Aromatic polyimide and polybenzoxazole membranes for the fractionation of aromatic/aliphatic hydrocarbons by pervaporation

A series of aromatic polyimides and polybenzoxazoles was synthesized and tested as membrane materials for separation of aromatic/aliphatic mixtures by pervaporation. Homo- and random copolyimides were synthesized by a two-step polycondensation of a total of 6 diamines and 4 dianhydrides. Polybenzoxazoles and poly(benzoxazole-co-imide)s were prepared by the thermal rearrangement of ortho-functionalized polyimide films. Pervaporation experiments were conducted at 80°C with either toluene/n-heptane or benzene/n-heptane mixtures as feed streams. All polymers were selective towards the aromatic hydrocarbon, whose permeability could be changed by approximately 4 orders of magnitude depending on the chemical structure of the diamine. Pure-liquid sorption experiments at 25°C revealed that these large changes in hydrocarbon permeability were mainly caused by changes in the diffusion coefficient. The performance of the polymers was compared with previous results reported for other polymers, as well as with the performance of a commercial polyimide (Matrimid ^®).