Fast Vibrational Modes and Slow Heterogeneous Dynamics in Polymers and Viscous Liquids

TL;DR

This paper reviews molecular dynamics simulations revealing how vibrational modes relate to dynamic heterogeneity and relaxation in glass-forming polymers and liquids, highlighting the predictive power of vibrational dynamics.

Contribution

It demonstrates the correlation between vibrational dynamics and slow relaxation, providing new insights into the microscopic heterogeneity in glassy materials.

Findings

Strong correlation between vibrational dynamics and relaxation.

Breakdown of the Stokes--Einstein relation in glass-formers.

Identification of regions with correlated displacements.

Abstract

Many systems, including polymers and molecular liquids, when adequately cooled and/or compressed, solidify into a disordered solid, i.e., a glass. The~transition is not abrupt, featuring progressive decrease of the microscopic mobility and huge slowing down of the relaxation.} A~distinctive aspect of glass-forming materials is the microscopic dynamical heterogeneity (DH), i.e., the presence of regions with almost immobile particles coexisting with others where highly mobile ones are located. Following the first compelling evidence of a strong correlation between vibrational dynamics and ultraslow relaxation, we posed the question if the vibrational dynamics encodes predictive information on DH. Here, we review our results, drawn from molecular-dynamics numerical simulation of polymeric and molecular glass-formers, with a special focus on both the breakdown of the Stokes--Einstein relation between diffusion and viscosity, and the size of the regions with correlated displacements.