Phonons and relaxations in oxide and polymeric glasses

Abstract

Abstract 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 relaxation time deduced from Brillouin data compares well with that deduced from neutron measurements; it appears different from that associated with the α relaxation near T g. Moreover, the density of excitations extracted from the dynamic structure factor demonstrates a conversion of high-frequency modes to low-frequency modes when the temperature increases. Our results suggest that the anharmonicity must be taken into account to explain the low-energy excitations in glasses.