  Entangled polymers are deformed by a strong shear flow. The shape of the
polymer, called the form factor, is measured by small angle neutron scattering.
However, the real-space molecular structure is not directly available from the
reciprocal-space data, due to the phase problem. Instead, the data has to be
fitted with a theoretical model of the molecule. We approximate the unknown
structure using piecewise straight segments, from which we derive an analytical
form factor. We fit it to our data on a semi-dilute entangled polystyrene
solution under in situ shear flow. The character of the deformation is shown to
lie between that of a single ideal chain (viscous) and a cross-linked network
(elastic rubber). Furthermore, we use the fitted structure to estimate the
mechanical stress, and find a fairly good agreement with rheology literature.
