Effects of non-TR-able codiamines and rearrangement conditions on the chain packing and gas separation performance of thermally rearranged poly(benzoxazole-co-imide) membranes

In present work, the incorporation of nonrearrangable codiamines (mPDA, DMB or TFMB) with different substituted groups into an ortho-hydroxypolyimide (HPI) precursor was applied to modify the gas separation and mechanical performances of resultant thermally rearranged (TR) polybenzoxazole membranes. Evolution of the thermal rearrangement for ortho-hydroxy copolyimide precursors was effectively monitored by the TGA-FTIR, WAXD and PALS measurements. The incorporated non-TR-able codiamines and the thermal treatment protocols have a great effect on the chains packing behavior, free volume, mechanical properties as well as the gas separation behavior of resulted TR-PBOI membranes. These TR-PBOI membranes show high tensile strength of 117-160 MPa and good elongation at break of 7.5-9.0% as thermally treated at 400 °C for 2 h. The t-TR400-2 membrane comprising the TFMB codiamine exhibits a synergistic effect of high gas permeability and high gas pairs selectivity, which is mainly attributed to the loose chains packing resulted from the thermal rearrangement and the substituted bulky -CF3 group that resulting a high fractional free volume (FFV≈0.15). The systematic structure/property relationship studies serve as a guide of materials and process development of commercial TR membranes for gas separation applications.