(627e) Understanding Polyether Sulfone Membrane Formation Via Nonsolvent Induced Phase Separation By Dissipative Particle Dynamics (DPD) Simulations
Nonsolvent induced phase separation (NIPS), in which a polymer collapses from solution into a thin film upon exposure to a nonsolvent, is the most widely adopted method to fabricate porous polymeric membranes. The link between the NIPS process variables and final membrane pore structure remains surprisingly empirical and relies mainly on continuum thermodynamics and transport models. In this work, we take a molecular approach using dissipative particle dynamics (DPD) simulation to investigate the early stages of membrane formation via the NIPS process. The model parameters were chosen to represent the polyether sulfone/methyl-2-pyrrolidinone/water (PES/NMP/water) system, which has been applied commonly in industry. Millions of particles were used to create an initial model of the sharp interface between the nonsolvent (water) and the polymer solution (PES/NMP), and the dynamics were followed for approximately hundreds of microseconds. The effects of polymer concentration and polymer molecular weight were investigated. The simulations showed clear evidence of microstructure formation, with the growth of polymer-rich and polymer-lean (fluid) domains, due to the mass transfer between the solvent and nonsolvent. An overall compaction of the polymer in the direction normal to the interface as well as a porous sub-layer were also observed. Compared with the polymer chain length, the polymer concentration had a stronger influence on the morphology; the surface layer became significantly denser, with smaller fluid domains, as the polymer concentration increased. Our findings are consistent with several observations and hypotheses about the interfacial processes that occur during a NIPS process and their effects on membrane pore structure.