Experimental evidence of mosaic structure in strongly supercooled molecular liquids

F Caporaletti,A I Chumakov,S Capaccioli,G Monaco,S Valenti,M Mikolasek

doi:10.1038/s41467-021-22154-8

Abstract

When a liquid is cooled to produce a glass its dynamics, dominated by the structural relaxation, become very slow, and at the glass-transition temperature Tg its characteristic relaxation time is about 100 s. At slightly elevated temperatures (~1.2 Tg) however, a second process known as the Johari-Goldstein relaxation, βJG, decouples from the structural one and remains much faster than it down to Tg. While it is known that the βJG-process is strongly coupled to the structural relaxation, its dedicated role in the glass-transition remains under debate. Here we use an experimental technique that permits us to investigate the spatial and temporal properties of the βJG relaxation, and give evidence that the molecules participating in it are highly mobile and spatially connected in a system-spanning, percolating cluster. This correlation of structural and dynamical properties provides strong experimental support for a picture, drawn from theoretical studies, of an intermittent mosaic structure in the deeply supercooled liquid phase.

Highlights

When a liquid is cooled to produce a glass its dynamics, dominated by the structural relaxation, become very slow, and at the glass-transition temperature Tg its characteristic relaxation time is about 100 s
TDI is sensitive to the fastest relaxation process active in the explored time window: given its dynamical range of fewer than two decades in time, it was not possible to detect more than one relaxation process per experimental curve even when both the α and the βJG process were expected to be active
The α-process in 1-propanol is still diffusive around Tαβ while the microscopic dynamics associated with the βJG-process is clearly restricted (n > 2) already at a relatively high T (122.5 K ≃ 1.25 Tg), giving broader significance to similar observations reported for the other two glass-formers studied using TDI: 5M2H12 and OTP8,9

Summary

Introduction

When a liquid is cooled to produce a glass its dynamics, dominated by the structural relaxation, become very slow, and at the glass-transition temperature Tg its characteristic relaxation time is about 100 s. We use an experimental technique that permits us to investigate the spatial and temporal properties of the βJG relaxation, and give evidence that the molecules participating in it are highly mobile and spatially connected in a system-spanning, percolating cluster This correlation of structural and dynamical properties provides strong experimental support for a picture, drawn from theoretical studies, of an intermittent mosaic structure in the deeply supercooled liquid phase. According to the coupling model, the βJG-relaxation consists of a distribution of elementary processes evolving with time by increasing the number of participating molecular units and culminating in the cooperative α-relaxation: the α-relaxation and βJG-relaxation share many relevant properties[2] In another model based on the hypothesis of strong dynamical heterogeneity of the supercooled liquid, the βJGrelaxation originates from the population exchange between regions of tightly confined molecules and of loosely confined ones[11]. This cluster evolves on the timescale of the βJG-process, and careful experimental design is required to study it

Methods

Results

Conclusion