Impact of Nonsolvent-Solvent Affinity on Membrane Morphology and Microstructure: Unraveling the Transition from Traversing Pore to Closed Void Structures

The commonly used nonsolvent-induced phase separation process for creating polymer membranes lacks microscopically solidified mechanisms. This study employs dissipative particle dynamics simulations to investigate the solidification dynamics and the impact of nonsolvent-solvent affinity on membrane morphology and microstructure. Strong nonsolvent-solvent affinity triggers active nonsolvent-solvent exchange and membrane solidification via nonsolvent-induced precipitation, resulting in a traversing pore structure. In contrast, weak affinity restricts exchange, leading to solidification primarily through solvent loss-induced oversaturation and resulting in a closed void structure. The membrane's microstructure is closely linked to the solidified polymer conformations, with smaller polymer sizes observed in membranes with low crystallite content compared to those with high crystallite content. Polymer sizes are smaller in coil-like conformations compared to those in interfolding conformations. Increasing nonsolvent-solvent affinity promotes a dominant nonsolvent-solvent exchange mechanism, leading to faster solidification, lower crystallinity, and poorer polymer alignment with coil-like conformations. The differences in macroscopic membrane morphology and microscopic polymer conformation illustrate how solidification varies with strong and weak affinity.