Preparationandcharacterizationofsulfonatedpolyethe

Preparationandcharacterizationofsulfonatedpoly(etheretherketoneketone)protonexchangemembranesforfuelcellapplicationXianfengLia,ChangpengLiub,HuiLua,ChengjiZhaoa,ZheWanga,WeiXingb,HuiNaa,∗aAlanG.MacDiarmidInstitute,CollegeofChemistry,JilinUniversity,Changchun130012,PRChinabChangchunInstituteofAppliedChemistry,ChineseAcademyofSciences,Changchun130022,PRChinaAbstractNovelsulfonatedpoly(etheretherketoneketones)(SPEEKKs)basedmembraneshavebeenpreparedandevaluatedforprotonexchangemembranes(PEMs).Directlyaromaticandnucleophilicsubstitutingpolycondensationwasadaptedtocontrolthesulfonatedprocess.SPEEKKswithdifferentsulfonateddegreeswerepreparedbyadjustingthecontentofsulfonatedmonomer.AlltheSPEEKKscaneas-ilybecastintotoughmembranesandshowhighthermalstability.SPEEKKsmembranesexhibitconductivities(25◦C)from3.6×10−4to0.05S/cm,(80◦C)from0.001to0.06S/cm;waterswellingfrom9.14to26.71%,ion-exchangecapacities(IEC)from0.63to1.57mmol/gandmethanoldiffusioncoefficientsat25◦Cfrom6.6×10−7to8.6×10−7cm2/s.ThesediffusingcoefficientsaremuchlowerthanthatofNafion®(2×10−6cm2/s)andmakeSPEEKKmembraneswithagoodalternativethatcanreduceproblemswhichassociateswithhighmethanolcrossoverindirectmethanolfuelcells.©2005ElsevierB.V.Allrightsreserved.Keywords:Protonexchangemembrane;Poly(etheretherketoneketone);Sulfonation1.IntroductionProtonexchangemembranefuelcells(PEMFC)arepromisingpowersourcesforbothstationaryanddynamicapplications[1,2].Protonexchangemembrane(PEM)re-gardedasthekeycomponentoffuelcellsystemisveryimportant,becauseonlystablemembranecanwithstandtheharshingchemicalandphysicalenvironment.Intraditional,membraneseverusedinfuelcellareperfluorinatedpoly-mers(Nafion).Eventhoughthesemembraneshavedemon-stratedgoodperformanceandlong-termstability,theirhighcostandmethanol-crossovermakethemunpracticalforlarge-scaleproduction[3,4].Therefore,alternativePEMFCmate-rialsareafield,whichworthsforsearchingon.∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](H.Na).Poly(aryletherketones)(PAEKs)arehigh-performanceengineeringthermoplastics.Theyhaveexcellentmechani-calpropertiesandhighthermalstability.Thus,sulfonatedPAEKshavebeenexaminedasalternativePEMmaterials[5–8].However,thesematerialsusuallypreparedwithpost-sulfonatingpolymers,whichnotonlydegradethemechani-calandthermalstability,butalsolackthecontrollingofsul-fonatedprocess.Inourpreviouswork,sulfonatedpoly(etheretherketoneketone)(PEEK)polymericmembraneswerepreparedbydirectsynthesisofsulfonatedmonomer,andeval-uatedforfuelcellapplications[5].Thismethodhasproventobemoreadvantageousthanpost-sulfonationduetoitseaseofcontroloverthedegreeofsulfonationinthecopolymer-izationstep.Alsothismethodavoidscross-linkingandothersidereactions[5,6].Poly(etheretherketoneketone)s,mem-bersofPAEKfamilyhaveexcellentproperties,andincludespecialthermalandmechanicalpropertiesincomparisonwithPEEK.Inthisstudy,sulfonatedPEEKKs(SPEEKKs)membranesarepreparedbythemethodofdirectsynthesisofsulfonatedmonomerandevaluatedforfuelcellapplications.2.Experimental2.1.MonomersynthesisThesynthesisofsulfonatedmonomer(1,4-bi(3-sodiumsulfonate-4-fluorobenzoyl)benzene)hadaccordedtoapro-ceduredescribedinourpreviouswork[9–11].AsshowninScheme1,the1,4-bi(4-fluorobenzoyl)benzenewasfirstsul-fonatedbyfumingsulfuricacid,followedbyneutralizingwithNaOHandprecipitatingwithNaCl.Thecrudeprod-uctwasrecrystallizedwiththemixtureofmethanolandwater.Theyieldofthismonomerwas81%.ThechemicalstructurewasconfirmedbyFT-IRand1HNMR.IR(KBr,cm−1):1656(C=O),1211,1093,621(Ar-SO3Na).1HNMR(500MHz,DMSO,ppm):8.12–8.15(dd2.3Hz,4.5Hz),7.87(s)7.81–7.84(m2.5Hz,2.0Hz,5.3Hz),7.34–7.38(dd5.3Hz,5.3Hz).13CNMR(500Hz,DMSO,ppm):194.74,163.34,161.29,141.01,133.98,133.91,132.81,131.66,130.29and117.53.2.2.SynthesisofsulfonatedpolymersAsshowninScheme2,differentsulfonateddegreesofSPEEKKswerepreparedwiththearomaticnucle-ophilicsubstitutioncondensationof3,3,5,5-tetrmethyldiphenyl-4,4-diolwithdifferentratioof1,4-bi(3-sodiumsulfonate-4-fluorobenzoyl)benzene(monomerm)to1,4-bi(4-fluorobenzoyl)benzene(monomerk)inaDMSO/toluenesolventsystem.Thetemperatureofroundflaskwasmain-tainedat140◦Cfor3hinordertoremoveallthewaterazeotropicallywithtoluenefromreactionmixture.Then,toraise170◦Cfor6h,theproductswereprecipitatedfromace-tone.Theinorganicsaltswereremovedfromboilingwater.Thedegreesofsulfonationwerecontrolledbyreactingdiffer-entamountofmonomersmandk,whicharelistedinTable1.TheviscositiesofpolymersweremeasuredinDMFat25±1◦Cwithpolymersconcentrationof5mg/dl.Thevis-cositiesofpolymersindicatehighmolecularweightofresult-ingpolymers.Table1displaysaresult,whichisthehighertheratioofmonomersmtok,thehighertheviscosityofthepolymer.Becauseoftheelectron-withdrawingsulfonategroups,SO3Na,increasesthedensityofpositivechargeofthecarbonatomthatconnectswiththefluorineatom,andthusenhancestheelectrophilicreactivity,although,thesterichindranceofsodiumsulfonatedgroupsdecreasesthereactiv-ity[26].Thedegreeofsulfonation(Ds),whichisthenumberofsulfonatedgroupsperrepeatingunit,wasdeterminedbytitrationmethod[5].Priortothetitr