Physical ageing and the Johari–Goldstein relaxation in molecular glasses

Based on some of our earlier dielectric relaxation studies during structural relaxation of molecular glasses, we describe certain features specific to the change in the Johari–Goldstein (JG) relaxation and examine their consequences for understanding of the molecular mechanism of the JG process. The parameter for the distribution of relaxation times increases slightly (loss curve becomes narrower) and the relaxation rate either remains constant or increases on ageing. In all cases, contribution to permittivity from the JG relaxation, ΔεJG decreases with time, with a rate constant k according to a relation, ΔεJG (t) = ΔεJG (t → ∞) − [ΔεJG (t = 0) − ΔεJG ( t → ∞)][1− exp[−(kt)]. This reduces to an equation of the type ΔεJG(t) = a + b exp(− kt) where a and b are constants of the glass as well are dependent on the thermal history of quenching of the glassy system. On decreasing the temperature, the relaxation rate for a rigid molecular glass follows the Arrhenius equation in a range that extends from liquid to glass, but deviates from it as a result of structural relaxation of the two alcohols, while the distribution of relaxation times decreases. The variation of ΔεJG with temperature shows an increase in slope on heating through Tg or else a deep and broad minimum before Tg is reached and the slope increases are remarkably similar to the changes observed for volume and thermodynamic properties on heating a glass. These findings need to be considered for a molecular mechanism in the potential energy view of JG relaxation, in analyzing the physical ageing of the α-relaxation process. It is argued that a recent suggestion for considering the JG relaxation within the picture of potential energy landscape may not be inconsistent with its molecular origin and dynamics in localized regions of structurally inhomogeneous glass. But there is still need to determine how the apparent dynamic heterogeneity evident from the broad distribution of the JG relaxation times can be reconciled with the dynamic heterogeneity used to explain less-broad spectra of the α − relaxation process.