Proton Conducting Crosslinked Polymer Electrolyte

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ProtonConductingCrosslinkedPolymerElectrolyteMembranesBasedonSBSBlockCopolymer

DongKyuRoh,JongKwanKoh,WonSeokChi,YongGunShul,JongHakKimDepartmentofChemicalandBiomolecularEngineering,YonseiUniversity,262Seongsanno,Seodaemun-gu,Seoul120-749,SouthKorea

Received7April2010;accepted5December2010DOI10.1002/app.33937Publishedonline11April2011inWileyOnlineLibrary(wileyonlinelibrary.com).

ABSTRACT:Aseriesofcrosslinkedpolymerelectrolytemembraneswithcontrolledstructureswerepreparedbasedonpoly(styrene-b-butadiene-b-styrene)(SBS)triblockcopolymerandasulfonatedmonomer,2-sulfoethylmeth-acrylate(SEMA).SBSmembraneswerethermallycross-linkedwithSEMAinthepresenceofathermal-initiator,4,40-azobis(4-cyanovalericacid)(ACVA),asconfirmedbyFT-IRspectroscopy.Thewateruptakeandionexchangecapacity(IEC)ofmembranesincreasedalmostlinearlywithSEMAconcentrationsduetotheincreaseofSOÀ3groups.However,theprotonconductivityofmembranesincreasedlinearlyupto33wt%ofSEMA,abovewhichitabruptlyjumpedto0.04S/cmpresumablyduetothefor-mationofwell-developedprotonchannels.Microphase-separatedmorphologyandamorphousstructuresofcross-linkedSBS/SEMAmembraneswereobservedusingwideangleX-rayscattering(WAXS),smallangleX-rayscatter-ing(SAXS),andtransmissionelectronmicroscopy(TEM).Themembranesexhibitedgoodmechanicalpropertiesandhighthermalstabilityupto250󰀂C,asdeterminedbyauni-

versaltestingmachine(UTM)andthermalgravimetricanalysis(TGA),respectively.VC

2011WileyPeriodicals,Inc.J

ApplPolymSci121:3283–3291,2011

Keywords:blockcopolymers;crosslinking;protonconductivity;ionomers;polymerelectrolytemembrane

INTRODUCTIONPolymerelectrolytesarematerialsofcurrentinter-estbecauseoftheirhighionicconductivity,stableelectrochemicalcharacteristics,andexcellentme-chanicalproperties.Inpolymer-basedelectrochemi-caltechnologies,thesecomplexesareappliedtosolid-statelithiumbatteries,dye-sensitizedsolarcells,chemicalsensors,andelectrochromicdevi-ces.1–4Thesematerialswerealsosuccessfullyused

asaseparationmembraneforolefin/paraffinmix-tures,usingreversibleinteractionsbetweenmetalionsandolefinmolecules.5Inparticular,polymer

electrolytemembraneswithprotonconductingpropertieshavebeenextensivelyinvestigatedforapplicationstofuelcells.6–15

Fuelcellsproduceelectricpowerthroughachemicalreactionoffuelandoxygen,andthustheyhavebeenattractingsignificantattentionasacleansourceofenergy.Polymerelectrolytemem-branefuelcells(PEMFC)havebeenextensivelystudiedforapplicationsinelectricvehicles,resi-dentialpowersources,andportabledevices.Per-fluorinatedpolymermembranescontainingsul-fonicacidgroups,e.g.,Nafion,arewidelyacknowledgedasanelectrolytemembraneduetotheirexcellentthermal,mechanical,andelectro-chemicalproperties.Greatmembranepropertiesstemfromthestructuresinwhichthemembranesconsist,ofahydrophobicfluorocarbonbackboneandhydrophilicsulfonicpendantsidechains.However,manyconcernshavebeenraisedfortheiruseinfuelcellapplications.Forexample,highcost,syntheticdifficulty,toxicintermediates,lossofprotonconductivityathightemperatures(>100󰀂C),andhighmethanolcrossoverareserious

drawbacksforpracticalapplicationsinfuelcells.Therefore,therehasbeenagreatdealofresearchinthedevelopmentofalternativeprotonconduct-ingmembranestotheperfluorinatedmem-branes.6–18

Microphaseseparationofacopolymercanbeusedtocreatewell-definedperiodicmicrodomainsofcon-trolledmorphologyonthenanoscale.Inparticular,theuseofaself-organized,nanophase-separated

Correspondenceto:J.H.Kim(jonghak@yonsei.ac.kr).Contractgrantsponsors:MinistryofKnowledgeEconomythroughtheManpowerDevelopmentProgramforEnergy,NewandRenewableEnergyRandDProgram;contractgrantnumber:2009T100100606.Contractgrantsponsors:WorkforceDevelopmentPrograminStrategicTechnologyofKoreaInstituteforAdvancementinTechnology(KIAT),LowObservableTechnologyResearchCenterprogramoftheDefenseAcquisitionProgramAdministrationandAgencyforDefenseDevelopment.

JournalofAppliedPolymerScience,Vol.121,3283–3291(2011)VC2011WileyPeriodicals,Inc.structurecanallowforabettercontrolofboththesulfonationdegreeandthedistributionofsulfonicacidgroupsinpolymerelectrolytemembranes.Therefore,theprotonconductingpropertiesofpoly-merelectrolytemembraneswithaphase-separatedstructureandself-organizedionicaggregatesarehigherthanthoseofhomogeneousmaterials.19The

advantagesoftheuseofamicrophase-separatedblockcopolymerinclude:controloftheswellingoftheionconductingdomainsbythesurroundingnonconductingdomains,loweringofthemethanolpermeabilityduetodecreasedswelling,andhighmechanicalstabilityduetotheinertmatrixofnon-sulfonatedsegments.19–22