(500b) Dynamics of Individual Chains In Linear Polyethylene Liquids Under Shear

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.