Effect of large hydrostatic pressure on the dielectric loss spectrum of type- a glass formers.

New dielectric spectroscopy results are reported for propylene carbonate (PC), glycerol, and threitol, measured at very high (1.8 GPa) pressure. These glass formers all exhibit an excess wing in their dielectric spectrum above T(g). We show that the shape of the alpha peak and excess wing of PC are invariant to pressure and temperature, when compared at a fixed value of the alpha -relaxation time. However, for the hydrogen-bonded liquids, there is a marked breakdown of this temperature-pressure superpositioning, due to a change in chemical structure (i.e., concentration of hydrogen bonds) with change of temperature or pressure. For all these materials, we can conclude that the excess wing is merely a secondary relaxation, masked under ordinary conditions by the intense, overlapping alpha peak.