Gengxin Liu

Letter to the Editor: Sufficiently entangled polymers do show shear strain localization at high enough Weissenberg numbers

This Letter concludes that the recent data of Li et al. [J. Rheol. 57, 1411–1428 (2013)] are entirely consistent with the previous observations of the occurrence and absence of shear banding during startup shear and nonquiescent relaxation after large stepwise shear. In other words, based on the linear viscoelastic characteristics of these solutions depicted in Fig. 5(a) of Li et al., we find their results to follow from the previous analysis: One insufficiently entangled solution naturally exhibited homogeneous shear under the explored conditions. The two more entangled solutions did not exhibit shear banding and nonquiescent relaxation, because the samples appear to have significant polydispersity in the molecular weight distribution and because the applied shear rates were much lower than those needed to produce shear banding. Thus, the observations of Li et al. support rather than refute the existing knowledge concerning nonlinear rheological responses of entangled polymer solutions to startup and stepwise shear.

Failure behavior after stepwise uniaxial extension of entangled polymer melts

This work studies how stepwise extension of various well-entangled polymer melts produce mechanical/structural breakdowns during stress relaxation. Depending on how stepwise extension is imposed on five different styrene-butadiene random copolymers, two different forms of specimen failure are observed. When a step extension is produced with a low Hencky rate or to a low strain below some thresholds, the sample breaks up rather sharply after an appreciable period of induction during which the stress relaxes quiescently. After step extension, the sample draws and undergoes unsustainable necking due to shear yielding, if the step extension is produced with a Hencky rate higher than the Rouse relaxation rate and the magnitude is beyond a Hencky strain of 1.5. Moreover, introduction of long-chain branching suppresses the elastic breakup, postponing it to Hencky strains beyond 2.5. The clearly identifiable characteristics of the elastic yielding may be understood in terms of some speculative interpretations. More convincing explanations have yet to come from future computer experiments that hopefully the present work is able to motivate.