Wall slip in primitive chain network simulations of shear startup of entangled polymers and its effect on the shear stress undershoot

TL;DR

This study uses an extended primitive chain network model to investigate how wall slip affects the shear stress undershoot in entangled polymers during startup shear flows, revealing that slip weakens the undershoot and disrupts molecular tumbling coherence.

Contribution

It introduces a model incorporating wall slip in entangled polymer simulations, providing new insights into the mechanisms behind stress undershoot phenomena.

Findings

Wall slip weakens the shear stress undershoot.

Disentanglement reduces molecular tumbling coherence.

Slip influences transient stress behavior in polymers.

Abstract

In some recent experiments on entangled polymers of stress growth in startup of fast shear flows an undershoot in the shear stress is observed following the overshoot, i.e., before approaching the steady state. Whereas tumbling of the entangled chain was proposed to be at its origin, here we investigate another possible cause for the stress undershoot, i.e., slippage at the interface between polymer and solid wall. To this end, we extend the primitive chain network model to include slip at the interface between entangled polymeric liquids and solid walls with grafted polymers. We determine the slip velocity at the wall, and the shear rate in the bulk, by imposing that the shear stress in the bulk polymers is equal to that resulting from the polymers grafted at the wall. After confirming that the predicted results for the steady state are reasonable, we examine the transient behavior. The simulations confirm that slippage weakens the magnitude of the stress overshoot, as reported earlier. The undershoot is also weakened, or even disappears, because of a reduced coherence in molecular tumbling. In other words, the disentanglement between grafted and bulk chains, occurring throughout the stress overshoot region, does not contribute to the stress undershoot.