THERMAL ANALYSIS METHODS 1 Technique

Thermal analysis methods are defined as those techniques in which a property of the analyte is determined as a function of an externally applied temperature... 

Any study of the polymerization kinetics of a bisbenzocyclobutene monomer is complicated by the lack of understanding of the resulting polymer s structure and the fact that as the polymerization proceeds, the reaction mixture crosslinks and vitrifies. This vitrification limits somewhat the number of quantitative methods which can be used to study the bisbenzocyclobutene polymerization kinetics. Some techniques are however useful under these constraints and good kinetic results have been obtained by both infrared and thermal analysis methods. 

The development of thermal analysis methods in materials research has led to a plethora of new methodologies since the elaboration of the first thermal method by by Le Chatelier and Robert-Austen [16,86], Thermal analysis consists of a group of techniques in which a physical property of a material is measured as a function of temperature at the same time when the substance is subjected to a controlled increase, or in some cases, decrease of temperature. Temperature-programmed techniques, such as DTA [87-89], TGA [87], DSC [53,90], TPR [91,92], and TPD [93-96], contribute to perform a more complete characterization of materials. 

Thermal analysis methods are defined as those techniques in which a property of the analyte is determined as a function of an externally applied temperature. Regardless of the observable parameter measured, the usual practice requires that the physical property and the sample temperature are recorded continually and automatically and that the sample temperature is altered at a predetermined rate. Thermal reactions can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. Such methodology has found widespread use in the pharmaceutical industry for the characterization of compound purity, polymorphism, solvation, degradation, and excipient compatibility. ... 

The use of thermal analysis techniques has increased rapidly in the past ten years and their field of application is widening continuously. This new book provides an overview of the principal thermal analysis methods and their application in major areas of importance, and will bring the reader up-to-date with the latest advances in the field. Special Publication No. 117 Hardcover viii+296 pages ISBN 0 85186 375 2... 

Differential scanning calorimetry (DSC) is one of the best known techniques among a group called thermal analysis methods others include differential thermal analysis, d5mamic mechanical analysis, and thermogravimetric analysis methods all of which are covered in the following sections. 

Probably the most important characteristic of military and commercial explosives and solid rocket propellants is performance as related to end use and safety. Performance can be described by a variety of conventional properties such as thermal stability, shock sensitivity, friction sensitivity, explosive power, burning, or detonation rate, and so on. Thermal analysis methods, according to Maycock (51), show great promise for providing information on both these conventional properties and other parameters of explosive and propellant systems. The thermal properties have been determined mainly by TG and DTA techniques and isothermal or adiabatic constant-volume decomposition. Physical processes in pseudostable ma-... 

The usefulness of the EGD-EGA techniques can be extended by combining the various types of detectors, as given in Figure 8.2, with other thermal analysis methods. These multiple techniques offer a savings in time and effort, and since data are taken at the same time on the same sample, the results are more likely to be comparable than if they are taken separately on two or more different samples. Examples of the more common multiple techniques are given in Figure 8.5 and Table 8.4. 

The identification, structural and thermal characterization of new polymorphs is an important topic in solid-state chemistry and requires a battery of techniques that includes X-ray diffraction and spectroscopic methods, in addition to thermal analysis methods and dissolution techniques to determine solubility trends. Such studies are described by Caira in Chapter 16, as well as more recent theoretical techniques aimed at the prediction of the crystal structures of new polymorphs. Crystal polymorphism is particularly important in pharmaceutical products, so there is an emphasis on this area. Systems displaying solvatomorphism (the ability of a substance to exist in two or more crystalline phases arising from differences in their solvation states) molecular inclusion and isostructurality (the inverse of polymorphism) are also given due attention in this chapter. 

For the above reasons, it is probably preferable that the samples be totally quenched to obtain a fully amorphous sample, and then to anneal the samples to achieve the correct crystallization profile. This is of course possible only for a polymer that can be quenched to its fully amorphous state. In some cases, the rate of crystallization for the polymers may be so high that some amount of crystallization is unavoidable. Most thermal analysis methods used for measuring crystallinity, such as the DSC or DMA methods, preclude the availability of data relating the heat of fusion or the storage modulus against the crystallinity [2]. Such data may not be always available especially for polymers that either do not fully crystallize or cannot be fully amorphous. An alternative method to determine the absolute crystallinity value would be to use the X-ray diffraction technique [. ]. 

The thermal behavior of coal may also be investigated by use of the differential thermal analysis method. In this technique, the differential change in temperature that occurs when a coal is heated at a given rate is noted to that for a reference material. The derivative is plotted against temperature to produce the thermogram a positive slope indicates an exothermic reaction whereas a negative slope indicates an endothermic reaction. 

IS often used to investigate the composition of solid phases in equilibrium with salt solutions. This method has been reviewed in (23), where [see also (24)] least-squares methods for evaluating the composition of the solid phase from wet residue data (or initial composition data) and solubilities are described. In principle, the same method can be used with systems of other types. Many other techniques for examination of solids, in particular X-ray, optical, and thermal analysis methods, are used in conjunction with chemical analyses (including the wet residues method). 

Thermal analysis methods can be broadly defined as analytical techniques that study the behaviour of materials as a function of temperature [1]. These are rapidly expanding in both breadth (number of thermal analysis-associated techniques) and in depth (increased applications). Conventional thermal analysis techniques include DSC, DTA, TGA, thermomechanical analysis, and dynamic mechanical analysis (DMA). Thermal analysis of a material can be either destructive or non-destructive, but in almost all cases subtle and dramatic changes accompany the introduction of thermal energy. Thermal analysis can offer advantages over other analytical techniques including variability with respect to application of thermal energy (step-wise, cyclic, continuous, etc.), small sample size, the material can be in any solid form - gel, liquid, glass, solid, ease of variability and control of sample preparation, ease and variability of atmosphere, it is relatively rapid, and instrumentation is moderately priced. Most often, thermal analysis data are used in conjunction with results from other techniques. 

The technique referred to as DSC is specifically that described next and any other thermal analysis method, in particular Boersma-type DTA, which may have been described as differential scanning calorimetry in the literature will be referred to as indirect DSC . 

The calorimetric or differential thermal analysis methods indicate the amount of crystallinity by the size of the area associated with the peak that occurs in the scans (see Fig. 1-34 and 1-35). These areas can be compared to those for a polymer with known crystallinity. The technique is rapid and quite precise. It does, however, require a first-class analytical device, which is not inexpensive. 

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