Materials Science and Engineering A 370 (2004) 273–277

Relaxation processes in a glassy polymer containing methanol molecules

S. Etiennea,b,∗, S. Testua,b, L. Davidc, C. Ménissezc, E. Duvald

a LPM, UMR CNRS 7556, Ecole des Mines, Parc de Saurupt, 54042 Nancy Cedex, Franceb Ecole Européene d’Ingénieurs en Génie des Matériaux, 6 Rue Bastien Lepage, BP 630, 54010 Nancy Cedex, France

c GEMPPM, UMR CNRS 5510, INSA, 20 Avenue Albert Einstein, 69621 Villeurbanne Cedex, Franced LPCML, UMR CNRS 5620, Université Lyon I, 69622 Villeurbanne Cedex, France

Received 12 July 2002

Abstract

The interaction of small guest molecules with the molecular degrees of freedom of a glassy polymer matrix is investigated, i.e. (i) thedynamics of these guest molecules and (ii) their effect on the secondary ‘�’ and main ‘�’ relaxations of the host polymeric matrix. Thesystem considered here is the glassy poly(methyl metacrylate) modified by introduction and desorption of methanol. The dynamics of thesystem is observed by means of low-frequency mechanical and wide band dielectric spectroscopies. The presence of small molecules inthe PMMA glassy matrix induces several effects, namely (i) a strong relaxation peak develops at low temperature (near 120 K at 1 Hz andcalled�m in the following), (ii) the strength of the� relaxation is increased while the temperature of the maximum shifts towards the lowtemperatures, (iii) a sharp peak appears superimposed on the� peak, and finally (iv) the� relaxation shifts towards the low temperatures.Dielectric and mechanical spectroscopies results are in agreement and make it possible to capture the dynamical behavior in a wide frequencyrange (eight decades). The experimental results are explained on the basis of physical concepts recently introduced in the physics of glassymatter: cooperativity and nanoheterogeneity. In particular, the low-temperature relaxation process�m is attributed to cooperative motions inmethanol clusters which form in the nanoheterogeneous polymeric matrix, in agreement with small angle X-rays scattering and low-frequencyRaman scattering observations recently reported.© 2003 Elsevier B.V. All rights reserved.

Keywords: Secondary relaxation; Structural relaxation; Glass transition; Mechanical spectroscopy; Dielectric spectroscopy; PMMA

1. Introduction

The degrees of freedom in glassy materials are responsi-ble for localized and long range atomic (or molecular) rear-rangements. The former and the latter processes lead to thesub Tg or so-called Johari–Goldstein� relaxation and themain or� relaxation, respectively. The� relaxation processexhibited by polymeric materials is long since recognized tobe particularly sensitive on additives (see, for example, thepioneer work of Nielsen et al.[1]) and an intensive researchwork has been devoted to this topic. The dynamic elasticmodulus of such materials is also shown to be affected bythe thermomechanical history, namely physical aging[2,3],thermal quenching[3,4] and plastic deformation[3,4]. Thephysical explanation at the microscopic level, is that the

∗ Corresponding author. Tel.:+33-3-83-58-4029;fax: +33-3-83-57-9794.

E-mail address: serge.etienne@eeigm.inpl-nancy.fr (S. Etienne).

cooperativity effect is depressed when the disorder is increa-sed. The effect of small guest molecules is less documented.

In this work the interaction of guest molecules (methanol)with the molecular degrees of freedom of the glassy matrix(PMMA) is investigated, i.e. (i) the dynamics of these guestmolecules and ii) their effect on the� and� relaxations ofthe polymeric matrix by means of mechanical and dielec-tric spectroscopy which provide a complementary set ofinformation. The aim of this work is to analyze the set of ex-perimental results provided by wide band relaxation spectro-scopies within the frame of concepts recently introduced inthe physics of the glassy state, namely the non homogeneousstructure of the glassy matrix at the nanometric scale andthe cooperativity strength of molecular rearrangements. Theguest methanol molecules in PMMA considered in this workare not simply regarded as additives because they can enterthe polymeric matrix and then be completely removed, thusleaving this glassy network in a new structural state througha so-called methanol assisted aging at room temperature.

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2003.07.015

274 S. Etienne et al. / Materials Science and Engineering A 370 (2004) 273–277

2. Materials, techniques and experimental procedures

2.1. Material

The poly(methyl methacrylate), abbreviated as PMMA,used for mechanical was slightly crosslinked by trimethylol-1,1,1-propane trimethacrylate moieties and kindly providedby Altulor company, while the material used for dielectricexperiments was a high purity grade with no additives (clin-ical quality) purchased from Goodfellow company.

The specimens were cut into suitable shapes for mechan-ical and dielectric spectroscopies, i.e. 10 mm× 1 mm ×0.1 mm platelets and disks with a diameter about 20 and0.3 mm thick, respectively.

2.2. Techniques

The dynamic shear modulusG∗(jω) measurements werecarried out by means of a low-frequency mechanical spectro-meter specially designed to work with small-size specimens.This set-up makes it possible to perform measurements ofG∗(jω) in the frequency range extending from 10−5 Hz up toa few Hz and in the temperature domain 90–470 K. The de-termination of the dynamic modulus was assessed by Fourierintegral calculation usingn integration periods. The numbern is usually chosen equal to 4. The data processing yieldsthe real partG′(ω) and the tanϕ(ω) coefficient. The relativestrain amplitude did not exceed 10−5.

The measurements of the complex dielectric constantε∗(jω) were performed in a temperature controlled cell inthe range 80–450 K (Oxford cryostat, equipped with an Ox-ford Intelligent temperature controller model 503) workingunder nitrogen atmosphere. Measurements were taken inthe frequency range 10−2 to 106 Hz, using a system com-posed of a frequency response analyzer (Solartron 1255

0.20

0.15

0.10

0.05

0.00100 200 300 400

0

1

2

3α relaxation

β relaxation

methanol

G'(G

Pa)

Temperature (K)

tan(

ϕ)

Fig. 1. Effect of guest methanol molecules on the complex modulus as a function of temperature of PMMA measured in isochronal condition (frequency= 1 Hz). The heating rate is 1 K/min. Normally PMMA aged specimen (�), PMMA containing methanol molecules (�).

FRAs), a dielectric interface (Solartron 1296) with a dielec-tric reference module (Solartron 1296 1). The sample wasplaced in a sample holder (Solartron 1296 2A) consistingof two parallel electrodes (diameter 20 mm) with a guardring. The thickness of the sample was about 50�m. Themeasurements were performed with an ac-level 1 V rms andfour integration periods.

Thus, by combining dielectric and mechanical relaxationspectroscopies, it was possible to capture the dynamical be-havior over a 10 decades wide frequency spectrum.

2.3. Experimental procedures

The material was tested by mechanical spectroscopy indifferent states according the history described as follows:(i) after aging for a long time at room temperature, (ii) af-ter immersion in methanol until saturation and (iii) aftercomplete removing by evaporation the methanol moleculesfrom step (ii) at room temperature. Step (iii) will be calledmethanol-assisted aging in the following. The material wassubjected to a similar history when tested by means of di-electric spectroscopy.

3. Experimental results

3.1. Mechanical spectroscopy results

The relaxation processes observed in PMMA are at in-creasing temperature: (i) a weak effect near 160 K attributedto the presence of water molecules, (ii) a so-called� orsecondary relaxation process near 300 K, the so-called� ormain relaxation near 400 K, then a (iii) tendency of the mod-ulus to exhibit a plateau attributed to entropic elasticity andthe (iv) the flow of the macromolecular chains[5].

S. Etienne et al. / Materials Science and Engineering A 370 (2004) 273–277 275

Fig. 1allows to compare the spectrum exhibited by a sam-ple annealed at room temperature and by the same sampleafter immersion in methanol. The measurement temperatureon the specimen containing methanol did not exceed 300 Kto keep the methanol amount constant (seeSection 3.2), sothat this paper is dealing with the effect of guest methanolmolecules on the low temperature part of the spectrum un-til the low-temperature tail of the� relaxation. It is obviousthat the presence of methanol molecules is responsible for(i) the occurrence of an extra peak at low temperature (near140 K at 1 Hz) and (ii) a shift towards the low temperaturesof the� relaxation process and of the outset of the� relax-ation process. The effect of the frequency measurement onthe temperature of the maximum makes it possible to deducethe characteristics of the relaxation processes: the apparentactivation energy and the pre-exponential factor exhibitedby the low-temperature peak and the� process are reportedin Table 1.

3.2. Dielectric spectroscopy results

Similar investigation as described above is performed bywide band dielectric spectroscopy. The complex permittiv-ity spectra exhibited by a specimen containing methanolmolecules were taken in the frequency range 10−2 up to105 Hz. Three relaxation processes are clearly observedon the spectra displayed inFig. 2 (similar results werealready quoted in the previous section), namely (i) thelow-temperature process ascribed to thermally activatedmotions of methanol molecules, (ii) an intermediate tem-perature process identified as the� process and (iii) a lossmaximum attributed to desorption of methanol molecules

Fig. 2. Imaginary part of the dielectric permittivity as a function of temperature measured at different frequencies. The PMMA sample was saturated withmethanol molecules (i.e. about 20 wt.%). The measurements were performed at different increasing temperatures by frequency scanning. The equivalentheating rate is about 0.005 K/min.

Table 1characteristics of the relaxation processes

Relaxation process

�m � �∗

DielectricAct. energy (kJ/mol) 61 52 74Pre-exponetial factor (s) 7× 10−25

Temperature (K) 125 250 300

MechanicalAct. energy (kJ/mol) 69 110 70Pre-exponetial factor (s) 5× 10−25

Temperature (K) 125

�m: relaxation attributed to methanol molecules;�: secondary relaxationof PMMA (with methanol);�∗: secondary relaxation of PMMA (withoutmethanol)[5].

on heating. Only the outset o the� process occurring athigher temperatures is captured on this plot.

These loss peaks are plotted as a relaxation map shown inFig. 3. It is obvious that the process attributed to methanoldesorption exhibits a very large apparent activation energyand therefore is not regarded as a relaxation process. Fromthis plot, the characteristics of the processes are deducedand reported inTable 1. These values are in agreement withthose quoted inSection 3.1.

The effect of methanol molecules on the dynamics ofthe system appears clearly on the isochronal spectra shownin Fig. 4. When compared to the loss spectrum of the as-received specimen aged at room temperature, the one of thespecimen containing methanol molecules exhibits an extralow-temperature peak, as already mentioned, and a decreas-ing of the� relaxation temperature. During evaporation of

276 S. Etienne et al. / Materials Science and Engineering A 370 (2004) 273–277

methanol

relaxation

β relaxation

methanol

desorption

Fig. 3. Relaxation map drawn fromFig. 2.

methanol

β relaxation

α relaxation

Fig. 4. Comparison of spectra exhibited by: (i) a normally aged specimen (�); (ii) a first run on a specimen containing the guest methanol molecules(�); the arrow corresponds to a isothermal aging at room temperature (atmospheric pressure) for 24 h. The first run up to 400 K was followed by asecond run (+). The equivalent heating rate is about 0.005 K/min.

methanol at room temperature the loss coefficient is de-pressed and becomes lower than the one of the as-receivedmaterial. It is worth to notice that similar features are ob-served by mechanical spectroscopy as reported elsewhere[6].

4. Discussion

The diffusion coefficient (i.e. the dynamics) of methanolmolecules in PMMA is affected by the thermomechanicalhistory of the polymeric material[7,8]. It is obvious fromthis work that the dynamics of the polymeric PMMA ma-trix is strongly affected by the presence of guest methanolmolecules. Different aspects are to be explained, namely the

dynamics of the glassy polymeric matrix when the guestmolecules are present and (ii) the change of the dynamicswhen the guest molecules are removed, by comparison withthe initial, as-received state.

The low-temperature peak, ascribed to the thermomechan-ical motion of methanol molecules, exhibits a departure fromthe Arrhenius behavior and a very low preexponential factor.These features reflect the cooperative nature of the motionsexperienced by methanol molecules. This cooperativity canoriginate from interaction (i) among methanol moleculesthemselves or (ii) between methanol and the PMMA moi-eties via hydrogen bonding to the carbonyl group. In fact, thetemperature range of the peak is in agreement with the es-timated glass transition temperature of methanol, i.e. 105 K[9], which is in favor of hypothesis (i). It means that the

S. Etienne et al. / Materials Science and Engineering A 370 (2004) 273–277 277

guest molecules form clusters in the polymeric glassy ma-trix, which is to be explained at a molecular scale. It wasproposed, on the basis of low-frequency Raman scattering[10] that a glassy network is heterogeneous at a nanometricscale: cohesive domains (size about 4 nm) are separated byless cohesive zones or channels. Within this frame of amor-phous matter modeling, the clusters are expected to form inthe channels.

The second effect induced by the presence of methanolis the decreasing in the temperature of the� relaxation ob-served inFigs. 1, 4while the apparent activation energy ob-served by mechanical spectroscopy is increased from 70 upto 110 kJ/mole. This reflects a particular interaction betweenpolymer units and solute molecules, the result of which isthe increase of cooperativity in the� molecular motions. Theincrease of the apparent activation energy of the� processis not observed by dielectrical spectroscopy experiments.This apparent contradiction should originate from differentcoupling strength with mechanical or electrical field[11]or more probably because of the low heating rate used forlarge frequency scan of the dielectric permittivity. Becauseof this low heating rate (about 5× 10−3 K/min), a struc-tural relaxation is expected to occur during the temperaturescan, inducing an additional shift of the peak towards hightemperature as the measurement frequency increases, thusreducing the apparent activation energy of the� process.

The third effect is the decrease of the� relaxation tem-perature: the guest methanol molecules behaving as a plasti-cizer make easier the cooperative rearrangements of PMMAsegments.

This model explains also the effect of methanol-assistedaging of glassy PMMA reported elsewhere[6] the loss coef-ficient is depressed at low temperature but increased at hightemperature when compared with the reference specimen.The aging is heterogeneous in nature, like the permeation ofmethanol molecules in the polymeric network assumed tobe non homogeneous at the nanometric scale. It is proposedthat as guest molecules evaporate, the less cohesive zonesundergo a more effective structural relaxation. It means thatthe amorphous network becomes more homogeneous at thenanometric scale. This idea is supported by low-frequency

Raman scattering and small angle X-rays scattering experi-ments[6,12].

5. Conclusion

Small molecules in a glassy polymer is shown to en-hance the relaxation rate of the glassy matrix and inducesan additional low-temperature relaxation process. This pro-cess is non-Arrhenian and is ascribed to cooperative motionsof guest molecules forming clusters. These features are ex-plained within the frame of a non homogeneous model ofthe glassy network at a nanoscopic scale and the concept ofcooperative motions in disordered matter. The detailed de-scription of the molecular rearrangements involved requiresadditional information. This information is expected fromNMR experiments now in progress.

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