Cross-Linked Thermally Rearranged Polybenzoxazole Derived from a Phenolphthalin-Based Polyimide for Gas Separation

A cross-linked thermally rearranged polybenzoxazole (XTR-PBO) was synthesized from a polyimide using a phenolphthalin-based diamine, 2-(bis(3-amino-4-hydroxyphenyl)methyl) benzoic acid (AHPBA), and a dianhydride, 4,4′-hexafluoroisopropylphthalic anhydride (6FDA). Hydrogen bonds between the carboxyl groups of AHPBA enabled 6FDA-AHPBA a high glass transition temperature of 405 °C. Therefore, the thermally induced decarboxylation cross-linking and polymer chain rearrangement took place at a glassy state. This was expected to mitigate pore collapse of asymmetric 6FDA-AHPBA membranes during thermal treatment. The decarboxylation cross-linking and thermal rearrangement mechanisms of 6FDA-AHPBA were studied and compared with those of a phenolphthalein-based polyimide, 6FDA-DAP. Because the lactone ring of DAP was less stable than the –COOH of AHPBA, 6FDA-DAP had a higher cross-linking density than 6FDA-AHPBA upon the process of making XTR-PBO. This led to a higher TR conversion rate of the PBO polymers derived from 6FDA-AHPBA than 6FDA-DAP. However, at the same TR temperatures, 6FDA-DAP-derived XTR-PBO showed higher gas permeabilities with lower gas selectivities. This might be attributed to the higher cross-linking densities of 6FDA-DAP that could preserve free volumes better during the TR process and the higher free volume of 6FDA-DAP than 6FDA-AHPBA. The XTR-PBO produced at 475 °C from 6FDA-AHPBA showed the best gas separation properties over the “2008 Robeson Upper limit” with a permeability of CO2 of 8790 Barrer and a selectivity of CO2/CH4 of 15.73.