Mixed gas solubility in glassy polymers

The evaluation of the mixed gas solubility in glassy polymers is of fundamental importance in several applications such as in the development gas separation membranes. For rubbery polymers the usual equation of state (EoS) models, in their multicomponent version, can be applied to calculate multiple gas solubility in the equilibrium phases; for the case of glassy polymers, a different thermodynamic tool has to be used. The general approach called Non Equilibrium Thermodynamics of Glassy Polymers (NET-GP) [1], coupled to the Lattice Fluid (LF) model for the representation of substances [2], was showed to be able to predict the solubility of gases and vapors in glassy polymers at various pressures based on pure component parameters. The approach is here applied to the modeling of multicomponent gas solubility in a glassy polymeric matrix, to predict the solubility of CH4/CO2 mixtures in Poly(phenyleneoxide) (PPO), C2H4/CO2 and N2O/CO2 in poly(methylmetacrylate) (PMMA), at various pressures, for which experimental data are available. The data show that the solubility of mixed gases differ significantly from the corresponding pure component values: the N2O content in the PMMA is lowered by the increase of CO2 partial pressure; the same effect is observed for the CO2/CH4 pair in PPO. The NELF model is able to predict the experimental behavior observed for mixed gases solubility in both systems; the agreement is indeed remarkable in both cases in the whole pressure range investigated. The present approach allows also to estimate predictively the mixed gas solubility-selectivity of N2/CO2, CO2/CH4 and H2/CO2 mixtures in several glassy polymers including polyimides such as Matrimid, allowing to detect significant deviations from the ideal selectivity behavior and estimate the selectivity of membrane materials.