REPRESENTATION OF GAS SOLUBILITY IN GLASSY-POLYMERS BY A CONCENTRATION TEMPERATURE SUPERPOSITION PRINCIPLE

A new relation is derived to express the dependence of gas solubility in polymers on pressure, under conditions where the polymers are significantly plasticized by the penetrant gases. This relation shows that, for such conditions, the solubility isotherms have an inflection point which indicates a change in the solution mechanism. At pressures below the inflection point the solubility isotherms exhibit the nonlinearity characteristic of the "dual-mode sorption" behavior of gases in glassy polymers. At higher pressures, by contrast, the solubility isotherms become linear, as observed for gases in "rubbery" polymers in the Henry's law limit. This transition is attributed to the fact that plasticizing penetrants can depress the glass transition temperature of penetrant/polymer systems to the temperature of the isotherms at sufficiently high concentrations. The new solubility relation describes satisfactorily the solubility isotherms for CO2 in several glassy polymers reported by Chiou, Maeda, and Paul and by Kamiya et al. These investigators also suggested the above mechanism.