Journal of Membrane Science 251 (2005) 51–57

Tunable CO2 transport through mixed polyether membranes

Nikunj P. Patela, Marcus A. Huntc, Sheng Lin-Gibsond,Sidi Bencherifd, Richard J. Spontaka, b, ∗

a Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695, USAb Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA

c Fiber and Polymer Science Program, North Carolina State University, Raleigh, NC 27695, USAd Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

Received 23 September 2004; received in revised form 31 October 2004; accepted 5 November 2004

Abstract

Gas-separation membranes composed of polyethers such as poly(ethylene glycol)diacrylate (PEGda) or poly(propylene glycol)diacrylate(PPGda) exhibit high CO2 solubility selectivity, which makes them attractive for use in H2 and air purification. In this work, we investigate thef PEGdal wever,r sport. Thep r rule ofm Gda matrixd©

K

1

etwog[SCptmI

f

ly,ne-

uri-ies,uip-andable

iv-dnfr

that

0d

actors governing CO2 and H2 transport in mixed polyether matrices. Addition of semicrystalline poly(ethylene oxide)s to amorphousowers the net CO2 permeability and CO2/H2 selectivity due to crystal formation. Gas permeation through the amorphous fraction, hoemains unaffected, confirming the existence of a molecular weight limit below which the entire membrane participates in gas tranermeabilities of CO2 and H2, as well as their activation energy of permeation, in miscible PEGda/PPGda blends follow the lineaixtures over the temperature range explored. Incorporation of amine moieties employed in liquid membranes into either the PEuring crosslinking or the PEG backbone generally reduces CO2/H2 selectivity but occasionally improves CO2 permeability.2004 Elsevier B.V. All rights reserved.

eywords:Polyether; Gas-separation membrane; Reverse selectivity

. Introduction

The increasing interest in H2 as the next-generation fuelxpected to replace dwindling petroleum reserves[1–3] con-inues to drive the development of purification strategies byhich to separate H2 from other light gases. Since mostf the worldwide H2 supply derives directly from syntheticas (“syngas”) via the two-stage water-gas shift reaction

4], removal of CO2 from H2 is of paramount importance.uch separation can be achieved by exposing a high-pressureO2/H2 gas stream to a glassy polymer membrane, whichermits more rapid permeation of H2 molecules (due to

heir smaller size and, hence, higher diffusivity) than CO2olecules through the rigid, size-sieving polymer matrix[5].

n this scenario, however, requisite post-repressurization of

∗ Corresponding author. Tel.: +1 919 515 4200;ax: +1 919 515 3465/7724.

E-mail address:rich spontak@ncsu.edu (R.J. Spontak).

the H2 permeate is economically prohibitive. Alternativethe mixed CO2/H2 stream can be subjected to liquid amibased membranes[6] or zeolitic adsorbents[7] designedto remove CO2 as the permeate, thereby yielding a pfied, high-pressure H2 retentate. While these technologare currently employed for H2, as well as air, purificationthey suffer from inherent drawbacks that can promote eqment corrosion, system fouling, energy consumptionenvironmental contamination. Development of comparpolymer membranes capable of removing CO2 from mixedCO2/H2 streams requires a high CO2 solubility (S) selectiv-ity (αS = SCO2/SH2 > 1), since the corresponding diffusity (D) selectivity (αD = DCO2/DH2) is less than unity anthe net membrane selectivity (αCO2/H2) required for desigand assessment purposes, given byαSαD in the context osolution-diffusion molecular transport[8], must be greatethan unity.

Initial gas-transport studies[9–11] of commercial poly(ether-b-amide) multiblock copolymers have established

376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.

oi:10.1016/j.memsci.2004.11.003

52 N.P. Patel et al. / Journal of Membrane Science 251 (2005) 51–57

these microphase-separated materials exhibit high CO2/H2selectivity due to high CO2 solubility in the relativelylow-molecular-weight (rubbery) poly(ethylene glycol)(PEG) microdomains. This high solubility is attributed tospecific interactions between the penetrant CO2 molecules,each possessing a quadrupolar moment due to the differencein electronegativity between the C and O atoms[12], andthe ether linkages located along the PEG backbone. Dueto the limited number of copolymers available, however,a systematic analysis addressing the effects of copolymercomposition and block length on CO2/H2 selectivity couldnot be performed. In an effort to overcome this shortcoming,we have explored CO2 and H2 transport through miscibleblends of a microphase-ordered triblock copolymer with apoly(ethylene oxide) (PEO) midblock and PEG oligomersdiffering in molecular weight[13]. Our results confirm thatthe locus of CO2 transport occurs through rubbery polyethermicrodomains and that polyether crystallinity is particularlydetrimental to CO2 solubility and, hence, selectivity. Thedeleterious role of PEO crystallinity in CO2 transport islikewise reflected in the recent report of Lin and Freeman[14]. In similar fashion, the glassy or semicrystalline“hard” microdomains of block copolymer membranes mayinadvertently compromise gas-separation efficacy whileserving as physical crosslinks to provide the membrane withs ea las-t havep et anesc condc

2

2

y-s 00g vely),a er( hylm d2 ur-c seda gen-e iseu eo

2

dm)w withI in

detail elsewhere[18]. Briefly, 5 g PEO1, a 2.2 equiv. ofIEM (1.71 g) and 0.2 mL TEA (0.2 mL) were reacted in≈15 mL dichloromethane over freshly activated molecularsieves (≈3 g) for 4 days at ambient temperature. The so-lution was filtered over alumina, and then dried in a vac-uum oven overnight. According to MALDI-TOF MS, thenumber- and weight-average molecular weights were 1299and 1316 g/mol, respectively, whereas the number-averagemolecular weight derived from1H NMR was 1249 g/mol.The chemical structures of the non-proprietary membraneprecursors employed in this study are displayed inFig. 1.Mixed membranes composed of PEGda with either a PEOhomopolymer, the PPGda or the AEMA, as well as mem-branes of the amine-containing PEGudm oligomer, wereprepared according to the procedure detailed earlier forpure PEGda membranes[16]. For each formulation exam-ined, predetermined amounts of the constituent species weremixed together at 23◦C to form a homogeneous solution,which was subsequently degassed under vacuum and sub-jected to free-radical crosslinking in the presence of 0.5 wt%AIBN for ∼18 h at 80◦C. The resultant defect-free mem-branes measured between 40 and 200�m in thickness, andtheir gas permeabilities exhibited no dependence on filmthickness over this range. Pure-gas permeation tests wereperformed at various temperatures and an upstream pres-s 5 e,v -m ecific

F em-p (b)p da),( leneglycol)urethane dimethacrylate (PEGudm). Molecular weight characteris-tics, whenever appropriate, are included in the text.

ufficient mechanical integrity[15]. This trade-off can bvoided through the use of chemically crosslinked (eomeric) PEG diacrylate (PEGda) membranes, as wereviously demonstrated[16,17]. In this work, we explor

he gas-transport properties of mixed polyether membromposed of crosslinked PEGda and modified with a seomponent, such as another polyether or an amine.

. Experimental

.1. Materials

A liquid PEGda oligomer (M= 700 g/mol), three semicrtalline PEO homopolymers (M= 1000, 10,000 and 100,0/mol, designated PEO1, PEO10 and PEO100, respectiliquid poly(propylene glycol) diacrylate (PPGda) oligomM= 900 g/mol), triethylamine (TEA), 2-isocyanatoetethacrylate (IEM), 2,2′-azobisisobutyronitrile (AIBN) an-aminoethyl methacrylate (AEMA) hydrochloride were phased from Aldrich Chemical (Milwaukee, WI) and us-received. Several commercial acrylated amines wererously supplied by Sartomer, Inc. (Exton, PA) and likewsed without further purification. The CO2 and H2 gases werbtained from National Specialty Gases (Durham, NC).

.2. Methods

A crosslinkable PEG urethane dimethacrylate (PEGuas synthesized by reacting the PEO1 homopolymer

EM and TEA according to the procedure described

ure of 6.8901× 10 Pa (6.8 atm) using the constant-volumariable-pressure technique[19], in which a specimen reains under vacuum until it is exposed to a gas at a sp

ig. 1. Chemical structures of the principal membrane precursorsloyed in this study: (a) poly(ethylene glycol)diacrylate (PEGda),oly(ethylene oxide) (PEO), (c) poly(propylene glycol)diacrylate (PPGd) 2-aminoethyl methacrylate (AEMA) hydrochloride and (e) poly(ethy

N.P. Patel et al. / Journal of Membrane Science 251 (2005) 51–57 53

pressure. The gas permeability (P) thus measured is deter-mined from

P = Vl

ART p

(dp

dt

)(1)

whereV is the downstream volume,l the membrane thick-ness,A the membrane area,R the universal gas constant,T the absolute temperature, p the transmembrane pressure(=p2 −p1, wherep2 andp1 are the upstream and downstreampressures, respectively), and dp/dt the steady rate at whichpressure increases on the downstream side. Temperature wascontrolled to within±1◦C with a thermal regulator connectedto a heater in the permeation cell wall, andp1 was held con-stant at 0.00709275× 105 Pa (0.007 atm) (so that p≈p2).The estimated uncertainty in reportedP values is less than10%. Differential scanning calorimetry (DSC) was conductedon select specimens with a TA DSC Q1000 instrument oper-ated at a heating rate of 10◦C/min under helium.

3. Results and discussion

The permeabilities of CO2 and H2 in mixed PEGda/PEOmembranes containing 10 wt.% PEO are presented as func-tions of PEG/PEO molecular weight (M) at 23◦C in Fig. 2aand clearly reveal that the incorporation of a chemically iden-t gnif-i ofo nt

Fm EO.T ess f pureP

are not included here for this reason.) The decrease in CO2permeability evident inFig. 2a can be explained in terms ofmolecular transport through a dense polymer membrane, asdictated by the solution-diffusion mechanism proposed byGraham[8]. Once a penetrant molecule absorbs on the high-pressure (upstream) side of the membrane, it diffuses throughthe membrane due to an applied pressure gradient and ul-timately desorbs on the low-pressure (downstream) side ofthe membrane. When the downstream pressure can be ne-glected (as it is here) and Fickian diffusion constitutes therate-limiting mechanistic step, the permeability of speciesican be expressed as

Pi = Di × Si (2)

whereDi andSi represent the diffusivity and solubility, re-spectively, ofi. It immediately follows from Eq.(2) that a re-duction inPmust be the result of a corresponding reductionin eitherD orS(or a combination of bothD andS). Exclud-ing the acrylate terminal groups on the PEGda molecules andthe hydroxyl terminal groups on the PEO molecules, the onlydifference between the PEGda oligomer and PEO homopoly-mers is the number of repeat units along the backbone (n inFig. 1) and, thus, the ability of the chains to enter into registerand crystallize[20]. The molecular weight of the PEGda issufficiently low to ensure that this oligomer exists as a liquid,w allines

xedmt bsta-c y tra-v ret PEOc le fornc e topt on in-t erl cces-sW y ofHm n to-g o(c solu-b ef( re-fl losepP

ical, yet crystallizable, polyether species promotes a sicant reduction in CO2 permeability. (The permeabilitiesther light gases, such as O2 and N2, consistently lie betwee

hose of CO2 and H2 in pure PEGda membranes[16,17]and

ig. 2. (a) The permeabilities of CO2 (©) and H2 (�) as a function of PEOolecular weight (M) in PEGda/PEO membranes containing 10 wt.% Phe corresponding CO2/H2 selectivities are displayed in (b). The solid linerve to connect the data, and the dashed lines indicate the limits o

EGda. Note that 1 Barrer = 10−10 cm3 (STP) cm/cm2 s cmHg. α

hereas the three PEO homopolymers are semicrystolids.

The formation of PEO crystals in PEGda/PEO miembranes impactsPCO2 and, to a much lesser extent,PH2 in

wo ways. Crystals generally behave as impermeable oles that hinder the motion of penetrant molecules as theerse the membrane[21], thereby promoting a longer, moortuous diffusive pathway. In this case, the existence ofrystals in the PEGda/PEO membranes is responsibet reductions in bothDCO2 andDH2. Moreover, the PEOhains immobilized within the crystals are inaccessiblenetrant molecules. Since the high solubility of CO2 within

hese linear polyether membranes depends sensitivelyeractions between penetrant CO2 molecules and the ethinkages along the polyether backbone, reduced chain aibility is accompanied by a concurrent decrease inSCO2.hile the presence of PEO crystals affects the solubilit

2 in similar fashion, the corresponding reduction inSH2 isuch less pronounced. These two considerations takeether explain why bothPCO2 andPH2, as well as their ratidesignated the CO2/H2 selectivity,αCO2/H2, in Fig. 2b), de-rease in the presence of crystallizable PEO. Since theility of a penetrant species (S) is proportional to the volum

raction of amorphous material (φa) [22] and the diffusivityD) scales asφm

a (wherem is an empirical parameter thatects the presence of immobilized chain segments in croximity to crystals[23]), it follows that αCO2/H2 for theEGda/PEO mixed membranes can be written as

CO2/H2 = α0φ ma (3)

54 N.P. Patel et al. / Journal of Membrane Science 251 (2005) 51–57

Fig. 3. (a) DSC thermograms collected from the three PEGda/PEO blendsdescribed inFig. 2.The corresponding crystallinities (Xc) are presented as afunction of PEO molecular weight in (b). The solid line in (b) is a logarithmicregression to the data.

whereα0 is the CO2/H2 selectivity of the fully amorphousPEGda membrane. Values ofφa can be discerned from thedegree of crystallinity (Xc) discerned from DSC analysis ofthe membranes.

According to the DSC thermograms displayed inFig. 3a(first heat cycle), all three PEGda/PEO blends exhibit peakmelting temperatures (Tm’s) located between 56 and 59◦C.In two instances, the melting peak is either split (PEO1) orbroadened (PEO100), revealing the coexistence of differentcrystal populations. Values ofTm measured for the neat ho-mopolymers are 46, 63 and 65◦C, respectively, indicatingthat the PEGda oligomer serves to shift theseTm’s to a rela-tively narrow range (which is retained upon thermal cycling)and thus alter the extent to which the PEO chains enter intoregister[24]. The area under the melting peak provides thespecific heat of melting ( hm), which when normalized withrespect to the specific heat of melting for 100% crystallinePEO (186.2 J/g from Ref.[25]) yieldsXc. This measure ofcrystallinity, displayed as a function ofM for the PEGda/PEOmembranes and pure PEO homopolymers inFig. 3b, is con-verted toφa from the mass densities of the PEGda oligomer(1.12 g/cm3 according to the manufacturer) and the amor-phous and crystalline fractions of the PEO homopolymers(1.05 and 1.23 g/cm3, respectively[26]) at ambient temper-ature. The resultant variation ofαCO2/H2 with φa is showni Eq.( dp mor-p nt of

Fig. 4. CO2/H2 selectivity presented as a function of the amorphous polymerfraction (φa) discerned from the degree of crystallinity (Xc) measured bythermal calorimetry. The solid line is a regressed fit of Eq.(3) to the data.

M. The value of m ascertained by fitting Eq.(3) to thedata is 3.2[27]. Comparable analysis[13] of gas transportthrough a poly(styrene-b-ethylene oxide-b-styrene) (SEOS)triblock copolymer imbibed with a low-molecular-weightPEG oligomer (M= 400 g/mol) yields m≈ 4 (inclusive ofcontributions from the glassy S), which is in fair quantita-tive agreement with the result obtained above for the presentcrosslinked PEGda/PEO blends.

The results displayed inFigs. 2–4confirm that the CO2/H2selectivity of amorphous polyethers possessing an ethyleneoxide repeat unit and increasing in molecular weight ulti-mately reaches a limit beyond whichαCO2/H2 does not changewithin experimental uncertainty. This limit, first proposed[17] on the basis of gas transport in pure PEGda membranesdiffering in M up to 700 g/mol, reflects the importance ofend-group contributions in low-M PEG oligomers and com-promises the potential utility of these materials in H2 purifica-tion. Strategies developed[17] to overcome this shortcominginvolve temperature variation, sinceαS increases whileαDdraws closer to unity, with decreasing temperature. Anotherapproach employs the addition of surface-modified nanoscalefillers, such as fumed silica and layered silicates. Althoughthese fillers generally improve the mechanical properties ofthese membranes and occasionally improve CO2 solubilityat very high CO2 pressures, they typically reduce both CO2p rear O)i estst nityfc dP a-rs wert Gdaa therec

P

n Fig. 4 and demonstrates that the functional form of3) accurately describes CO2/H2 selectivity in these mixeolyether membranes and that the selectivity of the ahous fraction of the PEO homopolymers is independe

ermeability and CO2/H2 selectivity at ambient temperatund modest CO2 upstream pressures. O’Neill et al.[28] haveeported that the solubility of poly(propylene oxide) (PPn CO2 is much higher than that of PEO, which sugghat rubbery PPGda exhibits a substantially higher affior CO2 than PEGda. Indeed, our previous efforts[29] haveonfirmed that the permeability of CO2 in the crosslinkePGda described in Section2 is 85% higher than the comp

ably crosslinked PEGda at 23◦C. Unfortunately, the CO2/H2electivity of the PPGda membrane is substantially lohan that of the PEGda membrane. Blending both PEnd PPGda prior to crosslinking results in mixed polyelastomers whose CO2 and H2 permeabilities at 23◦C mostlosely follow the linear rule of mixtures, viz.

i = WPEGPi,PEG+ (1 − WPEG)Pi,PPG (4)

N.P. Patel et al. / Journal of Membrane Science 251 (2005) 51–57 55

Fig. 5. Variation of (a) CO2 permeability, (b) H2 permeability and (c)CO2/H2 selectivity with composition (WPEG) in PEGda/PPGda mixed mem-branes at three different temperatures (in◦C): 23 (©), 35 (�) and 50 (♦). Thesolid lines in (a) and (b) are predictions based on the linear rule of mixtures(Eq.(4)), whereas those in (c) are predicted from Eq.(5) in the text.

whereWPEG is the mass percent of PEGda andPi,j denotesthe permeability of gasi in polymerj.

This result is likewise reflected inFig. 5a and b, whichshows the dependence of CO2 and H2 permeabilities, respec-tively, on PEGda content in PEGda/PPGda membranes atthree different temperatures. The linearity of the isothermaldata sets under these various conditions confirms the gen-eral applicability of Eq.(4) to PEGda/PPGda mixed mem-branes, which remain, on the basis of two independent ob-servations, homogeneous. Differential scanning calorimetryindicates that (i) the glass transition temperatures (Tg’s) ofthe neat PEGda and PPGda materials are−40 and−44◦C,respectively, which are substantially higher than the reported[26] values for PEO (−66◦C) and PPO (−68◦C) due tocrosslinking-induced chain rigidification and (ii) theTg’sof the PEGda/PPGda blends lie within this narrow range(data not shown). Moreover, films of the neat polymers andtheir blends appear optically transparent. While we recognizethat PEG/PPG blends may exhibit complex phase behaviordue to hydrogen bonding[30,31], we have no evidence tosuggest that phase demixing occurs within the crosslinked

PEGda/PPGda mixed membranes over the range of condi-tions investigated here. The composition-dependent CO2/H2selectivity corresponding to the CO2 and H2 permeabilitiespresented inFig. 5a and b, respectively, is provided as func-tions of WPEG and temperature inFig. 5c. Predictions ofαCO2/H2 derived from Eq.(4), namely

αCO2/H2 = WPEGPCO2,PEG+ (1 − WPEG)PCO2,PPG

WPEGPH2,PEG+ (1 − WPEG)PH2,PPG(5)

are included for completeness.Permeability data such as those shown inFig. 5a and b

can likewise be analyzed in terms of their temperature depen-dence, since permeation is typically described as a thermallyactivated transport mechanism exhibiting Arrhenius behavior[32], in which case

P = P0 exp(−EP/RT ) (6)

Here,P0 is a pre-exponential factor andEP is the activa-tion energy for permeation, which is equal to the sum of theheat of sorption ( HS) and the activation energy of diffu-sion (ED) in the event thatSandD likewise (and expectedly)exhibit Arrhenius behavior. The temperature dependence ofCO2 and H2 permeation in the neat PEGda and PPGda mem-branes, as well as one of their blends, is evident inFig. 6aand confirms that the data are accurately described by Eq.(6).V

F -b nes atthree differentWPEG(in wt.%): 100 (circles), 25 (triangles) and 0 (diamonds).Values of the activation energy of permeation (EP) extracted from data setssuch as those in (a) by Eq.(6) in the text (solid lines) are provided as afunction ofWPEG in (b). The solid lines in (b) are linear regressions to thedata.

alues ofEP discerned for CO2 and H2 permeation by fitting

ig. 6. (a) Dependence of CO2 and H2 permeability (open and filled symols, respectively) on temperature for PEGda/PPGda mixed membra

56 N.P. Patel et al. / Journal of Membrane Science 251 (2005) 51–57

Eq. (6) to data such as those inFig. 6a are displayed as afunction of membrane composition inFig. 6b. These resultsreveal a very interesting trend: values ofEP for CO2 andH2 increase linearly with increasingWPEG, but the changein EP for H2 is much more pronounced. In fact, the changein EP for CO2 transport is almost negligible, which impliesthat HS +ED is nearly constant. If CO2 is more soluble inPPG than PEG and since HS for CO2 in PEG and PEOis negative (indicative of exothermic mixing and estimated[33,34] to be about−13 kJ/mol independent of pressure), itfollows that HS for CO2 should be more negative in the PPGmatrix. Thus, forEP to remain nearly constant,ED must behigher in the pure PEG membrane than in the analogous PPGmembrane, in agreement with intuitive expectations based onchain-packing (free-volume) considerations.

Now we turn our attention to H2 permeation in PEGdaand PPGda membranes. Sorption of a penetrant moleculeproceeds by a two-step process wherein the molecule firstcondenses and then mixes in a nanovoid created within thepolymer [35]. In this scenario, HS is the sum of the twocorresponding enthalpy changes: Hcondand Hmix, respec-tively. For nonpolar, permanent gases such as H2, Hcondis usually negligible so that HS is dominated by Hmix,which, in turn, is approximately zero if a penetrant gas doesnot interact to any appreciable extent with the polymer ma-t ro,id asei overr n us-i tureaH ,a dam asureoc ionalm

hereh er orP g ana red.R ht logi-c anesb therm s intoP t.%A uc-t in-c lda st7 iala kingt e

additive, which increased the CO2 permeability of the PEGdamembrane to∼81 Barrers) and increased H2 permeability sothat the corresponding CO2/H2 selectivities generally mea-sure between 5.5 and 7.3. These amine-induced reductionsin gas-separation efficacy suggest that the amine moieties arenot interacting with penetrant CO2 molecules, in which casethey may be inadvertently interacting with the ether linkagesof neighboring polyether units or irreversibly forming carba-mates in the presence of CO2. In addition, we cannot dismissthe possibility that the amine groups are chemically modifiedduring the free-radical crosslinking reaction, but this possi-bility is beyond the scope of the present work.

Incorporation of amine groups into the membrane precur-sor may provide an alternative route by which to introduceamine functionality into polyether membranes. This avenueis examined through the use of the PEGudm precursor de-picted inFig. 1e. The PEGudm oligomer, a model materialdesigned for use in hydrogels for biomedical applications[18], consists of a PEG backbone (M= 1000 g/mol) with ure-thane methacrylate termini. Resultant PEGudm membranesgenerated after AIBN-induced free-radical crosslinking arefound to exhibit unique gas-transport properties: the CO2 andH2 permeabilities are both significantly reduced (to 43 and5.7 Barrers, respectively), and the CO2/H2 selectivity onlymodestly decreases (to 7.5), at 23◦C. Two points must bec s: (i)t rys-t da.U yn-t ixedmt el or-t esei

4

anesb PEOh dam -i ofP sm alc ctioni itiono bilityo isv ndentC ior,i iono inl angei tion

rix. Thus, HS for H2 is expected a priori to be close to zen which case the increase inEP with increasingWPEG evi-ent inFig. 6b is attributed almost exclusively to an incre

n ED, which is consistent with the conclusion drawn abegarding CO2 permeation. These implications of theEP dataeported here are amenable to experimental verificationg gravimetric sorption techniques. With the temperand composition dependence ofEP established for CO2 and2 permeation, it is possible to tailor the CO2 permeability ins well as the CO2/H2 selectivity of, a given PEGda/PPGixed membrane. Since permeability constitutes a mef throughputwhereas selectivity is a measure ofquality, bothharacteristics must be simultaneously considered in ratembrane design.Thus far, the mixed polyether membranes examined

ave consisted of PEGda and either PEO homopolymPGda oligomer. In this section, the effect of introducinmine moiety into the PEGda network is briefly exploecall that contemporary CO2 removal is achieved throug

he use of liquid amine membranes. To avoid the technoal and environmental drawbacks of such liquid membrut incorporate their functionality into the present polyeembranes, we have introduced several reactive amineEGda prior to free-radical crosslinking. Addition of 10 wEMA (seeFig. 1d), for instance, promotes a slight red

ion in CO2 permeability (from 70 to 64 Barrers), but anrease in H2 permeability (from 7.4 to 9.0 Barrers), to yienet decrease in CO2/H2 selectivity (from 9.5 to a mode

.2) at 23◦C. Similarly, incorporation of several commerccrylated amines into the PEGda matrix prior to crosslin

ypically results in reduced CO2 permeability (except for on

onsidered in evaluating the importance of these findinghe PEGudm oligomer is waxy and, by inference, semicalline at 23◦C and (ii) the membrane contains no PEGsing a shorter (liquid) PEG oligomer in the PEGudm s

hesis or addition of PEGudm to PEGda to produce a membrane is anticipated to improveαCO2/H2. In addition,

he concurrent reductions in CO2 and H2 permeabilities arikewise intriguing and merit further investigation. Unfunately, insufficient PEGudm is available to explore thssues at this time.

. Conclusions

We have prepared a variety of mixed polyether membrased, for the most part, on a PEG oligomer. Blendingomopolymers varying in molecular weight into the PEGatrix prior to crosslinking reduces both CO2 permeabil

ty and CO2/H2 selectivity due solely to the formationEO crystals. Moreover, the CO2/H2 selectivity increasearginally, while the crystallinity discerned from therm

alorimetry decreases (and, hence, the amorphous francreases), with increasing PEO molecular weight. Addf PPGda into the PEGda matrix increases the permeaf CO2 and H2 according to the rule of mixtures, whicherified at several temperatures. The temperature-depeO2 and H2 permeabilities both exhibit Arrhenius behav

n which case values ofEP have been extracted as a functf membrane composition. WhileEP generally increases

inear fashion with increasing PEGda content, the chn EP for H2 is much more pronounced. This observa

N.P. Patel et al. / Journal of Membrane Science 251 (2005) 51–57 57

Fig. 7. Robeson-type trade-off analysis[36] between CO2/H2 selectivity andCO2 permeability in a wide variety of conventional homopolymers (©), aswell as in the PPGda- and amine-modified PEGda membranes (� and�,respectively) and the single PEGudm membrane (�) investigated in thisstudy. The dashed line serves as a guide to the eye indicating the apparentupper limit of the trade-off, whereas the solid line is a linear regression ofthe present PEGda/PPGda data.

is consistent with greater CO2 solubility in PPG relative toPEG. Lastly, chemical incorporation of amine functionali-ties into the PEGda matrix, as well as the synthesis of aurethane methacrylate-terminated PEG oligomer, results inmembranes that exhibit marginally lower CO2/H2 selectivity,but occasionally higher CO2 permeability. Nonetheless, thesemixed membranes still exhibit CO2 selectivity, as evidencedin Fig. 7, which shows the selectivity–permeability trade-offintroduced in the seminal work of Robeson[36]. This studydemonstrates that the future development of CO2-selectivemembranes based on linear polyethers requires a multicom-ponent approach wherein systematic component variation[37] is needed to ascertain the underlying physical and chem-ical properties.

Acknowledgments

This work was supported by the U.S. Department ofEnergy under Contract No. DE-FG02-99ER14991. Certaincommercial materials and equipment are identified in thispaper in order to specify adequately the experimental proce-dure. In no case does such identification imply recommenda-tion by the National Institute of Standards and Technologynor does it imply that the materials or equipment identifiedi

R

964.84

74.

[8] T. Graham, Philos. Mag. 32 (1966) 401.[9] K.-I. Okamoto, M. Fujii, S. Okamiyo, H. Suzuki, K. Tanaka, H.

Kita, Macromolecules 28 (1995) 6950.[10] V.I. Bondar, B.D. Freeman, I. Pinnau, J. Polym. Sci. B: Polym. Phys.

37 (1999) 2463;V.I. Bondar, B.D. Freeman, I. Pinnau, J. Polym. Sci. B: Polym. Phys.38 (2000) 2051.

[11] J.H. Kim, S.Y. Ha, S.Y. Nam, J.W. Rhim, K.H. Baek, Y.M. Lee, J.Membr. Sci. 186 (2001) 97.

[12] B. Garzon, S. Lago, C. Vega, E. de Miguel, L.F. Rull, J. Chem.Phys. 101 (1994) 4166.

[13] N.P. Patel, R.J. Spontak, Macromolecules 37 (2004) 2829.[14] H. Lin, B.D. Freeman, J. Membr. Sci. 239 (2004) 105.[15] G. Holden, N.R. Legge, R.P. Quirk, H.E. Schroeder (Eds.), Thermo-

plastic Elastomers, 2nd ed., Hanser, Munich, 1996.[16] N.P. Patel, A.C. Miller, R.J. Spontak, Adv. Mater. 15 (2003)

729.[17] N.P. Patel, A.C. Miller, R.J. Spontak, Adv. Funct. Mater. 14 (2004)

699.[18] S. Lin-Gibson, S. Bencherif, J.A. Cooper, S.J. Wetzel, J.M. An-

tonucci, B.M. Vogel, F. Horkay, N.R. Washburn, Biomacromolecules5 (2004) 1280.

[19] R.M. Felder, G.S. Huvard, in: R. Fava (Ed.), Permeation, Diffusionand Sorption of Gases and Vapors, vol. 16C, Academic Press, NewYork, 1978, p. 315.

[20] K. Pielichowski, K. Flejtuchm, J. Macromol. Sci. B: Phys. 43 (2004)459.

[21] J.H. Petropoulos, J. Membr. Sci. 53 (1990) 229.[22] A.S. Michaels, H.J. Bixler, J. Polym. Sci. 50 (1961) 393.[23] D.H. Weinkauf, D.R. Paul, in: W.J. Koros (Ed.), Barrier Polymers

, DC,

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eferences

[1] S. Young, Nature 414 (2001) 487.[2] J.M. Ogden, Phys. Today 55 (2002) 69.[3] R.D. Cortright, R.R. Davda, J.A. Dumesic, Nature 418 (2002)[4] W.C. Kratz, D.L. Rarig, J.M. Pietratonio, AIChE Symp. Ser.

(1988) 36.[5] S.A. Stern, J. Membr. Sci. 94 (1994) 1.[6] J.T. Yeh, H.W. Pennline, K.P. Resnik, Energ. Fuel. 15 (2001) 2[7] A. Tavolaro, E. Drioli, Adv. Mater. 11 (1999) 975.

and Barrier Structures, American Chemical Society, Washington1990, pp. 60–91.

24] M.S. Lisowski, Q. Liu, J. Cho, J. Runt, F. Yeh, B.S. Hsiao, Mamolecules 33 (2000) 4842.

25] H.Z. Geng, R. Rosen, B. Zheng, H. Shimoda, L. Fleming, J.O. Zhou, Adv. Mater. 14 (2002) 1387.

26] D.W. van Krevelen, Properties of Polymers, 3rd ed., Elsevier, Aterdam, 1997.

27] The most physically meaningful independent values ofm giving riseto m (= mCO2 − mH2) in Eq. (3) are determined from the permation data inFig. 2a to be 3.2 for CO2 and 0 for H2, indicating thathe presence of PEO crystals influences the molecular transpCO2, but has negligible effect on H2 permeation, in the PEGda/PEblends investigated. From a sensitivity analysis of the data,m isfound to range from 2.9 to 3.8 for CO2 and from−0.3 to 0.6 forH2, but m remains constant at 3.2.

28] M.L. O’Neill, Q. Cao, M. Fang, K.P. Johnston, S.P. Wilkinson, CSmith, J.L. Kerschner, S.H. Jureller, Ind. Eng. Chem. Res. 37 (13067.

29] N.P. Patel, C.M. Aberg, A.M. Sanchez, M.D. Capracotta, J.D. MaR.J. Spontak, Polymer 45 (2004) 5941.

30] M.M. Coleman, J.F. Graf, P.C. Painter, Specific Interactions anMiscibility of Polymer Blends, Technomic, Lancaster, PA, 1991

31] S. Eckert, G. Meier, I. Alig, Phys. Chem. Chem. Phys. 4 (203743.

32] J.H. Petropoulos, in: D.R. Paul, Y.P. Yampol’skii (Eds.), PolymGas Separation Membranes, CRC Press, Boca Raton, FL, 19917–83.

33] K. Hata, Kobunshi Ronbunshu 58 (2001) 714.34] N.P. Patel, J.M. Zielinski, J. Samseth, R.J. Spontak, Macro

Chem. Phys. 205 (2004) 2409.35] K. Ghosal, B.D. Freeman, Polym. Adv. Technol. 5 (1994) 673.36] L.M. Robeson, J. Membr. Sci. 62 (1991) 165.37] S.J. Metz, M.H.V. Mulder, M. Wessling, Macromolecules 37 (20

4590.