Influence of temperature on gas solubility in thermally rearranged (TR) polymersby Stevens, KA; Smith, ZP; Gleason, KL; Galizia, M; Paul, DR; Freeman, BD
Thermally rearranged (TR) polymers have been the subject of many fundamental studies, but the effect of TR conversion on temperature-dependent transport properties is largely unexplored. Sorption isotherms for N-2, CH4, and CO2 in HAB-6FDA polyimide and its TR analogs were measured at temperatures ranging from -10 degrees C to 50 degrees C and pressures up to 27 atm. Solubilities increase with decreasing temperature for each gas and sample tested. At low TR conversions, the sorption process initially becomes less exothermic. However, enthalpies of sorption do not significantly change with TR conversion after the initial stages of rearrangement. Enthalpies of sorption in TR polymers are qualitatively similar to those of other high free volume materials. Solubility selectivity for CO2/CH4 at 10 atm did not change with temperature due to similar enthalpies of sorption for CO2 and CH4. Sorption data were fit to the dual mode model at different temperatures, and model parameters were correlated with polymer and penetrant properties.