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

## TL;DR

This paper studies how the polarity of monomers affects polymer dynamics, glass transition, and ionic conductivity in polyether electrolytes.

## Contribution

The study reveals two distinct salt-induced structures that influence polymer dynamics and ionic conductivity in polyether electrolytes.

## Key findings

- Salt concentration elevates glass transition temperature linearly, unrelated to static dielectric constants.
- Salt-induced intrachain and interchain polymer-ion complexes affect chain packing and dynamics.
- Polymer electrolytes show disentanglement and unchanged relaxation time separation with salt addition.

## Abstract

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
εsPBO≈4
)/Lithium bis­(trifluoromethanesulfonyl)­imide
(LiTFSI), poly­(propylene oxide) (PPO,
εsPPO≈7
)/LiTFSI, and poly­(epichlorohydrin) (PECH,
εsPECH≈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, τα
, 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 τα
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.