METASTABLE LIQUID-LIQUID AND SOLID-LIQUID PHASE BOUNDARIES IN POLYMER-SOLVENT-NONSOLVENT SYSTEMS

In general liquid-liquid demixing processes are responsible for the po rous morphology of membranes obtained by immersion precipitation. For rapidly crystallizing polymers, solid-liquid demixing processes also g enerate porous morphologies. In this study, the interference of both p hase transitions has been analyzed theoretically using the Flory-Huggi ns theory for ternary polymer solutions. It is demonstrated that four main thermodynamic and kinetic parameters are important for the struct ure formation in solution: the thermodynamic driving force for crystal lization, the ratio of the molar volumes of the solvent and the nonsol vent, the polymer-solvent interaction parameter, and the rate of cryst allization of the polymer compared to the rate of solvent-nonsolvent e xchange. An analysis of the relevance of each of these parameters for the membrane morphology is presented. (C) 1997 John Wiley & Sons, Inc.