TheeffectofZnCl2ontheformationofpolysulfonemembran

journalofMEMBRANESCIENCEELSEVIERJournalofMembraneScience119(1996)59-64TheeffectofZnC12ontheformationofpolysulfonemembraneSueRyeonKima,KewHoLeeb,MuShikJhona,*aCenterforMolecularScienceandDepartmentofChemistry,KoreaAdvancedInstituteofScienceandTechnology,373-1,Kusung-Dong,Yusung-Gu,Taejon,305-701,SouthKoreaAdvancedPolymerDivision,KoreaResearchInstituteofChemicalTechnology,P.O.Box107,Daedeog-Danji,Taejeon,305-343,SouthKoreaReceived3November1995;accepted19March1996AbstractInthepreparationofultrafiltrationmembranes,theeffectoftheadditionofZnC12inacastingsolution,polysulfone(PSf)/N-methylpyrrolidone(NMP)solution,wasinvestigatedonthemembraneperformance.PSfisanunchargedpolymer,butPSf/NMPwithZnC12showedanelectroviscousbehavior.Theadditionof3wt%ZnC1ztoPSfinducedaredshiftof3cm-1inthe1152.2cm-1peakwhichwasassignedtoasymmetricvibrationalstretchingbecauseofZnC12interactedwithPSfchain.TheadditionofZnC12increasedthereducedviscosityanddecreasedthecriticalconcentration,resultingfromtheassociationbetweenextendedchains.ThePSf/NMPsolutionwithZnC12requiredlesswaterfortheprecipitationpointthansaltlesssolutiondid.ItwasobservedthattheadditionofZnC12increasedtherejectionrateandloweredthemolecularweightcut-off(MWCO)ofPSfmembrane.Keywords:Polysulfone;ZnCI2;Castingsolution;Reducedviscosity1.IntroductionDuetoitsmechanicalstrength,thermostabilityandstabilityagainstchemicals,polysulfone(PSf)hasbeenwidelyusedasamembranematerial.ThestructureofPSfmembranepreparedbyphaseinver-sionisasymmetric[1-3].Itisimportanttocharacter-izetheskinlayeroftheasymmetricmembranebe-causethestructureandpropertyoftheskinlayerdeterminethemembraneperformance[4].Smolderetal.[5]suggestedthatthecharacteristicoftheskinlayerisinfluencedbythestructureofthecastingsolutionbecauseoftherapidexchangeofsolvent*Correspondingauthor.andnonsolventwhenacastingsolutionisimmersedinacoagulationbathcontaininganonsolvent.Smidetal.[6]foundthattheminimalskinthicknessofthemembraneisreducedwhenahigherintrinsicviscos-ityofthepolymerisused,leadingtoadecreaseinmembraneresistanceandanincreaseinwaterfluxthroughthemembrane.Usuallyachangeoftheintrinsicviscosityofapolymersolutionresultsinachangeofthedimensionofthepolymerchaininthesolution.Therefore,itwillbeinterestingtorelatethestructureofpolymerincastingsolutionwiththemembraneperformanceandadditivesarethemostactivefactorwhichcanaffectthestructureandstateofthecastingsolutions.Inorganicsaltadditivesincastingsolutionareconsideredtochangethesolventpropertiesand/ortheinteractionbetweenthemacromoleculechains.In0376-7388/96/$15.00Copyright©1996ElsevierScienceB.V.Allrightsreserved.PIIS0376-7388(96)00113-560S.R.Kimetal./JournalqfMembraneScience119(1996)59-64practice,theadditionofinorganicsaltstocastingsolutionswasreportedtobeveryeffectivetopreparemembraneswithhigherperformance[3,7-10].Fromthispointofview,ZnC12canbeoneofcandidatesfortheadditive.ItisreportedthatZnC12stronglyinteractswithpolymer[11]andenhancesthemem-braneperformance[3,8].Inthispaper,theeffectofadditionofZnC12inthePSfcastingsolutiononthestructuralchangeofPSfchainandtheperformanceofPSfmembranewerestudiedwiththechemicalbehaviorofZnC12inPSf/NMPsolution.2.Experimental2.1.MaterialsPolysulfone(PSf,UdelP-3500)wasobtainedbyreprecipitatingPSf/NMPsolutioninethanolforthemeasurementofreducedviscosity.N-Methylpyr-rolidone(NMP)andZnC12weresuppliedbyAldrichCo.andFlukaCo.,respectively.Polyvinylpyrroli-done(PVP,MW40000)andpolyvinylalcohol(PVA,MW78000and108000)whichwereusedforrejectionexperimentsweresuppliedbyFlukaCo.UltrapurewaterwasmanufacturedbyanMPIwatersystem.2.2.ViscometryViscositiesofdilutePSfsolutions(below0.15g/dl)weremeasuredinatemperature-controlledwaterbathbyusinganUbbelohdeviscometer(AVS400,Schott-Ger'zitt,Germany)at20+0.01°C.ViscositiesofconcentratedPSfsolution(1-30wt%)weremeasuredbyusingBrookfieldViscome-ter(DV-II).Theusedcellwasjacketedbywaterinordertocontrolthetemperature.AbsoluteviscositiesofPSfsolutionswithvariousconcentrationsofZnCI2weremeasured.TheZnCI2concentrationusedwasbelow3wt%becauseofthesolubilitylimitationofZnC12inNMP.Thecriticalconcentrationwasdeter-minedbymeasuringabsoluteviscosities,asde-scribedbelow.Likethemostpolymers,theabsoluteviscosityofthePSfsolutionisthehyperbolicfunc-tionofthepolymerconcentration.Thelogarithmicplotoftheviscosityofthepolymersolutionagainstthelogarithmicpolymerconcentrationgivestwoslopesofstraightlinewhichisbrokenatthecriticalconcentration[12].TheplotofcriticalconcentrationsagainsttheZnC12concentrationwillbesuggestedinnextsection.2.3.IRspectroscopyInordertounderstandtheinteractionbetweenZnC12andPSf,Fouriertransforminfrared(FT-IR)spectraofPSffilmwithvariousZnC12concentra-tionswereobtainedbyaFT-IRspectrophotometer(BomemInc.,MB100).Thefilmswerepreparedbydrying1wt%PSfinNMPwithfourdifferentcon-centrationsofZnC12castonaglassplateat140°Cfor2days.2.4.PhasediagramSeveralcompositionsofPSf/NMPweretitratedwithwater.Theendpointsweretakenasthemini-mumcontentsofwaterwhichmakethesolutionturbid.Fromtheseendpoints,thephasediagramsofPSf/NMP/waterandPSf/NMP(ZnC12)/waterwereobtai