Onset of topological constraints in polymer melts: A mode analysis by neutron spin echo spectroscopy

By neutron spin echo spectroscopy we have investigated the dynamic structure factors for single chain relaxation in polymer melts varying the molecular weight through the transition from unrestricted Rouse motion to entanglement controlled behavior. From an analysis of the structure factors with respect to the different relaxation modes we found that, depending on their spatial extension in relation to the entanglement length, large-scale relaxations are successively suppressed with increasing molecular weight. A comparison with macroscopic diffusion and viscosity data yields excellent internal consistency.