Correlation of structural and Johari-Goldstein relaxations in systems vitrifying along isobaric and isothermal paths

The effect of isobaric cooling (over the range 190-350 K) and isothermal compression (up to 700 MPa) on structural ?- and secondary ?-relaxations has been studied for low molecular weight glass-forming systems. The shape of the ?-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 ??(T, P). The invariance of shape at constant ??(T, P) involved also the excess wing, i.e. the process showing up at the high-frequency tail of the ?-loss peak in systems with no well-resolved ?-process. Likewise, systems where the excess wing evolved to a well-resolved ?-peak showed that the timescale of the ?- process was strongly related to that of the ?-peak. Also in this case, once a given value ??(T, P) was fixed, a corresponding value ?? (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 ?- and Johari-Goldstein relaxation over a wide interval of temperature and density.