Phase behavior and mechanism of membrane formation for polyimide/DMSO/water system

A macrovoid-free, sponge-like porous membrane was prepared from polyimide/DMSO/water system, whereas a finger-like membrane was obtained from a polyimide/NMP/water system by phase inversion. The sponge-like membrane structure was investigated by the thermodynamics and kinetics of the phase separation process. The rates of phase separation for both systems were similar, but their phase diagrams were significantly different. The distinct features of the phase diagram for the polyimide/DMSO/water system are: (1) extremely narrow miscibility gap, (2) close gelation point to the binodal curve, and (3) almost parallel tie-line passing through the gelation point to the polymer–solvent axis. These properties hinder the phase separation process to proceed further, i.e. the growth of the polymer-lean phase is, thus frozen or stopped in the early stages. Therefore, the membrane morphology will be determined at an earlier stage in the polyimide/DMSO/water system than polyimide/NMP/water, resulting in a macrovoid-free, sponge-like membrane.