Sorption of C2H6/CO2 gas mixtures in amorphous glassy polyphenylene oxide: Modelling of thermodynamics and of the multicomponent diffusion process

This contribution addresses the experimental characterization and the theoretical interpretation of the sorption thermodynamics and diffusion kinetics of a binary gas mixture in a glassy amorphous polymer. Specifically, the multicomponent diffusion of C₂H₆/CO₂ mixtures with a mole fraction of CO₂ $\cong 0.525$ mol mol⁻¹ has been investigated in a film of Poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) at 308.15 K and at several values of sub-atmospheric pressure. Static experiments were conducted in a closed volume, coupling FTIR spectroscopy in the transmission mode and barometry. Competitive sorption produces a decrease of the equilibrium concentration of each component of the mixture in PPO with respect to the corresponding cases of pure gas sorption performed at pressure values equal to the partial pressure values in the gas mixture. Moreover, during multicomponent diffusion, carbon dioxide attains concentration values exceeding the final sorption equilibrium value, displaying an overshoot in the sorption kinetics curve. To interpret these experimental findings, the glassy polymer phase has been modelled with a lattice fluid model according to the Non-Equilibrium Thermodynamics of Glassy Polymers (NET-GP) theory and, for each penetrant, its chemical potential gradient has been considered as the driving force for diffusion. Theoretical analysis well predicts the competitive sorption thermodynamics and the supra-equilibrium loading of carbon dioxide during multicomponent diffusion. Notably, the model shows that osmotic, reverse and barrier diffusion of carbon dioxide occur at short times prior to the overshoot. Implications of the theoretical analysis on separation performances of a PPO based membranes have also been addressed.