A. Wischnewski, M. Monkenbusch, D. Richter

The salient features of the motions of chains in high molecular weight polymer melts are well described by the famous Reptation model of DeGennes. The central ingredient of this model is a contorted virtual tube ( of diameter d) around the contour of each polymer chain. The diffusion of chain segments feels only little influence of this confining tube for times too short for the mean squared displacements (r**2(t)) to reach its "walls". In that time regime the chain dynamics feels the environment only through a simple local friction -segmental diffusion follows the standard ROUSE behaviour. However, when (r**2(t = Te)) becomes comparable to d the tube constraints become active and the segmental diffusion slows down. This slowing down at a crossover time Te would be directly observable if the mean squared displacements of some (10 A)**2 could be measured over a time range of several nanoseconds. For the first time it has been possible to measure this effect on (r**2(t)) directly by analyzing the incoherent scattering from hydrogen in long-chain polyethylene segments in the proper range of momentum transfer and time using the Jülich neutron spinecho spectrometer.