Control of viscoelasticity and ion transport in dynamic polymer networks using lithium salts

Vitrimers are dynamic polymer networks which can offer self-healing and unique physical properties relative to conventional polymers. Here, the properties of polyethylene oxide vitrimers with added salt were characterized with vinylogous urethane (VU) bonds which are catalyzed by the presence of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. Precise linker lengths ethylene glycol (xEG, x = 2-12) were prepared, and stress relaxation experiments show that the characteristic relaxation time is accelerated by a factor of ~70 due to added salt. As the crosslinking density of the vitrimers increases with shorter linker lengths, stress relaxation times increased with increasing T_g. The characteristic relaxation times do not superimpose when normalized by T_g, while the ionic conductivity collapses onto a universal curve indicating that conductivity is still dominated by segmental dynamics. In addition, ⁷Li solid state NMR and density functional theory (DFT) simulations show that the preference of Li ions to solvate with oxygen atoms along the ethylene glycol chains or with the VU bond site depends on the linker length, such that longer chains have a greater ability to solvate the ions.