a48
Ind. Eng. Chem. Res. 1993,32, 848-853
MATERIALS AND INTERFACES Design of Pervaporation Membrane for Organic-Liquid Separation Based on Solubility Control by Plasma-Graft Filling Polymerization Technique Takeo Yamaguchi,’ Shin-ichi Nakao, and Shoji Kimura Department of Chemical Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
We have prepared the filling-polymerized membrane for pervaporation of organic-liquid mixtures by the plasma-graft polymerization technique. The membrane is composed of two different polymers: a porous substrate which can suppress membrane swelling and a grafted polymer which forms in the pores of the substrate and exhibits selectivity due to its solubility. The objectives of the present study are to design a suitable membrane for an organic-mixture system by the control of the filling-polymer solubility. Specifically, a porous high-density polyethylene membrane and poly (methylacrylate/acrylamide)copolymer were employed as the porous substrate and grafted polymer,respectively, and grafted copolymersolubility was predicted by Hansen solubility parameters (HSP). The grafted polymer composition and its solubility behavior could be controlled by varying the monomer composition, and the solubility change was in accordance with the prediction by HSP. Pervaporation performance through the membranes showed the same tendency as solubility results. We concluded that an optimum pervaporation membrane can be designed on the basis of solubility control through use of these techniques for polymerization and prediction.
,a, and 611(Hansen andBeerbower, 1971): the contributions
Introduction In the separation of an organic-liquid mixture by an organic membrane, selectivity is generally caused by solubility difference between the feed but reduction of selectivity occurs due to membrane swelling; hence both suppression of the swelling and solubility control of membrane are important for successful separation. We have already proposed a new type of membrane for organic-liquid separation, called the filling-polymerized membrane (Yamaguchi et al., 1991). The membrane proposed is composed of two materials: the porous substrate and the filling polymer which fills the pores of the substrate. The porous substrate is inert to organic liquids, and the filling polymer is soluble with one component in the feed. The filling polymer exhibits permselectivity due to the solubility difference, and the porous substrate matrix restrains the swellingof the filling polymer due to its mechanical strength. Employing a high-density polyethylene (HDPE) membrane and poly(methy1acrylate) (poly(MA))as the porous substrate and grafted polymer, respectively, we have succeeded in preparation of filling-polymerizedmembrane proposed by the plasma-graft polymerization technique. We also succeeded in reduction of membrane swelling by the substrate matrix and increased the pervaporation selectivity by the reduction of swelling (Yamaguchi et al., 1991). In order to design a pervaporation membrane for specific organic-liquid systems,membrane-solubilitycontrol is also indispensable as well as suppression of the membrane swelling. Although the solubility parameter includes theoretical inaccuracy, the parameter has been used in many areas concerning a solubility prediction for ita usefulness and simplicity. The Hansen solubility parameters are defined as a solubility parameter divided into three contributions, bd,
from dispersion forces, permanent dipoles, and hydrogen bondings, respectively. In Hansen space, which consists of bd, b,, and bh axes, any solvent has a specific point and any polymer has a specific sphere. The polymer is soluble with a solvent inside the sphere and insoluble outside the sphere. Cabasso (1983)determined the solubilityof a polymeric alloy membrane and predicted the preferentially permeable components by pervaporation using Hansen solubility parameters. Mulder et al. (1982)determined the Hansen solubility parametersof various celluloseester membranes by means of the group contribution method,and explained the differences in membrane pervaporation properties qualitatively in terms of the parameter concept. We determinedthe Hansen solubilityparameters of the MA grafted filling-polymerized membrane, which corresponds to the grafted polymer solubility, by sorption experiments and found that the parameter could be use to predict the preferentially permeate component and to choose the organic-mixture systemswhich the membrane can separate (Yamaguchi et al., 1992). However, when we use a single polymer as the grafted polymer which exhibits selectivity,the solubilityvariation is limited, and we cannot design a suitable membrane for a separating system. In the present study, two kinds of monomers whose polymers have different solubilities were randomly copolymerized as the grafted polymer in order to vary the membrane solubility. The solubility parameters of random copolymer could be calculated as the intermediate between that of the homopolymer constituents (Knox, 1977;Barton, 1985), and Ho et al. (1991)confirmed this assumption through experiments on the methacrylonitrilelmethacrylic acid copolymer system. We also predict the grafted copolymer solubility change by the parameters, and give a suitable solubility for a separating system to the membrane.
Q888-5885/9312632-Q848$04.OO/Q 0 1993 American Chemical Society
Q
arbitrary sphere of
poly(AAm)
I
predicted sphere of
.
poly(MAIAAm)
cyclohexane
Figurn 1. prediction of Hansen solubility parameters of poly(MA/ AAm) copolymer: water; methanol; ethanol; isopropyl alcohol; acetone; benzene; cnrbon tetrachloride; cyclohexane,
Objectives of the present study are to control the solubility behavior of the grafted polymer and to design a suitable membrane for an organic-mixture system. The sphere of poly(MA) in Hansen space was determined in the previous paper (Yamaguchi et al., 1992).The dispersion forces,ad, are nearly constant for most solvents, and we cnn discuss the projection of the two-dimensional grid, 6,-6h (Yamaguchi et al., 1992). Poly(AAm) is widely known as a hydrophilic polymer, and it is soluble in water, but insoluble in most organic liquids. This suggests that the sphere of poly(AAm) is near the point of water, and the sphere of poly(MA/AAm) random copolymer is between the two spheres, as illustrated in Figure 1. The MAlAAm random copolymer's sphere will move in the direction of water with increase in poly(AAm) compcaition,andwewill beable topredictcopolymersolubility on the basis of this assumption. Copolymer solubility to the substances in the poly(MA) sphere, such as acetone, benzene, and carbon tetrachloride, will decreased, and copolymer solubility to the substances in the poly(AAm) sphere, such as water, will increase in keeping with the poly(AAm) ratio in the grafted polymer. Methanol, ethanol, and 2-propanol, whose parameters are between the poly(MA) and poly(AAm) spheres, are inside the predicted sphere of poly(MA/AAm)as shown in Figure 1, and an optimum copolymer composition which shows the maximumvalue of solubilityto them will exist. Solubility to cyclohexane with a (ap, ah) value of (0,O.l) will show low values over the entire range of compositions.
Experimental Section Membrane Preparation. Materials: As a porous substrate, porous high-density polyethylene (HDPE) film supplied by Tonen Chemical Co. Ltd. was used. The substrate had a thickness of 5 pm, pore size of 0.02 pm, and porosity of 35%. The graft monomers used were methyl acrylate (MA) and acrylamide (AAm).Both were purified by distillation under vacuum and recrystallization by benzene, respectively. The monomers for monomer mixtures were dissolved in water and then degassed by repeated freezing
Ind. Eng. Chem. Res., Vol. 32, No.5, 1993 849 and thawing. Total monomer concentration in water was 5 w t 9% ,and the compcaitions of the monomer mixtures were varied. Plasma-Graft Polymerization. The graftingprwss has been described in detail elsewhere (Yamaguchi et al., 1991). Theamountofgraftingwasmeasuredasthe weight of grafted polymer per 1 cm2 of porous substrate (mg/ cm2),and the composition of the grafted copolymer was estimated by ultimate analysis of N/C. Morphological Analysis. (1) Transmission Electron Microscopy: The morphological details of the membrane cross section were observed by transmission electron microscopy. The membrane (amount of grafting 1.85 mg/cm2;poly(AAm) ratio in grafted copolymer 37.3 w t %) was sliced, and an ultrathin sample was stained with tetraoxidized ruthenium to distinguish poly(MA/ AAm) copolymer from the pores or HDPE crystals of the substrate. (2) Fourier TransformInfrared (FT-IR) Analysis: The compositions of the entire membrane and its surface only were analyzed by FT-IR total reflection (TR) and attenuated total reflection (ATR). In ATR analysis, ZnSe or Ge crystals were used as the internal reflection element (IRE), and the angle of incidence was varied within the range 35-60 OC to vary the penetration depth. (3) Electron Spectroscopy for Chemical Analysis (ESCA): The grafted copolymer composition of the surface region less than 10 nm from the surface was measured by ESCA, using the C(1s) area ratio. Solubility Measurement. Solubility coefficients of the membrane in eight kinds of liquids (benzene, cyclohexane, carbon tetrachloride, acetone,methanol, ethanol, 2-propanol, and water) were measured. The grafted copolymer compositions were varied, and the amounts of grafting were controlled between 1.6 and 1.9 mg/cm2.The membrane was immersed in a pure liquid at 25 'C for more than 20 hand then taken from the liquid and wiped with filter paper which had been soaked in the liquid beforehandtoremove thesurface freesolvent. The swollen membrane was weighed immediately, and the weight was plotted against time, with the value at zero time extrap olated. The degree of solubility was measured in terms of the solubility coefficient S (g of liquid/g of dry membrane). Pervaporation. The pervaporation apparatus was described elsewhere (Yamaguchi et al., 1991). The membranes, whosegrdtedcopolymercompositionswerevaried, and amounts of grafting were controlled between 1.9 and 2.5 mg/cm2, were employed. (1) Pervaporation of Single Liquid Systems: Pervaporation of a single liquid was carried out through the membranes. As feedliquid, benzene, cyclohexane,earbon tetrachloride, acetone, methanol, ethanol, and 2-propanol were employed,and the feed-liquidtemperature was fixed at 25 O C . (2) Permeation and Separation of Binary Liquid Systems: As feed mixtures (A/B), ethanoVcyclohexane, methanoY2-propano1,and methanovacetone50/50 wt % mixtures were employed. The separation performance was measured a t 25 "C in terms of permeation rate and separation factor 01 defined as a,,B = (Y(100- X))/((100- Y)X) (1) where Y and X denote the weight fraction of the A component in the permeate and feed, respectively. Results and Discussion MembranePreparation. (1) Grafting Results: The relationship between graftingtime and amount of grafting
850 Ind. Eng. Chem. Rea., VoL 32, No. 5,1993
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Figure2 RehtionshipbetweMgr&ing timeandamount ofgrafting (monomer concentration 5 wt mo;grafting temperature 30 W. IN I
F i y r e l . Transmissionelectronmicrophotographof the membrane moss section (amount of grafting 1.85 mg/crn2;poly(AArn) ratio in grafted copolymer 37.3 w t 7%).
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Figure 3. Relationship between AAm ratio in monomer and poly( M m ) ratio in grafted copolymer.
is shown in Figure 2. The amount of graftiig increased inporportiontograftiitimeinallmonomercompositions. Grafting rate decreased with increasing AAm ratio in monomer. Although in a system of poly(propy1ene)-graftcopoly(acry1ic acid/acrylamide) the grafting rate had a maximum point varying with the monomer composition through plasma-graft polymerization (Hirotsu and Nakajima, 1988),this system had no maximum point. Whereas the porous substrate was nontransparent, all of the grafted membranes obtained were transparent. These results suggest that pores of the substratefilm were filled with grafted polymer whose refractive indexes were nearly the same as that of the the substrate material. (2) Whole Composition of the Grafted Copolymer: The relationship between monomer composition and the grafted polymer compositioncalculatedfromthe N/C ratio and amount of grafting is shown in Figure 3. These data include about 10% standard errors in their estimation. Above an AAm ratio in monomer of 30 wt % , the poly(AAm) ratio in grafted polymer increases with increasing AAm ratio. Below an AAm ratio of 30 wt %, the grafted polymer is almost pure poly(MA), and not much AAm is grafted. The figure shows that the amount of grafting has no great effect on the copolymer composition below 2.5 mg/ cm2. The amount of monomer in the monomer solution was so great that the difference between monomer cornposition before and after grafting was negligible. These results showed that grafted polymer composition was controllable by varying the monomer composition.
Morphological Analysis. Although we already conf i e d that MA grafted polymer filled in the pores (Yamaguchi et al., 1991), we have to analyze the distribution of MA/AAm grafted copolymer and the difference of copolymer composition in the direction of membrane cross section. (1) Transmission Electron Microscopy (TEM): A TEM cross-sectional microphotograph of the membrane (amount of grafting 1.85 mg/cm*; poly(AAm) ratio in grafted copolymer 37.3 wt %) is shown in Figure 4. The poresofthesubstratefilmandthegaps betweenthecrystal regions of polyethylene were stained and appear black in the photograph; poly(MA/AAm) copolymer and crystah were not stained and appear white. As revealed in the previous paper (Yamaguchi et al., 1991), the TEM photograph of the substrate film was almost black. The graftedmembraneshowninFigure4hasfewerblackparts, andthemagnificationshowsthatthedarkpartacomespond to microfibrils of polyethylene, while the white parts correspond to poly(MA/AAm)copolymer. This indicates that graft polymerizationtmkplacein thesubstrate pores, and the pores were filled with grafted copolymer. (2) FT-IR Analysis: FT-IR/ATR spectra of the membranes are shown in Figure 5. The amount of grafting in these membranes is 0.54-1.09 mg/cm2, and the poly(acrylamide) ratios in grafted copolymer are 15.4, 37.3, and 58.5 wt 7%. ZnSe was used as IRE, and the angle of incidence was 45' in the spectra. The peaks at 1480,1670, and 1730 cm-' are characteristic peaks of polyethylene, poly(AAm), and poly(MA), respectively. Their wavenumbers are similar, and the penetration depths of the peaks are almost the same (polyethylene, 1.35 pm; poly(AAm), 1.20 pm; poly(MA), 1.16 pm).
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 861
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60
80
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Figure 7. Relationship between monomer composition and grafted polymer composition of the whole membrane and ita surface (amount of grafting 0.9-2.3 mg/cm2).
1
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1800
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1400
1600
Wave number [ern.' 1
Figure 5. FT-IR/ATR spectra of the membranes (poly(AAm)ratio is grafted copolymer: (a) 15.4 wt %; (b) 37.3 w t %; (c) 58.5 wt % (amount of grafting 0.56-1.09 mg/cm2). 100 grafled copolymer 0 15.4wtC
0 37.3 WC
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Figure 6. Relationship between penetration depth and poly(AAm) peak ratio in grafted copolymer from FT-IR/ATR analysis (amount of grafting 0.56-1.09 mg/cm2;the dotted line shows the peak ratio of poly(AAm) 37.3 wt % membrane from TR).
The peak ratio generally exhibits a correlation with the polymer weight ratio. The relationship between the penetration depth and peak ratio of the grafted polymer is shown in Figure 6. The peak ratio is defined by the following equation: peak ratio = lOOP,/(P, + Pz)
(2)
where P1 is the peak height of poly(AAm) and PZis the peak height of poly(MA). The penetration depths are varied by changing IRE or angle of incidence and are calculated as the value at 1700 cm-1. The dotted line shows the peak ratio from the TR spectrum of the 37.3 wt % poly(AAm) membrane. The TR spectra of the 15.4 and 58.5 wt % poly(AAm) membranes could not be measured quantitatively because the absorbances of poly(MA) and poly(AAm) were above 2. The peak ratios of these membranes were almost independent of penetration depth, and the result from the TR spectrum shows the same ratio as that from the
ATR spectrum. The results show that the grafted copolymer composition was homogeneousin the direction of the membrane cross section. The peak ratios rose consistently with poly(AAm) weight ratio. If the grafted polymer was formed only on the substrate surface,the thickness of 0.5 mg/cm2grafted polymer would be about 4 pm from the substrate surface,and no substrate peak could be detected. However, the spectrum shows a substrate peak, and the results demonstrate that grafted polymer formed in the substrate pores, as indicated by the TEM photograph. (3) ESCA Analysis: The relationship between monomer composition and surface copolymer composition determined from ESCA analysis is shown in Figure 7.The poly(acry1amide)to poly(MA) ratio was measured from the area ratio of the C(1s) peak. The whole-membrane compositions are also shown. Differences in grafted copolymer composition between the whole membrane and its surface are very small, in accordance with the results from FT-IR analysis. Solubility Behavior. Relationships between poly(AAm)ratio in grafted copolymerand membrane solubility for seven kinds of liquids are shown in Figure 8. Solubility with water increased along with poly(AAm) ratio due to the increasing hydrophilicity of the grafted copolymer. Solubility with substances whose Hansen solubility parameters are in the sphere of poly(MA),such as acetone, benzene, and carbon tetrachloride, lessened consistently with poly(AAm) ratio in grafted copolymer. The grafted copolymer had optimum copolymer composition for the solubility with substances whose parameters are between the sphere of poly(MA) and water, such as methanol, ethanol, and 2-propanol. Solubilitywith substanceswhose parameters are outside of the space occupied by the poly(MA/AAm) spheres as well as the interval between the two spheres, such as cyclohexanewith (ap, ah) values of (0, 0.11,showed low values regardless of the grafted copolymer composition. These results are in agreement with the prediction based on Hansen solubility parameters shown in Figure 1and suggest solubility control. Pervaporation Property. (1) Permeation of Single Liquid Systems: The relationship between permeation rate and poly(AAm) ratio in grafted copolymer is shown in Figure 9. Permeation rates of acetone and benzene decrease with increasing poly(AAm) ratio in grafted copolymer,and the cyclohexane rate showslow values over the entire range. The grafted copolymerhad an optimum copolymer composition for methanol permeability. The
852 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 2
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2-propanol acetone
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Figure 10. Results of pervaporation of ethanoVcyclohexanemixtures through the membranes. Relationship between grafted copolymer composition and permeation rate or separation factor.
0
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20
40
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Figure 8. Relationship between grafted copolymercomposition and solubility coefficient (liquid temperature 25 "C).
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20
40
60
80
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Poly(AAm) ratio in grafled copolymer [wt%]
Figure 11. Results of pervaporation of methanol/2-propanol mixtures through the membranes. Relationship between grafted copolymer composition and permeation rate or separation factor.
0
20
40
60
80
100
Poly(AAm) ratio in grafted copolymer [wt%l
Figure 9. Relationship between grafted copolymercomposition and permeation rates of single components by pervaporation (feed temperature 25 "C).
dependence of the permeation rate on the grafted copolymer composition shows the same tendency as the dependence of the solubility, as shown in Figure 8. Regarding the permeation rates of ethanol, 2-propanol, and carbon tetrachloride, the influence of the grafted copolymer composition also shows the same tendency as in the case of solubility, but the values are very low. The reason for this may be low diffusivity through the membrane. (2) Separation of Binary Liquid Systems: The relationship between copolymer composition in grafted polymer and separation factor or permeation rate for the case using ethanol/cyclohexane mixture as feed is shown in Figure 10. In the case of single-component permeations, the membrane has an optimum composition in which solubility to ethanol shows the maximum, and the solubility to cyclohexane remains negligible over the entire range of composition. The separation results show that both the permeation rate and separation factor have a maximum point in varying copolymer composition. The
graft copolymerized membrane with a poly(AAm)ratio in grafted polymer of 17 wt 76 shows extremely high ethanol selectivity because the membrane has only ethanol solubility in the feed, and a relatively low permeation rate is shown because both of the feed components have low solubility for the membrane illustrated in Figure 8. Pervaporation separation results for the methanol/2propanol mixture are shown in Figure 11. In the single liquid system, the membrane has the maximum point of solubility for both of the feed components,and the degree of solubilityfor methanol is higher than that for 2-propanol. Regarding single permeation,the membrane has a suitable composition for the permeability of both of the two components, and methanol permeability far exceeds 2-propanol permeability. . The membrane has optimum composition for both selectivity and permeability. At a poly(AAm) ratio of 17 wt %, methanol selectivity is extremely high, and the membrane also shows the maximum permeation rate. The membrane condenses methanol from 48.5 wt 5% feed to 97.8 wt 76 in permeate. In the case of the methanol/acetone mixture, pervaporation results are shown in Figure 12. Although the membrane also has suitable compositions for both selectivity and permeability, they differ from each other. The membrane solubility with acetone lessens with poly(AAm) ratio, and methanol solubility has a maximum point in varying the composition. For the above reasons, the
Ind. Eng. Chem. Res., Vol. 32,No. 5,1993 8S3
Nomenclature MA. methylacrylate AAm: acrylamide
a NE
1.0
3
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.-* C
0.5
&
0
50
100
HDPE high-density polyethylene IRE internal reflection element HSP: Hansen solubility parameters S: solubility coefficient [g of liquid/g of dry membrane] Sd, S,, ah: Hansen solubility parameter contributions from nonpolar interaction,permanent-dipole-permanenbdipole interaction, and hydrogen bonding, respectively [(cal/ cm3)0.5] a: separation factor P1: peak height at 1670cm-l in FT-IR spectra (characteristic peak of poly(AAm)) Pz:peak height at 1730cm-l in FT-IR spectra (characteristic peak of poly(MA))
Poly(AAm) ratio in grafted copolymer [wt%]
Figure 12. Results of pervaporation of methanol/acetone mixtures through the membranes. Relationship between grafted copolymer composition and permeation rate or separation factor.
membrane which has maximum solubility for methanol shows maximum selectivity for methanol, and the poly(AAm) composition which shows maximum permeability is below that at which methanol solubility is maximum.
Conclusions We reached the following conclusions. (1)HDPE-graft-MAIAAm copolymerized membrane was prepared by means of the plasma-graft filling polymerizationtechnique. The grafted polymer composition could be controlled by varying the monomer composition. (2) The grafted copolymer solubility could be controlled by changing the copolymer composition,and the solubility behavior was in accordance with the prediction based on Hansen solubility parameters. (3) The permeability of a single component by pervaporation could be controlled through the copolymer composition as well as solubility control. There was an optimum copolymer composition for both selectivity and permeability of a binary mixture system, and we found that an optimum pervaporation membrane for a separating system could be designed on the basis of solubilitycontrol.
Acknowledgment The authors thank Tonen Chemical Co. Ltd. for supplying the porous high-density polyethylene fiims, Toray Research Center for taking the TEM microphotograph, and Ms. Aiko Nakao of the Institute of Physical and Chemical Research (RIKEN) for ESCA analysis.
Literature Cited Barton, A. F. M. Correlation of Polymer Cohesion Parameters and Other Properties. In Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL 1985; pp 401-423. Cabasso, I. Organic Liquid Mixtures Separation by Permselective Polymer Membranes. 1. Selection and Characteristics of Dense Isotropic Membranes Employed in the Pervaporation Process. Znd. Eng. Chem. Prod. Res. Dev. 1983,22,313-319. Hansen, C.; Beerbower, A. Solubility Parameters. Kirk-Othmer Encyclopedia of Chemical Technology, 2nd ed.; Mark, H. F., Mcketta, Jr., J. J., Othmer, D. F. Eds.; Wiley: New York, 1971; Suppl. Vol., pp 889-910. Hirotsu, R.; Nakajima, S. Water-Ethanol Permseparation by Pervaporation through the Plasma Graft CopolymericMembranes of Acrylic Acid and Acrylamide J. Appl. Polym. Sci. 1988,36,177189. Ho,B.4.; Chin, W.-K.; Lee, Y.-D. Solubility Parameter of Polymethacrylonitrile, Poly(methacry1icacid) and Methacrylonitrile/ Methacrylic Acid Copolymer J. Appl. Polym. Sci. 1991,42,94106. Knox, B. H. Bimodal Character of Polyester-Solvent Interactions. 1. Evaluation of the Solubility Parameters of the Aromatic and the Aliphatic Ester Residues of Poly(ethy1eneTerephthalate). J. Appl. Polym. Sci. 1977,21,225-247. Mulder, M. H.V.; Kruitz, F.; Smolders, C. A., Separation of Isomeric Xylenes by Pervaporation through CelluloseEster Membranes J. Membr. Sci. 1982,11,349-363. Yamaguchi, T.;Nakao, S.; Kimura, S. Plasma-Graft Filling Polymerization: Preparation of a New Type of Pervaporation Membrane for Organic Liquid Mixtures. Macromolecules 1991, 24,5522-5527. Yamaguchi, T.; Nakao, S.; Kimura, S. Solubility and Pervaporation Properties of the Filling Polymerized Membrane Prepared by Plasma-Graft Polymerization for Pervaporation of Organic-Liquid Mixtures. Znd. Eng. Chem. Res. 1992,31,1914-1919. Received for review September 1, 1992 Revised manuscript received January 21,1993 Accepted February 3, 1993
