Ternary Phase-Field Simulations of the Skin-Sublayer Structures in Poly(vinylidene fluoride) Microporous Membranes Prepared by a Nonsolvent-Induced Phase Separation

In this paper, the existing phase-field model based on the nonsolvent-induced phase separation (NIPS) method was optimized. Two-dimensional simulations using the relevant parameters of a poly(vinylidene fluoride) (PVDF) membrane system were carried out, simulating and analyzing the effects of changes in initial concentrations, concentration fluctuations, and diffusion rates of the solvent on the skin layer and sublayer structures of the membranes. These simulations modeled the process of preparing PVDF microporous membranes by the NIPS method to better understand the structural development of PVDF microporous membranes under different conditions. It was found that dense skin layers were formed at the mass-transfer exchange interface of the PVDF microporous membranes, whose number increased with the decrease of the concentration fluctuation, which has little effect on the structure of the sublayer. The initial concentration of PVDF and the diffusion rate of the solvent had a little impact on the number of skin layers yet played a relatively large role in the formation time of the skin layers and the structure of the sublayers. Also, the validity of the model was verified by corresponding experiments. Hence, the model can be applied to other PVDF ternary membrane systems by modifying specific thermodynamic and kinetic parameters.