Molecular motions in polyisobutylene: A neutron spin-echo and dielectric investigation

We present neutron spin-echo and dielectric results on the local dynamics of polyisobutylene. The dielectric spectra reveal the existence of a so far unknown secondary relaxation process being distinctly different to previous theoretical predictions. Neutron spectra have been taken over a large range in momentum transfer Q and temperature. At the Q value of the first structure factor maximum the dynamic pair correlation function is selective for interchain motions. There the neutron spectra display the same temperature dependence and shape as the classical rheological data taken in the terminal zone and do not follow the temperature laws based on spectroscopic results. A quantitative evaluation combining the information content of the dielectric and neutron results reveals a small stepwidth of 0.5-0.9 Angstrom involved in the secondary process. The alpha process is diffusive and follows the Gaussian approximation resulting in a sublinear time development of associated mean squared displacements.