  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.
