Permeability and Permselectivity of Gases in Fluorinated Polyimides

Abstract

Permeability, solubility and diffusion coefficients [P, S, and D (=P/S)] of gases were mea sured at 35°C and 10 atm for a series of polyimides prepared from four acid dianhydrides and three diamines. The introduction of CF3 groups decreased the packing density of polymer segments, resulting in a small increase in S and large increases in D and P. A roughly linear relationship was observed between In Dco2 and the inverse of the fraction of “free space” (or expansion volume). The diffusivity selectivity was significantly different among the polyimides, while the solubility selectivity changed only slightly. For the CO2/CH4 system, polyimides having lower local segmental mobility tended to have higher diffusivity selectivity. The introduction of -C (CF3)2- linkages and/or CF3 substituent groups was effective for CO2/CH4 and O2/N2 separations, but not for H2/CO and H2/CH4 separations, because in the latter cases a moderate increase in PH2 was followed by significant decreases in PH2/PCO and PH2/PCH4. © 1990, The Society of Fiber Science and Technology, Japan. All rights reserved.