Crosslinked thermally rearranged polybenzoxazole derived from phenolphthalein-based polyimide for gas separation

We have developed a novel ortho-hydroxyl diamine 3,3-diaminophenolphthalein and formed a polyimide (Ac-6FDA-DAP) via chemical imidization between this compound and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride. The acetate groups and lactone ring of the diamino moiety degraded and subsequently formed a biphenyl crosslinked polyimide, enabling subsequent conversion of the thermally crosslinked polyimide into a polybenzoxazole (PBO) with superior CO2 separation performances. Compared to the polyimide precursor, the CO2 permeability of the TR PBO, thermally rearranged at 400 °C, increased by at least 21 folds (30.1 to 613 Barrer), with 10% decrement in the CO2/CH4 selectivity (36.7 to 33.5). The CO2/CH4 separation performances of the TR PBO surpassed the “2008 Robeson Upper Bound”, and no plasticization was observed at a CO2 pressure up to 30 atm for pure CO2 and 40 atm for 50:50 CO2 and CH4 gas mixtures. The TR-PBO polymer derived from Ac- 6FDA-DAP showed a strong potential for carbon capture and natural gas treatment.