Glass transition and density fluctuations in the fragile glass former orthoterphenyl - art. no. 061502

High-resolution Brillouin light scattering is used to measure the dynamic s tructure factor of the fragile glass former orthoterphenyl (OTP) in a wide temperature range around the glass transition region and up to the boiling point. The whole set of spectra is described in terms of a phenomenological generalized hydrodynamic model. In the supercooled phase, we show the cont emporary existence of the structural process, whose main features come out to be consistent with the results obtained with other spectroscopies, and o f a secondary, activated process, which occurs on the 10(-11) s time scale and has a low activation energy (E-a(f) = 0.28 kcal/mol). This latter proce ss, which is also present in the glassy phase and seems to be insensitive t o the glass transition, is attributed to the coupling between the density m odes and intramolecular degrees of freedom. In the normal liquid phase, the two processes merge together, and the resulting characteristic time is no longer consistent with those derived with other spectroscopies. The analysi s points to the conclusion that, for what concerns the long-wavelength dens ity fluctuations in fragile glass formers such as OTP, the universal dynami cal features related to the glass transition come out clearly only in the s upercooled phase and at frequencies tower than approximate to 10(6) Hz.