Multicomponent gas sorption in glassy polymers

Abstract

Glassy polymers are nonequilibrium solids whose properties may change slowly due to time-dependent variation in the excess volume and enthalpy functions of these materials. Measurable changes in the gas sorption behavior of samples exposed to different time–temperature histories below the polymer's glass transition temperature can be explained primarily in terms of differences between the excess volume of different samples. A simple physical and mathematical model will be described which relates excess sorption in glasses, compared to rubbery polymers, to sorption into excess volume frozen into the quenched matrix. The model will be explained for pure component gases and then generalized to treat multicomponent sorption data for the CO₂/C₂H₄ and CO₂/N₂O systems in poly(methyl methacrylate). The rather complex mixed gas behavior can be predicted quantitatively using only pure component sorption data in conjunction with the generalized model for multicomponent gases. The model, based on the concept of competition of sorbing penetrants for the excess volume present in the glass, is successful for the two systems described above for total pressures up to 300 psia.