Prediction of gas permeability coefficients of copolyimides by group contribution methods

Group contribution methods have been tested for several polyimide structures to calculate their gas permeability coefficients, but predictive studies on copolyimides have not been reported to date. The aim of this work is to design novel copolyimide structures with better separation properties than the upper bound performance of currently available polymers. For this purpose, the group contribution methods of Park and Paul and Alentiev et al. were used to predict the permeability coefficients for 30 different copolyimides that had been prepared and had gas permeability data available in the literature. The results have shown that the method of Alentiev et al. give better predictions for copolyimides. Next, the method of Alentiev et al. was used to predict the permeability coefficients of H₂, O₂, N₂, CO₂, and CH₄ gases at 35°C for more than 2200 copolyimide structures. The diamine/diamine or dianhydride/dianhydride ratios were also varied as 50/50, 25/75, and 75/25. The ideal selectivity values were calculated for O₂/N₂, CO₂/CH₄, CO₂/N₂ and H₂/CO₂ separation applications and the results were evaluated in terms of the permeability-selectivity plots to propose copolyimide structures with enhanced separation properties. A unique feature of most of the outstanding structures emerged in this study is the sulphone containing diamines such as p-BAPS and pDDS.