Comment on: "Relating Chain Conformations to Extensional Stress in Entangled Polymer Melts"

TL;DR

This paper critiques a recent study claiming a direct link between chain entropy decrease and extensional stress in entangled polymer melts, arguing that the original analysis was incomplete and misleading.

Contribution

It clarifies that intrachain entropic stress does not match the total macroscopic stress and emphasizes the importance of comprehensive data analysis.

Findings

The original claim is based on limited and questionable data analysis.

Proper examination of full simulation trajectories contradicts the original conclusion.

Intrachain entropic stress does not directly correspond to macroscopic extensional stress.

Abstract

Based on non-equilibrium molecular dynamics simulations of entangled polymer melts, a recent Letter [Phys. Rev. Lett. $\textbf{121}$, 047801 (2018), arXiv:1806.09509] claims that the rising extensional stress is quantitatively consistent with the decreasing entropy of chains at the equilibrium entanglement length. We point out that exactly the opposite is true: the intrachain entropic stress arising from individual entanglement strands generally does not agree with the total "macroscopic" stress. The conclusion of the Letter is based on an incomplete and questionable analysis of a limited range of the simulation trajectory. The opposite conclusion should have been drawn from their data, had they examined the full simulation trajectory in a proper way.