| Popis: |
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. |
