Deviation from Gaussian behavior in the self-correlation function of the proton motion in polybutadiene

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