Reverse dynamic calorimetry of a viscous ionic liquid

We compare the time scale of thermal relaxation with that of the electric modulus in the deeply supercooled regime of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate. Thermal relaxation refers to the process of configurational temperatures of the slow degrees of freedom equilibrating toward the vibrational temperature, which is a reliable indicator for the time scale of structural relaxation. Energy is supplied to the sample by absorption from a sinusoidal electric field with amplitude as high as 387 kV/cm and frequencies in the 0.2 Hz-56 kHz range, analogous to microwave heating. The time resolved response of configurational temperature as well as the low field dielectric properties are measured in a single high field impedance setup. Near T_g, we find that the macroscopic field (or modulus M) relaxes considerably faster than the structure (in terms of thermal relaxation, solvation dynamics, and probe rotation), although the liquid is entirely composed of mobile ions.