Thin film composite (TFC) polymer membranes are ubiquitous in membrane-based liquid separation processes, especially in reverse osmosis (RO). While the ultrathin polyamide separating layer employed in TFC membranes has exhibited excellent performance in many liquid separation processes, the polymer support has been identified as a bottleneck to practical applications. In this work, we report a highly porous, thermally and chemically robust support comprising a thermally rearranged polymer which is combined with a polyamide active layer to form a thermally rearranged, thin film composite (TR-TFC) polymer membrane for general use in liquid separation, and for environmentally-friendly power generation. The precursor polymer has good processability for scale-up of synthesis and membrane fabrication. After thermal rearrangement, the developed TR-TFC membranes containing polybenzoxazole-co-imide can be utilized in separations in any organic liquids, including under harsh environments such as dimethyl formamide even at elevated temperatures, with a remarkable performance. Moreover, the membrane achieves 40 W m(-2) of power density through pressure retarded osmosis using a concentrated brine, similar to those obtained from RO plants. These results points to the possibilities for next-generation TFC polymer membranes for general use in liquid separation and power generation.