Evaluating the Relationships between Molecular Interactions and Polymer Chain Dynamics across Phase Boundaries in Solid Polymer Electrolytes

Fundamental studies on the structure–property relationships of solid polymer electrolytes (SPE) are required to further our understanding of the parameters impacting their ionic conductivity. This work investigates the interplay between molecular interactions and polymer chain dynamics of an electrolyte model system based on poly(ε-caprolactone)(PCL):LiTFSI across structural phase boundaries with salt concentrations varying from 0 to 65 wt %. To evaluate the thermodynamic behavior of PCL:LiTFSI and enable a clear overview of the phase environment of the system through the concentration range, construction of the phase diagram of PCL:LiTFSI is realized. Infrared (IR) spectroscopy experiments allowed probing of the polymer:salt molecular interactions. Spectroscopic identification of salt saturation at 27 wt % which corresponds to the beginning of a totally amorphous phase hints at the ideal concentration range to optimize ionic transport. Coordination thermodynamic parameters were evaluated through variable-temperature Fourier transformed infrared spectroscopy (FT-IR) experiments. Relationships between the free energy of coordination between the lithium ion and PCL and phase boundaries, as well as relationships between the change in coordination entropy and conductivity activation energy, were elucidated. Polymer chain dynamics was evaluated through oscillatory rheology. Relationships between the polymer terminal relaxation times and ionic conductivities identify an abnormal plasticization behavior at phase inversion (24 wt %). Finally, an integrated interpretation of molecular interactions, polymer chain dynamics, and phase environment of the PCL:LiTFSI and their relationship with ionic behavior is realized, highlighting the importance of each of these parameters.