Bulletin of the American Physical Society
2023 APS March Meeting
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session G23: Nonequilibrium Structures and Dynamics of Polymeric Materials II
11:30 AM–2:30 PM,
Tuesday, March 7, 2023
Room: Room 215
Sponsoring
Unit:
DPOLY
Chair: Tad Koga, Stony Brook University
Abstract: G23.00007 : Nonlinear Rheology of Unentangled Polymer Melts
12:42 PM–1:18 PM
Presenter:
Hiroshi Watanabe
(Kyoto Univ)
Authors:
Hiroshi Watanabe
(Kyoto Univ)
Yumi Matsumiya
(Kyoto Univ.)
Takeshi Sato
(Kyoto Univ.)
Obviously, the ζ-reduction is a local phenomenon occurring in a length scale well below the entanglement length. Thus, we have investigated details of ζ-reduction and other non-equilibrium features for unentangled melts being free from any nonlinearity of entanglement dynamics. Specifically, the extensional viscosity ηE of an unentangled polystyrene melt was found to exhibit hardening at the Weissenberg number Wi ~ 0.5 and then softening at Wi > 1. The standard Rouse model and its modification for the finite extensible nonlinear elasticity (FENE) cannot reproduce this hardening-softening crossover. However, the model with FENE and ζ-reduction can describe the data given that ζ is chosen to properly decrease with increasing stress. This result demonstrates importance of FENE and ζ-reduction in the non-equilibrium dynamics of chains in unentangled melt (and in entangled melt, too). An additional feature is found for the viscosity η and the first normal stress difference coefficient Ψ1 under fast shear. The Rouse analysis with FENE and ζ-reduction indicates that a factor (η/η0)2 should agree with the Ψ1/ Ψ1,0 ratio if ζ and the mean-square Brownian force intensity B satisfy the fluctuation-dissipation theorem, B = 2ζkBT. However, the unentangled PS melt does not show this agreement, and thus the non-equilibrium behavior emerges also for B. Further information obtained from rheo-dielectric experiments will be presented on site.
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