Correlations between Penetrant Properties and Infinite Dilution Gas Solubility in Glassy Polymers: NELF Model Derivation

An analysis of the empirical correlations that relate the solubility coefficient of gases at infinite dilution in glassy polymers to penetrant properties has been performed, by considering gas sorption as a succession of three steps: condensation of the penetrant, dissolution in the equilibrium polymeric phase, and departure of the system from equilibrium. The infinite dilution solubility coefficient is obtained from the contributions of the three steps, the first two of which involve standard equilibrium thermodynamics, while the latter requires nonequilibrium considerations and is described by using the nonequilibrium lattice fluid (NELF) model. The various terms have been evaluated for several gases in polycarbonate, polysulfone, poly(phenylene oxide), and poly(methyl methacrylate). It has been found that the largest term in the expression of solubility is due to condensation, which explains the observed correlation between solubility and critical temperature, while the out of equilibrium contribution to solubility, which generally gives numerical values lower than the condensation term, is in turn more strongly correlated to the penetrant critical volume.