Basis of Permeability/Selectivity Tradeoff Relations in Polymeric Gas Separation Membranes

Gas separation properties of polymer membrane materials follow distinct tradeoff relations: more permeable polymers are generally less selective and vice versa. Robeson 1 identified the best combinations of permeability and selectivity for important binary gas pairs (O 2 /N 2, CO 2 /CH 4, H 2 /N 2, etc.) and represented these permeability/selectivity combinations empirically as α A/B = β A/B, where P A and P B are the permeability coefficients of the more permeable and less permeable gases, respectively, α A/B is selectivity (= P A / P B ), and λ A/B and β A/B are empirical parameters. This report provides a fundamental theory for this observation. In the theory, λ A/B depends only on gas size. β A/B depends on λ A/B, gas condensability, and one adjustable parameter.