Molecular-Dynamics Simulation of a Glassy Polymer Melt: Rouse Model and Cage Effect

TL;DR

This study uses molecular dynamics simulations to analyze how the caging effect influences the Rouse model's applicability in describing the dynamics of a glassy polymer melt, highlighting the scale-dependent validity of the theory.

Contribution

It demonstrates that the Rouse model accurately describes polymer melt dynamics above the caging scale and compares monomer displacements with Lennard-Jones mixtures to show connectivity effects.

Findings

Rouse theory applies above the caging length and time scales.

Caging causes a two-step mean square displacement behavior.

Connectivity influences monomer displacement compared to Lennard-Jones mixtures.

Abstract

We report results of molecular-dynamics simulations for a glassy polymer melt consisting of short, linear bead-spring chains. It was shown in previous work that this onset of the glassy slowing down is compatible with the predictions of the mode coupling theory. The physical process of `caging' of a monomer by its spatial neighbors leads to a distinct two step behavior in the particle mean square displacements. In this work we analyze the effects of this caging process on the Rouse description of the melt's dynamics. We show that the Rouse theory is applicable for length and time scales above the typical scales for the caging process. Futhermore, the monomer displacement is compared with simulation results for a binary Lennard-Jones mixture to point out the differences which are introduced by the connectivity of the particles.