Effects of salt addition on viscoelasticity in conductive dynamic networks

We have investigated a series of dynamic networks made from precise ethylene glycols (2, 3 or 4 repeat units), boric acid, and added Li salt. The resultant boronic ester crosslinks are typically associative dynamic covalent bonds, but can coordinate with anions in the presence of salt which alters the topology and rheology of the networks. We studied a range of linker lengths and Li:ethylene oxide (Li:EO) ratios to understand the effect of these parameters on conductivity and viscoelasticity. The storage modulus decreases from 10 to 0.5 MPa at room temperature with added salt, contrary to linear PEO, attributed to boron-anion interactions which decrease the number of elastically active boron centers and lead to a non-Arrhenius temperature dependence of stress relaxation times. The crossover of G’ and G” drops from 100 C (at 1 Hz) in a neutral system to 15 C with added salt. Conductivities up to 3 x 10^-4 S/cm were measured at 90 C and go through a maximum which is attributed to the competition of added salt and increasing Tg. By tuning the linker length of the networks from 2 to 3 to 4 repeat units, a systematic increase in conductivity and decrease in modulus is observed due to mesh size and Tg effects.