MicrostructuresinphaseinversionmembranesPart2Thero

JournalofMembraneScrence,73(1992)277-292ElsevlerSciencePubhshersBV,Amsterdam277Microstructuresinphaseinversionmembranes.Part2.Theroleofapolymericadditive*R.M.Boom,I.M.Wlenk,Th.vandenBoomgaardandCA.SmoldersDepartmentofChemicalTechnology,UnwersltyofTwente,P0Box217,7500AEEnschede(TheNetherlands)(ReceivedJuly31,191,acceptedmrevisedformApril16,1992)AbstractMembraneswerepreparedfromacastmgsolutionofawater-solublepolymer,poly(vmylpyrrohdone)(PVP),andamembraneformingpolymer,poly(ethersulfone),ml-methyl-2-pyrrohdone(NMP)assolventbyimmersingthemmmixturesofwaterandNMPItwasfoundthattheaddltlonofPVPtotheternarysystemsuppressestheformatlonofmacrovoldsmthesub-layer,whiletheultraflltratlon-typetop-layerconsistsofacloselypackedlayerofnodulesUsingamodelformasstransfermthisquaternarysystem,it1spossibletoexplamtheeffectsoftheadditiveonmacrovoldformationStrongmdlcatlonsarefoundthattheappearanceofanodularstructuremthetop-layerfollowsamechanismofspmodaldecomposltlondunngtheveryearlystagesofthelmmerslonstepKeywordsmembranepreparationandstructure,theory,thermodynamics,macrovolds,phase-mverslonmembranesIntroductionPhasemverslonisthemostimportantpro-cesstopreparesymmetricorasymmetrlcmem-branesBecauseoftheslgmficanceoflmmer-slonpreclpltatlon(phasemverslon),themechanismofformatlonofthesemembraneshasbeenthesubjectofextensiveinvestlgatlon[1,2]Inrecent,years,Reuversetal[1]devel-opedamodelforthedescrlptlonofmasstrans-ferduringthelmmerslonstepTwotypesofde-CorrespondencetoRMBoom,DepartmentofChemicalTechnology,UniversityofTwente,P0Box217,750OAEEnschede,TheNetherlands*PaperpresentedattheIntSympon“ProgressmMem-braneScienceandTechnology”,Enschede,TheNether-lands,June2528,199l0376-7388/92/$050001992ElsevlerSciencePubhshersBVmixingoccurringduringimmersionpreclpltatlonfollowedfromthismodel.mstan-taneousdemlxmganddelayeddemixmg.Tak-mgthismodelofmasstransfermcombmatlonwithhquld-liquidphaseseparation,effectsofvariationsmthecomposltlonofthecastingso-lutionandthecoagulationbathcouldbeexplainedOnthebasisofthedistinctionbetweende-layedandmstantaneousdemlxmg,Reuversetalproposedamechanismfortheformatlonoflargefingerhkecavltles(macrovolds),oftenoccurringmthesub-layeroflmmerslonpreclp-ltatlonmembranes(seeprecedingPart1).Al-thoughthismodel1sstrictlyvahdonlyforter-narysystemsconslstmgofamembraneformmgpolymer,asolventandanon-solventforthepolymer,someeffectsofthead&tlonofafourthAllrightsreserved278RMBoometal/JMembraneSCL73(1992)277-292componenttothesystemcanbeexplainedwith1t.Inordertoobtainanoptimalmembranestructure,anad&tlve(afourthcomponent)isfrequentlyused.Usually,theadditiveisaweaknon-solventforthepolymer,eg.glycerolmasystemconsistingofpolysulfone,DMAcandwater,ormaleicacidmasystemofcelluloseacetate,&oxaneandwater.Suchanad&tlvetothecastingsolutionbringstheuutlalcompo-sitionofthecastingsolutionnearertothebm-odalAccordmgtothemechanismproposedbyReuversetal.,thisdecreasesthetendencyofthesolutiontoformmacrovoids.Inothersystems,polymericad&tlvesareusedWell-knownistheadditionofpoly(vmylpyrrohdone)toasystemconsistingofpolysul-fone,andasolventsuchasDMAc[31.Themostimportanteffectsofthistypeofadditivearesuppressionofmacrovoids,improvedmtercon-nectivltyoftheporesandhigherporositlesmthetop-layerandthesub-layerFurther,itap-pearsthatthetop-layersofthesemembraneshaveanodularstructureReuvers’modelcannotaccountfortheeffectsofthistypeofadditive.Inthispaperwewillshowsomeoftheeffectsoftheadditionofpoly(vinylpyrrohdone)tothesystemconsistingofpoly(ethersulfone)andl-methyl-2-pyrrohdone,coagulatedwithwater.InPartAofthispaperthemechanismunder-lyingmacrovoidformationwillbediscussed.Wewillpresentamodeltodescribemasstransferinaquaternarysystemconsistingoftwomis-ciblepolymersmacommonsolventprecipl-tatedbyanon-solventforoneofthepolymers.Onthebasisofthismodel,wewillcometoamechanismthatcanexplainsomeoftheeffectsofthepolymericadditiveInPartB,theoccur-renceofnodularstructuresmUF-typetop-lay-erswillbe&scussedExperimentalevidencewillbegivenforapossiblespmodalmechanismofformationofthesenodularstructuresmtop-layers(A)MacrovoidformationinsystemswithamacromolecularadditiveTheoretrcalconslderatuvuThesystemwewanttoinvestigateconsistsoffourcomponents.lAnon-solventforthepolymer,uutlallypres-entmthecoagulationbath(component1).lAsolventforthepolymer,uutlallypresentmthepolymersolution,andmsomecasesmthecoagulationbath(component2)lThemembraneformingpolymer(compo-nent3),presentonlymthepolymersolutionandmthefinalmembranelThemacromolecularadditiveused(compo-nent4).Weassumethattheadditiveismisciblewithallcomponentspresent.Atypicalmodelsystemforthisisthesystemusedmourexperiments:water(l)-1-methyl-2-pyrrohdone(2)-poly(ethersulfone)(3)-poly(vinylpyrrohdone)(4)ThermodynamicsWehavemoursystemtwomacromolecularcomponents,bothpresentathighconcentra-tionsmthesamesolution.Typicalconcentra-tionsmcastingsolutionsare15to20weightpercentmembraneformingpolymerand10to15weightpercentadditive.Thetwopolymersbotharefarabovetheiroverlapconcentrationsandformentangled,intertwinedcoils,sinceweknowthatthetwopolymersarewellmiscible.Thisimpl