Metastable Liquid–Liquid and Solid–Liquid Phase Boundaries in Polymer–Solvent–Nonsolvent Systems

In general liquid–liquid demixing processes are responsible for the porous morphology of membranes obtained by immersion precipitation. For rapidly crystallizing polymers, solid–liquid demixing processes also generate porous morphologies. In this study, the interference of both phase transitions has been analyzed theoretically using the Flory–Huggins theory for ternary polymer solutions. It is demonstrated that four main thermodynamic and kinetic parameters are important for the structure formation in solution: the thermodynamic driving force for crystallization, the ratio of the molar volumes of the solvent and the nonsolvent, the polymer–solvent interaction parameter, and the rate of crystallization of the polymer compared to the rate of solvent–nonsolvent exchange. An analysis of the relevance of each of these parameters for the membrane morphology is presented. q 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 763–770, 1997