The flow of viscoelastic materials is usually interpreted as resulting from intramolecular properties. Typically, the non‐linear flow behaviour and sluggish relaxation dynamics in entangled polymers are interpreted by a disentanglement process. This molecular interpretation has never been validated by direct observation. We report here on in situ observations of polymer melts under steady‐state shear flow using neutron scattering and particle tracking velocimetry. It is shown that the chains remain largely undeformed under steady‐state shear flow whereas wall slippage and shear‐banding are identified in both entangled and unentangled polymer melts. These observations are of prime importance; they reveal that the flow mechanism and its viscoelastic signature reflect a collective effect and not properties of individual chains.
