Effect of Isomerism on Molecular Packing and Gas Transport Properties of Poly(benzoxazole-<i>co</i>-imide)s

A facile approach to synthesize poly­(benzoxazole-<i>co</i>-imide)­s without thermal rearrangement at high temperature is proposed. Poly­(benzoxazole-<i>co</i>-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-<i>co</i>-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 (<i>T</i>_gs) of up 306 °C. The effect of chain isomerism on molecular packing and physical and gas transport properties of the poly­(benzoxazole-<i>co</i>-imide)­s was investigated. The <i>para</i>-connecting isomers exhibited higher molecular weights (<i>M</i>_ws), better mechanical properties, higher <i>T</i>_gs, higher chain packing order and better overall performance for CO₂/CH₄ and CO₂/N₂ separations as compared to the corresponding <i>meta</i>-connecting ones. This study guides molecular architecture to improve particular membrane separation performance by introducing either <i>para</i>- or <i>meta</i>-connections into polymeric main chains

Mechanically Tough, Thermally Rearranged (TR) Random/Block Poly(benzoxazole-<i>co</i>-imide) Gas Separation Membranes

Insufficient mechanical properties are one of the major obstacles for the commercialization of ultrahigh permeability thermally rearranged (TR) membranes in large-scale gas separation applications. The incorporation of preformed benzoxazole/benzimidazole units into <i>o</i>-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-<i>co</i>-imide) copolymers (TR-PBOI) were investigated for gas separation. The effects of the chemical structures, copolymerization modes, and thermal holding time of <i>o</i>-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

Durable Sulfonated Poly(benzothiazole-<i>co</i>-benzimidazole) Proton Exchange Membranes

Two series of random sulfonated poly­(benzothiazole-<i>co</i>-benzimidazole) polymers (sPBT-BI) with 70% and 60% degree of sulfonation were evaluated as proton exchange membranes. sPBT was also prepared for a comparative study. The mechanical properties of sPBT-BI were greatly enhanced by incorporation of benzimidazole (BI); sPBT-BI70-10 showed a tensile strength of 125 MPa and elongation at break of 38.9%, an increase of 56.5% and 145%, respectively, compared with sPBT. The solubility, dimensional stability, thermal properties, and oxidative stability of sPBT-BI were also improved. The ionic clusters of sPBT-BI membranes in both AFM phase images and TEM images became narrower with increasing amounts of BI while containing the same molar amount of sulfonic acid groups. This resulted in lower dimensional swelling and higher mechanical strength, but the proton conductivity decreased. However, high proton conductivity was achieved by incorporating an appropriate content of BI. PEMFC H₂/air single cell performances and durabilities were improved by incorporation of 5% of BI units in sPBT