Anisotropy of the monomer random walk in a polymer melt: local-order and connectivity effects

TL;DR

This study uses molecular dynamics simulations to analyze how local order and bond connectivity influence the anisotropic motion of monomers in polymer melts, revealing complex interplay affecting microscopic dynamics.

Contribution

It provides a detailed analysis of local order and connectivity effects on monomer anisotropy in polymer melts, distinguishing their individual and combined roles.

Findings

Connectivity anisotropy peaks at short times and decays as t^{-1/2}.

Local geometry anisotropy is influenced by cage shape and connectivity.

Local order alone does not fully explain monomer rattling behavior.

Abstract

The random walk of a bonded monomer in a polymer melt is anisotropic due to local order and bond connectivity. We investigate both effects by molecular-dynamics simulations on melts of fully-flexible linear chains ranging from dimers ($M=2$) up to entangled polymers ($M=200$). The corresponding atomic liquid is also considered as reference system. To disentangle the influence of the local geometry and the bond arrangements, and reveal their interplay, we define suitable measures of the anisotropy emphasising either the former or the latter aspect. Connectivity anisotropy, as measured by the correlation between the initial bond orientation and the direction of the subsequent monomer displacement, shows a slight enhancement due to the local order at times shorter than the structural relaxation time. At intermediate times - when the monomer displacement is comparable to the bond length - a pronounced peak and then decays slowly as $t^{-1/2}$, becoming negligible when the displacements is as large as about five bond lengths, i.e. about four monomer diameters or three Kuhn lengths. Local-geometry anisotropy, as measured by the correlation between the initial orientation of a characteristic axis of the Voronoi cell and the subsequent monomer dynamics, is affected at shorter times than the structural relaxation time by the cage shape with antagonistic disturbance by the connectivity. Differently, at longer times, the connectivity favours the persistence of the local-geometry anisotropy which vanishes when the monomer displacement exceeds the bond length. Our results strongly suggest that the sole consideration of the local order is not enough to understand the microscopic origin of the rattling amplitude of the trapped monomer in the cage of the neighbours.