Methyl group dynamics in glassy polymers by neutron scattering: from classical to quantum motions

Abstract

Recent experimental neutron scattering results have stimulated a new revival of the interest on methyl group dynamics in glasses and polymer systems. The existence of quantum rotational tunnelling of methyl groups in polymers was expected for a long time. Only very recently, these processes have been directly observed by high-resolution neutron scattering techniques (IN16, ILL Grenoble). The goal of this paper is to revise and summarise the work done in these topics over the last few years by means of neutron scattering. It will be shown that the results obtained, for both quantum and classical regimes, in different polymers can be consistently described in terms of a model which assumes a pure single-particle threefold potential for methyl group dynamics together with a distribution of potential barriers. The microscopic origin of such distributions, which should be associated to the disorder present in any structural glass, can be investigated by molecular dynamics simulations in realistic polymers. It is concluded that the width of the distribution is mainly controlled by the non-bonded interactions.