(98ay) Atomistic Simulation of Dynamics of Individual Molecules in Entangled Polymers Undergoing Homogenous Shear Flow

Nonequilibrium molecular dynamics (NEMD) simulations of an entangled linear polyethylene (C₄₀₀H₈₀₂) were performed to study the chain dynamics over a wide range of Weissenberg numbers (Wi). The distribution of the end-to-end vector, |R_ete|, exhibited Gaussian behavior at low Wi. At high Wi the distribution was bimodal with two peaks associated with rotation and stretching of the chains. To understand the underlying physics of the aforementioned distributions, the relevant system time scales were determined. Specifically, the autocorrelation function of the unit end-to-end vector showed a periodic behavior at high Wi which is associated with the onset of rotation of individual chains within a background of highly stretched chains. A functional form of A exp(-t/τ_R)cos(2πt/τ_rot) was fitted to the autocorrelation function data to extract the Rouse (τ_R) and rotational (τ_rot) relaxation times of the system. It is shown that the Rouse time calculated using this method is considerably different than the one calculated using a stretched exponential (as in previous works[1]) for all shear rates. Both the Rouse and rotational relaxation times exhibited a shear-thinning behavior that scaled as Wi ^{- 0.75} at high shear rates. The number and lifetime of entanglements, which is related to the average time of creation and destruction of entanglements, was also computed as a function Wi. This characteristic timescale exhibited a shear-thinning behavior which scaled as Wi ^{- 0.63} at high shear rates. The total number of entanglements also has a shear-thinning behavior at high Wi. The implications of these findings will be discussed in this presentation with regard to convective constraint release and contour fluctuations in reputation theory at high Wi.