GAS SORPTION AND TRANSPORT IN GLASSY-POLYMERS

Gas sorption and transport in amorphous polymers above their glass transition temperature are adequately described phenomenologically by Henry's and Fick's laws. However, below the glass transition temperature an additional sorption mechanism develops which follows the Langmuir isotherm, and, thus, there are two populations of gas molecules which may be regarded to be in equilibrium with each other. The Langmuir mechanism evidently arises from the nonequilibrium nature of the glassy state. Molecules sorbed by this mechanism have less diffusional mobility than molecules sorbed by the Henry's law mode. Because of these differences, the analyses of transient permeation experiments are more complex for glassy polymers compared to those used widely for rubbery polymers. A more comprehensive phenomenological model for transport of gases in glassy polymers is developed which has been successfully used to interpret the observed dependence of the permeability coefficient and diffusion time lag on upstream gas pressure. It is seen that such observations provide a unique method to probe the physical structure of the glassy state and that knowledge in this area is increasing rapidly.