DEVIATION FROM GAUSSIAN BEHAVIOR IN THE SELF-CORRELATION FUNCTION OF THE PROTON MOTION IN POLYBUTADIENE

In this article neutron-scattering experiments using the backscatterin g instrument IN13 on the glass-forming polymer polybutadiene are repor ted. As compared to other neutron spectrometers IN13 has a large range of wave vectors Q accessible. Therefore, a better statement can be ma de about the spatial extent of the observed motions. In order to obtai n reliable results for the Q dependence a multiple scattering correcti on of the intensities has been performed. The Q dependence of the inco herent scattering from polybutadiene shows strong deviations from the Gaussian approximation at high Q. These deviations are interpreted und er different microscopical model assumptions. Finally, a picture of a distribution of anharmonic potentials can give account for the observe d deviation from Gaussian behavior in the incoherent intermediate scat tering function at low temperatures. At higher temperatures the fast m otion in these potentials is superimposed by a long-range diffusional motion. The latter may be extracted assuming a model for the fast moti on. It shows certain characteristics of fractal diffusion but defies a simple scaling approach.