Inelastic fast relaxation in a weakly fragile polymer glass near Tg

Incoherent inelastic neutron scattering experiments on the weakly fragile polymer glass polyisobutylene (PIB) show a fast inelastic relaxation component at temperatures exceeding the glass transition temperature, Tg = 200 K. The temperature dependence of the inelastic excess excitation is similar to that of the quasielastic relaxation observed usually in other glass formers near Tg. We are investigating the elastic and inelastic form factors for Q values up to 6 Å-1 and for the temperature range Tg - 200 K < T < Tg + 100 K. We interpret our observations as being due to changes of the torsional librations of the polymer backbone. Furthermore we calculate the hydrogen weighted vibrational density of states up to 100 meV. In the low frequency range we find an intensity contribution which does not follow a Q2 dependence.