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SynopsisCalculations have been carried out, based on Flory-Huggins solution theory, to analyze the behavior of the ternary nonsolvent-solvent-polymer phase diagram for typical membraneforming 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 phasediagram 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.

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SynopsisA derivation is presented of a ternary diffusion model to describe the mass transfer processes associated with the quench bath period of the phase inversion process for membrane formation. The complete governing equations, initial conditions, and boundary conditions in the casting film and coagulation bath are presented. Equations for ternary chemical potentials and diffusion coefficients are consistently based on constant specific volume formulations. The model is applied to the analysis of mass transfer paths and their effects on membrane structure formation. Precipitation times are determined for given sets of conditions by superposing calculated mass transfer paths on the ternary phase diagram and observing when the miscibility gap is crossed. Comparisons are made with an earlier reported study on the membrane-forming system: water-acetone-cellulose acetate (CA). Agreement between predicted and measured precipitation times is found to be excellent. The polymer film composition profile a t the moment of precipitation is shown to be a useful indicator of both skin and sublayer structures, allowing distinctions to be made between conditions leading to spongelike and fingerlike morphologies. The influence of model parameters on the mass transfer paths and associated polymer profiles is also discussed.

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