Abstract
The diffusion coefficients and the solubility coefficients of oxygen, nitrogen and carbon dioxide below 1.3 atm were measured for seven polyimides, chemical structures of which were systematically changed. The solubility is better described according to a dual-mode sorption model, rather than Henry's law, from the sorption experiments. The diffusivities correlated with packing density (reciprocal of specific free volume) of the polymer. except in the case of two polyimides. The packing density of these two polymers may not be successfully calculated by Bondi's method. However, all the diffusion coefficients decreased linearly with an increase in the cohesive energy density (CEO), calculated by the group contribution method of van Krevelen. These results suggest that gas diffusion in polyimides correlates with CEO better than with the packing density. Therefore, the cohesive energy density can be considered a more reliable indicator ofthe efficient gas separation. In this study, the reason why the cohesive energy density shows a more reliable correlation to the transport coefficient is also discussed.