Preparation and gas transport properties of triptycene-containing polybenzoxazole (PBO)-based polymers derived from thermal rearrangement (TR) and thermal cyclodehydration (TC) processes

Abstract

Polybenzoxazoles (PBOs), such as thermally rearranged (TR) polymers, have been shown to have excellent gas separation performance. Herein we report the preparation and transport properties of two new series of PBO-based polymers that were thermally derived from triptycene-containing o-hydroxy polyimide and polyamide precursors via a thermal rearrangement (TR) process and a thermal cyclodehydration (TC) process, respectively. Incorporation of triptycene units into poly(hydroxyimide) precursor structures led to a significant increase of fractional free volume and created ultrafine microporosity in the converted PBO-based TR polymers, which enabled both high gas permeabilities and high selectivities. Although the TC process of the poly(hydroxyamide) precursor led to moderate improvement in the separation performance of the resulting triptycene-containing PBO polymers as compared to the TR process, the PBO films converted via the TC process exhibited excellent mechanical properties superior to many other TR polymers previously reported in the literature as well as the triptycene-containing TR polymers in this study. In particular, the PBO film thermally rearranged at 450 °C showed a H₂ pure gas permeability of 810 barrer, a CO₂ permeability of 270 barrer, and CO₂/CH₄ and H₂/CH₄ selectivities of 67 and 200, respectively, at 35 °C and 11 atm, which are far beyond the upper bound limits.