Influence of backbone structure on proton conductivity and supramolecular channel in aliphatic sulphonated polyimides

In this study, we synthesised a novel alkyl sulphonated polyimide (ASPI) using an aliphatic dianhydride monomer containing cyclobutane moiety (ASPI-7-H), and investigated influence of backbone conformation of aliphatic ASPIs on proton conductivity and supramolecular channel formation. Molecular simulations revealed that ASPI-7-H adopted distorted backbone geometry, distinct from reported rigid or bent aliphatic ASPIs. Polymer backbone in ASPI-7-H thin film was aligned parallel to the substrate. Moreover, ASPI-7-H thin film formed an anisotropic supramolecular lamellar structure upon humidification, resulting in high proton conductivity of 9.0 × 10−2 S cm−1 at 298 K and 95% relative humidity. Compared with other aliphatic ASPIs, higher water uptake was required to induce lamellar structure formation, reflecting distorted backbone. Obtained results demonstrate that while lyotropic liquid-crystalline behaviour is governed by multiple factors, including backbone rigidity, hydration, and degree of supramolecular organisation, backbone rigidity plays a key role in modulating hydration threshold and continuity of transport pathways.