  We use computer simulations to study the relaxation of strongly deformed
highly entangled polymer melts in the non-linear viscoelastic regime, focusing
on anisotropic chain conformations after isochoric elongation. The Doi-Edwards
tube model and its GLaMM extension, incorporating contour length fluctuation
and convective constraint release, predict a retraction of the polymer chain
extension in all directions, setting in immediately after deformation. This
prediction has been challenged by experiment, simulation, and other theoretical
studies, questioning the general validity of the tube concept. For very long
chains we observe the initial contraction of the chain extension parallel and
perpendicular to the stretching direction. However, the effect is significantly
weaker than predicted by the GLaMM model. We also show that the first
anisotropic term of an expansion of the 2D scattering function qualitatively
agrees to predictions of the GLaMM model, providing an option for direct
experimental tests.
