Glass Transition in o-Terphenyl

Volumetric and thermal measurements on its undercooled liquid indicate that o-terphenyl undergoes a glass transition in the vicinity of −30°C. This glass temperature is about 60°C higher than that predicted by correlations applicable to simple molecular glass formers. The viscosity of the undercooled liquid was measured over the temperature range 20° to −16°C. These data and the higher-temperature data of other investigators are described approximately (between 250° and −16°C) by the equation η=4.65×10−4exp689/(T−231) P.The apparent activation energy for viscous flow increases from about ¼ of the heat of vaporization ΔHv at the highest part to about 5 ΔHv at the lowest part of the temperature range. The rates of volume relaxation in the vicinity of the glass transition temperature were measured. Each relaxation isotherm could be described with a single time constant over volume changes ranging up to 0.2%. In a cylindrical dilatometer, with a large length-to-diameter ratio, the relaxation rates apparently were limited by a flow process reflecting the adhesion of terphenyl to the dilatometer walls. Approximate values of the shear viscosity were inferred from a model for this process. Apparently, the homogeneous nucleation frequency of crystals in amorphous o-terphenyl does not reach a measurable level. The undercooling dependence of the rate of advance of the crystallization front was measured.