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)/-gamma-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 L(c). Meanwhile, the general sponge-like structure thickness L(gs) underneath the membrane surface is almost a constant. And the morphology of thickness-gradient membranes IS Illustrated with three different thickness regions