Synthesis and characterization of biomimetic hybrid membranes based on a side chain liquid crystalline poly(2-oxazoline) for selective proton transport

Abstract

Designing biomimetic membranes with controlled and selective ion transport pathways is essential for next-generation electrochemical and energy-conversion systems. In this work, the cation transport properties of hybrid membranes composed of a side chain liquid crystalline poly(2-oxazoline), poly(2-(3,4,5-tris(4-dodecyloxybenzyloxy)phenyl)-2-oxazoline (PTOx40), supported on a polyester fabric were investigated. Polymer columns were homeotropically oriented by thermal treatment. X-ray diffraction revealed that hydrophobic substrates (fluorinated ethylene propylene resin and silanized glass) promoted better homeotropic orientation of the polymer columns than hydrophilic substrates (untreated glass). Wettability studies indicated comparable absorption behaviour for water and methanol in all membranes. Methanol uptake was consistently lower than that of water, highlighting their suitability for methanol-based hydrogen systems. Compared with Nafion® 117, these PTOx40-based membranes exhibited superior dimensional stability and a hydrophobic character. Cation transport was examined through electrochemical impedance spectroscopy (EIS), permeability tests, and linear sweep voltammetry (LSV). Although EIS confirmed the non-ionic nature of these membranes, LSV measurements demonstrated that oriented membranes exhibited lower resistance densities than unoriented ones. This strong dependence on column alignment underscores the role of columnar self-assembly in facilitating selective ion transport. The PTOx40-based membranes exhibited lower absolute conductivity than Nafion® 117. However, they showed remarkable proton selectivity, highlighting their potential for proton-transfer applications in artificial photosynthesis and sustainable energy technologies.