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, f m,␤ , the dielectric relaxation strength, ⌬⑀ ␤ , and the distribution of relaxation time parameters, ␣ and ␤, for the Johari-Goldstein process were determined. The parameter ␤ was found to be equal to 1 and independent of both the temperature and time, ⌬⑀ ␤ initially decreased on increasing the temperature, reached a minimum value at ϳ145.6 K, and then increased. The plot of f m,␤ against the reciprocal temperature decreased in slope and at ϳ140 K became linear. This indicates that f m,␤ increases on structural relaxation. In the course of the annealing at 131.6 K, ⌬⑀ ␤ of the quenched sample decreased with time, approaching a plateau value. It is described by an equation, ⌬⑀ ␤ (t)ϭ⌬⑀ ␤ (t→ϱ)ϩ͓⌬⑀ ␤ (tϭ0)Ϫ⌬⑀ ␤ (t →ϱ)͔exp͓Ϫ(t/ )͔, where t is the time, and ͑ϭ3.5 ks͒ is the characteristic time. It is pointed out that contrary to the earlier finding, o-terphenyl shows a ␤ 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 ␤ relaxation, indicates that this mechanism is inconsistent with the known increase in the equilibrium permittivity on cooling.

Kinetics of spontaneous change in the localized motions of D-sorbitol glass

“…1͑a͒ that the height of the Љ peak spontaneously decreases with time, as has been found in several earlier studies. These spectra were analyzed using the Novocontrol WINFIT program used previously 23,26 by fitting the real and imaginary parts of the equation for the complex permittivity,…”

“…In the earlier studies, 6,[8][9][10]23 it had been found that as a glass becomes denser on structural relaxation and its T f decreases, f m,JG does not change within the detection limits of the single frequency measurements and the dielectric spectra could not be determined in the required short time in those studies. 6,[8][9][10] It has now been found 12,20,24 that when a quenched sample structurally relaxes, f m,JG increases and ⌬ JG decreases.…”

“…But the manner in which ⌬ JG , f m,JG , and ␣ JG change on increase in density as a result of structural relaxation is distinguished from the manner in which they change on deliberately increasing the density, as discussed earlier. 22 ͒ Also, when the effects of both the time and temperature were studied by heating a quenched sample at a fixed rate, 23,26 it was found that changes in ⌬ JG , f m,JG , and ␣ JG were partly due to structural relaxation at a fixed temperature and partly due to increase in temperature at a fixed structure. Their respective magnitudes varied with the temperature.…”

“…27,40 Characteristics of the JG relaxation have been recently reviewed. 1,2 Briefly, ͑i͒ the ␣-relaxation process appears to evolve from it at a temperature where its relaxation rate is in the microsecond range, 16,41 a temperature termed as the Donth temperature ͑for the merging of the ␣-and ␤-relaxation processes͒, ͑ii͒ the height of the dielectric relaxation peak Љ m,JG and its relaxation strength ⌬ JG decreases on structural relaxation or aging of a glass, [8][9][10]12,24,28 ͑iii͒ its spectral half width, which is much larger than that of the ␣-relaxation process, increases with a decrease in the temperature, [6][7][8][9][10][12][13][14][15]23,26,29 ͑iv͒ its relaxation rate follows an Arrhenius temperature dependence at temperatures [6][7][8][9][13][14][15][16]18,19,25,[29][30][31]42 far below the Donth temperature, with an activation energy and preexponent compatible with that for hindered molecular reorientation, ͑v͒ the ⌬ JG against temperature plot shows a feature similar to that of enthalpy relaxation on heating, 17,18,26 and an elbow-shape change in the slope occurs a...…”

Relaxations and nano-phase-separation in ultraviscous heptanol-alkyl halide mixture

“…Currently, four features of the ␣-relaxation process are regarded as characteristic of liquids; ͑i͒ It occurs with an asymmetric distribution of times, 5,6 which is interpreted in terms of dynamic heterogeneity; 7,8 ͑ii͒ the temperature dependence of its relaxation time follows a Vogel-Fulcher-Tammann type relation; [9][10][11] ͑iii͒ its dielectric relaxation strength increases on cooling; and ͑iv͒ its relaxation time is quantitatively related to the relaxation time of the Johari-Goldstein ͑JG͒ process, 12,13 which is the faster and localized motion whose occurrence leads to the large-scale diffusion involved in the ␣ relaxation, [14][15][16] and whose equilibrium dielectric properties show features that are characteristic of the ␣-relaxation process. 17 The dielectric relaxation time of the ␣-relaxation process is regarded as directly proportional to according to Maxwell's relation = / G ϱ , where G ϱ is the instantaneous shear modulus of the liquid. But recent studies have shown that G ϱ itself varies with the temperature, as the liquid's structure varies.…”