Relation between the dispersion of α-relaxation and the time scale of β-relaxation at the glass transition

Dielectric spectra of several typical molecular glass-formers, showing one or more secondary processes resolved in the glassy state, have been measured at different temperatures. We found that the genuine Johari–Goldstein β-relaxation is connected to the structural relaxation. In fact, a clear correlation was found between the structural relaxation time, the Johari–Goldstein relaxation time and the dispersion of the structural relaxation (i.e. its Kohlrausch parameter). Moreover, for a group of epoxide oligomers the steepness index is correlated to the broadness of the structural peak and to the time separation between the structural and the Johari–Goldstein relaxation. These results support the idea that the Johari–Goldstein relaxation acts as a precursor of the structural relaxation. A rationale for our results is provided by the coupling model.