very exciting because usually the introduction of fillers does not permit to improve

the compromise stiffness/fracture deformation but rather to reduce it. However, the

modification of layered silicates with ionic liquids such as imidazolium or phosphonium

functionalized with long alkyl chains is a way to enhance the compromise between stiffness

and ability to deformation.

With the imidazolium cation, the symmetric configuration with the both alkyl chains

(C18-C18 or C22-C22) is the most suitable one since a modulus increase of + 25% and + 40%

with the imidazolium treated-montmorillonites MMT-C18C18Im and MMT-C22C22Im

respectively is obtained respect to the neat matrix. In the case of MMT-C18C22Im, the slight

increase of modulus can be explained by the combination of bromide (Br-) and iodide (I-)

anions, which come respectively from (C22) and (C18) alkyl chains. The mechanical results are

in good correlation with the surface properties that demonstrate a higher hydrophobic

character with alkyl chains having the same chain length as well with the state of dispersion

of nanoplatelets within the polymer matrix.

In the case of PE/phosphonium treated-montmorillonite nanocomposites, the increase

of modulus is between 10% and 30% for only one percent of introduced fillers into the

polyethylene matrix. The lowest increase of modulus (4%) is obviously obtained with the

bromide anion (Br-) that led to the lowest hydrophobicity of the filler. On the contrary, a

better compatibility between polyethylene and MMT-P I- seems to be checked like considered

from the surface properties since a modulus increase of 35% is measured. The use of

hexafluorophosphate (PF6-) leads to an intermediate modulus with an increase of 20%.


Page 160

IV.1.4 Conclusions

In this work, new surfactants based on alkyltriphenyl phosphonium and dialkyl

imidazolium salts have been used to prepare highly thermally stable organophilic

montmorillonites. It was demonstrated that the use of ionic liquid modified-montmorillonites

induces an increase in the thermal stability (+15°C) and the mechanical properties of

nanocomposites. The stiffness was found to be improved without reducing the ultimate

properties such as strain and stress at break. Moreover, the ionic liquids can be tuned by a lot

of possible combinations between cation and anion that play a very important role on the

physical properties of the polymer.


Page 161

IV.2 Ionic Liquids as Interfacial Agents in PVDF-based nanocomposites

Different ionic liquids based on alkyltriphenylphosphonium and imidazolium-

functionalized either with two alkyl chains or with a fluorinated ligand have been synthesized

and used as interfacial agents for the layered silicates. The effect of the chemical nature of the

organic cation on the morphology and the physical properties of the polyvinylidene fluoride

(PVDF) nanocomposites has been studied. The influence of ionic liquids on polymorphic

crystalline forms, i.e. α and β phases of the polymer matrix was discussed.

IV.2.1 Introduction

For many years, research in the field of polymer/clay nanocomposites has been widely

extended [20-21]. The nanoscale of nanoplatelets leads to an increase of surface to volume

ratio generating a very huge amount of interface. The success of nanofillers dispersion into

polymer matrix implies the tailoring of interactions between fillers and matrix. The objective

is to process organic-inorganic nanomaterials for which all the polymer matrix chains are in a

confined configuration with the consequences on chain mobility and on final properties such

as thermal [1], mechanical [2] and barrier properties [5].

Polyvinylidene fluoride (PVDF) is a polymer commonly used in electronic and

chemical industries due to its excellent chemical, thermal stabilities and mechanical

properties. From a morphology view, it has several crystalline forms, i.e. α, β, γ, and

δ [22]. While the α form is the most commonly generated one; the β form is expected due to

the resulting dielectrical properties and piezoelectric applications of PVDF materials. Several

ways could be followed to obtain the β form; i) the application of a strain, ii) the use of an

electric field [23], iii) the growth from a PVDF solution, and iv) the introduction of inorganic

fillers [24-25]. For this last route, the influence of the modified clays on the mechanical

properties has been widely studied in the literature. It has been shown that the use of

Cloisite® 6A and 20A [26-27], i.e. clays modified by alkylammonium ions, promote the

formation of β phase and a significant increase in the storage modulus is obtained with fillers

contents from 1.5 to 7 wt%. Shah et al. showed that the addition of Cloisite® 30B (5 wt%)

increases Young’s modulus of 40% and strain at break of 250% [28]. However, the great

disadvantage of these ammonium salts is their poor thermal stability as degradation starts


Page 162

from 180°C [8, 13, 14], which is the temperature limit for processing clay-based PVDF

nanocomposites from melt intercalation. Recently, a new alternative to conventional

ammonium emerges gradually with the ionic liquids (ILs). In particular, the imidazolium and

phosphonium salts known to possess an excellent thermal stability [11]. Patro et al. [29] used

phosphonium and pyridinium ionic liquids as intercalant agents and have achieved an increase

of 50 and 150% for Young’s modulus and the strain at break, respectively.

In this work, the interfacial interactions between the fluorinated matrix and the

lamellar silicates, i.e. montmorillonite, were tailored from a relevant selection of own-

synthesized ionic liquids. The selected ILs have either an alkyl ligand (imidazolium and

phosphonium) or a fluorinated one (imidazolium) to match the PVDF matrix nature. Then, the

key role of interfacial interactions will be argued in terms of morphology, crystallinity and

mechanical properties of nanocomposites based on layered silicates modified by these new

surfactants called ILs.

IV.2.2 Experimental

IV.2.2.1 Materials

A sodic montmorillonite, denoted Nanofil 757 (MMT), i.e. an aluminosilicate with

intercalated sodium ions, was chosen as pristine clay. This one was provided by Süd Chemie

Co. The Nanofil 757 has a cationic exchange capacity of 95 meq/100g and could be described

by the following formula Na0,65[Al,Fe]4Si8O20(OH)4. All chemicals necessary for the

synthesis of ionic liquids, i.e. triphenylphosphine (95%), imidazole (99.5%), iodooctadecyl

(95%), and all the solvents (toluene, sodium methanoate, pentane, acetonitrile, THF) were

supplied from Aldrich and used as received. 1-iodo-1H,1H,2H,2H-perfluorododecane (97%)

was purchased at ABCR Co. The poly(vinylidene fluoride) used in this study, PVDF, was

supplied from Arkema under the trade name Kynar 740 (Melt Flow: 1.1 g/10 min at 230°C).

IV.2.2.2 Synthesis of ionic liquids

IV.2.2.2.1 Synthesis of ILs with long alkyl chains

The procedures detailed for synthesizing the imidazolium and phosphonium salts were

identical to the method used by Livi et al. [11]. The synthesis of salts was checked by 13C

NMR spectroscopy collected on a Bruker AC 250 (250 MHz) spectrometer. The assignment

of 1H NMR resonance peaks is reported below.


Page 163

Imidazolium iodide 1H NMR (CDCl3): δ 0.75-0.90 (m, 6H, 2CH3), 1.15-1.30 (m, 64H,

32 CH2), 1.80-1.90 (m, 2H, NCH2CH2), 4.30 (t, 2H, CH2N=), 7.45 (m, 1H, H arom), 7.65 (m,

1H, H arom), 9.15 (s (b), 1H, H arom).

Phosphonium iodide 1H NMR (CDCl3): δ 0.8-0.90 (m, 3H, CH3); 1.10-1.35 (m, 28,

CH2Me); 1.50-1.70 (m, 4H, PCH2(CH2)2); 3.50-3.70 (m, 2H, PCH2); 7.70-7.90 (m, 15H, H

arom).

IV.2.2.2.2 Synthesis of imidazolium functionalized with perfluorinated chain

1 equiv. of methylimidazole (3g) and 1 equiv. of 1-Iodo-1H,1H,2H,2H-

perfluorododecane (7.3 g) were placed in a 100 mL flask under a positive nitrogen pressure,

and magnetic stirring for 72 h at 120°C in toluene (40mL). A yellow precipitate was formed.

The reaction mixture was then filtered, washed repeatedly with pentane. Most of the solvent

was removed under vacuum. The synthesis of salts was confirmed by 1H NMR. 1H NMR (CDCN): δ 2.79 (CH2-CF2); 3.79 (CH3N-); 4.45 (CH2CH2N-); 7.35 (CH); 7.42

(CH); 8.60 (N-CH=N).

IV.2.2.3 Organic modification

The montmorillonite (2 g, 1.9 meq) was dispersed in 400 mL of deionised water. The

amount of surfactant added was about 2 CEC, based on the cation exchanged capacity (CEC =

95 meq/100 g) of the MMT used [12]. This dispersion was mixed and stirred vigorously at

80°C for 6 h, followed by filtration and continuous washing at 80°C with deionised water

until no iodide ions were detected using an aqueous silver nitrate (AgNO3) solution. The

solvent was removed by evaporation under vacuum. The modified montmorillonite was then

dried for 12 hours at a temperature below 80°C. The following references were used to denote

the different types of modified montmorillonites: MMT-Na+ for the pristine montmorillonite,

MMT-P for phosphonium montmorillonite as octadecyltriphenylphosphonium iodide was

used as interfacial agent. An imidazolium montmorillonite, denoted MMT-I, was obtained as

N-octadecyl-N’-octadecylimidazolium iodide was used as intercalation agent. MMT-IC12F

denotes the montmorillonite exchanged with imidazolium functionalized with the

perfluorinated chain. The chemical structure of synthesized phosphonium and imidazolium salts

are described in Table IV-3.


Page 164

Table IV-3 – Pristine and ionic liquid-modified montmorillonites (MMT) Trade name

References

Intercalant

Nanofil 757

MMT-Na+

MMT-I

MMT-P

PC18H37

I

MMT-IC12F

N NC18H37

I

C18H37

N N(CH2)2(CF2)9CF3

I

H3C

IV.2.3 Results and discussion

IV.2.3.1 Characterization of ILs exchanged montmorillonites

IV.2.3.1.1 Thermal stability of ionic liquid-modified montmorillonites

The thermal stability of organically modified montmorillonites was characterized by

Thermogravimetric Analysis (TGA). Figure IV-12 displays the evolution of the weight loss as a

function of temperature performed on the three montmorillonites exchanged either with

imidazolium and phosphonium cations functionalized with long alkyl chains or with

imidazolium cation functionalized with perfluorinated chain (C12). The degradation

temperatures of physically adsorbed and intercalated species are summarized in Table IV-4.


Page 165

Table IV-4 – Degradation temperatures of the physisorbed and intercalated species for the treated montmorillonites (determined from the maxima of the weight loss by temperature derivative)

Sample Physisorbed temperature

(°C)

Intercalated temperature

(°C) MMT-P 330 510 MMT-I 320 420/480

MMT-IC12F 280 460

As reported in a previous study [11], imidazolium-treated and phosphonium-modified

montmorillonites, MMT-I and MMT-P, have an excellent thermal stability, which can be

useful for processing nanocomposites at high temperatures. Indeed, the organic species,

physically adsorbed on the clay surface are decomposed at around 320-330°C, whereas the

thermal degradation of the species ionically exchanged between the clay layers is delayed in a

range between 420 and 510°C. In the case of MMT-IC12F, the use of a perfluorinated chain

associated with the methylimidazole ring does not induce a better thermal stability. On the

opposite, a decrease of the thermal stability is observed with two degradation peaks at about

280°C for the physisorbed species and 460°C for the intercalated species. This phenomenon

could be attributed to the volatilization of short fluorinated chains.

��

�

�

��

��

��

��

��

��

��

40

60

80

100

120

We

igh

t (%

)

0 200 400 600 800

Temperature (°C)

� MMT-P� MMT-I� MMT-IC12F


Figure IV-12 – Weight loss as a function of temperature of the (■) MMT-I, (●) MMT-P, (○) MMT-IC12F (heating rate: 20 K.min-1, nitrogen atmosphere).


Page 166

IV.2.3.1.2 Structural analysis of ionic liquid-modified montmorillonites

To evaluate the organic chain organization in the galleries of the lamellar silicates, the

X-ray diffraction analysis was performed on MMT-P, MMT-I and MMT-IC12F and the

corresponding spectra are reported in Figure IV-13.

0 1 2 3 4 5 6 7 8 9 10

0

200

400

600

800

1000

1200

1400

1600

1.7 nm

4.2 nm

Inte

nsity (

u.a

)

2θ

MMT-P

MMT-I

MMT-IC12F

3.7 nm

Figure IV-13 – X-Ray diffraction spectra of ionic liquid-modified MMT:

(a)MMT-P; (b) MMT-I; (c) MMT-IC12F.

It is well known that before any treatment, the basal spacing, d001, measured on the

sodic montmorillonite is 1.2 nm [31]. After cationic exchange in water with the phosphonium

salts, the MMT-P displays a (001) diffraction peak at 2.1°2θ, corresponding to an interlayer

distance of 4.2 nm. This value could be explained by the swelling of layered silicates due to a

larger steric volume associated with the three benzyl rings and the alkyl chain. For the MMT-

I, the diffraction peak situated at 2.4°2θ is related to a distance of 3.7 nm, characteristic of a

full trans-trans conformation of the alkyl chain. On the other side, the montmorillonite

functionalized with one perfluorinated chain, MMT-IC12F shows an interlayer distance of 1.7

nm in agreement with its shorter alkyl chain length.


Page 167

IV.2.3.1.3 Surface energy of ionic liquid-treated montmorillonites


pressed clay powder are collected in Table IV-5.

Table IV-5 – Polar and dispersive components of the surface energy on pristine and exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders)

Montmorillonite Θwater (°) ΘCH2I2 (°) γ polar (mN.m-1)

γ dispersive (mN.m-1)

γ total (mN.m-1)

MMT-Na+ 22.9 ±0.9 33.6 ±0.8 30 43 73 MMT-P 88.9 ±0.1 49.4 ±0.6 2 35 37 MMT-I 92.8 ±0.1 55.5 ±0.6 1 31 32

MMT-IC12F 83.6 ±0.2 99.4 ±0.2 14 9 23

After organic treatment in water by the imidazolium and phosphonium ionic liquids

with long alkyl chains, a significant decrease of the surface energy of organically modified

clays is obtained. The modified montmorillonites became more hydrophobic with a surface

energy characteristic of a polyolefin surface. The use of fluorinated chain, even shorter

combined with the imidazolium cation makes the montmorillonite more hydrophobic. In fact,

the surface energy of MMT-IC12F is two times lower than MMT-I and MMT-P ones close to

polytetrafluoroethylene one.


Page 168

IV.2.3.2 Effect of interfacial interactions on the material physical properties

The combination of the X-ray diffraction with transmission electron microscopy is

required to determine the material structuration which is tuned from the interfacial

interactions, i.e. the IL chemical nature.

IV.2.3.2.1 On the morphology of the PVDF nanocomposites

• Structural analysis of nanocomposites

WAXS analysis was used to evidence of the intercalation of PVDF chains between

platelets. The diffractograms performed on the PVDF/MMT-P, PVDF/MMT-I, and

PVDF/MMT-IC12F nanocomposites are reported in Figure IV-14.

0 1 2 3 4 5 6 7 8 9 10

0

100

200

300

400

500

600

700

800

900

1000

3.8 nm

Inte

nsity (

u.a

)

2θ

PVDF/MMT-P

PVDF/MMT-I

PVDF/MMT-IC12F

3.8 nm

Figure IV-14 – X-Ray diffraction spectra of nanocomposites: (a)PVDF/MMT-P; (b) PVDF/MMT-I; (c) PVDF/MMT-IC12F.

The X-ray spectra are very different as a function of the chemical nature of IL. Only

the fluorinated IL used for the montmorillonite modification displays a flat pattern which can

be interpreted as a well dispersed morphology, i.e. known as exfoliated morphology. On the

opposite, both ILs based on imidazolium or phosphonium cations with octadecyl chains do

not show additional swelling of nanoplatelets compared to the IL-modified nanoclays. An

intercalated structure seems to be kept.


Page 169

• Transmission electronic microscopy analysis (TEM) of nanocomposites

The consequence of the chemical nature of the interfacial agents on the resulting

morphology issued from the clay layers distribution has been studied by transmission

electronic microscopy. TEM micrographs of the nanocomposites prepared by melt

intercalation are presented in Figure IV-15.

1 µm 1 µm 1 µm Figure IV-15 – TEM micrographs of (a) PVDF/MMT-I, (b) PVDF/MMT-P, (c) PVDF/MMT-IC12F

TEM images performed on the PVDF/MMT-P and PVDF/MMT-I nanocomposites

reveal that nanoclays remain stacked as tactoids based on few layers homogeneously

distributed within the PVDF matrix. Nevertheless, the swelling of primary particles, i.e.

tactoïds, by the PVDF chains remains, as shown by X-ray diffraction analysis, low in the case

of phosphonium and even non-existing for imidazolium ions. In these two cases, no swelling

with PVDF chains of the tactoïds occurred. ILs, as imidazolium or phosphonium ions

functionalized with long alkyl chains, lead to the nanocomposites having an intercalated

structure.

The modification of montmorillonite with imidazolium cations functionalized with a

fluorinated chain seems to be the most suitable to induce a fine dispersion of clay layers into

the fluorinated polymer matrix. This excellent dispersion is explained by the hydrophobicity

of the modified montmorillonite higher than MMT-I and MMT-P and by a higher affinity of

interfacial agent towards matrix chains. In fact, one can expect a better compatibility of the

fluorinated ligand with the PVDF chains, which is the driving force for the later ones to

diffuse into the galleries in the molten state.

(a) (b) (c)


Page 170

IV.2.3.2.2 Crystallinity of PVDF based nanocomposites

• Effect of the ionic liquids on the crystallinity

Owing to the numerous crystalline phases of PVDF, an effect of interfacial agent on

the crystallinity of the fluorinated matrix could be expected. As a consequence, in a first step,

ionic liquids were added to the PVDF matrix (1wt%) using the same protocol as for the

nanocomposites. The chemical nature of the cation associated to ionic liquid, imidazolium or

phosphonium plays a significant role on the crystalline microstructure of PVDF as evidenced

by WAXD analysis (Figure IV-16).

10 15 20 25 30

0

1000

2000

3000

4000

5000

6000

2θ

(a) PVDF

0

200

400

600

800

1000

1200

1400

1600

Inte

nsit

y (

u.a

) (b) PVDF-P

0

500

1000

1500

2000

2500

(c) PVDF-I

Figure IV-16 – WAXD patterns of (a) PVDF and PVDF/ionic liquid blends (b) PVDF-P (c) PVDF-I


Page 171

According to the literature, for the crystallographic structure of the PVDF [29, 32], the

main diffraction peaks corresponding to α-phase are observed at 17.8, 18.4, 19.9, and 25.8°

2θ are attributed to the following reflections (100), (020), (110), and (201), respectively. The

addition of a small amount (1wt%) of imidazolium salt does not modify the X-ray spectrum of

the neat PVDF. Indeed, the diffraction peaks are related to the formation of the α phase. On

the other hand, after addition of 1wt% phosphonium salt in the PVDF matrix, a additional

peak at 20.7° 2θ is detected. This peak corresponding at (110) plane is significant of the

formation of β crystalline form.

The melting temperature (Tm) and the crystallinity ratio (%X) of the PVDF-ionic

liquid blends are reported in Table IV-6.

Table IV-6 – Differential scanning calorimetry analyses of PVDF and PVDF-ionic liquid blends, PVDF-I and PVDF-P (1wt%)

Material ∆Hm (J/g) ∆Hc (J/g) Τm (°C) Τc (°C) Xc (%) PVDF 38 42 165 140 37

PVDF-P 33 36 172 141 32 PVDF -I 38 41 165 141 37

The chemical nature of interfacial agents and the interactions between ionic liquid and

matrix predominate. As previously demonstrated by X-Ray diffraction, as imidazolium ionic

liquid is added to the PVDF matrix, the α form is kept with a melting temperature of 165°C

and the melting enthalpy is similar to the PVDF matrix one. The phosphonium ionic liquid

generates an increase in melting temperature of 7°C and a significant decrease in the

crystallinity ratio of the polymer matrix in agreement with literature reported on modified

clays/PVDF nanocomposites [29]. The higher melting point associated to the β form

compared to α phase one can be due to a higher thermal stability [33] or to a higher perfection

of crystals [34]. In any case, the phosphonium ionic liquid modifies the crystalline form of the

polyvinylidene fluoride acting as a βgene agent as described in the literature [35-36].


Page 172

Generally, the β form crystal is generated by applying a mechanical strain or an

electrical field, by adding of inorganic fillers, or by using of lithium salts with amounts above

2.5% in membrane applications. Nevertheless, this is the first time that a phosphonium ionic

liquid is used to generate this β form.

• Effect of ionic liquid exchanged montmorillonites on the crystallinity

The effect of the chemical nature of the organic cation of the ionic liquid combined to

the inorganic filler on the crystalline structure of PVDF was also studied. The X-Ray

diffraction analyses of nanocomposites filled with IL-modified montmorillonites are given in

Figure IV-17.

10 15 20 25 30

0100020003000400050006000

2θ

PVDF

0500

10001500200025003000

PVDF/MMT

0

500

1000

1500

2000

Inte

nsit

y (

u.a

)

PVDF/MMT-P

0500

10001500200025003000

PVDF/MMT-I

0

500

1000

1500

2000 PVDF/MMT-IC12F

Figure IV-17 – WAXD patterns of PVDF/IL-modified nanoclay (1wt%) nanocomposites

(a) PVDF/MMT-IC12F, (b) PVDF/MMT-I, (c) PVDF/MMT-P, (d) PVDF/MMT, (e) PVDF


Page 173

The addition of untreated montmorillonite does not change the neat matrix WAXS

pattern, i.e. the presence of pristine MMT does not disturb the crystalline microstructure,

based on α phase. According to the results reported previously for PVDF/LI blends, the

WAXS pattern of PVDF/MMT-I is similar to the PVDF one, i.e. no change in crystalline

form is induced by MMT-I. However, as the imidazolium cation is functionalized with a

fluorinated chain, a change in the crystalline form of PVDF is observed, i.e. the use of a

fluorinated imidazolium ionic liquid promotes the formation of the β phase. On the X-ray

diffraction pattern of phosphonium treated montmorillonite/PVDF nanocomposites, the

diffraction peak at 20.7° attributed to the β phase is also present. As modified clays are not

washed, physically adsorbed ionic liquid remains on the external montmorillonite platelet

surface, and could induce the formation of β-form crystal.

To confirm the effects related to phosphonium and imidazolium treated

montmorillonites on the formation of α and β phases, X-ray diffraction analysis and

differential scanning calorimetry (DSC) analysis on the nanocomposites were carried out

(Table IV-7).

Table IV-7 – Differential scanning calorimetry analysis for PVDF and nanocomposites denoted PVDF/MMT-I, PVDF/MMT-P, PVDF/MMT-IC12F (1wt%)

Material ∆Hm (J/g) ∆Hc (J/g) Τm (°C) Τc (°C) Xc (%) PVDF 38 42 165 140 37

PVDF/MMT-P 35 36 172 150 34 PVDF/MMT-I 39 41 166 148 38

PVDF/MMT-IC12F 32 33 174 149 31

These DSC analyses are in agreement with X-ray diffraction results since the use of

ionic liquids based on phosphonium or imidazolium cations functionalized by a fluorinated

chain as interfacial agents leads to lower crystallinity ratios and higher melting temperatures

associated to the formation of β phase. This result could provide a new perspective on the use

of ionic liquids in the field of membranes as β crystalline form contribute to better dielectrical

and thermal properties. Moreover, additional improvement could be achieved due to the

presence of clay nanoplatelets.


Page 174

IV.2.3.2.3 Mechanical properties of PVDF based nanocomposites

In addition to the characterization of the morphology and resulting crystallinity of

nanomaterials based on IL-modified silicate nanolayers, the mechanical properties were

analyzed by performing uniaxial tensile tests described in Table IV-8.

Table IV-8 – Tensile properties of the ionic liquid/ poly(vinylidene fluoride) blends and ionic liquid modified montmorillonites- poly(vinylidene fluoride) nanocomposites at room temperature (10 mm.min-1)

Sample Young’s modulus (MPa)

Strain at break (%)

PVDF 940 36 PVDF-P PVDF-I

760 700

30 37

PVDF-MMT 900 30 PVDF/MMT-P 750 32 PVDF/MMT-I 800

80

PVDF/MMT-I C12F 800 250

The values of the moduli and strain at break are not governed by the nature of the

crystalline forms of PVDF, i.e. α or β. In fact, the imidazolium cation has the same

plasticizing effect on the mechanical properties as it is functionalized with alkyl chains that

promotes the α form or with a fluorinated chain that promotes the β form. The phosphonium

cation that generates the β form has the same plasticizing effect and does not improve the

mechanical behavior of PVDF. The mechanical properties are mainly dependent on the

material morphology. The stiffness is less reduced and the failure properties are slightly

enhanced for the nanocomposite showing an intercalated morphology obtained with the ionic

liquids based on long alkyl chains imidazolium cation. The excellent compromise obtained

between stiffness and strength at break is achieved on the nanocomposites containing the

montmorillonite modified with perfluoroalkylimidazolium, i.e MMT-IC12F. As a

consequence, the exfoliated structure is the right morphology to get the best mechanical

behaviour.


Page 175

IV.2.4 Conclusions

In this work, phosphonium and imidazolium ionic liquids were used as interfacial

agents to enhance the physico-chemical interactions within the matrix and to help the lamellar

silicates intercalation in the polymer matrix. The compatibility brought by the cation is the

key parameter that governs the interactions and generates an exfoliated morphology

responsible for the excellent compromise between stiffness and failure behaviour. Thus, a

better dispersion of clay nanolayers and a plasticizing effect are observed with the use of

fluorinated imidazolium ionic liquid due to the higher compatibility of the fluorinated ligand

with the PVDF matrix. Moreover, the chemical nature of organic cation has a strong effect on

the crystalline form of PVDF. In fact, a small amount (1 wt%) of phosphonium and

fluorinated chain-functionalized imidazolium salts promotes the formation of the β phase and

opens new applications in the field of membranes for fuel cells.


Page 176

Conclusions of chapter IV In this chapter, many imidazolium and phosphonium ionic liquids have been

synthesized and used as modifiers agents of layered silicates.

In the first part, we have demonstrated that the use of montmorillonites modified with

imidazolim and phosphonium IL enables improved thermal stability and mechanical

properties with a good compromise between rigidity and failure strain of nanocomposites

PE/MMT.

We have also investigated the compatibility formed by the nature of the organic cation

is the main parameter controlling the physico-chemical interactions within the matrix and

contributes to a better distribution of clays in the polymer matrix. Thus, we have observed that

the addition of only 1 wt% of MMT treated with imidazolium functionalized by

perfluorinated chain leads to exfoliation of clay layers in the polymer resulting in a

plastification of the matrix. In addition, we have also demonstrated that the interfacial

chemical agents, especially fluorinated imidazolium and phosphonium ionic liquids favor the

formation of β form and open new perspectives in the field of membranes for fuell cells.


Page 177

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(2009); 47:2590–2599. [25] D. Shah, P. Maiti, D.D. Jiang, C.A. Batt , E.P. Giannelis, et al. Adv Mater (2005); 17(5):525–528. [26] L. Priya, J. P. Jog, Journal of Applied Polymer Science (2003); 89:2036–2040. [27] L. Priya, J. P. Jog, J. Polym Sci Part B Polym Phys (2003); 41:31–38. [28] D. Shah, P. Maiti, E. Gunn, D. Schmidt, D.D. Jiang, C.A. Batt, et al. Adv Mater (2004); 16(14):1173–1177. [29] T. U. Patro, M. V. Mhalgi, D. V. Khakhar and A. Misra, Polymer (2008); 49:3486–3499. [30] Y. Rosenberg, A. Siegmann, M. Narkis, S. Shkolnik, Journal of Applied Polymer Science (1991); 43:535–541. [31] H. He, J. Duchet, J. Galy, J.F. Gerard, J.colloid Interf Sci. (2006); 295:202. [32] D.J. Lin, C.L. Chang, F.M. Huang, L.P. Cheng, Polymer (2003); 44:413–422. [33] J. Buckley, P. Cebe, D. Cherdack, J. Crawford, B.S. Ince, M. Jenkins, et al Polymer (2003); 47:2411–2422. [34] S. Hellinckx, JC. Bauwens Colloid Polym Sci (1995); 273:219–26 [35] B. Wunderlich, Macromolecular physics , 3, (1980) Academic press. [36] W.M. Prest, D.J. Luca, J Appl Phys (1975); 46:4136


Page 178

Conclusion générale

Page 179

CONCLUSION GENERALE L'objectif premier de ce travail a été de valoriser et de mettre en avant les effets

bénéfiques et les différentes possibilités proposées par les liquides ioniques dans le domaine

des polymères que ce soit en tant qu'agents renforçants, plastifiants ou encore agents

interfaciaux pour les silicates lamellaires.

Dans un premier temps, nous avons mis en évidence pour la première fois la

structuration des liquides ioniques dans une matrice polymère. Nous avons ainsi démontré que

la nature chimique du cation organique: pyridinium, imidazolium ou phosphonium ainsi que

l'effet de l'anion jouent un rôle essentiel sur les différentes structurations obtenues. En effet,

nous avons observé des morphologies différentes allant de la formation d'agrégats de clusters

ioniques en ce qui concerne l'utilisation du liquide ionique pyridinium (C18Py) à une

excellente dispersion à l'échelle du nanomètre dans le cas du phosphonium (C18P) en passant

par une morphologie co-continue pour l'imidazolium (C18C18Im). Les relations

morphologies/propriétés physiques et mécaniques ont également été établies. Ainsi, nous

avons constaté que le liquide ionique phosphonium le mieux dispersé dans la matrice fluorée

mène à une nette amélioration des propriétés mécaniques du matériau avec une augmentation

du module et de la déformation à la rupture de +160 et +190% respectivement. Les analyses

SAXS et MET ont contribué à une meilleure connaissance de la structuration du matériau

sous sollicitation.

Dans un second temps, l'utilisation des liquides ioniques comme agents intercalants en

remplacement des ammoniums conventionnels a également été discuté. Nous avons ainsi

démontré une meilleure stabilité thermique des argiles modifiées par les liquides ioniques ce

qui permet d'élargir le champ d'utilisation des silicates lamellaires dans le domaine des

nanocomposites thermoplastiques/argiles nécessitant des températures de mise en oeuvre plus

élevées. Nous avons aussi développé un procédé propre de modification des surfaces des

silicates lamellaires basé sur la combinaison CO2 supercritique-eau-liquide ionique avec pour

résultats une amélioration de la stabilité thermique des argiles organiquement modifiées par

les liquides ioniques.


Page 180

Dans une dernière partie, nous avons montré les effets de ces charges organiquement

modifiées sur les propriétés thermiques et mécaniques de nanocomposites préparés par

intercalation à l'état fondu. Ainsi, nous avons observé que l'influence de la longueur des

chaînes et de l'anion joue un rôle crucial sur la stabilité thermique intrinsèque des liquides

ioniques ainsi que sur les propriétés thermiques et mécaniques des polymères, notamment

dans le cas d'une polyoléfine (PEhd). Ensuite dans une matrice fluorée, le polyfluorure de

vinylidène (PVDF), nous avons démontré que l'utilisation d'un imidazolium fonctionalisé par

une chaîne perfluorée a un effet similaire à celui du cation phosphonium sur la structure

polymorphe du PVDF c'est à dire que leur utilisation en très faible quantité engendre la

formation de la phase β, favorable aux propriétés diélectriques ce qui offre de nouvelles

perspectives dans le domaine de l'énergie et en particulier dans celui des membranes de piles

à combustible. Une exfoliation des feuillets d'argiles dans la matrice polymère ainsi qu'une

augmentation de la déformation à la rupture est également obtenue dans le cas de la

montmorillonite modifiée par l'imidazolium fluoré.

Néanmoins, le coût et l'accessibilité aux liquides ioniques désirés limitent

considérablement leur utilisation dans les polymères. C'est pour ces raisons qu'il est

nécessaire d'intensifier les recherches sur les liquides ioniques afin d'apporter de la

compréhension et de prouver que les avantages des liquides ioniques sont beaucoup plus

importants que leurs inconvénients. Ce travail n'est qu'un aperçu du vrai potentiel des liquides

ioniques en science des polymères. En effet, les différentes combinaisons cations/anions ainsi

que les différentes fonctionnalisations possibles nous laissent penser qu'il est concevable en

théorie de synthétiser une infinité de liquides ioniques à propriétés spécifiques en fonction de

la matrice sélectionnée.

FOLIO ADMINISTRATIF

THESE SOUTENUE DEVANT L'INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE

LYON

NOM : LIVI DATE de SOUTENANCE : … … 2010 Prénoms : Sébastien TITRE :

IONIC LIQUIDS : MULTIFUNCTIONAL AGENTS OF THE POLYMER MATRICES NATURE : Doctorat Numéro d'ordre : 2010-ISAL- Ecole doctorale : Matériaux de Lyon Spécialité : Matériaux Polymères et Composites Cote B.I.U. - Lyon : T 50/210/19 / et bis CLASSE : RESUME : Une excellente stabilité thermique, une faible pression de vapeur saturante, une ininflammabilité, une bonne conductivité ionique ainsi que les différentes combinaisons cations/anions possibles font des liquides ioniques l'objet d'un engouement grandissant de la Recherche. De part ces avantages, les LI se présentent comme une nouvelle voie dans le domaine des polymères, et en particulier dans le milieu des nanocomposites où leur utilisation est essentiellement limitée à la fonction de surfactant des silicates lamellaires. Néanmoins, avant de pouvoir prétendre à un statut d'alternative, il est nécessaire de mettre en évidence les effets bénéfiques de leur utilisation sur les propriétés finales des matériaux polymères. Dans un premier temps, l’objectif de ce travail a été de synthétiser des liquides ioniques différents par la nature de leur cation et anion mais tous porteurs de longues chaînes alkyles afin de permettre une meilleure compatibilité avec la matrice. Ensuite, les excellentes propriétés intrinsèques des liquides ioniques ont motivé leur utilisation comme agents structurants dans une dispersion aqueuse fluorée. Ainsi, leur rôle d’agents ioniques sur la morphologie, les propriétés physiques, thermiques et mécaniques a été étudié. Dans une seconde partie, les liquides ioniques ont été utilisés comme agents intercalants des silicates lamellaires puis confrontés aux surfactants conventionnels dans le but de préparer des argiles thermiquement stables pour la préparation de nanocomposites thermoplastiques/argiles. Dans une dernière partie, une faible quantité de ces argiles organiquement modifiées ont été introduites par intercalation à l'état fondu dans deux matrices différentes afin de mettre en évidence les effets de ces nouveaux agents interfaciaux sur les propriétés finales du matériau. MOTS-CLES : Liquides ioniques ; Nanocomposites ; Silicates lamellaires ; Agents structurants ; CO2 supercritique Laboratoire (s) de recherche : Institut des Matériaux Polymères / Laboratoire des Matériaux Macromoléculaires UMR 5223 INSA de Lyon Directeurs de thèse: Jannick DUCHET-RUMEAU – Jean- François GERARD Président de jury : … Composition du jury : DUCHET-RUMEAU Jannick Professeur (INSA Lyon) – Directrice de thèse GALY Jocelyne DR CNRS (INSA Lyon) – Examinateur GANTILLON Barbara Dr (Société TEFAL) – Examinateur GERARD Jean-François Professeur (INSA Lyon) – Co-directeur de thèse PHAM Thi Nhàn Maître de Conférences (Université de Caen) – Examinateur PLUMMER John Christopher Professeur (Ecole Polytechnique Fédérale de Lausanne) – Rapporteur SEGUELA Roland DR CNRS (Université de Lille 1) – Rapporteur

Documents

Ionic liquids: multifunctional agents of the polymer …theses.insa-lyon.fr/publication/2010ISAL0101/these.pdfIonic Liquids: Multifunctional agents of the polymer matrices Abstract