Consequences of Delayed Chain Retraction on the Rheology and Stretch Dynamics of Entangled Polymer Liquids under Continuous Nonlinear Shear Deformation

Abstract

We generalize our recent new ideas for the continuous startup shear rheology of entangled polymer liquids in the transient stress overshoot regime to formulate a theory for the full constitutive response and nonequilibrium dynamics over all time scales, deformation rates, and degrees of entanglement. The convective constraint release (CCR) idea that chain retraction locally triggers disentanglement in a shear-rate-dependent manner is significantly modified to be physically consistent with our nonclassical treatment of delayed retraction due to an entanglement grip force. A detailed numerical study of the predictions of the theory is presented for the full stress–strain response, scaling behavior of the stress overshoot and undershoot features, orientational stress, primitive path contour length dynamics, and nonequilibrium steady-state properties spanning the slow and fast nonlinear deformation regimes. For deformations slow enough there is little or no chain stretch, our results are qualitatively the same as in prior tube-based models. However, under fast deformation conditions, we make qualitatively new predictions for all rheological and dynamic properties that are not contained in any existing models. No-fit-parameter quantitative comparisons are made with experimental and simulation studies, and very good agreement is found; testable predictions are made. Strong connections between properties (stress and degree of chain stretch) at the overshoot and in the steady state are found, which suggests common physics exists at the elastic–viscous crossover (stress overshoot) and for long time flow associated with a delayed onset of primitive path retraction and emergence of CCR.

Nonlinear shear of entangled polymers from nonequilibrium molecular dynamics

“…a) as well as the corresponding data for PS melts. In particular, the model has the correct scaling exponent, as reported in Refs . The model achieves this because the intermolecular gripping force delays chain retraction relative to the GLaMM model for moderate values of by requiring strands to exceed a critical tension.…”

“…Comparison of our MD results with experiments by Costanzo et al (PS melt and solution), Schweizer et al (PS melt), Menezes and Graessley (PBD solutions), Boukany et al (SBR melts) and Ravindranath and Wang (PBD solutions), along with the GLaMM and Xie and Schweizer models. The strain at peak stress against for PS and low Z (a) and high Z (b); the peak stress against peak strain (c); and the peak stress against for PS and low Z (d) and high Z (e).…”

“…We ascribe the PS behavior to the low N eK value of PS melts and KG chains. The model of Xie and Schweizer captures the strain of peak stress for PS melts and KG simulations, but overpredicts the stress peak in experiments and simulations. However, the Xie and Schweizer model has the correct shape over a wide range of flow rates for the high Z data despite its overprediction relative to these experiments.…”

From Wall Slip to Bulk Shear Banding in Entangled Polymer Solutions

“…Scenario (b) remains to be worked out for the challenging case of nonquiescent relaxation after step shear of entangled melts. In contrast to (a) and (b), the theory for scenario (c) is less quantitative and still under development . However, it may have plausibly captured the correct physics, and thus should be taken seriously.…”

“…The observations of shear strain localization in both startup shear and stepwise shear also motivated a different kind of interpretation: The termination of homogeneous deformation occurs upon localized yielding through breakdown of the entanglement network upon force imbalance between the intrachain entropic retraction force and the intermolecular grip force . While the theoretical development along this line is still at its infancy, promising progress is being made …”