Molecular Mechanisms for Conformational and Rheological Responses of Entangled Polymer Melts to Startup Shear

Correction: Finite cohesion due to chain entanglement in polymer melts

Correction for 'Finite cohesion due to chain entanglement in polymer melts' by Shiwang Cheng et al., Soft Matter, 2016, 12, 3340-3351.

Correction: Nonlinear rheology of entangled polymers at turning point

Correction for 'Nonlinear rheology of entangled polymers at turning point' by Shi-Qing Wang et al., Soft Matter, 2015, 11, 1454-1458.

Finite cohesion due to chain entanglement in polymer melts

Three different types of experiments, quiescent stress relaxation, delayed rate-switching during stress relaxation, and elastic recovery after step strain, are carried out in this work to elucidate the existence of a finite cohesion barrier against free chain retraction in entangled polymers. Our experiments show that there is little hastened stress relaxation from step-wise shear up to γ = 0.7 and step-wise extension up to the stretching ratio λ = 1.5 at any time before or after the Rouse time. In contrast, a noticeable stress drop stemming from the built-in barrier-free chain retraction is predicted using the GLaMM model. In other words, the experiment reveals a threshold magnitude of step-wise deformation below which the stress relaxation follows identical dynamics whereas the GLaMM or Doi-Edwards model indicates a monotonic acceleration of the stress relaxation dynamics as a function of the magnitude of the step-wise deformation. Furthermore, a sudden application of startup extension during different stages of stress relaxation after a step-wise extension, i.e. the delayed rate-switching experiment, shows that the geometric condensation of entanglement strands in the cross-sectional area survives beyond the reptation time τd that is over 100 times the Rouse time τR. Our results point to the existence of a cohesion barrier that can prevent free chain retraction upon moderate deformation in well-entangled polymer melts.

Tube Dynamics Works for Randomly Entangled Rings

The tube model is the cornerstone of molecular theory for polymer rheology. We test its microscopic assumptions by simulating topologically equilibrated ring polymers, whose dynamics is free from end segment relaxation. We show that a closed-form expression derived from the tube model adapted to ring polymers quantitatively predicts the segmental mean squared displacements over the entire range of time scales from local motion to complete equilibration, with a time-independent local friction factor.

Simulating Startup Shear of Entangled Polymer Melts

Start-up shear rheology is a standard experiment used for characterizing polymer flow, and to test various models of polymer dynamics. A rich phenomenology is developed for behavior of entangled monodisperse linear polymers in such tests, documenting shear stress overshoots as a function of shear rates and molecular weights. A tube theory does a reasonable qualitative job at describing these phenomena, although it involves several drastic approximations and the agreement can be fortuitous. Recently, Lu and co-workers published several papers [e.g., Lu ACS Macro Lett. 2014, 3, 569-573] reporting results from molecular dynamics simulations of linear entangled polymers, which contradict both theory and experiment. On the basis of these observations, they made very serious conclusions about the tube theory, which seem to be premature. In this letter, we repeat simulations of Lu et al. and systematically show that neither their simulation results nor their comparison with theory is confirmed.