Abstract
Gas permeation and sorption in amorphous polymers above their glass transition temperature are adequately described by Henry's and Fick's laws. However, below the glass transition temperature, sorption is apparently composed of two mechanisms, the first described by Henry's law dissolution and the second by Langmuir's law of adsorption, known as the “dual sorption model”. The Langmuir mechanism evidently arises from the non-equilibrium nature of the glassy state and the capacity of Langmuir mode of sorption is related to the unrelaxed volume in the glassy state. Molecules sorbed by this mode have less diffusional mobility than molecules sorbed by the Henry's law mode. Because of these differences, the analyses of transient permeation expriments are more complex for glassy polymes compared to those used widely for rubbery polymers. The “dual mobility model” for transport of gases in glassy polymers is developed here which has been successfully used to interpret the observed dependence of the apparent permeability and diffusion coefficient on upstream gas pressure. The state of knowledge about gas permeation and sorption in glassy polymers is reviewed. It is seen that such observations provide a new approach to probe the physical structure of the glassy state.
