- 12:55 PM

Dynamics of Individual Chains In Linear Polyethylene Liquids Under Shear

Jun Mo Kim, David Keffer, Bamin Khomami, and Brian Edwards. Chemical and Biomolecular Engineering, The University of Tennessee, 419 Dougherty Hall, Knoxville, TN 37996-2200

Many previous studies investigated dynamics of polymer chains under shear. In this work, we examined dynamics of individual polymer chains in linear polyethylene liquids under shear using nonequilibrium molecular dynamics (NEMD) simulation and Brownian dynamics (BD) simulation.  As expected, the probability distribution function of the end-to-end vector's magnitude follows Gaussian behavior at low shear rate. As the shear rate increases to intermediate values, individual polymer chains continue to be stretched on average, but also begin to rotate with the local fluid kinematics. Therefore, the end-to-end vector distribution begins to exhibit non-Gaussian behavior. Indeed, we observed a bimodal probability distribution function at intermediate and high values of the shear rate. The first peak of the distribution as low magnitudes of the end-to-end vector is associated with the rotational motion of the chains, and the peak at higher values of the end-to-end vector corresponds to the degree of chain elongation.

We also calculated time auto- and cross-correlation functions of each component of end-to-end vector with respect to itself and the other components, and extracted multiple time scales of the chain dynamics, associated with various physical mechanisms including the rotational motion of the chains. These time scales vary strongly with shear rate.

We compared NEMD simulation results with BD simulation results mapping atomistic configurations to the bead-rod model. Each mapped atomistic chain was also classified 'Stretched, Dumbbell, Half Dumbbell, Kink, Fold, Coil' according to its configuration and compared with BD results.  Furthermore, we calculated the time that each chain spends in the four quadrants of its coordinate system, and found that an individual chain spends more time with positive orientation with respect to the direction of flow as the shear rate increases. For example, the chains spent 66% of the time at a positive orientation at the highest shear rate examined, while only 55% of the time at the lowest value of shear rate examined.

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