Effect of Ethylene Glycol Monobutyl Ether on Skin LayerFormation Kinetics of Asymmetric Membranes

Xin Li,1 Cuixian Chen,2 Jiding Li,2 Yi Su3

1Beijing Institute of Technology, Beijing 1000812Department of Chemical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China3Institute of Process Engineering, Chinese Acadamy of Sciences, Beijing

Received 14 June 2008; accepted 2 March 2009DOI 10.1002/app.30367Published online 27 April 2009 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The present work was concerned with skinlayer formation kinetics of Polyethersulfone with Cardo(PES-C) asymmetric membranes by the phase inversionmethod. Gelation process was observed using an online op-tical microscope–CCD camera experimental system, theresult indicated that a single linear correlation betweensquare value of precipitation front move (X2) and time (t)did not exist for the whole gelation process, which was dif-ferent from the traditional conclusion. The effect of Ethyleneglycol monobutyl ether (EGME) on skin layer gelation pro-cess was investigated, and the skin layer formation kineticsequation was determined, which could be described withgelation rate constant equation. The gelation rate constant

decreased with the increase of concentration of EGME.EGME concentrations affected the viscous flow activationenergy, and the increase of viscous flow activation energyled to the decrease of gelation rate. The change trend of thegelation rate was exactly the same as that of the flux. Theresults helped to understand the asymmetric membraneskin layer formation process better, which was importantfor controlling the membrane separation properties anddesigning the membranes for a specific application. VVC 2009Wiley Periodicals, Inc. J Appl Polym Sci 113: 2392–2396, 2009

Key words: formation kinetics; skin layer; asymmetricmembrane; effect of EGME; wet phase inversion

INTRODUCTION

Asymmetric polymer membranes have numerousapplications, such as food, chemical processing,wastewater handling, drug delivery medium, artifi-cial human organs etc., because of their high effi-ciency and energy conservation. Phase-inversionmethod is often used for preparing asymmetric mem-branes.1,2 In the phase-inversion process, theexchange of solvent and non-solvent between castingsolution and the coagulation bath includes a kineticsprocess.3 To get a membrane with good penetrationproperty, an additive is usually added to the castingsolution. It is presented that addition of additive hasan effect on thermodynamic and kinetic process in themembrane formation and thereby affects the mem-brane structure and properties,4 many investigatorsattempt to elucidate the effect of addition on themembrane-forming mechanism.

It is important for exploring the membrane forma-tion mechanisms with the help of membrane formationkinetics. The experimental techniques investigate thekinetics of membrane formation are light transmittancemeasurement and optical microscopy,5–8 are reportedto be used to investigate the kinetics of membrane for-mation up to present. The two experimental techniques

are not real time processing system and precipitationprocess can not be described continuously. Time andthe precipitation move are measured by manual opera-tion. Time distance of data adopting is about severalseconds or several minutes, which is too slow to obtainprecipitation data in the skin layer stage. Usually askin layer is as thin as several microns or even lessthan 1 lm. Thus the formation of the skin layer is veryfast (within several seconds or even less than 1 s) andis difficult to be observed.9 Several research studieshave measured the rate of precipitation of asymmetricmembrane solutions.10–15 But the effect of EGME onskin layer formation kinetics has not still been studied.

PES-C membranes were prepared via phase inver-sion process with with N-Methyl-2-Pyrrolidone dime-thylacetamide (DMAc) as a solvent and Ethyleneglycol monobutyl ether (EGME) as an additive. PES-Cis a newly developed engineering thermoplastic andmembrane material with a high glass transition tem-perature(Tg ¼ 255�C), and it shows excellent compre-hensive properties and outstanding thermal stability,and it has been widely identified as a high-perform-ance polymer. A new optical microscope-CCD cameraexperimental system was used to observe the forma-tion rate of skin layer. In comparison with the previousmethods, this new approach is capable of describingmembrane formation intuitively, and the gelation pro-cess of skin layer stage can be observed clearly. Anattempt is made to present a quantitative descriptionof the effect of EGME on skin layer formation kinetics

Journal of Applied Polymer Science, Vol. 113, 2392–2396 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: X. Li (klkxlx@163.com).

Figure 1 Comparison between SEM photo and Gelation kinetics photo. (a) The evolution of gelation kinetics photo, PES-C: 16 wt %; Solvent: DMAC;OA: 6 wt %; Gelation temperature: 60�C, t was 0, 0.5, 5, 30, 40, 60 s, respectively. (b) The leftphoto was SEM photo, the right one was gelation kinetics photo. PESC: 16 wt %; Solvent: DMAC; OA: 6 wt %; Gelationtemperature: 60�C. (c) The left photo was SEM photo, the right one was gelation kinetics photo. PSF: 18 wt %, Solvent:NMP, Gelation temperature: 60�C.

EFFECT OF EGME ON SKIN LAYER FORMATION KINETICS OF ASYMMETRIC MEMBRANES 2393

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of asymmetric membranes. The results presented heremay provide a better understanding formation mech-anism of asymmetric membranes.

EXPERIMENTAL

Materials

PES-C (g ¼ 0.6 m3/kg) was obtained from Chang-chun Institute of Applied Chemistry Chinese Acad-emy of Sciences. DMAC and EGME were obtainedfrom Xingchenweiye chemical reagent company. Allthe solvents in the experiment were commercial andused without further purification.

Observation of gelation process

The gelation rate of casting solution was determinedby using an online optical microscope–CCD cameraexperimental system (OM–CC system). This systemcontains an Olympus IX71 optical microscope(Olym-pus, Japan), a Basler A101fCCD camera (Basler AG,Ahrensburg, Germany), a personal computer, andtwo specially designed microscope slides. The gela-tion process can be observed clearly, especially inskin layer stage. The magnification was from 120�to 2000�. The images are automatically captured bythe camera and stored in a computer using a highspeed digital video recording software (Video Sa-vant 4.0, Industries, Canada) and processed using animage processing software (ImageJ, National Insti-tute of Mental Health, Bethesda, MD).

Rheological property

The casting solution viscosity was measured using aBrookfield DV-IIþCP (Brookfield, USA)viscometervaried from 25�C to 65�C arranged at intervals of5�C.

RESULTS AND DISCUSSION

Comparison OM–CC images withSEM micrographs

A drop of the casting solution was dropped betweentwo special microscope accessories and then the pre-cipitant (distilled water) was introduced on theseaccessories.When the distilled water began to touchwith the casting solution,the precipitation processoccurred. The whole precipitation process wasobserved and recorded with an optical microscopephotographically and CCD camera, as showed inFigure 1(a). SEM photo were compared with thephotograph taken from OM–CC in Figure 1(b,c), themembrane structure photos observed on the com-puter were similar as the SEM micrographs, whichindicated that the membrane formation processobserved with optical microscope and real mem-brane formation were almost uniform.

Analysis of the gelation rate curve

The relationship between X2 and t was showed in Fig-ure 2, From Figure 2, it could be seen that X2–t curvedidn’t present a perfect straight line of passingthrough the original point, which was not consistentwith the commonly accepted conclusion.13,14 The tra-ditional opinions considered to be a single straightlinear correlation between X2 and t in the whole gela-tion process.7

The reason is that X and t is measured by manualoperation in previous study. The manual operationmethod usually spent about several seconds orminutes in each time distance to capture the data.The long time distance hided the true precipitationprocess and led to the line relationship, which couldbe reflected in Figure 3, when time span was 10 s, a

Figure 2 Variation of the square value of precipitationfront move with time: PES-C: 16 wt %.

Figure 3 The square value of precipitation front move withtime: PES-C: 16 wt %, when time was 20, 30, 40, 50, and 60 s.

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perfect line passing through the original point wasobtained, the slope of line was 8,549.

In the membrane cross-section, three layers could beclearly distinguished as Figure 1 showed: a dense skinlayer, a medium dense transition layer, a loose fingerstructured support layer. Because of inhomogeneity ofasymmetric membrane cross-section, the resistance ofexchange between solvent and gelation mediumchanged gradually with the increase of precipitationtime and thereby gelation rate constant (slope rate)changed. The traditional method could not measuregelation process of skin layer and transition layer intime. Since the skin layer was actually taking the mostimportant role for an asymmetric membrane, it mightlead to misunderstanding for traditional method todescribe the whole membrane formation process.

Effect of EGME on membrane skin layerformation kinetics

Figure 4 showed the effect of EGME on the gelationrate of skin layer, and EGME concentrations are 2, 4,8, and 10 wt %, respectively. The result showed thatX2 increases linearly with the increase of t, gelationrate constant decreases with the increase of EGMEconcentration. The reason was that addition of EGMEincreased viscosity of casting solution, which limited

movement of macromolecular segment. Thus, the dif-fusion of precipitation into the casting solutionbecame slower, which delayed the time of phase sepa-ration time and led to the decrease of gelation rate.

X2 could be described using equations (X2 ¼ kt þb), equations were listed in Table I. X was the move-ment of the precipitation front, t was the gelationtime, k was gelation rate constant, R was the correla-tion coefficient, value of R closed to 1 indicated excel-lent linear reliability. From Table I, it could be seenthat correlation coefficients of the regression equa-tions all presented exceeded 0.99. As shown in TableI, when EGME concentration was 4 wt %, the gelationrate constant skin layer was 5014, which was differentwith the gelation rate constant shown in Figure 3(8549). The result showed that gelation rate was notsame during three different layer formation processes.Since the support layer was the dominant structurefrom a geometric point, skin layer and transition layeronly occupy a small proportion of the whole mem-brane thickness. Because one straight line equationcould not described the whole gelation process, theresult of the gelation kinetics prediceted by the lineequation was incorrect. Actually, each layer shouldhave a different gelation rate constant individually.

Figure 4 Variation of the square value of skin layer pre-cipitation front move with time: PES-C: 16 wt %.

TABLE IThe Correlation Equations of Polymer Skin Layer

Between the Square of Gelation Front Motion (X2) andGelation Time (t) with Various EGME Concentration

Concentration of EGME Equations R

EGME ¼ 2 w t% X2 ¼ 7286t 0.9982EGME ¼ 4 wt % X2 ¼ 5014t 0.9979EGME ¼ 8 wt % X2 ¼ 3502t 0.9983EGME ¼ 10 wt % X2 ¼ 2550t 0.9956

Figure 5 Relationship between viscosity of cast solutionand temperature.

TABLE IIRelationship Between the Viscous Flow Activation

Energy and the EGME Concentration

Concentrationof EGME Arrhenius equations

Viscous flowactivation energy,

Eg (KJ/mol)

EGME ¼ 0 wt % Ln(g) ¼ 2393.28/T-1.78 19.91EGME ¼ 2 wt % Ln(g) ¼ 2458.05/T-1.95 20.45EGME ¼ 4 wt % Ln(g) ¼ 2528.00/T-2.09 21.03EGME ¼ 8 wt % Ln(g) ¼ 2688.45/T-2.38 22.37EGME ¼ 10 wt % Ln(g) ¼ 2837.38/T-2.54 23.61

EFFECT OF EGME ON SKIN LAYER FORMATION KINETICS OF ASYMMETRIC MEMBRANES 2395

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Relationship between viscous flow activationenergy and gelation rate

The viscosity of cast solution with various EGME con-centrations was measured at various temperatures. InFigure 5, the natural logarithm value of the viscosityof cast solution was plotted against the reciprocalvalue of the temperature. It could be concluded that astraight line was obtained at a given EGME concentra-tion. The rheological data of the cast solution at vari-ous temperatures could be described using Arrheniusequation. Table II gave the Arrhenius equations andviscous flow activation energy at various EGME con-centrations. Viscous flow activation energy increasedwith the increase of EGME concentrations. In Figure6, viscous flow activation energy and precipitationfront move at 0.5 s were plotted against the EGMEconcentration. When viscous flow activation energywas bigger, more energy was needed for the diffusiv-ity between solvent and nonsolvent molecules. Sincethe diffusivity of the solvent molecules through theprecipitated polymer layer strongly affected the finalmembrane morphology, it was possible to obtaineddifferent membrane morphology by simply control-ling the EGME concentration.13

CONCLUSION

The effect of EGME on skin layer gelation process ofasymmetric membranes had been investigated using

PES-C/EGME/DMAC cast solution. This resultextended the observation to the gelation process ofthe skin layer, which was very important for control-ling membrane characteristics and performances. Therelation between X2 and t with various EGME concen-trations was determined, gelation rate constant equa-tions of membrane skin layer formation wereobtained. EGME concentration strongly affected thegelation rate of skin layer, the gelation rate constantdecreased with the increase of concentration ofEGME. Rheological property of the cast solution hadbeen investigated. The results showed that the rela-tionship between the apparent viscosity and tempera-ture could be described with the Arrhenius equation.Further the EGME concentrations affected the viscousflow activation energy.

The authors of this article would like to thank the NationalHigh Technology Research and Development Program ofChina (863 Program) (No. 2003AA328020), the Major StateBasic Research Development Program of China (973 Pro-gram) (No. 2003CB61570), the National Natural ScienceFoundation(No.20806008), The second excellent youngscholars research Fund of Beijing Institute of Technology(No. 2008YS0501) for financial support of this research.

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Figure 6 Relationship between the viscous flow activa-tion energy and the gelation rate.

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