InfuenceoftheadditionofPVPonthemorphologyofasymmet

JournalofMembraneScience236(2004)203–207ShortcommunicationInfluenceoftheadditionofPVPonthemorphologyofasymmetricpolyimidephaseinversionmembranes:effectofPVPmolecularweightSeongHyunYooa,JongHakKima,JaeYoungJhob,JongokWonc,YongSooKanga,∗aCenterforFacilitatedTransportMembranes,KoreaInstituteofScienceandTechnology,P.O.Box131,Cheongryang,Seoul130-650,SouthKoreabDivisionofChemicalEngineering,SeoulNationalUniversity,SouthKoreacDepartmentofAppliedChemistry,SejongUniversity,Seoul143-747,SouthKoreaReceived25August2003;receivedinrevisedform30January2004;accepted9February2004AbstractAmacrovoid-free,porousmembranewithasponge-likestructurewasobtainedbyphaseinversionofapolyimide(PI)/
-butyrolactone(GBL)/watersystem,whereasamembranewithafinger-likestructurewasobtainedfromaPI/N-methyl-2-pyrrolidinone(NMP)/watersystem.Inthisstudy,theeffectofthemolecularweightofpoly(vinylpyrrolidone)(PVP)additive(Mw:360kand40k)intwosystemsonmembranemorphologyhasbeeninvestigatedbythedifferencesinphaseseparationratesandmiscibilitygaps.OnadditionofPVPintothePIcastingsolutions,thetwosystemsexhibitedsignificantlydifferentmembranemorphologies;theformeraugmentedmacrovoidformationwhilethelattersuppressedformationoffinger-likestructures.ThemembranemorphologyandthephaseseparationratesignificantlydependonthemolecularweightofPVPinPI/NMP/watersystemwhereasthosedoinsignificantlyinPI/GBL/watersystem.ThecloudpointsrepresentingthemiscibilitygapdonotdependonthemolecularweightofPVPinbothsystemsverymuch.Therefore,itissuggestedthatthemorphologychangeinthePI/NMP/watersystemismostlyduetothechangeinthephaseseparationratecausedbythechangeinthesolutionviscosityuponadditionofPVP.However,inthePI/GBL/watersystem,themolecularweighteffectofPVPisnotassignificantasinthePI/NMP/watersystem,andthemorphologychangeseemstobeprimarilyduetothechangeinthemiscibilitygap.©2004ElsevierB.V.Allrightsreserved.Keywords:Phaseinversion;Polyimide;PVP;Additive;Membranestructure1.IntroductionMostpolymericmembraneswithasymmetricstructuresarepreparedbyimmersionprecipitation[1–5].Inthispro-cess,aninitiallysingle-phasehomogeneouspolymersolu-tioniscastontoasuitablesubstratetoformathinfilmandisthenimmersedinacoagulationbath,whichresultsindiffusiveexchangesofsolventandnonsolventandtheso-lidificationofthepolymer.Thusthemorphologyandtrans-portpropertiesofthemembranestronglydependonboththethermodynamicsandthekineticsofthephaseseparationprocess[6–12].∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](Y.S.Kang).Macrovoidsarequiteoftenobservedontheasymmet-ricmembranesmadebythephaseinversionprocess.Theycanbefavorableformicrofiltrationandultrafiltrationmem-branesbygivinggenerallyhigherflux.However,theymustbeavoidedforhigh-pressureoperationssuchasgassepara-tionandreverseosmosis,becausetheycanbeweakpointsleadingtomembranecollapse.Therefore,therehavebeenreportedmanystudiesforthemacrovoidformationmecha-nismaswellasthemethodstodepressmacrovoids[13–22].Withsomereferencetomembraneformationmecha-nism,therehavebeenproposedafewmethodstodepressmacrovoidformation.Oneofthesemethodsistheadditionofpolymericadditivetothemembranecastingsolution.Thismethodindeedshowedquitesuccessfulresultsintermsofreducingmacrovoid[18].Awatersolublepolymersuchaspoly(vinylpyrrolidone)(PVP)[18–21]orpoly(ethylene0376-7388/$–seefrontmatter©2004ElsevierB.V.Allrightsreserved.doi:10.1016/j.memsci.2004.02.017204S.H.Yooetal./JournalofMembraneScience236(2004)203–207glycol)(PEG)[13]hasbeenusedfrequentlyforthestudyofpolymeradditiveeffect.AsystematicstudyhasbeencarriedoutbyBoometal.onthemorphologicalchangeofmembranesbytheadditionofPVP[18].Theyhavestudiedthesystemofpoly(ethersulfone)(PES)/PVP/NMPwithacoagulationmediumofwatercontaining50–80wt.%NMP.TheyexplainedtheeffectofPVPonthemembranestruc-tureintermsofthechangeoftendencyforprecipitation(delayedorinstantaneous)upontheadditionofPVP.Theeffectofadditivesonthestructuresofpolymericmembranesisstronglydependentonanumberoffactors.Severalresearchershavereportedthattheadditionofathirdcomponentintothedopesolutioncaneitherinduceorsuppressmacrovoidformationinasymmetricmembranes[7,15,16,19].Inthepreviousstudy[19],wefoundthattheeffectofadditionofPVPtoapolyimide(PI)solutiononmacrovoidformationdependedonthetypeofsolvent.Here,themolecularweighteffectofPVP(360kand40k)isin-vestigatedintermsofboththemiscibilitygapmeasuredbycloudpointsandthekineticsofthephaseseparationprocessmonitoredbylighttransmittanceandrheologicalmeasure-ments.2.ExperimentalThepolyimide(PI;P-84,Lenzing)wassuppliedasa27wt.%solutioninDMF.Itwasprecipitatedinmethanolanddriedforatleast24hinavacuumovenbeforeuse.Itsnumberandweightaveragemolecularweightswerefoundbygelpermeationchromatographytobe33,400and75,600g/mol,respectively.Twosamplesofpoly(vinylpyrrolidone)(PVP;Polyscience)wereusedasreceived;theirweightaveragemolecularweightswerefoundtobe40,000(denotedhereafterasPVP40k)and360,000(de-notedhereafterasPVP360k).N-Methyl-2-pyrrolidinone(NMP;Aldrich)and
-butyrolactone(GBL;Aldrich)wereusedassolventswithoutanyfurtherpurification.Distilledwa