DENSE POLYMER FILM AND MEMBRANE FORMATION VIA THE DRY-CAST PROCESS .1. MODEL DEVELOPMENT

The dry-cast membrane-formation process is a major phase-inversion tec hnique by which asymmetric membranes are manufactured. In this paper a fully predictive model which incorporates coupled heat and mass trans fer is developed to describe the evaporation of both solvent and nonso lvent from an initially homogeneous polymer/solvent/nonsolvent system. This unsteady-state, one-dimensional, coupled heat- and mass-transpor t model allows for local film shrinkage owing to excess volume of mixi ng effects as well as evaporative solvent and nonsolvent loss. The mod el can predict composition paths into the ternary phase diagram which determine the onset of phase transition. The ternary phase diagram is predicted using the Flory-Huggins theory allowing for composition-depe ndent interaction parameters. The model is applied to the well-charact erized cellulose acetate/acetone/water system for which sufficient exp erimental data are available to permit determination of the friction c oefficients in the ternary mass-transport model. The model is solved n umerically using a software package based on the method of lines which is capable of handling moving boundary problems. The modeling studies indicate that for a given polymer/solvent/nonsolvent/support system, the most influential parameters are the gas-phase mass transport, init ial cast film thickness, and initial composition. Of particular import ance, the model can predict the general morphological characteristics associated with the formation of dense polymer films and symmetric as well as asymmetric membranes.