We report evidence from broadband dielectric spectroscopy that the dynamics of the primary alpha- and secondary Johari-Goldstein (JG) beta-processes are strongly correlated in different glass-forming systems over a wide temperature T and pressure P range, in contrast with the widespread opinion of statistical independence of these processes. The alpha-beta mutual dependence is quantitatively confirmed by (a) the overall superposition of spectra measured at different T-P combinations but with an invariant alpha-relaxation time; (b) the contemporary scaling of the isothermal-pressure and isobaric-temperature dependences of the alpha-and beta-relaxation times as plotted versus the reduced variable Tg(P)/T where Tg is the glass transition temperature. These novel and model-independent evidences indicate the relevance of the JG relaxation phenomenon in glass transition, often overlooked by most current theories.

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The effect of isobaric cooling (over the range 190–350 K) and isothermal compression (up to 700 MPa) on structural α- and secondary β-relaxations has been studied for low molecular weight glass-forming systems. The shape of the α-loss peak was found to change with temperature T and pressure P but to be constant for a combination of T and P giving the same τα(T,P). The invariance of shape at constant τα(T,P) involved also the excess wing, i.e. the process showing up at the high-frequency tail of the α-loss peak in systems with no well-resolved β-process. Likewise, systems where the excess wing evolved to a well-resolved β-peak showed that the timescale of the β-process was strongly related to that of the α-peak. Also in this case, once a given value τα(T,P) was fixed, a corresponding value τβ(T,P) was found for different T and P. Same results were found also for a binary mixture of a polar rigid molecule dissolved in an apolar solvent, i.e. a model system for Johari–Goldstein intermolecular relaxation. These evidences imply that a strong correlation exists between structural α- and Johari–Goldstein relaxation over a wide interval of temperature and density.

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We investigated, by means of dielectric spectroscopy, the relaxation dynamics of glass forming binary mixtures composed by the quite rigid polar molecules tert-butylpyridine dissolved in the apolar solvent tristyrene. By changing the relative concentration of the components we observed a transition from a relaxation scenario with a structural process and an excess wing to that with a structural process and a well resolved secondary process. Another relaxation process, slower than the latter, was observed, well below Tg. Our detailed analysis evidenced that the secondary relaxation with shorter relaxation time can be identified as the Johari-Goldstein relaxation for all the mixtures, whereas the new relaxation process was attributed to a different type of motion of tert-butylpyridine needing a larger amount of free volume for the molecular rotation.