A model for wet-casting polymeric membranes incorporating nonequilibrium interfacial dynamics, vitrification and convection

A new model is developed for wet-casting polymeric membranes that address how the concentrations at the interface between the casting solution and nonsolvent bath adjust from initial nonequilibrium to equilibrium values on the binodal. Properly describing the evolution of the interface concentrations enables this new model to predict vitrification, which has been observed experimentally but not predicted heretofore. This new model also incorporates densification-induced convection that arises owing to density changes associated with the concentration gradients and contributes to the mass-transfer fluxes. The predictions for the cellulose acetate, acetone, and water system indicate that densification-induced convection can increase the mass-transfer flux by nearly two orders-of-magnitude shortly after initiating wet-casting. This increased mass-transfer flux can have a marked effect on the properties of the functional layer of asymmetric membranes that is formed early in the casting process. The predictions for initial casting-solution thicknesses of 75 and 125 μm are markedly different. When densification-induced convection is included, the 125 μm film is predicted to enter well into the metastable region, thereby allowing supersaturation that promotes macrovoid defects. Hence, this new model provides an explanation for the effect of casting-solution thickness on the occurrence of macrovoids.