(178t) Mechanistic Dynamics of Single Chains in Dense Liquids Under Shear Flow

Many models have been developed to describe
the bulk-averaged rheological and morphological properties of polymeric
liquids. However they are still inadequate to describe many material properties
at high strain rates. By tracking and comparing the motions of individual
molecules as functions of strain rate, new ideas might emerge which will allow
for the extension of existing models to relevant phenomena at atomic length and
timescales.  In this study, we focused on the mechanistic dynamics of
chains over a wide range of shear rates.  In the process of explaining the
maximum value in the mean-squared, end-to-end vector vs. Weissenberg number (Wei)
curve, we hypothesized that substantial rotational motion of individual chains
at high Wei effectively lowers the magnitude of mean-squared, end-to-end
vector:  as the chains tumble through periodic cycles, they adopt
hairpin-like configurations in which the chain ends pass very close to each
other. This hypothesis also explains the disparate behavior of the probability
distribution of the magnitude of end-to-end vector between low Wei and
high Wei. At low Wei, the probability distribution displays a
Gaussian behavior that is associated with the pre-averaged rheological
theories. At high Wei, however, it is divided into two peaks, which are related
to rotation and stretching of chains, respectively. The magnitude of end-to-end
vector and orientation angle vs. time at high Wei depicts the details of
the rotational or tumbling motion of individual chains and these two properties
are correlated. We saw that the magnitude of end-to-end vector changed from a nearly
fully-extended chain to tightly-coiled structure as functions of time, and that
chain ends pass each other very closely during the tumbling cycle. From
calculation of the time correlation of the components of end-to-end vector and
its Fourier transformation, we extracted multiple timescales associated with dynamics
of chain motion:  these are associated with the frequency of rotation and
the decay of the rotational correlation.  These timescales will be very
important factor to develop new model and to understand the response of complex
polymeric liquids in the future.