Primary relaxation in regular and irregular glass-forming liquids studied through their timescale steepness and curvature

The thermodynamic and kinetic features of the structural-glass transformation in molecular liquids are discussed. A study was conducted on the temperature behavior of the primary relaxation timescale derived from the measurements of dielectric response and dc conductivity above the glass-transformation temperature Tg. A theoretical–experimental analysis is elaborated via description of the first-order derivative (steepness) and second-order derivative (curvature) data on the timescale, available from the literature. Stress is put on the timescale curvature, which conventionally divides all supercooled molecular liquids into regular (propylene carbonate type) and irregular (salol type) glass formers through the corresponding jump and kink, observed at the crossover temperature Tc. For regular liquids, the temperature behavior of the curvature is accurately described through the cluster fluctuation (heterostructured) model and also tested by the defect diffusion model. The regular liquids are shown to be driven to the glass state by the configurational entropy, controlled by thermal fluctuations. The irregular glass-forming liquids are governed by strong (unspecified) kinetic effects. Their timescale curvature signals the dynamic-type instability, located near Tg and attributed to a non-trivial ergodicity breaking crossover, common to all supercooled liquids. As by-products, the curvature jump is predicted in all SCLs and the validity domains for different known models are established.