High-frequency dynamic 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 Omega and the width Gamma of the inelastic 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 upsilon(infinity). Consistently with the results obtained in different glass-forming 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 acousticlike excitations progressively loose their propagative nature. In the glass, upsilon(infinity)(T) 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 upsilon(o) and upsilon(infinity) (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.