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Journal of Membrane Science 378 (2011) 444– 452

Contents lists available at ScienceDirect

Journal of Membrane Science

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

Effect of varying solvents compositions on morphology and gas permeationproperties on membranes blends for CO2 separation from natural gas

Sikander Rafiqa,∗, Zakaria Mana, Abdulhalim Mauluda, Nawshad Muhammada, Saikat Maitrab

a Department of Chemical Engineering, Universiti Teknologi PETRONAS, Tronoh 31750, Malaysiab Ceramic Engineering Division, Govt. College of Engineering and Ceramic Technology, West Bengal University of Technology, 73, A.C. Banerje Lane, Kolkata 700010, India

a r t i c l e i n f o

Article history:Received 25 March 2011Received in revised form 13 May 2011Accepted 15 May 2011Available online 20 May 2011

Keywords:Polysulfone/polyimideN-methyl-2-pyrrolidone/dichloromethaneSolubility parameterCO2/CH4 selectivity

a b s t r a c t

Asymmetric polysulfone/polyimide (PSF/PI) blended membranes were developed at compositions of80/20, 50/50 and 20/80 for N-methyl-2-pyrrolidone/dichloromethane (DCM/NMP) solvent mixtures, inorder to understand the mechanism of membrane formation for CO2/CH4 separation performance. Scan-ning electron microscopy (SEM) investigations showed that the membrane surfaces for all compositionswere homogenous and their cross-sections showed non-porous top layer and porous substructure. Mor-phological analysis was further correlated with solubility parameter approach and coagulation value.Results showed that for NMP/DCM solvent mixtures, the solubility parameter difference to ethanoland polymeric membranes was in the order of 80/20 < 50/50 < 20/80. Among various compositions ofNMP/DCM solvent mixtures, membranes prepared from (20/80) composition showed the lowest coagu-lation value. The compatibility of all the membrane blends was confirmed by using differential scanningcalorimetry (DSC). Thermogravimetric analysis (TGA) showed that thermal stability was improved withPI addition in the membranes blends and PSF/PI-20% exhibited the maximum thermal degradation value(Td) of 549 ◦C. While gas permeation results showed maximum CO2/CH4 selectivity values in the range of28.70–28.22 at 2–10 bar feed pressure respectively for PSF/PI-20% prepared from NMP/DCM with solventcomposition of (80/20).

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Membrane technology has been actively practiced for manyyears in the separation of CO2 from natural gas as it offersnumerous advantages over other techniques, which make it quitesuitable for gas separation [1–5]. In this regard, fabrication ofefficient membranes is most important parameter in determin-ing the advantageous nature and efficiency over other separationtechniques. So in the fabrication of membrane development, per-meability and selectivity parameters are of great attention [6]. Forthis reason, membranes acquiring high permeance and satisfactoryselectivity are key issues for efficient CO2 separation.

Many glassy polymeric materials have been used for the fab-ricating membranes for gas separation based upon industrial orscientific work. PSF has been considered to have good thermal,mechanical and chemical stability along with satisfactory gas per-meability and permselectivities [7–9]. These aspects along withits comparative low price recognized PSF as the selection foruse as a standard material in membrane fabrication. The limi-tation of achieving high thermal stability led to the use of high

∗ Corresponding author. Tel.: +60 175453294; fax: +60 53656176.E-mail address: sikanderrr@hotmail.com (S. Rafiq).

performance glassy polymer such as polyimides. These aromaticpolymers exhibit excellent thermal and mechanical stability. Theyhave relatively high glass transition temperature compared to PSFand has attracted many researchers for improved gas permeabil-ities and selectivities [10–12]. However PI is highly susceptibleto plasticization in CO2 atmosphere resulting in reduced perms-electivity of the membrane for operating pressure of above 8 bar[13]. In plasticization, the sorbed CO2 molecules swell the polymermatrix thereby reducing the interaction of adjacent fragments andadjacent chains. This tends to increase the CO2 diffusion free vol-ume, ultimately resulting in increased permeability with decreasedpermselectivity upon increasing feed pressure [14,15]. Macroscop-ically, due to plasticization in the membranes, the glass transitiontemperature (Tg) is reduced while softness and ductility is consid-erably increased [16]. In addition, the high price of PI compared toPSF limits its use for gas separation [17,18]. It has been observedfrom the past studies that PSF exhibits high resistance towardsplasticization above operating pressure of 30 bar while maintain-ing good permselectivity and low cost [19,20]. Thus blending ofPSF and PI polymers is likely to reduce plasticization and impartsuperior properties in the membrane for gas separation [21,22].This approach of blending may optimize the properties of polymersthat belong to glassy families, in terms of gas separation, thermalstability and cost of production.

0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.memsci.2011.05.025

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S. Rafiq et al. / Journal of Membrane Science 378 (2011) 444– 452 445

The use of asymmetric membranes particularly for gas sepa-ration applications is of great interest because of high gas fluxgenerated by these membranes as compared to dense symmetricmembranes. In these asymmetric membranes, high permeation isa consequence of thin dense top skin supported by thick poroussub-layer to provide mechanical strength due to which these mem-branes are favored for commercial use in gas separation processes[23,24]. Dry/wet phase inversion technique was employed for thefabricating these types of asymmetric membranes which involveddrying of the casting solution via evaporation under controlledenvironment followed by wet phase in which casted solution isallowed to precipitate by solvent exchange in a non-solvent pre-cipitation bath [25]. Pinnau and Koros [26] optimized the processof phase inversion forced evaporation technique to prepare thinskin supported by open cell substructure having nodular transi-tion layer. The solvent exchange process had a great influenceon the development of uncontrolled skin thickness. Lin et al.[27] prepared PVDF membrane using mixture of soft and harshcoagulant (water + DMSO). The membrane was prepared by rapidsolvent exchange that exhibited finger-like macro voids rather thansponge-like structures due to the presence of harsh coagulant.

Few attempts were made to explain the mechanism of mem-brane development on account of solubility parameter andcoagulation value. Cheng et al. [28] found smaller solubilityparameter difference between PMMA, solvent and a non-solventproduced finger-like substructure due to fast demixing of the cast-ing solution in non-solvent. However Chun et al. [29] reported thatlarger difference in solubility parameter between the solvent andpolymer and small solubility difference with between the solventand non-solvent produced more porous structures. Based on paststudies on solvent effects parameter, it showed that mechanismsof asymmetric membranes are highly influenced by these solubilityparameter difference and coagulation value measurements.

There are reports on PSF/PI blends based upon dense mem-branes, however, to the best of author knowledge; there are noreports on the study related to asymmetric PSF/PI membraneblends [30]. Hence the main objective of the present study is toinvestigate the fabrication of asymmetric PSF/PI blended mem-branes by various solvent mixtures composition through phaseinversion technique followed by characterization and CO2/CH4 sep-aration performance.

2. Experimental

2.1. Material and methods

Powdered form of PSF Udel® P-1800 was purchased fromSolvay Advanced Polymers, L.L.C, U.S. Powdered PI Matrimid® 5218having 3,3�,4,4�-benzophenone tetracarboxylic dianhydride anddiaminophenylindane monomers.was obtained from HuntsmanAdvanced Materials Americas Inc. The soluble solvents includingN-methyl-2-pyrrolidone (NMP) having 99% purity b.p. 204.3 ◦C,dichloromethane (DCM) with purity of 99%, b.p. 40 ◦C and ethanolwith 99.8% purity was supplied by Merck. Fig. 1(a–d) shows thestructures of polymers and the solvents used.

2.2. Development of asymmetric membrane by phase inversiontechnique

PSF and PI were dried for 24 h prior to their use. The castingsolution was prepared by dissolving 25 g of polymer in the solventto develop membranes of different compositions. The composi-tion of NMP and DCM in the casting solution was varied in variouscompositions in order to understand the effect of solvents on themorphology of the developed membranes. The various composi-

tions of the polymeric blends and the solvents are shown in Table 1.The reaction mixture was stirred in a round bottomed vessel at35 ◦C for 24 h to ensure complete dissolution of polymers in order toproduce a complete homogenous solution. The homogeneous cast-ing solution was then subjected to ultrasonic degasser in TranssonicDigital S, Elma® for 5 h to remove any bubbles from the agitation.This casting solution was then casted on a casting glass and thethickness of the membrane was adjusted to 150 �m using the cast-ing knife at ambient conditions. Stream of nitrogen was suppliedabove the casting solution for 15 s in back and forth motion forevaporation. The casting solution on the glass plate was immersedin a coagulant bath of ethanol at room temperature until the mem-brane was completely detached from the glass plate. The coagulantinduced the precipitation of the membrane film by this phase inver-sion technique. The membrane was air dried for 12 h followed bydrying at room temperature for another 2 days to ensure completedrying of the developed asymmetric membrane. The densities ofthe membranes were then measured by buoyancy measurementapproach in which the membrane sample was weighted in air fol-lowed by weighing after immersing in water at 25 ◦C. Volumes ofthe membrane sample were obtained from the weight differencedivided by the density of water. Eventually dividing membranesample weight in air from its volume, density of membrane samplewas obtained [31] (Table 1).

2.3. Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) enabled the observationof the membrane cross sections and surfaces. The effectivenessof the blend was determined by observation on the photographs.The air dried membrane samples were fractured under cryogenicconditions using liquid nitrogen and dried at 21 ◦C atmosphere.The fractured specimens were coated with gold–palladium alloy(60:40) by using Polaron Range SC7640 before the SEM pho-tographs were taken. Photographs were obtained from LEO 430VPSEM analyzer to examine the morphology of asymmetric mem-branes. Membrane thickness was measured on SEM image usingthe software of SEM equipment.

2.4. Coagulation value determination

Coagulation value determination was carried out by means oftitration of the casting solution having 1 g of polymer and 49 gof solvents against ethanol as a coagulant. The solvents NMP andDCM were added together and stirred for 10 min in order to ensurecomplete mixing. Then the solvents were added to the powderedpolymer and stirred for another 1 h. This dope solution was thentitrated slowly with the non solvent under agitation until the ini-tially clear solution became visually cloudy [32]. The quantity ingrams of the ethanol required for the dope solution to becomecloudy was the coagulation value of the sample.

2.5. Differential scanning calorimetry (DSC)

Each membrane was examined for measuring Tg values underPerkin Elmer, DSC Pyris-1 calorimeter. Samples were prepared bytaking small piece of a membrane in aluminum pan about 10–15 mgin weight followed by thermal scanning in a range of 50–400 ◦C ata heating rate of 10 ◦C rise per minute.

2.6. Solubility parameter estimation

Solubility parameter was calculated to express the nature andmagnitude of the interaction force working between the polymersand solvents. For that purpose, group contribution method was

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446 S. Rafiq et al. / Journal of Membrane Science 378 (2011) 444– 452

C

CH 3

S O

CH 3O

O

O

N CH3

(c) (a)

N

O

O

O

N

CH3 CH3

CH3

O

O

C

Cl

HH

Cl

(b) (d)

Fig. 1. Structures of (a) PSF, Udel® P-1800, (b) PI, Matrimid® 5218, (c) NMP, and (d) DCM.

used to calculate the overall solubility parameter and is writtenas [33]:

ı =√

ı2d

+ ı2P + ı2

h(1)

and

ıd =∑ Fdi

V; ıp =

√∑F2

pi

V; ıh =

√∑ Ehi

V(2)

where ıd, ıp, ıh are the dispersive, polar and hydrogen bonding sol-ubility parameters, respectively, calculated by Van-Kravelen andHoftyzer’s method [34]. The numerical values assigned to eachstructural component of the organic compounds can be obtainedreadily from Table 2. The table indicated the functional groups of thepolymers, solvents and the non-solvent used in the present studyand the number of times any functional group appearing was notedin frequency [34]. On the other hand, according to Barton, the totalsolubility parameter was calculated by using this formula [35]:

ı2 =(∑

ıid�i

)2+

(∑ıi

P�i)2

+(∑

ıih�i

)2(3)

where ı and � is the volume fraction for i species. The total solubilityparameter difference between the casting solution and the ethanol

was calculated by the following equation based upon Hansen’s sol-ubility parameter equation written as [36]:

�ıij =√

(ıi,d − ıj,d)2 + (ıi,p − ıj,p)2 + (ıi,h − ıj,h)2 (4)

where i is the solute and j is the solvent. The parameter follows therule that the smaller �ıij value, the greater the affinity betweensolute and the solvent.

2.7. Thermogravimetry

Thermogravemetric analaysis (TGA) of the membranes wereanalyzed on Perkin Elmer, TGA-7 for evaluating their thermal sta-bility. Samples weighing about 10–15 mg were run from ambientto 900 ◦C with a heating rate of 5 ◦C/min in liquid nitrogen at20 mL/min.

2.8. Gas permeation evaluation

The permeation experimentation for CO2 and CH4 evaluationwas performed on membrane gas permeation testing unit overrange of 2–10 bar pressure. The test unit consisted up of a stainlesssteel paired disk between which the membrane with an effectivearea of 14.54 cm2 was placed, which was tightened by O-rings andflanges to eliminate the chances of gas leakage. Before perform-

Table 1Composition and thickness of developed PSF/PI blended membranes.

Solvent compositions in casting solution (NMP/DCM) (%) Polymer blends Membrane thickness (SEM) (�m) Membrane density (g/cm3)

PSF (%) PI (%)

80/20 100 0 40 ± 0.8 1.15295 5 40 ± 0.5 1.15390 10 36 ± 0.6 1.14985 15 41 ± 0.7 1.15480 20 36 ± 0.5 1.148

50/50 100 0 75 ± 0.6 1.18095 5 73 ± 0.5 1.17990 10 80 ± 0.5 1.18285 15 83 ± 0.6 1.18480 20 80 ± 0.8 1.183

20/80 100 0 136 ± 0.7 1.22695 5 135 ± 0.6 1.22590 10 139 ± 0.8 1.22885 15 138 ± 0.8 1.22780 20 133 ± 0.9 1.225

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Table 2Calculation of overall solubility parameter using molar attraction constants at 25 ◦C for polymers and solvents functional group components by using Eq. (1).

Polymers/solvents Functional groups Components Frequency Overall solubility (MPa)1/2

Fdi (J1/2 cm3/2 mol−1) Fpi (J1/2 cm3/2 mol−1) Fhi (J mol−1)

PSF

CH3 420 0 0 2

19.47

C −70 0 0 1

O 100 401 3000 1

S

O

O

591 0 13490 1

1270 110 0 4

PI

Tertiary amine 20 800 5000 2

12.54C O 290 770 2000 5

CH 200 0 0 9

CH3 420 0 0 3

C 70 0 0 11

1270 110 0 1

NMP

C O 290 770 2000 1

8.528CH2 270 0 0 3

CH3 420 0 0 1

Tertiary amine 20 800 5000 1

DCMCH2 270 0 0 1

25.30

Cl 450 550 400 2

Ethanol

CH3 420 0 0 125.58

CH2 270 0 0 1

OH 210 500 20,000 1

ing the test, the testing unit was completely vacuumed for 0.5 h toremove the residual gases and impurities at a pressure of ≥0.1 bar.CO2 and CH4 gas permeation on the permeate side was measuredby using bubble flow meter [37]. For accuracy, each permeationtest was performed twice at steady state. The permeance of CO2and CH4 gases was calculated by the following formula:

PCO2

l= JCO2

�pCO2

;PCH4

l= JCH4

�pCH4

(5)

where J is the flux of CO2 and CH4 gases, �p is the differential partialpressure of across the membrane for the gases and l denotes the

thickness of the membrane. Thus the ideal selectivity (˛CO2/CH4) is

calculated by taking the ratios of CO2 and CH4 permeance [38].

˛CO2/CH4= PCO2 /l

PCH4 /l(6)

3. Results and discussion

In the present study, various morphological asymmetric mem-branes were prepared with various compositions of PSF/PI blendsas shown in Table 1. The suitable choice of solvents and non sol-vents holds the key role in optimization of developed membranes.The surface and cross sectional views of the development asym-metric membranes of PSF/PI blends are shown in the Figs. 2 and 3,respectively. The structures showed homogenous surfaces of

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448 S. Rafiq et al. / Journal of Membrane Science 378 (2011) 444– 452

Fig. 2. SEM images of membranes surfaces (a) PSF, (b) PSF/PI-10%, (c) PSF/PI-20 and cross sections, (d) PSF, (e) PSF/PI-10%, and (f) PSF/PI-20.

Table 3Solubility parameter differences between coagulant, solvents and polymers.

Components Solubility parameter (MPa)1/2 Solubility parameter difference

For solvent mixtures ıd ıp ıh ımix With coagulant [�ımix−�ıethanol]

NMP/DCM(80/20) 16.36 12.45 7.72 21.96 11.70NMP/DCM(50/50) 16.97 13.98 6.33 22.88 12.29NMP/DCM(20/80) 17.71 15.99 4.74 24.33 15.82

For polymer mixtures ıd ıp ıh ımix With solvent mixture [�ımix−�ıpolymer]

NMP/DCM (80/20) NMP/DCM (50/50) NMP/DCM (80/20)

PSF/PI(100/0) 18.18 1.66 6.77 19.47 10.98 12.39 13.48PSF/PI (95/5) 18.01 2.06 6.77 19.35 10.56 11.97 14.08PSF/PI (90/10) 17.84 2.46 6.77 19.23 10.14 11.56 13.68PSF/PI (85/15) 17.67 2.86 6.77 19.13 9.73 11.15 13.29PSF/PI (80/20) 17.49 3.26 6.77 19.03 9.31 10.74 12.90

pure and blended asymmetric membranes indicating towards thecompatibility between the two glassy polymers. PSF, PSF/PI-10%membranes showed dead end pores of different sizes but thesekinds of pores were not seen in PSF/PI-20% thus making them moreresilient to harsh conditions. The cross-sectional views of the devel-oped membranes showed top skin layer supported by closed-cellsub-layers in the structures [39]. However, with different solventcomposition and polymeric blends produced different morpholo-gies of membranes in terms of skin and sub-porous layers.

The miscibility among the solvents and non-solvents con-stituents was addressed to evaluate the mechanism of membranemorphology. The affinity of the PSF/PI and NMP/DCM (solvents)and ethanol (non-solvent) was examined on the basis of solubil-ity parameter difference approach. Three different compositions ofNMP/DCM solvent mixtures 80/20, 50/50 and 20/80 (also shownin Table 1) were used in membrane formation, which would affectthe solubility parameter of the casting solution. Table 3 shows thesolubility parameter for each solvent mixture. It was found that

the solubility parameter of NMP/DCM solvent mixtures compo-sition with 20/80 was greater than 50/50 and 80/20. The tablefurther indicated that each solvent mixture had dissimilar solu-bility parameter difference i.e. for ethanol [�ımix−�ıethanol] andpolymeric membranes [�ımix−�ı polymer]. It showed that forNMP/DCM solvent mixtures, the solubility parameter difference toethanol as coagulant [�ımix−�ıethanol] and polymeric membrane[�ımix−�ıpolymer] was in the order of 80/20 < 50/50 < 20/80. Therewas decrease in solubility parameter difference as the PI contents inPSF composition were increased for each of NMP/DCM mixtures. Soas the �ı(m-polymer) value decreased, the time required to removethe solvent from the developed membranes increased. It resultedin delayed demixing, as the casted layer on the glass plate wasimmersed in ethanol bath [40–42]. Porous sub-structures with thinskin layer were supposed to be appear in the case of NMP/DCM(80/20) than with (50/50) and (20/80) [43]. It was observed fromSEM images (Fig. 3) that in comparison to NMP/DCM (50/50) and(20/80), membranes prepared with NMP/DCM (80/20) has thin

Fig. 3. SEM images of PSF/PI-20% membranes cross sections with NMP/DCM solvents mixtures (a) 80/20, (b) 50/50, and (c) 20/80.

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S. Rafiq et al. / Journal of Membrane Science 378 (2011) 444– 452 449

(i)

(ii)

8

10

12

14

16

18

20

20/8050/5080/20

Solvent Ratio (NMP/DCM)

Coa

gula

tion

Val

ue (g

)

11

12

13

14

15

16

17

Δδ

(m - ethanol) (M

Pa) 1/2

9

10

11

12

13

20/8050/5080/20Solvent Ratio (NMP/DCM)

Δδ (m

- P

SF/

PI-8

0/20

) (M

Pa)

1/2

11

12

13

14

15

16

Δδ

(m - ethanol) (M

Pa) 1/2

Fig. 4. Solubility parameter difference of solvent mixtures with ethanol forPSF/PI-80/20% blended membrane against (i) coagulation value, and (ii) solubilityparameter difference of solvent mixtures with PSF/PI-80/20% membrane.

skin structure with small size of pores in their substructure witha few eddies seen at the bottom part of the cross-section. This indi-cated that low boiling DCM solvent in least solvent composition ofNMP/DCM (80/20) helped in reducing the skin structure while NMPcontrolling the rate of evaporation causing delayed demixing.

For further investigations of the various compositions of solventeffects used on the rate of demixing, the coagulation value and thesolubility parameter difference with respect to non solvent ethanolwas plotted in Fig. 4(i). The graph showed that among variouscompositions of NMP/DCM solvent mixtures, (80/20) compositionshowed smallest [�ımix−�ıethanol] value that corresponded tohighest coagulation value in contrast to (50/50) and (20/80). Lowercoagulation value indicated that once the casting layer of mem-brane is immersed in ethanol bath, it take less time for demixingand more porous in the membranes substructure prepared fromNMP/DCM (20/80) [44] which was observed by SEM images (Fig. 3).Fig. 4(ii) indicated that for PSF/PI-20% membranes, the solubilityparameter difference is minimum for solvent mixture of NMP/DCM(80/20) and whereas NMP/DCM (20/80) showed the maximum.

The glass transition temperature (Tg) values from the DSC curvesfor the PSF/PI blended membranes has been reported in Table 4. DSCwas carried out for at a heating rate of 10 ◦C/min in order to observethe compatibility of PSF/PI blends. The miscibility of the polymersin the molecular level could be confirmed as all the compositions

(i)

(ii)

1000900800700600500400300200100

(e)

(d)

(c)

(b)

(a)

Wei

ght L

oss

(%)

900800700600500400300200100

(b)

(c)

(d)

(e)

(a)

-dw

/dt (

% C

-1)

Temperature (oC)

Temperature (oC)

Fig. 5. (i) Thermogravimetric (TG) and (ii) derivatives of thermogravimetric curvesof (a) PSF, (b) PSF/PI-5%, (c) PSF/PI-10%, (d) PSF/PI-15%, and (e) PSF/PI-20% at heatingrate of 5 ◦C/min.

exhibited distinct single Tg values [45]. Tg value of each membraneblend increased with the increase in PI content in the membraneindicating improved stability. This is due to the fact that pure PIhas Tg of 302 ◦C, so by addition of PI in the PSF membrane causedincrease in Tg values for the membrane blends.

Thermal stability behaviour is quite important factor in orderto carry out the various separation processes at higher tempera-tures. For that purpose thermogravimetric analysis was performedfor the membrane upon the addition of PI contents in PSF mem-branes. Fig. 5 showed the results obtained for all compositions ofmembrane blends at a heating rate of 5 ◦C/min. It was interest-ing to observe distinctive trends particularly after 380 ◦C. It hasbeen found that with the increase in the composition of PI in themembrane blends, the maximum peak thermal degradation (Td)increased gradually which is obtained by taking the derivatives ofchange in consecutive weights to the heating rate and then plot-ting against temperature. For each PSF/PI membrane blend, singlepeak was found which indicated the miscibility of the two poly-mers. The Td value for PSF membrane was found to be 510 ◦C, whichincreased gradually with addition of PI contents in the membranesfrom 520 ◦C to 549 ◦C for PSF/PI-5% and PSF/PI-20%, respectively(Table 4). This increase in the peak temperatures can be related tothe effect of heat transfer in the material. It might be correlated tothe enhanced blending of the polymers with the increase in the PIcontent resulting in the improvement in thermal stability.

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Table 4Glass transition temperature (Tg) of PSF/PI membranes blends.

Membrane samples Polymer blends Glass transition, Tg (◦C) Maximum thermal degradation temperature, Td (◦C)

PSF (%) PI (%)

1 100 0 185.33 5102 95 5 190.85 5203 90 10 198.67 5374 85 15 204.77 5445 80 20 209.09 549

1086420

5

10

15

20

25

30

35

40

45

50

CO

2 Per

mea

nce

(GP

U)

Pressur e (Bar )

NM P/DCM (80 /20 ) NM P/DCM (50 /50 ) NM P/DCM (20 /80 )

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

CH

4 Perm

eance (GP

U)

Fig. 6. Permeance of CO2 and CH4 for PSF/PI-20% blended membranes prepared bydifferent solvent compositions.

The developed asymmetric blended membranes at various sol-vent compositions were evaluated on the basis of gas separationcharacteristics, by determining the permeances of CO2 and CH4gases and their selectivity against various feed pressures. Fig. 6shows that each membrane with various compositions showed sig-nificant difference in their permeation behaviour for both the gases.It was observed that the permeation of CO2 and CH4 was increasedwith the decrease of the DCM composition in the NMP/DCM solventcomposition. The increase in the permeance for all the membraneson the basis of NMP/DCM solvents composition was in the orderof 20/80 < 50/50 < 80/20. The permeation value decreased from39.34 GPU to 35.03 GPU with increase in the pressure from 2 to10 bar for solvent composition of NMP/DCM (80/20) followed by(50/50) from 30.46 GPU to 24.92 GPU and (20/80) from 25.30 GPUto 20.03 GPU (Fig. 6). It was further seen that with the increase inthe pressure for both the gases, the permeance value decreased

108642-10

-5

0

5

10

15

20

25

30

35

CO

2 Per

mea

nce

(GP

U)

Pressure (Bar)

PSF PSF/PI-5% PSF/PI-10% PSF/PI-15% PSF/PI-20%

0.8

1.0

1.2

1.4

1.6

CH

4 Perm

eance (GP

U)

Fig. 7. Effect of PSF/PI membrane blends on permeance of CO2 and CH4 at variousfeed pressures at various feed pressures.

showing the absence of membrane swelling at higher pressure[2]. This indicated that the developed membranes are resistantto harsh conditions. The permeation of gases was further evalu-ated by SEM images (Fig. 3). The sub porous layer of membranesprepared solvent composition of NMP/DCM (80/20) and (50/50)look similar but the former appeared with thin top selective skinlayer as compared to thick top skins of NMP/DCM (50/50) and(20/80). This characteristic also explains the difference in perme-ance of the gases in the membranes. The CO2 and CH4 permeationfor PSF/PI-20% membranes for each of the three compositionsshowed maximum value in comparison of the other PSF and PSF/PImembrane blends (Fig. 6). It was observed that as PI contents inPSF membrane was increased, CO2 permeance improved signifi-cantly in comparison to improved CH4 gas permeance (Fig. 7). Themembranes prepared from NMP/DCM (20/80) exhibited the low-est CO2 and CH4 gas permeance values among 50/50 and 80/20.

Fig. 8. Comparison of ideal selectivity of CO2/CH4 at various feed pressures for NMP/DCM solvents composition of (20/80): (a)–(e); (50/50): (f)–(j) and (80/20): (k)–(o).

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Fig. 7 still indicated that PSF/PI-20% (having maximum additionof PI contents) exhibited maximum permeance as compared toother PSF/PI compositions, prepared with NMP/DCM (20/80). Fig. 7showed that PSF/PI-20% membrane exhibited maximum perme-ance of 25.30 GPU at 2 bar pressure that decreased to 20.03 GPU at10 bar pressure. This increased CO2 permeance with the increasingPI contents in PSF/PI membrane blends was due to the high affinityfor CO2 in matrix. The linear structure of CO2 gas along with polarmolecular orientation exhibit relatively smaller kinetic diameter of3.3 A compared to CH4 gas molecules. These slow moving CH4 withkinetic diameter of 3.8 A have tetrahedral and non-polar orienta-tion. This high CO2 permeance might be related to the increasedsolubility of CO2 as compared to CH4 molecules. The ideal selectiv-ity (˛CO2/CH4

) graph for all the developed membranes with variousNMP/DCM solvent compositions against various feed pressureshave been shown in Fig. 8. It was observed that the selectivity valuesdecreased with the as the pressure increased from 2 bar to 10 barunder the running conditions. For NMP/DCM (80/20), PSF/PI 20%blended membrane showed the highest selectivity from 2 to 10 barpressure, respectively, (˛CO2/CH4

) = 28.70 − 28.22 in comparisonto other membranes. This declining trend of selectivity againstincreasing pressure has also been observed earlier [46–48]. It wasfurther noticed that the percentage decrease in selectivity value forsolvent mixture of NMP/DCM (80/20) from 2 to 10 bar pressure wasfound to be minimum i.e. 1.67 as compared to 7.88 for NMP/DCM(50/50) and 13.70 for NMP/DCM (20/80). This indicated that themembranes prepared by using solvent compositions of NMP/DCM(80/20) showed better performance at higher pressures withoutlosing much selectivity. The effect of polymer blending along withvarying solvents mixture composition proved to be useful tech-nique that provided deep inside to improved chemical and thermalstability for the polymeric membranes.

4. Conclusion

Asymmetric PSF/PI blended membranes were fabricated by atvarious proportions of PI ranging from 5% to 20% in PSF membraneby phase inversion technique. The effects of solvents NMP/DCMon the fabrication of the membranes were evaluated by usingthree different composition of solvent composition (80/20, 50/50and 20/80). Surface morphology of all the developed membranesshowed homogeneity and uniformity in all the developed mem-brane blends with porous sub-layer and variable skin thickness.It was observed that by increasing the composition DCM in sol-vent mixtures of NMP/DCM, the skin thickness increased NMP/DCM(80/20) < 50/50) < (20/80). Based on the solubility parameter dif-ference, the [�ımix−�ıethanol] and [�ımix−�ıpolymer] was in theorder of 80/20 < 50/50 < 20/80. The Tg value of each membraneblends increased with the increase in PI content in the membraneindicating their compatibility among the other PSF/PI membranes.TGA analysis showed increments in Td values for each membraneblends compared to pure PSF which indicated improvement inthermal stability upon the addition of PI contents in the blendedmembranes and PSF/PI-20% exhibited the maximum Td value.Based upon NMP/DCM solvents composition, the increase in thepermeance for all the fabricated membranes was of the order of20/80 < 50/50 < 80/20. The permeance of value decreased with theincrease in operating pressures from 2 to 10 bar depicting theabsence of membrane swelling at higher pressures. It was foundthat for the NMP/DCM solvent composition of (80/20), PSF/PI 20%membrane showed the highest selectivity of 28.70–28.22 from 2to 10 bar pressure, respectively, showing a minimum decrease of1.67% in selectivity values as compared to other membranes pre-pared from NMP/DCM (50/50) and NMP/DCM (20/80). This effectof blending methodology of glassy polymers helped in membranes

optimization with thermal, physical and gas permeation propertiesalong with cost-effective practicability.

Acknowledgment

The authors acknowledge the financial support provided by Uni-versiti Teknologi PETRONAS in carrying out this work.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.memsci.2011.05.025.

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