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