Pure- and mixed-gas permeation of CO₂ and CH₄ in thermally rearranged polymers based on 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) and 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)

Permeability coefficients for pure CO₂, pure CH₄, and CO₂/CH₄ mixtures containing 50% CO₂ are reported for a polyimide synthesized from 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) and 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and for three thermally-rearranged (TR) derivatives thereof. Permeability measurements were made at 35°C for fugacities ranging from 4 to 25atm. The permeability of CO₂ and CH₄ increased as the degree of TR conversion increased. For example, CO₂ permeability at 10atm increased by a factor of 30 between the unconverted polyimide and its TR analog converted at 450°C. In pure-gas experiments, CO₂ was observed to plasticize the unconverted polyimide, but it did not appear to plasticize the TR polymers. In mixed-gas experiments, dual-mode competitive sorption caused a depression in CH₄ permeability, with very little change in CO₂ permeability. In addition, plasticization by CO₂ was evident in the CH₄ mixed-gas permeability trends, but its impact was small in contrast with dual-mode competitive effects. Consequently, CO₂/CH₄ mixed-gas permeability selectivity was higher than the ideal selectivity, calculated as the ratio of pure gas permeability coefficients. The dual-mode sorption and permeation model was fit to the experimental data. Dual-mode model parameters and model predictions are reported, along with their confidence intervals. By comparing the dual-mode model predictions with the experimental mixed-gas data, the degree of CO₂-induced plasticization was observed to decrease as the degree of TR conversion increased and was completely absent (within experimental uncertainty) for the TR polymer converted at 450°C.