Effect of hexafluoroisopropylidene group contents and treatment temperature on the performance of thermally rearranged poly(hydroxyamide)s membranes

Soluble aromatic polyamide films were prepared with different molar ratios of 2,2'-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (APAF) and 3,3'-dihydroxy-4,4'-diaminobiphenyl (HAB), and then in-situ treated at different temperature to obtain thermally rearranged polybenzoxazole (TR-PBO) membranes with ultra-high gas permeability. Compared to polyamide precursors, the obtained TR-PBO membranes exhibited bimodal cavity distribution. The size of smaller cavities, ranging from around 3.3 angstrom to around 4.1 angstrom, increased with APAF content. And the larger cavities of 5.6-6.0 angstrom were almost constant in size, irrespective of thermal treatment temperature or precursor structure. It was found that increasing thermal treatment temperature could prompt smaller cavities to coalesce into bigger ones, thus resulting in better gas permeability. Higher APAF content results in higher gas permeability but inferior resistance to CO2 plasticization, however, too low APAF content is also not beneficial to plasticization resistance due to the lower thermal conversion ratio caused by close-packed array of HAB-TPC. It was found that with APAF/HAB molar ratio of 5/5, the prepared TR-PBO membranes exhibited better resistance to CO2 plasticization as well as comparable mechanical properties and stable permeation performance.