Characterization of the secondary relaxations in glass-forming liquids. The contribution of the Starkweather line and of the Coupling Model to the rationalization of TSDC results

•The Starkweather (or zero activation entropy) line can be used to identify the secondary relaxations as studied by TSDC.•A link is established between the Coupling Model theory and the Starkweather line.•A new criterion was introduced to distinguish the Johari-Goldstein relaxations from the fast secondary relaxations.•Using the Coupling Model it was shown that belonging to the Starkweather line and having βKWW = 1 are independent indicators of non-cooperativeness. This work addresses the study by thermostimulated depolarization currents (TSDC) of Johari-Goldstein (JG) β-relaxations in glass forming systems. Here we analyze the differences between the β-relaxation and the fast secondary (γ, δ, ε, …) relaxations on the one hand, and the α-relaxation on the other. It is shown that TSDC is able to characterize JG although this mobility is often close to, or even partially overlapped with, the α-relaxation. It is also shown that the Starkweather line or zero entropy line (ZEL) can be used as a reference element to identify the secondary relaxations: non-compliance with the ZEL is an indicator of cooperativity. We finally showed that the non-cooperativeness of the Johari-Goldstein relaxation is highlighted by the joint consideration of the Coupling Model and the Starkweather line. [Display omitted]