Undershoot Recovery in Polystyrene Melts: Effects of Annealing on Repeated Shear Startup

Entangled polymer melts undergo a transient stress undershoot, following the well-known overshoot, during shear startup, as established with several experiments using primarily nearly monodisperse linear polystyrenes (PS) and their mutual blends of different molar mass. While the microscopic origin of the undershoot remains debated, growing evidence supports a connection to chain tumbling, as proposed by [Costanzo, S. Macromolecules 2016, 49(10), 3925-3935.] through their tumbling-enabling Ianniruberto-Marrucci (IM) model and further supported by simulations and other modeling approaches. The current view is that undershoots reflect the cyclic orientation-retraction dynamics of chains with the initial chain orientation state playing a decisive role. Here, we investigate both overshoot and undershoot behavior systematically across a series of PS melts across the unentangled-entangled transition (with the average number of entanglements ranging from 1.8 to 16.7) using a modular cone-partitioned-plate (CPP) geometry, and compare the experimental data with the tumbling-enabling Ianniruberto-Marrucci (IM) model. Subsequently, repeated shear startup protocols with increasing rest (annealing) times between sequential tests were applied to probe the undershoot evolution. Our results demonstrate that undershoots re-emerge only after unexpectedly long annealing times at sufficiently large shear rates (Wi_R ≥ 10), highlighting persistent structural memory effects. These findings provide new insights on how segmental reorientation affects the undershoot recovery of sheared polymer melts and contributes toward assessing and improving the constitutive description of nonlinear polymer dynamics.