Multinuclear Solid-State NMR, DSC, and Conductivity Studies of Solid Polymer Electrolytes Based on Polyurethane/Poly(dimethylsiloxane) Segmented Copolymers

Abstract

Solid polymer electrolytes based on polyurethane/poly(dimethylsiloxane) segmented copolymers (PS55) have been characterized by differential scanning calorimetry (DSC), ionic conductivity, and multinuclear solid-state NMR measurements. The results of DSC measurements indicate the formation of transient cross-links between Li+ ions and the ether oxygens on complexation with LiClO4, resulting in an increase in the soft segment Tg. However, the soft segment Tg remains almost invariant at high salt concentration. There is a conductivity jump at around 310−330 K that the behavior of ionic conductivity changes from Arrehnius- to Vogel−Tamman−Fulcher (VTF)-type behavior. Below this jump temperature, the conductivity follows Arrehnius-like behavior, implying a diffusing mechanism for transport of the charge carriers where the charge carriers are decoupled from the segmental motion of the polymer chain. By contrast, the diffusion of charge carrier is assisted by the segmental motions of the polymer chains above the jump temperature, suggested by the VTF-like behavior. At high salt concentration, the ionic conductivity decreases due to the formation of ion pairs and/or ion clusters. Solid-state 13C NMR results from cross-polarization time constant (TCH) measurements along with two-dimensional (2D) WISE NMR suggest that a significant decrease in the mobility of the soft segment as the salt is added. Polysiloxane backbone is not affected until at a higher salt concentration, as observed by the line width change in the 29Si NMR spectrum. The onset temperature of 7Li motional line narrowing is correlated with the soft segment Tg. The activation energies obtained from ionic conductivity, 7Li line width, and T1 measurements indicate that there is a strong correlation between the ionic conductivity of the solid polymer electrolyte and the mobile lithium cation.