Influence of the addition of PVP on the morphology of asymmetric polyimide phase inversion membranes: effect of PVP molecular weight

A macrovoid-free, porous membrane with a sponge-like structure was obtained by phase inversion of a polyimide (PI)/γ-butyrolactone (GBL)/water system, whereas a membrane with a finger-like structure was obtained from a PI/N-methyl-2-pyrrolidinone (NMP)/water system. In this study, the effect of the molecular weight of poly(vinyl pyrrolidone) (PVP) additive (Mw: 360k and 40k) in two systems on membrane morphology has been investigated by the differences in phase separation rates and miscibility gaps. On addition of PVP into the PI casting solutions, the two systems exhibited significantly different membrane morphologies; the former augmented macrovoid formation while the latter suppressed formation of finger-like structures. The membrane morphology and the phase separation rate significantly depend on the molecular weight of PVP in PI/NMP/water system whereas those do insignificantly in PI/GBL/water system. The cloud points representing the miscibility gap do not depend on the molecular weight of PVP in both systems very much. Therefore, it is suggested that the morphology change in the PI/NMP/water system is mostly due to the change in the phase separation rate caused by the change in the solution viscosity upon addition of PVP. However, in the PI/GBL/water system, the molecular weight effect of PVP is not as significant as in the PI/NMP/water system, and the morphology change seems to be primarily due to the change in the miscibility gap.