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Journal of Membrane Science 378 (2011) 194– 202

Contents lists available at ScienceDirect

Journal of Membrane Science

j ourna l ho me pag e: www.elsev ier .com/ locate /memsci

haracterization of polydimethylsiloxane pervaporation membranes usingmall-angle neutron scattering

hanshyam L. Jadava, Vinod K. Aswalb, Puyam S. Singha,∗

RO Membrane Division, Central Salt & Marine Chemicals Research Institute (Council of Scientific and Industrial Research), G.B. Marg, Bhavnagar, Gujarat 364002, IndiaSolid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

r t i c l e i n f o

rticle history:eceived 31 January 2011eceived in revised form 28 April 2011ccepted 5 May 2011vailable online 12 May 2011

eywords:olydimethylsiloxane

a b s t r a c t

We report characterization of polydimethylsiloxane (PDMS) pervaporation membranes prepared fromvarious conditions using the small-angle neutron scattering (SANS). The PDMS membranes were pre-pared by cross-linking reactions between hydroxyl terminated polydimethylsiloxane (HPDMS) andpolymethylhydrosiloxane with pendant hydride (PHMS). The radius of gyration (Rg) of HPDMS and PHMSpolymer chains determined from the SANS data analysis is found to be similar with the size of about12 A. Upon the initial cross-linking reaction at 25 ◦C, the Rg of the polymer was increased to 31 A. Thefinal membrane structure obtained after the completion of reaction is comprised of interacted polymer

◦

ervaporationembraneanostructureANS

chains of the Rg values in the 55–61 A range. With increasing the reaction temperature to 40 C, about atwo-fold increase in the chain length of polymer and polymer chain clustering was observed in the mem-brane structure. Such membrane exhibited high separation factor (˛), of about 100–140 for hydrophobicorganics over water. Similar high separation factor was observed for the membrane obtained by curingat 150 ◦C, which also has longer polymer chain and a larger polymer chain clusters. Thin film membranescoated over a porous support have loose membrane structures and show poor organic selectivity.

. Introduction

Wastewater generated from various chemical and petroleumndustries is a serious environmental problem. The wastewater

ainly consists of volatile organic compounds (VOCs), which arehlorinated and aromatic compounds causing health problems toumans. Since the concentration of these VOCs in the wastew-ter is usually very low, conventional distillation process is notuitably applied. The pervaporation membrane process is suitableor removing the trace VOCs from the wastewater [1–5]. Besides,he pervaporation process is applied in other areas like dehydra-ion of solvents and separation of close boiling point mixtures ofzeotropes [6–8]. Membranes with high organic permeability andow water permeability are required in the process for separation ofOCs from water. A typical organic permselective membrane mate-ial is polydimethylsiloxane (PDMS) also known as silicone rubber9], which has an alternating –O–Si–O– unit structure and has veryood stability in operation. PDMS has a flexible rubber like char-

cter with a very low glass transition temperature. Many researchroups have studied different PDMS membranes in pervaporationeparation of the organics form wastewater including proprietary

∗ Corresponding author. Tel.: +91 278 2566511; fax: +91 278 2567562.E-mail address: puyam@csmcri.org (P.S. Singh).

376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2011.05.006

© 2011 Elsevier B.V. All rights reserved.

PDMS membranes from GKSS Germany, GE, Beijing Huaer Co. Ltd.,etc. [10–13]. Different functional group terminated PDMS [13–15]have also been used to improve the membrane performance interms of selectivity and flux. Different techniques like pore-fillingand pressing have been employed to prepare defect-free symmet-ric or composite PDMS membranes for enhanced selectivity andflux [13,16].

It is important to characterize the PDMS membrane structure atthe nanometer length scale to understand the relationship betweenthe membrane structure and performance. Even though there arenumerous studies on the performance of the PDMS membranes, theunderstanding of membrane structure is still lacking. In this paperwe report the pervaporation separation performance study of var-ious PDMS hydrophobic membranes for the separation of organicsfrom water along with detailed study of the membranes polymerstructure at the nanometer length scale using small-angle neu-tron scattering (SANS). The separation performance studies of themembranes were carried out using a ternary mixture of methanol,benzene and dichloromethane in water. SANS studies were carriedout on the PDMS membranes starting from the initial reactants tothe final cross-linked PDMS membranes under different conditions.

The PDMS membranes were prepared at various conditions usingdifferent solvents, different reaction temperatures and curing atdifferent temperatures with or without vacuum treatment prior tocuring.

G.L. Jadav et al. / Journal of Membrane Science 378 (2011) 194– 202 195

Table 1Details of PDMS membranes prepared at different conditions.

Membrane sample Solvent Polymer concentration(%, w/w)

Reactiontemperature (◦C)

Vacuumtreatment (h)

Curingtemperature (◦C)

Curingtime (h)

S-1 Chloroform 10 25 1 80 1S-2 Toluene 10 25 1 80 1S-3 Hexane 10 25 1 80 1S-4 Hexane 8 25 1 80 1S-5 Hexane 4 25 1 80 1S-6 Toluene 10 25 Nil 80 1S-7 Toluene 10 25 1 150 1S-8 Toluene 10 40 1 80 1

S

2

2

0ddibtr

2

lsTavmoTwPrimr

i1hdbmtccnofinont1wt

S-9 Toluene 10 70

-4 and S-5 were thin membrane films coated over a porous polysulfone support.

. Experimental

.1. Materials

Hydroxy terminated polydimethylsiloxane (HPDMS) of.97 g/ml density, 18,000–22,000 cSt viscosity; polymethylhy-rosiloxane (PHMS) of 1.006 g/ml density, 12–45 cSt viscosity;ibutyltin dilaurate were obtained from the Sigma–Aldrich Chem-

cal Co. The other chemicals used (n-hexane, toluene, chloroform,enzene, methanol, and dichloromethane) were obtained fromhe S.D. Fine-Chem Ltd., India. All the chemicals were of analyticaleagent grade and used without any further purification.

.2. Membrane preparation

The preparation procedure of PDMS membrane is given as fol-ows. The HPDMS and PHMS reactants were dissolved in an organicolvent (toluene or chloroform or n-hexane) and mixed thoroughly.he mixture ratio of the HPDMS to PHMS was 10:1 (w/w). The cat-lyst dibutyltin dilaurate was added to the mixture followed byigorous stirring. The quantity of the catalyst was 3 wt.% of the totalass of HPDMS and PHMS. The polymer mixture was then vigor-

usly mixed for about an hour to ensure a highly viscous solution.he polymer solution was then poured in a petridish and the solventas allowed to evaporate at 25 ◦C. The resulting dry cross-linked

DMS membrane was subjected to vacuum at 25 ◦C to remove un-eacted and extra solvent present. The membrane was then curedn an oven at 80 ◦C or 150 ◦C for 1 h for further cross-linking. The

embrane was finally peeled off from the petridish to get a smooth,ubbery and non sticky transparent membrane.

The details of the membrane preparation conditions are givenn Table 1. S-1–S-3 were the membrane samples prepared from0 wt.% polymer concentration in chloroform, toluene and n-exane, respectively. Samples S-4 and S-5 were prepared byeposition of PDMS film over a porous polysulfone support [17]y dip coating and casting methods, respectively. In the dip coatingethod, the polysulfone support was immersed in n-hexane solu-

ion of 8 wt.% polymer concentration for about 30 s while in theasting method, a dilute n-hexane solution of 4 wt.% polymer con-entration was allowed to evaporate on the porous support overight. Subsequently they were held in air at 25 ◦C for dryness inrder to coat a thin film of PDMS over the porous support andnally degassed and cured at 80 ◦C as above. Sample S-6 was aeat membrane with same conditions as in case of S-2 but with-ut the vacuum treatment. The sample numbers S-7–S-9 were alsoeat membranes prepared from 10 wt.% polymer concentration in

oluene. S-7 was prepared using a higher curing temperature of50 ◦C after the cross-linking reaction at 25 ◦C, while S-8 and S-9ere prepared by the cross-linking reaction at 40 and 70 ◦C, respec-

ively, prior to curing at 80 ◦C.

1 80 1

2.3. Small angle neutron scattering measurements and dataanalysis

The SANS measurements from the samples over the wave vec-tor range (Q) of 0.015–0.35 A−1 were taken at 25 ◦C for about 12 hon the SANS instrument at the Dhruva reactor, BARC, Mumbai,India. Q is defined as (4�/�) sin �, where 2� is the scattering angleand � is the wavelength of incident radiation. Throughout the dataanalysis, corrections were made for instrumental smearing and theincoherent scattering. Initial polymer reactants, HPDMS and PMHSwere separately dissolved in CDCl3 to give dilute solution systemof 15 wt.% polymer concentration each for the measurements. Themixed dilute solutions of HPDMS and PMHS (4, 8 and 10 wt.%) inCDCl3 were directly used for the measurements to probe interac-tion between HPDMS and PMHS. The mixture ratio of the HPDMSto PHMS and amount of catalyst added was the same, which isdescribed in the membrane preparation section. The preparationsof the membrane samples for the SANS measurement were as thefollowing. The PDMS membrane films were cut into small pieces.A specific amount of the sample pieces were then put into theSANS measuring cuvette. CDCl3 was added to the sample cuvette togive 20% (w/w) solution concentration. In case of composite mem-branes the thin PDMS layer was first extracted from the polysulfonesupport layer using dichloromethane solution. The extracted sam-ples were then dried to remove excess of dichloromethane andcut into small pieces. Required amount of the sample pieces andCDCl3 was then put into the measuring cuvette to maintain 20%(w/w) concentration. Hereafter, the sample cuvettes were vigor-ously shaked to ensure removal of all air bubbles present in it if anyand kept for 24 h at room temperature before performing the scat-tering experiments. After the treatment of the sample in CDCl3, veryhigh swellings of polymeric membranes were observed. Thus, thescattering measurements were carried out on these highly swelledhomogeneous polymeric membrane samples in CDCl3.

The absolute scattering intensity of the chain conformation isobtained from the following relation according to the Debye model[18,19].

I(Q ) = �(�p − �s)2V2 2x2

[x − 1 + exp (−x)]; x = Q 2R2g (1)

where � is the number density; �p and �s are the scattering den-sities of polymer and the solvent, respectively, and (�p–�s)2 isreferred as the scattering contrast; V and Rg are the volume andradius of gyration of the polymer chain, respectively.

Kratky plot (Q2I(Q) vs. Q) is used to identify the Gaussian poly-mer chain conformation. Since the structure factor for Gaussian

chains varies like I(Q) ∼ 1/Q2 at high-Q, this plot tends to a hor-izontal asymptote at high-Q. Thus, deviation from a horizontalasymptotic behavior indicates a non-Gaussian characteristic for thescattering chains

196 G.L. Jadav et al. / Journal of Membrane Science 378 (2011) 194– 202

y perv

2

cTfttuTmms(mpte

bttwHot(Hp

Fig. 1. (i) Schematic diagram of fabricated laborator

.4. Pervaporation separation experiments

All pervaporation experiments were carried out with a fabri-ated laboratory unit as shown in a schematic diagram, Fig. 1(i).he membrane was fixed into a membrane cell (M) of a plate andrame configuration (Fig. 1(ii)). The feed solution was kept in con-act with the membrane film at atmospheric pressure by circulatinghe feed with a help of peristaltic pump while a vacuum is appliedsing a two-stage rotary pump to the other side of the membrane.he downstream pressure at the vacuum side was continuouslyeasured by a pressure transducer (T) and was displayed by theicropirani gauge (PG) read out. The downstream permeate pres-

ure was about 267 Pa. Permeate (PR) was collected in the cold trapsTR) cooled in liquid nitrogen, through a two way valve. The per-

eate collection was by switching between the two cold traps inarallel, so that the connection between the permeate side andhe vacuum pump (VP) was never closed during the separationxperiment. The effective z area was 18.86 cm2.

Ternary mixture solution of methanol, dichloromethane andenzene in water was used as a feed solution for the pervapora-ion separation studies. The experiments were carried out at roomemperature. The concentration of methanol was 50% in water,hile for dichloromethane and benzene it was 2% each in water.igher amount of methanol was used to overcome low solubilityf dichloromethane and benzene in water. The concentrations of

he feed and permeate were analyzed by gas chromatograph (GC)Trace GC Ultra, Thermo) equipped with a polar capillary columnP-PLOT U. Isopropanol was used as an internal standard com-ound in the GC analysis.

aporation test rig; (ii) sketch of the membrane cell.

The separation factor of mixture was expressed as

= P0/F0

Pw/Fw(2)

where P0 and Pw represent the organic and water concentrationsin permeate, respectively (wt.%), F0 and Fw represent the organicand water concentrations in feed, respectively (wt.%).

3. Results and discussion

3.1. Membrane formation process

The absolute scattering intensity of the polymer chain accordingto the Debye model as given in Eq. (1) was fitted to the SANS data ofthe initial HPDMS and PMHS polymer reactants. The fitted curvesto the SANS data were excellent as shown in Fig. 2. The radius ofgyration (Rg) of the reactant HPDMS and PMHS, given by the Debyefit are found to be similar with the size of about 12 A. The cross-linking reaction between the terminal hydroxyl group of HPDMSand pendant hydride group of PMHS were probed by the scatteringmeasurements on the mixed systems of 4, 8 and 10 wt.% of thepolymer solution concentrations in CDCl3. The mixture ratio of theHPDMS to PHMS was 10:1 (w/w) and amount of catalyst addedwas 3 wt.% of the total polymer concentration as described in theexperimental section. These solutions were homogeneous liquid

and the solutions scattering were strong enough to perform formfactor fits. The Debye model fits to the data were excellent as shownin Fig. 3. The Rg of polymer chain given by the Debye model fit for the4 wt.% polymer solution (sample INDTA) is 14 A which is larger than

G.L. Jadav et al. / Journal of Membrane Science 378 (2011) 194– 202 197

Log

I(Q

), cm

-1

Log Q/ Å-1

2x10-1

3

4

5

6

2 3 4 5 6 7 8 90.1

2 3

PDMS PMHS fit_Debye

FPi

tsP8rwo

laKplsftpif

3s

u

Log

I(Q

), c

m-1

Log Q/Å-1

68

0.1

2

4

681

2

4

68

2 3 4 5 6 7 8 90.1

2 3

INDT C INDT B INDT A fit_debye

Fig. 3. Debye fits to the SANS profiles of the reaction intermediates formed by the

ig. 2. Debye fits to the SANS profiles of the initial reactant solutions (HPDMS andMHS). The data have been shifted to vertical direction by an arbitrary unit to showndividual plots distinctly.

he initial Rg of individual polymer reactants. This implies the largertructure formation by the cross-linking reaction of HPDMS withHMS. The Rg values observed for INDTB and INDTC samples from

and 10 wt.% polymer solution concentrations are 18 A and 31 A,espectively. This means that the cross-linking reaction increasesith the polymer concentration, which led to increase in the size

f polymer chain networks.The Gaussian characteristic of the chain network upon cross-

inking was found to be changed as identified by horizontalsymptote at different Q range of the Kratky plot. Fig. 4 showsratky plots of the polymer solutions. The Q value, at which thelateau value is reached, is directly related to the statistical segment

ength of Gaussian chain. As revealed from Fig. 4, the Q value corre-ponding to Gaussian segmental length is found at about 0.25 A−1

or the initial reactants. The Q value is reduced to about 0.12 A−1 forhe reacted polymers indicating the Gaussian segmental length ofolymer chain networks is increased upon the crosslinking which

s in agreement with the increase in the chain length as observedrom the Rg values.

.2. Characterization of the membranes prepared using different

olvents

Neat membranes, designated as S-1–S-3 samples were preparedsing chloroform, toluene and n-hexane solvents, respectively.

0.03 0

0.02 0

0.01 0

0.00 0

0.300.250.200.150.100.050.00

PDMS PMHS

Q2 I (

Q) [

Å-2

cm

-1]

Q (Å-1)

2 -2

-1

(a)

Fig. 4. Kratky profiles of (a) the reactants (HPDMS and PMHS); (

reaction between HPDMS and PMHS. INDTA, INDTB and INDTC samples are from 4,8 and 10% polymer solutions, respectively, in CDCl3. The data have been shifted tovertical direction by an arbitrary unit to show individual plots distinctly.

Fig. 5 shows the scattering profiles collected from the mem-branes. Polymer chain conformation according to the Debye chainmodel was fitted to the scattering data. There are good agree-ments between the data and the fits throughout the whole Q range0.015–0.35 A−1. This indicates the highly mobile polymer chains inthe PDMS membrane film in agreement with the rubbery nature ofthe polymer at room temperature. There is a slight deviation of thechain model fit to the data at the Q of 0.05–0.06 A−1 which showsan apparent minimum due to the oscillation of non-Gaussian scat-tering. Fig. 5a shows the Debye fits to the scattering data of themembrane samples S-1–S-3. The Rg values obtained from the fitare, respectively, 61, 58 and 55 A for S-1, S-2 and S-3 samples. Thelowest Rg value of the sample S-3 suggests that the S-3 sample hasthe least degree of polymerisation among the samples.

With the increase in the Rg value from 30 to 131 A it is not sur-prising that the I(0) value increases from 1.1 to 11.6 cm−1 since I(0)increases with Rg according to the Debye model given in Eq. (1).

The differences in polymer chain nanostructure of the mem-

branes are also evident in the Kratky plots [I(Q)Q2 vs. Q]. Fig. 5bshows the Kratky plots for the S-1, S-2 and S-3 membrane sam-ples. As shown in Fig. 5b, a small-angle peak at the Q range ofabout 0.0375–0.0398 A−1 was observed in the Kratky plots of the

0.015

0.010

0.005

0.000

0.300.250.200.150.100.050.00

INDTC INDTB INDTA

QI (

Q) [

Å c

m]

Q (Å-1)

(b)

b) the reaction intermediates (INDTA, INDTB and INDTC).

198 G.L. Jadav et al. / Journal of Membrane Science 378 (2011) 194– 202

2

3

456

1

2

3

456

10

2

2 3 4 5 6 7 8 90.1

2 3

S- 1 S- 2 S- 3 Debye fit

(a)

Log

I(Q

), cm

-1

Log Q/Å-1

0.006

0.004

0.002

0.000

0.200.150.100.050.00

S-1

S-2

S-3

0.03 75 Å-1

0.039 8 Å-1

Q2 I(

Q) [

Å-2

cm

-1]

Q (Å-1)

(b)

Fig. 5. (a) SANS profiles and (b) Kratky profiles of the S-1, S-2 and S-3 ‘PDMS’mhd

sa(Tpcpi1

3t

c4Shatfisfb

Table 2The radius of gyration using Debye chain model (Rg) and the scattering intensityI(0).

Sample Rg (Å) I(0) (cm−1)

S-1 61 3.8S-2 58 2.9S-3 55 2.8S-4 Not determined Not determinedS-5 30 1.1S-6 49 2.5S-7 75 6.0

◦ ◦

embranes prepared from chloroform, toluene and hexane, respectively. The dataave been shifted to vertical direction by an arbitrary unit to show individual plotsistinctly.

amples. The small-angle peak of the membrane can be interpreteds observed in other systems [20] as due to the polymer mesh sizechain clusters) of cross-linked structure of polymeric membrane.he peak position can be related to the mesh size of the cross-linkedolymeric membrane, larger the Q value means smaller mesh sizeomprised of small clustering of polymer chains. The small-angleeak position is at the larger Q value of 0.0398 A−1 for sample S-3

ndicating relatively the smaller mesh size of 158 A as compared to68 A mesh size of the S-1 and S-2 samples.

.3. Characterization of the composite membranes comprised ofhin PDMS film coated over polysulfone support

Composite membranes were prepared by coating a thin film ofross-linked PDMS film over a porous polysulfone support. The S-

sample was prepared using a dip coating technique while the-5 sample was prepared by a solution casting technique using n-exane solvent as described in the experimental section. Fig. 6and b shows the scattering profiles collected from the thin PDMSop layers of the composite membranes. The profiles of thin PDMS

lms are clearly different from the profile of the neat membraneample prepared using the same solvent. The scattering intensitiesor the thin films at the low Q range (0.015–0.06 A−1) are found toe lower than that for the neat membrane in spite of same sam-

S-8 131 11.6S-9 35 1.84

ple amount used for the SANS measurement. This implies that theoverall average size of scattering domains of PDMS from the com-posite membranes is smaller than that of neat PDMS sample sinceI(0) = �(�p − �s)2V2. For the S-4 membrane obtained by dip coatingmethod, the Debye chain model is not fitted well to the data whilethere is good agreement between the data and the fit for S-5 sampleobtained by solution casting method. The Rg value obtained by theDebye chain model fit is 30 A for the S-5 sample, which is nearlyhalf of the size value of the neat membrane sample. The oscillationof non-Gaussian scattering is also observed in the profiles of thinPDMS films. Further insights on the differences of polymer chainnanostructures are revealed by the Kratky plots. The mesh sizedue to polymer clustering in the thin film prepared by dip-coatingmethod is 262 A which is larger than that of the neat membrane, andis about twice the mesh size of the solution casted neat membrane.The rapid evaporation of solvent in case of dip-coating techniqueperhaps influences to the higher degree of polymer chain clusteringof the cross-linked membrane.

3.4. Characterization of the membranes prepared by curing atdifferent conditions

Samples S-6 and S-7 were neat membranes prepared usingtoluene as solvent at different curing conditions. In case of S-6 membrane the curing at 80 ◦C was performed in presence ofentrapped solvent while in case of S-7 membrane the solvent wasremoved by degassing prior to curing at higher temperature of150 ◦C. Fig. 7 shows the scattering profiles collected from the mem-branes. The Debye chain model is applied to perform a form factorfit to the scattering data and there are good agreements betweenthe data and the fits throughout the whole Q range except theapparent minimum at the Q range of 0.05–0.06 A−1 similar to theother membranes discussed above. As given in Table 2, the mem-brane samples S-6 and S-7 have the Rg values of 49 A and 75 A,respectively, suggesting that relatively shorter and longer polymerchain network respectively. As shown in the Kratky plots, Fig. 7b,the small-angle peak observed for the samples S-6 and S-7 is at∼0.041 and ∼0.033 A−1, respectively. This implies the mesh size ofthe S-7 sample is larger than that of S-6 sample. Thus, the mem-brane cured in presence of entrapped solvent leads to a less polymerchain clusters while the membrane cured at higher temperature inabsence of the solvent possess longer polymer chain network witha higher polymer chain clusters.

3.5. Characterization of the membranes prepared by cross-linkingat higher temperatures

In this preparation method, the cross-linking reaction of the

reactants is at higher temperature of 40 C or 70 C while otherpreparation parameters are the same as that of the normal prepa-ration method. The membranes obtained by the initial reaction at40 ◦C and 70 ◦C are designated as S-8 and S-9 samples, respectively.

G.L. Jadav et al. / Journal of Membrane Science 378 (2011) 194– 202 199

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exhibits the highest separation factor of organics over waterwith the least water permeability of 31 × 10−9 mole m m−2 s−1.For this membrane, the separation factors of benzene/water,

Table 3Benzene, dichloromethane, methanol and water permeabilities for the membranesfrom the pervaporation experiments with the feed of 100H2O: 56.3CH3OH: 0.9C6H6:0.8CH2Cl2.

Membrane Thickness (�m) Flux (×10−9 mole m m−2 s−1)

Benzene DCM Methanol Water

S-1 203 4.22 5.60 70.48 36.94S-2 213 5.83 6.94 72.63 30.82S-3 222 9.44 6.96 53.12 69.99S-4 36 0.55 0.82 56.59 77.25S-5 90 1.49 2.07 110.03 88.76

ig. 6. SANS profiles of the PDMS membranes coated over polysulfone using (a) dipnd (c) Kratky plots for the membranes. The data in the plots of (c) have been shifte

ig. 8a shows the scattering profiles collected from the membranes.he scattering intensity profile for the S-8 membrane at the low Qange (0.015–0.06 A−1) is found to be steeper and higher than thatf the neat membrane. In case of the S-9 membrane, the scatteringntensity at the low Q range is lower. This implies that the over-ll average size of scattering domains is larger for S-8 and smalleror S-9. The Rg values obtained by the Debye chain model fit are31 A and 35 A for S-8 and S-9 samples, respectively. A higher cross-

inking density resulting into longer polymer chain network (asvident from larger Rg values) is observed for S-8 sample mem-rane obtained from the cross-linking reaction at 40 ◦C as comparedo the neat membrane structure prepared by reacting at room tem-erature. However, further increase of the reaction temperature to0 ◦C did not favour to the increase of the cross-linking density.he S-9 membrane prepared at 70 ◦ C has the smaller Rg value.ig. 8b shows the Kratky plots. The small-angle peaks are observedt Q values of ∼0.030 A−1 and 0.042 A−1 for S-8 and S-9 samples,espectively. This means that the sample S-8 has a larger mesh sizef 210 A of polymer chain clusters.

.6. Pervaporation separation performance of the membranes inhe separation of organics from water

The separation performance of the membranes in terms of theeparation factors and permeability are presented in Fig. 9 andable 3, respectively. The permeability was calculated in terms ofole m m−2 s−1 using the liquid permeance data and membrane

ng technique (S-4 membrane), (b) evaporation casting technique (S-5 membrane),ertical direction by an arbitrary unit to show individual plots distinctly.

thicknesses. There were differences in the separation performancesof the membranes prepared from different conditions. The relation-ship between the membrane performance data and the nanoscalestructure observed by SANS is described as the following.

The membranes prepared using different solvents: S-1, S-2and S-3 neat membranes were of about 200–220 �m thick-nesses prepared using chloroform, toluene and n-hexane sol-vent, respectively. Among the membranes the S-2 membrane

S-6 230 9.25 6.79 48.65 88.76S-7 215 11.79 14.86 51.34 23.05S-8 220 7.91 7.05 56.39 13.89S-9 219 6.00 7.15 74.70 82.82

200 G.L. Jadav et al. / Journal of Membrane Science 378 (2011) 194– 202

0.200.150.100.050.00

0.004

0.002

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S-7

S-6(b)

Q (Å-1)

Q2 I(

Q) [

Å-2

cm

-1]

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S-6 S-7 Debye fit

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Fig. 7. (a) SANS profiles and (b) Kratky profiles of the S-6 membrane prepared bycuring at 80 ◦C without the vacuum treatment and the S-7 membrane with the vac-uis

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Fig. 8. (a) SANS profiles and (b) Kratky profiles of the S-8 and the S-9 membranes◦ ◦

um treatment prior to curing at higher temperature of 150 ◦C. The data of the plotn the inset and (b) have been shifted to vertical direction by an arbitrary unit tohow individual plots distinctly.

ichloromethane/water and methanol/water were found to be6, 40 and 5, respectively. The water permeability of the S-1embrane was 37 × 10−9 mole m m−2 s−1 which is slightly higher

han that of the S-2 membrane. The flux was found to be theighest for the S-3 membrane with the water permeability of0 × 10−9 mole m m−2 s−1 which is nearly double of the water per-eability of the other membranes. The high water flux and less

rganic selectivity of the S-3 membrane may be related with itsoose nanostructure as observed by the SANS studies which showshat the S-3 membrane structure has less interaction between thendividual polymer chains and shorter chain length of the polymer.

The composite membranes: The S-4 and S-5 membranesrepared by coating PDMS layer on porous polymer supportxhibited have very low permeation of hydrophobic benzenend dichloromethane molecules but higher permeation of polarolecules of water and methanol. Therefore the selectivity of

enzene and dichloromethane over water was poor in these mem-ranes. For these composite membranes, the separation factors ofenzene and dichloromethane to water were found to be about 2–4

hich is decreased by about ten times from that of neat membrane.

he composite membranes have structures with large defects dueo polymer chain clustering or shorter polymer chain length asevealed by the SANS data analysis. Therefore, apparently, the com-

prepared with from the cross-linking reaction at 40 C and 70 C, respectively. Thedata of the plot in the inset and (b) have been shifted to vertical direction by anarbitrary unit to show individual plots distinctly.

posite membranes structures are not as dense and homogeneousstructure as that of the neat membrane even though they are pre-pared from the same initial reactants.

The membranes prepared by different curing: The water per-meability of the S-6 membrane which was cured at 80 ◦C withoutapplying the vacuum treatment was nearly about thrice the waterpermeability of the membrane prepared at normal condition.The permeability of organics was however approximately sim-ilar between the S-6 and the normal membrane, consequentlythe separation factors of benzene/water, dichloromethane/waterand methanol/water were found to be lower as 25, 14 and 1,respectively, for the S-6 membrane. The S-7 membrane which wascured at higher temperature of 150 ◦C with the vacuum treat-ment has the water permeability of 23 mole m m−2 s−1 whichis about one-fourth of the water permeability of the S-6 andeven lower than 31 mole m m−2 s−1 of the normal membrane.The permeability of hydrophobic organic was found to be higherfor the S-7 membrane resulting to high hydrophobic organicselectivity for the S-7 membrane. The separation factors of ben-zene/water and dichloromethane/water for the S-7 membranewere found to be 125 and 120, respectively, which is aboutthree times of the separation factors obtained by the normalneat membranes (S1, S2, and S3) cured at 80 ◦C subsequent to

casting and degassing at room temperature. The improved selec-tivity towards the hydrophobic organics for the S-7 membranemay be due to the relatively denser polymer nanostructure asobserved from the SANS data analysis which shows a longer

G.L. Jadav et al. / Journal of Membran

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40 ◦C which also led to the formation of membrane with dense

ig. 9. Separation factor (˛) of (a) benzene/water, (b) dichloromethane/water andc) methanol/water for the PDMS membranes prepared at different conditions.

olymer chain. The dense polymer nanostructure perhaps actss barrier to polar molecules while allowing the transport ofydrophobic molecules via sorption–diffusion–desorption pro-ess.

The membranes prepared by cross-linking at higher tempera-ures: The S-8 and S-9 membranes were prepared starting from theross-linking reaction at 40 ◦C and 70 ◦C, respectively, to study theffect of the reaction temperature to the membrane structure anderformance. Between the S-8 and S-9 membranes, S-8 exhibiteduperior performance in terms of the organic selectivity similar tohat of the S-7 membrane cured at higher temperature of 150 ◦C but

he water permeability was reduced to about one-sixth of the waterermeability given by the S-9 membrane. The S-9 membrane exhib-

ted separation factors of benzene/water, dichloromethane/water

e Science 378 (2011) 194– 202 201

and methanol/water as 18, 16 and 2, respectively only. Again asin case of S-7 membrane, the high organic selectivity for the S-8membrane may be due to the relatively denser polymer nano-structure as a result of longer polymer chain as observed fromthe SANS data. This implies that the polymer nanostructure ofthe membrane is denser upon increasing the reaction temper-ature to a certain temperature beyond which the cross-linkingreaction is disfavored resulting into loose-nanostructure of themembrane.

4. Conclusions

The PDMS pervaporation membranes were prepared from thereaction of a hydroxyl terminated polydimethylsiloxane (HPDMS)and polymethylhydrosiloxane with pendant hydride (PHMS). Theperformances of the membranes were evaluated from the pervapo-ration experiments carried out using a ternary mixture of benzene,dichloromethane and methanol in water. The Rg values obtainedfrom the SANS data analysis based on Debye chain model was12 A for both the reactants HPDMS and PHMS. Normally the reac-tion was performed in solution using chloroform or toluene orn-hexane as solvent at room temperature after which the poly-mer solution was casted to produce a symmetric film and finallydegassed and cured at 80 ◦C to produce the neat membrane. Thereaction between the reactants leading to progressive increase inthe polymer chain length was observed by the SANS data. The finalmembrane structures obtained were comprised of interacted poly-mer chains of the larger Rg values, which were about 5 times of theRg values of the polymer reactants. Among the neat membranesprepared, the membranes prepared using toluene or chloroformas solvents have higher organic selectivity than the membraneprepared using n-hexane as solvent. The separation factors ben-zene/water and dichloromethane/water and methanol/water of 46,40 and 5, respectively, were observed for the membrane obtainedusing toluene as solvent. The relative flux was found to be higherfor the membrane prepared using n-hexane as solvent with thewater permeability of 70 × 10−9 mole m m−2 s−1 (nearly double ofthe water permeability of the other membranes) which agrees wellwith its loose polymer nanostructure as observed by Kratky mod-els. The structure–performance studies of thin PDMS film coatedover a porous support by dip coating and solution casting meth-ods were also performed. For these membranes, the separationfactors of benzene to water and dichloromethane to water wereabout one-tenth of the separation factors given by the neat mem-brane which is in corroboration with the SANS results, whichshow structures with larger defects in case of composite mem-branes.

In the normal preparation of the neat membrane, vacuum isapplied to remove the entrapped solvent from the membranefilm prior to the curing. It was observed that the curing of themembrane in presence of entrapped solvent (without vacuumtreatment) had also resulted loose membrane structure with slightdecrease in polymer chain length, which subsequently showed lessorganic selectivity. A high separation factor of hydrophobic organ-ics, benzene/water and dichloromethane/water of 126 and 120,respectively, was observed for the membrane obtained by curingat 150 ◦C with prior vacuum treatment which is in accord with thedenser polymer nanostructure as observed by the SANS data. TheRg values of the polymer chain was found to be increasing uponincreasing the reaction temperature from room temperature to

polymer nanostructure. Such membrane also showed high sepa-ration factor of hydrophobic organics over water (benzene/waterand dichloromethane/water).

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cknowledgment

Financial assistance for this work from Department of Science &echnology, Government of India (Sanction No. SR/S1/PC-15/2006)s gratefully acknowledged.

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