Light-scattering study of the liquid-glass transition in propylene carbonate.

Abstract

Depolarized light-scattering spectra of propylene carbonate were obtained in the frequency range 0.2 GHz--4 THz at temperatures from 350 to 135 K. Analysis of the resulting susceptibility spectra revealed reasonable agreement with the predictions of the idealized mode coupling theory, yielding critical exponents a=0.29, b=0.50, and an exponent parameter \ensuremath{\lambda}=0.78\ifmmode\pm\else\textpm\fi{}0.05. A scaling analysis demonstrated critical slowing down of the scaling frequencies from both above and below ${\mathit{T}}_{\mathit{C}}$, with the scaling frequency extrapolating to zero at ${\mathit{T}}_{\mathit{C}}$=187\ifmmode\pm\else\textpm\fi{}5 K. The \ensuremath{\alpha}-relaxation peak was fit to a Kohlrausch function, which gave \ensuremath{\beta}=0.77\ifmmode\pm\else\textpm\fi{}0.05 for 210\ensuremath{\le}T\ensuremath{\le}350 K with no sign of systematic temperature dependence. An extended mode coupling theory analysis was also carried out which corroborated the results obtained with the idealized version. Polarized Brillouin spectra were also obtained, and were analyzed with a generalized hydrodynamics approach using low-frequency sound velocity values determined separately in an ultrasonics experiment. The results indicate that the Cole-Davidson function is not an adequate model for the structural relaxation due to the neglect of \ensuremath{\beta}-relaxation processes, so that this analysis cannot provide a meaningful estimate of ${\mathit{T}}_{\mathit{C}}$. Furthermore, the \ensuremath{\alpha}-relaxation time associated with Brillouin scattering was found to be about five times shorter than that probed by depolarized light scattering.