Molecular Engineering in Benzobisazole-Linked Polymers: Unveiling the Linkage Effect on Proton Conductivity and Transport Pathways.

Abstract

Benzobisazole-linked polymers hold potential as proton-exchange electrolytes; however, their structural rigidity and restricted chain dynamics limit their practical applications. This study investigates the linkage motif effect in three acid-doped, vinylene-bridged zwitterionic benzobisazole-based polymers: benzobisoxazole (PA@v-ZLP-NO), benzobisthiazole (PA@v-ZLP-NS), and benzobisimidazole (PA@v-ZLP-NN), synthesized via aldol polycondensation, which exhibit distinct thermal and morphological properties to overcome this limitation. Interestingly, although PA@v-ZLP-NN is less electron-deficient than PA@v-ZLP-NO and PA@v-ZLP-NS, it shows superior proton conductivity of 2.0 × 10^-2 S/cm at 80°C and 98% relative humidity, which is about 1.3 times higher than PA@v-ZLP-NS and 5.1 times higher than PA@v-ZLP-NO under identical conditions. The improved performance of PA@v-ZLP-NN stems from the presence of NH groups, which enable additional sites for hydrogen bonding in synergy with SO₃ ^- ions, self-protonation, and dynamic proton transfer via acid-base interactions, thereby forming efficient proton-transfer pathways. Their low activation energy (0.11-0.25 eV) supports Grotthuss-type proton transport, highlighting their potential as superior proton-exchange electrolytes.