A model for dry- and wet-casting of polymeric membranes incorporating convection due to densification: Application to microvoid formation

The dry-casting process and the wet-casting process are two typical phase-inversion techniques for manufacturing synthetic polymeric membranes. Although extensive modeling studies have been done for both casting processes in order to achieve an optimization of a membrane recipe, all models developed heretofore allow for mass transfer only by diffusion. A proper model for membrane casting should incorporate both the diffusive and convective contributions to the mass transfer fluxes. Therefore, the objective of this thesis is the developments of a dry-casting model and a wet-casting model based on the fundamental and general approach to construct well-defined mass-transfer problems incorporating both convection and diffusion.; This new more general approach produces well-defined description of wet- and dry-casting processes that are solvable with currently available PDE solvers and accurately describe the effects of density variation in the system. Non-equilibrium thermodynamics allows further generalization of this approach to multicomponent mass-transfer problems. The predictions of the dry-casting model developed with this general approach show much better agreement with experimental data in the literature for the CA/acetone/water system. The new wet-casting model predicts the presence of a metastable region in the casting solution depending on the initial thickness that is not predicted by model that incorporates only diffusive mass transfer. Low-gravity experiments using a newly developed membrane casting apparatus show that macrovoids are formed in the CA/acetone/water casting solution when a metastable region is predicted by the new wet-casting model. Furthermore, membrane casting experiments that incorporated surfactant in the precipitation bath reveal that macrovoid formation is strongly associated with the coalescence of microdoplets having a high surface energy in the metastable region of the casting solution. Therefore, both the experimental and modeling results support the coalescence-induced coalescence macrovoid formation mechanism.