Topological Linking Drives Anomalous Thickening of Ring Polymers In Weak Extensional Flows

TL;DR

This study uses molecular dynamics simulations to reveal that topological links between ring polymers in weak extensional flows cause significant viscosity thickening and extreme elongation, especially near certain flow conditions.

Contribution

It uncovers the role of flow-driven topological linking in causing anomalous thickening and elongation of ring polymers, a phenomenon not previously understood.

Findings

Linked rings experience larger drag forces and elongate more.

Fraction of linked rings peaks at Wi~1.

Thickening correlates with the formation of topological links.

Abstract

Molecular dynamics simulations confirm recent extensional flow experiments showing ring polymer melts exhibit strong extension-rate thickening of the viscosity at Weissenberg numbers $Wi<<1$. Thickening coincides with the extreme elongation of a minority population of rings that grows with $Wi$. The large susceptibility of some rings to extend is due to a flow-driven formation of topological links that connect multiple rings into supramolecular chains. Links form spontaneously with a longer delay at lower $Wi$ and are pulled tight and stabilized by the flow. Once linked, these composite objects experience larger drag forces than individual rings, driving their strong elongation. The fraction of linked rings generated by flow depends non-monotonically on $Wi$, increasing to a maximum when $Wi\sim1$ before rapidly decreasing when the strain rate approaches the relaxation rate of the smallest ring loops $\sim 1/\tau_e$.