Retainment of pore connectivity in membranes prepared with vapor-induced phase separation

Abstract

The present work shows that the nascent lacy (bi-continuous) structure, formed by the vapor-induced phase separation (VIPS) of polysulfone (PSf) solution, could be effectively retained when 2-pyrrolidinone (2P) was employed as the solvent for polysulfone. With 2P as the solvent the PSf membranes contained uniform lacy structure throughout the whole cross-section, while with NMP the nascent lacy structure transformed to cell-like and the membrane pore connectivity was lost. The employment of 2P increased dramatically the viscosity and elasticity of the polymer-rich phase formed after VIPS. The effect of 2P on preserving lacy structure and enhancing the viscoelasticity of polymer-rich phase was further verified by investigating the formation of poly(methyl methacrylate) (PMMA) membranes with VIPS. The increased viscoelasticity of the polymer-rich phase with the employment of 2P played important roles in retaining the nascent lacy structure. The increase in viscosity slowed down the coarsening of the phase-separated domains, and the increase in elasticity made the polymer-rich domains easier to gel, reducing the time allowing for the domains to coarsen. For the system of PSf/2P, the domain coarsening was slowed down to allow the gelation of the polymer-rich phase to occur before the collapse of lacy structure. For the system of PMMA/2P, the increase in the viscoelasticity of the polymer-rich domains was enough to bring about their gelation almost immediately after phase separation; therefore, no domain coarsening was detected. Also, we propose that the ability of 2P molecules to form hydrogen bonding with PSf or PMMA chains, water molecules, and other 2P molecules was the reason for the dramatic increase in the viscoelasticity of the polymer-rich domains.