Cross-link density influence on the relaxations in glass- and gel-forming polyurethanes by neutron and Brillouin scattering

An experimental investigation of the relaxational processes related to the glass transition is presented in a family of polymers where the cross-link density as well as the length between cross links can be varied. We used inelastic light and neutron scattering to determine the relaxation times in the ${10}^{-8}–{10}^{-12}$ s range. The relaxation time determined from neutron scattering exhibits a $Q^{-n}$ dependence, with $n\simeq 4$, and is more sensitive to variations of the cross-link density than Brillouin scattering is. However, the relaxation times measured by both techniques split from the structural relaxation determined at lower frequencies; they have a nearly Arrhenius behavior with approximately the same activation energy. The two high-frequency techniques probe the same secondary relaxation process. The dependence on the microscopical parameters of this process suggests that the corresponding relaxing entity can be identified as a small portion of the arms of the triol.