Molecular design of a series of gemini-type zwitterionic amphiphiles with various linker lengths: control of their self-organisation for developing gyroid nanostructured proton conductive membranes

Abstract

The development of advanced proton-conductive membranes is considered a significant challenge in fuel cell technology. To induce high proton conductivity (10⁻²–10⁻¹ S cm⁻¹), it is important to construct a macroscopically continuous pathway for protons in the membranes. Recently, we successfully created gyroid nanostructured polymer films with a three-dimensional continuous proton conductive pathway, which exhibited high ionic conductivity on the order of 10⁻⁵–10⁻¹ S cm⁻¹ depending on the water content. These films were prepared by self-organisation of gemini-type zwitterionic amphiphiles with polymerisable groups into bicontinuous cubic phases in the presence of appropriate amounts of acid, HTf₂N, and water, followed by in situ polymerisation. Herein, we investigated the effects of the n values of the –(CH₂)_n– linker in gemini-type zwitterionic amphiphiles on bicontinuous cubic phases. Results revealed that n = 4–6 was suitable for designing gemini-type zwitterionic amphiphiles that can form bicontinuous cubic phases. Moreover, we successfully developed a new method to change the mesophase pattern of mixtures of these zwitterionic amphiphiles and HTf₂N from smectic to bicontinuous cubic phases by placing them under controlled relative humidity conditions. Using this method, a gyroid nanostructured polymer film was successfully obtained from a gemini-type zwitterionic amphiphile with polymerisable groups. A high ionic conductivity of 1.8 × 10⁻² S cm⁻¹ was observed when the water content in the film was 13.3 wt%. The present study is expected to provide important insights into the next-generation design of proton-conductive membranes with innovative functions and properties.