Effect of thermal annealing on a novel polyamide–imide polymer membrane for aggressive acid gas separations

Abstract

A fluorinated, 6FDA based polyamide–imide is investigated for the purification of CH4 from CO2 and H2S containing gas streams. Dense polymer films were thermally annealed and showed that increased annealing temperatures at constant annealing time caused transport behavior that does not resemble physical aging. Free volume increased after annealing at 200 °C for 24 h relative to annealing at 150 °C for the same time. CO2 and CH4 permeabilities and diffusivities did not decrease as a result of the higher annealing temperature, and in fact, were shown to increase slightly. A change to the intrinsic microstructure that cannot be described by simple, densification based physical aging is hypothesized to be the reason for this trend. Furthermore, annealing increased CO2 induced plasticization resistance and a temperature of 200 °C was shown to have the greatest effect on plasticization suppression. Annealing at 200 °C for 24 h suppressed pure gas CO2 plasticization up to 450 psia. Fluorescence spectroscopy revealed increased intramolecular charge transfer, which is presumably due to increased electron conjugation over the N-phenyl bond. Additionally, intermolecular charge transfer increased with thermal annealing, as inferred from fluorescence intensity measurements and XRD patterns. 50/50 CO2/CH4 mixed gas permeation measurements reveal stable separation performance up to 1000 psia. Ternary mixed gas feeds containing toluene/CO2/CH4 and H2S/CO2/CH4 show antiplasticization, but more importantly, selectivity losses due to plasticization did not occur up to 900 psia of total feed pressure. These results show that the polyamide–imide family represents a promising class of separation materials for aggressive acid gas purifications.