(618f) Effect of Thermal Treatment on the Structure and Gas Transport Properties of a Triptycene-Based Polybenzoxazole Exhibiting Configurational Free Volume

Polymer membranes exhibiting superior stability at temperatures higher than 200°C would improve the thermal and economic efficiency of hydrogen separation from pre-combustion syngas in the Integrated Gasification Combustion Cycle (IGGC), as well as of H₂/CH₄ separation from refinery processes.

The effect of thermal treatment at 220°C for 10 days on the structure and transport properties of 30mm thick films of a triptycene-based polybenzoxazole (TPBO) was investigated experimentally and theoretically. In sharp contrast with the behavior of other rigid glassy polymers, such as PIMs and Matrimid polyimide, which lose most of their permeability upon exposure to high temperatures for a few hours, gas and water vapor diffusion and permeability coefficients in TPBO decrease by just 20%, while sorption is virtually unaffected by thermal treatment. Remarkably, the CO₂/CH₄ selectivity exhibit a negligible change. Fluorescence spectroscopy, WAXD and FTIR analysis indicate that, in contrast with typical behavior of glassy polymers, TPBO does not experience accelerated physical aging, and rule out formation of intermolecular charge transfer complexes upon thermal treatment. According with this physical picture, the diffusion coefficient of penetrant molecules sorbed in the Langmuir’s mode, D_H, does not change after treatment. Small molecule diffusivity and permeability decline is caused by a decrease in polymer chain mobility, which makes more difficult opening gaps to allow penetrant diffusion jumps. According to this picture, the Henry’s mode diffusion coefficient, D_D, substantially decreases upon thermal treatment. The higher stability exhibited by TPBO relative to other high Tg glassy polymers is ascribed to the presence of configurational free volume, which is not related to the non-equilibrium transient conformation, but to the molecular configuration and, as such, it is not relaxed upon protracted exposure to high temperatures.