Open and anisotropic soft regions in a model polymer glass

TL;DR

This study uses Molecular Dynamics simulations to analyze the vibrational dynamics of a model polymer glass, revealing that soft monomers tend to form anisotropic, quasi-local clusters with specific structural properties.

Contribution

It introduces a severe threshold for identifying soft monomers, enabling the study of their clustering behavior and anisotropic shapes in limited-size systems.

Findings

Soft monomers form quasi-local clusters involving up to 15 monomers.

Clusters are open, tenuous, with fractal dimension less than 2.

Clusters exhibit strong anisotropy and linear correlation between eigenvalues.

Abstract

The vibrational dynamics of a model polymer glass is studied by Molecular Dynamics simulations. The focus is on the "soft" monomers with high participation to the lower-frequency vibrational modes contributing to the thermodynamic anomalies of glasses. To better evidence their role, the threshold to qualify monomers as soft is made severe, allowing for the use of systems with limited size. A marked tendency of soft monomers to form quasi-local clusters involving up to 15 monomers is evidenced. Each chain contributes to a cluster up to about three monomers and a single cluster involves monomer belonging to about 2-3 chains. Clusters with monomers belonging to a single chain are rare. The open and tenuous character of the clusters is revealed by their fractal dimension $d_f < 2$. The inertia tensor of the soft clusters evidences their strong anisotropy in shape and remarkable linear correlation of the two largest eigenvalues. Owing to the limited size of the system, finite-size effects, as well as dependence of the results on the adopted polymer length, cannot be ruled out.