Journal of Scientific & Industrial Research Vol. 62. July 2003 , pp 666-677

Prospects of Siloxane Membrane Technology for Gas Separation - A Review

B S R Reddy* and U Senthilkumar

Industrial Chemistry Laboratory, Central Leather Research Institlltc, Adyar, Chennai 600020

. Sil oxane membrancs havc b een . long utili zed industri all y lor the separati on 01 gas mi xtures due to I!!; very high PCI meatl on rate. But thc selec tivity 0 1' thiS membrane tor a parti cul ar gas mi xture is very low. In order to impro ve it s selec!i vil Y value wilhout drast icall y reducing its permeation rate, lot of literature based on the chemicil l structure modilicil tions is avai lable. These modifi cations often leild to an improvement 01 selectivity value at the cost 01 permeability value .. or. vlcc-versa. Thi S rev lcw dcals with the factors affecting the permeability value and an IIp-to-datc chemica l lllotlili catlO n 01 siloxa nc membrancs and its transport parameters.

Keywords: Siloxilnc mcmbrane. Gas separat ion, Transport parametcrs

Introduction

r~ : : "-,,;; The . di scovery of vulcani zati on by Goodyear in ~ . 1839 transfo rmed natura l rubber into an essential

World wide rubber commod ities, such as balloons, bicyc le tyres , auto , tyres, etc., led to the puzzling

. observa ti on of air loss from infl ated rubber containers in the absence of any detec tab le wa ll defects. Grahan1 I .. had i'eported that rubber is pe rmeabl e to d iffe rent gases in diffe rent degrees and is independent of gas viscos ity . Many additi onal aspec t of research on gas and vapour permeati on through rubbers, whose princ ipa l obj ec tives were to understand the cause of gas leakage and fi nd means to e liminate it , were conducted be tween 1877- 1945. The research work carri ed out during thi s pe riod es tab li shed that rubber was both gas / vapour permeabl e and selecti ve, and it a lso la id the ground work fo r our understanding of the mechani sms and kine tics o f mo lecul ar transport through po lymers]--'. The past decade has seen the rapid deve lopment o f important laboratory and industri a l app l icat ions of pe rmse lec ti ve synthe ti c polymeri c membranes fo r mo lecu la r separations. Cryogen ic di still ati on, adso rpti on, and c hemica l separati on are well es tab li shed technoiogies in thi s fi e ld . More recentl y, membranes have j o ined the li st o r avai lab le techno logies for the sepa rati on o f gases. Me mbrane separat ion offers the specific advantage o f low e nergy use. s implic ity , ease o f operati on, limited

" AUl ho r lor co rrcspondencc

space requirements as we ll as eco no mic benefits. Although membrane separat ion is not expected to complete ly rep lace the es tab li shed tec hnol ogies, it offe rs signi ficant benefits and wi ll be a competitive system in specific ex isting applicati ons as we ll as in new applicati ons .

The chemica l s tructures of the se po lymeric membranes range fro m simpl e hydrocarbons ( li ke po lyethylene or po lypropy lene) to po la r struc ture (like po lyamides) or ionic structures in w hic l ca ti ons (or) ani ons are attached to the back bone. Performance of these membranes depends upon the phys ioche mi cal interactions ( ionic inte rac tions, dipo la r inte racti o ns and Vander Waal fo rces) between the pe rmeati on spec ies and the me mbrane mate ri a ls'i. T he potential application of a po lymeri c membrane for gas separati on depends on two paramete rs, viz. pe rmeability, the ab ility of gaseous mo lecule to pass through the membrane, and se lectivity, the ability of the membrane to se lec tive ly a ll ow a particul ar gas mo lecule to pass through it from a mi xture o f gases. For commerc ia l application, these two paramete rs should be as large as poss ibl e, however, majo rity of the polymeric membranes show a typ ical trade off behav iour6

•7 i. e., hi gh permeability v~ l u e combi ned

with low selec tivity va lue and vice versa . Figure I shows a typica l tradeoff behaviour of diffe rent me mbranes for oxygen/nitrogen gas mi xture. Separati on of gaseous and vo latile liqu id mixture by membrane permeati on continues to be the subjec t of

REDDY & SENTHILKUMAR: SILOXANE MEMBRANE TECHNOLOGY 667

intensive research. Some of the polymeric membranes that have been wide ly used for gas separation are shown in Table I.

Transport Mechanism

Gas separation us ing polymeric membranes function as a s imple method for purifying commerc iall y important and often difficult to separate gaseous mixtures. In the ir ideal form, membrane

LIpstream

-\ r \­p Downstream

Knudsen

Upstream

Down stream

appears to act as a molecular sca le filter that produces permeate containing pure A and a non-permeate containing pure B from a gaseous mixture of A and B . Real membrane can approach thi s s impli city and separation efficiency by involving more complex operations of recycling some of the permeate (o r) non-permeate stream for high purity require ments, s ince perfect separat ion of A and B typicall y cannot be achieved in a single pass l

) .

Upstream

Downstream

Table 1- Some of the polymeric membranes used for gas separat ion

Stru cture

JBh-I~~_~_o+ '):=:!II ~ 11

Br CH:I Hr

o 0 \I CF3 \I C I C

-t<Q)-N~ ~T-©C ~N~l (' CF~ C II J II o 0

Cellu lose :1Celale

Elhyl cellul ose

Pulyu illleth yl silolxalle

Pol ylrilllelhyl si lyl rropYlle

1.2

1.36

122

0.82

19.4

800

97 10

0.2

0.18

35.6

0 . 15

5.8

400

6):190

4.9

4.23

440

4.75

11 6

3800

37,000

0.2 1

0. 126

28.2

0 .1 5

12.4

1200

18,400

Ref.

8

9

10

II

12

13

14

668 J SCI IND RES VOL 62 JULY 2003

Gas separation, in general, can .be performed using membranes based on anyone of the three genera l tran sport mechani sms, viz. Knudsen diffu sion, Mo lecul ar sieving, and Solution-diffus ion mechani sms (Figure 2). Knudsen diffusion re lies on separation based on the difference in the molecular weight of the diffus ing gases . In thi s separation the se lecti vity for a particul ar gas is proporti onal to the square root of the inverse rati o of the molecular

. . . ' 6 17 M I I weight of permeatmg gas mixture . . 0 ecu ar siev ing mechanism depends on the pore diameters that are c reated in the membranes, which are inbetween the size of the gas particle to be separated IS. Gas molecule with sma ller size pass readil y compared to the large r s ize molecules . In solut ion-diffus ion mechani sm, there is no continuous passage of gas molecule through the membrane but it largely depends on the penetrant-sca le trans ient gaps formed in the po lyme r matri x due to thermall y ag itated cha in segmental moti on. The penetrants undergo random jumps due to higher concentration at the upstream face than the downstream face, a diffusion flu x occurs towards the dow~stream face ' 9

.

T he maj ority of the membranes that are studied in the laboratory and the membranes that are currently commerc ia ll y ava ilable operate according to the so luti on-d iffu sion mechani sm20.

The mode l concept for so luti on-di ffu sion mechani sm was first arti culated by Graham '

15 f-'

10 I--

8

IS. 6 ~

O2

N2 4 o •

3,-

2f- .0

1·5 f-

1<> .1 .1 .1

0 ·001 0 ·01 0 ·10

.1

1·0

quali tatively. According to hi s concept the materia l transport occurs in three steps, viz. so rption, diffus ion, and desorpti on. The rate of gas permeating through the membrane depends on these th ree paramete rs. Consistent with Grahams view of the process the thermally agitated motion of chain segment compnsmg the polymer membrane generates penetrant-scale transient gaps by whic h sorbed penetrants diffuse from up steam to the down stream face of the membrane.

A quantitative relationship for the diffu sion of gas through a non-porous membrane was g iven by Fick21. He observed that the quantity of gas (1) which passes through the membrane per un it t ime and unit area is proportional to the diffe rence in pressure between the input side (PI ) and permeate side (P2) and is inversely proporti onal to the th ie kness of the membrane (d).

J = Px (Pr P2) ld ,

V[cm3(STP)] d [c m]

Px = A[cm2

] t[s] t,P[cm Hg]

where, Px is the proportiona lity constant, ca lled permeability coeff ic ient of the given polymer membrane fo r the g iven gas X, it is a substance

• o

.1

10

o·

• -GLASS

o-RUBBER

,I

100

-

-

---

. .. --

,I

10000

Figure 2 - 'Tradeoff" curve fo r oxygen permeability against selecti vity factor a:(02/N2)

.."

REDDY & SENTHILKUMAR: SILOXANE MEMBRANE TECHNOLOGY 669

spec ific quantity and is characteri stic for each membrane materia l, V - permeate vo lume of the gas, d - membrane thickness, A - membrane area, t - time, and t:, p - pressure di ffe rence between input and permeate sides. Genera lly the permeability coeffic ient was expressed in Barrer and the unit is [cm' (STP) cmf c m2 s c m Hg] x 10.10

.

Yon Wroblewski22 showed that the permeability coeffic ient (P) can be viewed as the product of solubility coeffi c ient (S), a thermodynamic parameter, and di ffu sion coeffic ient (D), a kinetic parameter.

P = D x S.

Experimenta l methods and apparatus for the measurement of P, 0 and S have been described by many in vestigators23-26. C ritical discuss ion, including descri ption and ana lys is of many techniques with methods of calculation and sources and minimizati on of errors has been g iven by Fe lder and Hu vard26

.

T he se lecti vity a (a lso called separati on fac tor) of a membrane mate ri a l fo r a pair of gases X and Y is defined as the ratio of the permeability coeffi c ients fo r X and y27 .

a = Px / Py.

Theoretical Approach

The theore tical approaches fo r the descripti on of gas permeat ion/d iffus ion through the po lymeric membranes have been di scussed in numerous rev i ews2~ .. , 1. T he permeation of a gas molecul e

through a polymeric membrane large ly depends on the nature of the gas molecule and its inte rac ti on with the po lymer in ques tion. The molecular structure re lated factor, such as po larity, hydrogen bonding, cohes ive energy, density, chain fl ex ibility, ste ri c hind rance, s ide group substituti on, and crystallinity a ll ff" h . . 11·1S a ect t e permeatIO n process JI1 some manner' - .. .

Unfo rtunate ly, these fac tors because of their interre lating nature are not eas il y separable from one another. Th us , it is di fficult to iso late the indi vidual effec ts.

Pye and Hoehn 36 at Dupont deri ved a qualitati ve rule based on the ir pi oneering work. According to them, "when chang ing the structure within a family of po lyme rs inhibiting inte rsegmental packing, whil e s imultaneously hindering the backbone mobility, tend to produce a des irabl e trade off between producti vity and permselecti vity changes". Curren tly, thi s is the most re li able guide fo r understanding the structure­permeability studies of a given famil y of po lymers.

Increased packing inhibition can be detected by an increase in the free vo lume fracti on in the po lymer matri x,7.3g. Inhibition of segmental and sub-segmenta l motion can be detec ted spectroscopicall y by the increase in glass trans ition temperature (Tg) or sub-Tg (Ty) values37 .. 19

.

The macroscopic space which a polymer occupies was not comple te ly fill ed by its chains and gaps were formed which cannot be filled due to conformational constraints. A fraction of these gaps are large enough to accommodate gas parti c les and the sum of these gaps are called the spec if ic free vo lume(V-Vo/

OA3. The spec if ic free vo lume can be

obta ined from the differe nce be tween the tota l specif ic volume (V) and the occupied vo lu me (Vo). The tota l specific vo lume can be obtained from the experimentall y determined densities of the membrane and the occupied volume, primari Iy compri ses the e lec tron cloud of the atoms of the po lymer, can be estimated from group contribution method.~~ T he rati o of the spec ific free volume to the spec if ic vo lu me of a polymer sample is defined as the frac ti onal free vo lume (FFV). This free space is mobile and exists as more or less random di sconnected packets~ 2.~3

FFV = (V - Vo) / v.

It was shown that the permeability and d iffusion coeffi c ients do not correlate with the density or the glass transiti on temperature of the membrane . T here is, however, a re lat ive ly good corre lati on between the inverse of the fracti onal free vo lume in the poly mer and the logarithm of the permeability coeffic i e n t~ 5 .~6 . As the free volume of amorphous polymer is genera lly much la rger than that of c rystall ine po lymers, mostly amorphous po lyme rs were used for gas separati on.

Theoretica l mode l, such as that of Pace and Datyner

47 which takes the structure of po lymer chains

into account are more suitable . According to the m the transport of gas particles occur by leaps between the gaps present in the polymer matri x. T he therma l motion of certa in segments of the po lymer chain open up a suffic iently large channe l to a ne ig hboring gap. The gas parti c les can then diffuse th rough these channe ls. Once the channe l closes afte rwards the jump has been successfull y conc luded. In thi s scenario the se lecti vity of a membrane materia l depends on the control of these leap c hanne ls. Large openings due to substanti a l segment mot ions permit unrestric ted passage to a ll gas parti c les, whereas morc

670 J SCI INO RES VOL 62 JULY 2003

limited motion permits the passage of smaller species move frequently than of large particles. Therefore, by modifying the local chain flexibility by introducing subst ituents or by varying the length of the critical segments within the repeat unit of the polymer a llows the optimization of the polymer for a given separation task .

The facts presented so far conclude that significant improvements in permeability and select ivity of a polymer can no longer be achieved by more or less random structural vari ations of bas ic structures. A plausible sugges tion for furthe r development of polymer membrane stems from Koros

).j~ el al.· · . They recommend contro l of the ITlean diameter and the distribution of diameter of the gaps between the po lymer chain s, which make up the free vo lume. According to the sugges ti on from Koros et al. a narrower di stributi on of gap diameter could be ach ieved with a certai n type of polymer a rchitecture . Polymer chain that consists of alternating bulky and fl at units should be preferentially packed in such a manner that the bulky groups act as spacers and prevent tight packing of the flat groups and thus leave gaps in the vicinity of the f lat chain segments . The effect of the various molecular structura l factors on the diffusion constant can be examined in terms of a simp lifi ed free volume concept

42. For instance the

addition of a side group to the backbone of a polymer chain could cause a c hange in its chain stiffness. At the same time, it may have altered the polarity and hydrogen bonding characteristics of the chain. As a resul t of the changi ng molecular inte ractions and chain to chain di stance, it may bring about variation in free vo lume. Hence the gas diffusivity of the modified polymer cha in would take on a different va lue at the new free vo lume .

Although cons iderable data and genera l corre lations re lati ng st ructure and permeabi I ity ex ists, the re are no trul y quantitative re lationships to gu ide deta iled struc ture-permeability optimi zati on. The rev iew studi es how introduction of different fun cti onal groups affect the permeability and se lect ivity value for pol y(dimethyl s iloxane)

membrane.

Polyfdimethylsiloxane) [PDMS] Membrane for Gas Separation

The app lication of s il oxane polymers in the area of synthetic membrane is of special interest because of very hi gh permeation ra tes ex hibited by POMS for a range of gases.j~ . The limitati on with this polymer is

its poor selectivity value for a particul ar gas in a mixture of gases. It is more permeable to many organic vapours than to supercriticaI gases, such as nitrogen in membrane-based vapour separation application 50

. It has unique properties, such as thermal stability, hydrophobicity , and the fl exibi lity for the incorporation of different functional groups into Si-O I · k J 3 5 J 52 F . I I" III age . ". or commercia app Icatlon , a membrane should have a permeabili ty value of at leas t one Barrer and a selectivity value f five.

Merkel ef al .Jl

studi ed the permeability of po ly(dimethylsiloxane) (POMS), f ill er free composite membrane crosslinked at 30 nCo The values obtained by them for diffe rent gases a re shown in Table 2 . It is seen from Table 2 that the solubility coefficient affects the permeability value much more than the diffu sion coefficient. Generally, diffusion se lectivity inc reases as the difference in the relative size of two penetrants increases and the so lubility se lectivity increases as the d iffe rence in condensability be tween the two pene trants in a mixture increases53

. Hence, trade off ex its between solub il ity se lec tivity and diffus ivity se lectivity with the overall selectivity depending on the relative magnitudes of these two terms.

Based on the size (Table 3) of the permeat ing gas the pe rmeabi lity coefficient for the smalle r s ize permanent gases, like H2, O 2 and N 2 were expected to have high value compare to the larger s ize more condensable gases, like CO2 , CH4, C2H6, and C3Hs. But condensable gases were observed to have very high permeability value in POMS . When penetrant pressure and the refore penetrant concentration in the polymer increases the tendency to plasticize a polymer matrix inc reases, particularly fo r a strong sorbi ng penetrants. Plasticization refers to an increase in penetrant diffus ivity resulting from increased local

Table 2 - Permeability of POMS membrane for dilTerenl gases

Penetrant Penneabilily So lu bility Oillusivi ty [BaITer) (CIl1\ STP)/cIW' .aJlIl J rCI1l2/S)

H2 890 ± 30 0.05 ± 0.008 140 ± 5

O2 800 ± 20 0.18 ± 0.0 1 34 ± I

400 ± 10 0.09 ± 0.008 34 ± I

CO2 3800 ± 70 1.29 ± 0.0 I 22 ± I

CH~ 1200 ±40 0.42 ± 0.01 22 ± I

C1H(, 3300 ± 100 2.2 ± 0.02 I U±O.3

C,Hx 4100 ± 300 5.0 ± 0.07 5. 1 ± 0.3

i

1

REDDY & SENTHILKUMAR: SILOXANE MEMBRANE TECHNOLOGY 671

Table 3- Molecul ar sieving di ameter

Molecule He

Sieving diameter (A") 2.6 2.89

NO

3. 17 3.3

segmenta l motion of the po lymer matrix . On the other hand, high penetrant pressure acting on the polymer can s lightly compress the po lymer matri x, thereby reducing the amount of free vo lume ava il able and reducing penetrant diffusion coeffi c ients. As a result of inte rac tions between these factors the pe rmeability coeffi c ient of low sorbing penetrants, such as H2 which do not pl astiCize POMS have low permeability va lue compare to the more soluble penetrants whic h induce s ignificant plasti c izati on.

F rom the result s, one can observe that a rubbery po lyme r, such as POMS has poor ability to s ieve mo lecul es based on s ize, making diffus ivity a weak fun cti on of penetrant s ize . Consequently, for rubbery po lymer the overa ll se lec ti vity is often governed by di ffe rence in penetrant so lubility. C riti ca l te mperature is used as a scaling fac tor for penetrant condensab ility, he nce pene trants w ith hi ghe r c riti ca l te mpe rature are more so lubl e in po l ymers 5~. Many resea rc he rs have attempted to improve the se lec ti vity va lue fo r oxygen/nit rogen gas mi xture w ith out dras tica ll y reduc ing the pe rmeati on coeffic ie nt by func ti ona li zati on of PDMS . h d 'ff '\4-19 w it I e re nt groups

Functionalized Poly(Organosiloxane) Membrane for Gas Separation

Ste rn el al.60 have studied the permeability va lue fo r different hydrocarbon func tionalized poly(dimethylsil oxane). The POMS was modified with di ffe rent bul ky hyd rocarbon groups, li ke e thyl, propyl, octyl, and phenyl in the side chain as we ll as in the backbone chain . They observed that as the bulkiness of the subslitution group increases the glass transiti on te mperature of the po lymer inc reases, which shows that the po lymer looses its f lex ibility and the intersegmenta l backbone c hain mobility is hindered . With the substituti on of inc reas ingly bulkie r hydrocarbon functi onal groups, like e thy l, propyl, octyl, and pheny l groups in the side chain replac ing the pendent methy l groups the permeability va lue for He, O 2, CO2, and CH~ were fo und to decrease markedl y compared to the unsubstituted PDMS. For exampl e the permeab ility va lue dec reases for oxygen gas from 933 Barre r (unsubsti tu ted PDMS) to 3 12

Ar

3.4

CO

3.76 3.9

C~H x

4.3

Barrer for ethyl substituted , 383 Barre r fo r propyl substituted, 190 Barrer for octyl substituted, and 32 Barrer for phenyl substituted , and simil arl y for CO2

gas the permeability changes from 455 3 to 239 Barrer. The se lectivity va lue for 0 2/N2 improved fro m 2.0 to 3. 1 with the inc reas ing ly bulky substitution.

In a similar manner they observed that the permeability value decreased with the substituti on of diffe rent a lkyl groups, like ethyl, hexy l, oClyl, and phenyl groups in the back bone chain of POMS fo r He, O2, CO2, and CH4 gases. The permeability va lue for oxygen decreases from 933 to I I Barre r and fo r CO2 from 4553 to 64 bare r. The se lecti vity va lue was improved from 2.0 - 3.3 for 0 2/N2 gas mi xture .

A sil oxane conta ining po lyamide was first reported by Speck6J who described that introducti on of organosiloxane group into the po lyamide backbone led to the inc rease of fl ex ibility of the po lyme r cha in . Po lys iloxane conta ining amide group was prepared by va rious researchers62.6.1 . Kovacs el ul.6H5 have investigated aromati c polyamides conta ining s ily l moieties in the po lymer backbone to obta in processable po lymers which were stabl e to the rmo­ox idation.

Takeo el al.66

copolymers (Scheme have prepared mulli block I ) conta ining POMS and

+CO--r()r-CO+NH-;()r-O-@-NH-C01()l-C07m~H ~ ~ . ~CH (cH;, ~

J ' I ;;-f-N H- (CH ) - Si - O-Si- CH, n 2"3 I I

Scheme I CH, CH,

aromatic po lyamide using 3,4-diaminodiphenyl ethe r, isophthaloy l chloride, and amino propy l te rminated PDMS. The amount of POMS in the copo lymer was va ried fro m 26 to 75 wt per cent. The prepared copolymer was cast into transparent films us ing ,N ' ­dimethy lacetamide so lutions. The oxygen and nitrogen permeability va lues for these po lymeric membranes were found to inc rease with the inc rease in s il oxane content. For 75 wt per cent of s il oxane in the copo lymer the oxygen permeability was shown to

be 224 bare r and a (02/N2) = 2.3, whereas for 46 wt pe r cent of s il oxane in the copo lymer the oxygen

permeability was found to be 4 1 Barre r and a (02/N l)

672 J SCI IND RES VOL 62 JULY 2003

= 2.4. The introduction o f bulky and rig id amide group inhibits the loca l segmental motion and decrease the permeability value with a slight increase in the se lectivity va lue .

Ki yotsukuri et al.67 have synthes ized siloxane

containing aliphatic polyamides by melt

polycondensation of amino propyl terminated

tetramethyldisiloxane with dicarboxylic acid having

various chain length (HOOC-(CHz),,-COOH, where

Il = I to 10 (Sche me 2) and studied its permeation

C~ CH3 . I I --.:I.-

+NH-{CH ) - Si-O-Si -(CH2) -NH -CO-{CHzln - C0--rm 23 I I 3

CH3 CH3 Scheme 2

properties for oxygen and nitrogen gases. The

permeability value for oxygen and nitrogen gas was

found to decrease as the chain length of dicarboxylic

ac id inc reases, for oxygen (20.4 Barre r) and nitrogen

(9. 15 Barre r) and the selectivity value was found to be

high (ex va lue for O2 I Nz = 2.7) for Il = 6 and low

(ex va lue fo r Oz/N } = 2.4) for Il = 12. As expected the

se lectivity inc reases as the chain segmental motion

decrease due to the substituti on of bulky groups which hinder the backbone mobility . When the chain

length becomes too long as in the case of n = 12 the

inhib iti on to its backbone mobility decreases which

may lead to decrease in mobility se lect ivity and hence

the se lectivity would have dec reased. The magnitude

of pe rmeability va lue is hi gh in the case of s il oxane

contai ning aromatic polyamide group compared to the

a liphati c polyamide group and thi s inc rease may be

attributed to the packing inhibiti on of bulky groups

which may lead to larger free vo lume and hence

increase in permeab ility va lue .

Po lyimides generally have rig id chain structure, resulting in low gas permeab ilitl 8

. The rig idity of the polymer chain reduces the segmenta l motion of the chain and plays a role as a good barri er against gas transport with inc rease in diffusion se lecti vity . To overcome the limit for use as mate ri a ls for gas separati on, many copo lymers with fl ex ible segments,

h ' l d h' d 69-7 1 suc as S l ox ane an et e r units were prepare . Incorporati on of siloxane unit to polyimides makes it possib le to inc rease the so lubility and processability. Several kinds of synthet ic methods to incorporate silox<lne groups into the polyimides have been

h· db ' h 7?· 74 ac leve y varIOU S researc ers - .

Robeson ef al7 5 synthesized poly(imide­siloxane) me mbrane from pyromelliti c dianhydrides,

methylene diamines and 1,3-bis(3-amino propyl) tetramethyl di sil oxane by polycondensation react ion . The permeability value for oxygen and nitrogen gas for these membrane was found to be 7.5 and 3.41

Barrer, respective ly. The separation factor, ex for 0 2/N2 gas mixture was found to be 2.2 .

Bott et al.76 have synthes ized po\y(i mide­siloxane) membrane by the pol ycondesation reac tion of 3,3 ',4,4'-benzophenone tetracarboxy lic dianhydrides and 1,3-bi s(3-amino propyl) te tramethy ldi siloxane . The presence of bulky rig id group is expec ted to decrease the segmental motion and hence decrease the permeability value. As expected the permeability value for oxygen and nitrogen gas for th is me mbrane was found to be very low, 1. 1 and 0 .26, respective ly. However the mobility se lecti vity was greatly

enhanced and the separation factor ex for 0 11N2 gases was found to be very high , i. e. , 4.3.

Arnold et al. 77 have synthes ized pol y (imide­s iloxane) copo lymers by poly condensa ti on reaction of 2,2-bis[ 4-(3 ,4-dicarbox yphenoxy )pheny I ]propane dianhydrides, m-pheny lened iamine and amine terminated PDMS . The permeability va lue for oxygen and nitrogen gas for thi s membrane was found to be 43.4 and 18.87 Barrer, respecti ve ly . The separation

factor ex(02/N2) was found to be in the range of 2.3. The introduct ion of bulky phenyl group lTlay inhibit the chain packing which lTlay be the reason for the high permeability value in thi s membrane .

A s imil ar type of me mbrane was prepared by C hen et 0 1.78 They have synthes ized po ly(s il oxane­

imide) copolymer usi ng pyrome lliti c c1ianhydrides (PMDA), 4 ,4 '-oxydianiline (ODA) and amine

te rminated polydimethylsiloxane. Effe t of PDMS

content on the membrane morphol ogy was studied , using X-ray diffrac tion technique . Both permeability

value and diffusivity vallie were found to increase

with inc rease in the PDMS content.

Polotskaya et al. 79 has synthes ized po lysi loxane­polyimide copolymers optionally conta lI1lI1g polyketone, pol yether, f1uoroalkyl, and polythioether

groups. The polymers were synthes ized by two step condensation procedure of aromat ic di an hydrides with y-amino propyl bi s-te rminated oligosil oxanes and

a lly l aromatic diamines, fo llowed by cyc lization of

the formed polyamic ac ids. The oxygen and nitrogen

permeability of these membranes we re found to be

high and the separation factor for oxygen and nitrogen were found to be around 2.

REDDY & SE THI LKUMAR: SILOXANE MEMBRANE TECHNOLOGY 673

Yamada et al.80 synthes ized sil oxane containing polyimide membranes with various chemi cal structure and siloxane content for thermally stabl e gas separati on membrane. The dependence of permeability and permselectivity on the sil oxane content as well as the chemical structure of the polyimide was studied. They found that the permeability va lue of gases like oxygen, nitrogen, carbon di oxide, and methane increases with the si loxane content , while the permselecti vity decreases .

Monique el {/1. ~1 synthes ized hybrid polyimide­sil oxane containing diffe rent proporti on of silica by polycondensati on of pyromellitic dianhydrides, amino sil ane (aminopropyl trimethoxys il ane and amino­propyl methyl dimethoxys il ane) and tetra­methoxys il ane. The materi als were characteri zed by TGA, 1R, and 2<JSi-NMR techni ques. Gas permeation measurements were studied at vari ous temperatures fo r H2, CO2, and N2. It was . fo und that the permeability of the two materi als, prepared wi th two diffe rent amino sil ane, vary with the sil ane content and the nature of the amino sil ane. The gas transport was fo und to be therma ll y ac ti vated and the acti vati on energy va lue vary with the sil ane content of the materi al.

Sil oxanes containing both amide and imide groups were fo und to have exce llent permeability va lue co mpared to the sil oxane polymers containing either ami de or imide group alone. Lee el al82

synthes ized polyamide-imide branched sil oxane membranes (Scheme 3) fro m 4,4 ' -(hexafluoro-

OF,

+::~!%l~N-@-o-@+m+-N¢r2rj~-@-o-@F I II 0 0 0

fC tt;J,O C!i\ 01,

cH,-~,-o+f'- o-~t'-O-(CH21,- N H.,

CH, CH, e H, Scheme 3

isopropy lidene) di phthali c anh ydride (6FOA), p,p '­oxydianiline (OOA) and aminopropy l-terminated oligo meri c dillleth yisil oxane (OOMS) using N-methyl pyrroli cl inone (NMP) and tetrahyd rofuran (THF) as a ,·olvent. With thi s membrane they fo und the permeab i I ity va lue as 456 Barer for oxygen gas and as 23 1 barer for ni trogen gas at 25"C. The selecti vity valu e towards oxyge n and nitrogen (ex for OlIN2) gas with thi s membrane was fou nd to .be 2. 1. Thi s membrane was fo und to have hi gh permeab ilit y va lue compared to the membranes prepared from the poly(ami de-sil oxa ne) or po ly( imide-sil oxane) mem­branes. Thi s diffe rence in permeab ility va lue can be att ributed to decrease in pack ing density due to the

incorporation of OOMS as a side chain into the backbone of polyamide-imide group, leading to more free volume within the pol ymer matri x.

Higashi et al.83 synthesized poly(amide-i mide) contallllllg siloxane membrane from trimel­liticanhydride chloride, 4.4 '-oxydi aniline and ami ne terminated oligomeric dimethylsiloxane. The permeabili ty va lue for oxygen and nitrogen gas for thi s membrane was found to be 120 and 5 1.72 Barrer, respecti vely. The selectivity value was fou nd to be 2.3.

Stern et al .60 observed that the incorporati on of phenyl group in the sil oxane unit increased the selecti vity value for 0 2/N 2. Kawakami et 01.84

synthesized p-oligo organosil oxane substi tuted styrene (Scheme 4) using p-vinyl phenyl magnes iu m

chloride and substituted chloros il anes . These polymers were obtained by free rad ical polymerization and cast into thin films using THF. They found that the polymer with less sil oxane content give a brittle film and higher sil oxane content were waxy and lacked film fo rming property. The polymer with Il = I form tough film and showed good result for oxygen with permeability va lue of 100 Barrer and considerabl y hi gh separati on facto r of 2.8 fo r oxygen and nitrogen gas pairs.

Kawakami et al.R4 have also studied the effect of the permeati on behav iour of oligos il oxane substituents (o ligodimethyl sil oxane of branched structure, cyc lic structure and those having di fferent alkyl group at co-terminal) on po lystyrene by int roducing these groups at the p-pos iti on of the po lystyrene. They found that the structure of 17-

oli gos il oxane substituents had greater e ffect not onl y on glass transition temperature of the polymer but also on the oxygen permeati on behav iour through the polymer film. In most of the substituted polymers, it was observed that there is increase in permeabili ty va lue and it may due to increase in diffu sion coefficient. It was also observed that introduction of cyc li c structure in to oli god imethyl sil oxane structures results in decrease in permeability coefficient of the polymer compared with those having same nu mber of sili con atoms In the linear subst ituenls. The

674 J SCIIND RES VOL 62 JULY 2003

introducti on of bulky groups like phenyl, triphenyl, substituted sil oxanes at the (O-terminal lowers the permeability coeffi cient of oxygen irrespecti ve of the higher concentrati on of siloxane linkage in the polymers. Replac in g the styrene unit with vinyl and benzy l type substituents the permeability coeffi cient was found to increase compared with the styrene type polymer hav ing the same silicon atoms. The permeability va lue fo r oxygen gas in the oligos il oxane substituted polystyrene ranges from 47-71 Barrer, whereas for nitrogen gas it ranges from 5.6-25 Barrer. The separati on factor fo r oxygen and nitrogen ranges from 2.7-4 .0.

ft was also concluded that the structure of the side chain is very important in determining the oxygen permeability coe ffi cient through the polymer films. The trimeth yl siloxy l group, espec iall y at the terminal pl ace plays an essenti al role in enhancing the permea bility coeffi cient by enhancing the diffusion coefficient84

. The spacer oli godimethyl siloxane or methylene linkage is important to give the terminal trimethyl sil oxy l group max imum mobility. Replacement of methyl group of terminal trimethyl sil oxy l group by other group lowers the mobility of the terminal by substituted sil oxy l group, resulting in low .r permeability coeffi cient of oxygen through the polymer film .

Vari ous researcherss.' -88 have described a

syntheti c method to prepare polys iloxane/polystyrene copo lymers. They al so showed that these polymers coul d be modified by the incorporati on of the highl y ster ica ll y demanding tri s (trimethyl sil yl) methyl subst ituents (Me,Si),C. The DSC analys is showed that the modification increases the rigidity of the polymers. The effec t is espec ially dramatic in the case of silyl substituted polys il oxane copolymer. The steric bul k group, tri s( trimethyl sil yl)methyl in polys iloxane polymer renders 8 per cent of Si-H bonds in po ly( methyl hydro sil oxane), which is one of the reac tant , unreacti ve towards the hydrosil ylation process. The SiH bonds of the resultant copolymer can be utili zed for cross linking of the linear polys il oxane to prepare membranes.

Brown and Price89 have synthesized PDMS -polystyrene block copolymers. This synthes is in vo lves chl oromethyl terminated polys iloxane as initiator fo r atom transfer radi cal polymeri zati on. The copolymer formed was charac terized by NMR. The

thermal properties of these polymers were studied by DSC analys is.

Ashworth et a i .90 synthes ized polys il oxane

containing ester functionalities by the platinu m catalysed hydrosi lylation reacti on They have incorporated different ester functionali ties -(CH2)­COOR, (R = -Me, -CF" -Et, Pr and -CH2COMe), -(CH2),-CHMeC0 2Et, -(CH2)2 -CH(C0 2Meh -(CH2h­CH(C02Eth -(CH2)2C02CH=CHC02 CH2CH=CH2 and -(CH2hC02C6HsC02CH2CH=CH2 into the poly methyl hydrogen siloxane (Me,SiO-(MeS i(H)O)Il­SiMe,) under the anhydrous condi tion. They found that the amount of catalyst and temperature pl ay a significant role in the product formati on. The products were well charac teri zed by TR and IH and I:lC NMR techniques.

Ashworth el a l .t) 1 also sy nthes ized a seri es of polyorganos iloxane membranes functi onalized to vari ous degrees of ester group by platin um catalysed hydros il ylati on reacti on, Me,SiO[MeS i(H)O]x[MeS i « CH2):lCOOMe)0 ]ySiMe, (Scheme 5). Catalysed

Scheme 5

crosslinking of the remall1l11g Si-H unit 111 the functi onali zed polymer with a, (O-dihydroxy poly(dimethylsil oxane) lead to the formati on of membrane. The Tg value of these polymers was found to be around -1 20 °C. The permeabilities fo r CO2, CH .. , O2, and N2 show a slight decrease with increase in ester functionality. The solubility value for CH." O2, and N2 remains constant and that for CO2 was found to increase with increas ing ester functi onality. The selectivity of CO2 over CH.j was found to increase with increas ing ester functi onal ity as a resul t of improved solubility selecti vity. The diffusivity selecti vity remained virtually unchanged. In the case of O2 and N2 the selectivity for O2 over N2 slightl y decreased with greater ester functi onality due to the reducti on in the mobility se lecti vity.

Tronc el a i .92 synthes ized tai lor-made polys iloxanes with anchoring groups by polycondensation using hydros il ylati on reacti on. They prepared copolymers of dimethyl sil oxa ne segments of different lengths regul arl y separated by one

REDDY & SENTHILKUMAR: SILOXANE MEMBRANE TECHNOLOGY 675

bi sphenol A unit. They achieved thi s by performing hydrosil ylation of 2,2 ' -diallylbisphenol-A with hydride terminated polydimethylsil oxanes in the presence of Pt catalyst. The influence of several factors including the control of the [Si-H]/[double bond] rati o and the protection of -OH groups on the molecul ar weight di stribution of the polymer was reported. Side reacti on, such as isomerization of the doubl e bond and O-silylation was studied by I H and 29Si NMR techniques.

Conclusions

Functi onalization of poly(dimethylsil oxane) with diffe rent organic groups affect the permeability and se lecti vity value for oxygen and nitrogen gas has been reviewed. From the results, one can find that the permeability and selec ti vity va lue for similar sized and less condensable penetrants, like oxygen and nitrogen gas mixture, highl y depends on the mobility selec ti vity. The mobility selecti vity can be increased by introducing bulky substituents whi ch hinder the backbone mobility and decrease the intersegmental motion but the permeability va lue decrease. Solubility selecti vity pl ays a major role for hi ghl y condensable penetrants like, CO:? and CI-I 4 and it can be increased by substituti on of polar groups in the polymer chain .

Current and Future Industrial Applications for Membrane Technology

Petroc hemi ca l and gas producers, as well as environmental service industr ies recogni ze the advantages of appl yin g memb rane tec hnology to process operati ons . . Membrane-based gas separations have already made significant commercial contributions in vari ous indus tri al appl ications, such as the production of nitrogen enri ched air fo r inert gas blanketing appli cati ons; CO2 removal in natural gas treatment; hydrogen recovery during indust ri al process operations; and oxygen enri chment. For example, stri ngent S02 removal from smelter gas streams, H2S, and water removal from natura l gas, and separat ions of hydrocarbon and chl oro­flu orocarbon vapours from air. It was recogni zed th at the nex t generation of gas sepa rat ion membranes will not only require heightened se lecti vity to improve the qua lit y of the separat ions, but also improved permeab ilit y to permit increased production levels for indust ri al requi rements.

RefeJ'enccs Graham T, On the absorpti on and dialytic sepa rat ion of gases by co ll ()id septa. Philos Mag, 32 ( I g ( 6) 40 I.

2 Daynes H A, The process or di ffu sion th rough a rubber membrane, Proc Roy Soc, 97 ( 1920) 286.

3 Barrer R M, Nature of the diffusion process in ru bber. Nature, 140 ( 1937) 106.

4 Mitchell J K, On the permeati on of gases, Alii J M ed Sci , 25 ( 1833) 100.

5 Koros W J & Fleming G K, Membrane-based gas separati on. J M elllbr Sci, 83 ( 1993) I.

6 Robeson L M, Correlat ion o r separati on factor vs. permeability for polymeri c membranes, .I M embr Sci . 62 ( 199 1) 165.

7 Freeman B 0 , Basis of permeability/selecti vity tradeoff relati ons in polymeri c gas separati on membrane, Macrolllolecules, 32 ( 1999) 375 .

8 Barbari T A, PaulO R & Koros W J, Polymeri c membranes based on bisphenol-A for gas separati on, .I M elllbr Sci , 42 ( 1989) 69.

9 Muru ganandam N, PaulO R & Koros W J, Gas so rption and transpo rt in substituted polycarbonates . .I POIVlII Sci Part B: PolYIIl Phys, 25 ( 1987) 1999.

10 Tanaka K. Okano M, Toshino H, Ki ta H & Okamoto K I, Erfect of methyl substituents on permeab ility and permselecti vity of gases in polyi mides prepared from meth yl-substituted phenylene diamines, .I PO/VII I Sci Par t R: PolYI/l Phy.\' , 30 ( 1992) 907.

I I Puleo A C, PaulO R & Kelley S S, The effect of degree of acetylatio n on gas sorpti on and transport behaviour in cellulose acetate, .I M elll br Sci. 47 ( 1989) 30 I.

12 Houde A Y & Stern S A, Permeabil ity of eth yl cell ul ose to li ght gases: Effect of ethoxy content , .I M elllhr Sci . 92 ( 1994) 95.

13 Merkel T C, Bonder V I, Naga i K, FreemJn 13 0 & Pi nn au I. Gas sorpti on, diffusion and permeati on in poly(di methylsiloxane) , .I PolYIII Sci Par I B : Polrlll Ph ,·s. 38 (2000) 4 15.

14 Masuda T, Isobe E, Higashimura T & T~lkada K. Pol y ll -(tri methyl sil yl)- I-propyne] A new hi gh polymer synthesized with transiti on-meta l catalys ts and charac terized by extremely hi gh gas permeabil ity . .I Alii Chelll Soc. 105 ( 1983) 7473 .

15 Baker R W, Cuss ler E L. Eykam[J W. Korns W J & Strathman 1-1 , M elli b rail e sClm/'(/t ioll .1'\'.1' / 1'11 1.1' :

developlllellls ([nd / 11111/,(, di reu ioll s (Noyes Corporati on. Park Ridge. NJ) 199 1.

Recellt Data

16 Uhlhorn R J R. Keizer K & Burggraa f A J, Gas and surface difru sion in modi lied ga mma alU'll ina systems . .I M<'IlIiJ r Sci , 46 ( I n9) 225 .

17 Kesting R E & Fritzsche A K. Polwller ic gas sepamlioll lIIelll brall es (Wiley , New York) 1993.

IS Koresh J & So lTer A, The carbon m()iL:cul ar sieve membranes: General properti es and the permeability () r methane I hydrogen mixtures. Sel l Sci Tee/Ill o /, 22 ( 19R7) 973.

19 Korus W J & Hell ums M W. Tmll .I,/ IIJI'I I lw l lel'l ies,

Ell cvc!ojJedia O/ IIO IYlll er science. 2"d ed (Wil ey- Int erscience

Pu bli shers, New York , NY) Surpl Vol 1%9.

20 Maier G, Gas separat ion with pol ymer- membranes. Allgell '

Chelll 1111 Ed. 37 ( 1999) 2960_

676 J SCI IND RES VOL 62 JULY 2003

21 Fick A. Ann Ph),s Chell1, 94 ( 1955) 94.

22 VOIl Wroblewski S, Ann Ph),s Chelll, 8 ( I g79) 29.

23 Rogers C E, In Engineering design fo r plastics, edited by E Bacr (Reinhold Publication Co rporati on, New York) 1964.

24 Crank J & Park G S, DiJjilsion in polYlllers (Academi c Press, , ew York ) 1968.

25 Bixlcr 1-1 J & Sweeting 0 J. In Science and technology of polrlll er fi/IlIS, Vol 2 edited by 0 J Sweeting (Wi ley­In terscience, New York) 197 1.

26 Felder R M & I-Iu vard G S, Methods of experilllental physics (Academi c Press , New York) 1980.

27 Ghosal K & Freeman B D, Gas separation using polymeric mcmbranes: An overview, PolYIII Adv Technol, 5 ( 19CJ4) 673.

2X Stern S A, Polymers for gas separati ons: The nex t decade, J Melllbr Sci, 94 ( 1994) I .

2CJ PaulO R & Yampolskii Yu P, PolYllleric gas separation melllbralles (CRC Press, Boca Raton, FL) 1994.

30 Brandrup J & Immergut E 1-1 , PolYllIer hand book, 3rtl cd (Wil ey, New York ) 1989.

3 1 Freeman B D & Pinnau I, Polymeric matcrial s for gas scpa rat ion. ACS S I 'III/} Ser, 733 ( 1999) I.

32 Hoehn H 1-1 , Material science oj"sWilhetic lII elllhranes. ed ited by D R Loyds , ACS Syrnp. Ser. 269 (Ameri can Chemical Society Washington, DC) 1995.

33 Chern R T, Sheu F R, Jia L, Stannetl V T & I-I opfenberg 1-1 B. Transpo rt of gases in unmodified and aryl-brominated 2,6-dimethyl-I ,4-po ly(phenylene ox ide), J Melllbr Sci, 35 ( 1987) 103.

. \4 Stern S A, Mi Y. Yamamoto & St Clair A K, Structure­permcabilit y relationships of polyimide membranes: Applicat ions to the separation of gas mi xtures. J PolYIIl Sci Part B: PolYIII Ph)".\" Ed, 27 ( 1989) I g87.

35 rvle Hatti e J S. Koros W J & Paul D R, Gas transport propcrties of po lysulfones. I. Role of symmctry of methyl group placemcnt on bi sphenol rings, PolYlller, 32 (199 1) 841.

36 Pye D G & Hoehn 1-1 H, Mcasurcment of gas permeability of polymcrs. II . ApparatLi s. J Appl POIVIII Sci, 22 ( 1976) 287.

37 Korus W J. Co leman & Walker D R B, Controll ed permeabili ty polymer membranes, Annll Rev Mate r Sci , 22 (1991 ) 47

38 Sheu F R, Chcrn R T, Stannctt V T & Hopfenberg H B, Transport or carbon dioxide and mcthane in glassy aro mati c polyes ters, J Po/nil Sci Parr 13: PO/YIII PIn's Ed, 26 ( 1988) 8li3 .

39 Yce A F & Smith S A, Molecular structure effects on the dynamic mechanical spectra of polycarbonates, Macroli lOlecllles, 14 ( 19g J ) 54.

40 Shah,V M, Stern S -A & Ludov ice P J, Estimati on of the free vo lume in polymers by means of monle carlo techniques , Macl"OlIIo lewles. 22 ( I n9) 4660.

4 1 Odian G, Prillciples of polwllerizalioll , 3rt'cd (Wiley, New York ) 199 I.

42 Lee W M, Se lect ion of barrier materials fro m molecular structure. POIVlII Ellg Sci, 20 ( 1 9~0) 65.

43 Van Krevelen D W & Hortyzer P J, Properties of l}olYlllers, 3r

t! cd (Elsevier Publishing Co., Amsterdam) 1990.

44 Hell ums M W, Koros W J. Hu sk G R & Paul D R, Gas tr,lnspon in halogen-containing aromati c polyearbonales, J Allpl 1'01.1'111 Sci. 43 ( 1991 ) 1997.

45 Park J Y & Paul D R, Correlation and prediction of gas permeability in glassy pol ymer membrane materials via a modifi ed free volume based group con tribu ti on met hod. J Melllbr Sci, 125 ( 1997) 23.

46 Teplykov V & Meares P, Correlation aspeCtS of selective gas permeabilities of polymeri c materials and membranes, Cas Sep Purif, 4 (1990) 66.

47 Pace R J & Datyner A, Stati stical mechan ical model for diffusion of simple penetrants in pol ymcrs. I. Theory, .I PolYIII Sci Part B: PolYIII Phys Ed, 17 ( 1979) 437.

48 Koros W J, Fleming G K, Jord an S M, Ki lll T H & Hoehn H H, Polymeri c membrane materials for so lution-diffusion based permeation separations, Prog PO/YIII Sci, 13 ( 1998) 339.

49 Freeman B D & Pinnau I, SeparatiOli of gases using solubi lity-select ive polymers, Trellds POIVIII Sci. 5 ( 1997) 167.

50 Baker R W, Yoshioka N, Mohr J M & Khan A J, Separ~\lion of organic vapours from air, J Melllbr Sci, 3J ( 1987) 259.

5 I Stefan B. Josette C, Mathali e G, Gerard M. Philippe 0 , Andre V S & Caro l V, Poly-functionali zati on of po lysi loxanes, new industrial opportu niti es, J Co{/t('d Fahr, 27 (1998) 309.

52 Merkel T C, Gupta R P, Turk B S & Freema n B D. Mixed­gas permeation of syngas components in poly(dimethylsiloxane) and poly( I-trimeth yls il yl- I-propync) at elevated temperature, .I Mell1br Sci, 191 (200 I ) ~5.

53 Reid R C, Prausnit z J M & Poling B E. The 1}I"OI}erties of" gases alld liqllids , (McG raw Hill. New York) 1987 .

54 Vigo F, Gnessi A, Ma lastesta F. Uliana C & Costa G, Gas separation membranes prepared by styrene-grafted polytrimethyl silylpropyne, Sep Sci Tecl/l lO l, 32 ( 1997) 2335.

55 Kimura H, Nagasaki Y, Kalo M, Okamoto A. Mori ya N & Nakayama I, Gas pcrmeation properties o f po ly(si lamine) mcmbrane. Polwll Adv Technol, 1:1 ( 1997) 52<).

56 Takashi M, Seijiobata & Tadashi U, Rel ationship between fractal microphase separation and permselecti vit y of hlock copo lymer membranes containing po ly(dilllct hyisiloxane) , J Pol)'1II Sci Part B : PolYIII Phys. 38 (2000) 5X4.

57 Jain R, Adsorbent filled membrane for ai r separation , Poll"fll Mater Sci Ellg, 77 ( J 997) 330.

58 Hendel R A, HSII J T & Regen S L In;,ighl inlo the fabricati on on hi ghly permse lecli ve po lymer surfac tant composite membranes , POIYIlI Mater Sci Eng , 77 ( I ()()7) 3 18.

59 Ettouney H & Majeed U, Permeability funct ions of pure and mixture gases in sili cone rubber and po lysa l fone membranes: Dependence on pressure and composite, J Ivlelllbr Sci. 135 ( 1997) 25 I.

60 Stern S A, Shah V M & Hardy B J, St rllctwc- permeabilit y relationship in silicone polymer membrane, J Polrlll Sci Part B: PolYIl1 Phys, 25 ( 1987) 1263.

61 Speck S B, Linear polyamide from di sub,t itutcd malonic acid, .I Org Chell1 , 18 ( 1953) 689.

62 Furuzono T, Seki K, Ki shi da A, Ohshigc T, Wakik. Maruyama I & Akash i M. Novel function al polymers: Poly(d imethylsi loxane)-polyamide mul tiblock copo lymer. III. Synthesis and surface properties of disil ox<lne-aromatic polyamide multiblock copo lymer, J AI}Ji/ POll'llI Sci, 59 ( 1996) 1059.

REDDY & SENTH ILK UMAR: SILOXANE MEMBRANE TECHNOLOGY 677

63 Tnkeo M, Yasumi K. Kazunori W, Ki shida A, Furuzona T, Maruyama I & Akas hi M, Novel functional polymers: Pol y(d i methylsiloxane)-polyamide multi block copo lymer. IV. Gas permeability and thermomechanical properti es of aramide-sili cone resins, J Ap/Ji PO/YIII Sci, 59 ( 1996) 1067.

64 Kovacs H N, Delman A D & Simms B B, Silicon-containing amide, benzi midazole, hydrazide, and oxad iazole polymers, J PO/YIII Sci . A- I, 6 ( 1968) 2 103.

65 Kovacs H N, Delman A D & Simms B B, Thermal stabi lity of silicon-containing amide, benzimidazo le, hyd razide, and oxadiazo le po lymers, .1 PO/VIII Sci, A-I , 6 ( 1968) 2 11 7.

66 Takeo M, Uchida T, Ki shid a A, Furu zo no T, Maru yama I & Akashi M, Novel functi onal polymers: poly(di methylsiloxane)-polyamide multi block copolymer. VIII. Ox ygen permeability of Aramid-sili cone membrane in a gas membrane-liqu id system, .I ApI'/ PO/.l'1II Sci , 64 ( 1997) 11 53.

67 Kiyotsukuri T, Tsutsumi N & Ayama K, Preparati on and properties of silicon containing polyamides , .1 PO/VlI1 Sci Pari A: Po/vm Chel l1 , 25 ( 1987) 159 1.

68 Bower G M & Frost L M. Aromati c polyimides, .I Poi.l'lII Sci Pari A: Po/rill Chelll. 1 ( 196:1) 3 135.

(,9 Park H B. I-I a S Y & Lee Y M, Percobtion behaviour of gas permeabilit y in ri gid-Ilex ib le block copolymer membranes. .I Mell1br Sci . 177 (2000) 143.

70 Okamoto K, Fujii M, Okamyo S, Suzuki H, Tunaku K & Kita 1-1 , Gas permeation properties of poly(ether-imide), Macrolllo/eclI/es. 28 ( 1995) 6950.

71 Moh ite S S, Maldar N N & Marvel C S, Synthesis amJ characteriza ti on of silicon-co nt aining phenylated so lu ble arall1ids , J PO/YIIl Sci Pari A : PO/YIl1 ChCIII , 26 ( 1 9~g ) 2777.

72 Desc hl er U. Kleinschmit P & Panster P, 3-chloropropy ltrialkoxysilanes - key intermed iates for the producti on of organofuncti onali zed sil anes and polys il oxa nes . Angell' Chelll 1111 Ed, 25 ( I n6) 23 6.

73 Kim T H. Koros W J, Husk G R & O' Brien K C, Reverse permselCCliv ity of nit rogen over methane in aromatic po lyi mides . .I ApI'/ PolYIIl Sci,34 ( 1 9~7) 1767.

74 Stern S A. Liu Y & Feld W A, Structu re/permeabi li ty relati onshi ps of polyimides with branched or ex tended diamines moieties, .I PO/VIII Sci Pari B: .PO/VIII Phys, 31 ( 1993) 939 .

75 Roheson L M, Burgoyne W F. Langsam M. Savoca A C & Tien C F. High performance polymers for membrane separati on, PolYlller, 35 ( 1994) 4970.

76 BOll R H, Summers J D, Arnold C A. Tay lor L T, Ward T C & McGrath J E, ·Synthes is and characteri sti cs of novel poly( imide siloxane) segmented copolymer, .I Adhes, 23 ( 191<7) 67.

77 Arnold C A. Summers J D, Chen Y P. BOll R H, Chen D & McGrath J E. Structure - property behav iou r of soluble polyimide-poly(dimeth ylsiloxane) segmented co polymers, Po/vlller . 30 ( 1988) 986.

78 Chen S H, Lee M H & Lai J y , Pol ys il oxaneimide membranes: Gas transport properties, Eur Po/rill .I. 32 ( 1996) 1403.

79 Polotskaya G A, Martynenkov A A & Trolim()v A E. Synthesis and use of poly(sil oxane-imides) in gas separat ion membranes, Zh Prikl Khilll , 70 ( 1997) 32 1.

80 Yamada Y, Furukawa N & Tujita Y, Structure and gas separation properties of silicone containing polyamide, High Pelfo rm Poiym, 9 (1997) 145.

8 1 Monique S, Jean-Christophe S, Chantal L, Isbell a P & Chri st ian G, Gas separati on properti es of hybrid imide­sil oxane copol ymers with various silica co ntent , .I Melllbr Sci , 161 ( 1999) 157.

82 Lee Y B, Park H B, Shim J K & Lee Y M, Synthesis and characterizati on of pol yamide-imide branched siloxane and its gas separati on, .I App/ PO/),III Sci, 74 ( 1999) 965.

83 Higashi F & Nishi T. High molecu lar weig ht poly(p­phenylenetetraphthalamide) by CaCI, promoted direct polycondensati on with thionyl chl oride in N-meth yl pyrrolidone, J Polym Sci Pari A : Palmi. Chem, 26 ( 1988) 3235.

84 Kawakami Y, Akoi T. Hisada H, Yamanura Y & Yamashit a Y, Poly(p-disiloxane substituted styrene)s as materi als for oxygen permeable membrane, PO/.\'III COIII/ IIIIII . 26 ( 1985) 133 .

85 Kowalewska A, Wodzimierz, Staczyk A, Boil eau S, Lestcl L & Davidsmith J, Novel pol ymer systems wit h very bu lky organosi li con side chai n substituents, Poll'lller, 40 ( 1999) 8 13.

86 Lauter U, Kantor S W, Schmidt R K & Muck ni ght W J. Vin yl substituted silphenylene siloxa ne copo lymers: Novel hi gh temperature elastomers, Mac/'OlIl u/ecII/l's. 32 (1 999) 3426.

87 Kim H J, Jeong Y S & Lee Y S, Membranes composed 01 carboxylated poly(vi nylchloride) and poly( dimcthyl silo­xane)-graft- po lystyrene Preparation and gas pcrmeability. .I Illd Ellg Chelll , 5 ( 1999) W.

88 Mishima S, Kaneoka H & Nakagama T. Charact eri zati on and pervaporation of chl orinated hydrocarbon - water mixture wit h Iluoro alkyl methacrylate - grafted PDM S membrane. J App/ PO/Ill Sci , 71 ( 1999) 273.

89 Brown D A & Price G J. Preparati on and thermal properties of block copol ymers of PDMS wit h styrene or methyl methacrylate using ATRP. Po/vlller, 42 (200 I) 6767.

90 Ashworth A J, Brisdon B J. Reddy B S R. Zafar I & England R, Pol y organo -sil oxanes contain ing both ester and hydridc function alities: a detailed study by 1 H and I~C NMR spectroscopy, Bril Po/ym .I, 21 ( 1989) 49 1.

9 1 Ashworth A J, Brisdon B J. England R. Reddy B S R & Zafar I, The penmelecti vit y 01 po lyorganosi loxanes containing ester functionaliti es . .f Memb Sci. S6 ( 199 1) 2 17.

92 Tronc F, Lestel L & Boileau S, Polyco ndensati on usi ng hydrosilyl ation : A tool for preparing tail or-made polysiloxanes wi th anchorin g groups, Po 1I'111 er, 41 (2000) 5039.