Anomalous properties of the local dynamics in polymer glasses

TL;DR

This study reveals that local high-frequency dynamics in polymer glasses, specifically the Johari-Goldstein process, are linked to density fluctuations and can predict macroscopic properties, challenging traditional volume and entropy explanations.

Contribution

It demonstrates that local density fluctuations, rather than free volume or entropy, govern the dynamics in polymer glasses and introduces a model based on an asymmetric double well potential.

Findings

High frequency relaxation correlates or anti-correlates with density.

Local dynamics are governed by density fluctuations, not free volume.

Properties of local fluctuations can predict macroscopic behavior.

Abstract

The emergence of nanoscience has increased the importance of experiments able to probe the very local structure of materials, especially for disordered and heterogeneous systems. This is technologically important; for example, the nanoscale structure of glassy polymers has a direct correlation with their macroscopic physical properties. We have discovered how a local, high frequency dynamic process can be used to monitor and even predict macroscopic behavior in glassy polymers. Polyvinylethylenes vitrified by different chemical and thermodynamic pathways exhibit different densities in the glassy state. We find that the rate and amplitude of a high frequency relaxation mode (the Johari-Goldstein process involving local motion of segments of the chain backbone) can either correlate or anti-correlate with the density. This implies that neither the unoccupied (free) volume nor the configurational entropy governs the local dynamics in any general sense. Rather it is the magnitude of the fluctuations in local density that underlie these nm-scale motions. We show how properties of the dynamics and the density fluctuations can both be interpreted in terms of an asymmetric double well potential. Finally, since fluctuations underlie the macroscopic properties, we argue that information about the latter should be obtainable from characterization of the local dynamics.