Using Brownian dynamics simulation and calculations of the most sophisticated version of the tube model

the GLaMM model

we explore the near equilibrium and fast startup shear rheological behavior of entangled polymer fluids.In the nearly linear flow regime a detailed comparison between the rheometric measurements and GLaMM theoretical calculations is performed.The overall agreement is found to be satisfactory but several minor disagreements

e.g.

the measurements and calculations reveal totally different stress relaxation dynamics on the time scale in units of t/R.However

during startup shear

an explicit comparison between the simulation and the theoretical description from the GLaMM theory shows irreconcilable differences on the time scale in units of t/d

which confirms our previous results

but the difference can disappear in the nearly linear flow regime.In addition

the results of our simulations about the relationship between the strain max at the overshoot point and WiR or t/R are also compared and found to behave in a very similar manner with several literature data sets but disagree with those predicted by the GLaMM theory.These findings call into question the barrier-free chain retraction mechanism as envisioned in the tube model.