Search

We present an experimental investigation of the relaxational processes related to the glass transition in a family of polymers where the molecular architecture can be varied. The samples are crosslinked polyurethanes where the crosslink density, as well as the length between crosslinks, can be modified. Inelastic neutron scattering was used to determine the relaxation times in the 10−8-10−12s 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 β process probed by dielectric spectroscopy. Moreover, the dependence on the microscopic parameters of this process suggests that the corresponding relaxing entity can be identified as a smali portion of the arms of the triol, in agreement with the explanation suggested independently for the relaxational process investigated by dielectric measurements.

We present the results of inelastic light scattering measurements (Brillouin and low-frequency Raman scattering) and neutron scattering in two oxide glasses (one phosphosilicate and one heavy-metal oxide) and in polyurethane samples in which the number of cross-links has been varied. These measurements have been performed across the glass transition in solid and undercooled liquid phases. The results showed vibrational and relaxational contributions arising in the so-called boson peak and in the quasielastic contributions respectively. The softening of the vibrational contribution with increasing temperature must be taken into account in order to extract the relaxational contribution. The correlations between the position of the boson peak measured by Raman or neutron scattering and the hypersound velocity measured by Brillouin scattering on the one hand and between the quasielastic intensity and the hypersonic attenuation on the other hand are seen in the oxide glasses. In polyurethane the relax...

In the low-frequency regime, the Raman spectrum of most glasses is dominated by a strong broad line which is usually called the Boson peak. The physical origin of this peak is still a matter for discussion. Several attempts have been made in order to connect the Boson peak to the vibrational density of states. Various interpretations have been proposed in recent years, in which the Boson peak is related to structural characteristics. In order to investigate the relationship between the vibrational dynamics and the medium-range order in glasses, we have studied the Brillouin and Raman scattering of several samples of oxide glasses prepared and stabilized under very different conditions. A study of the first sharp diffraction peak (FSDP) on the same samples shows that the two features, FSDP and Boson peak, are not clearly correlated. The results are discussed on the basis of the relationship between the mean free path of phonons and the low-frequency Raman intensity.