Localized relaxation in a glass and the minimum in its orientational polarization contribution

Abstract

The dielectric permittivity and loss spectra of the glassy state of 5-methyl-2-hexanol obtained by quenching it from the liquid state has been studied. In one experiment, the spectra were studied at different temperatures as the quenched sample was heated at 0.1 K/min from 105.3 to 160.5 K. In the second experiment, the quenched sample was heated from 77 to 131.6 K and kept at that temperature for 14.6 ks. The relaxation rate, fm,β, the dielectric relaxation strength, Δεβ, and the distribution of relaxation time parameters, α and β, for the Johari–Goldstein process were determined. The parameter β was found to be equal to 1 and independent of both the temperature and time, Δεβ initially decreased on increasing the temperature, reached a minimum value at ∼145.6 K, and then increased. The plot of fm,β against the reciprocal temperature decreased in slope and at ∼140 K became linear. This indicates that fm,β increases on structural relaxation. In the course of the annealing at 131.6 K, Δεβ of the quenched sample decreased with time, approaching a plateau value. It is described by an equation, Δεβ(t)=Δεβ(t→∞)+[Δεβ(t=0)−Δεβ(t→∞)]exp[−(t/τ)], where t is the time, and τ (=3.5 ks) is the characteristic time. It is pointed out that contrary to the earlier finding, o-terphenyl shows a β relaxation in the equilibrium liquid state. A consideration of dielectric permittivity arising from small-angle motions of all molecules, which has been suggested as an alternative mechanism for the localized motions seen as β relaxation, indicates that this mechanism is inconsistent with the known increase in the equilibrium permittivity on cooling.