A method to determine the kinetics of a supercooled liquid by temperature scanning measurements applied to (Li,Na)acetate and GeO2

Temperature scanning techniques are frequently used to measure glass transitions. For such a measurement, the width of the glass transition regime depends on the fragility of the substance. This paper discusses the degree to which the transition regime becomes smaller when the temperature scan-rate is decreased. Based on the general properties of supercooled liquids it is shown that this decrease may vary by a factor of two, from the most strong to the most fragile substances. To test this prediction, scanning calorimetry data are reported for germanium oxide, a strong glass-forming system, and (Li,Na)acetate, a fragile system. By considering the scanning rate dependence of the width, the second derivative of the glass temperature with respect to the relaxation time can be determined from the data. The parameters of the Vogel-Fulcher equation can be specified with this result from temperature scanning data alone, and the numerical results are compared with equilibrium viscosity measurements. The method demonstrated here to determine the kinetic parameters of a supercooled liquid is well suited for a regime of longer relaxation times, whereas in the case of a constant temperature experiment it is difficult to ensure that a sample is in thermal equilibrium. (C) 1999 Elsevier Science B.V. All rights reserved.