Atomic motions in the $\alpha\beta$-region of glass-forming polymers: Molecular versus Mode Coupling Theory approach

TL;DR

This study combines molecular dynamics simulations and Mode Coupling Theory to analyze localized atomic motions in glass-forming polymers, revealing the interplay of different mechanisms affecting relaxation processes.

Contribution

It demonstrates the applicability of Mode Coupling Theory to polymer relaxations and links local conformational changes to secondary relaxations.

Findings

Localized motions occur alongside diffusive relaxation.

Mode Coupling Theory can be applied with an unusually large exponent.

Competition between packing and intramolecular barriers influences dynamics.

Abstract

We present fully atomistic Molecular Dynamics simulation results on a main-chain polymer, 1,4-Polybutadiene, in the merging region of the $\alpha$- and $beta$-relaxations. A real space analysis reveals the occurrence of localized motions (``$\beta$-like'') in addition to the diffusive structural relaxation. A molecular approach provides a direct connection between the local conformational changes reflected in the atomic motions and the secondary relaxations in this polymer. Such local processes occur just in the time window where the $\beta$-process of the Mode Coupling Theory is expected. We show that the application of this theory is still possible, and yields an unusually large value of the exponent parameter. This result might originate from the competition between two mechanisms for dynamic arrest: intermolecular packing and intramolecular barriers for local conformational changes (``$\beta$-like'').