ENSCHEDE, The Netherlands.

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 phase inversion process is of a highly complex character,in 3