Abstract
The heterogeneous character of the Johari-Goldstein (JG) relaxation is evidenced by molecular-dynamics simulation of a model polymer system. A double-peaked evolution of dynamic heterogeneity (DH), with maxima located at JG and structural relaxation time scales, is observed and mechanistically explained. {The short-time single-particle displacement during JG relaxation weakly correlates with the long-time one observed during structural relaxation}.
Highlights
Avoiding crystallization, polymers and liquids freeze into a microscopically disordered solid-like state, a glass.[1]
We focus on linear polymers where there are no side groups, and the secondary relaxations are usually ascribed to some movement of short lengths of the main chain like limited vibrational oscillation about their mean position or crankshaft motion.[2,11−13] The secondary relaxation in linear polymers is thought to be a genuine manifestation of the Johari−Goldstein (JG) β-relaxation,[5] a special class of secondary relaxations having strong connections to the α-relaxation in properties, and advocated to be a universal feature of the glass transition.[4,6−8,14]
This has been recently substantiated by studying the invariance of the relation between αrelaxation and β-relaxation in metallic glasses to variations of pressure and temperature by molecular-dynamics (MD) simulations combined with the dynamic mechanical spectroscopy method.[23]
Summary
Polymers and liquids freeze into a microscopically disordered solid-like state, a glass.[1]. The existence of spatial correlations between dynamic fluctuations in short, dynamic heterogeneity (DH) has been revealed by experiments and numerical studies, for example, see the reviews in the literature.[26−32] In particular, the presence of DH in the α-relaxation regime has been studied in bulk polymers by, for example, multidimensional NMR33 and simulations,[34] and tuned by nanoconfinement.[31,32] Of particular interest to the present study are the findings that DHs at both α- and β-relaxation time scales have been reported in colloids as clusters of faster-moving particles,[35] and numerical studies found heterogeneous dynamics of JG relaxation,[36] supporting previous suggestions.[18] On even shorter time scales, picosecond DH, observed by incoherent quasielastic neutron scattering, allowed the evaluation of the characteristic time scale of primary relaxation of molecular liquids.[9] it has been noted that JG involves a broad distribution of processes with those occurring at longer times being characterized by a longer length scale.[8,22]. Our findings capture a new correspondence between JG and structural relaxation in polymers
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