Brillouin light scattering is used for studying the spectrum of density fluctuations of the glass-forming epoxy resin diglycidyl ether of bisphenol-A. Spectra at different temperatures ranging from the glassy to the liquid phase are obtained from a direct subtraction of depolarized from polarized spectra. In addition to the structural relaxation, evidence is given of a fast secondary relaxation process, which affects Brillouin spectra also at temperatures lower than that of the glass transition T g . For the elaboration of isotropic spectra, we exploit the possibility of using the same relaxation function gained from dielectric spectra taken from the same sample. The temperature behavior of the relaxation strength shows the existence of an onset for the structural relaxation, located at a temperature about 93 K higher than T g , consistent with the results of previous dielectric spectroscopy and depolarized light scattering investigations. The role of secondary relaxations of intramolecular nature in the mode-coupling analysis of real glass formers is also discussed.
The dynamics of the fragile glass-forming liquid diglycidyl ether of bisphenol-A was studied by depolarized Rayleigh-Brillouin light-scattering and photon correlation spectroscopy above the glass transition, in the temperature range from 261 to 473 K and in the frequency range from 1 Hz to 300 GHz. The structural (alpha-) relaxation process was revealed and no signature of the secondary relaxation previously evidenced by dielectric spectroscopy at about 0.1 GHz was observed. The characteristic time of the alpha process differs from that determined by dielectric spectroscopy of an amount, which increases with increasing temperature. The relaxation times were compared with viscosity data to test the predictions of the classic Stokes-Einstein-Debye model. The tau proportional, variant eta behavior was verified for dielectric data, while a fractional power law of viscosity tau proportional, variant eta(0.89) was obtained for light-scattering relaxation times, extending over more than seven decades in viscosity and time. This deviation of light scattering from viscosity data could be interpreted in terms of cooperative motion in the supercooled liquid with a characteristic length xi(a) proportional, variant(T-T0)(-v) where T(0)=229 K is the Vogel temperature and v is close to 2 / 3 which is consistent with the prediction of the fluctuation theory of glass transition.
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 ⍀ and the width ⌫ of the inelastic peaks of the spectra have been obtained for different values of the scattering vector Q. A linear behavior of ⍀(Q) for QϽ4 nm Ϫ1 has been revealed, allowing the determination of the unrelaxed sound velocity v ϱ . Consistently with the results obtained in different glass-forming systems, the Q dependence of ⌫ is well represented by a Q 2 law. For QϾ5 nm Ϫ1 the values of ⌫ overtake those of ⍀ and the acousticlike excitations progressively loose their propagative nature. In the glass, v ϱ (T) compares well with previous Brillouin Light Scattering ͑BLS͒ 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 v o and v ϱ ͑in the liquid phase͒ and from the ratio between elastic and inelastic intensities of inelastic x-ray scattering spectra ͑in the whole investigated temperature range͒. Both temperature and Q behavior of this quantity might be consistently interpreted in the framework of the mode coupling theory. ͓S1063-651X͑99͒10004-7͔
Measurements of the dielectric constant of poly(n-butyl acrylate) have been taken in the frequency range 100 Hz-3 GHz, for temperatures between 254 and 359 K. Analysis of dielectric data and their comparison with results recently obtained on the same system by Brillouin light scattering have evidenced a two-step relaxation process. Dielectric spectra account for the behaviour of the slower process, i.e. the structural relaxation, while Brillouin spectra are particularly sensitive to the faster process, i.e. the secondary relaxation. The temperature behaviour of these processes is not in accordance with the predictions of the mode-coupling theory, while it can be suitably interpreted by a model that relates the high-frequency dependence of the dielectric loss curve to the local chain dynamics and the low-frequency dependence to the intermolecular correlation.
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