Solvent and Thermal History Effects on Gas Transport in TR Polymers and Their Precursors

HAB-6FDA polyimide was synthesized by a two-step polycondensation method with chemical imidization. Dense polymer films were solution cast from tetrahydrofuran (THF), acetone, and N,N-dimethylacedamide (DMAc). Films cast from THF and acetone were cast in a solvent saturated glove bag then dried at 200°C for 24 h. Two separate films were cast from DMAc in a vacuum oven using different heating and vacuum protocols, with one utilizing a gas purge and the other a temperature ramp under partial vacuum. Each precursor film was used to prepare thermally rearranged (TR) polymers. The rearrangement reaction was performed at 400°C and resulted in a 54% conversion to the polybenzoxazole TR product. Pure CH₄ and CO₂ gas permeability measurements were made on both the precursor and the TR polymer. While the ideal CH₄/CO₂ selectivity remained constant for all four polyimide films, the different casting procedures resulted in a 40% difference in permeability coefficients, with the film prepared from DMAc under partial vacuum having the lowest permeability coefficients for both gases and the DMAc film cast using a gas purge having the highest. The respective TR polymer films exhibited the same trend in permeability with a 37% variation with only minimal differences in selectivity. These trends have been attributed free volume variations due to the different casting techniques and the inability to thermally “reset” the history of the polyimide precursor.