Diffusion‐controlled formation of porous structures in ternary polymer systems

We propose a theory for the appearance of two‐phase structures during the formation of polymer membranes from a casting solution immersed in a coagulant bath. Our model is based on diffusion induced phase separation at the spinodal in the ternary nonsolvent‐solvent‐polymer system. A simplified treatment of the interdiffusion process by the diffusion layer method permits the formulation of criteria for the formation of two‐phase structures in the course of the solvent‐coagulant exchange. Our criteria are expressed in terms of the composition dependence of the chemical potentials in the stable and metastable region of the ternary phase diagram. Comparison with experimental results shows qualitative similarities with theoretical predictions.