Other Methods of Thermal Analysis

Commercial equipment is available for monitoring other properties as a function of temperature. In dynamic mechanical analysis (DMA), tensile or shear modulus and loss are measured. In dielectric analysis, dielectric constant and loss tangent are followed. Both provide information about the time response of polymers by applying an oscillating field at a predetermined frequency. At each temperature, the mechanical or electric field test is run at a number of frequencies covering two or more decades. Mechanical frequencies up to 200 Hz are common. Electrical tests can cover greater ranges, say 10-10,000 Hz. 

Dielectric spectroscopy measures the dielectric properties of a medium placed in an electric field as a function of frequency. Practically in an experiment, a polymer film of known thickness is placed between two metal electrodes and then an AC electric field is applied across the polymer film at a known frequency. The measured response is related to the interaction of the external electric field with an electric dipole present in a sample, expressed in terms of its permittivity. This measurement can be used to describe a number of important characteristics of polymers. 

Thermally stimulated current (TSC) has been used to characterize various thermal processes [9]. In this method, a sample is subjected to a high voltage that orients dipoles in the material. The tanperature is lowered with the voltage applied, trapping the polarized dipoles. When the sample is reheated, small currents are detected as the material relaxes. TSC can be operated in such a way as to obtain a complete map of relaxation times versus tanperature. Even thin films of coatings and adhesives are conveniently studied by this method. 

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TG analysis also represents a powerful adjunct to the other methods of thermal analysis, since a combination of either a DTA or a DSC study with a TG determination can be used in the assignment of observed thermal events. Desolvation processes or decomposition reactions must be accompanied by weight changes, and they can be thus identified by a TG weight loss over the same temperature range. On the other hand, solid-liquid and solid-solid phase transformations are not accompanied by any loss of sample mass and would not register in a TG thermogram. 

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