Effect of sonication on polymeric membranes

Journal of Membrane Science 181 (2001) 213–220

Effect of sonication on polymeric membranes

Isabelle Masselina, Xavier Chasseraya, Laurence Durand-Bourlierb,Jean-Michel Lainéb, Pierre-Yves Syzaretc, Daniel Lemordanta,∗

a Laboratoire de Physicochimie des Interfaces et des Mileux Réactionnels (EA2098), Faculté des Sciences et Techniques,Parc de Grandmont, 37200 Tours, France

b Lyonnaise des Eaux, CIRSEE, 38 av. Président Wilson, 78230 Le Pecq, Francec Laboratoire de Microscopie électronique, Faculté de Médecine, 2 bis Bd Tonnellé, 37032 Tours Cédex, France

Received 27 March 2000; received in revised form 27 April 2000; accepted 25 July 2000

Abstract

This article studies the effect of 47 kHz ultrasonic (US) waves on polymeric membranes immersed in an aqueous bath. Themembranes under study are made from three different polymers: polyethersulfone (PES), polyvinylidenefluoride (PVDF) andpolyacrylonitrile (PAN) and present various molecular weight cut-off (MWCO). The evolution of the polymeric structureexposed to US was followed by the measurement of the water permeability and theAk/1xparameter which represents the ratioof surface porosity to thickness. Results showed that important variations occurred on certain membranes after irradiation.In addition, microscopic imaging using field emission electron scanning microscopy (FESEM) was performed on irradiatedmembranes in order to visualize the nature of the degradation. An image analysis method gives the evolution of the poredensity, porosity and pore size distribution of a homogeneous area of this membrane before and after irradiation.

It has been shown that, over the three materials tested, only the PES is affected by the ultrasonic treatment over all itssurface, whereas the others present no significant change in the measured parameters except the PAN (50 kDa) and PVDF(40 kDa) membranes whose edges are affected. In conclusion, in spite of their great efficiency in enhancing filtration processes,ultrasonic waves have to be used with care as the polymeric material itself is sensitive to the ultrasonic waves at the chosenfrequency. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Ultrasound; Polymeric membrane; Ultrafiltration; Porosity to thickness ratio; Image analysis

1. Introduction

The mechanical and physical effects of ultrasounds(US) have found many applications in several indus-trial sectors such as chemical or biological industries[1–4], semi-conductor manufacturing [5] and foodtechnologies [6,7]. They are just as easily used indewatering and drying materials, to enhance filtra-tion, [8–11] assist heat transfer [12], degas liquids

∗ Corresponding author. Fax:+33-247-3669-60.E-mail address:[email protected] (D. Lemordant).

[13,14], accelerate extraction processes [15,16], de-grade chemical contaminants in water [17] and toenhance many processes where diffusion takes place[7,18]. The beneficial use of US is achieved throughits chemical effects on products, or mechanical andphysical effects on processes. General applications in-clude acceleration of chemical reactions, degradationof polymers, and polymerization reactions [19,20].

Ultrasonic physical effects and sonochemical ef-fects are also used in membrane technology inorder to clean surfaces [21], to prevent formationof filter cake and to enhance filtration rates [22].

0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0376-7388(00)00534-2

214 I. Masselin et al. / Journal of Membrane Science 181 (2001) 213–220

Nomenclature

Ak porosity of the membraneD diffusivity [m2 s−1]J flux [L m2 s−1]Lp hydraulic permeability [L m2 h−1 bar−1]1P transmembrane pressure [bar]r radius [nm]t time [s]1x membrane thickness [m]1r relative variation of a parameterσn−1 standard deviation

Subscriptw water

Superscript0 relative to initial conditions

Cleaning procedures are significantly enhanced bycavitation and acoustic streaming induced by ultra-sonic waves [23]. High-intensity acoustic radiationscause various changes as they propagate through amedium. These changes occur as a result of sev-eral mechanisms, which can be summarized asfollows:

1. Heating: it is generally accepted that the over-all temperature increase due to the absorption ofsound waves or to the adiabatic compression ofthe medium in the sound field, is dependent on thenature of the medium and the liquid/membraneinterface.

2. Structural effects: when fluids are placed underhigh-intensity sound fields (frequency> 18 kHz),the dynamic agitation and shear stresses producedaffect their structural properties, particularly theirviscosity [10,11,24].

3. Compression and rarefaction: when high-intensityacoustic energy waves travel through a solidmedium, rapid and successive compression andrarefaction occurs, the rates of which depend ontheir frequency. Dense material usually “fracture”under acoustic stress.

4. Turbulence: high-intensity US in low-viscosity liq-uids and gases produce violent agitation which canbe used to disperse particles [25]. At liquid/solid

or gas/solid interfaces, acoustic waves cause ex-treme turbulence known as “acoustic streaming” or“micro-streaming” [26].

5. Cavitation: acoustic cavitation is the formation,growth, and violent collapse of small bubbles orvoids in liquids as a result of pressure fluctua-tion. Among other effects, cavitation may causeenhanced polymerization or depolymerization re-actions by temporarily dispersing aggregates orby permanently breaking chemical bonds in poly-meric chains [27,28]. Cavitation can also damagesurfaces. Determination of the exact mechanismof damage has been an area of considerable study[5]. Plesset [29] speculated that shock waves radi-ating from collapsing bubbles could be the causeof damages but he noted that the collapse wouldhave to occur very close to the substrate and thatthe vapor bubble could not contain large quantitiesof gas. He also suggested that liquid jets emanat-ing from collapsing bubbles could be a potentialcause of surface damage. Indeed, extremely highvelocities and temperature are associated with cav-ity implosion or collapse. Experiments by Naudeand Ellis [30] using high-speed photography haveshown that erosion is caused by high-speed jetsfrom bubble implosion at a surface, rather thanfrom extreme pressures and temperature resultingfrom cavity collapse. Properties of the liquid andgas can also affect the degree and intensity ofcavitation in ultrasonic baths.

We noticed that membranes cleaned by means ofan ordinary laboratory ultrasonic bath, presented anunexpected behavior after irradiation. For this rea-son, we decided to investigate the effect of ultrasonicirradiations on polymeric materials. The aim of thisarticle is to study the effect of 47 kHz ultrasonicwaves on polyethersulfone (PES), polyvinylidenefluo-ride (PVDF) and polyacrylonitrile (PAN) membranesimmersed in an aqueous bath. The evolution of mem-branes exposed to US irradiation was followed bythe measurement of the permeability to water andthe porosity to thickness ratio (Ak/1x). In addition,microscopic imaging, using field emission electronscanning microscopy (FESEM), was performed on themost damaged membranes in order to visualize thenature of the degradation and to analyze the evolutionof the pore density, porosity and pore size distribution.

I. Masselin et al. / Journal of Membrane Science 181 (2001) 213–220 215

Table 1Membranes under study

Material MWCO(kDa)

Membranereference

Polyethersulfone (PES) 3 PES310 PES1030 PES30

100 PES100

Polyvinylidenefluoride (PVDF) 40 PVDF4040 PAN40

Polyacrylonitrile (PAN) 50 (ref 3042) PAN50a50 (ref 3050) PAN50b

2. Experimental

2.1. Membranes

Asymmetric PES, PVDF and PAN ultrafiltrationmembranes are supplied by Techsep (Rhone-Poulenc,France). The MWCO of the membranes used arereported in Table 1. Before all experiments, mem-branes are rinsed with de-ionized water (resistivity≈15 M� cm), until the permeability remains constant.

2.2. Permeability measurements

The permeabilityLp of a membrane is defined by:

Lp = Jw

1P(1)

whereJw is the water flux and1P the applied pres-sure. The permeability of membranesLp is given inL m−2 h−1 bar−1. Measurements were performed atroom temperature (295±2 K) using a 200 ml Amiconcell. The effective surface, accessible to the solvent is28.3 cm2 andLp is determined from the measurementof the time required to obtain a fixed volume of per-meate at a given pressure. The applied pressure rangesfrom 0.3–1.1 bars.

2.3. Ak/∆x measurements by diffusion experiments

Diffusion of small molecules or ions, performed atthe pseudo-stationary state, is a convenient method toobtain theAk/1x ratio whereAk represents the poros-ity of the membrane and1x its thickness, or more pre-cisely, the mean path for diffusing solutes. An accurate

value ofAk/1x can be deduced from the determinationof the solute concentration versus time as shown pre-viously [31,32]. For this study, the diffusing solute ishydrochloric acid (HCl) dissolved in 0.1 mol/l NaCl,used as a supporting electrolyte. The diffusion of sol-vated protons is monitored by pH measurements. Theuse of a supporting electrolyte is useful in order to ob-tain a better reproducibility of measurements by fixingthe ionic strength of the solution and the charge den-sity on the pore walls. Due to the presence of metallicrings acting as membrane holders in diffusion experi-ments, only the central part of the membrane surfaceis accessible to the solute. Thus,Ak/1x measurementscharacterize only a 12.6 cm2 area in the central sectionof the membrane.

2.4. Irradiation

In order to investigate the effect of ultrasonic waves,membranes are placed in a beaker immersed in the ul-trasonic bath and filled with de-ionized water. The fre-quency of the ultrasonic waves is fixed at 47 kHz. Thetotal duration of the ultrasonic treatment is 2 h, dividedas follows: three times 5 min, three times 10 min, threetimes 15 min and finally 30 min. The determination ofLp andAk/1x values are realized after each irradiationperiod in order to monitor the membrane parametersversus the total irradiation time and the duration.

2.5. Microscopic images

Microscopic observation of the membrane surfaceis performed by using a field emission scanning elec-tron microscope (FESEM) on a LEO gemini 982 ap-paratus. Using images at the nominal magnification of80,000, we measured the porosity, pore density andpore size distribution of the membrane PES100 beforeand after the two hours of ultrasonic treatment. Ana-lyzed images are chosen from “homogeneous” partsof the membrane surface, i.e. area which do not ex-hibit large holes or cracks (see Fig. 4b). The analysisof the binarized image is based on the labellisation ofeach pore of the membrane surface, followed by themeasurement of the pore area.

Each pore is assumed to be circular and discon-nected from its neighbors. The pore density (numberof pores per unit area) is deduced from the number oflabelled pores on a given surface. The porosity is the


total porous surface divided by the total image mem-brane area. The pore size distribution is calculatedfrom the occurrence of pores in a given area interval.The frequency is defined as the ratio of the numberof pores with radius betweenr andr + i (where i =1 nm is the value of the selected interval) to the totalpore number. More details about these image analysismethods are given elsewhere [33].

3. Results and discussion

3.1. Permeability and diffusion measurements

The variation of permeability of a membrane, rela-tively to its initial value,1rLp is defined by:

1rLp = 100Lp(t) − L0

p

L0p

whereLp(t) andL0p are, respectively, the permeability

at time t and at timet = 0. The variation of1rLp,with the irradiation time is reported in Fig. 1.

With the exception of the PAN40 membrane whichexhibits only a small increase in permeability (c.a.10%), all membranes undergo large permeability vari-ations. The degradation of the membrane occurs dur-ing the first 5 min period of exposure. The largest

Fig. 1. Relative variations of the permeability1rLP, expressed in% of the initial value, versus the irradiation time. Membranes: (s)PES3; (4) PES10; (h) PES30; ( ) PES100; (e) PVDF40;(d) PAN40; (m) PAN50a; (j) PAN50b.

degradations concern the PES membranes of lowestMWCO. The PVDF40 membrane has an intermedi-ate behavior between PES and PAN membranes. Afterthe first period of irradiation, a plateau of permeabilityis observed, meaning that no subsequent degradationoccurs. To conclude, PES membranes are strongly af-fected by the US irradiation as the permeability is morethan 10 times higher after the first 5 min of exposure.

The variation ofAk/1x, relatively to its initial value,is defined by:

1r

(Ak

1x

)= 100

Ak/1x(t) − Ak/1x(t = 0)

Ak/1x(t = 0)

The variation of1r(Ak/1x) with irradiation time is il-lustrated in Fig. 2. A similar pattern is observed forthe porosity to thickness and the permeability varia-tions, as only PES membranes exhibit relative poros-ity variation over 100% even after the first 5 min ofexposure. The PES3, PES100 and PES30 membranesseem to be more strongly affected than the PES100.On the other hand, the PVDF and PAN membranesshow little or no variations.

Figs. 1 and 2 underline the different behavior of thetested membranes after irradiation: PAN and PVDFmembranes present no significant evolution forAk/1x,whereas the evolution of their permeability is large.This can be explained by the difference between thearea taken into account in each parameter: permeabi-lity measurements take into account all the surface areaexposed to the solvent (28.3 cm2) whereas the diffu-

Fig. 2. Variations of1r(Ak/1x), expressed in % of the initialvalue, versus the irradiation time. Membranes: (s) PES3; (4)PES10; (h) PES30; ( ) PES100; (e) PVDF40; (d) PAN40;(m) PAN50a; (j) PAN50b.


sion measurements take into account only the centralpart of the membrane surface (12.6 cm2). One mayconclude that the external edges of the membrane aremore sensitive to ultrasonic waves than its central sec-tion.

Comparing the values obtained for these two param-eters, we can also notice that ultrasonic waves have amore pronounced effect on the membrane surfaces per-meability (10–10 000%) than on porosity (0–500%).

3.2. Microscopic images

In order to visualize the way by which the ultrasonicwaves affect the PES membrane, the PES100 mem-brane was chosen as a test membrane as its pores areclearly visible by FESEM. Pictures were taken beforeand after a 2 h irradiation treatment. Some of them arereported in Figs. 3 and 4.

The picture reported in Fig. 3 concerns a non-irradiated membrane taken at a nominal magnification

Fig. 3. Microscopic image of a non-irradiated PES100 membrane; magnification: 80,000.

of 80,000. Non-overlapping pores of homogeneoussizes (mean pore radius: 4.5 nm) are regularly dis-tributed all over the investigated surface. In Fig. 4,are displayed two pictures of an irradiated PES mem-brane, obtained at nominal magnifications of 80,000(Fig. 4a) and 550 (Fig. 4b). As seen in Fig. 4a, thepore size is little affected by the ultrasonic wave butcracks occur between adjacent pores. On the left partof the picture a large rift (725 nm× 25–46 nm) isvisible which will certainly have a pronounced effecton the permeability and porosity. Fig. 4b representsan example of large holes (136mm × 60mm) whichare found occasionally over the membrane surface. Inconnection with what has been previously suggested,such holes, normally found on the edges of the mem-brane disk, could effectively lead to a very largeincrease in permeability. This hole is large enoughfor the membrane substructure to be visible. In thepresence of numerous large holes, formed probablyby the junction of several cracks, the permeability and


Fig. 4. Microscopic images of the PES100 membrane irradiated by ultrasonic waves; (a) “homogeneous” area of the membrane surface,at a magnification of 80,000; (b) highly damaged area, at a magnification of 550.


Table 2Effect of a 2 h period of US irradiation on the pore density(1014 m−2), the porosity (%) and the mean pore radius (nm) fortwo PES100 membranesa

Pore density(1014 m−2)

Porosity(%)

Mean poreradius (nm)

Before 2.08± 0.50 1.30± 0.30 4.47± 0.102.56 ± 0.50 1.68± 0.35 4.57± 0.05

After 8.92 ± 1.20 9.36± 1.60 5.78± 0.108.97 ± 0.80 8.91± 1.60 5.62± 0.30

a All parameters are determined by microscopic image analysis.

porosity values should strongly increase. Moreover,the hydraulic pressure applied during the permeabil-ity measurement will contribute to temporary enlargeholes and cracks. As this occurs during the measure-ment of the permeability, the differences betweenLpandAk/1x values for the concerned membranes couldbe explained.

In Table 2, the pore density, porosity and mean poreradius values for two PES100 membrane samples arepresented. The first value is given before any irradi-ation and the second one, after 2 h of ultrasonic irra-diation. For each sample two images are taken fromtwo different parts of the membrane. Results show thatthe two selected areas do not differ significantly. Af-ter the ultrasonic treatment, the mean pore radius un-dergoes a 30% increase but the porosity and the poredensity increase, respectively, by a factor 6.1 (515%)and 3.9 (286%). These large variations show that thesurface of the membranes has been strongly modified,all the more since the images chosen for this analy-sis are not taken from highly damaged parts of themembrane surface as observed on Fig. 4b. As largeholes are not taken into account for this analysis, re-sults in Table 2 do not depict the whole membranedegradation.

Furthermore, as seen on the graph reported inFig. 5, the pore distribution is modified by the ultra-sonic treatment. Pores lying between 1 and 4 nm inradius become less frequent in the distribution and,simultaneously the occurrence of larger pores (ra-dius > 4 nm) become more frequent. Nevertheless, asthe overall pore density is multiplied by a factor of3.9 after the ultrasonic treatment, this means that thedamage caused to the membrane is related as well tothe formation of small pores ranging from 1 to 4 nmas to the apparition of large pores, mainly in the range

Fig. 5. Pore size distribution (nm) of the PES100 membrane beforeand after 2 h of ultrasonic irradiation: (j) non-irradiated sample,(h) irradiated sample.

5 to 15 nm. This explains why the mean pore radiusundergoes little variation after the US treatment.

4. Conclusion

Many of the observed trends in polymer degrada-tion can be explained by theories such as apparitionof frictional forces, shear gradients and impacts dueto cavitationnal collapse [28]. In general, the modesof stress reaction in the solid state can be classified as(i) comminution of polymers (e.g. grinding, milling,crushing, etc.); (ii) machining (drilling, cutting, slic-ing, etc.); (iii) stretching; and (iv) other means suchas fatigue, tear and wear [27].

This work shows that, over the three membrane ma-terials tested, only the PES is affected by the ultra-sonic treatment over its entire surface. PVDF and PANmembranes are more resistant and present less dam-ages at the exception of the PAN50a and the PVDF40membranes for which the edges are more affectedthan the central section. Results also show that thedegradation of the membrane surfaces under ultra-sonic stress lead to an increase in pore radius for largepores, an overall increase in pore density and poro-sity and to the formation of large cracks preferentiallyat the edges of the membrane samples. Any increasein pore density, especially when large pores are con-cerned may favor the formation of cracks resultingfrom the interconnection of neighboring pores. Theselarge cracks will bring a large contribution to the in-crease in permeability and porosity observed for mostmembranes.


From these findings, it is clear that, in spite of theirgreat efficiency in enhancing permeation of fouledmembranes, ultrasounds have to be used with care.The nature of the polymeric material as well as the ul-trasonic wave frequency and intensity have to be takeninto account.

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Effect of sonication on polymeric membranes