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Prediction of the glass transition temperature of water solutions: comparison of different models

The glass transition temperature (Tg) of a sample is an important parameter that determines its stability during storage. While Tg can be measured by a variety of methods, it is a time-consuming procedure, especially if the sample is to be kept at subzero temperatures, in anhydrous conditions, or if sampling a portion of the specimen for analysis is cumbersome. Hence, predicting rather than directly measuring Tg as a function of the content of the constituents of a blend, mixture, or solution can be a powerful tool. Two main models for predicting Tg have been proposed: Couchman-Karasz (C-K) and Gordon-Taylor (G-T) formalisms. However, many aspects of both are theoretical/terminological in nature, and substantial controversy exists about the various experimental approaches to measuring Tg as well. Here, we compare C-K and G-T formalisms, as well as related problems that arise from the variety of definitions and methods of measuring Tg. Water-trehalose solutions are used as an example for application of the analysis. However, the same conclusions can be expanded to any other solutions so thermodynamical parameters (Tg, DeltaCp, and k) of 20 other commonly used solutes are provided. Practical pitfalls related to determining water content, including experimental data on thermal gravimetry, are also discussed.

Brillouin scattering study of molten zinc chloride

Polarized and depolarized Brillouin scattering experiments on molten ZnCl2 were performed between 300 and 600 degrees C in different geometries. VV spectra measured in backscattering and small angle scattering were analyzed with conventional viscoelastic theory using either a Debye or a Cole-Davidson model for the memory function. We also analyzed in the same way the temperature dependence of the transverse Brillouin lines detected in a 90 degrees VH geometry. We show that the Cole-Davidson memory function yields a consistent interpretation of all the spectra. The resulting shear and longitudinal relaxation times are equal within their error bars, and are about 2.5 times smaller than the alpha relaxation time previously determined. The static shear viscosity values deduced from the analysis of the propagating transverse waves agree, at all temperatures, with the measured viscosity values.

The Microscopic Basis of the Glass Transition in Polymers from Neutron Scattering Studies

Recent neutron scattering experiments on the microscopic dynamics of polymers below and above the glass transition temperature T(g) are reviewed. The results presented cover different dynamic processes appearing in glasses: local motions, vibrations, and different relaxation processes such as alpha- and beta-relaxation. For the alpha-relaxation, which occurs above T(g), it is possible to extend the time-temperature superposition principle, which is valid for polymers on a macroscopic scale, to the microscopic time scale. However, this principle is not applicable for temperatures approaching T(g). Below T(g), an inelastic excitation at a frequency of some hundred gigahertz (on the order of several wave numbers), the "boson peak," survives from a quasi-elastic overdamped scattering law at high temperatures. The connection between this boson peak and the fast dynamic process appearing near T(g) is discussed.