NEUTRON-SCATTERING EXPERIMENTS ON THE GLASS-TRANSITION OF POLYMERS

Neutron scattering experiments have been performed to explore the dynamics of polymers near the glass transition. The experimental data show three distinct types of relaxations: (1) Susceptibility spectra exhibit a contribution in addition to phonons around a temperature independent frequency. (2) Neutron-spin-echo (NSE) experiments show a slow relaxation of the stretched exponential type. Above 220 K, the characteristic time of this process strictly follows the Vogel-Fulcher dependence of viscosity data indicating the direct connection between microscopic and macroscopic relaxation. (3) Below 220 K, the temperature dependence of the relaxation observed by NSE changes to an Arrhenius form while the viscosity still follows the Vogel-Fulcher law. Analysis of the Q dependence from IN13 backscattering data reveals a change of mechanism at the crossover. The experimental findings will be discussed emphasizing the comparison with the mode coupling theory. Aspects of agreement as well as deviations will be pointed out in the discussion.