(50c) Comparison of Individual Chain Dynamics of a Short-Chain Polyethylene Dense Liquid with Equivalent Free-Draining Dilute Solution Using An Atomistic and Mesoscopic Level Approach

Observing the motion of individual molecules
in polymeric liquids and comparing macroscopic properties from different levels
of description allows for the development of more accurate rheological and
topological models of the dynamics of these materials. In this study, the
motions of single chains in the dense liquid (C₇₈H₁₅₈)
and equivalent free-draining dilute solution of bead-rod chains are
investigated and also various properties of both systems are examined. The
mean-squared, end-to-end vector of the dense liquid increases as Weissenberg (Wei)
number increases and obtains maximum value at high Wei. It shows similar
behavior and especially attains maximum value at the same value of Wei
in dilute solution. We proposed that rotation or tumbling of single chains
plays an important role and explains maximum at high Wei in these two
different systems. The probability distribution of magnitude of end-to-end
vector in both systems has Gaussian behavior at low Wei; however, it is
non-Gaussian at high Wei in both liquids, with a decidedly bimodal
character in the dense liquid.  The probability distribution of magnitude
of end-to-end vector in the dense liquid displays two peaks which are
associated with rotation and stretching of the individual chains while a very wide
distribution with a single long-extension peak is exhibited by the dilute
solution. Based on time trajectories of individual chains for magnitude of
end-to-end vector and the individual chain orientation angles, the
quasi-periodic rotation or tumbling motion of the individual chains in each liquid
is different. In dilute solution, chains retract and almost collapse to tightly
coiled configurations during the tumbling cycle. On the other hand, they rotate
in hairpin configurations in the dense liquid.  We observed that the
characteristic timescales, calculated from auto- and cross-correlations between
components of end-to-end vector, have qualitatively similar behavior in both liquids
and decreased as functions of Wei.  The characteristic frequencies
of molecular rotation of the dense liquid changed from Wei^-0.75
to Wei^-0.66 after Wei = 2 in the dense liquid. This is
caused by a decrease in elastic response of system due to relaxation of the rotational
correlation. Similarly, characteristic frequencies reveal similar behavior (Wei^-0.68
and Wei^-0.66) in dilute solution. Other properties, such as
probability distribution of representative chain configuration classes,
positive and negative chain orientations, and so on, are also compared to
explore similarities and differences between these two systems.