  Polymer architecture plays critical roles in both bulk rheological properties
and microscale macromolecular dynamics in entangled polymer solutions and
composites. Ring polymers, in particular, have been the topic of much debate
due to the inability of the celebrated reptation model to capture their
observed dynamics. Macrorheology and differential dynamic microscopy (DDM) are
powerful methods to determine entangled polymer dynamics across scales, yet
they typically require different samples under different conditions, preventing
direct coupling of bulk rheological properties to the underlying macromolecular
dynamics. Here, we perform macrorheology on composites of highly-overlapping
DNA and dextran polymers, focusing on the role of DNA topology (rings versus
linear chains) as well as the relative volume fractions of DNA and dextran. On
the same samples under the same conditions, we perform DDM and single-molecule
tracking on embedded fluorescent-labeled DNA molecules immediately before and
after bulk measurements. We show DNA-dextran composites exhibit unexpected
non-monotonic dependences of bulk viscoelasticity and molecular-level transport
properties on the fraction of DNA comprising the composites, with
characteristics that are strongly dependent on the DNA topology. We rationalize
our results as arising from stretching and bundling of linear DNA versus
compaction, swelling, and threading of rings driven by dextran-mediated
depletion interactions.
