title: Finite cohesion due to chain entanglement in polymer melts.
abstract: 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 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 τd that is over 100 times the Rouse time τR. Our results point to the existence of a cohesion barrier that can prevent free chain retraction upon moderate deformation in well-entangled polymer melts.
authors.affiliation: Department of Polymer Science, University of Akron, Akron, Ohio 44325-3909, USA. swang@uakron.edu.
authors.forename: Shiwang
authors.initials: S
authors.lastname: Cheng
authors.forename: Yuyuan
authors.initials: Y
authors.lastname: Lu
authors.forename: Gengxin
authors.initials: G
authors.lastname: Liu
authors.forename: Shi-Qing
authors.initials: SQ
authors.lastname: Wang