Effects of chemical transition of polyetherimide membranes having an integrally skinned asymmetric structure

Integrally skinned asymmetric polyetherimide (PEI) membranes have been prepared from an N-methyl-2-pyrrolidone (NMP)/water system by using the phase inversion process involving partial solvent evaporation, and the morphological characteristics for the separation of organic mixtures were investigated by pervaporation. In this work, to estimate the effects of chemical transition on the skin layer of the prepared asymmetric PEI membranes, oxidizing treatment was carried out by using sodium hydroxide aqueous solution. The results showed that the overall prepared membranes exhibited an asymmetric structure composed of a dense skin layer and a porous sublayer of finger-like type. By increasing partial solvent evaporation and oxidizing treatment time, the skin layer became denser as compared with a nascent membrane. On the other hand, when a dense skin layer underwent chemical transition, the morphology of the skin layer progressed and accelerated toward small size particles, which was in good agreement with experimental results for the separation of IPA and water mixtures.