TRANSPORT OF GASES THROUGH POLYMERIC MEMBRANES

In this paper tailor-made polymers like polyoxadiazoles, polytriazoles and polyimides are compared with conventional polymers as gas separation membrane materials. Because their permeation characteristics as a function of the free volume does not compare too well with that of the conventional ones the term free volume was reconsidered. Based on a unique relation between the diffusion coefficient and the jump of the heat capacity at the glass transition temperature T(g), the transport mechanism in the polymer matrix is considered to occur in jump motions from one cavity to an adjacent one. This mechanism was reconfirmed by a molecular dynamics study. It is shown that mass transport at high penetrant concentrations is not time-independent but a phenomenon with transient characteristics. This transient character is also found in the volumetric behaviour during sorption. The polymer matrix is described as a network with topological constraints which changes its state upon high swelling stresses induced by the sorbed penetrant.