Sorption and transport of small gas molecules in thermally rearranged (TR) polybenzoxazole membranes based on 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF) and 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA)

The gas solubility of thermally rearranged polybenzoxazole (TR-PBO) membranes and precursor polymer membranes was determined for five representative small gas molecules, H2, N2, O2, CH4, and CO2, at 35 °C and pressures up to 23 atm. Precursor membranes that thermally rearranged to TR-PBO were prepared from 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF) and 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) using three different imidization methods. Sorption isotherms of TR-PBO followed the dual-mode sorption model, which is regarded as a typical model for glassy polymers. The Henry's law coefficient (kD), Langmuir affinity parameter (b), and Langmuir capacity parameter (C′H) were determined using the dual-mode sorption equation. During the thermal rearrangement process, excess free volume in the polymer membrane matrix increased and improved molecular transport was observed. A similar trend of increasing solubility was observed during the thermal rearrangement process. The gas permeability and diffusivity of TR-PBO membranes were also studied using the solution-diffusion model.