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 Ω and the width Γ of the inelastic peaks of the spectra have been obtained for different values of the scattering vector Q. A linear behavior of Ω(Q) for Q<4nm^-1 has been revealed, allowing the determination of the unrelaxed sound velocity v_∞. Consistently with the results obtained in different glass-forming systems, the Q dependence of Γ is well represented by a Q² law. For Q>5nm^-1 the values of Γ overtake those of Ω and the acousticlike excitations progressively loose their propagative nature. In the glass, v_∞(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 vₒ and v_∞ (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.