Simple Glass Formers Research Articles

Neutron and light scattering study of relaxation dynamics in a glass-forming fragile molecular liquid

We present experimental results on the structural relaxation dynamics of a fragile molecular glass, propylene carbonate, from quasielastic neutron and inelastic light scattering investigations of the supercooled liquid. The data show the power law and the scaled dynamics predicted in recent mode-coupling theories for the glass transition. As expected, the critical behaviour occurs at a temperature much higher than the calorimetric glass transition temperature. The relaxation is found to be considerably stretched despite the high probe frequencies (GHz) and there is no indication of any crossover to exponential decay. This is in contrast to the single relaxation time behaviour expected at short relaxation times/high frequencies in phase-space models and also contrary to the reported general behaviour of simple glass-formers. Comparing with data of ionic glass-formers of similar fragility we conclude that there is no simple relation between nonexponentiality of the relaxation decay and degree of fragility as has been suggested.

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=4.65×10−4exp[689/(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.