Gas transport properties and structural order of poly(4,4′-oxydiphenylene piromelliteimide) in composite membranes

Abstract

Abstract Specific physico-chemical properties of aromatic polyimides determine their high selectivity in gas separation and relatively low gas permeability. Gas transport properties of insoluble polyimides may be improved in the process of membrane formation. A two-stage process was used for making composite membranes comprised of poly(4,4′-oxydiphenylene pyromelliteimide) (PI) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). In the first stage the polyamic acid (PAA)/PPO membrane was formed by casting a 5% PAA solution on the surface of the PPO support. The second stage of solid-phase imidization led to obtaining a PI/PPO membrane. Catalytic, chemical and thermal methods of imidization with heating to 150°C were compared. It was established that the catalytic method is the simplest and most reliable in the preparation of membranes with the best gas transport properties. The interaction between the PAA prepolymer and benzimideazole (BI) catalyst was studied. The formation of a PAA:BI complex is favourable to imide ring formation at lower temperature. The PI/PPO membrane was analysed on the basis of the resistance model. It was shown that resistance to gas flow through the PI/PPO composite membrane is determined principally by the zone of homogeneous film in the PI top layer. The analytical data on the membrane structure was verified by electron microscopy data. It was concluded that low-temperature solid-phase catalytic imidization is a promising method for making composite membranes based on insoluble polyimides.