Observing the chain stretch transition in a highly entangled polyisoprene melt using transient extensional rheometry (original) (raw)

Shear and extensional rheology of entangled polymer melts: Similarities and differences

2012

This work extends our previous understanding concerning the nonlinear responses of entangled polymer solutions and melts to large external deformation in both simple shear and uniaxial extension. Many similarities have recently been identified for both step strain and startup continuous deformation, including elastic yielding, i.e., chain disentanglement after cessation of shear or extension, and emergence of a yield point during startup deformation that involves a deformation rate in excess of the dominant molecular relaxation rate. At a sufficiently high constant Hencky rate, uniaxial extension of an entangled melt is known to produce window-glass-like rupture. The present study provides evidence against the speculation that chain entanglements tie up into "dead knots" in constant-rate extension because of the exponentially growing chain stretching with time. In particular, it is shown that even Instron-style tensile stretching, i.e., extending a specimen by applying a constant velocity on both ends, results in rupture. Yet, in the same rate range, the same entangled melt only yields in simple shear, and the resulting shear banding is clearly not a characteristic of rupture. Thus, we conclude that chain entanglements respond to simple shear in the manner of yielding whereas uniaxial extension is rather effective in causing some entanglements to lock up, making it impossible for the entanglement network to yield at high rates.

Chain Retraction in Highly Entangled Stretched Polymer Melts

Physical Review Letters

We use computer simulations to study the relaxation of strongly deformed highly entangled polymer melts in the nonlinear viscoelastic regime, focusing on anisotropic chain conformations after isochoric elongation. The Doi-Edwards tube model and its Graham-Likhtman-McLeish-Milner (GLaMM) extension, incorporating contour length fluctuation and convective constraint release, predict a retraction of the polymer chain extension in all directions, setting in immediately after deformation. This prediction has been challenged by experiment, simulation, and other theoretical studies, questioning the general validity of the tube concept. For very long chains we observe the initial contraction of the chain extension parallel and perpendicular to the stretching direction. However, the effect is significantly weaker than predicted by the GLaMM model. We also show that the first anisotropic term of an expansion of the 2D scattering function qualitatively agrees to predictions of the GLaMM model, providing an option for direct experimental tests.

Exploring rheological responses to uniaxial stretching of various entangled polyisoprene melts

Journal of Rheology, 2019

The present study is carried out to determine whether marginally entangled melts behave differently in both startup uniaxial extension and stepwise extension and how entangled solutions and melts with a comparable number of entanglements per chain behave similarly or differently. It is found that there is still a yield-to-rupture transition in the weakly entangled polyisoprene (PI) melts. Specifically, even a PI melt with fewer than five entanglements per chain was found to undergo melt rupture as if there was chain crosslinking. On the other hand, such weakly entangled melts were unable to show a type of strain localization called tensile decohesion. Moreover, the PI melts with the relatively low molecular weights were observed to remain essentially intact after stepwise extension of a magnitude for which well-entangled PI melts of high molecular weight undergo elastic breakup. Finally, it was demonstrated that an entangled solution shows a stronger elastic response than entangled melt of comparable degree of entanglement.

Chain Deformation in Entangled Polymer Melts at Re-entrant Corners

…, 2010

Using SANS to map the deformation of individual polymer chains in the vicinity of re-entrant corners in a contraction-expansion flow, we show that stress singularities at such corners, predicted by formulations of fluid dynamics that lack a molecular basis, do not cause extreme deformation of the chains. Multiscale modeling based on a nonlinear tube theory incorporating appropriate relaxation processes quantitatively reproduces the observed scattering, thus providing further evidence for the universality of the tube model for polymer flow.

Step Shear of Entangled Linear Polymer Melts: New Experimental Evidence for Elastic Yielding

Macromolecules, 2009

This work studies the most basic and important behavior of entangled linear polymer melts in sudden large shear deformations. In particular, melt elasticity resulting from the large step shear is extensively shown to produce cohesive breakdown. Unlike entangled solutions studied in Macromolecules 2007, 40, 8031, the residual elastic forces in sheared melts struggle quiescently for a significant induction period before bringing down the entanglement network. The induction time for the elastic yielding can be much longer than the longest Rouse relaxation time τ R , making it difficult to associate this cohesive failure with a chain retraction process envisioned in the tube theory. The cohesive failure also occurs upon a step strain produced at rates too slow to produce chain stretching, again making it unreasonable to invoke the concept of chain retraction due to chain stretching.

Unusual Nonlinear Effects in the Rheology of Entangled Polymer Melts

Progress of Theoretical Physics Supplement, 2008

We discuss the maxima in stress which were recently observed in the start up of shearing flows of entangled polymer melts with a complex branched structure. More than one maximum has been observed, and maxima can be found at extremely large values of deformation, of the order of several hundred units of shear. Furthermore, such maxima may disappear upon repeating the experiment, to come up again only after an extremely long resting time. By assuming that such high-deformation maxima are due to stretching of the internal segments of the branched structure, we develop a model able to estimate the unusual location of the maxima. The physical model underlying the calculations is also consistent with the observed effect of the resting time.

Universal scaling behavior in startup shear of entangled linear polymer melts

Journal of Rheology, 2009

We have studied stress overshoot behavior in startup shear of four monodisperse polymer melts with a range of chain entanglement from Z = 24 to 160 entanglement points per chain. In the elastic deformation regime defined by ␥ R Ͼ 1, where R is the Rouse relaxation time, ͑i͒ the peak shear stress max scales with the time t max at the peak to Ϫ1/2 power, in contrast to an exponent of Ϫ1/4 in the viscoelastic regime ͑for ␥ R Ͻ 1͒, ͑ii͒ max changes linearly with the elapsed strain at the stress peak ␥ max , which scales with the applied shear rate as ␥ 1/3 , ͑iii͒ a supermaster curve collapses time-dependent shear stress growth curves up to the stress maximum at all shear rates for all the four styrene-butadiene rubber samples.

Finite cohesion due to chain entanglement in polymer melts

Soft matter, 2016

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 exte...