Pervaporation membranes for ethanol-water mixture prepared by plasma polymerization of fluorocarbons. II. Perfluoropropane membranes

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Journal of Membrane Sczence, 69 (1992) 109-120 Elsevler Science Publishers B V , Amsterdam

Pervaporation membranes for ethanol-water mixture prepared by plasma polymerization of fluorocarbons. II.* Perfluoropropane membranes Toshlo Masuoka, Takashi Iwatsubo and Kensaku Mlzoguchl Department of Maternal Desgn and Engtneermg, Research Instttute for Polymers and Textdes, l-1-4 Hgashs Tsukuba, Ibarakc 305 (Japan) (Received February 11,1991, accepted m revised form December 10,199l)

Abstract Pervaporatlon membranes for ethanol-water tnnary mixtures were prepared by plasma polymerization of perfluoropropane (PFP) The plasma-polymerized thm films were deposited onto porous polysulfone (PS ) filters as substrates with an average pore size from 0 1 to 0 45 pm By adding argon carrier to PFP system, fluonne/carbon elemental ratios (F/C) of the produced membranes evaluated by X-ray photoelectron spectroscopy (XPS), showed a maximum value, then shghtly decreased at lngher partial pressure of Ar This tendency was recogmzed more clearly by comparmg the summation of -CF, and -CF,peak area percentages based on a whole Cl, peak area As a measure of hydrophobiclty, tins value 1s more mtelhgible than the dmect F/C ratlo The influences of substrate pore-size, plasma-treatment time and hydrophobicity of the membranes on the separation capablhty were studied The ethanol-separation coefficient, ( (yEtow), of PFP membranes Increased slightly mth decrease of the average pore-size of the substrates, but treatment kme chd not apparently affect the cyEtOHWe classified the prepared membranes into two classes, 1 e the membranes showmg higher and lower permeation fluxes than 0 5 kg/m2-hr The (YEtoHwas more evidently observed to increase mth membrane hydrophoblclty for the former class of PFP membranes We suggest that, at least, three separation schemes might be necessary to understand the correlations found m each group Keywords composite membranes, liquid permeablbty and separations, membrane preparation and structure, pervaporatlon, plasma polymenzed membranes

Introduction Recently, the development of pervaporation membranes, particularly for ethanol-water mixtures, has been attractive to many memCorrespondence to To&no Masuoka, Department of Material Design and Engmeenng, Research Institute for Polymers and Textiles, l-l-4 I-hgasln, Tsukuba, Ibaralci 305, Japan *Part I has been pubhshed as Ref [ 131

brane scientists because of the industrial lmportance [l-lo]. On the other hand, the lowtemperature plasma (LTP ) technique has been applied to many thin film fields, such as electronic devices, medical materials, and separation membranes, as well [ 11,13,14] LTP 1s normally generated by electric glow discharge m low-pressure gases. Under lowpressure conditions, orgamc vapors or monomers can be decomposed and excited into re-

0376-7388/92/$05 00 0 1992 Elsevler Science Pubhshers B V All rights reserved

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active species such as ions, electrons, radicals, metastables. These species eventually produce orgamc polymer deposltlon on the surfaces of the various solid substrates exposed to plasma. This process is designated as plasma polymerization. Substantially, any organic compound, or monomer can be plasma-polymerized, as far as it can be vaporized. So the plasma polymenzation method can simply be considered as a membrane preparation method with almost perfect freedom. However, it must be kept in mind that plasma polymerization can only produce thin films, which may not exhibit separation capability without substrate membranes. The deposited thin films are often sensitive to destruction due to the insufficient mechanical strength. There are also some hmltations concernmg the quality of the thm films. First, plasma polymers do not always display homogeneous and stable forms. Under specific reaction conditions, formation of powdery or oily polymers has been frequently reported [ 151. Second, it is difficult to predict the chemical structures of the polymers formed before the experiments are performed. To pick up one particular choice from a wide range of reaction conditions, including monomer selection, general behavior of plasma polymerization should be understood Third, membrane properties are also subject to aging phenomena, probably due to reactive groups such as polymer radicals and peroxides built up during deposition of the polymers. The half-life period has been observed to be days or even months [ 111. In spite of such limitations the applicability of the technique is still enormous, not only for preparation of separation membranes but also m many other industrial fields as mentioned above. In this study, we mvestigated perfluoropropane (PFP ) plasma polymerization and membrane preparation to evaluate the effects of de-

T Masuoka et al /J Membrane See 69 (1992) 109-120

properties on PV membrane posited performance for ethanol-water mixture separation. PFP plasma-polymerized membranes were obtained under various conditions, with or without argon carrier, on membrane substrates having micropores of different average sizes. The PFP films may fulfill extreme cases without any hydrophilic structures, which were always mcorporated mto membranes deposited from a mixed monomer system [ 131. It may thus be interesting to see if the correlation between membrane hydrophobicity and permselectlvity for ethanol found m the mixed monomer system 1svalid also for the PFP system Experimental Mater&s and membrane preparatzon As substrates for plasma polymerized thin films, polysulfone (PS) filters produced by Brunswick Co were used with average pore sizes of 0 1,0.2 and 0.45 pm. These substrates have asymmetric structures, m which a relatively dense layer of ca. 0.1 pm thick on one side is supported by a spongehke structure of ca 50 pm thick. Plasma polymerized films were deposited on the dense skin layer of the substrates PFP and argon gases were purchased from Matheson Co. and Nippon Sanso Co., and used without further purification. Figure 1 shows the schematic diagram of the tubular-type LTP reactor used for plasma polymerization of PFP. This apparatus was made of Pyrex glass, and the sizes were 360 mm long and 70 mm l.d., large enough to treat the PS substrates LTP was excited by 13.56 MHz-radiofrequency power supplied to aluminum outer electrodes wound around the upstream side of the reactor tube The outer electrodes were 1 cm wide, and the separation gap was 4 cm. The monomer and carrier gases were supphed into the plasma region from the upstream side of the electrodes through a fretted glass disc. A PS substrate was placed at the downstream side, the surface of which was perpen-

111

T Masuoka et al /J Membrane SCL 69 (1992) 109-120

To pumpmg system

To RF generator

aner

Substrateholder (Al)

qas

Electrcdermgs

Fig 1 Plasma polymenzatlon reactor with radiofrequency (RF) outerelectrodes

&cular to the gas stream to obtain homogeneous deposition, especially in the central region of the substrate. We usually exposed a substrate to PFP plasma for more than two hours to achieve complete coverage with a polymerized film of sufficient thickness. The PS substrates apparently exhibited no deterioration m mechamcal strength, even after such long plasma treatments.

TABLE 1

Characterrzatzon of membranes X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) were used to characterize the plasma polymerized film. In XPS experiments, Shimadzu’s model 750 spectrometer with Mg (Ka) X-ray source was used and deconvolution analysis of C1, peaks as well as normal elemental analysis were performed to study the detailed structures of the plasma polymers. We assumed that C1, peaks consisted of the five component peaks listed in Table 1 The FTIR spectrum of a deposited membrane on a substrate was obtained by PerkmElmer’s model 1800. Absorption peaks from the

List of XPS C1, component peaks with assignments assumed for deconvolutlon analysis Bmdmg energy (eV) Assignments 295 4 292 0

-C*F3 -C*F2-CF2-

287 9

-b’F-+

286 5

_ *_ F,_ +c’

285 0

- *_ ++

*The carbon atom correspondmg to the C,, peak at the bmdmg energy listed

PFP polymer could be separated from peaks of the PS substrate by a difference spectrum technique and an attenuated total reflectance (ATR) method using a KRS-5 element of 45 ’ reflective angle To observe the deposition layer, scanning electron microscope (SEM), was used (S-500A model, Hitachi Electric Inc.) Deposited membrane thickness was measured by SEM, weight method or multiple internal reflectance mterferometry. The inter-

112

ferometer used is the model-2 system by Mizojlri Optics Inc. In this experiment, membranes were deposited on a glass shde instead of the PS substrates. In the weight method, an alummum foil substrate was used to measure the weight of deposition Measurement of pervaporatron performance PV performance was mainly measured for ethanol aqueous solutions at 40’ C by usmg the apparatus schematically shown in Fig. 2. The volume of the feed solution reservoir was about 100 cm3, and the diffusion area of a test membrane was 9.8 cm2 The apparatus is evacuated alternately through two liquid-nitrogen traps m parallel with a mechanical pump. The test solution was agitated by a magnetic stirrer be-

Fig 2 Pervaporatlon (PV) test apparatus (A) feed solution vessel, (B) porous .&unless disc, (C) PV membrane, (D) magnetic stirrer, (E) glass stopper, (F) inlet for feed solution, (G) tube connected wth liquid mtrogen traps and pumping system

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neath a constant temperature bath. The permeate was condensed m a trap, weighed to estimate the permeation flux (kg/m2-hr), and mjected into a gaschromatography (GC) to measure the ethanol separation coefficient ( %tCM ) The GC column used was 3 m long, packed with Porapak-PS The c~zt0i-iwas calculated from the followmg equation, aEtOH

=

[ YEtOH/Yw

I/ [xEtOH/xw

1

where X and Y were weight percentages of ethanol (EtOH) or water (w) in the feed solution and permeate, respectively Results and discussion General behavror of hexafluoroethaneallylumme system In our previous studies, we found that the deposition rate of hexafluoroethane (HFE) polymer was much accelerated by addition of hydrogen or hydrogen-containing compounds [ 11-131 Without hydrogen, no substantial deposition can be observed due to etching by the HFE plasma itself By addmg allylamme (AA) to the system, the deposition rate also increased [ 131. The fluorme/carbon elemental ratio (F/C) of the plasma-polymerized membrane tends to mcrease with AA addition. However, the possible maximum (2.0) of the F/C ratio for a polytetrafluoroethylene-like structure could not be achieved. The real maximum observed was only ca. 13. This was explained by incorporation of AA segments into a fluorocarbon chain to constitute an mcongruous combmation of hydrophilic and hydrophobic structures. Since a clear correlation between hydrophobicity, i.e the F/C ratio and the aztou values was recognized, the existence of hydrophilic AA segments is not desirable for ethanol separation. Another important characteristic of HFEAA polymers, revealed by SEM, was that the

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deposltlons were not always homogeneous. A particle-like internal structure could be often observed, although the membranes apparently looked homogeneous Formation of particle-like structures has been mainly reported for unsaturated monomers and is usually explained by a gaseous polymerization mechanism [ 11,151. Such heterogeneous deposition should be avoided to prevent crack formation due to mternal stress, which can be detected by membrane curling [ 111 Thus it 1sinteresting to search for a possibility to further increase the F/C ratio, i.e. improvement of ~~zton,by using only hydrophobic monomers like PFP. PFP was found to form a quite homogeneous and flexible membrane Plasmu polymerlzatton of perfluoropropane PFP) The PFP monomer does not contain hydrogen, nitrogen, or oxygen Thus the PFP plasma may be expected to form more hydrophobic films than HFE-AA plasma. PFP can also be polymerized faster than HFE, smce the untial steps of fluorocarbon polymerization are considered to be the bond fissions of C-C and C-F in a monomer molecule [ 111. The typical deposition rate of PFP at 75 PmHg, 40 W (R.a&o Frequency, RF, power ) was 20 A/min. It was measured by a weight method using an aluminum foil substrate This value was, at least, one order of magnitude slower than that of the HFE-AA system [ 131 It is curious that the deposition rate estimated by using PS substrate was 180 A/mm, rune times faster. Since SEM observation did not confirm such a phenomenon, the polymer substrate might be fluormated by the PFP plasma. In fact, plasma surface fluorination of polyethylene has been already reported by e g. Anand et al [ 161. Thus the weight of the fluorme atoms chemically linked with the substrate carbons may be much higher than that of real

polymer deposition. By XPS, fluorine could be detected even on the other that surface of the substrate that had not been m direct contact with the PFP-plasma. Therefore, it may be concluded that fluorine species may diffuse even mto the inner surfaces of the porous substrate, probably due to the slow membrane growth. At higher monomer pressure, we added argon as a carrier gas to help the expansion of the LTP region. Figure 3 shows the influence of this addition on the PFP polymer structures An mcrease of the F/C ratio was clearly observed with the increase of argon partial pressure. Argon may also act as an energy absorber and thus reduce undesirable monomer decomposition. Thus ethanol separation performance should be improved, provided that the correlation between membrane hydrophobiclty and (~zton found m HFE-AA plasma-polymerization holds.

PFP partial

0

0

100 UmHq

l

200 umHg

1 Partial

pressure

Pressure

0

2 0 Ratlo

(Ar/PFP)

Fig 3 Effect of argon carrier on structure of plasma-polymerized PFP analyzed by XPS C( -CF,+ -CF,-)% = summation of -CF, and -CF,- peak area percentages based on total C,, peak area, F/C = fluorine/carbon elemental ratlo, polymenzed at 40 W (13 56 MHz-RF power)

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Frg. 3. We also calculated the summation of -CF, and -CF2- peak area percentages [C (-CF, + -CF2-) % ] based on total C1, peak area. These peaks are representative of the most hydrophobic structures m fluorocarbon polymers. The Z(-CF3+-CF,-)% values were m the range of 47 to 60%, higher than the maxlmum value, of ca 40% for the HFE-AA polymers. These results show that the PFP polymers were certainly more hydrophobic.

Churactewatron of PFP membranes The elemental and deconvolutlon analyses by XPS were carried out for plasma-polymerized PFP membranes. Generally speaking, when complete coverage with a PFP polymer was achieved, the elemental ratio F/C was higher than that of a HFE-AA membrane, as predlcted. The F/C values were distributed from 16 to 1.8 m a relatively narrow range, as shown in

(A)

I

3000

I

2000

I

I

1000

1500

Wave Number

(cm

-1

400

)

Fig 4 FTIR spectra of PS substrate surface treated by PFP plasma (A) untreated PS, (B ) treated for 2 hr, (C ) difference spectrum [ = (B)- (A)] (PFPpolymer)

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115

Fig 5 Plasma-polymenzed PFP membranes observed by SEM (A) untreated PS substrate (pore size = 0 45 pm) Preparatlon condltlons 75 hmHg PFP pressure, 40 W Treatment tune (B) 30 mm, (C) 2 hr, (D) 4 hr

As shown in Fig. 4, FTIR spectroscopy gave relatively simple absorption spectra. Polar groups such as ammo and mtril groups, which could be observed m HFE-AA polymers, were not found. Only an intense C-F stretching (at ca. 1250 cm-l) and other fluorocarbon-chainrelated bands (at ca. 750 cm-l ) were detected Therefore, all the spectra of PFP polymers obtamed under various reaction conditions gave quite similar features. Figure 5 shows SEM photos of PFP plasma-

polymerized films on PS substrate. The depositlon layer apparently provided a smooth surface. The mternal stress of such membranes was thought to be negligible, because no membrane curling was observed [ 111. After a 4-hr treatment, no more pores were visible on the substrate surface, as shown in Fig. 5 (D). Pervaporatwn performance of PFP membranes Deposition time of PFP polymer did not effeet separation performance, as shown m Fig.

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0

1 2 4 5 PFP plasma treatment tune (hr)

6

Fig 6 Effect of deposltlon time on separation performance for 4 8 wt % ethanol solution at 40°C PFP pressure = 75 PmHg, 40 W, 2 hr deposltlon on PS substrate of 0 45 pm pore size

/

“”

Substrate pore

0

size

Membrane SCL 69 (1992) 109-120

(urn)

50 100 Ethanol concentration Of feed (wt %)

Fig 7 Effect of substrate pore size on separation coefficient ( aEtoH) of plasma-polymenzed PFP membranes for ethanol solutions at 40” C

6 In this case, the average pore size of the PS substrate was 0.45 ,um,and relatively large. This result demonstrates the unique characteristics of the PFP membrane, i.e. high permeation flux,

40

I 0

5 Separatmn

cceffnxent

10

(aEtOH)

Fig 8 Relatlonshlp between chemical structures of plasmapolymenzed PFP membranes onto 0 1 pm pore size substrate and separation coefficient (o!atoH) for 4 5 wt % ethanol solution at 40 ’ C

although the substrate pores were completely covered by the deposited layer, as observed by SEM (Fig 5) Considering the correlation between c&ton and F/C ratio found m the HFEAA system, the (~ston values were not as high as expected The deposition rate of the HFE-AA system is much higher than that of the PFP system Thus we could safely use the PS substrate with the larger pore size of 0.45 pm. Previously, we thought that to support a thinner deposition layer, the substrate pore size should be smaller, especially for the plasma-polymerized PFP membranes. Figure 7 shows that the c~ston was found to be slightly higher for the PFP membranes prepared on the smaller poor-size substrate. Such tendency could not be found in the HFE-AA system, in which the deposition layer was thick enough to cover the substrate pores. The thickness of the PFP deposition was ca. 0.64 pm. This may be enough to cover 0.45 pm pores, but may,

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TABLE 2 Pervaporatlon properties of plasma-polymerized methods

PFP” membranes compared with other membranes prepared by similar

Material

Preparation method

PFP/PSb

Plasma polymerization (PFP with Ar carrier) Plasma polymerization (mixed monomer system) Plasma polymerization (stepwlse deposltlon ) Plasma polymerization Plasma polymerization Coating with slhcone 011by plasma polymerization

HFE-AA' / PS HFE/AA/PS TFEd/PS PFP-TFE/PP” Siloxanesf/PPp

J (kg/m’-hr)

Reference

70

03

This work

60

06

U31

0 l-+05’

15+07

P31

02 05 0 015

1181

%*OIih

70 15 o-t11 01 18 Ok

WI

WI

“Perfluoropropane monomer bPolysulfone substrate ‘Hexafluoroethane-allylamme system dTetrafluoroethylene monomer “Polypropylene substrate [ 141 fHexamethyltnslloxane monomer %Nodetailed descnptlon m the literature hPerformance for 4 8 wt % ethanol solution at 40°C ‘Increased with HFE-plasma treatment tune JImtlal + steady-state values for the same ethanol solution but at 28°C kFor ca 4 wt % ethanol solution at 25°C

however, not be stable enough as an active layer for a PV membrane As ethanol concentration increased, c~xtondecreased to one at ca 50 wt.%. Probably, the PS substrate itself may be damaged by high concentration of ethanol. In Fig. 8, mcluding data from PFP membranes prepared under various comhtlons by using 0.1 pm pore-size substrates, (~xton for a 4.8 wt.% solution is plotted agamst Z (-CF,+ -CF,-) % from Ci, peak deconvolution by XPS It is not clear whether the correlation between F/C ratio and axton exists In contrast, the deconvolution data exhibit pretty dlstmct tendency for membranes exhibiting higher flux than 0 5 kg/m2-hr (shown by open circles). It may be suggested that the membranes containmg more -CF3 and -CF2- groups show higher permselectivity. Table 2 shows a comparison of PV membranes for ethanol/water mixture separation

prepared by similar plasma polymerization methods. It is mdicated that a higher permeation rate while maintaining a high axton may be characteristic for plasma-polymerized PFP membranes. Mechanism of ethanol enrtchment by plasmapolymewed fluorocarbon membranes Through the results mentioned above we have recognized the importance of hydrophobic structures in the membranes prepared by fluorocarbon LTP We should again emphasize that plasma polymerized membranes may form a suitable model system to establish the structure-separation relationship for various chemical structures. As a measure of hydrophoblclty the elemental ratio F/C of a membrane surface is not always reliable. Particularly for highly hydrophobic membranes, like plasma-polymerized

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For PFP plasma-polymerized membranes, there are three possible permeation mechanisms, (a) through the deposited polymer bulk, (b) through hydrophobic pores with bulk hquid flow, and (c) through smaller pores or gaps m the deposited layer without bulk liquid flow. These mechanisms are described in Fig. 9. Solution-diffusion, pore distillation, and surface diffusion models may correspond to these mechanisms, respectively. Mechanism (a) should dominate for membranes with a thick deposition layer like HFEAA membranes. However, mechamsm (c) should also have some contribution m cases where the deposition layers have heterogeneous structures. It should be noted that PFP membranes with a lower flux, exhibited relatively lower ~~Et0i-i values, m the range from 2.0 to 3.0, which were not as high as expected from correlation with the values found in HFE-AA polymers [ 131 In this case, mechanism (a) may be predominant. Despite the higher affimty between the membranes and ethanol, permselectivity may not be improved due to the higher mobility of the small water molecules through the membranes [ 91. The most attractive cases are PFP membranes showmg high axton and flux values, simultaneously. It may be concluded that the high ~~Et0i-iof PFP membranes can be attributed to

PFP membranes, X (-CF, +-CF2-) %, which reflects the hydrophobic structures more directly, should be used It has been thought that separation can be achieved by differences in either solubihty and/ or diffusivity in/through a membrane arising from a difference m size or structure of the permeates [ 4-61 Hydrophobic membranes prepared by fluorocarbon plasmas may absorb ethanol more preferentially, eventually showmg higher separation coefficients. Such a selective interaction should become stronger with membrane hydrophobicity [lo] Some PFP membranes exhibited unusually high permeation rates, more than 6 kg/m’-hr, with axton still bemg larger than 4 Therefore, for ethanol-permselective membranes, a different mechamsm, a kmd of distillation through the small pores of a membrane, should not be precluded, even though such membranes were found to be very hydrophobic and impenetrable to a dilute ethanol solution. However, conventional porous hydrophobic membranes, such as polytetrafluoroethylene and polypropylene filters, did not show such high (~Eton values as observed for the PFP membranes. The axton values were less than 4.0 under the same separation conditions. The results suggest that some specific mechanisms should be considered to explain the higher axton of PFP membranes Solution bulk _-----mm--

[al

Lb1

[cl

Fig 9 Separation schemes of plasma-polymerized membranes [a] through the deposited polymer bulk, [b] through drophobic pores with bulk flmd flow, and [c] through smaller pores or gaps m the deposltlon without bulk fluid flow

hy-

T Masuoka et al /J Membrane Scz 69 (1992) 109-120

mechanism (c) operating with mechanism (b). If so, there must be some optimum pore size suitable for separation according to scheme (c ) [ 181. The situation at the membrane-liquid interface may be somewhat similar to the preferential sorption model proposed for reverse osmotis membranes [ 171 In all mechanisms, it is obvious that separation performance should be limited by mass transfer phenomena and/or vapor-liquid equilibrium. For plasma-polymerized membranes, the importance of the porous structures of the substrate as well as the membrane surface properties should be recognized Conclusion Ethanol-permselective membranes prepared by PFP plasma polymerization with or without argon carrier gas were studied and the followmg conclusions were obtamed (1)By using PFP monomers, the resultant deposited polymers were more hydrophobic than the HFE-AA polymers previously reported. (2) To compare the hydrophobicity of PFP membranes, the summation of -CF3 and -CF,peak area percentages based on the whole area of the C1, peak by XPS gave a more mtelhgible measure than the direct elemental ratio F/C (3) The separation coefficient of the membranes showmg a higher flux than 0.5 kg/m’hr tends to increase with the hydrophobicity (4) The other class of membranes showing low flux values, and in which such tendency was not evident, may correspond to membranes covered completely with PFP polymer deposition. References 1

S Klmura and T Nomura, Pervaporatlon of orgamc substance water system with .&cone rubber membrane, Maku (Membrane), 8 (1983) 177

119 A Wenzlaff, K W Boddeker and K Hattenbach, Pervaporatlon of water-ethanol through Ion exchange membranes, J Membrane Scl ,22 (1985) 333 J Neel, Q T Nguyen, R Clement and L Le Blanc, Fractlonatlon of binary hquld mixture by contmuous pervaporatlon, J Membrane Scl., 15 (1983) 43 M H V Mulder and C A Smolders, On the mechamsm of separation of ethanol/water mixtures by pervaporation I Calculations of concentration profiles, J Membrane Scl ,17 (1984) 289 5 M H V Mulder, A C M Franken and C A Smolders, On the mechamsm of separation on ethanol/water mixtures by pervaporatlon II Experlmental concentration profdes, J Membrane Scl ,23 (1985) 41 M H V Mulder, T Franken and C A Smolders, Preferential sorption versus preferential permeablhty m pervaporatlon, J Membrane Scl ,22 (1985) 155 M H V Mulder, J Oude Hentikman, H Hegeman and C A Smolders, Ethanol-water separation by pervaporatlon, J Membrane Scl ,16 (1983) 269 E Nagy, 0 Borlal and A UJhldy, Membrane permeation of water-alcohol binary mixtures, J Membrane SCl, 7 (1980) 109 H J C te Hennepe, D Bargeman, M H V Mulder and CA Smolders, Zeohte-filled slhcone rubber membranes Part 1 Membrane preparation and pervaporatlon results, J Membrane Scl ,35 (1987) 39 10 K Ishlhara and K Matsm, Pervaporatlon of ethanolwater mixture through composite membranes composed of styrene-fluoroalkyl acrylate graft copolymers and cross linked polydlmethylsdoxane membrane, J Appl Polym Scl ,34 (1987) 437 11 H Yasuda, Plasma polymenzatlon, Academic Press, New York, NY, 1985 12 T Masuoka and H Yasuda, Plasma polymenzatlon of hexafluoroethane,J Polym Scl , Polym Chem Ed, 20 (1982)2633 13 T Masuoka, T Iwatsubo, S Hongyou and K Mlzoguchl, Pervaporatlon membrane for ethanol-water mixture prepared by plasma polymermatlon of hexafluoroethane-allylamme system, Kagaku Kogaku Ronbunsyu, 16 (1990) 447 14 T Masuoka, 0 Hlrasa, Y Suda and M Ohmshl, Plasma surface graft of N,N-dlmethylacrylamlde onto porous polypropylene membrane, Radlat Phys Chem ,33 (1989) 421 15 H Kobayashl, A T Bell and M Shen, Formation of amorphous powder during the polymerlzatlon of ethylene m a radio-frequency discharge, J Appl Polym Scl ,17 (1973) 885 16 M Anand, R E Cohen and RF Baddour, Surface modlficatlon of low density polyethylene m a fluorine gas plasma, Polymer, 22 (1981) 361

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120 17

18 19

S SounraJan, The mechamsm of demmerahzatlon of aqueous sodium chlonde solutions by flow, under pressure, through porous membranes, Ind Eng Chem Fundam ,2 (1963) 51 T Masuoka, T Iwatsubo and K Mlzoguchl, to be pubhshed, J Appl Polym Scl T Masuoka, M Suzuki, T Iwatsubo and K MIZO-

20

guchl, Pervaporatlon membranes by plasma surface treatment for separation of organic volatiles m water, Prepr 3rd SPSJ Int Polym Conf , Nagoya, Sot Plym Scl , Japan, 1990, p 50 T Kashlwam, K Okabe and K Oklta, Separation of ethanol from ethanol/water mixtures by plasma-polymerized membranes from slhcone compounds, J Membrane Scl ,36 (1988) 353