2016 Volume 44 Issue 1 Pages 65-68
Lu et al [ACS Macro Lett. 2014, 3, 569-573; Macromolecules 2015, 48, 4164-4173] performed the Kremer-Grest (KG) bead-spring simulation for entangled polymer melt to report that the stress overshoot under start up shear is not induced by the subchain orientation but by the subchain stretch, even at the shear rates well below the reciprocal Rouse time, 1/τR. Apart from the contradiction to the tube picture, their result violates the stress-optical rule (SOR), and thus, it contradicts the conventional view in further coarse-grained models that assumes local equilibration of the subchain. In this study, KG simulation was conducted for melts of bead-spring chains each composed of 200 beads. The results obtained under shear at the rates around 1/τR were fully consistent with SOR. Namely, the stress overshoot is invoked by the subchain orientation rather than the subchain stretch. Those results are consistent with experiments and tube theories but contradict to the results of Lu et al.