Inelastic neutron scattering experiments on polyurethane: microscopic origin of the beta relaxational process

We present an experimental investigation of the relaxational processes rela ted to the glass transition in a family of polymers where the molecular arc hitecture can be varied. The samples are crosslinked polyurethanes where th e crosslink density, as well as the length between crosslinks, can be modif ied. Inelastic neutron scattering was used to determine the relaxation time s in the 10(-8) 10(-12) s range. The times measured by this technique split from the structural relaxation determined at lower frequencies. Inelastic neutron scattering is sensitive to a secondary relaxation process, which is in the continuity of the beta process probed by dielectric spectroscopy. M oreover, the dependence on the microscopic parameters of this process sugge sts that the corresponding relaxing entity can be identified as a small por tion of the arms of the triol, in agreement with the explanation suggested independently for the relaxational process investigated by dielectric measu rements.