Connection between slow and fast dynamics of molecular liquids around the glass transition

TL;DR

This study reveals a fundamental link between slow and fast molecular dynamics near the glass transition, showing that the mean-square displacement correlates with fragility at specific timescales, supporting elastic models of glass formation.

Contribution

It establishes an intrinsic Lindemann criterion for molecular liquids and uncovers a direct connection between MSD and fragility at nanosecond timescales.

Findings

MSD is invariant along the glass-transition line at studied pressures.

A one-to-one MSD-fragility correlation exists at ~4 ns timescale.

The correlation does not hold at shorter times.

Abstract

The mean-square displacement (MSD) was measured by neutron scattering at various temperatures and pressures for a number of molecular glass-forming liquids. The MSD is invariant along the glass-transition line at the pressure studied, thus establishing an ``intrinsic'' Lindemann criterion for any given liquid. A one-to-one connection between the MSD's temperature dependence and the liquid's fragility is found when the MSD is evaluated on a time scale of approximately 4 nanoseconds, but does not hold when the MSD is evaluated at shorter times. The findings are discussed in terms of the elastic model and the role of relaxations, and the correlations between slow and fast dynamics are addressed.