Far IR spectra and heat capacities for propylene carbonate and propylene glycol, and the connection to the dielectric response function

A joint analysis of far infrared and Hz - MHz dielectric absorption spectra suggested by us for aqueous propylene glycol (PG) solutions, and extended by Cole and colleagues to 10 GHz, is applied to pure propylene carbonate (PC) for which microwave absorption data are also available. We include heat capacity and fusion entropy studies to obtain a Kauzmann (ideal glass transition) temperature for PC, which agrees well with the Vogel-Tammann-Fulcher (VTF) To parameter. The VTF parameters show PC to be a more fragile liquid than PG, though not as fragile as previously reported by Borjesson et al from very short time relaxation data. This is a consequence of a sharp departure, at -1.7 T_o (relaxation time -10^-9s) from the temperature dependence predicted by the VTF equation which is similar to that observed at the same T/T_o value in fragile molten salt systems, and also to that observed at the same relaxation time, but higher T/T_o in less fragile PG solutions. It corresponds to the crossover predicted by Goldstein to occur as the potential energy hypersurface becomes relevant to the system dynamics (and, as such, it sets the limit to validity of the currently popular mode coupling theory.) At about the same temperature, the energy absorption due to the dielectric relaxation process starts to overlap that due to damped molecular libration, and the two processes thereafter become progressively merged as temperature increases into and through the normal liquid range.