Kinetic modeling and simulation of non-solvent induced phase separation: Immersion precipitation of PVC-based casting solution in a finite salt coagulation bath

A mathematical model is developed to describe the phase inversion kinetics for the fabrication of porous PVC-based polymeric membranes under finite coagulation bath condition. The model accounts for the non-quasi-stationary diffusion of non-solvent across the solidified membrane, time-variant non-solvent concentration in the coagulation bath and swelling of the membrane while phase inversion. An analytical solution is obtained for the pseudo-steady-state model, whereas the unsteady-state phase inversion model is converted by finite integral transform to a set of first-order coupled ordinary differential equations, which are solved by the numerical means. The above models are tested with the real-time movement of the solidification front using PVC/N-methyl pyrrolidine (NMP)-based casting solution involving Pluronic F127 and bentonite additives, and CaCl₂, NaCl and KCl salt coagulation bath as well as demineralized water. The proposed pseudo-steady-state and unsteady phase inversion models predict diffusivity of the non-solvent through membrane matrix and the diffusivity follows the following order for a given casting solution: KCl > NaCl > CaCl₂ > water. The results reveal that the combined effect of Pluronic F127 and bentonite in PVC-NMP-based casting solution dominates over individual additives and it results from the higher non-solvent diffusivity with fast liquid-liquid demixing as well as the enhancement in membrane swelling.