The deviation from the conventional dual-mode sorption and mobility model for a gas in glassy polymer membranes has separately been studied thus far, and to simulate sorption and diffusion behavior, an extended dual-mode sorption model and a modified dual-mode mobility model, respectively, have been proposed independently.  However, simultaneous deviation from the conventional dual-mode sorption and mobility model was observed in cases of CO₂ in poly (4-methyl-l-pentene) membrane at 20 °C, in polystyrene membrane at 60 °C and 70 °C and in cellulose triacetate membrane at 50 °C and 60 °C. The plasticization effect of sorbed CO₂ on both the sorption and diffusion processes tends to be brought about in glassy polymer membranes near the glass transition temperature. The behavior was simulated based on the concept that only one population of sorted gas molecules should exist.

The permeability to CO₂ in an equimolar mixture of CO₂ and air for homogeneous poly (4-methyl-l-pentene) membranes was found to be smaller than that for pure CO₂ at the same CO₂ upstream pressure. The permeability to O₂ for the mixed gas was increased with increasing upstream gas pressure, although for pure O₂ the permeability was independent of upstream pressure Such experimental evidence is consistent with the free-volume model prediction rather than a modified dual-mode model, where the free-volume model has the premise that only one population of the sorbed gas exists.  The increase in permeabilities for the coexisting O₂ might result from the membrane plasticization action of sorbed CO₂.  The depression of CO₂ permeability in the gas mixture case might be ascribed to the hydrostatic pressure effect applied to the system, as suggested by the free-volume model.