Thermally rearranged (TR) polymers, first reported by Park et al. , have attracted increasing attention in natural gas sweetening . They offer an unusual combination of high gas permeability and selectivity, which, in some cases surpass the upper bound reported by Robeson in 2008 . TR polymers can be prepared via thermal rearrangement of solvent-soluble polyimides to form insoluble poly-benzoxazoles.
In this work, hydrogen, nitrogen, methane and carbon dioxide sorption isotherms in three TR samples, obtained via thermal conversion of HAB-6FDA polyimide, were theoretically interpreted using the Non-Equilibrium Lattice Fluid (NELF) model . Since PVT data in the rubbery region are not accessible for HAB-6FDA and its thermally rearranged analogs, the polymer characteristic lattice fluid parameters, T*, p* and ρ*, were estimated as reported by Galizia et al. , using a collection of infinite dilution sorption data available at multiple temperatures. The model outcomes show good agreement with the experimental sorption isotherms for both the pristine polyimide and the corresponding thermally rearranged samples. Interestingly, the model confirms by a completely independent perspective that the increase in gas solubility in TR samples is due mainly to entropic factors, i.e. the increase of excess free volume upon thermal rearrangement. Finally, the model results indicate that TR samples have, in the presence of carbon dioxide, greater dimensional stability relative to HAB-6FDA polyimide.
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