Thermal rearrangement of hydroxyl-containing polyimides in solid state formed microporous polybenzoxazoles showing extraordinarily fast molecular transport for small gas molecules. Their microporous structure and size distribution can be tuned easily by varying the chemical structure of the precursor hydroxyl−polyimide and by using different thermal treatment protocols. This manuscript reports, for the first time, the synthesis of ether containing polybenzoxazole, that is, poly(ether−benzoxazole) (PEBO) membranes by thermal rearrangement of a novel fluorinated poly(o-hydroxy ether−imide). The effect of increased chain flexibility on the physical and transport properties of the resultant thermally rearranged (TR) polymer membranes for different thermal treatment protocols (e.g., final temperature and thermal dwell time) have been examined and reported in detail.
A novel strategy to tune the cavity size and free volume of thermally rearranged polybenzoxazole (TR-PBO) copolymer membranes by transesterification cross-linking reaction of o-hydroxy polyimide precursors with 1,4-butylene glycol in the solid state is demonstrated in this study. During the thermal rearrangement (TR) process at high temperatures, loose diester interchain cross-linkers are prone to degrade while formation of a much more rigid cross-linked structure occurs following or alongside the imide-to-benzoxazole rearrangement. As a result, a synergistic effect of high permeability and high selectivity appeared to be created in one step, and the newly synthesized cross-linked TR-PBO membranes exhibited outstanding gas separation performance, surpassing the so-called 2008 upper bound for CO 2 /CH 4 separation.
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