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Colloids and Surfaces A: Physicochem. Eng. Aspects 297 (2007) 267–274

Development of crosslinked poly(ether-block-amide) membranefor CO2/CH4 separation�

S. Sridhar a,b, R. Suryamurali b, B. Smitha b, T.M. Aminabhavi a,∗a Membrane Separations Division, Center of Excellence in Polymer Science, Karnatak University, Dharwad 580003, India

b Membrane Separations Group, Chemical Engineering Division, Indian Institute of Chemical Technology, Hyderabad 500007, India

Received 17 August 2006; received in revised form 10 October 2006; accepted 23 October 2006Available online 27 October 2006

bstract

Thin film composite membranes of poly(ether-block-amide) (PEBAX-1657) were prepared on PVDF ultra-porous substrate by solution castingnd solvent evaporation method. The membrane was crosslinked by a novel method using 2% (v/v) 2,4-toluylene diisocyanate (TDI) in hexaneedium. Sorption studies were carried out in pure water to confirm crosslinking. Both the unmodified and modified membranes were characterized

y scanning electron microscopy (SEM) to study the morphologies of the surface and cross-section. X-ray diffraction (XRD) experiments werearried out to understand the intermolecular interactions and separation mechanisms. Thermal stability of the membrane was assessed by differentialcanning calorimetry (DSC). Single gas permeabilities of CO2 and CH4 and the resultant ideal selectivities were determined for unmodified PEBAX-657 membrane using an indigenously built high-pressure gas separation manifold. A comparison was made with PEBAX-2533 membrane, alsorepared in this study, to evaluate the effect of rigid block content in the polymers. As the pressure was varied from 10 to 40 kg/cm2, the permeance

nd ideal selectivity for the unmodified PEBAX-1657 were found to range from 3.0 to 4.8 GPU and 18 to 25, respectively. On the other handEBAX-1657 membrane crosslinked for 5 min exhibited only one-fifth the permeance (0.6–0.9 GPU) but almost twice the selectivity (38–47). Theffect of crosslinking time (0–60 min) and binary feed mixture composition (5–20% CO2) on performance of membrane was studied at a constantressure. PEBAX-1657 was found to be promising for the separation of CO2 from CH4. 2006 Elsevier B.V. All rights reserved.

teriza

m[afbpsctbo

eywords: Gas permeation; PEBAX polymer; Crosslinking; Membrane charac

. Introduction

Over the past two decades, gas separation using polymericembranes has drawn a great deal of interest from researchers

ecause it offers advantages such as low energy costs and envi-onmental benignity [1]. This is especially true for hydrocarboneparations performed in the petrochemical industry, which gen-rally incur heavy operating costs [2]. The membrane-basedethod is particularly attractive for the removal of heavier

pecies present in dilute concentrations, such as CO2 and H2Srom the lighter but major constituent, CH4, in natural gas.

The development of inexpensive but up-scalable polymericembranes capable of giving high separation factors and per-eabilities, is a challenging task. Numerous attempts have been

� This paper is CEPS communication # 168.∗ Corresponding author. Tel.: +91 836 2215372; fax: +91 836 2771275.

E-mail address: aminabhavi@yahoo.com (T.M. Aminabhavi).

tcpMbis

927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2006.10.054

tion; CO2/CH4 separation

ade to develop efficient membranes for CO2/CH4 separation3–6]. Amongst the polymers reported, polyimides and cellulosecetate have been widely explored for CO2 and H2S removalrom natural gas [7]. Poly(ether-block-amide) (PEBA) resin isest known under the trademark PEBAX, and is a thermo-lastic elastomer combining linear chains of rigid polyamideegments interspaced with flexible polyether segments. Thisrystalline/amorphous structure creates a blend of properties ofhermoplastics and rubbers. It was believed that the hard amidelock provides the mechanical strength, whereas gas transportccurs primarily through the soft ether block [8].

It is thus clear that the prime focus of researchers has beeno improve the permeation properties of the polymers withoutompromising on selectivity. In order to achieve it, the basicolymers have been subjected to a number of modifications.

any researchers have focused on modifying the polymer

y incorporating inorganic materials into the organic matrixn order to improve the mechanical properties and thermaltability [9–14]. An improvement in the aforementioned

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68 S. Sridhar et al. / Colloids and Surfaces A: P

rocess is the sol–gel technique wherein the inorganic phaseorms a very fine dispersion at the molecular level [15,16].

uller et al. [17] modified PEBAX-2533 and 4011 polymersy silver incorporation using AgClO4 and AgBF4 and reportedelectivities as high as 110 for ethane/ethylene separation forEBAX-2533 loaded with 36% Ag+.

Crosslinking is widely used as a tool for modifying basicolymers in order to attain enhanced selectivities and renderhe polymer suitable for various applications [18,19]. How-ver, reports on the preparation and application of crosslinkedEBAX for separating CO2/CH4 mixtures are none. Thus, in theresent study, PEBAX-1657 membranes were synthesized androsslinked using 2,4-toluylene diisocyanate for varying inter-als of time. To ensure the occurrence of crosslinking, WAXDtudies were conducted. The effect of crosslinking on the thermalroperties of the membranes was investigated by DSC and theorphological changes were inspected using SEM. The objec-

ive of the study was to investigate the gas permeation propertiesf TDI-crosslinked PEBAX-1657 as well as to evaluate the effectf operating parameters on gas permeation properties of thenmodified and crosslinked membranes. Results observed fornmodified membrane were compared with those obtained forther grade of PEBAX polymer, viz., 2533, which had a lowerolyamide content.

. Experimental

.1. Materials

PEBAX polymer grades were purchased from Atofina Chem-cals, France with 1657 grade specified to have a polyetherontent of 60% and 2533 grade about 80% [20]. 2,4-Toluyleneiisocyanate, dimethyl formamide, m-cresol and isopropanololvents were purchased from Loba Chemie, Mumbai, India.oly(vinylidene fluoride) (PVDF) polymer was supplied byermionics Membranes Pvt. Ltd., Vadodara, India. Cylindersf pure methane and carbon dioxide gases of >99.9% purityere supplied by BOC Gases Ltd., Hyderabad, India, whereasinary mixtures were supplied by Inox Air Products, Mumbai,ndia.

.2. Membrane preparation

Ultra-porous PVDF substrate was prepared by the phasenversion technique from 25% (w/v) solution of the polymer in,N-dimethyl formamide. The film was cast onto a non-wovenolyester membrane support fabric and gelled in an ice-coldater bath for 5 min. The molecular weight cut off (MWCO) ofVDF support was determined to be approximately 10,000 Dasing polyethylene glycol aqueous solution.

Thin film composite (TFC) membranes of PEBAX-1657nd 2533 grades were prepared on PVDF ultrafiltration sup-ort by solution casting and solvent evaporation technique. A

olution of 20% (w/v) of PEBAX was prepared in a solventixture comprising 30 vol% m-cresol + 70% isopropanol, and

he bubble-free solution was cast onto the PVDF support to theesired thickness using a doctor’s blade. Solvent was evaporated

2

t

ochem. Eng. Aspects 297 (2007) 267–274

n an oven at 150 ◦C for 3–5 min to obtain PEBAX-htc (highemperature cured) TFC membrane. The 20% (w/v) polymeroncentration in the casting solution was optimized to maintain aiscosity of at least 3.0 dL/g. Below this concentration, the poreenetration in PVDF support was excessive, which renderedefects in the PEBAX membranes.

Since PEBAX-1657 exhibited better ideal selectivity forO2/CH4 system and is a relatively less investigated poly-er than 2533, it was chosen for further studies involving

rosslinking, characterization and permeability measurements.rosslinking of PEBAX-1657 was carried out by immersing

he membrane in a 2% (v/v) solution of TDI in hexane forarying time intervals. The membranes were removed fromhe bath at appropriate time and washed thoroughly in distilledater for an hour. The membranes were then dried in oven

t 60 ◦C followed by vacuum drying for a period of 24 h toemove the residual solvent present, if any. The chemicaltructure of PEBAX-1657 polymer and a model representinghe crosslinking mechanism between terminal hydroxyl groupsf the polymer and the isocyanate groups of TDI to producerethane linkages is given in Fig. 1.

.3. Membrane characterization

PEBAX-1657 layer of the TFC membrane was peeled offrom the PVDF-polyester support and then subjected to charac-erization studies in order to clearly understand the influence ofhe crosslinked skin layer on the separation performance of CO2nd CH4 gases and also to avoid overlap of peaks arising fromther polymers constituting the composite. Only the proposedeaction mechanism in Fig. 1 is valid, since PEBAX is the denseelective layer in the composite, whereas PVDF and polyesteron-woven layers are highly porous and their reaction with TDIoes not carry any significance, as they would allow free per-eation of all the feed gas molecules even in crosslinked state.

.3.1. Sorption experimentsEquilibrium sorption of unmodified PEBAX-1657 and the

embrane crosslinked for 5 min in TDI/hexane medium werebtained by soaking small pre-weighed strips of the polymerlms in pure water for a period of 48 h or more until equilib-ium was established as indicated by constant weight over longerime period. The surface of soaked films was quickly wipedith tissue paper to remove the adhering liquid droplets and theass sorbed by the membrane was determined using a Sartorius

lectronic balance (accuracy 10−4 g). The % sorption was thenalculated by the equation:

sorption =

⎛⎜⎜⎜⎝

weight of wet membrane

− weight of dry membrane

weight of dry membrane

⎞⎟⎟⎟⎠ × 100 (1)

.3.2. XRDA Siemens D 5000 powder X-ray diffractometer was used

o study the solid state morphology of plain and modified

S. Sridhar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 297 (2007) 267–274 269

rossli

Pgwi

2

bti

2

1mc

2

h1c

wd

t

2

mmdtbusdcs

Fig. 1. Structural representation of (a) PEBAX and (b) c

EBAX-1657. The X-rays of 1.5406 A wavelength wereenerated by a Cu K� source. The angle (2θ) of diffractionas varied from 0◦ to 65◦ to identify the crystal structure and

ntermolecular distances between the intersegmental chains.

.3.3. DSCDSC thermogram of the synthesized PEBAX-1657 mem-

ranes were recorded on a Mettler Toledo 821e instrument in theemperature range of 25–600 ◦C at a heating rate of 10 ◦C/minn nitrogen atmosphere.

.3.4. SEMThe surface and cross-sectional morphology of the PEBAX-

657 membranes were studied by SEM using a Hitachi S2150icroscope. To obtain smooth cross-section, the samples were

ut in liquid nitrogen.

.3.5. Mechanical strength

A universal testing machine (UTM) with an operating

ead load of 5 kN, grip length of 5 cm and testing speed of2.5 mm/min was used for tensile strength measurement. Theross-sectional area of the crosslinked PEBAX-1657 of known

hcci

nking of PEBAX-1657 with 2,4-toluylene diisocyanate.

idth and thickness was calculated and the tensile strength wasetermined using the formula:

ensile strength = load at break

cross-sectional area(2)

.4. Permeability studies

A schematic diagram of the high-pressure gas separationanifold used in permeability studies is given in Fig. 2. A per-eability cell of SS 316 having an effective area of 42 cm2 was

esigned and fabricated in-house. Feed and permeate lines inhe manifold were made of 1/4 in. SS piping connected togethery means of compression fittings. The carrier gas cylinder wassed only to determine the permeation of methane at lower pres-ures (10–20 kg/cm2) by the continuous flow method, which isescribed in detail elsewhere along with the requisite analyti-al procedure [21]. The permeate flow was measured using aoap bubble meter or a wet gas meter, depending on its rate. The

ousing of the spiral module was also provided with 1/4 in. endonnections to enable the easy replacement of the permeabilityell with the module in the same manifold. The feed was keptn continuous flowing condition through partial opening of the

270 S. Sridhar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 297 (2007) 267–274

for g

nad(

rdTs

K

wpgpiSa

α

3

3

3

sui1

3

ip1r

(thiment in the intensity of the crystalline peak gives an indicationof shrinkage in cell size or intersegmental spacing, which wouldimprove the selective permeation property of the membrane[22].

Fig. 2. Experimental manifold

eedle valve on the retentate line, with the pressure maintainedt a constant value by throttling the regulator of the feed cylin-er. The experiments were conducted at ambient temperature30 ◦C) and repeated thrice to ensure reproducibility of data.

Instead of the permeability coefficient, the permeance (K) iseported throughout this work, since it was difficult to accuratelyetermine the exact thickness of the PEBAX skin layer in theFC membrane as well as its extent of penetration into the PVDFupport. The permeance was calculated as

= Q

tA(P1 − P2)(3)

here Q is the volume of permeated gas ((cm3 (STP))), t theermeation time (s), A the effective membrane area (cm2) foras permeation, P1 and P2 are the feed side and permeate sideartial pressures (cmHg), respectively. The unit of permeances denoted as GPU [1 GPU = 10−6 (cm3 (STP)/cm2 s cmHg)].electivity was determined as the ratio of permeances of CO2nd CH4, respectively, by using the equation:

= KCO2

KCH4

(4)

. Results and discussion

.1. Membrane characterization

.1.1. Sorption studies

PEBAX-1657 is a hydrophilic polymer, which showed con-

iderable affinity to water. The equilibrium water sorption ofnmodified PEBAX-1657 was found to be 21.2%. After soak-ng in TDI–hexane bath for 5 min, the sorption was reduced to2.9% indicating the occurrence of crosslinking. F

as permeability measurement.

.1.2. XRDFig. 3 shows the X-ray diffraction patterns of plain and mod-

fied PEBAX-1657. The unmodified PEBAX (Fig. 3(a)) is aartially crystalline polymer, with a narrow diffraction peak at8◦ of 2θ corresponding to crystalline region and amorphousegions at 14◦, 23◦ and 27◦ of 2θ.

Although the pattern obtained for the modified PEBAXFig. 3(b)) is similar, one can observe a more intense diffrac-ion pattern at 18◦ of 2θ indicating that the crosslinking reactionas brought the polymer chains closer to one another resultingn specific arrangement of the polymer matrix. This enhance-

ig. 3. X-ray diffractograms of (a) PEBAX-1657 and (b) crosslinked PEBAX.

S. Sridhar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 297 (2007) 267–274 271

F

3

pacai1maaogb

3

PmcntTptfpctwc

F(

cloPTcostPtFtf

3

ig. 4. DSC thermogram of (a) PEBAX-1657 and (b) crosslinked PEBAX.

.1.3. DSCFor unmodified PEBAX-1657 (Fig. 4(a)), two endothermic

eaks are evident, whose maxima occur approximately at 50nd 165 ◦C. These endotherms can be attributed to fusion of therystalline fraction of the blocks of poly(ethylene oxide) (PEO)nd polyamide (PA), respectively. The peak observed at 42 ◦Cs the melting temperature (Tm) of PEO, whereas the peak at65 ◦C corresponds to the Tm of PA present in the block copoly-er. For the modified PEBAX (Fig. 4(b)), the endothermic peak

ttributed to the crystalline fraction shows a shift to higher valuet around 60 ◦C after crosslinking, whereas the melting peakf polyamide appears invariably at 240 ◦C. These results sug-est that the carbonyl groups of TDI interact with the polyetherlocks resulting in a crosslinked structure.

.1.4. SEMSEM pictures of PVDF, PEBAX-1657 and crosslinked

EBAX are shown in Fig. 5. The surface and cross-sectionalorphologies of PVDF, PEBAX and crosslinked PEBAX are

ombined together for comparison. From Fig. 5(a), it can beoted that the surface morphology of PVDF shows the poreshat are distributed uniformly across the membrane surface.he pores in the parent PVDF ultrafiltration membrane disap-ear upon coating it with PEBAX as evidenced in Fig. 5(b) (athe same magnification factor). Fig. 5(c), representing the sur-ace morphology of crosslinked PEBAX, shows a homogeneousattern without any agglomerations or fractures indicating that

rosslinking of the polymer did not have a significant impact onhe morphology. The dark areas are aromatic-rich hard segments,hile the uniform matrix is enriched in the aliphatic polyether

omponent.

tac

ig. 5. SEM photographs representing the surface morphologies of (a) PVDF,b) PEBAX-1657 and (c) crosslinked PEBAX.

The cross-sectional view of PVDF, PEBAX-1657 androsslinked PEBAX are shown in Fig. 6. Fig. 6(a) shows twoayers representing the presence of micropores of PVDF coatedn a macroporous polyester non-woven fabric support. TheEBAX coated onto PVDF support is displayed in Fig. 6(b).his layer clearly appears dense and defect-free in the magnifiedross-sectional view. In Fig. 6(c), the cross-sectional viewf crosslinked PEBAX matrix is displayed, which clearlyhows two layers of varying color intensities pertaining tohe brighter non-porous PEBAX skin layer and darker porousVDF substrate. The penetration of crosslinked PEBAX across

he entire cross-section of the PVDF layer is visible. Sinceig. 6(c) shows a magnification of the intersection of only the

op two layers of PEBAX and PVDF, the polyester non-wovenabric is not visible.

.2. Permeability results

Pure gas permeability experiments were carried out withwo polymer grades, PEBAX-1657 and 2533, available with theuthors, which had considerable differences in the rigid blockontent. Table 1 compares the rigid block (polyamide) content

272 S. Sridhar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 297 (2007) 267–274

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TP

Mt

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Table 2Effect of crosslinking time on tensile strength and permeation properties ofPEBAX-1657 membrane

Crosslinkingtime (min)

Tensilestrength (MPa)

Permeance (K) (GPUa) Ideal selectivity,KCO2 /KCH4CO2 CH4

0 36 3.7 0.17 21.21 37 1.7 6.2 × 10−2 27.45 48 0.75 1.73 × 10−2 43.1

10 54 0.48 1.02 × 10−2 47.230 59 0.26 5.1 × 10−3 50.66

uffmdotf6gpPstuctfkcMvbh

3

mup

ig. 6. SEM pictures representing the cross-sectional morphologies of (a)VDF, (b) PEBAX-1657 and (c) crosslinked PEBAX.

f both the grades and the permeability results, which exhibitigher permeability of 5.5 barrers for PEBAX-2533 as against.7 barrers observed for PEBAX-1657. The former also gaveower selectivity of 16.2 due to greater content of polyetherlock (80%), which is the soft and more permeable block inhe copolymer.

.2.1. Effect of crosslinking timeThe TFC membrane made from PEBAX-1657 polymer was

rosslinked using 2,4-toluylene diisocyanate for varying inter-als of time ranging from 1 to 60 min. These membranes weressessed for their mechanical stability and subjected to single gasermeability measurements at a constant pressure of 20 kg/cm2

sing pure CO2 and CH4 gas cylinders. Table 2 compares theerformance of the pristine and crosslinked PEBAX-1657 mem-ranes in terms of their CO2 and CH4 permeance and idealelectivity. The tensile strengths of the membranes are also tab-

able 1ermeation properties of two grades of PEBAX polymers at 20 kg/cm2

embraneype

Polyamidecontent (%)

Permeance (K) (GPUa) Selectivity,KCO2 /KCH4CO2 CH4

EBAX-1657 40 3.7 0.17 21.2EBAX-2533 20 5.5 0.34 16.2

a 1 GPU = 10−6 [cm3 (STP)/cm2 s cmHg] and feed pressure = 20 kg/cm2.

tticgbpfiipop

0 62 0.12 2.3 × 10−3 52.4

a 1 GPU = 10−6 [cm3 (STP)/cm2 s cmHg] and feed pressure = 20 kg/cm2.

lated. It can be noted that an increase in the crosslinking timerom 0 to 60 min caused a substantial rise in the tensile strengthrom 36 to 62 MPa. This rise in the mechanical strength of theembrane can be attributed to the formation of urethane groups

ue to the interaction between TDI and terminal hydroxyl groupsf PEBAX. With an increase in the crosslinking time, the selec-ivity of the membrane towards CO2 also increased from 21.2or unmodified PEBAX-1657 to 52.4 for PEBAX crosslinked for0 min. With an increase in the crosslinking time, the carbonylroups of TDI tend to interact with all the available reactiveolyether units resulting in the compaction of chains of theEBAX polymer thereby contributing to the enhancement inelectivity. On the other hand, increasing the crosslinking dura-ion resulted in a fall in the CO2 permeance from 3.7 GPU fornmodified membrane to 0.12 GPU in case of PEBAX-1657rosslinked for 60 min. In the current study, the crosslinkingime for studying the effect of operating parameters such aseed pressure and feed composition has been restricted to 5 mineeping in view the drastic drop in permeance resulting uponrosslinking the membrane for durations greater than 5 min.oreover, keeping commercial membrane casting machines in

iew, an immersion time of greater than 5 min in TDI–hexaneath would not be feasible, especially due to evaporation of theighly volatile hexane.

.2.2. Effect of feed pressureThe effect of feed pressure has been studied for PEBAX-1657

embrane crosslinked for 5 min and has been compared with thenmodified PEBAX. The CO2 and CH4 permeances have beenlotted against feed pressure in Fig. 7, whereas Fig. 8 representshe effect on selectivity. As the pressure was increased from 10o 40 kg/cm2 the permeance for the unmodified PEBAX-1657ncreased from 3.06 to 4.8 GPU whereas that of the TDI-rosslinked membrane varied from 0.6 to 0.9 GPU (Fig. 7). Aradual increase in the permeance with rising pressure maye attributed to increased sorption of the polar CO2 gas in theolymer membrane, which already has a preferential affinityor this gas. Hydrogen bonding between oxygen atoms presentn CO2 gas and the amide functional groups in PEBAX-1657

s also expected to occur. Earlier, we have reported increasedropylene sorption in ethyl cellulose membrane under similarperating conditions [23]. Gas transport through a non-porousermselective membrane follows the solution-diffusion mecha-

S. Sridhar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 297 (2007) 267–274 273

Fc

nCihtmoraoPTsi

3

mb(04

Fc

Fm

tssHspprrt

4

csi

ig. 7. Variation of CO2 permeance with feed pressure for unmodified androsslinked PEBAX-1657 membranes.

ism and hence the sorption of the non-polar and non-interactingH4 molecule remains more or less the same in spite of the

ncreasing pressure [24]. The diffusion coefficient of CO2 is alsoigher than CH4 since its kinetic diameter is 3.3 A as comparedo 3.8 A for CH4 [24]. At all feed pressures, the TDI-crosslinkedembranes showed lower permeance than the unmodified

ne due to the close compaction of chains occurring througheaction of the crosslinker with the polymer matrix. However,

much improved selectivity in the range 37.6–46.7 wasbserved in case of crosslinked PEBAX compared to pristineEBAX-1657 which exhibited selectivity of 17.8–24.8 (Fig. 8).hese results are in accordance with the XRD studies, whichhow an improvement in the intensity of the crystalline peakndicating compaction of the chains in crosslinked PEBAX.

.2.3. Effect of feed composition of binary mixtureThe effect of varying composition of binary CO2/CH4 gas

ixture on performance of crosslinked PEBAX-1657 mem-

rane was carried out at 20 kg/cm2 and ambient temperature28–30 ◦C). Fig. 9 shows a rise in CO2 permeance from 0.4 to.8 GPU and subsequent enhancement in selectivity from 31.7 to0.2, upon increasing the CO2 concentration in the feed from 2

ig. 8. Variation of CO2 selectivity with feed pressure for unmodified androsslinked PEBAX-1657 membranes.

tcfTSeofmiwnaleflmtCbg

ig. 9. Effect of feed composition on performance of crosslinked PEBAX-1657embranes at 20 kg/cm2.

o 20 mol%. Ideally, it is expected that membrane characteristicsuch as permeability and selectivity would remain constant irre-pective of whether the feed is a pure gas or a binary mixture.owever, in actual practice, coupling effect amongst the feed

pecies takes place resulting in a loss in selectivity [25]. In theresent case, a transfer of momentum takes place from the fasterermeating CO2 molecule to the relatively slow CH4 moleculeesulting in dragging of the latter through the membrane bar-ier. This results in retarded CO2 permeation and increased CH4ransport leading to a reduction in selectivity.

. Conclusions

The present study has demonstrated the possibility of usingrosslinked TFC membranes of PEBAX-1657 for CO2/CH4eparation. The greater concentration of rigid polyamide blockn PEBAX-1657 made it more selective but less permeablehan PEBAX-2533 which had twice the soft polyether blockontent. The crosslinking was expected to occur through theormation of urethane linkages between carbonyl groups ofDI and hydroxyl groups of polyether segments of PEBAX.orption studies confirmed the occurrence of crosslinking byxhibiting reduced water sorption values after the reactionf PEBAX-1657 with TDI. The membrane was crosslinkedor varying intervals of time and subjected to permeabilityeasurements and mechanical strength analysis. Of the vary-

ng crosslinking times, PEBAX-1657 crosslinked for 5 minas considered ideal for further studies. SEM studies showedo disruption in the homogeneity of the surface morphologypart from clearly indicating the penetration of the PEBAXayer into the porous PVDF substrate. The CO2 sorption wasnhanced with increasing feed pressure, which improved theux and selectivity. The crosslinked PEBAX showed lower per-eance but higher selectivites than unmodified membranes due

o the close compaction of chains. Compared to single gases,O2 permeance and selectivity dropped marginally in case ofinary mixtures due to coupling effect between CO2 and CH4ases.

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74 S. Sridhar et al. / Colloids and Surfaces A: P

Both pristine and crosslinked PEBAX-1657 membranesppear promising for natural gas purification, since selectivi-ies for H2S/CH4 and H2O/CH4 are expected to be much higherhan the reasonably high values obtained in the present study forO2/CH4 system. The crosslinked PEBAX-1657 can be madeommercially viable by increasing its flux through reduction ofts effective thickness atop the TFC membrane.

cknowledgements

Professor T.M. Aminabhavi thanks the University Grantsommission, New Delhi (Grant No. F1-41/2001/CPP-II) for anancial support to establish the Center of Excellence in Poly-er Science at Karnatak University, Dharwad. Mr. S. Sridhar

hanks the Department of Scientific and Industrial ResearchDSIR), New Delhi for a financial grant to IICT during theourse of this work. Dr. K. Kamal, Dr. S. Nistandra of DSIR andr. S.J. Chopra of University of Petroleum and Energy Stud-

es, Dehradun are thanked for their encouragement. The supportendered by Dr. M. Ramakrishna of IICT and the EIL team ofr. P.K. Sen, Dr. S. Banik and Mr. Dipak Sarkar is gratefully

cknowledged.

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[

[

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