Do Repeated Shear Startup Runs of Polymeric Liquids Reveal Structural Changes?

Using the integral equation of the Doi-Edwards theory that only accounts for tube orientation of entangled linear polymers, we explore the behavior of the stress maximum typically observed in shear startup as a function of the waiting time t_w between repeated startup runs. Depending on whether the first run is interrupted before the maximum, or sufficiently beyond it, the magnitude of the peak in the second run comes out nonmonotonic with t_w or else monotonically increasing, respectively. A similar behavior has been observed in several experiments and commonly attributed to structural changes of the entangled network. By emphasizing the role played by the mere orientation of the network strands in faithfully reproducing all the observed behavior, at least qualitatively, our results help to put things in better perspective.