Dynamics of density fluctuations in glass forming liquids and polymers as measured by light scattering

In the static light-scattering patterns of glass-forming polymers a strong q-dependent (q=(4π/λ) sin θ/2) “excess” scattering is generally observed also at temperatures above T g . Consequently, the Landau-Placzek ratio (LPR) in the Rayleigh-Brillouin spectra is much higher than theoretically predicted for viscoelastic liquids. It is shown that these features are also observed in low molecular weight glass-forming liquids, namely o-terphenyl and chlophen, which were investigated by Brillouin spectroscopy over a wide temperature inverval ranging from shortly below the glass transition temperature to very deep into the melt region. The experimentally determined Brillouin shifts and line widths are consistently describable using updated theories of light scattering from viscoelastic liquids in the hypersonic regime. However, the LPR is much higher than expected from theory, therefore simple liquids behave in the neighborhood of T g very much like polymers. To explain this behavior we introduce an order parameter in the melt whose mean square fluctuation gives rise to “excess” scattering. It is postulated than in the supercooled liquid above T g “pre-freezing” phenomena occur that can be qualitatively discussed in the framework of recent free volume glass transition theories. This concept is supported by data from photon correlation spectroscopy and angular dependent elastic light-scattering measurements.