  Ring polymers exhibit unique flow properties due to their closed chain
topology. Despite recent progress, we have not yet achieved a full
understanding of the nonequilibrium flow behavior of rings in nondilute
solutions where intermolecular interactions greatly influence chain dynamics.
In this work, we directly observe the dynamics of DNA rings in semidilute
ring-linear polymer blends using single molecule techniques. We systematically
investigate ring polymer relaxation dynamics from high extension and transient
and steady-state stretching dynamics in planar extensional flow for a series of
ring-linear blends with varying ring fraction. Our results show multiple
molecular sub-populations for ring relaxation in ring-linear blends, as well as
large conformational fluctuations for rings in steady extensional flow, even
long after the initial transient stretching process has subsided. We further
quantify the magnitude and characteristic timescales of ring conformational
fluctuations as a function of blend composition. Interestingly, we find that
the magnitude of ring conformational fluctuations follows a non-monotonic
response with increasing ring fraction, first increasing at low ring fraction
and then substantially decreasing at large ring fraction in ring-linear blends.
A unique set of ring polymer conformations are observed during the transient
stretching process, which highlights the prevalence of molecular individualism
and supports the notion of complex intermolecular interactions in ring-linear
polymer blends. Together with results from molecular simulations, our results
suggest that ring conformational fluctuations arise due to ring-linear
threading and intermolecular hydrodynamic interactions (HI). Taken together,
our results provide a new molecular understanding of ring polymer dynamics in
ring-linear blends in nonequilibrium flow.
