Free volume model and tradeoff relations of gas permeability and selectivity in glassy polymers

A simple free volume model for permeability (P) and diffusivity (D) of polymers has been used to obtain various linear correlations of the type log Pi versus log Pj or log Di versus log Dj for gas pairs Mi and Mj. These correlations are proven for several hundred glassy polymers and various gas pairs. An important implication of this result is that it predicts the well-known tradeoff relation between permeability Pi and permselectivity Pi/Pj considered by many authors. Possible ways for improving the combination of these two parameters of membrane polymeric materials are discussed. In particular, the role of sorption selectivity and its enthalpic and entropic contributions are emphasized.