Designing Macrovoid-Free Polysulfone (PSf) Ultrafiltration (UF) Membranes with Tetraoxaundecane (TOU) Solvent

Formation of macrovoids in the membrane structure is a common occurrence in the process of nonsolvent-induced phase separation (NIPS), yet it is often undesired due to low structural homogeneity, poor performance, and introduction of weak mechanical points. Here, we introduce an ether-based solvent, 2,5,7,10-tetraoxaundecane (TOU), for fabrication of macrovoid-free polysulfone (PSf) ultrafiltration (UF) membranes. We investigate how strong polymer-solvent affinity, high chain entanglement, and high viscosity influence the phase-inversion process and promote the formation of homogeneous membrane structures during NIPS. Membranes prepared from PSf/TOU dope solutions exhibited stable sponge-like morphology, without macrovoids, in contrast to PSf membranes prepared from the more common solvent N-methyl-2-pyrrolidone (NMP), which typically display large fingerlike macrovoids. For PSf/TOU systems, mixing ethanol and water in the coagulation bath led to fabrication of porous, permeable membranes while maintaining the macrovoid-free morphology. Membrane performance was further optimized by changing the polymer concentration and the addition of a pore-forming agent in the dope solution. The best-performing membranes showed pure water permeance between 149 and 204 L·m^-2·h^-1·bar^-1 and a bovine serum albumin rejection around 90%. This work establishes TOU as a previously unexplored yet effective solvent for NIPS fabrication of PSf UF membranes, enabling precise control over membrane structure and effective suppression of macrovoid formation and ultimately broadening the design space for PSf membranes.