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Insufficient mechanical properties are one of the major obstacles for the commercialization of ultrahigh permeability thermally rearranged (TR) membranes in largescale gas separation applications. The incorporation of preformed benzoxazole/benzimidazole units into o-hydroxy copolyimide precursors, which themselves subsequently thermally rearrange to form additional benzoxazole units, were prepared for the first time. Using commercially available monomers, mechanically tough membranes prepared from random and block TR poly(benzoxazole-co-imide) copolymers (TR-PBOI) were investigated for gas separation. The effects of the chemical structures, copolymerization modes, and thermal holding time of o-hydroxy copolyimides on the molecular packing and properties, including gas transport, for the resulting TR-PBOI membranes have been examined in detail. After treatment at 400°C, tough TR-PBOI membranes exhibited tensile strengths of 71.4−113.9 MPa and elongation at break of 5.1−16.1%. Moreover, they presented higher or comparable gas transport performance as compared to those tough/robust TR membranes reported previously. Reported for the first time is a comparative investigation of the copolymerization mode (random or block) on membrane properties. The novel polymer architecture and systematic property studies promote a better understanding of the materials and process development of commercial TR membranes for gas separation applications.

A facile approach to synthesize poly-(benzoxazole-co-imide)s without thermal rearrangement at high temperature is proposed. Poly(benzoxazole-co-imide)s with improved mechanical and solution-processable properties were prepared through polycondensation of 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) with three synthesized novel benzoxazole-containing diamines and a commercial diamine. These poly(benzoxazole-co-imide)s had high tensile strengths of 110.3−122.0 MPa and good elongation at break of 11.9−26.3%, good thermal stability and high glass transition temperatures (T g s) of up 306 °C. The effect of chain isomerism on molecular packing and physical and gas transport properties of the poly(benzoxazole-co-imide)s was investigated. The para-connecting isomers exhibited higher molecular weights (M w s), better mechanical properties, higher T g s, higher chain packing order and better overall performance for CO 2 /CH 4 and CO 2 /N 2 separations as compared to the corresponding meta-connecting ones. This study guides molecular architecture to improve particular membrane separation performance by introducing either para-or meta-connections into polymeric main chains.

Thermally rearranged polybenzoxazoles (TR-PBO) are some of the most promising materials for gas separation because of their microporous and bimodal cavities that offer high gas transport performance. However, the brittleness of fully converted TR-PBO membranes has impeded their widespread industrial implementation. In this study, we prepared novel, thermally rearranged poly-(benzoxazole-co-imide) membranes (TR-PBOI) with improved mechanical strength and good gas separation performance. These membranes are based on two commercially available TR-able diamines and two non-TR-able diamines with various compositions and different polymer rigidities. TR-PBOI membranes with the appropriate ratio of PBO and PI displayed a high fractional free volume and therefore exceptional gas separation properties (CO 2 permeability over 300 barrer and CO 2 /N 2 ideal selectivity above 20); both these values were higher than those of the corresponding original TR-PBO membranes. Furthermore, a substantial improvement in the mechanical properties of TR-PBOI membranes relative to their TR-PBO counterparts was observed.