Molecular simulation was used to model the d-spacing and fractional free volume of 6FDA-durene，6FDA-pPDA and their copolymer 6FDA-durene/pPDA. Calculation results were in agreement with experimental data. Grand canonical Monte Carlo (GCMC) calculations were performed to simulate the sorption of O2

N2

CH4 and CO2 in the polyimide. The simulation results indicated that the COMPASS force field was suitable to describe the sorption of O2

N2 and CH4. The calculated solubility coefficients of O2

N2 and CH4 were in agreement with experiments. But the solubility coefficient of CO2 exhibited high deviation (about 50%) from the literature value. The main reasons might be: (1) swelling effect of CO2 was not considered in the simulation process; (2) COMPASS force field could not describe the interaction of CO2 and —CF3 of the 6FDA-polyimides. The calculated solubility coefficients decreased in the sequence of the inherent condensabilities of the gases

namely

CO2

CH4

O2 and N2. The solubility coefficients in different polyimide increased with increasing fractional free volume. The 6FDA-polyimides showed better solubility selectivity for CO2/CH4 than O2/N2. Copolymerization could not improve the solubility selectivity remarkably.