ANALYSIS OF NONSOLVENT SOLVENT POLYMER PHASE-DIAGRAMS AND THEIR RELEVANCE TO MEMBRANE FORMATION MODELING

Calculations have been carried out, based on Flory-Huggins solution theory, to analyze the behavior of the ternary nonsolvent-solvent-polymer phase diagram for typical membrane-forming systems. Consideration is given to the behavior of the spinodal as well as binodal curves, tie-line slopes, and critical points as a function of various parameters, most especially those related to the concentration dependency of the interaction parameters. Implications regarding membrane structure formation are discussed, and the suitability of different functional forms for the interaction parameter concentration dependence is also analyzed. The net result of these calculations is to demonstrate the importance of the various parameters in controlling the phase-diagram behavior and particularly to show the critical role of the concentration dependence of the solvent-polymer interaction parameter in affecting the nature of the miscibility gap.