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

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 (gamma) over dot, i.e., (gamma) over dot > tau(-1)(d), where tau(d) 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 tau(-1)(d) < (gamma) over dot < tau(-1)(R) range, where tau(R) 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 (gamma) over dot < tau(-1)(R), the stress overshoot is associated with the global stretch of the chains rather than with their segmental orientation. While several subsequent studies by other groups did not support the results of Lu et al., the issue remains open, mainly due to the use of mesoscopic coarse -grained models in all these simulations which may give rise to quantitatively inaccurate results especially at high flow rates. In order to resolve this issue and clarify the possible causes of the controversy, in this study we conducted direct atomistic nonequilibrium molecular dynamics (NEMD) simulations of an entangled polymer melt under start-up shear -flow. Our results unambiguously show that the segmental orientation is the primary molecular mechanism leading to the stress overshoot. We also determine and discuss various structural properties relevant for the quantitative analysis of the stress response of flowing polymeric systems.