A DIFFUSION-CONTROLLED MODEL FOR WET-CASTING MEMBRANE FORMATION

A simplified analytical treatment of membrane formation mechanism, following the Paul method, is proposed using a casting solution immersed in a coagulation bath. This model assumes a discontinuous system consisting of parallel layers in the membrane with each layer proceeding rapidly to phase inversion. It allows the measurement of local composition variation at each layer depending on the flux ratio of solvent to nonsolvent. It is difficult to test the model unless a direct method of estimating the flux ratio at each layer is developed. However, some conclusions can be drawn about the applicability of the suggested model according to the type of membrane. For a homogeneous membrane, the flux ratio is independent of position. For an asymmetric membrane, the flux ratio has two values: the larger value for the skin layer and the other for the sublayer. Experimental results are compared with the values predicted by this pseudobinary mathematical model, and show fairly good agreement.