Unveiling the competitive diffusion of binary gas mixtures in polymers: The case of carbon dioxide and alkanes in nanoporous-crystalline polyphenylene oxide

The competitive diffusion of multicomponent gases in polymeric materials has been long debated and rarely observed experimentally. In this contribution, we explore the diffusion of binary gas mixtures of CO2 and either CH4, C2H6 or C3H8 in semicrystalline Poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films at sub atmospheric pressure. FTIR Spectroscopy in the transmission mode was exploited to investigate both the gas and the polymer phases coupling this technique with Barometry. The IR signals of methane, ethane and propane were isolated for the first time in a polymer phase and their molar absorptivity calibrated from pure gas sorption experiments. Then, during sorption tests involving binary gas mixtures, the same IR signals were used to quantify the simultaneous diffusion of the two gases in the polymer phase and their solubility at thermodynamic equilibrium. The experimental conditions were carefully chosen to prevent any sorption induced modification of the polymer structure. For the first time, in addition to the analysis of sorption equilibrium, a complete picture of the sorption process of a binary gas mixture in polymer films is given and the mutual effects of the diffusing species during the sorption process is unveiled.