Dielectric anomalies in the β‐relaxation of glassy 1,4‐polybutadiene

Abstract

We have measured the dielectric relaxation of the polymeric glass-former 1,4-polybutadiene in the glassy and supercooled liquid state. The loss spectra E"(w) of the p-relaxation are measured between the lower detection limit at T = 125 K and T = 225 K > T p with Tp = 210 K being the a-P-merging temperature. The related storage data E'(w) indicates that the pprocess is a signature of supressing the higher frequency polarization with increasing temperature, rather than giving rise to additional polarizability at frequencies below the average relaxation frequency of the P-process. This observation, as confirmed by data obtained for o-terphenyl and salol, contradicts the previous understandings of the mechanism underlying the Johari-Goldstein-type P-process in glasses. A tentative link between the p-process and high frequency vibrational modes is established.

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

“…34,35 Accordingly, ⌬⑀ ␤ of a glass should increase with decrease in T. The finding here is the opposite, i.e., ⌬⑀ ␤ decreases on cooling a glass, in agreement with the earlier studies. 5,6,[22][23][24][25][26][27] As mentioned earlier here, the density of a glass increases on isothermal structural relaxation. Therefore, if the angle of reorientation of molecules did not change, as concluded recently 34,35 ⌬⑀ ␤ would increase on an increase in density during structural relaxation.…”

“…6 of Ref. 24, ⑀Љ showed a peak at about 200 K, but ⑀Ј continued to increase with decreasing temperature. This is quite the opposite of the known dielectric relaxation behavior of materials, namely, that dielectric relaxation decreases ⑀Ј and that (d⑀Ј/dT) is negative at a temperature where ⑀Љ shows a peak.…”

“…All these observations show that, contrary to the earlier conclusions, ␤ relaxation does occur in the equilibrium liquid state of o-terphenyl. It is also worth pointing out that in the study of o-terphenyl, 24 particularly in the ⑀Ј and ⑀Љ plots in Fig. 6 of Ref.…”

“…On the basis of a recent study, it has been suggested that the ␤-relaxation process in some cases may occur only in the quenched state of the glass, and that it may vanish on annealing the glass and it is not observed in the equilibrium liquid state. 24,25 The suggestion had been based on the finding that the ␤-relaxation peak of magnitude ⑀Љϳ13ϫ10 Ϫ5 observed in the quenched glassy state of o-terphenyl at Tϳ205 K vanished after the o-terphenyl glass was annealed for 24 h at 234 K, 12 K below its glass-softening temperature, T g , of 246 K ͑see the inset in Fig. 5 in Ref.…”

“…Evidence for this has been found for some glasses in some experiments, 22,23 but not in all glasses and other experiments. 5,6,[23][24][25][26][27] Structural relaxation of a glass has been found to decrease its ⌬⑀ ␤ , as the state of its equilibrium liquid is approached at a constant temperature in all cases. 6,[22][23][24][25][26][27] Thus, although the extent of thermodynamic metastability, as measured by the frozen-in volume, enthalpy, and entropy of a glass, raises ⌬⑀ ␤ , the ␤ process itself remains a property of the liquid state.…”