470577 Atomistic Simulation of Dynamics of Individual Molecules in Entangled Polymers Undergoing Homogenous Shear Flow
Nonequilibrium molecular dynamics (NEMD) simulations of entangled linear C400H802 and C700H1402 polyethylene melts were performed to investigate the chain dynamics over a wide range of shear rates . The rheological and dynamical responses of these liquids can be classified roughly according to three shear rate regimes; namely, , , and , where τd and τe are the disengagement and entanglement time of the liquid. Under quiescent conditions, the liquid dynamics are described well by the reptation theory of Doi and Edwards. Over the intermediate shear rate range, shear-induced rotational motion of the individual chain molecules became a dominant relaxation mechanism as the number of chain entanglements decreased dramatically with increasing , ultimately resulting in a plateau in the shear stress profile. A new timescale became evident that was associated with the period of the rotation/retraction cycles of the individual molecules. In the third region, the rotational motion of the chains became the sole relaxation mode of the system as the number of entanglements was reduced to a level too low to support conventional reptation theory. The decorrelation (τd) and rotational (τrot) relaxation times of the system were extracted by fitting a functional form of a exp(-t/τd)cos(2πt/τrot) to the end-to-end vector autocorrelation function data. Both the decorrelation and rotational characteristic times exhibited a shear-thinning behavior that scaled as Wi - 0.76 at high shear rates, regardless of the chain length. Wi is the dimensionless shear rate defined as . The number of entanglements was also computed as a function Wi. It is shown that this characteristic timescale has a shear-thinning behavior which scaled as Wi - 0.35 at high shear rates for both liquids. The implications of these findings will be presented with regard to convective constraint release, Chain orientation, and chain stretch over a wide range of shear rates.
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