  We investigate with numerical simulations the molecular origin of viscosity
in melts of flexible and semiflexible oligomer rings in comparison to
corresponding systems with linear chains. The strong increase of viscosity with
ring stiffness is linked to the formation of entangled clusters, which dissolve
under shear. This shear-induced breakup and alignment of rings in the flow
direction lead to pronounced shear-thinning and non-Newtonian behavior. In
melts of linear chains, the viscosity can be associated with the (average)
number of entanglements between chains, which also dissolve under shear. While
blends of flexible and semiflexible rings are mixed at rest, the two species
separate under flow. This phenomenon has potential applications in microfluidic
devices to segregate ring polymers of similar mass and chemical composition by
their bending rigidity.
