Modeling gas permeability and diffusivity in HAB-6FDA polyimide and its thermally rearranged analogs

Gas permeability in HAB-6FDA polyimide and its thermally rearranged analogs was described using a thermodynamic model based on the non-equilibrium lattice fluid (NELF) theory. This study is part of an ongoing effort to describe gas sorption and transport behavior of TR polymers theoretically. Hydrogen, nitrogen and methane permeability over a broad range of pressures (up to 32 atm) and temperatures (−10 to 50 °C) was calculated with one adjustable parameter at each temperature, i.e., the infinite dilution mobility coefficient. For highly soluble, swelling gases, such as CO₂, matrix plasticization was accounted for by a second adjustable parameter, the plasticization factor, which describes the dependence of penetrant mobility on concentration. Model parameters correlate with membrane structure and gas properties. At fixed temperature, the infinite dilution mobility correlates with penetrant critical volume and polymer fractional free volume. For each penetrant, the temperature dependence of infinite dilution mobility is described by the Arrhenius law. Based on the modeling results, unique separation performance of TR polymers is a manifestation of their strong size-sieving ability. Finally, diffusion coefficients and ideal selectivities were predicted with no adjustable parameters.