Understanding how the molecular conformation of polymer molecules change with uniaxial extension, relaxation, and molecular architecture as well as blend dispersity is relevant both from an application and fundamental science point of view: appropriate processing conditions depend strongly on the molecular conformation and detailed information about molecular conformations may provide tests for polymer models and theories for the molecular interactions in the melt. In this thesis we show three examples of how the combination of controlled non-linear uniaxial extension and controlled relaxation following extension in combination with scattering techniques can provide deep insight to the conformation of polymer molecules during flow and relaxation. We study the relaxation of a mono-disperse melt of relatively short, but entangled, linear polystyrene using small-angle neutron scattering and show that the recently published framework of spherical harmonics expansion is sensitive enough to chain length changes during relaxation to resolve chain retraction as proposed by Doi and Edwards even for short molecules. We also study the relaxation of local orientation probed by wide-angle X-ray scattering in a bi-disperse polystyrene melt relative to that in the pure melt of the short component and find that the local orientation in the blend is larger and that the local orientation relaxes as a power law with the same exponent in both melts. Finally, we study end-deuterated three-armed polystyrene stars in small-angle neutron scattering to test the hypothesis that branched polymers take a quasi-linear molecular conformation during fast extensional flow and that the quasi-linear conformation last well into the relaxation. We find that at least the scattering patterns corresponding to short relaxation times are consistent with a quasi-linear conformation.