Entangled chain dynamics of polymer knots in extensional flow.

Area of Science:

• Polymer Physics
• Soft Matter Physics
• Computational Chemistry

Background:

• Polymer chains in solution exhibit complex behaviors influenced by topology, hydrodynamics, and elasticity.
• Understanding the impact of topological constraints like knots on polymer dynamics is crucial for predicting solution properties.

Purpose of the Study:

• To develop and utilize a coarse-grained molecular dynamics model to investigate the influence of polymer knots on chain dynamics and rheology under extensional flow.
• To quantify the effect of entanglements and knot tightening on polymer stretching and induced stress.

Main Methods:

• Formulation of a coarse-grained molecular dynamics model incorporating hydrodynamic interactions, thermal fluctuations, nonlinear elasticity, and solvent-mediated excluded volume interactions.
• Inclusion of explicit geometric detection and tracking of chain entanglements to model topology-preserving interactions.
• Numerical and computational methods to simulate polymer behavior in strong extensional flow (Deborah number De=1.6).

Main Results:

• Polymer knots significantly slow down chain stretching by obstructing natural, unraveling, and flow-induced motions through entanglements.
• Knotted polymer chains exhibit smaller steady-state lengths and reduced polymer-induced stress compared to topologically trivial chains.
• The rate of knot tightening was identified as a key molecular process affecting the solution's rheology.

Conclusions:

• Topological complexity, specifically the presence of knots, fundamentally alters polymer behavior in flow.
• The developed model provides insights into the molecular mechanisms by which knots influence macroscopic rheological properties.
• This study highlights the importance of considering chain topology in the rheological characterization of polymer solutions.