Modeling Gas and Vapor Sorption and Swelling in Triptycene-Based Polybenzoxazole: Evidence for Entropy-Driven Sorption Behavior

The nonequilibrium lattice fluid model was used to describe and sometimes predict nitrogen, methane, carbon dioxide, ethane, and water vapor sorption at multiple temperatures (5-50 °C) and pressures (up to 32 atm) in novel triptycene-based polybenzoxazole (TPBO) prepared via a thermal rearrangement (TR) process from an ortho-functional polyimide precursor. The polymer lattice fluid parameters were determined using a few sorption data in the limit of infinite dilution and used to predict the solubility of nonswelling gases at several temperatures with no adjustable parameter. To calculate the solubility of swelling gases, the polymer-penetrant interaction parameter was adjusted to experimental sorption data at low pressure at a reference temperature. The second adjustable parameter, that is, the swelling coefficient, was calculated at each temperature using only one experimental sorption datum at high pressure. TPBO exhibits better dimensional stability upon exposure to swelling penetrants relative to previously reported TR polymers. Finally, it was demonstrated that the larger sorption capacity exhibited by TPBO relative to iptycene-free TR polymers has a purely entropic origin.