Relation between the dispersion of alpha-relaxation and the time scale of beta-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-Gold stein P-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. (C) 2007 Elsevier B.V. All rights reserved.