Effectofmolecularweightofpolymericadditivesonforma

JournalofMembraneScience243(2004)45–57Effectofmolecularweightofpolymericadditivesonformation,permeationpropertiesandhypochloritetreatmentofasymmetricpolyacrylonitrilemembranesBumsukJunga,∗,JoonKiYoona,b,BokyungKima,Hee-WooRheebaPolymerHybridsResearchCenters,KoreaInstituteofScienceandTechnology,P.O.Box131,Cheongryang,Seoul130-650,SouthKoreabDepartmentofChemicalandBiomolecularEngineering,SogangUniversity,Shinsu-Dong1,Mapo-Ku,Seoul121-742,SouthKoreaReceived5February2004;accepted2June2004Availableonline17August2004AbstractAsymmetricpolyacrylonitrilemembraneswerepreparedviaphaseinversionprocessfromcastingsolutioncomposedofsynthesizedpolyacrylonitrile(PAN),dimethylsulfoxide(DMSO),andpoly(vinylpyrrolidone)(PVP)ofdifferentmolecularweights.TheeffectofmolecularweightofPVPinthecastingsolutiononthemorphology,waterfluxwasinvestigated.TheeffectofhypochloritetreatmentonPVPwhichwasentrappedinporewasinvestigated.AsthehighermolecularweightofPVPisadded,thethickertoplayeroftheasymmetricmembraneis.ThetoplayerofthemembraneandthesuppressionofmacrovoidformationstronglydependonthemolecularweightofPVP.TheadditionwithdifferentmolecularweightofPVPsalsochangespurewaterpermeability.ItisalsofoundthatthesubstantialamountofPVPisentrappedinthepores,thewaterfluxdecreases.Theeffectofpore-fillingdependsonthemolecularweightoftheadditive.Usingsodiumhypochloritesolution,theentrappedPVPwasabletobepartiallyremoved.TheeffectivenessoftheselectivebleachispronouncedforahighermolecularweightofPVP,whichresultsinhigherwaterpermeability,comparedtountreatedmembranes.©2004ElsevierB.V.Allrightsreserved.Keywords:Ultrafiltration;Additive;Microporousandporousmembranes;Morphology;Membranecharacterization;Hypochloritetreatment1.IntroductionSinceLoebandSouriajanfirstintroducedphaseinversionmethod[1],muchinvestigationhasbeenmadeforunder-standingthemechanismofformationofasymmetricmem-branes.Phaseinversionisthemostextensivelyusedtech-niqueforthepreparationofasymmetricmembranes,whichisthatcastsolutionfilmonasubstrateisimmersedandisprecipitatedinwaterbath.Duringtheprocess,thesolventinthecastingsolutionfilmisexchangedwithnonsolventandphaseseparationoccursinthefilm.Itusuallyturnsouttobeacharacteristicmorphologyofasymmetricmembraneshowingadensetoplayerandporoussublayer[2].Aso-calledasymmetricmembranehavebeenwidelyusedforgasseparationandliquidseparation,becausethethintoplayerplaysaroleofselectivebarrierfilm,andtheporous∗Correspondingauthor.Fax:[phone].E-mailaddress:[e-mail](B.Jung).sublayer,inwhichincludesmacrovoids,poresandmicrop-ores,offersgoodmechanicalstrength.Itiswellknownthattheformationofsymmetricmembranedependsonkineticparameterssuchasexchangeratebetweensolventandnon-solvent,kineticsofphaseseparation,aswellasonthermody-namicparameterssuchasphasediagrams,polymer–solventinteractions,solvent–nonsolventinteractionsandinterfacialstability.Thus,thematerialselectionsuchaspolymers,sol-vents,andnonsolventsisveryimportantforthefabricationofasymmetricmembranes,accordingtoitsapplications[3].Polyacrylonitrile(PAN)isoneofpopularmembranema-terialsforwatertreatment,becauseofsufficientlychemicalstability,hydrophilicityandgoodsolubilitytocommonsol-vents.Duetothehighlyhydrophilicpropertiesamongothermembranematerialssuchaspolysulfone,polyethersulfone,polyethylene,polypropylene,ithasknownaslowfoulingmembranesforaqueousfiltrationandhasalreadycommer-cialized[4].Comparedtootherpolymermaterials,PANhasalsogoodresistanceagainstchlorine,andcleaningagentssuchassodiumhypochlorite,sodiumhydroxide.Neverthe-0376-7388/$–seefrontmatter©2004ElsevierB.V.Allrightsreserved.doi:10.1016/j.memsci.2004.06.01146B.Jungetal./JournalofMembraneScience243(2004)45–57less,theapplicationsofPANmembranehavebeensome-timeshinderedbythebrittlenessandporecollapsewhenitisdried[5].Ithasbeenwellknownthatthemorphologycontrolcanbemadebyaddingsmallamountofadditives.TheadditionofionicsaltssuchasLiCl,ZnCl2andorganicacidsuchasaceticacid,propionicacidcausesmacrovoidformation.Forexample,theadditionofLiClcauselagermacrovoids,andthehighertheamountofLiClisadded,thelargermacrovoidisformed[6].Itisgenerallyacceptedthatthemacrovoidsaresuppressedasorganicacidsareadded,becausetheacidsformacid–basecomplexwithbasicpolarsolventssuchasNMP,DMF,DMAc[7].Furthertotheinorganiclowmolec-ularadditive,polymeradditivesarewidelyusedforthestructurecontrolofmembranessuchaspolyvinylpyrroidone(PVP),polyethyleneglycol(PEG)forthefabricationoful-trafiltrationandmicrofiltrationmembranes[8].PVP,whichisknownasanontoxicmaterialwidelyusedforbiomedicalapplications,isoneofgoodpolymeraddi-tives,becausePVPismisciblewithmembranematerialsandisquitewellsolubleinwateraswellassolvents.Infact,ithasbeenwidelystudiedthattheeffectofPVPadditiontopoly-mersolutionontheformationofphaseinversionmembraneisaccountedforpromotionofmicropores.Duringthephaseinversionprocess,itisassumedthatthehydrophilicadditive,PVPisdissolvedoutbywaterandthesiteswherePVPexistbecomemicropores.Besidestheformationofmicropores,ithasbeengenerallyacceptedthattheporosityincreasesandthemacrovoidformationdisappearsa