  The rheology of molecular brushes remains challenging to control due to the
multiple length scales and relaxation processes involved and the lack of direct
observation of molecular conformation during flow. We use molecular dynamics
simulations to determine the shear thinning of unentangled bottlebrush polymers
with varying architecture, from linear chains to combs, to densely grafted
bottlebrushes, to star-like and star polymers. We find shear thinning exponents
in line with theoretical and experimental results and characterize the shape
and orientation of bottlebrushes in steady-state flow. Many shape parameters
derived from the gyration tensor show molecular alignment with the flow for all
systems. Yet, the orientation of individual bonds is what most strongly
correlates with the architecture-dependent shear-thinning exponents. In densely
grafted bottlebrushes, the packing of side chains prevents alignment with the
flow, causing a reduction in shear thinning. The molecular insight from our
simulations is useful to tune the architecture of bottlebrushes to control
their rheology.
