Short Communication

768

Received: 30 June 2009, Revised: 23 August 2009, Accepted: 25 August 2009, Published online in Wiley Online Library: 30 September 2009

(wileyonlinelibrary.com) DOI: 10.1002/pat.1556

Use of block copolymer as compatibilizer inpolyimide/zeolite composite membranes

Rajkumar Patela, Jung Tae Parka, Hyun Pyo Honga, Jong Hak Kima

and Byoung Ryul Mina*

In this work, we introduced a diblock copolymer (dBC

Polym. Adv

), i.e., polystyrene-b-poly(hydroxyl ethyl acrylate) (PS-b-PHEA) asa compatibilizer to enhance interfacial adhesion between PI and zeolite in PI/Zeolite/dBC (1/0.1/0.05wt%) membranefor gas separation. FT-IR spectroscopy showed the formation of hydrogen bonding interactions of the carbonyl andthe hydroxyl in dBC with both PI and zeolite. The differential scanning calorimeter (DSC) study showed that the glasstransition temperature (Tg) of PI increased upon the introduction of dBC, indicating specific interactions in the mixedmatrix membranes. The gas permeabilities of H2, N2, O2, and CO2 through PI/zeolite 5A/dBCmembranes were reducedbut the permselectivity were increased compared to neat PI membrane. Copyright � 2009 John Wiley & Sons, Ltd.

Keywords: mixed matrix membrane; zeolite; diblock copolymer; polyimide; permeability

* Correspondence to: B. R. Min, Department of Chemical and Biomolecular

Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-

749, South Korea.

E-mail: minbr345@yonsei.ac.kr

a R. Patel, J. T. Park, H. P. Hong, J. H. Kim, B. R. Min

Department of Chemical and Biomolecular Engineering, Yonsei University,

262 Seongsanno, Seodaemun-gu, Seoul 120-749, South Korea

INTRODUCTION

Polymer based organic-inorganic composite membrane hasreceived much attention since last a few decades in the field ofmembrane processes. This is inspired by the easy process abilityof organic polymer with excellent separation properties ofinorganic materials.[1–9] There was a significant progress in therubbery polymer-zeolite mixed matrix membrane,[10] whichshowed a significant increase in O2/N2 selectivity, especially athigh-zeolite loading. These membranes are not practicallyattractive because a rubbery polymer might lack mechanicalstability and desirable inherent transport properties relative torigid glassy polymer at high temperatures. However, the mainobstacle to improve gas separation performance in the case ofglassy polymer membranes is the poor polymer-molecular sieveinteraction, leading to the formation of voids between the twophases.[5,10–12]

In order to prepare compatible mixed matrix membrane withbetter selectivity, various approaches have been reported.Surface modification of zeolite with silane coupling agent wasperformed to improve adhesion with polymer matrix.[13–15]

However, significant improvement in permselectivity was notobserved despite indications of good coupling between silaneand zeolite, as reported by Duval et. al.[15] Another methodreported by Mahajan et. al is the high-processing temperatureclose to glass transition temperature (Tg) of polymer matrix in thepresence of non-volatile solvent, which acts as a plastisizer inorder to maintain the polymer flexibility during membraneformation.[16] The drawback of this approach is to find suitablenon-volatile solvent with a high-boiling point to match thetemperature requirement during membrane formation. Analternative way of enhancing compatibilization by simul-taneously forming hydrogen bonding in the mixed matrixmembrane in the presence of 2,4,6-triaminopyrimidine (TAP) wasreported by Yong et. Al.[17] It showed that the O2/N2 and CO2/N2

selectivity of Matrimid/zeolite 4A/TAP membranes increased

. Technol. 2011, 22 768–772 Copyright � 200

around 3-fold compared with pure Matrimid membrane, whileCO2 and O2 permeability decreased around 40-fold.In this work, polystyrene-b-poly(hydroxyl ethyl acrylate) (PS-b-

PHEA) diblock copolymer was synthesized by atom transferradical polymerization (ATRP) and used as a compatibilizingagent for eliminating interfacial voids in the mixed matrixmembrane. Zeolite 5A, having a pore size of 4.5 A, was chosen asa molecular sieve since the mixed matrix membrane containingzeolite 5A showed higher O2/N2 selectivity than that containing4A.[18] The carbonyl and hydroxyl of the diblock copolymer (dBC)are expected to interact simultaneously with PI and zeolite.Interactions, morphologies, and gas transport properties of PI/Zeolite 5A/dBC membranes are reported in this paper.

EXPERIMENTAL

Materials

Matrimid 5218 CH polyimide (PI) was obtained from CibaGeigy. Styrene (99%). 2-Hydroxyl ethyl acrylate (HEA; 99%),1,1,4,7,10,10-hexamethyl triethylene tetramine (HMTETA; 99%),copper(I) chloride (CuCl; 99%), and methyl 2-bromopropionate(MBP) were purchased from Aldrich and used as received withoutfurther purification. The molecular sieve used was zeolite 5A(Aldrich) and its particle size was in the range of 3.5 to 4mm, asdetermined by SEM. Zeolite was dehydrated at 2508C undervacuum before use to remove moisture and organic residue.

9 John Wiley & Sons, Ltd.

Scheme 1. Synthesis route of PS-b-PHEA diblock copolymer (dBC).

USE OF BLOCK COPOLYMER AS COMPATIBILIZER

Synthesis of PS-b-PHEA diblock copolymer

First, 20 g of styrene, 0.296 g of CuCl, and 1.24ml of HMTETA wereadded in a 250ml pear-shaped flask, and then the green mixturewas stirred until a homogeneous solution was obtained. Nitrogenwas purged to the solution for 30min, and then 0.22ml ofMBP was added. The mixture was placed in an 1108C oil bath for5 hr. After polymerization, the resultant polymer was diluted withtetrahydrofuran (THF). After the passage of the solution througha column with activated Al2O3 to remove the catalyst, it wasprecipitated into methanol. Macroinitiator, i.e., polystyrenecontaining Br as an end group (PS-Br) was obtained and driedin a vacuum oven overnight at room temperature.Secondly, 6 g of PS–Br was dissolved in 10ml of toluene with

stirring. HEA (7 g), 0.0888 g of CuCl, and 0.372ml of HMTETA wereadded to the solution. The green mixture was stirred until ahomogeneous solution was obtained and was purged withnitrogen for 30min. The mixture was placed in a 508C oil bath for7 hr. After polymerization, the resultant block copolymer wasdiluted with THF. After the passage of the solution through acolumn with activated Al2O3 to remove the catalyst, it wasprecipitated into methanol. The PS-b-PHEA diblock copolymerwas obtained and dried in a vacuum oven overnight at roomtemperature.

Membrane preparation

PI/Zeolite/dBC mixed matrix membrane with 1/0.1/0.05 weightratio was prepared by conventional solution casting technique.The required amounts of zeolite were stirred in NMP and thensonicated for 10min. PI and dBC were added simultaneously tothe solution (5% w/v) and stirred well until complete dissolution,followed by casting on a glass petri dish. It was dried overnight at808C and then kept in a vacuum oven at 1208C for 2 days toremove the final traces of the residual solvent.

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CHARACTERIZATION

FT-IR spectra of the sampleswere collectedwith an Excalibur seriesFTIR instrument (DIGLABCo., Hannover, Germany) in the frequencyrange of 4000–600 cm�1with an attenuated total reflection facility.The glass transition temperature (Tg) of the membranes wasdetermined by differential scanning calorimeter (DSC Q2000 fromTA Instruments). A small piece of the membrane was heated at aheating rate of 108C/min from �808C to 3478C under nitrogenatmosphere. The sample was then cooled down to �808C andheated again to 3478C with the same procedure for second scan.The second scan thermogram was used to determine the glasstransition temperature (Tg) of the membrane. The cross sectional(fractured in liquidnitrogen)morphologiesof themembraneswereexamined using scanning electron microscopy (SEM, HitachiS4200). The membranes were gold-coated prior to SEM measure-ments. Gas permeability instrument consisted of a membrane cellwith an effective permeation area of 14.7 cm2, a gas flow system,constant temperature bath, and flow measurement equipment.Thepermeationofgaseswasmeasuredbyamassflowmeter (MFM,Brooks, Japan) with capacities of 1000 sccm and the data wererecorded on a personal computer. The feed side pressure rangedfrom0.2 to0.6MPaandthepermeatesidepressurewasvacuumataconstant temperature of 358C. All the tests were conducted underthe permeate side pressure with below 0.1 mbar.

Polym. Adv. Technol. 2011, 22 768–772 Copyright � 2009 John Wiley

RESULTS AND DISCUSSION

Synthesis of dBC

The synthetic route of PS-b-PHEA diblock copolymer is outlined inScheme 1. The first step that produced bromine-terminated PSinvolved the homopolymerization of styrene in bulk at 1108C for5 h in the presence of MBP/CuCl/HMTETA. The obtained PSexhibited a narrow molecular distribution (polydispersityindex¼ 1.3) and a molecular weight of 14,000 g/mol, whichwas determined by GPC. The polymerization yield was as high as90%. The PS-b-PHEA diblock copolymer was then synthesizedwith PS–Br and CuCl/HMTETA as a macroinitiator and catalyst/ligand, respectively. The resultant diblock copolymer showed anarrow molecular distribution (polydispersity index¼ 1.4) and amolecular weight of 24,000 g/mol. The 1H-NMR spectrum showedthat the diblock copolymer has a composition of 56: 44wt % inPS-b-PHEA, and the synthesis via ATRP is successful.[19]

Hydrogen bonding interactions

Figure 1 presents the FT-IR spectra of PS-b-PHEA, PI/Zeolite, PI/Zeolite/dBC, and PI. For PS-b-PHEA diblock copolymer, newstretching bands appeared at 1723 and 1160 cm�1, which wereassigned to –C––O and C–O of PHEA, respectively. These FT-IRspectroscopy results are clear evidences of the sequentialsynthesis of the diblock copolymer by ATRP. The stretchingbands of dBC shifted to 1708 and 1150 cm�1 in the PI/Zeolite/dBC membrane, and thus indicate that the ester groups of PHEAparticipated in the hydrogen bond interactions in themembranes. The hydroxyl peaks of dBC at 3392 cm�1 alsoshifted to 3361 cm�1 in the mixed matrix membrane, implyinghydrogen bond interaction between the carbonyl group of PIand the hydroxyl group of dBC.[17,20] The shift in the carbonylpeak of 1717 and 1723 cm�1 in PI and dBC, respectively, to1708 cm�1 in PI/Zeolite/dBC indicates the hydrogen bondinteractions of dBC with PI and zeolite. There was no differenceof the carbonyl peak at 1717 cm�1 between PI and PI/Zeolite,implying the absence of any interactions between PI andzeolite. Thus, it is concluded that the presence of dBC helps inimproving the compatibility of the mixed matrix membranes,reducing interfacial voids.The effect of dBC on the mixed matrix membrane was

investigated using DSC thermogram, as presented in Fig. 2. Tgvalues of PI and PI/zeolite were 305 and 3068C, respectively. Themarginal increase in Tg by 18C upon the incorporation of zeoliteinto the membrane suggests that there is hardly any interactionbetween PI and zeolite 5A particles. This was supported by thesimilar study of polycarbonate (PC)/zeolite 4A[21] and poly(ethersulfone) (PES)/zeolite 4A mixed matrix membrane.[22] The PI/

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Figure 2. DSC traces of the PI, PI/Zeolite, dBC, and PI/Zeolite/dBC at a

heating rate of 108C/min in N2.

Figure 1. FT-IR spectra of PI, PI/Zeolite, PI/Zeolite/dBC, and dBC.

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Zeolite 5A/dBC membrane exhibited a higher Tg of 3138C thancompared with PI/zeolite membrane 3058C. The increase in Tg by88C with the introduction of dBC results from the increase inpolymer chain rigidity, which might be due to the interaction ofdBC with zeolite and the polymer matrix. Sen et. al reported thatPC/pNA/zeolite 4A had a higher Tg compared to PC/pNA butlower than PC/zeolite 4A.[21] The introduction of zeolite increasedthe Tg of the membrane unlike our systems in which dBCincorporation increases the glass transition of the system. Thisindicates that the dBC plays an important role of compatibilizerbetween the polymer matrix and the inorganic zeolite 4Aparticles.

Membrane morphology

Scheme 2 presents the pictorial demonstration of possibleinteraction of dBC with zeolite and PI. The dBC has pendantfunctional group consisting of ester linkage with end functionalhydroxyl group. The carbonyl group present in the ester linkagemight have hydrogen bond interactions with the hydroxyl groupof zeolite. The hydroxyl group of dBC might have hydrogen bondlinkage with the carbonyl group of the PI mixed matrixmembrane. It should also be noted that the HEA domains ofdBC behave as rubber at room temperature, which is beneficialfor compatabilization of the blend system.Figure 3 shows the SEM cross sectional images of PI/Zeolite, PI/

Zeolite/dBC, and PI/dBC. The dBC is soluble in NMP, which is thesolvent of choice for the PI matrix. Thus, the dope solutions werealways homogenous. When dBC with 0.05 wt% was added to themixed matrix membrane, it was observed that the interfacialvoids were absent. It was also observed that the interfacialcontact between PI and zeolite was improved upon theintroduction of dBC. This result is attributed to (1) the hydrogenbonding interactions of dBC with zeolite and PI matrix and (2) therubbery properties of PHEA chains in dBC. There was hardly anymacrophase separation visible in the PI/dBC matrix, indicatinggood miscibility between PI and dBC.

Gas permeation properties

Gas permeation properties of PI, PI/dBC, PI/Zeolite, and PI/Zeolite/dBC membranes were presented in Table 1. The gaspermeation study shows that the permeability of PI/Zeolite washigher than the pristine PI membrane. This result may be relatedto the interfacial void formation between zeolite particles and PImatrix. When voids are formed, the gas molecules can passthrough the voids without any resistance against permeation.Therefore, the permeability of the membranes containing emptyvoids is generally higher than pure PI membranes. Upon theaddition of dBC, its interaction with matrix and inorganic fillerreduced the interfacial voids, which might be the reason of thedecrease in the gas permeation. Another reason of the reducedgas permeability might be pore blockage by the dBC. This is alsosupported by DSC study, which showed increased Tg resultingfrom the increase in polymer chain rigidity and the diffusivity ofthe larger penetrants was reduced as compared to smaller ones.As a result, the selectivity of oxygen/nitrogen increased by morethan 3-fold. If hydrogen bonds are formed between polymerchains and dBC additive, the free volume of polymers decreases,which results in a decrease in their gas permeability and increasein their gas permselectivity.[17,23]

009 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2011, 22 768–772

Scheme 2. Hydrogen bonding interactions of dBC with zeolite and PI.

Figure 3. SEM cross-sectional images of membranes; A, (A1) PI/Zeolite membrane, B, (B1) PI/Zeolite/dBC membrane and C, (C1) PI/dBC membrane.

Table 1. Gas permeation results through the membranes

Membrane

Permeability (Barrer) Selectivity

H2 O2 N2 CO2 O2/N2 CO2/N2 H2/CO2

PI 16 (�1.8) 1.5 (�0.2) 0.25 (�0.03) 5.76 (�0.8) 6.0 23.04 2.77PI/dBC 6.9 (�0.4) 1.0 (�0.15) 0.065 (�0.01) 1.39 (�0.1) 15.4 21.38 4.96PI/Zeolite 20 (�1.3) 2.8 (�0.2) 0.52 (�0.06) 10.5 (�1.2) 5.38 20.19 1.9PI/Zeolite/dBC 6.7 (�0.7) 0.8 (�0.1) 0.04 (�0.005) 1.2 (�0.11) 20 30 5.58

USE OF BLOCK COPOLYMER AS COMPATIBILIZER

7

CONCLUSION

In this work, mixed matrix membranes were prepared for gasseparation by solution blending method. PS-b-PHEA diblockcopolymer synthesized via ATRP process was used as acompatibilizer in the mixed matrix membrane to enhance

Polym. Adv. Technol. 2011, 22 768–772 Copyright � 2009 John Wiley

interfacial properties of glassy PI matrix and zeolite particles.The carbonyl and hydroxyl groups present in the dBC interactsimultaneously with zeolite and PI polymer matrix, as revealedby FT-IR spectroscopy. As a result, the gas permeability of H2,N2, O2, and CO2 was reduced but permselectivity wasincreased.

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