Anisotropy in the form factor of entangled polymer solutions under shear

The structure of entangled polymers is perturbed by strong shear, resulting in a non-linear response. However, practical limitations make it difficult to probe the molecular structure far from equilibrium, and its understanding is currently incomplete. Here we report \emph{in situ} measurements of the form factor of a semi-dilute polymer solution under a steady shear flow, as revealed by small angle neutron scattering. A simple analytical formula is derived with only four parameters, giving an excellent fit to the anisotropic 2D scattering pattern in the flow-vorticity plane. There emerges a length scale below which the polymer is not perturbed by shear, while above it there is elongation along the flow and contraction perpendicular to it. Similar patterns have earlier been reported for polymer melts, which now amounts to two greatly different dynamical systems producing the same time-averaged form factor.