Identifying the genuine Johari-Goldstein β-relaxation by cooling, compressing, and aging small molecular glass-formers

The different nature of multiple secondary (beta- and gamma-) relaxations for the glass-formers dipropyleneglycol-dibenzoate and benzoin-isobutylether has been investigated by varying temperature and pressure in the equilibrium liquid and glassy states, as well as by monitoring the structural recovery after a rapid quenching of the liquid to form a glass. For both systems the behavior of the slower P-relaxation mimicked that of the alpha-relaxation: its time scale was strongly affected by pressure variation and its behavior below T-g was sensitive to thermal history and to aging. On the contrary the gamma-relaxation time was negligibly dependent on pressure and on aging. The correlation between the dynamic properties of the alpha-process and those of the slower beta-process indicates that the latter was likely the genuine Johari-Goldstein (JG) intermolecular relaxation. Moreover, a good agreement was found between the beta-relaxation time and the independent relaxation time tau(0) of the coupling model (CM), leading to predictive quantitative relations between the JG and the structural relaxation, supported also by the experimental data of many other systems, in both liquid and glassy state. (c) 2005 Elsevier B.V. All rights reserved.