Diffusion phenomena in ternary systems polymer-nonsolvent-solvent

Diffusion in a boundary between a polymer + solvent soln. and nonsolvent was treated by accounting for the presence of the four diffusion coeffs. that describe the isothermal transport process in a three-component system. Diffusion equations were integrated assuming a concn. dependence of diffusion coeffs. that account for the thermodn. conditions on the cross-diffusion terms. The presence of non-zero cross terms promotes an incongruent diffusion of polymer whose concn. increases at the boundary between the polymer + solvent soln. and the non-solvent. Although the authors' model describes diffusion in the range of homogeneous soln., this incongruent polymer diffusion is a process similar to that promoted by the solvent evapn. from the polymer + solvent film that some authors suggested as an intermediate step before the film immersion into the coagulation bath to obtain good asym. membranes.