Fabrication and characterization of BTDA-TDI/MDI (P84) co-polyimide membranes for the pervaporation dehydration of isopropanol

Asymmetric membranes with superior selectivity and relatively high flux for pervaporative dehydration of isopropanol (IPA) were successfully developed from a novel material P84 co-polyimide by a dry-wet phase inversion method. The best separation performance has a flux of 432 g/m2 h and a separation factor of 3508 at 60 °C for a feed stream containing 85 wt.% IPA. The membrane shows imperceptible degree of swelling even at high feed water concentrations. Polymer dopes with or without non-solvent additives were studied and yielded different membrane skin layer structure. It is found that heat treatment at elevated temperatures can effectively enhance membrane performance because it not only smoothens membrane skin layer roughness but also reduces membrane defects and the d-space value of polymeric chains as indicated by XRD, FESEM, AFM and gas permeation tests. However, asymmetric P84 co-polyimide membranes' performance was influenced by the history of thermal cycling. The testing cycle from 100 °C to 60 °C has a more stable performance compared with that from 60 °C to 100 °C, indicating that a conditioning at a higher temperature is recommended. Thickness-dependent pervaporation performance was observed when investigating dense P84 membranes, and the pervaporation data from asymmetric P84 membranes are consistent with this observation.