A BRILLOUIN-SCATTERING STUDY OF AN EXTREMELY FRAGILE MOLECULAR GLASS FORMER

Brillouin scattering results on the structural relaxation in fragile glass-forming propylene carbonate are presented. A new analyzing technique has been employed in which the full shape of the Brillouin spectrum was compared with a general hydrodynamic theory that has been modified to include non-exponential relaxation. The relaxation is found to be considerably stretched and time-temperature scaling is obeyed. This is by contrast with the reported crossover towards exponentially decay at high frequencies/high temperatures which has been observed previously with Brillouin scattering in other fragile systems such as glass-forming ionic liquids and ionic aqueous solutions. Such a crossover is also expected at sufficiently high temperatures both in mode-coupling and recent geometric phase-space percolation theories. It is concluded that there is no simple relation between the non-exponentiality of the relaxation and the degree of fragility as has been suggested.