Microscopic understanding of the Johari-Goldstein β relaxation gained from nuclear γ -resonance time-domain-interferometry experiments
| Autor: | K L Ngai |
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| Rok vydání: | 2021 |
| Předmět: | |
| Zdroj: | Physical Review E. 104 |
| ISSN: | 2470-0053 2470-0045 |
| Popis: | Traditionally the study of dynamics of glass-forming materials has been focused on the structural \ensuremath{\alpha} relaxation. However, in recent years experimental evidence has revealed that a secondary \ensuremath{\beta} relaxation belonging to a special class, called the Johari-Goldstein (JG) \ensuremath{\beta} relaxation, has properties strongly linked to the primary \ensuremath{\alpha} relaxation. By invoking the principle of causality, the relation implies the JG \ensuremath{\beta} relaxation is fundamental and indispensable for generating the \ensuremath{\alpha} relaxation, and the properties of the latter are inherited from the former. The JG \ensuremath{\beta} relaxation is observed together with the \ensuremath{\alpha} relaxation mostly by dielectric spectroscopy. The macroscopic nature of the data allows the use of arbitrary or unproven procedures to analyze the data. Thus the results characterizing the JG \ensuremath{\beta} relaxation and the relation of its relaxation time ${\ensuremath{\tau}}_{\ensuremath{\beta}}$ to the \ensuremath{\alpha}-relaxation time ${\ensuremath{\tau}}_{\ensuremath{\alpha}}$ obtained can be equivocal and controversial. Coming to the rescue is the nuclear resonance time-domain-interferometry (TDI) technique covering a wide time range (${10}^{\ensuremath{-}9}--{10}^{\ensuremath{-}5}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$) and a scattering vector $q$ range ($9.6--40\phantom{\rule{0.16em}{0ex}}\mathrm{n}{\mathrm{m}}^{\ensuremath{-}1}$). TDI experiments have been carried out on four glass formers, ortho-terphenyl [M. Saito et al., Phys. Rev. Lett. 109, 115705 (2012)], polybutadiene [T. Kanaya et al., J. Chem. Phys. 140, 144906 (2014)], 5-methyl-2-hexanol [F. Caporaletti et al., Sci. Rep. 9, 14319 (2019)], and 1-propanol [F. Caporaletti et al., Nat. Commun. 12, 1867 (2021)]. In this paper the TDI data are reexamined in conjunction with dielectric and neutron scattering data. The results show the JG \ensuremath{\beta} relaxation observed by dielectric spectroscopy is heterogeneous and comprises processes with different length scales. A process with a longer length scale has a longer relaxation time. TDI data also prove the primitive relaxation time ${\ensuremath{\tau}}_{0}$ of the coupling model falls within the distribution of the TDI $q$-dependent JG \ensuremath{\beta}-relaxation times. This important finding explains why the experimental dielectric JG \ensuremath{\beta}-relaxation times ${\ensuremath{\tau}}_{\ensuremath{\beta}}(T,P)$ is approximately equal to ${\ensuremath{\tau}}_{0}(T,P)$ as found in many glass formers at various temperature $T$ and pressure $P$. The result, ${\ensuremath{\tau}}_{\ensuremath{\beta}}(T,P)\ensuremath{\approx}{\ensuremath{\tau}}_{0}(T,P)$, in turn explains why the ratio ${\ensuremath{\tau}}_{\ensuremath{\alpha}}(T,P)/{\ensuremath{\tau}}_{\ensuremath{\beta}}(T,P)$ is invariant to changes of $T$ and pressure $P$ at constant ${\ensuremath{\tau}}_{\ensuremath{\alpha}}(T,P)$, the \ensuremath{\alpha}-relaxation time. |
| Databáze: | OpenAIRE |
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