JournalofMembraneScience474(2015)156–166ContentslistsavailableatScienceDirectJournalofMembraneSciencejournalhomepage:www.elsevier.com/locate/memsciConstructingCO2transportpassagewaysinMatrimidsmembranesusingnanohydrogelsforef?cientcarboncaptureXueqinLia,b,MeidiWanga,ShaofeiWanga,b,YifanLia,b,ZhongyiJianga,b,RuiliGuod,HongWua,b,c,n,XingZhongCaoe,JingYange,BaoyiWangeKeyLaboratoryforGreenChemicalTechnologyofMinistryofEducation,SchoolofChemicalEngineeringandTechnology,TianjinUniversity,Tianjin300072,ChinabCollaborativeInnovationCenterofChemicalScienceandEngineering(Tianjin),Tianjin300072,ChinacTianjinKeyLaboratoryofMembraneScienceandDesalinationTechnology,TianjinUniversity,Tianjin300072,ChinadKeyLaboratoryforGreenProcessofChemicalEngineeringofXinjiangBingtuan,SchoolofChemistryandChemicalEngineering,ShiheziUniversity,Xinjiang,Shihezi832003,ChinaeKeyLaboratoryofNuclearAnalysisTechniques,InstituteofHighEnergyPhysics,ChineseAcademyofSciences,Beijing100049,ChinaaarticleinfoArticlehistory:Received18June2014Receivedinrevisedform27September2014Accepted4October2014Availableonline14October2014Keywords:MatrimidsNanohydrogelsCompositemembranesWatercontentCO2separationabstractCompositemembraneswerefabricatedbyincorporatingpoly(N-isopropylacrylamide)nanohydrogels(NHs)intoMatrimids5218matrixtoimprovetheseparationperformanceforCO2/CH4andCO2/N2mixtures.Themembraneswerecharacterizedbyafouriertransforminfraredspectrometer(FT-IR),scanningelectronmicroscopy(SEM),tensiletest,dynamicmechanicalanalysis(DMA),X-raydiffraction(XRD),positronannihilationlifetimespectroscopy(PALS),thestaticcontactangleandwatercontentmeasurement.Theincorporationofnanohydrogelsincreasedthefractionalfreevolumeofthecompositemembranes,wateruptakeandwaterretentioncapacity.ThecompositemembranesdisplayedbetterperformancethanthepureMatrimidsmembrane.ThenanohydrogelshomogeneouslyembeddedintheMatrimidsmatrixactedaswaterreservoirstonotonlyprovidemorewaterfordissolvingCO2,butalsoconstructinterconnectedCO2transportpassageways.Theas-preparedMatrimids/NHs-20compositemembraneshowedCO2/CH4andCO2/N2selectivitiesof61and52withaCO2permeabilityof278Barrer,surpassingorbeingclosetothe2008Robesonupperboundlines.&2014PublishedbyElsevierB.V.1.IntroductionThedevelopmentofenergy-ef?cientandscalableCO2capturetechnologieshasrecentlybecomeanimportantworldwideissue[1,2].Amongvariouscapturetechnologies,membraneseparationisanattractivealternativetosuchconventionaltechniquesasabsorption,adsorptionandcryogenicdistillationforitshighenergyef?ciencyandenvironmentalsustainability[3–6].Formanygasseparationapplications(biogasandfuelgas)involvingCO2capture,thegasmixtureisoftenmoistorsometimesevensaturatedwithwatervapor[2,7–9].ThesolubilityofCO2inwaterisanorderofmagnitudehigherthanthatofCH4andN2,leadingtoahighergaspermselectivity.Moreover,watertypicallyswellsthepolymermatrixandincreasesthepolymerchainmobility,thusreducingthegasdiffusionresistance[10–15].TheimmobilizedwaterretainedinthepolymermatrixformstransportnCorrespondingauthorat:KeyLaboratoryforGreenChemicalTechnologyofMinistryofEducation,SchoolofChemicalEngineeringandTechnology,TianjinUniversity,Tianjin300072,China.Tel./fax:t862223500086.E-mailaddress:wuhong@tju.edu.cn(H.Wu).http://dx.doi.org/10.1016/j.memsci.2014.10.0030376-7388/&2014PublishedbyElsevierB.V.passagewaysforgaspermeationthroughthemembrane.ThepermeabilityofCO2inwaterhasbeenfoundtobeashighas1983Barrer,whichisthreetosixordersofmagnitudehigherthanthatinpolymermatrix[16,17].Therefore,thepresenceofwatervaporisexpectedtoincreasebothsolubilityanddiffusivityofthegasinthemembranes.Water-swollenhydrogelmembraneshavebeenstudiedforCO2separationinconsiderationofthefavorablesolubilityofCO2inwater[18–21].ParkandLee[22]preparedawater-swollenpoly(vinylalcohol)(PVA)hydrogelmembranecrosslinkedbyglutar-aldehyde.ThepermeationrateofCO2throughthePVAmembranewas1011GPUwithanapproximateCO2/N2selectivityof80.JiangandYuan[23]preparedawater-swollencellulosemembranewhichshowedahighpermeationrateofCO2withaCO2/CH4selectivityof30andaCO2/N2selectivityof50.Theyconcludedthatthepermeationrateofagasthroughawater-swollencellulosemembranedependedonboththegassolubilityanddiffusivityinthewaterexistinginthemembrane.Fengetal.[16]studiedaseriesofhydrogelmembranesincludingpoly(vinylalcohol),chit-osan,carboxylmethylcellulose,alginicacidandpoly(vinylamine),andfoundthatgaspermeabilityinthesewater-swollenhydrogelX.Lietal./JournalofMembraneScience474(2015)156–166157membraneswasthreetosixordersofmagnitudehigherthanthatindrymembranes.Itwasfoundthatwaterofferedtransportpassagewaysforgaspermeationthroughthemembrane.There-fore,thegaspermselectivitycanbeimprovedsigni?cantlybyincreasingwatercontentinthemembranes.However,thepre-senceofalargeamountofwaterinhydrogelmembranesusuallycausessuchproblemsaslowmechanicalproperty(Young'smod-ulus,$103Pa),pooroperationstabilityandlowCO2/gasselectivity,thuslimitingtheirindustrialapplication[16,24,25].Thecommercialpolyimides,oneofthemostwidelyusedglassypolymermembranes[26],areattractivematerialsforgasseparationowingtotheirexcellentmechanicalproperty(Young'smodulus,$106Pa),operationstabilityandgasseparationperformance,especiallyforCO2separation[27,28].However,thehydrophobicaromaticbackboneofpolyimiderestrictstheamountofwaterinpolymermatrix,andtheincreaseofpermeabilityisthuslimitedunderwetconditions.Exploringnewmembraneswithhighwatercontent,goodmechanicalpropertiesandoperationstabilitysimultaneouslyishighlydesirable.Compositemembranescomprisingapolymerbulkphase(con-tinuousphase)anda?llerphase(dispersedphase)provideapromisingsolutionfortheabove-mentionedstickyproblems[29,30].Theuseofappropriate?llerswithmultifunctionalproper-tiesprovidesthepossibilitytobetterdesignmembranestructure.Forexample,theincorporationofversatilehydrophilic?llersintopolymermatrixcanincreasewatercontentandthefractionalfreevolume,providingmoregastransportpassageways[31–33].Nano-hydrogelshavetriggeredconsiderableattentioninrecentyearsowingtotheirhighwatercontent,biocompatibilityanddesirablemechanicalproperties[34–36].Nanohydrogelscanabsorbandretainextremelyhighwatercontentofupto10–1000timesoftheiroriginalweightorvolumewithoutbeingdissolved[37–39].Thewater-absorbingabilityofnanohydrogelsarisesfromtheabundanthydrophilicfunctionalgroupsonpolymerbackbone,whiletheirindissolubilityinwaterisduetocross-linksbetweenpolymerchains.However,nanohydrogelshavenotbeenconsideredasmulti-functional?llersincompositemembranesforef?cientCO2capture.Inthiswork,nanohydrogelswereexploitedasversatile?llerstoenhanceCO2separationperformanceofthecompositemem-branes.Tobespeci?c,poly(N-isopropylacrylamide)nanohydrogels(NHs)withadiameterof$250nmweresynthesizedbyprecipita-tionpolymerizationmethod.TheNHswerethenembeddedintoMatrimidsmatrixtofabricateaseriesofcompositemembranes.Thewatercontentandthegasseparationperformanceofcompo-sitemembranesweresystematicallyinvestigated.Inaddition,thephysicochemicalpropertiesofthemembranesincludingmicro-structure,mechanicalpropertiesandfreevolumecharacteristicswereevaluatedindetail.Thisstudymayofferagenericstrategytoconstructidealmembranestructureandfabricatehighlyperme-ableandselectivemembraneforCO2capture.2.Experimentalandcharacterizationmethod2.1.MaterialsPolyimideresin(Matrimids5218)powderwaspurchasedfromAlfaAesar.N-isopropylacrylamide(NIPAM,98wt%)waspurchasedfromAladdinChemistryCo.Ltd.N,N0-Methylenebisacrylamide(BIS,99wt%)waspurchasedfromTianjinBodiChemicalIndustryCo.Ltd.Potassiumperoxodisulfate(KPS,99.5wt%)andN,N-Dimethylformamide(DMF,99.5wt%)werepurchasedfromTianjinGuangfuTechnologyDevelopmentCo.Ltd.Allreagentswereofanalyticalgradeandusedwithoutfurtherpuri?cation.Deionizedwaterwasusedthroughouttheexperiments.2.2.Synthesisofpoly(N-isopropylacrylamide)nanohydrogelsThecrosslinkedpoly(N-isopropylacrylamide)nanohydrogelswerepreparedbyaprecipitationpolymerizationmethod[40].Brie?y,NIPAMandBIS(NIPAM:BIS?14:9weightratio)weredissolvedindeionizedwater(2.8mg/mLNIPAMsolution)inafour-neck?askequippedwithanitrogeninletandathermometer.Nitrogenwasbubbledintothesolutiontoremoveoxygenandthe?askwaskeptat901Cfor30min.Then,KPS(NIPAM:KPS?14:1weightratio)wasdissolvedindeionizedwater(10.0mg/mLKPSsolution)andwasaddedintothe?asktoinitiatepolymerization.Afterbeingstirredfor6h,theresultantNHswerepuri?edbythreecyclesofcentrifugationfollowedbydryinginavacuumovenat501Cuntilconstantweightwasreached.2.3.MembranepreparationTheMatrimids/NHscompositemembranesandpureMatri-midsmembranewerefabricatedbyasolution-castingmethod.Priortomembranefabrication,MatrimidsandNHsweredriedinavacuumovenat501Cfor24hinordertoremovethewatercompletely.MatrimidspowderwasdissolvedinDMFundermagneticstirringatroomtemperaturefor12htoobtaina6wt%homogeneoussolution.Aspeci?edamountofNHswasdispersedintoDMFunderultrasonictreatmentfor30min.Afterbeingstirredvigorouslyforanother12h,themixedsolutionwascastontoa?atglassplate,dried?rstat501Cfor12handthen801Cforanother12h.ThepureMatrimidsmembranewasalsofabricatedbyexactlythesameprocedurewithouttheadditionofNHs.Forsimplicity,theobtainedcompositemembranesweredesig-natedasMatrimids/NHs-X,whereXintherangeof0–20wt%referredtotheweightpercentageofNHsrelativetotheweightofMatrimids.Themembranethicknesswasmeasuredbyadigitalmicrometerandvariedfrom70to100μm.2.4.NanohydrogelsandmembranecharacterizationThemorphologyofnanohydrogelswasobservedwithaTecnaiG2F20transmissionelectronmicroscopy(TEM).Theinfraredspectraofnanohydrogelsandmembranesintherangeof4000–400cmà1wererecordedonaBRUKERVertex70fouriertransforminfraredspectrometer(FT-IR)equippedwithahorizontalattenu-atedtransmissionaccessoryfornanohydrogelsandahorizontalattenuatedtotalre?ectanceaccessoryformembranes.Thecross-sectionalmorphologyofallmembraneswasobservedbyscanningelectronmicroscopy(SEM).TheimageswerecollectedusingaHitachiS-4800instrumentwithanacceleratingvoltageof15kV.ThemechanicalpropertiesofmembranesweremeasuredbyaChangchunKexinWDW-02tensiletestmachine.Thetestswereconductedatacrossheadspeedof5mm/minat251C.DynamicmechanicalpropertiesofmembranesweremeasuredbyaPerkin-Elmerdynamicmechanicalanalyzer(DMA).Thesamplesweremeasuredatafrequencyof1Hzandaheatingrateof51C/minintherangeof25–4001C.ThecrystallinestructureofmembranewasinvestigatedusinganX-raydiffraction(XRD)withaRigakuD/max2500v/pcintherangeof3–651atthescanrateof101/min.Theaveraged-spacingofMatrimidsmatrixwasevaluatedbasedonBragg'slawasfollows:nλ?2dsinθe1Twherenwasaninteger(1,2,3,…),λdenotestheX-raywavelength,drepresentedtheintersegmentalspacingbetweentwopolymerchainsandθstandsfortheX-raydiffractionangleofthepeak.Thermogravimetricanalysis(TGA)ofthemembraneswasconductedbyaNETZSCHTG209F3TGAinstrumentfromthe158X.Lietal./JournalofMembraneScience474(2015)156–166roomtemperatureto8001Cundernitrogen?owataheatingrateof101C/min.ThestaticcontactangleofthemembranewasmeasuredatroomtemperaturebyaJC2000CContactAngleMetercontactanglegoniometer.Positronannihilationlifetimespectroscopy(PALS)experimentwasdeterminedbyanEG&GORTECfast–fastcoincidencesystem(resolution201ps)atroomtemperature.Weuseda22Na(5?105Bq)radioactivepositronsourcewhichwassandwichedbytwopiecesofsample(thickness1.0mm).Morethan2?106coincidenceswerecollectedforeachsample.Onassumptionthatthelocationofo-PsoccursinaspherepotentialwellsurroundedbyanelectronlayerofaconstantthicknessΔr(0.1656nm),theradiusoffreevolumecavity(r3,r4)iscalculatedfromthepick-offannihilationlifetimeofo-Ps(τ3,τ4)bythefollowingsemi-empiricalequation:τ???????121àr??rtΔrt1??2πsin2πr??à1rtΔre2TThefractionalfreevolume(Fv)wasdeterminedbymultiplyingthevolumeofequivalentsphereandtheintensityofo-Ps(I3,I4):F?43πr3I4v3t333πr4I4e3TInordertoobtainthefractionalfreevolumeofpureMatrimidsmembraneinhumidi?edstate,thefractionalfreevolumewascalculatedbasedonthedensityofmembranes[41].Thedensityofsoakedmembranewasdeterminedbythebuoyancymethod.Siliconoilwithknowndensity(ρ0?0.971g/cm3)wasselectedastheauxiliaryliquid.Thevaluesofdensity(ρp)werecalculatedbythefollowingequation:ρp?MAMAàMLρ0e4TwhereMAandMLwerethemembraneweightsintheairandintheauxiliaryliquid,respectively.ThefractionalfreevolumeofsoakedpureMatrimidsmem-brane(Fh)wasestimatedbythefollowingequation,Fh?1à1:3υwρpe5TwhereρpwasthedensityofpureMatrimidsmembrane,νwwasvanderWaal'svolumeoftherepeatunitofMatrimidsing/cm3whichwascalculatedfromBondiradii[42].2.5.Measurementoftotalwater,freewater,boundwaterandwaterretentionThewateruptake(totalwater)andwaterstateinthemembraneweredeterminedaccordingtothemethodinliteratures[11,41].Eachmembranewasweighedtodeterminethe“humidi?ed”weight(Wh)aftersoakingindeionizedwatertoachievecompletehydrationatconstanttemperature.Freewaterhadessentiallythesamepropertyasbulkwaterandboundwaterbondedtothepolymermatrixviahydrogenbonds.Themembraneswere?rstheatedat1001Cinavacuumovenfor6htoremovefreewaterandreweighed(Wb),andthendriedinavacuumovenat1501Cfor6handreweighedagain(Wd).Immediatelyafterwateruptakemea-surement,thehydratedmembranewasmaintainedat401Cand20%RHinaclimateboxandwasweighed(Wht)attimet.Thecontentoftotalwater,freewater,boundwaterandwaterretentionwerecalculatedusingtheformulasTotalwatere%T?WhàWdWd?100e6TFreewatere%T?WhàWbWb?100e7TBoundwatere%T?WbàWdWd?100e8TWaterretentione%T?WhtàWdWd?100e9T2.6.SwellingpropertyofmembranesTheswellingpropertywasdeterminedbymeasuredthemembraneareadifferencesbeforeandaftersoakinginwater.Theswellingdegreewithanerrorwithin73%wascalculatedbasedonthefollowingequation:Swellingdegreee%T?AsàAdAd?100e10TwhereAsandAdweretheareasoftheswollenanddrymem-branes,respectively.2.7.GaspermeationtestsThegaspermselectivityofthemembraneswasmeasuredbyasetoftestequipmentsdescribedinthepreviousliteratures[43,44].Puregas(CO2,CH4andN2)andmixed-gas(CO2/CH4?30/70,vol%;CO2/N2?10/90,vol%)permeationexperimentswereconductedusingtheconventionalconstantpressure/variablevolumemethodunderbothhumidi?edanddryconditions[43,45].ThekineticsorptionbehaviorofwaterintheMatrimidsmem-braneshowedthatwatersorptionreachedequilibriumafter2weeks(Fig.S5).Therefore,thehumidi?edmembranessampleshadbeensoakedinwaterfor2weekstoabsorbsuf?cientwaterbeforetheyweretested.Inatypicalmeasurement,thefeedgaswassaturatedwithwatervaporthroughahumidi?erandthenpassedthroughanemptybottletoremovethecondensatewater.Meanwhile,thesweepgaswashumidi?edbypassingthroughawater-containingbottle.Forcomparison,drystategaspermeationexperimentswerealsoconducted,inwhichcasethefeedgasandsweepgasweredirectlyintroducedintomembranecell.N2wasusedassweepgaswhenthefeedgaswasCO2,CH4orCO2/CH4mixtures,whileCH4wasusedasthesweepgaswhenthefeedgaswasN2orCO2/N2mixtures.Thesweepgaswaskeptatatmo-sphericpressure.The?owofsweepgaswasmeasuredusingamass?owmeter.ThecompositionsofthefeedandpermeategaseswereanalyzedbyanAgilent6820gaschromatographequippedwithathermalconductivitydetector(TCD).Theeffectiveareaofcompositemembranesis12.56cm2.Thepermeability(Pi,Barrer,and1Barrer?10à10cm3(STP)cm/(cmscmHg))ofeachgaswasobtainedfromtheaveragevalueofatleastthricemeasurementsbyusingEq.(8)PQi?ΔilPiAe11TwhereQiisthevolumetric?owrateofgas‘i’(cm3/s)atstandardtemperatureΔandpressure(STP),listhemembranethickness(cm),piisthetransmembranepartialpressuredifferenceofgas‘i’(cmHg)betweenthefeedsideandpermeatesidepartialpressures,andAistheeffectivemembranearea.TheCO2/CH4andCO2/N2selectivities(αij)werecalculatedbythefollowingequation:αij?PiPje12TThediffusioncoef?cientandsolubilitycoef?cientofpuregas(CO2,CH4andN2)weremeasuredatsteadystateconditionsbythe“time-lag”method[45].Allexperimentswereperformedataconstanttemperatureof301C.Thecellexposedaneffectivemembraneareaof12.56cm2topermeation.Inallgaspermeationexperiments,perme-abilityresultswererecordedatsteadystateconditions.ThefeedX.Lietal./JournalofMembraneScience474(2015)156–166159pressurewas2barforallgases(CO2,CH4andN2),andthemeasure-mentwasconducted3timesforeachmembrane.3.Resultsanddiscussion3.1.Characterizationofthepoly(N-isopropylacrylamide)nanohydrogelsTheTEMimageinFig.1illustratedtheas-synthesizednanohy-drogelparticleswithadiameterofabout250nmintheirdriedstate.ThechemicalcompositionofNHswascharacterizedbyFT-IRasshowninFig.2.Thetwocharacteristicpeaksappearedat1545cmà1(N–Hbending)and1647cmà1(C–Ostretching)inFT-IRspectrumcon?rmedthatNHsweresuccessfullysynthesizedviaprecipitationpolymerization.3.2.MembranecharacterizationFig.1.TEMimageofNHs.15451647
40003600320028002400200016001200800400
Wavenumber (cm-1)
Fig.2.FT-IRspectrumofNHs.TheFT-IRspectraofallmembranesarepresentedinFig.3.ComparedwithpureMatrimidsmembrane,twonewabsorptionbandsat1647cmà1and1545cmà1wereobservedforthecompo-sitemembranes,suggestingthatNHsweresuccessfullyincorporatedintothepolymermatrix.IncompositemembranesdopedwithNHs,theadsorbedwaterwasdetectablebyO–Hstretchingbondwhichappearedasabroadat3000–3600cmà1.Thecharacteristicabsor-bancebandsappearedat3050–3100cmà1wereduetotheC–Hstretchingvibrationsofaromaticrings,whiletheonesobservedat2863cmà1and2956cmà1wereattributedtotheC–Hstretchingvibrationsofaliphaticrings.Thebandat1778cmà1and1683cmà1wereassignedtoC–Ostretchingbandsofketonegroupsandimidicgroups,respectively.BendingvibrationsofaliphaticC–Hbondspresentedat1395and1426cmà1.ThebendingvibrationsofC–CO–Cgroupscouldbeobservedat1297cmà1.Thepeakof1360cmà1wasduetothestretchingvibrationofC–Nintheimidegroup.ThepeakforMatrimids/NHsmembraneatdifferentloadingdisplayedsimilarsignals.Therefore,itcouldbeinterpretedthattherewasnochemicalinteractionbetweenpolymermatrixandNHs.TheSEMmicrographsinFig.4showthedispersionofNHswithinthemembrane.Fig.4(a)–(d)revealsthattheNHsmaintainedtheirpristinestructureanddispersedhomogeneouslyinMatrimidsmatrixTransmittance (%)Matrimid/PHMs-202956286317781683164715451426139512971360Matrimid/PHMs-15Transmittance (%)Matrimid/PHMs-10Matrimid/PHMs-5MatrimidTransmittance (%)1800
400035003000200015001000500170016001500140013001200
Wavenumber (cm-1) Wavenumber (cm-1)
Fig.3.FT-IRspectraof(a)pureMatrimidsmembraneandMatrimids/NHscompositemembranes(4000–400cmàm)and(b)theampli?cationofthelowwavenumberzone(1800–1200cmàm).
