Recentadvancesintheformationofphaseinversionmembra

journalofMEMBRANESCIENCEELSEVIERJournalofMembraneScience113(1996)361-371Recentadvancesintheformationofphaseinversionmembranesmadefromamorphousorsemi-crystallinepolymersI.M.Wienk,R.MBoom,M.A.M.Beerlage,A.M.W.Bulte,C.A.Smolders,H.StrathmannUniversityofTwente,P.O.Box2!7,7500AEEnschede,NetherlandsAccepted11September1995AbstractStructuralcharacteristicsinmembranesformedbydiffusioninducedphaseseparationprocessesarediscussed.Estab-lishedtheoriesonmembraneformationfromternarysystemscanbeextendedtodescribetheeffectsofhighorlowmolecularweightadditives.Amechanismfortheformationofnodularstructuresinthetoplayerofultrafiltrationmembranesispresented.Inthelastpartstructuresarisingfrompolymercrystallizationduringimmersionprecipitationarediscussed.Keywords:Membraneformation;Additives;Macrovoids;Nodules;Crystallizablepolymers1.IntroductionFormorethanthirtyyearsstructuralcharacteris-ticsofphaseinversionmembranesfromamorphouspolymershaveattainedmuchattention.Problemstobesolvedinasemi-quantitativewaywere:theab-senceorpresenceofaskinlayeranditsporosity,theoccurrenceofmacrovoidsandofnodularstructures.Abreakthroughinunderstandingtheoriginofthesestructuralelementswasobtainedby:(i)thermodynamiccalculationof3-componentphasediagrams(polymer/solvent/non-solvent),us-ingproperbinaryinteractionparameters,incombina-tionwith(ii)computationsofso-calledcompositionpathsfortracingthekineticsofphaseseparation,usingproperdiffusionequations,boundaryconditionsandfrictionalparametersbothforthepolymerfilmandthecoagulationbath.ElsevierScienceB.V.SSDI0376-7388(95)00256-1Recentattentioninresearchonmembraneforma-tionattheUniversityofTwentehasfocusedon:•theroleofadditives,bothhigh-andlow-molecu-larweight,onmembraneformation•furtherstudiesonmacrovoidformation•thebackgroundofformationofnodularstruc-tures.Apartfromthesestudiesonsystemswithamor-phouspolymers,theabovementionedcomputationalapproachofphaseseparationandcompositionpathshasbeenappliedalsotomembraneformationusingsystemswitharapidlycrystallizablepolymer.2.Membraneformation:stateoftheart2.1.PhasediagramsThestructureofphaseinversionmembranesre-sultsfromaphasechangeofinitiallystablesolutions362I.M.Wienketal./JournalofMembraneScience113(1996)361-371whichhavebeenbroughttoanunstablestate.Thestateandequilibriumcompositionsofpolymersolu-tionscanbewelldepictedinternaryphasediagrams.Inthemetastableregion,lyingbetweenthebinodalandspinodalcurves,phaseseparationwilltakeplacebynucleationandgrowth.Intheunstableregionofthephasediagramspinodaldecompositionwilloc-cur.Duringnucleationinmembraneformingsystemsthepolymerleanphasewillnucleateandthepoly-merrichphasewillbecontinuous.Theconcentrationofthepolymerrichphaseincreasesuntillocallytheglasstransitionofthesystemisreachedandvitrifica-tionofthesolutiontakesplace.Oncethepolymerrichphasehasattainedacertainrigidityprofoundchangesinmorphologyarenotpossibleanymore.Theultimatestructureofthisprocessisaporousmembrane[1].Thelocationofthebinodalcurveintheternaryphasediagram,enclosingthedemixinggap,canbefoundbycloud-pointmeasurementsorbymeasuringthelightscatteringofthepolymersolutionsforvaryingcompositions[2],oritcanbecalculatedifthethermodynamicinteractionparametersofthesys-temareknown.Tompa[3]andAltenaetal.[4]calculatedphasediagramsforternarysystemsofamorphouspolymersusingbinaryinteractionparam-eters.2.2.CompositionpathsPhaseseparationofapolymersolutioncanbeinducedby:•quenchingthesolutiontoalowertemperature•immersionofthesolutioninabathofnon-solvent•contactingthepolymersolutionwithavapourofthenon-solvent•evaporationofthesolvent.Formembraneformationimmersionprecipitationisthemostimportantandbeststudiedprocess.Phaseseparationprocessesbyevaporationorcontactwithanon-solventvapourarecloselyrelatedtotheimmer-sionprecipitationprocessbecausetheyarealsodif-fusioninduced.Uponimmersionofafilmofapolymersolutionintoanon-solventorcoagulationbaththenon-solventwilldiffuseintothepolymersolution,whereasthesolventdiffusesintothebath.Thediffusioncoeffi-cientofthepolymerismuchlowerandthereforethemobilityofthesepolymericmoleculeswillreachonlyforsmallerdistances.Fundamentalstudiesonmembraneformationbyimmersionprecipitationinternarysystemsofpoly-mer,solventandnon-solventhavebeencarriedoutbyCohenetal.[5]Reuversetal.[6],Tsayetal.[7]andRadovanovicetal.[8].Theydevelopedmasstransfermodelstodescribethediffusionprocessesforespeciallythefirstmomentsofimmersion.Suchakineticmodelresultsinacompositionpathintheternaryphasediagram.Thispathrepresentseitherthechangeincompositionofasmallvolumeintimeorthecompositionsatalllociinthefilmatacertainmoment.Combinedwiththeequilibriumthermody-namicsofthesystemitcanbedeterminedwhenandwherethecompositionpathwillcrossthebinodalcurve.Soitispossibletopredictatwhattimeafterimmersionphaseseparationwillstartneartheinter-faceandwhatthecompositionprofileofthepolymersolutionwillbeatthatmoment.BasedonthecalculatedcompositionpathsandsupportedbyexperimentaldataReuversetal.[6]dividedmembraneformationprocessesintotwogroups:instantaneousdemixingsystemsandsystemsthatshowdelayofdemixing.Ifforagiv