Intermediate Length Scale Dynamics in Polymer Melts

Abstract

We report on neutron spin echo investigations of the intermediate scale dynamics of polyisobutylene (PIB) studying both the self and the collective motions. The momentum transfer dependencies of the self correlation times are found to follow a Q−2/β law in agreement with the picture of Gaussian dynamics. In the full Q range of observation their temperature dependence is weaker than that of the rheological shift factor. Studying the single chain dynamic structure factor, deviations from the universal Rouse dynamics become evident at intermediate length scales. In solutions they can be directly related to intrachain viscosity effects arising from the intrachain rotational potential. Invoking a similar mechanism in the melt leads to a consistent description of the self and the single chain correlation functions suggesting that the weaker temperature dependence relates to configurational dynamics. The same is true for the stress relaxation times as seen in sound wave damping. The collective times show both temperature dependencies. At the structure factor peak they follow the temperature dependence of the viscosity but below the peak one finds the stress relaxation behaviour.