2018 Volume 46 Issue 1 Pages 23-28
In this study, viscosity growth of entangled polymers under startup of fast shear flows was investigated by means of primitive chain network simulations. In particular, we focused on the undershoot following the well-known overshoot. The simulations reasonably reproduced the viscosity growth data reported in the literature for polystyrene melts. To investigate the origin of the observed undershoot, stress was decomposed into orientational and stretch contributions through a decoupling approximation. The decoupled results show a tiny undershoot in both orientation and stretch. Molecular tumbling was also monitored, both for single molecules and in terms of an average end-to-end orientation angle. Results appear to confirm that tumbling causes the viscosity undershoot, consistently with the suggestion of Costanzo et al. [Macromolecules, 2016, 49, 3915].