Effect of salt and crosslinking density on viscoelasticity and conductivity of vitrimers

Covalent adaptive networks with a topology-conserving bond exchange mechanism (known as vitrimers) are emerging materials replacing conventional polymers due to their reprocessability and self-healing properties. Vitrimers are of key interest in applications such as electrolytes where viscoelasticity and conductivity are controlled by salt addition. In this study, vinylogous urethane (VU)-based vitrimers containing Li salts were prepared with different crosslinking densities using precise linker lengths of ethylene glycol. Stress relaxation experiments show that the ionic dynamic networks exhibited substantially faster relaxation dynamics with smaller characteristic relaxation times compared to neutral networks. Ionic dynamic networks materials exhibited an Arrhenius temperature dependence in relaxation times, which is consistent with expected behavior for vitrimers. In addition, it was found that relaxation times and shear moduli decrease with lower crosslinking densities. Electrochemical impedance spectroscopy was performed to investigate ionic charge transport depending on the network mesh size. Our results show that ionic conductivity increases as the linker length increases. Interestingly, the conductivity data collapse onto a universal curve regardless of crosslinking density when temperature is normalized by the glass transition temperature, which indicates that ionic charge transport is primarily controlled by the segmental chain dynamics of ethylene glycol. ⁷Li ssNMR suggests that vitrimers with longer linker lengths prefer Li-ethylene oxide coordination rather than Li-VU coordination based on chemical shifts. Overall, this work provides an improved understanding of the effect of salt and crosslinking density on properties of vitrimers, which is essential for applications including solid polymer electrolytes.