Pervaporation performance of oligosilylstyrene-polydimethylsiloxane membrane for separation of organics from water

iomtudof

MEMBRANE SCIENCE

ELSEVIER

Journal of Membrane Science 134 (1997) 209-217

Pervaporation performance of oligosilylstyrene-polydimethylsiloxane membrane for separation of organics from water a*

Wayne W.Y. Lau ' , Jennine Finlayson b, James M. Dickson b, Jianxiong Jiang c, Michael A. Brook c aDepartment of Chemical Engineering, National University of Singapore, Singapore, Singapore 119260 bDepartment of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ont., Canada LSS 4M1 eDepartment of Chemistry, McMaster University, 1280 Main Street West, Hamilton, Ont., Canada L8S 4MI Received 13 September 1996; received in revised form 14 April 1997; accepted 30 April 1997

Abstract A novel silicone rubber was prepared by crosslinking silylstyrene-oligomer containing -Sill groups with divinylpolydimethylsiloxane using Karstedt's catalyst at room temperature. Membranes cast from this silicone rubber were found capable of separating chlorinated hydrocarbons and aromatics from water containing trace amount of the organics through pervaporation (PV). Synthesis of this silylstyrene-oligomer, crosslinking it to divinyl-polydimethylsiloxane to produce the silicone rubber and preparation of membranes from this rubber are discussed. PV test results are presented.

Keywords: Novel silicone membrane; Removal of organics; Pervaporation

/OSiHez

I. Introduction Silicone rubber membranes prepared by crosslinking HO-terminated polydimethylsiloxane (PDMS) with chloro-oligosilylstyrene (COSS) [1-3] according to Scheme A below were found capable of removing trace organics from water by pervaporation (PV) [4]. The curing reaction in Scheme A was effected by moisture. Since moisture in the curing environment was difficult to control and HC1 was released during the curing reaction, the resulting membrane was thermodynamically unstable. In this study the chlorogroups in COSS were converted to hydride groups in Scheme B and an alternative silicone crosslinking *Corresponding author. 0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. 1-2

PII S0376-7388(97)0013

r

A

Silicone OSHeci] HOSil'4eTO/~]SiMeTOH

Olgomer"Si~(os~M: ,z; zn /

L

LiAIH~

B

,. [

E

OSiHe2H7

• 1.3 "~(0SHe~, / 0hg0mer Jn

Siticone "n u~ iMe~

"/S,~E,jO 1 L tuziHe2), n

0 gomer n= 2,3...

/ Silicone ~. Me2SiO OSiMe7

/OSiMe~

S~icone 0., -SiHe2 Pt- cotdys! .ISIS f 1 Oligomer [~(OSiMe2}.Jn

method involving platinum catalysed hydrosilation (Scheme C) was used, in which crosslinking takes

210

w.w.Y. Lau et al./Journal of Membrane Science 134 (1997) 209-217

place by a reaction between - S i l l of the oligomer and the vinyl groups in divinyl-terminated PDMS. This curing reaction proceeds at room temperature when catalysed by trace amount of Karstedrs catalyst. In this scheme, water is not a serious contaminant, and no toxic by-product is produced. Spacer arms of 10 carbon atoms in length were inserted into the oligomer to extend the hydride end groups farther into the reaction medium for more effective curing. Membranes were cast out of this novel silicone rubber and tested in PV experiments to remove organics from water. These membranes showed that they can separate methylene chloride, 1,2-dichloroethane, chloroform, toluene, p-xylene and cumene from water by pervaporation.

2.1. Preparation o f oligosilylstyrene

Trichlorosilylstyrene (TCSS Ph SIC13) prepared by a procedure reported elsewhere [1-3] was purified by distillation. Oligomerization of TCSS was catalysed by the addition of trifluoromethanesulfuric acid, TfOH (CF3SO3H), at - 1 5 ° C in the presence of trace amount of chloroform to yield COSS. Freshly distilled tetrahydrofuran (THF) was added to dissolve the dark red gel-like oligomer. _

CI / s f - cI

CI

Oligo-SiCI 3

Ph : ~ " cL,,.,,.L

s~ Cl

-'1-

mD 4

~--

t..,j,,,,,,,,sli [ cr"~L,

LJ~R ~ i" si-,,,-,cl ~c~ & ",,, Cl ~i Cl

CI .¢ s ' " cl "'-

c,

O ligo-Si(DmCI)3 m:0, 1, 2, 3,/, .... The purpose of this insertion of D4 spacer arms was to extend the functional groups farther into the reaction medium for more efficient crosslinking to take place.

2. Experimental

_

at 47°C allowed the insertion of D4 between -Si and -CI atoms in the SIC13 end groups through a slow reaction which took more than 48 h [3].

CF3SO3H

2.3. HSi-oligosilylstyrene

Oligo-Si(DmCI)3

H

""'"~ H

Jr- LiAIH 4

'~ H

H

H

Olig°-Si(DmH )3 TCSS

This conversion follows essentially the following stoichiometry: 4RSiC13 + 3LiA1H4 ~ 4RSiH3 + 3LiC1 + 3AIC13

P"-Y c,J, cr',~L CI CI

cJ cl

S'-c,

~, cl ~-"

cI

Oligo-SiCI 3 2.2. Modified COSS

Under the effect of TfOH, addition of octamethylcyclotetrasiloxane (D4) to the above reaction mixture

The quantity of LiA1H4 to be used can thus be calculated based on the amount of chlorine in the initial charge of TCSS. The required quantity of LiA1H4 was weighed out inside a dry box under a nitrogen atmosphere into a round-bottom flask containing a magnetic bar. In this experiment 13% excess LiA1H4 was used. Freshly distilled ethyl ether was added to disperse the LiAIH4 powder. The capped flask was placed in an acetone-dry ice bath at -78°C. The content in the first reactor, which held the initial

211

W.W.Y. Lau et al. /Journal of Membrane Science 134 (1997) 209-217

charge of TCSS and D4, was transferred using syringe and added slowly to the LiA1H4 at - 7 8 ° C with stirring under nitrogen. The flask content was finally allowed to settle to room temperature. The top liquid was carefully transferred to a separatory funnel. Distilled water was added to the funnel to wash by vigorous shaking. Gelatinous matter was formed and settled to the bottom, presumably AI(OH)3, LiOH and others which were removed. This washing was repeated until no more gelatinous matter was formed. Amount of this gelatinous matter would have been greatly reduced if only a slight excess of LiA1H4 was used. The oligomer was extracted from the above liquid portion using ethyl ether. This ether extract was purified first using a rotory evaporator at 100°C to remove ether, THF and water, and then by vacuum distillation. A sample of the purified oligomer was analysed. Its IR spectra confirmed the presence of - S i l l groups at wave number 2130 cm -1 and 1H NMR confirmed its presence at 4.7 ppm (JH-H 2.8 HZ) and 29Si NMR at - 6 . 5 ppm (Jsi-H 213 Hz).

ity in centistokes (cSt). In a typical A/B preparation with B1000 cSt, 1 g total mass of A and B (at various A/B ratios) was dissolved in hexane to make a 10 ml solution. Crosslinking began when a tiny drop (about 0.04 ml) of the Pt-catalyst was mixed into the solution. The solution was then poured into a glass petridish to allow the solvent to evaporate. This room temperature vulcanization (RTV) yielded a brown film in about 5 h. Postcuring the film in an oven at 60-80°C for few hours rendered the film less tacky. 2.5. Gel fraction

The postcured silicone rubber films were placed in sample bottles and soaked in 1,2-dichloromethane for one week. The solvent was drained off and the film samples were air dried at room temperature for several days. Loss in sample mass was determined. Mass A/mass B1000 cSt Sample mass, g Loss in mass, g Gel fraction

1/1 0.4525 0.0580 0.87

1/4 0.5471 0.0454 0.92

1/9 0.7062 0.0342 0.95

2.4. Crosslinking

.Silicone

Me2SiO/ "?SiMe= Olig°'Si(DnH )3

R-catalyst

(3-D rubber) Crosslinking experiments were carried out at various mass ratios of the HSi-oligosilylstyrene to divinyl-PDMS using Karstedt's catalyst. For convenience this HSi-oligosilylstyrene crosslinker will be referred to as A and divinyl-PDMS as B. B is available in different molecular weights indicated by their viscos-

With B100cSt no self-standing film could be formed in all the above A/B combinations. A/B1000 cSt formed films at A/B from 1/1 to 1/19. B5000 cSt and B10000 cSt also formed films at these combinations. 2.6. M e m b r a n e preparation

Membranes were prepared from above A/B combinations using the procedure described above except that here the curing (casting) solution was coated onto the surface of a piece of fiat polysulfone ultrafiltration membrane to form a composite membrane, with the UF membrane serving as a support. After a few hours at RTV the composite membrane was postcured at 6080°C for another few hours. The composite membrane was finally cut to size and mounted in a permeation cell for PV experiment. Two sets of PV data were collected, one set from membranes made of A/B in ratio of 1/4, 1/7 and 1/11 with B1000 cSt, B5000 cSt and B10000 cSt; the second set from A/B1000 cSt in the ratio of 1/4. Comparison between these two sets of membranes would show that the second set performed better.

212

W.W.Y. Lau et al./Journal of Membrane Science 134 (1997) 209-217

3. R e s u l t s a n d d i s c u s s i o n !r -- -- . . . .

Details of the PV test rig used in this study has been described elsewhere [4]. In essence it consists of three parallel permeation cells connected to a feed tank and their permeate sides connected to a vacuum pump. Each permeate stream is analysed by an on-line gas chromatograph and permeates are recycled. The whole system is controlled by a computer program which also does data-logging and data-reduction. Permeate samples are analysed after preset temperature and permeate pressure have been reached. The system is fully automatic and runs continuously according to an input algorithm. A schematic of this PV test rig is shown in Fig. 1. PV test data for results presented in Tables 1-4 were collected at a feed pressure of 5 psi. Controlled run temperatures and permeate stream pressures are included in these tables. The first set of data on the removal of 1,2-dichloroethane (1,2-DE) from feed water containing 100 ppm 1 , 2 - D E was collected from four different membranes:

Feed Mixture

II

~

_

_

a

,hlu

N6 .

_~ - ~

h

I I II III

.

.

. . . . . . .

J ~ I T - I

',,

,I i~ h .

T' .

IIt~ - --

.

_

t~_--_~Z

.

6

.

-7_

.

Z~_

.

_-_~.

rt.~ -n,

I-@

vopoor

15 f~

Li,r~l

. O~--

~ ..~,.I

rerrneme.

_-_-...-_-_7_-_=_~J

Z --_ Z Z ~

~ - --~

I"

I

I'II ~-~ ~ II .....

Cell

J

! I ~ i

i

!I

! I

~ ,

,

I

-" ----._--_Z._-__- Z - -

,

" , I

I

I

Z--J__J

Fig. 1. Schematic diagram of pervaporation apparatus. T Thermocouple, P - Pressure transducer, (1) water bath for temperature control, (2) membrane, (3) sintered metal plate, (4) pressure controller, (5) mass flow meter, (6) cold trap, (7) vacuum pump, (8) sample injector A, (9) sample injector B.

a commercially available silicone membrane made by GE, to serve as a basis of comparison, and three membranes made from A/B1000cSt, A/B5000cSt

Table 1 PV test results Tested at 30°C, 10 TorT

Tested at 30°C, 5 TOrT

GE silicone membranes Sample Film thickness, nun Selectivity Water flux, g/m2 h 1,2-DE, g/m2 h

1 0.3 1344 24.81 1.51

2 0.3 1394 17.97 1.08

3 0.3 1692 27.44 1.98

1 0.3 1472 28.00 2.20

2 0.3 1393 21.18 1.45

3 0.3 1743 30.96 2.89

HSi-oligosilylstyrene crosslinked PDMS membranes Mass A/mass B1000 cSt Coating thickness, mm Selectivity Water flux, g/m2 h 1,2-DE, g/m2 h

1/4 0.085 1233 54.68 3.94

1/7 0.09 1187 52.18 3.85

1/11 0.104 2098 15.79 1.84

1/4 0.085 1313 53.19 4.99

1/7 0.09 1205 54.56 4.95

1/11 0.104 2342 18.63 2.77

Mass A/mass B5000 cSt Coating thickness, mm Selectivity Water flux, g/m2 h 1,2-DE, g/mz h

1/4 0.05 1594 43.66 3.21

1/7 0.065 2089 24.74 2.33

1/11 0.084 1986 23.09 1.80

1/4 0.05 1728 53.09 4.90

1/7 0.065 2156 31.12 3.85

1/11 0.08 1894 27.46 2.73

Mass A/mass B10000 cSt Coating thickness, mm Selectivity Water flux, g/m2 h 1,2-DE, g/mz h

1/7 0.18 1609 17.09 1.70

1/11 0.16 654 85.39 4.07

1/15 0.165 2003 20.43 2.58

1/7 0.18 1723 21.83 3.14

1/11 0.16 618 92.08 5.23

1/15 0.165 2200 23.71 3.97

1,2-Dichloroethane ~120 ppm

Methylene chloride ~100ppm

Chloroform~170 ppm

Chloroform~500ppm

Chloroform~7OOppm

Chloroform ~ 4 0 0 p p m

Chloroform~250ppm

Chloroform~60ppm

C h l o r o f o r m s 2 5 ppm

Solute in feed water

30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C, 25°C, 25°C, 25°C, 30°C, 30°C, 30°C, 25°C, 25°C, 25°C, 30°C, 30°C, 30°C,

5Tog 10 To~ 15 T o g 5To~ 10To~ 15 To~ 5 To~ 10 To~ 15Tog 5To~ 10To~ 15To~ 5 Tog 10 To~ 15To~ 5 To~ 10To~ 15 To~ 5To~ 10 To~ 15 Tog 5To~ 10Tog 15 To~ 5To~ 10 To~ 15To~ 5To~ 10 Tog 15To~ 5To~ 10 To~ 15To~ 1137 1321 1428 1015 1141 1207

767 974 1053 825 974 1314 743 878 1113 858 998 1504 837 987 1069 807 950 932 963 1082 1055 1079 1186 1502 947 1063 42.95 31.29 20.84 58.66 47.81 37.68

62.84 54.40 62.99 49.18 34.27 25.05 62.92 54.62

47.51 49.56

42.78 37.52

47.55 45.42

57.15 51.26

71.21 55.06 52.48 68.77 56.21

5.69 3.42 2.12 7.23 5.15 3.45

6.10 4.42 3.75 4.98 3.12 4.37 7.85 5.49

21.46 9.83

25.51 21.43

18.66 9.39

11.60 8.35

1.52 1.00 0.81 3.76 2.55 1308 1650 1752 1590 1874 2448 1528 1843 2315 1545 1828 2042 1493 1643 2314 1455 1649 2090 1708 2060 2577 1747 1949 2260 1612 1701 2347 1700 1862 1837 1644 1707 1731

37.56 30.08 23.06 34.20 28.07 21.91 26.17 22.89 17.91 21.17 21.61 18.18 18.44 16.32 20.81 22.08 22.01 24.33 31.84 26.73 20.59 20.63 18.07 12.77 32.21 27.14 21.31 19.77 18.89 9.96 28.20 24.12 19.32

Water flux (g/m 2 h)

Selectivity t3

Organic flux (g/m 2 h)

Selectivity /3

Water flux (g/m2 h)

PV cell 2 (60 ~m coating)

PV cell 1 (22 ~ m coating)

Table 2 PV test results on removal of chlorinated organics from water

1.37 0.89 0.52 3.30 2.22 1.63 10.04 7.02 5.21 14.11 7.49 5.24 19.30 15.53 19.44 16.82 7.19 7.47 5.18 3.58 2.55 3.81 2.42 3.08 6.21 4.09 3.97 3.87 3.19 1.41 5.50 3.66 2.36

Organic flux (g/m2 h) 80.54 69.87 55.18 76.16 66.84 52.50 61.86 56.85 45.31 52.29 53.08 41.77 45.28 41.91 50.58 50.63 54.68 54.85 68.58 60.50 50.36 53.52 38.96 68.62 62.82 47.95 53.69 37.93 29.84 63.10 52.15 47.34

1041 1113 1643 1129 1376 1530 1114 1265 1308

Water flux (g/m2 h)

900 961 1130 902 1025 1458 973 1064 1492 962 1032 1516 923 1025 1319 909 1032 1253 1175 1313 1491 1157 1422

Selectivity fl

8.54 6.00 6.24 6.17 4.43 2.98 7.93 6.00 4.26

1.73 1.21 0.78 4.17 2.89 2.33 13.98 10.08 8.52 20.42 12.04 10.53 28.36 23.19 25.53 22.73 11.79 10.97 6.84 4.82 3.53 5.49 3.88

Organic flux (g/m 2 h)

PV cell 3 (20 I.tm coating)

q~o

"-.4

t-o

',o

Toluene ~ l l 0 p p m

Cumene ~ 3 0 p p m

p-Xylene ~ 3 0 p p m

Solute in feed water

25°C, 25°C, 25°C, 30°C, 30°C, 30°C, 25°C, 25°C, 25°C, 30°C, 30°C, 30°C, 25°C, 25°C, 25°C, 30°C, 30°C, 30°C,

5To~ 10To~ 15 To~ 5 To~ 10 To~ 15 To~ 5To~ 10 To~ 15To~ 5To~ 10 To~ 15 To~ 5To~ 10 To~ 15 Ton" 5To~ 10 To~ 15 To~ 1655 2282 3016 1386 1643 2255 1075 1356 1836 898 1299 1776 1730 1947 2612 2518 3011 3566

42.08 29.24 19.29 63.20 53.66 44.49 46.29 32.43 23.03 67.47 49.58 44.55 37.82 29.21 21.0 45.19 36.37 31.39

2.77 1.93 1.32 2.84 2.18 1.68 1.75 1.36 1.07 1.67 1.63 1.87 5.66 4.37 3.09 10.97 8.96 5.52

3303 4510 5636 2939 3370 4151 1901 2523 3413 1608 2514 2314 2702 3640 4685 3989 5777 6801

20.06 14.61 9.85 29.82 24.78 19.76 23.53 16.53 11.75 34.38 25.38 20.25 18.20 13.40 8.72 20.45 14.32 11.34

Water flux (g/m2 h)

Selectivity 3

Organic flux (g/m2 h)

Selectivity fl

Water flux (g/m 2 h)

PV cell 2 (60 ~ m coating)

PV cell 1 (22 ~ m coating)

Table 3 PV test results on removal of aromatics from water

2.44 1.80 1.18 2.63 1.92 1.31 1.67 1.24 0.95 1.67 1.61 1.30 4.68 3.47 2.08 7.87 7.37 5.53

Organic flux (g/m2 h)

2093 2683 3731 1605 2009 2364 1295 1490 2058 972 1474 1735 1624 2074 3029 2509 3052 3691

Selectivity fl

48.74 38.15 26.74 68.48 60.57 50.78 52.85 38.86 28.19 72.19 60.10 47.39 44.71 34.56 23.77 50.24 40.70 33.50

Water flux (g/m2 h)

PV cell 3 (20 ~ m coating)

3.52 2.69 1.98 3.30 2.51 1.83 2.26 1.72 1.38 2.12 2.23 1.94 6.91 5.10 3.63 12.18 11.07 8.82

Organic flux (g/m2 h)

vo

-~

~"

~"

~'~

"~

tO

W.W.Y Lau et al./Journal of Membrane Science 134 (1997) 209-217

215

Table 4 Water flux through the crosslinked PDMS membranes Water flux (g/m2 h)

Pure water feed data set 1

Pure water feed data set 2

25°C, 25°C, 25°C, 30°C, 30°C, 30°C, 30°C, 30°C, 30°C,

5 Torr 10 Torr 15 Tort 5 Torr 10 Torr 15 Torr 5 Torr 10 Torr 15 Torr

PV cell 1 (22 ~tm coating)

PV cell 2 (60 larn coating)

PV cell 3 (20 ~tm coating)

52.15 38.78 23.55 72.85 60.03 49.84 76.68 62.0

29.03 19.94 13.91 39.22 31.88 24.98 39.55 32.97 24.98

61.78 47.65 32.43 80.67 68.36 56.31 84.24 71.86 57.51

and A/B 10000 cSt at various A/B ratios. Results are summarised in Table 1, from which it can be seen that at 30°C and 5 Torr permeate pressure the GE silicone membrane removed 1,2-DE from feed water at rates less than 3 g/m 2 h while silicone membranes made in this study could remove 1,2-DE at rates between 3 and 5 g/m 2 h under the same experimental conditions. In general, membranes made from A/B 1000 cSt-1/4 performed better than the others. Also B1000cSt is fluid enough to flow at room temperature while the other PDMS of higher molecular weight are quite viscous for material transfer. Three membranes of different thickness were made from an A/B1000cSt-1/4 composition and extensively tested for the separation of a variety of organic compounds from feed water. This second set of PV test data is summarised in Tables 2-4. The thickness of the silicone coating on these membranes was determined

from Scanning Electron Micrographs (SEM) taken from cross sections of the membranes. One SEM is shown in Fig. 2. Membrane selectivity /3 is defined as / 3 = Xp(1 - X f ) / X f ( 1 - Xp) where Xp is mole fraction of solute in permeate and Xf is mole fraction of solute in feed. In this study feed to PV ceils was water containing trace amount (30-700 ppm) of the organics. /3 serves as an indicator of membrane selectivity for the organics. However, a more direct membrane performance indicator is the membrane flux (g/m 2 h) of organic solute through the membrane. In general, rate of permeation increases with increase in temperature, concentration of organic in the feed and lower permeate pressure, which are driving forces in this membrane transport process. The higher resistance of a thicker silicone membrane reduced flux and increased membrane selectivity, as shown in Fig. 3. The thicker

ing =olle Lrle Fig. 2. Scanning electron micrograph showing thickness of a silicone coating made from oligomer/PDMS1000 cSt = 1/4 by wt.

216

W.W.Y. Lau et al./Journal of Membrane Science 134 (1997) 209-217

7000 6000

~O00r

30"C 15 Torr

~0

o

[+ 3500[,~+

30 C

505TOrrpm

35 30

5000 J

in feed

~. 2500.

t,O00

~ +

~_~2000. "~ selectivity

---= 3000

"~

~,l.ene

2000

~20

500 20

I

l,O

-

of"

lOOO

Ppm1,2-0E ~

I

60

silicone membrane offered a higher resistance to water permeation and sorbed a larger amount of organics in it for transfer through a solution-diffusion mechanism. The permeation of water and organic molecules through the membrane are two competing processes, in which presence of one type hinders the passage of the other. Figs. 4 and 5 compare water fluxes when the feed was pure water alone to water flux when the water feed contained an organic solute. As the concentration of organics increased in the feed, more organic solute would be preferentially sorbed by the silicone coating. For this aromatics appear to exert a bigger effect than chloro-organics as indicated in Fig. 5. This can be explained, at least in part, as due to a greater affinity of the oligosilylstyrene membrane with the aromatics in view of the fact that the membrane also contains benzene rings in its chemical structure. The much higher selectivity for the aromatics (Table 3) lends further support to this view. For example at 30°C and 15 Torr permeate pressure, the 60 ~tm thick coating yielded for chloroform a # of 2577 while for toluene it was 6801.

,

l

,

123

Thickness (prn) Fig. 3. Membrane selectivity in relation to membrane thickness (oligomer/PDMS1000 cSt = 1/4).

S I

80

25_

/,,-

0

"--

c,oride

1000 1

~

•

,5oo"

0

w+eter flux .

I

2O N '

Y

o

15

i

0

gnmcflux

,

I

i

I

,

I

.

I

.

I

i

I

,

/, 5 6 7 8 9 1 0

I

mole fractionchloroform(xlOs/ in feed

ll

Fig. 4. Flux of water and chloroform through membrane made from oligomer/PDMS1000 cSt = 1/4.

60

30°C 50 pm coating ~0

A a.o E

30 in feed water

"~'~x -.,,,.+

~ 20

110p

f x ~ . ~ in feedwater

0

IIII'IIII

IIII'IIIII'

5

Permeate

I ' ' I ' I ' I I ' ' I I I I I '

10

15

II

20

Pressure(Tort}

Fig. 5. Effect of presence of organic solute on water flux through membrane made from oligomer/PDMS1000 cSt = 1/4.

W.W.Y Lau et aL/Journal of Membrane Science 134 (1997) 209-217

Before the PV cells were opened up, to prepare for other PV experiments, one final set of pure water flux data was collected. This final set (set 2, Table 4) appeared almost exactly the same as the initial set (set 1, Table 4), which was collected ten months earlier. This consistency in membrane performance strongly indicated that this oligosilylstyrene-PDMS coating, having gone through many experiments with chloro-organics and aromatic compounds in a period of more than ten months, remained intact chemically and physically in one integral whole.

4. Conclusions Silicone membranes made by crosslinking divinylpolydimethylsiloxane with a newly synthesized silylstyrene-oligomer containing -Sill functional groups are capable of separating chlorinated hydrocarbons and aromatics from water through pervaporation. These silicone membranes are chemically and mechanically stable.

217

Acknowledgements A sabbatical study grant from the National University of Singapore to one of the authors is greatly appreciated.

References [1] M.A. Brook, E Hulser and T. Sebastian, Oligo(trichlorosisyl)styrene: Highly functionalized silicone precursors, Macromolecules, 22 (1989) 3814. [2] M.A. Brook, P. Modi and J.M. Dickson, Silicon functionalized styrene polymers, Macromolecules, 26 (1993) 2624. [3] J. Jiang, M.A. Brook and J.M. Dickson, Substitution reactions at silicon under strongly acidic conditions: Ligand metathesis between methyltrichlorosilane and octamethylcyclotetra siloxane, Heteroatom chemistry, 5 (1994) 275. [4] C.K. Yeom, J.M. Dickson, M.A. Brook and J. Jiang, Development of crosslinked oligosisylstyrene pervaporation membranes for the removal of chlorohydrocarbons from water, ICOM-93 Proceeding, Heidelburg Germany, 1993, 5.11.