Abstract
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.