4.6 Article

Finite cohesion due to chain entanglement in polymer melts

Journal

SOFT MATTER
Volume 12, Issue 14, Pages 3340-3351

Publisher

ROYAL SOC CHEMISTRY
DOI: 10.1039/c6sm00142d

Keywords

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Funding

  1. NSF [DMR-1105135]
  2. National Natural Science Foundation of China [21120102037, 21334007, 21304097]

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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 gamma = 0.7 and step-wise extension up to the stretching ratio lambda = 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 tau(d) that is over 100 times the Rouse time tau(R). Our results point to the existence of a cohesion barrier that can prevent free chain retraction upon moderate deformation in well-entangled polymer melts.

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