Thermally stable polyimide isomers for membrane-based gas separations at elevated temperatures

The temperature dependence of gas sorption and transport properties is examined for two polyimide isomers. The permeabilities and solubilities of five gases in these materials are reported over an extensive temperature range from 35 to 325°C. Also, the activation energies for permeation, the heats of sorption, and the activation energies for diffusion obtained for both polyimides are compared and correlated with physical properties of the polymers and penetrants. The influence of temperature on the selective properties of these membrane materials is discussed for three gas separations; He/N₂, CO₂/CH_4, and O₂/N_2. Thorough analysis of these data provides insight into the influence of the subtle difference in chain structure of the two isomers. The performance of the 6FDA-6F_p DA as a separation membrane at high temperatures suggests that it is an outstanding candidate for use in novel elevated temperature applications. ©1995 John Wiley & Sons, Inc.