Pervaporative separation of aromatic/aliphatic mixtures with poly(siloxane-co-imide) and poly(ether-co-imide) membranes

Aromatic random copolyimides were synthesized and tested as membrane materials for the separation of a mixture of aromatic and aliphatic hydrocarbons by pervaporation. The polymers were synthesized by a two-step polycondensation route with a total of 4 aromatic dianhydrides, 4 aromatic diamines, and 3 diamino-terminated aliphatic oligomers containing either ether or siloxane units. Pervaporation experiments were conducted at two temperatures with toluene/n-heptane and benzene/n-heptane mixtures as feed streams. All polymers were selective toward the aromatic hydrocarbon. Introduction of siloxane units in the polymer generally led to very high hydrocarbon permeability coefficients, but caused a reduction in selectivity relative to that of the aromatic homopolyimide. Incorporation of ether units, on the other hand, did not generally cause such large increases in permeability, nor large decreases in selectivity. The performance of these materials was compared with previous results reported in the literature for other polymers.