CROSS-LINK DENSITY INFLUENCE ON THE RELAXATIONS IN GLASS-FORMING 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 c ross-link density as well as the length between cross links can be var ied. We used inelastic light and neutron scattering to determine the r elaxation times in the 10(-8)-10(-12) s range. The relaxation time det ermined from neutron scattering exhibits a Q(-n) dependence, with n si milar or equal to 4, and is more sensitive to variations of the cross- link density than Brillouin scattering is. However, the relaxation tim es measured by both techniques split from the structural relaxation de termined at lower frequencies; they have a nearly Arrhenius behavior w ith approximately the same activation energy. The two high-frequency t echniques probe the same secondary relaxation process. The dependence on the microscopical parameters of this process suggests that the corr esponding relaxing entity can be identified as a small portion of the arms of the triol.