We have determined the gas transport properties of He, H2, O2, N2, and CO2 for 6FDA‐durene homopolymer and 6FDA‐durene/pPDA copolyimides. The 6FDA‐durene exhibits the highest permeability with the lowest selectivity. Permeability of copolymers decreases with increasing 6FDA–pPDA content, while permselectivity increases with an increase in 6FDA–pPDA content. 6FDA‐durene/pPDA (50/50) and 6FDA‐durene/pPDA (20/80) materials have O2 and CO2 permeabilities greater than those calculated from the addition rule of the semilogarithmic equation. These higher deviations from the additional rule of the semilogarithmic equation are mainly attributed to the fact that these copolyimides have higher solubility coefficients than those calculated from the additive rule. The Tg s of 6FDA‐durene/pPDA copolyimides decrease with an increase in 6FDA–pPDA content. Tg s predicted from the Fox equation are lower than the experimental data, and the their difference increases with an increase in pPDA content, implying the copolyimides of 6FDA‐durene/pPDA may have greater interstitial space among chains because of the conformation difference, and thus create more fraction free volume compared with the ideal case of simple volume addition. Density measurements also suggest these two copolymers have greater free volumes and the fractions of free volume, which supporting the gas transport results. The thermal stability and β‐relaxation temperature have also been studied for these copolymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2703–2713, 2000

ABSTRACT:The gas transport properties of He, H 2 , O 2 , N 2 , and CO 2 for 6FDA-durene, 6FDA-mPDA, and their copolyimides have been investigated as a function of the composition, pressure, and temperature. The permeabilities, diffusion coefficients, and solubility coefficients follow the simple additional rules and Arrihenuis relationships for 6FDA-durene/mPDA polyimides. The permeability decreases with an increase in pressure except He, and in the kinetic diameters of the penetrant molecules with the order of He, CO 2 , O 2 , and N 2 . The diffusion coefficients of O 2 , N 2 , and CO 2 increase with increasing pressure and temperature. Interestingly, for these polyimides, the diffusion coefficient of O 2 is larger than that of CO 2 . This may be caused by the strong quadrupole moment of the CO 2 molecules or the difficulty in estimating the accurate diameter of CO 2 molecules. The solubility coefficients decrease in the order of inherent condensability of the penetrant gases, namely CO 2 , O 2 , and N 2 . In addition, WAXD results show that 6FDA-durene/mPDA polyimides are amorphous. The presence of a single glass transition temperature for these copolyimides indicates that these polyimides can be assumed to be random copolymers. They have excellent thermal stability exhibiting degradation temperature in a range of 493 to 548°C.