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29 March 2024
 
  » arxiv » cond-mat/0203207

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Dynamics and Rheology of a Supercooled Polymer Melt in Shear Flow
Ryoichi Yamamoto ; Akira Onuki ;
Date 9 Mar 2002
Subject Soft Condensed Matter; Materials Science | cond-mat.soft cond-mat.mtrl-sci
AbstractUsing molecular dynamics simulations, we study dynamics of a model polymer melt composed of short chains with bead number N=10 in supercooled states. In quiescent conditions, the stress relaxation function $G(t)$ is calculated, which exhibits a stretched exponential relaxation on the time scale of the $alpha$ relaxation time $ au_alpha$ and ultimately follows the Rouse dynamics characterized by the time $ au_{ m R} sim N^2 au_alpha$. After application of shear $gdot$, transient stress growth $sigma_{xy}(t)/gdot$ first obeys the linear growth $int_0^t dt’G(t’)$ for strain less than 0.1 but saturates into a non-Newtonian viscosity for larger strain. In steady states, shear-thinning and elongation of chains into ellipsoidal shapes take place for shear $gdot$ larger than $ au_{ m R}^{-1}$. In such strong shear, we find that the chains undergo random tumbling motion taking stretched and compact shapes alternatively. We examine the validity of the stress-optical relation between the anisotropic parts of the stress tensor and the dielectric tensor, which are violated in transient states due to the presence of a large glassy component of the stress. We furthermore introduce time-correlation functions in shear to calculate the shear-dependent relaxation times, $ au_alpha (T,gdot)$ and $ au_{ m R} (T,gdot)$, which decrease nonlinearly as functions of $gdot$ in the shear-thinning regime.
Source arXiv, cond-mat/0203207
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