On the multicomponent diffusion of gases in polymer membranes: the case of CO2–C3H8 in amorphous polyphenylene oxide and semicrystalline polyethylene terephthalate

This study is part of an ongoing effort to measure and understanding multicomponent gas diffusion in polymer membranes. The multicomponent diffusion of CO₂–C₃H₈ binary mixtures in amorphous polyphenylene oxide (PPO) and semicrystalline biaxially oriented polyethylene terephthalate (BOPET) is investigated here at 35 °C, sub atmospheric pressure and CO₂ composition in the gas phase (0.49, 0.57) mol mol⁻¹ approximately. To this aim we resort to a closed cell and two analytical techniques, barometry and FTIR Spectroscopy in the transmission mode. By measuring the pure gas solubilities in each polymer with the classical volumetric method, the infrared absorbance related to each penetrant in the polymer phase is calibrated. Then, during multicomponent sorption experiments, the concentration of each species is monitored directly in the polymer phase with FTIR Spectroscopy. Competitive sorption with propane depletes the concentration of carbon dioxide in both polymers with respect to unary sorption. The concentration of carbon dioxide reaches supra-equilibrium loading during co-diffusion with propane in both polymers because of mutual effects between the two diffusing components. The sorption thermodynamics of unary and binary gases is described with the Dual Mode sorption model. The model is then coupled with the Maxwell – Stefan diffusion theory to describe unary transport in both polymers and to predict multicomponent diffusion of CO₂–C₃H₈ in PPO. The concentration overshoot is interpreted as the sequential occurrence of osmotic – reverse – barrier diffusion. Details on this phenomenon and on the time evolution of the selectivity are given.