Neutron scattering and the glass transition in polymers - present status and future opportunities

We present results obtained by the exploration of molecular motions by neut ron scattering and outline future opportunities resulting from methodologic al progress in high-resolution neutron spectroscopy. Starting on methododic al grounds, we will commence with possible future developments in neutron s pin echo spectroscopy (NSE), the prime technique for the observation of mol ecular processes in the nanosecond domain. After describing the scope of ne utron scattering for experiments on glass-forming materials, we will presen t measurements of the self-correlation function, which include the beta -re laxation in polyisobutylene, and address the problem of the heterogeneity o f the alpha -process. Thereafter, we deal with different aspects of the pai r correlation function, starting from coherent scattering experiments on po lybutadiene, where the choice of the momentum transfer is selective for the observation of the alpha- or beta -relaxation. Then, we emphasize results on the structural relaxation obtained for the different polymers at the fir st structure factor peak. Finally, by example, we will address the capabili ty of neutron scattering, to deal with motions in polymer blends, where by judicious labeling the dynamics of the different components can be observed separately. (C) 2001 Published by Elsevier Science B.V.