Effect of salt on viscoelasticity and conductivity in vitrimers and dynamic networks

Polymer networks with associative dynamic bonds (vitrimers) have been investigated as recyclable and self-healing materials. We have developed vitrimers made from polyethylene glycol, dynamic boronic ester crosslinks, and added Li salt that function as Li-ion conductors. These networks were studied over a range of Li:ethylene oxide (Li:EO) ratios to understand the effect of salt concentration on conductivity and viscoelasticity. The storage modulus decreases from 10 to 0.5 MPa at room temperature with added salt, attributed to boron-anion interactions which decrease the number of elastically active boron centers (supported by ¹¹B NMR analysis). The temperature dependent moduli also show a crossover of G’ and G” which occurs at ~100 °C (at 1 Hz) in a neutral system and drops by 85 °C with added salt. Conductivities up to 3 x 10^-4 S/cm were measured (solvent-free) at 90 °C, and go through a maximum which is attributed to the competition of added salt and increasing Tg. The networks can also be dissolved in water back to monomer and recover conductivity after damage due to the dynamic bonds. Finally, we discuss how these vitrimers transition to dissociative networks with added salt due to anion-boron interactions which lead to non-Arrhenius stress relaxation times.