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Diffusion during the immersion precipitation process

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  • Physics


PII: S0011-9164(00)86004-X &s&&ion,32(1980)33-45 0 Ehwier Scientific PublishingCompany, Am&edam--PrintszdinTheN&&ds ASYMMETRIC MEHBRANE STRUCTURES AS A RESULT OF PHASE SEPARATION PHENOMENA Il. BROENS, F.W. ALTENA, C.A. SMOLDERS Twente University of Technology, Department of Chemical Engineering. ENSCHEDE, The Netherlands. and D.M. KOENHEN Wafilin Membrane Filtration Systems, Hardenberg, The Netherlands. ABSTRACT 1n this report we give a description of the membrane formaticn mechanism based on theoretical and experimental knowledge of phase separation phenomena in concentrated polymer solutions. WTe demon- strate that different types of phase separation are responsible for the build-up of the dense skin layer and the porous supporting layer in asymmetric membranes of several materials: cellulose acetate, polysulfone polyacrylonitrile and polydimethylphenylene- oxide. The formation of the porous sublayer will be explained in terms of liquid-liquid phase separation, coalescence and gelation. Special attention will be paid to the formation mechanism of the finger-like cavities in the sublayer. A possible explanation will be given on thermodynamic and kinetic grounds, for the formation of nodular structures in the skin of the membranes. I I~RO~UCT~ON The majority of the commercially available membranes are produced by the so-called phase inversion process, starting from a polymer solution which is precipitated in a non-solvent coagulation bath. The resultant membranes prepared in this way have an asymmetric structure, i.e. a very thin more or less dense skin is supported by a porous sublayer of the same material. The technique of producing membranes by the phase inversion method has reached a high degree of reproducibility. This development has been sustained by many systematic studies of the effect of relevant parameters and by the efforts to elucidate the mechanism of membrane formation l-12) The phas

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