Chain End Mobilities in Polymer Melts - A Computational Study

TL;DR

This study uses molecular dynamics simulations to examine the validity of the Rouse model's assumption of uniform monomer mobility in polymer melts, revealing minor deviations at terminal monomers due to local interactions.

Contribution

It provides a detailed computational analysis of monomer mobilities in PEO melts, challenging the uniform friction assumption of the Rouse model with new statistical methods.

Findings

Uniform mobility assumption holds approximately for PEO.

Microscopic dynamics are influenced by intra- and intermolecular interactions.

Terminal monomers show slight deviations due to escape mechanisms.

Abstract

The Rouse model can be regarded as the standard model to describe the dynamics of a short polymer chain under melt conditions. In this contribution, we explicitly check one of the fundamental assumptions of this model, namely that of a uniform friction coefficient for all monomers, on the basis of MD simulation data of a poly(ethylene oxide) (PEO) melt. This question immediately arises from the fact that in a real polymer melt the terminal monomers have on average more intermolecular neighbors than the central monomers, and one would expect that exactly these details affect the precise value of the friction coefficient. The mobilities are determined by our recently developed statistical method, which provides detailed insights about the local polymer dynamics. Moreover, it yields complementary information to that obtained from the mean square displacement (MSD) or the Rouse mode analysis. It turns out that the Rouse assumption of a uniform mobility is fulfilled to a good approximation for the PEO melt. However, a more detailed analysis reveals that the underlying microscopic dynamics is highly affected by different contributions from intra- and intermolecular excluded volume interactions, which cannot be taken into account by a modified friction coefficient. Minor deviations occur only for the terminal monomers on larger time scales, which can be attributed to the presence of two different escape mechanisms from their first coordination sphere. These effects remain elusive when studying the dynamics with the MSD only.