Neutron spin — echo investigations on the dynamics of polymer systems

Abstract

Neutron spin-— echo spectroscopy allows the observation of the long-range internal relaxation mechanism of macromolecules simultaneously in space and time. Thereby, it facilitates a microscopic study of molecular models applied for the explanation of macroscopic viscoelastic properties of polymer materials. After an outline of the method, experimental results on chain relaxation in polymer melts are discussed. It is shown that in the short-time regime, the Rouse model describes perfectly well the space and time dependence of the self- and the pair-correlation function. For longer times distinct deviations from the Rouse model towards slower relaxation are observed. These deviations exhibit a systematic dependence on the momentum transfer, Q, and thus show that beyond a certain length scale the relaxaation of density fluctuations within a given chain is strongly reduced. This intermediate dynamic length scale shows direct evidence of the existence of a well-defined entanglement distance in polymer melts, confirming thereby the essential assumption of areptation concept. Its value is found to be in excellent agreement with the entanglement distance obtained from the rheological measurement.