Watching shear thinning in creep: Entanglement-disentanglement transition

Abstract

We have carried out various rheometric experiments to probe the nonlinear rheological behavior of two well-entangled 1,4-polybutadiene solutions using both stress-controlled and rate-controlled startup shear. At a polymer concentration of 2% neither wall slip nor shear banding occurs, and there is no visible edge failure. At a moderate level of shear stress, the shear rate can take an induction time tind longer than the reptation time to rise toward an eventual steady-state value. The time tind for the entanglement-distanglement transition (EDT) shortens exponentially with increasing stress. The state of entanglement also changes in response to a sinusoidal stress σ0sinωt when its amplitude σ0 is beyond a threshold to produce a sufficiently high strain amplitude γ0 (>1). Moreover, the evolution of the state of chain entanglement during and after constant-stress creep has been studied using two different protocols. The first switches from creep to rate-controlled startup shear to determine the stress overshoot characteristic. In the second protocol, after creep to different stages, the sample is evaluated for its ability to undergo elastic recovery. We found that the overshoot response to the rate switching weakens and the ability to undergo elastic recoil deteriorates as the EDT progresses. These changes are clearly inherent because edge instability could not produce such effects. Thus, it is confirmed that an entanglement-disentanglement transition is a leading characteristic rheological response of these solutions.