Gas sorption and characterization of thermally rearranged polyimides based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)

The solubilities of H2, N2, O2, CH4, and CO2 were determined over a range of pressures at 35 °C in a glassy, amorphous, ortho-functional polyimide prepared from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). The HAB-6FDA polyimide was partially converted to its corresponding thermally rearranged (TR) polymer by thermal treatments at different times and temperatures. At 10 atm, solubility coefficients of H2, N2, O2, CH4, and CO2 increased by a factor of approximately two between the polyimide and the most highly converted TR polymer. Correlations between solubility and penetrant condensability were in good agreement with such correlations in other fluorinated polymers. Dual-mode sorption model parameters were determined from the sorption isotherms. The affinity constant, Henryʹs law solubility, and Langmuir capacity constant increased with gas condensability, and increases in the Langmuir capacity constant were observed as TR polymer conversion increased. Comparisons were made between the solubility selectivity of CO2/CH4, O2/N2, CH4/N2, and CO2/N2 with HAB-6FDA, its corresponding TR polymers, and with other polymers in the literature. Qualitatively, a decrease in solubility selectivity for gas pairs including CO2 correlates with imide and acetate loss during conversion.