Morphology transition of asymmetric flat sheet and thickness-gradient membranes by wet phase-inversion process

In this paper, asymmetric membranes are prepared with P84 (BTDA-TDI/MDI co-polyimide)/N-methyl-2-pyrrolidone (NMP)/γ-butyrolactone (GBL) and polyetherimide (PEI)/NMP/GBL casting solutions by wet phase-inversion process. And the transition of membrane morphology is investigated with different flat sheet and thickness-gradient membranes. For the thickness-gradient membranes, with the increase of membrane thickness, the membrane morphology changes from sponge-like to finger-like structure at the critical structure-transition thickness Lc. Meanwhile, the general sponge-like structure thickness Lgs underneath the membrane surface is almost a constant. And the morphology of thickness-gradient membranes is illustrated with three different thickness regions.For the flat sheet membranes prepared with PEI/NMP/GBL casting solutions, with the increase of GBL weight ratio wGBL, the transition of membrane morphology from finger-like to sponge-like structure occurs at the critical weight ratio of GBL (w* = 0.588). The presence of GBL increases the viscosity of the casting solutions and suppresses the tendency of macrovoid formation.For the thickness-gradient membranes prepared with PEI/NMP/GBL casting solutions, the presence of GBL retards the transition of membrane morphology. The increase of wGBL, results in the increase of Lc and Lgs. When wGBL is greater than w*, the transition of membrane morphology is not investigated and a fully sponge-like structure is formed.