Effect of glass structure on the dynamics of the secondary relaxation in diisobutyl and diisoctyl phthalates

The β relaxation is a principal source of information about the dynamics in the glassy state; however, the nature of this process remains a controversial issue. In this paper, we show that properties of the β relaxation measured below T_g are sensitive to the structure of the glass; that is, the thermodynamic path from the equilibrium liquid strongly affects the β relaxation times, their distribution, and the activation energy quantifying their temperature dependence. These results support the idea that the Johari-Goldstein β process is the precursor to the structural relaxation transpiring at longer times. We discuss the experimental findings in light of the heterogeneous and homogeneous scenarios for the β process.