  We investigate the molecular origin of shear-thinning in melts of flexible,
semiflexible and rigid oligomers with coarse-grained simulations of a sheared
melt. Alignment, stretching and tumbling modes or suppression of the latter all
contribute to understanding how macroscopic flow properties emerge from the
molecular level. By performing simulations of single chains in a shear flow, we
identify which of these phenomena are of collective nature and arise through
interchain interactions and which are already present in dilute systems.
Building upon these microscopic simulations we identify by means of the
Irving-Kirkwood formula the corresponding macroscopic stress tensor for a
non-Newtonian polymer fluid. Shear-thinning effects in oligomer melts are also
demonstrated by macroscopic simulations of a channel flow. The latter have been
obtained by the discontinuous Galerkin method approximating macroscopic polymer
flows. Our study confirms the influence of microscopic details in the molecular
structure of short polymers such as chain flexibility on macroscopic polymer
flows.
