Toward Predicting the Formation of Integral‐Asymmetric, Isoporous Diblock Copolymer Membranes

Abstract

The self‐assembly and nonsolvent‐induced phase separation (SNIPS) process of block copolymers and solvents enables the fabrication of integral‐asymmetric, isoporous membranes. An isoporous top layer is formed by evaporation‐induced self‐assembly (EISA) and imparts selectivity for ultrafiltration of functional macromolecules or water purification. This selective layer is supported by a macroporous bottom structure that is formed by nonsolvent‐induced phase separation (NIPS) providing mechanical stability. Thereby the permeability/selectivity tradeoff is optimized. The SNIPS fabrication involves various physical phenomena—e.g., evaporation, self‐assembly, macrophase separation, vitrification – and multiple structural, thermodynamic, kinetic, and process parameters. Optimizing membrane properties and rationally designing fabrication processes is a challenge which particle simulation can significantly contribute to. Using large‐scale particle simulations, it is observed that 1) a small incompatibility between matrix‐forming block of the copolymer and nonsolvent, 2) a glassy arrest that occurs at a smaller polymer concentration, or 3) a higher dynamical contrast between polymer and solvent results in a finer, spongy substructure, whereas the opposite parameter choice gives rise to larger macropores with an elongated shape. These observations are confirmed by comparison to experiments on polystyrene (PS)‐block‐poly(4‐vinylpyridine) (P4VP) diblock copolymer membranes, varying the chemical nature of the coagulant or the temperature of coagulation bath.