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.