We present neutron spin-echo and dielectric results on the local dynam
ics of polyisobutylene. The dielectric spectra reveal the existence of
a so far unknown secondary relaxation process being distinctly differ
ent to previous theoretical predictions. Neutron spectra have been tak
en over a large range in momentum transfer Q and temperature. At the Q
value of the first structure factor maximum the dynamic pair correlat
ion function is selective for interchain motions. There the neutron sp
ectra display the same temperature dependence and shape as the classic
al rheological data taken in the terminal zone and do not follow the t
emperature laws based on spectroscopic results. A quantitative evaluat
ion combining the information content of the dielectric and neutron re
sults reveals a small stepwidth of 0.5-0.9 Angstrom involved in the se
condary process. The alpha process is diffusive and follows the Gaussi
an approximation resulting in a sublinear time development of associat
ed mean squared displacements.