Direct microscopic observation of the entanglement distance in a polymer melt

Using neutron spin-echo spectroscopy and rheometry, we investigated the melt dynamics of poly(ethylene-propylene) alternating copolymers. The experiments show that beyond a certain length scale the relaxation of density fluctuations within a given chain is strongly reduced. The presence of this intermediate dynamic length scale implies the existence of a well-defined entanglement distance confirming the essential assumption of the reptation concept. Its value is found to be in excellent agreement with the entanglement distance obtained from the rheological measurement.