Formation of membranes by means of immersion precipitation

Using equations and boundary conditions derived in Part I1, calculations have been performed on the ternary diffusion processes that occur in a cellulose acetate (CA) -acetone casting solution immersed into a water bath. The necessary concentration-dependent thermodynamic and hydrodynamic parameters have been derived from experimental data on the three limiting binary mixtures. Calculations show that immersion of the polymer solution into the coagulation bath results in an instantaneous increase of the polymer concentration at the surface of the solution. For a CA-acetone casting solution the thickness of this concentrated surface layer will increase until the onset of liquid-liquid demixing by means of nucleation and growth of the diluted phase that fixes the asymmetric polymer distribution in the film. The moment of onset of the demixing process depends on the thickness of the film. However, addition of a certain minimum amount of water to the casting solution results in an instantaneous onset of liquid-liquid demixing upon immersion, yielding a membrane with a very thin skin layer. The model calculations have been confirmed by light transmission measurements performed on immersed casting solutions.