Recent advances in the formation of phase inversion membranes made from amorphous or semi-crystallin

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.Memb