High-frequency dynamics of glass-forming polybutadiene

Inelastic x-ray measurements of polybutadiene are reported, performed over a wide temperature range covering both the glass and the liquid phase. At each temperature, the frequency position [Formula Presented] and the width [Formula Presented] of the inelastic peaks of the spectra have been obtained for different values of the scattering vector [Formula Presented]. A linear behavior of [Formula Presented] for [Formula Presented] has been revealed, allowing the determination of the unrelaxed sound velocity [Formula Presented] Consistently with the results obtained in different glass-forming systems, the Q dependence of [Formula Presented] is well represented by a [Formula Presented] law. For [Formula Presented] the values of [Formula Presented] overtake those of [Formula Presented] and the acousticlike excitations progressively loose their propagative nature. In the glass, [Formula Presented] compares well with previous Brillouin Light Scattering (BLS) determinations, while in the liquid the BLS sound velocity shows a steeper temperature dependence related to the structural relaxation. The temperature behavior of the nonergodicity factor has been derived both from [Formula Presented] and [Formula Presented] (in the liquid phase) and from the ratio between elastic and inelastic intensities of inelastic x-ray scattering spectra (in the whole investigated temperature range). Both temperature and Q behavior of this quantity might be consistently interpreted in the framework of the mode coupling theory.