Change of the vibrational dynamics near the glass transition in polyisobutylene: Inelastic neutron scattering on a nonfragile polymer

Incoherent-inelastic-neutron-scattering studies were carried out on polyisobutylene, presumably the least-fragile polymeric glass former known so far. A fast dynamical process on a frequency scale of roughly 500 GHz is measured for temperatures higher than the glass transition essentially without being influenced by the {α} relaxation. The present data show that this fast dynamical process can be inelastic and thus has no relaxational, i.e., quasielastic character. This strongly underlines the importance of changes in the vibrational behavior of glasses near the glass-transition temperature. We propose a librational model relating steric hindrance with the observed inelastic scattering.