Dynamic Decoupling in Polymerized Ionic Liquids, Supercooled Melts, and Glasses

Abstract

We combine dielectric and viscoelastic measurements with novel statistical mechanical theories to establish and understand strong decoupling of cation activated dynamics from the polymer segmental relaxation in polymerized ionic liquids. Weakly ion-dependent (Li, Na, K) apparent power laws with fractional exponents between the two characteristic time scales (ionic and segmental) are observed and predicted in the supercooled regime. The ratio of segmental to ion relaxation times grows in a strongly non-Arrhenius manner with cooling. However, at high (fast) enough temperature (segmental relaxation), the theory predicts that decoupling continuously vanishes, and a linear Walden-like proportionality between the two relaxation times is recovered. The theoretical analysis spans 20 decades in time, including the glass state. The mechanistic key to the rich dynamics is the extent to which ion-activated barrier crossing is coupled with in-cage local polymer dynamical fluctuations that facilitate ion hopping.