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

Formation of Polyimide Membranes via Non-Solvent Induced Phase Separation: Insight from Molecular Dynamics Simulations

Molecular dynamics simulations were applied to investigate the formation of P84 polyimide membranes through the non-solvent induced phase separation (NIPS) process, considering two scenarios: one using a conventional organic solvent like n-methyl-2-pyrrolidone (NMP) and the other a greener alternative, γ-butyrolactone (GBL), with water serving as the non-solvent. Different compositions of polymer solutions were established along the binodal boundaries of the respective systems, derived from experimental cloud point data on the ternary phase diagram. The resulting polymer membranes were analyzed and compared in terms of their morphology. The wettability of their surfaces was notably affected by the polymer content in the initial casting solution and demonstrated a correlation with the Brunauer-Emmet-Teller (BET) specific surface area of the associated polymer nanostructures. The GBL solvent systems produced porous polymers qualitatively similar to those obtained with NMP, albeit with slightly narrower pore size distributions.

Ternary Phase-Field Simulation of Poly(vinylidene fluoride) Microporous Membrane Structures Prepared by Nonsolvent-Induced Phase Separation with Different Additives and Solvent Treatments

In this study, an existing ternary membrane system based on nonsolvent-induced phase separation (NIPS) with a phase-field model was optimized. To study and analyze the effects of different additives on the formation of the skin layer and the effects of the three solvents on membrane characterization under the same conditions, two-dimensional simulations of the relevant parameters of a poly(vinylidene fluoride) (PVDF) membrane system were performed. The specific application of quaternary substances in ternary membrane systems was elaborated by determining the cohesive energy density between the additives and solvents, followed by the interaction parameters χ under the joint effect of the two. The results showed that the PVDF microporous membrane formed a dense surface layer at the mass transfer exchange interface, and with an increase in the poly(ethylene glycol) (PEG) concentration, the phase separation of the skin layer was predominantly transformed from liquid-solid partitioning to liquid-liquid partitioning; the number of membrane pores increased with increasing poly(vinylpyrrolidone) (PVP) concentration. The N,N-dimethylacetamide (DMAc) solvent system had the most stable thermodynamic properties; the dimethyl sulfoxide (DMSO) solvent system had mostly large pores running through the membrane and exhibited a porous structure. Related experiments also validated the model. Therefore, this model can be applied to other PVDF ternary membrane systems to better understand the structural development of microporous PVDF membranes under different conditions.

Phase-Field Simulation of the Effect of Coagulation Bath Temperature on the Structure and Properties of Polyvinylidene Fluoride Microporous Membranes Prepared by a Nonsolvent-Induced Phase Separation

We used the phase-field model of the existing Nonsolvent Induced Phase Separation (NIPS) method to add the variable of temperature in simulating the changes in the process of membrane formation. The polyvinylidene fluoride (PVDF) membrane system was applied to examine the influence of coagulation bath temperature change on the skin-sublayer of the membrane structure, thereby elucidating the development process of membrane structure under different conditions and shedding light on the most suitable coagulation bath temperature ranges. It was found that as coagulation bath temperature increased, the number of interface pores in the outer skin layer decreased, but the size increased. As a result, it changed from the crack shape to round-hole shape, thus making the pore structure looser. In the sublayer, the mesh support structure was increased, which enhanced the mechanical strength of the membrane. Relevant experiments also verify the effectiveness of the model.