Interplay of chain dynamics and ion transport on mechanical behavior and conductivity in ionogels†

Abstract

Understanding the interplay among the mechanical behavior, ionic conductivity and chain dynamics of ionogels is essential for designing flexible conductors that exhibit both high conductivity and excellent mechanical properties. In this study, ionogels were synthesized via the radical polymerization of N,N′-dimethylacrylamide (DMAA) and methacrylic acid (MAAc) monomers in the presence of ionic liquid 1-ethyl-3-methylimidazolium trifluoromethane sulfonate ([EMIM][OTf]). By varying the mass content of ionic liquid within ionogels, we investigated the mechanical behavior and ionic conductivity at the macroscopic scale using tensile, rheological testing and electrochemical impedance spectroscopy, as well as the dynamic behavior of chain segments and ions within the network at the microscopic scale using broadband dielectric relaxation spectroscopy (BDS) over a broad temperature range. Our findings revealed that variations in ionic liquid concentration significantly affect mechanical performance, ionic conductivity, complex conductivity spectra, and complex permittivity spectra. These ionogels exhibited remarkable stretchability, adhesion, and strain-sensing capabilities. Analysis of BDS indicated that the temperature dependence of the hopping frequency (ω_H), the conductivity of free ions (σ_dc), and the relaxation time (τ_s) of chain segments conforms to the Vogel–Tammann–Fulcher (VTF) equation for ionogels with varying ionic liquid content. By correlating τ_s measured through rheological tests and BDS, we observed a transition from Arrhenius to VTF behavior, which shifts towards lower temperatures with increasing ionic liquid content. This study highlighted a strong coupling between σ_dc and ω_H, as well as between 1/τ_s and ω_H, at low ionic concentrations, facilitating high mechanical performance of the ionogels due to viscoelastic energy dissipation. However, as the ionic concentration increased, a slight decoupling of σ_dc and ω_H was noted, leading to a substantial reduction in the mechanical properties of the ionogels. Ultimately, these ionogels demonstrate potential as polymer electrolytes for applications in flexible wearable devices.