Gas separation performance and mechanical properties of thermally-rearranged polybenzoxazoles derived from an intrinsically microporous dihydroxyl-functionalized triptycene diamine-based polyimide.

An intrinsically microporous hydroxyl-functionalized polyimide (PIM-PI) made from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,6(7)-dihydroxy-3,7(6)-diaminotriptycene (DAT1-OH), was thermally converted to polybenzoxazole (PBO). The thermal rearrangement of the PIM-PI to PBO significantly increased the free volume, which was reflected by a boost in its microporosity as indicated by enhanced Brunauer-Emmett-Teller (BET) surface area from 167 to 405 m2 g−1. The increase in free volume noticeably improved the gas permeability but also resulted in reduced gas-pair selectivity. The fresh PBO membrane made by thermal treatment at 460 °C for 30 min (TRIP-TR-460-30) with a PBO conversion of 98% displayed a 20-fold higher CO 2 permeability of 840 barrer than the initial value of 43 barrer for the 6FDA-DAT1-OH polyimide at the expense of ~ 60% decrease in pure-gas CO 2 /CH 4 selectivity from 52 to 21. The TRIP-TR-460-30 PBO showed good performance for propylene/propane separation with pure-gas C 3 H 6 permeability of 21 barrer and C 3 H 6 /C 3 H 8 selectivity of 16 for a 28-days aged sample. When tested under mixed-gas conditions C 3 H 6 permeability dropped to 12.8 barrer and C 3 H 6 /C 3 H 8 selectivity of 8. TRIP-TR-460-30 PBO displayed mechanical properties comparable some rigid polyimides with tensile strength, Young's modulus and elongation at break of 58 MPa, 1.83 GPa and 4.3%, respectively. Image 1 • TR-derived PBO membranes made from 6FDA/triptycene OH-based diamine polyimide. • Fully converted PBO exhibited improved gas permeability but reduced selectivity. • Physical aging had large effect on gas permeability but small effect on selectivity. • PBO demonstrated performance located on the propylene/propane upper bound. [ABSTRACT FROM AUTHOR]