Entanglements in Quiescent and Sheared Polymer Melts

TL;DR

This study visualizes entanglements in polymer melts using molecular dynamics, revealing how shear flow causes chain bending and disentanglement, which correlates with stress overshoot phenomena.

Contribution

It introduces a method to detect entanglements via time-averaged interaction energy hotspots and analyzes their behavior under shear flow.

Findings

Entanglements appear as hot spots of high interaction energy.

Shear flow causes chain zigzagging and bending of entanglement hotspots.

Disentanglements occur as chains bend and approach chain ends, explaining stress overshoot.

Abstract

We visualize entanglements in polymer melts using molecular dynamics simulation. A bead at an entanglement interacts persistently for long times with the non-bonded beads (those excluding the adjacent ones in the same chain). The interaction energy of each bead with the non-bonded beads is averaged over a time interval $\tau$ much longer than microscopic times but shorter than the onset time of tube constraints $\tau_{\rm e}$. Entanglements can then be detected as hot spots consisting of several beads with relatively large values of the time-averaged interaction energy. We next apply a shear flow with rate much faster than the entangle motion. With increasing strain the chains take zigzag shapes and a half of the hot spots become bent. The chains are first stretched as a network but, as the bends approach the chain ends, disentanglements subsequently occur, leading to stress overshoot observed experimentally.