Dynamics and Rheology of a Supercooled Polymer Melt in Shear Flow

Abstract

Using 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 ${\tau }_{\alpha }$ and ultimately follows the Rouse dynamics characterized by the time ${\tau }_{\mathrm{R}}\sim N^2{\tau }_{\alpha }$. After application of shear $\gdot$, transient stress growth $σ_{xy}(t)/\gdot$ first obeys the linear growth ${\int }_0^{t}d{t}^{\prime }G(t^{\prime })$ 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 ${\tau }_{\mathrm{R}}^{-1}$. 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, $τ_α(T,\gdot)$ and $τ_{\rm R} (T,\gdot)$, which decrease nonlinearly as functions of $\gdot$ in the shear-thinning regime.

14 pages, 14 figures

References in corpus (1)

• Nonequilibrium dynamics and fluctuation-dissipation relation in a sheared fluid

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