Reverse calorimetry of a supercooled liquid: Propylene carbonate

Using propylene carbonate as an example for a glass-forming liquid with generic behaviour, a technique based on dielectric relaxation experiments at high electric fields is reviewed and explained in unprecedented detail. Analogous to microwave heating, the sample absorbs energy from the field, and the resulting configurational changes are measured and linked to the configurational modes of the heat capacity. Evidence is provided for the heterogeneous nature of the slow degrees of freedom that are responsible for the heat capacity step near Tg. Moreover, the thermal and dielectric relaxation times are not only subject to the same overall dispersion, but are locally correlated and the dielectric and thermal time constants are identical within a 15% margin. Only for the fastest few percent of the thermal relaxation times can deviations from these correlations be found.