Strategy to prepare skin-free and macrovoid-free polysulfone membranes via the NIPS process

The non-solvent induced phase separation (NIPS) process applied to polysulfone (PSf) casting solutions traditionally leads to the formation of membranes decorated with large macrovoids in their cross-section on top of which lies a dense skin offering a significant resistance to flow. Macrovoids, are considered weak points. They are prone to compaction leading to poor filtration performance over time. Here, we used 2-pyrrolidone (2P) to prepare skin-free PSf membranes by NIPS, and studied the effect of water as an additive to help suppress the formation of macrovoids. The surface images showed the presence of multiple pores while the cross-section images exhibited the successful suppression of macrovoids. The latter was attributed to the enhancement of the solution viscosity causing the mass transfer kinetics to be slowed down, consequently affecting the demixing rates. Moreover, the rheological behavior of the casting solutions revealed that high degree of chain entanglement helped mitigate macrovoid formation. Using 2P as the solvent for PSf permitted the retention of interconnected pores from the top surface, due to the high viscosity of the solution which prevented coarsening. The high porosity (>75%) and large pores (>0.3 μm) of the PSf membranes were still maintained. The permeability of membranes was maintained after suppressing the macrovoids but their mechanical attributes were improved. The feasibility of utilizing water to eliminate macrovoid growth was further validated when applied to other PSf/solvent systems which showed similar trends in the SEM images. These results imply that water as a casting solution additive can provide a highly cost-effective and simple strategy to obtain macrovoid-free p olysulfone membranes.