Studiesoncross-linkedpolyurethaneacrylate-basedelectrolyteconsisting ofreactivevinyl/divinyldiluents M.L.Digar,S.L.Hung,T.C.Wen * ,A.Gopalan 1 Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC Received13July2001;receivedinrevisedform23October2001;accepted25October2001 Abstract Aseriesofcross-linkedpolyurethaneacrylatePUA)electrolyteshavebeenpreparedbyusing4,4 0 -methylene bisphenyl isocyanate), polyethyleneglycol,hydroxyethylmethacrylateanddifferentreactivevinyl/divinyldiluents,suchasmethylmethacrylateMMA),ethyl acrylateandacrylonitrile,tripropyleneglycoldiacrylateTPGDA).TheelectrolyteswerepreparedbyUVradiationinducedcross-linkingof thePUA±diluentmixturefollowedbyswellinginaliquidelectrolyteLP-30).Dependinguponthecompositionofthecomponents,these electrolytesexhibitedawiderangeofmechanicalandelectricalproperties.ThesystemcontainingMMAasreactivediluentshowedhighest conductivity, but poor mechanical properties and stability in the liquid electrolyte. The TPGDA cross-linked system possesses a good combination of ionic conductivity and stability in liquid electrolytes. These systems showed good compatibility with Li-electrodes and suf®cientelectrochemicalstabilitytoallowsafeoperationinrechargeableLi-batteries. q 2002ElsevierScienceLtd.Allrightsreserved. Keywords:Polymerelectrolyte;Polyurethaneacrylate;Vinyldiluents 1. Introduction Researchefforthasbeenverymuchfocusedtodevelop polymer electrolyte PE) having combined high ionic conductivityanddimensionalstabilityforuseinrecharge- ableLi-batteries[1±3].Forbatteryapplications,highionic conductivity must be complemented by good dimensional stabilitysincethePEwillalsofunctionasseparatorinthe battery providing electrical insulation between the anode and the cathode. Of the many polymers, polyethylene oxide PEO) based electrolyte is the most widely studied. ButthemaindrawbackofPEO-basedelectrolyteisitslow conductivity 10 28 ±10 27 S/cm) at ambient temperature. ThehighdegreeofcrystallinityofPEO,whichisunfavor- able for the mobility of the ions in the salt-doped PEO systems, restricts PEO to have conductivity. Several attempts have been made to obtain amorphous polyether matriceswithhighionicconductivityoverawidetempera- turerange[2,3]. Besides PEO, several alternative systems like poly methyl methacrylate) PMMA), polyacrylonitrile)PAN), polyvinylidine ¯uoride) PVdF), polyvinyl chloride) PVC),etc.havebeeninvestigatedincludingthermoplastic polyurethaneTPU)[4±15].TheinterestinusingTPUas matrix for PE is related to the possibility of increasing mechanical strength of linear polyethers due to their phase-separated microstructure. The rubbery soft phase can dissolve alkali metal salts without formation of ionic clusters, which may be due to the interaction of ether oxygen with alkali metal ions [16,17]. Furthermore, the low glass transition temperature T g ) and hence higher segmental motion of the polyether soft segments leads to higher mobility of the dissolved ions. The hard segment domains,whichareintheglassystateandeitherdistributed orinterconnectedthroughouttherubberyphaseofthesoft segment,actasreinforcing®llerandhencecontributetothe dimensionalstabilityofthePE. Cross-linkingofsomeoftheunitsassidechainsisoneof thevariouswaystoimprovethemechanicalpropertiesofa polymer. It has been found that the comb-branched poly- mers based on phosphazene backbone and having PEO in the side chain are one of the highest conducting conven- tionalPEs[18,19].However,themechanicalpropertiesof thesesystemsareverypoorduetothepresenceof¯exible amorphousphase.Inthesesystems,cross-linkingwithpoly- ethylene glycol PEG) [20] or by g-ray induced cross- linking [21] results in dimensional stability. Cross-linked polymer networks prepared by radiation curing have Polymer432002)1615±1622 0032-3861/02/$-seefrontmatter q 2002ElsevierScienceLtd.Allrightsreserved. PII:S0032-386101)00737-6 www.elsevier.com/locate/polymer * Correspondingauthor.Tel.: 1886-6-2385-487;fax: 1886-6-2344-496. E-mail addresses: tcwen@mail.ncku.edu.twT.C.Wen), algopal_99@yahoo.comA.Gopalan). 1 Permanent address: Department of Industrial Chemistry, Alagappa University,Karaikudi630003,India.