Effect of Monomer Polarity on Polymer Dynamics, Glass Transition, and Ionic Conductivity of Polyether Electrolytes

Combining X-ray scattering, Fourier transform infrared spectroscopy (FT-IR), broadband dielectric spectroscopy (BDS), rheology, and differential scanning calorimetry (DSC), we investigate the ion solvation structures, the polymer dynamics, glass transition, and ionic conductivity of poly-(butylene oxide) (PBO, static dielectric constant ${\epsilon }_s^{\mathrm{PBO}}\approx 4$ )/Lithium bis-(trifluoromethanesulfonyl)-imide (LiTFSI), poly-(propylene oxide) (PPO, ${\epsilon }_s^{\mathrm{PPO}}\approx 7$ )/LiTFSI, and poly-(epichlorohydrin) (PECH, ${\epsilon }_s^{\mathrm{PECH}}\approx 15$ )/LiTFSI. While their molar conductivity follows the Walden rule, these polyether electrolytes exhibit intriguing features in glass transition and viscoelastic properties. In particular, BDS and rheology show clear slowing down of the structural relaxation time, τ_α , along with a large elevation in glass transition temperature, T _g, with salt concentration. DSC results, meanwhile, demonstrate a strong broadening of the T _g step or signs of phase separation. Interestingly, the elevation in T _g depends linearly on the salt concentration with a slope not correlating with their static dielectric constants. Furthermore, linear rheology shows an apparent "disentanglement" of these polymer electrolytes with salt concentration, while the separation between τ_α and the terminal relaxation remain almost constant. We explain these results through two types of salt-induced structures correlating with their different ion solvation structures, which contribute differently to the glass transition and the linear viscoelastic properties: (i) the intrachain polymer-ion complex that shortens the effective chain length and (ii) the interchain polymer-ion complex that bridges different chains. These results point to a crucial role of the ion solvation structure in the local chain packing, which in turn, influences the polymer dynamics, glass transition, and ionic conductivity.