LIGHT-SCATTERING STUDY OF THE LIQUID-GLASS TRANSITION IN PROPYLENE CARBONATE

Depolarized light-scattering spectra of propylene carbonate were obtai ned in the frequency range 0.2 GHz-4 THz at temperatures from 350 to 1 35 K. Analysis of the resulting susceptibility spectra revealed reason able agreement with the predictions of the idealized mode coupling the ory, yielding critical exponents a = 0.29, b = 0.50, and an exponent p arameter lambda = 0.78+/-0.05. A scaling analysis demonstrated critica l slowing down of the scaling frequencies from both above and below T( C), with the scaling frequency extrapolating to zero at T(C) = 187+/-5 K. The alpha-relaxation peak was fit to a Kohlrausch function, which gave beta = 0.77+/-0.05 for 210 less-than-or-equal-to T less-than-or-e qual-to 350 K with no sign of systematic temperature dependence. An ex tended mode coupling theory analysis was also carried out which corrob orated the results obtained with the idealized version. Polarized Bril louin spectra were also obtained, and were analyzed with a generalized hydrodynamics approach using low-frequency sound velocity values dete rmined separately in an ultrasonics experiment. The results indicate t hat the Cole-Davidson function is not an adequate model for the struct ural relaxation due to the neglect of beta-relaxation processes, so th at this analysis cannot provide a meaningful estimate of T(C). Further more, the alpha-relaxation time associated with Brillouin scattering w as found to be about five times shorter than that probed by depolarize d light scattering.