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)

Abstract

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.