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 e ach temperature, the frequency position Omega and the width Gamma of the in elastic peaks of the spectra have been obtained for different values of the scattering vector Q. A linear behavior of Omega(Q) for Q<4 nm(-1) has been revealed, allowing the determination of the unrelaxed sound velocity upsil on(infinity). Consistently with the results obtained in different glass-for ming systems, the Q dependence of Gamma is well represented by a Q(2) law. For Q>5 nm(-1) the values of Gamma overtake those of Omega and the acoustic like excitations progressively loose their propagative nature. In the glass , upsilon(infinity)(T) compares well with previous Brillouin Light Scatteri ng (BLS) determinations, while in the liquid the BLS sound velocity shows a steeper temperature dependence related to the structural relaxation. The t emperature behavior of the nonergodicity factor has been derived both from upsilon(o) and upsilon(infinity) (in the liquid phase) and from the ratio b etween elastic and inelastic intensities of inelastic x-ray scattering spec tra (in the whole investigated temperature range). Both temperature and Q b ehavior of this quantity might be consistently interpreted in the framework of the mode coupling theory.