Dielectric and mechanical relaxation of cresolphthalein–dimethylether

From pressure–volume–temperature measurements, in combination with published dielectric spectroscopy results, we assess the volume dependence of structural relaxation in cresolphthalein–dimethylether (KDE). Structural relaxation in KDE cannot be described as either a volume activated or thermally activated process, and unlike results for some other molecular glass formers, temperature is not the dominant control variable. The inflection point of the $PVT$ data yields a glass temperature, $Tg,$ that corresponds to a dielectric relaxation time equal to 10 s. There is a marked variation with pressure, $dTg/dP=307 K/GPa$ at low pressure. We also carried out dynamic mechanical measurements on KDE near $Tg.$ The shape of the mechanical and dielectric relaxation functions, as well as both the magnitude and temperature dependence of the respective relaxation times, are the same. Such equivalence is not generally expected, and arises herein due to the rigid structure of the KDE molecule.