A new feature of glass-forming liquids, i.e., long-range density fluctuations of the order of 100 nm, has been extensively characterized by means of static light scattering, photon correlation spectroscopy and Rayleigh-Brillouin spectroscopy in orthoterphenyl (OTP) and $1,1-\mathrm{di}{(4\mathrm{}}^{\prime }$-methoxy-$5^{\prime }\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{y}\mathrm{l}-\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{y}\mathrm{l})$-cyclohexane (BMMPC). These long-range density fluctuations result in the following unusual features observed in a light scattering experiment, which are not described by the existing theories: (i) strong q-dependent isotropic excess Rayleigh intensity, (ii) additional slow component in the polarized photon correlation function, and (iii) high Landau-Placzek ratio. These unusual features are equilibrium properties of the glass-forming liquids and depend only on temperature, provided that the sample has been equilibrated long enough. The temperature-dependent equilibration times were measured for BMMPC and are about 11 orders of magnitude longer than the α process. It was found that the glass-forming liquid OTP may occur in two states: with and without long-range density fluctuations (“clusters”). We have characterized the two states by static and dynamic light scattering in the temperature range from $T_g$ to $T_g+200\mathrm{}$ K. The relaxation times of the α process as well as the parameters of the Brillouin line are identical in both OTP with and without clusters. The α process (density fluctuations) in OTP was characterized by measuring either the polarized (VV) or depolarized (VH) correlation function, which are practically identical and q-independent. This feature, which is commonly observed in glass-forming liquids, is not fully explained by the existing theories.

• Received 2 January 2001

DOI:https://doi.org/10.1103/PhysRevE.63.061503