Revealing the rich dynamics of glass-forming systems by modification of composition and change of thermodynamic conditions

Abstract

Secondary relaxations have been classified into two types, depending on whether they are related to the structural α-relaxation in properties or not. Those secondary relaxations that are related to the α-relaxation may have fundamental importance, and are called the Johari–Goldstein (JG) β-relaxations. Two polar molecular glass-formers, one flexible and another rigid, dissolved in apolar host with higher glass transition temperature are studied by broadband dielectric spectroscopy at ambient and elevated pressure. The neat flexible glass-former diethylphthalate (DEP) has a resolved secondary relaxation which, unlike the α-relaxation, is insensitive to pressure and hence is not the JG β-relaxation. In the solution, the JG β-relaxation of DEP shows up in experiment and its relaxation time τβ is pressure and temperature dependent like τα. The result supports the universal presence of the JG β-relaxation in all glass-formers, and the separation between τα and τβ is determined by intermolecular interaction. The rigid glass-former is cyano-benzene (CNBz) and its secondary relaxation involves the entire molecule is necessarily the JG β-relaxation. The dielectric relaxation spectra obtained at a number of combinations of pressure and temperature at constant τα show not only unchanged is the frequency dispersion of the α-relaxation but also τβ. The remarkable results indicate that the JG β-relaxation bears a strong connection to the α-relaxation, and the two relaxations are inseparable when considering the dynamics of glass-forming systems. Experimentally, τα has been found to be a function of the product variables, T/ργ, where ρ is the density and γ is a material constant. From the \\invariance of the ratio, τα/τβ, to change of thermodynamic conditions seen in our experiment as well in other systems, it follows that τβ is also a function of T/ργ, with the same γ at least approximately. Since the JG β-relaxation is the precursor of the α-relaxation, causality implies that the T/ργ-dependence originates from the JG β-relaxation and is passed on to the α-relaxation.