Neutron scattering on partially deuterated polybutadiene

The molecular nature of the secondary relaxation (Johari-Goldstein relaxation) and its relationship with the a relaxation is in most cases still unknown. In order to access these processes on a molecular level, it is necessary to obtain spatial information of the relaxation. Through the momentum-transfer dependence of the dynamic structure factor S(Q, t), this information can be provided by quasielastic neutron scattering techniques. The large difference in scattering lengths between hydrogen and deuterium allows us to accentuate specific correlations between atoms in a polymer melt. Here, we report on recent results on a polybutadiene melt, where the double bond was hydrogeneous, while the methylene groups carried deuterons (d4h2-PB). In this way the correlations between the double bonds are emphasised. We will show that the double bond/double bond correlation function, generated in this way, shows the same temperature dependence as the viscosity at higher temperatures at the structure factor peak maximum, but at lower temperatures the spectra decay faster than expected from the viscosity time scale. However, the fully deuterated sample (d6-PB) has shown that the intermediate scattering function at the structure factor peak maximum shows the same temperature dependence over the complete temperature range. This indicates the prevalence of a different process observed in the d4h2-PB sample.