Effect of Chain Orientation and Stretch on the Stress Overshoot of Entangled Polymeric Materials under Start-Up Shear

Abstract

One of the most important nonlinear rheological phenomena in flowing polymeric materials is the stress overshoot under start-up shear at sufficiently high shear rates γ, i.e., γ > τd–1, where τd is the terminal relaxation time of system. According to the well-known tube theory for entangled polymeric materials, a linear relationship between the anisotropy of the stress and that of the birefringence holds for strain rates in the τd–1 < γ < τR–1 range, where τR is the Rouse time. This implies that in such a flow range the stress overshoot behavior of entangled polymers is accurately described in terms of the chain orientation, as confirmed by previous experiments. However, there has been some recent debate on this issue, following the study of Lu et al. [ACS Macro Lett. 2014, 3, 569−573], whose coarse-grained molecular dynamics simulations produced results in apparent conflict with existing theoretical and experimental predictions, which had suggested that even for γ < τR–1, the stress overshoot is asso...