Does Brillouin light scattering probe the primary glass transition process at temperatures well above glass transition?

The primary $α$-relaxation time $(τα)$ for molecular and polymeric glass formers probed by dielectric spectroscopy and two light scattering techniques (depolarized light scattering and photon correlation spectroscopy) relates to the decay of the torsional autocorrelation function computed by molecular dynamics simulation. It is well known that Brillouin light scattering spectroscopy (BLS) operating in gigahertz frequencies probes a fast (10–100 ps) relaxation of the longitudinal modulus $M∗$⁠. The characteristic relaxation time, irrespective of the fitting procedure, is faster than the $α$-relaxation which obeys the non-Arrhenius Vogel–Fulcher–Tammann equation. Albeit, this has been noticed, it remains a puzzling finding in glass forming systems. The available knowledge is based only on temperature dependent BLS experiments performed, however, at a single wave vector (frequency). Using a new BLS spectrometer, we studied the phonon dispersion at gigahertz frequencies in molecular [o-terphenyl (OTP)] and polymeric [polyisoprene (PI) and polypropylene (PP)] glass formers. We found that the hypersonic dispersion does relate to the glass transition dynamics but the disparity between the BLS-relaxation times and $τα$ is system dependent. In PI and PP, the former is more than one order of magnitude faster than $τα$⁠, whereas the two relaxation times become comparable in the case of OTP. The difference between the two relaxation times appears to relate to the “breadth” of the relaxation time distribution function. In OTP the $α$-relaxation process assumes a virtually single exponential decay at high temperatures well above the glass transition temperature, in clear contrast with the case of the amorphous bulk polymers.