Scaling of the dynamics of flexible Lennard-Jones chains

TL;DR

This paper tests the isomorph theory on flexible Lennard-Jones chains, confirming invariant dynamics along isomorphs and explaining power-law density scaling in polymers and alkanes.

Contribution

First detailed validation of isomorph theory applied to flexible chain molecules, demonstrating invariant dynamics and providing a theoretical basis for experimental scaling observations.

Findings

Confirmed existence of isomorphs in flexible Lennard-Jones chains

Demonstrated invariance of dynamics along isomorphs in reduced units

Explained power-law density scaling in polymers and alkanes

Abstract

The isomorph theory provides an explanation for the so-called power law density scaling which has been observed in many molecular and polymeric glass formers, both experimentally and in simulations. Power law density scaling (relaxation times and transport coefficients being functions of $\rho^{\gamma_S}/T$, where $\rho$ is density, $T$ is temperature, and $\gamma_S$ is a material specific scaling exponent) is an approximation to a more general scaling predicted by the isomorph theory. Furthermore, the isomorph theory provides an explanation for Rosenfeld scaling (relaxation times and transport coefficients being functions of excess entropy) which has been observed in simulations of both molecular and polymeric systems. Doing molecular dynamics simulations of flexible Lennard-Jones chains (LJC) with rigid bonds, we here provide the first detailed test of the isomorph theory applied to flexible chain molecules. We confirm the existence of isomorphs, which are curves in the phase diagram along which the dynamics is invariant in the appropriate reduced units. This holds not only for the relaxation times but also for the full time dependence of the dynamics, including chain specific dynamics such as the end-to-end vector autocorrelation function and the relaxation of the Rouse modes. As predicted by the isomorph theory, jumps between different state points on the same isomorph happen instantaneously without any slow relaxation. Since the LJC is a simple coarse-grained model for alkanes and polymers, our results provide a possible explanation for why power-law density scaling is observed experimentally in alkanes and many polymeric systems. The theory provides an independent method of determining the scaling exponent, which is usually treated as a empirical scaling parameter.