The influence of polynorbornene backbone structure on ion clustering, water uptake, and ion transport

Polynorbornene (PNB)-based anion exchange membranes have shown great potential in energy conversion and storage due to their exceptional alkaline stability. However, it is not well understood how the PNB polymer backbone structure affects key fundamental properties such as water absorption and ion transport that have an important influence on device performance. In this study, we designed and synthesized polymers based on bromine-containing polynorbornene (PNB) with similar ion exchange capacity (IEC), molecular weight, and an identical spacer length. The distinctive backbone structures were achieved through vinyl addition polymerization (VAP) and ring-opening metathesis polymerization (ROMP) followed by a hydrogenation reaction. By exposing the obtained bromine-containing polynorbornene thin films to trimethyl amine vapor, quaternary-ammonium (QA) groups were added to the polymer backbone via a four-carbon spacer. These QA-containing polynorbornene thin films allowed us to investigate how different backbone structures influence the properties of polyelectrolytes. We examined QA-VAP-PNB, QA-ROMP-PNB, and QA-hydrogenated ROMP-PNB thin films, all with a thickness of around 90nm, across a humidity range of 25%RH to 90%RH at 25°C. Interestingly, QA-hydrogenated ROMP-PNB exhibited the highest ionic conductivity in relation to water concentration and hydration number despite having comparable IEC, molecular weight, spacer length, and QA group. This can be attributed to the ionic clustering morphology within QA-hydrogenated ROMP-PNB, enabling efficient bromide transport with minimal water uptake and swelling ratio, as confirmed by mid-angle X-ray scattering (MAXS) and Grazing incidence mid-angle X-ray scattering (GI-MAXS). Furthermore, ion clustering facilitates efficient bromide transport in various water states, whether freezable or nonfreezable. The outcomes of this study underscore the significance of hydrophobicity and segmental mobility of the backbone structure in shaping ion clustering morphology, a key factor for facilitating effective anion transport. Ultimately, we believe this study offers valuable insights that can inform the design and synthesis of polyelectrolytes with tailored properties.