Investigation of the relation between viscoelastic properties of polysulfone solutions, phase inversion process and membrane morphology: The effect of solvent power

This study displays that the lacy structure could be effectively formed and retained in polysulfone (PSf) membrane morphology, prepared by non-solvent induced phase inversion process, by the adoption of poorer solvent as an additive. With 2-pyrrolidone (2P) as a solvent, the PSf membrane comprised a porous skin layer, lacy structure and fewer macrovoids, while with N-methyl pyrrolidone (NMP) a dense skin layer, finger-like macrovoids and cellular structure were dominant. The addition of 2P and water to PSf/NMP solution increased the viscoelasticity of the polymer solution. When water added to the PSf/NMP solution, the fully cellular structure formed in the PSf membranes due to the enhanced viscosity of the solution and occurring the delayed liquid-liquid demixing process. The enhanced viscoelasticity of the PSf/NMP solution with the utilization of 2P at various ratio slowed down the growth rate of demixed domains during the liquid-liquid demixing process due to the formation of hydrogen bonding between PSf chains and 2P molecules in the solution. The gelation of polymer-rich phase inhibited the coarsening of the phase-separated domains, and consequently, the lacy structure formed and locked in the nascent PSf membrane. The lacy structure formation, as well as the size and shape of the macrovoids in the PSf membranes, were controlled by the amounts of 2P presented in the systems. Also, the performance results showed that the flux is enhanced and salt rejection is declined with 2P content in the dope solution. While, the addition of water yielded reverse results, which they are in good agreement with the membrane morphology.