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

## TL;DR

The study explores how polymer melts recover from stress undershoots after repeated shearing, revealing long-lasting structural memory effects.

## Contribution

The paper demonstrates that undershoot recovery in polymer melts requires unexpectedly long annealing times, indicating persistent structural memory.

## Key findings

- Undershoots re-emerge only after long annealing times at high shear rates.
- Structural memory effects significantly influence the recovery of sheared polymer melts.
- Results highlight the role of segmental reorientation in nonlinear polymer dynamics.

## Abstract

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 [

CostanzoS.,

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 (WiR
≥ 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.