Neutron spin echo investigations on the dynamics of dense polymer systems

Abstract

We discuss experimental results on molecular motion in polymer melts, obtained by neutron spin echo spectroscopy (NSE). We show that in the short time regime the assumption of entropic restoring forces (Rouse model) describes well the space and time dependence of the pair correlation function of one chain molecule. For longer times we observe systematic deviations from the Rouse model, revealing the presence of a well defined intermediate dynamical length scale beyond which density fluctuations within a given chain are strongly reduced. Its value is found to be in excellent agreement with the entanglement distance obtained from rheological measurements. Measurements of the temperature dependence and polymer volume fraction dependence of the entanglement distance give insight into the molecular origin of entanglement constraints.