Differential thermal analysis described

The study of elastic and viscoelastic materials under conditions of cyclic stress or strain is called dynamic mechanical analysis, DMA. The periodic changes in either stress or strain permits the analysis of the dynamic response of the sample in the other variable. The analysis has certain parallels to the temperature-modulated differential thermal analysis described in Sect 4.4, where the dynamic response of the heat-flow rate is caused by the cyclic temperature change. In fact, much of the description of TMDSC was initially modeled on the more fully developed DMA. The instruments which measure stress versus strain as a function of frequency and temperature are called dynamic mechanical analyzers. The DMA is easily recognized as a further development of TMA. Its importance lies in the direct link of the experiment to the mechanical behavior of the samples. The difficulty of the technique lies in understanding the macroscopic measurement in terms of the microscopic origin. The... 

In addition to these standardised test methods set by regulation (in particular the transport regulations of dangerous substances), there are laboratory methods that can provide more details regarding substance behaviour. In particular, there is differential thermal analysis (DTA), thermal gravimetric analysis, calorimetry and thermomanometry, which will not be described here. 

In a manner similar to that just described for differential thermal analysis, DSC can be used to obtain useful and characteristic thermal and melting point data for crystal polymorphs or solvate species. This information is of great importance to the pharmaceutical industry since many compounds can crystallize in more than one structural modification, and the FDA is vitally concerned with this possibility. Although the primary means of polymorph or solvate characterization s centered around x-ray diffraction methodology, in suitable situations thermal analysis can be used to advantage. 

More advanced techniques are now available and section 4.2.1.2 described differential scanning calorimetry (DSC) and differential thermal analysis (DTA). DTA, in particular, is widely used for determination of liquidus and solidus points and an excellent case of its application is in the In-Pb system studied by Evans and Prince (1978) who used a DTA technique after Smith (1940). In this method the rate of heat transfer between specimen and furnace is maintained at a constant value and cooling curves determined during solidification. During the solidification process itself cooling rates of the order of 1.25°C min" were used. This particular paper is of great interest in that it shows a very precise determination of the liquidus, but clearly demonstrates the problems associated widi determining solidus temperatures. 

Differential thermal analysis (DTA) data was obtained using a DuPont 1090 thermal analyzer using 0.04 SCF/h of air as purging gas and heating rates of 10°C/min. All powder diffraction measurements were obtained with a Siemens D-500 diffractometer at a scan rate of l°/min using monochromatic Cu-K radiation. The preparation of DFCC mixtures containing sepiolite hSs been described elsewhere (4). 

As described in Section 3.3.2.1 on heat sensitivity, thermoanalytical methods are sufficiently sensitive as an early indication of incipient chemical decomposition or chemical reaction, that is, stability and incompatibility. Some research papers discuss the use of differential thermal analysis (DTA) and differential scanning calorimetry (DSC) for this purpose [20-22]. 

One of the simpler ways to obtain such information is called differential thermal analysis (DTA), and a typical apparatus is described in Figure 2.37. Basically, the polymer sample P and an inert reference material R are heated from the same source. Thermocouples measure the temperature of the polymer and that of the reference, and the temperature difference AT =TP- TR is then plotted as a function of the temperature of the polymer. 

The first measurements of temperature as a function of time during a cooling or heating process were made by J. F. E. Rudberg in Sweden in 1829. Other early workers were M. L. Frankenheim (1837) and H. Le Chatelier (1883 and 1887), both of whom seem to have been unaware of the earlier work.271 Le Chatelier was followed by W. C. Roberts-Austin, who initiated differential thermal analysis in 1899. The development of this technique, from its introduction to the 1970s, has been discussed.272 Hungarian work in thermal analysis over the period 1950-1990 has been described.273... 

On the basis of several analytical studies (differential thermal analysis, fluorescence, CPMAS solid-state NMR spectroscopy and others) [56-58] two models have been proposed to describe the structure of HCN-polymers, the Umemoto [59] and the Volker models [60]. In the Volker model, HCN polymerizes to extensive double-ladder rod-like structures, while a simpler mono-ladder pattern was hypothesized by Umemoto (Fig. 1). Irrespective of the structure assumed by HCN-polymers, a large panel of purine, imidazole and pyrimidine derivatives can be obtained by hydrolysis of these materials. In 1963, Lowe described the first example of acidic hydrolysis of the HCN-polymer (boiling 6.0 N HC1) to yield amino acids, carboxylic acids, adenine and hypoxanthine (Scheme 4). 

Enthalpy changes on adsorption and desorption of probe molecules on catalyst surfaces may also be followed by differential thermal analysis (DTA) (67) although this method has been used only sporadically in the past. The experimental techniques have been described by Landau and Molyneux (67) very recently. As an example, Bremer and Steinberg (68) observed three endothermic peaks during the desorption of pyridine from a MgO-Si02 catalyst these peaks were assigned as three different chemisorption states of pyridine. 

This monograph provides an introduction to scanning ther-moanalytical techniques such as differential thermal analysis (DTA), differential scanning calorimetry (DSC), dilatometry, and thermogravimetric analysis (TG). Elevated temperature pyrometry, as well as thermal conductivity/diffusivity and glass viscosity measurement techniques, described in later chapters, round out the topics related to thermal analysis. Ceramic materials are used predominantly as examples, yet the principles developed should be general to all materials. 

Spectroscopically pure peroxy titanium oxide was prepared by the method described in the literature, using spectrographically standardized titanium powder supplied by Johnson and Matthey, London, England. Anatase was then obtained by the thermal decomposition of the peroxy compound. Differential thermal analysis of the peroxy compound was carried out using an apparatus described by Pask and Warner 13 (fig. a). 

Thermogravimetry (TG), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) are the most frequently used techniques in lignin chemistry, although thermomechanical analysis (TMA) has also been used effectively in the analysis of thermal properties of lignin (Goring 1963). In this section, the principles of TG, DTA, and DSC, and their application to lignin are described. 

The next stage of characterization focuses upon the different phases present within the catalyst particle and their nature. Bulk, component structural information is determined principally by x-ray powder diffraction (XRD). In FCC catalysts, for example, XRD is used to determine the unit cell size of the zeolite component within the catalyst particle. The zeolite unit cell size is a function of the number of aluminum atoms in the framework and has been related to the coke selectivity and octane performance of the catalyst in commercial operations. Scanning electron microscopy (SEM) can provide information about the distribution of crystalline and chemical phases greater than lOOnm within the catalyst particle. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) can be used to obtain information on crystal transformations, decomposition, or chemical reactions within the particles. Cotterman, et al describe how the generation of this information can be used to understand an FCC catalyst system. 

Many relatively slow or static methods have been used to measure Tg. These include techniques for determining the density or specific volume of the polymer as a function of temperature (cf. Fig. 11-1) as well as measurements of refractive index, elastic modulus, and other properties. Differential thermal analysis and differential scanning calorimetry are widely used for this purpose at present, with simple extrapolative eorrections for the effects of heating or cording rates on the observed values of Tg. These two methods reflect the changes in specific heat of the polymer at the glass-to-rubber transition. Dynamic mechanical measurements, which are described in Section 11.5, are also widely employed for locating Tg. 

Mara. Kamei and Osada [109 described a detailed study of the thermal decomposition of TNT. They examined the decomposition by differential thermal analysis, thermogravimetry, infra-red spectroscopy. HSR and mass spectrometry. One of their most important findings was that TNT produced free radicals already in the vicinity of the melting point, that is SO C. The substances which promote the decomposition of TNT are free radicals which are stable at room temperature. They are insoluble in benzene or chloroform and are partly oxidized polymeric substances. 

The differential calorimetric curves (DSC) of the various crystalline forms of triamterene grown from organic solutions containing water and from absolute organic solutions, and the DSC curves of triamterene crystals dried under reduced pressure have been described. The differential thermal analysis-thermogravimetry analysis (DTA-TG) thermograms are also given. 

The decomposition temperatures of hydrates were measured by means of differential thermal analysis (DTA) under the conditions of excess gas in a stainless steel flask that was developed specially for the investigation of hydrate formation with a gaseous guest at high hydrostatic pressure. The hydrate decomposition temperature was measured with a chromel-alumel thermocouple to the accuracy of 0.3 K. The thermocouple was calibrated with the use of temperature standards. Pressure was measured with a Bourdon-tube pressure gauge. The error of the pressure measurements did not exceed 0.5 %. This procedure was described in more detail previously.The gases used in the investigation... 

Two other important milestones in the development of the modern thermobalance occurred in 1958 and 1964. A multifunctional instrument, called the Derivatograph. was described by Paulik 35) et al. in 1958. The instrument could record not only the TG curve, but also its first derivative (DTG) and the differential thermal analysis jDTA) curve. In 1964. Weide-mann (3) described the Mettler system, which was perhaps the most sophisticated thermobalance ever commercially available. This system is described in detail by Wiedemann and Bayer (8). 

The history of differential thermal analysis and differential scanning calorimetry, as well as thermal analysis, has been described in great detail by Mackenzie (10,11). 

Thermoelectrometry techniques are not wideiy employed in thermal analysis, in fact, they may be described as neglected techniques in comparison with the widely used thermogravimetry iTGl differential thermal analysis (DTA). and differential scanning calorimetry iDSC) techniques. Nevertheless, they are important for certain specific applications, many of which will be discussed here. 

The application of dielectric constant techniques to thermophysical measurement of solids has been used for a number of years (114, 115). The early uses of the technique involved isothermal measurements employing bridge methods. Recently, techniques have been developed that permit the measurement of the dielectric constant of a solid as a function of temperature, in a manner similar to other TA techniques. Chiu (116) used the term dynamic electrothermal analysis (ETA) to describe the measurement of both the capacitance and the dissipation factor of polymeric samples. Nottenburget al (117) developed an automated technique that permitted the rapid determination of the dielectric properties of a substance over a wide range of temperature and frequencies. This technique, which was called dynamic dielectric analysis (DDA), was modified to measure concurrently the DTA curve of the sample as well (117, 118). This new technique was called dynamic dielectric analysis-differential thermal analysis, DDA-DTA,... 

Differential thermal analysis was conducted for 2-methoxy-7,8,9,10-tetrahydropyrido[2,l-c][l,2,4]benzothiadiazine-5,5-dioxide, and it was found that this compound exists in two polymorphic forms, one of them being remarkably unstable. Detailed conformational analyses for hexahydro[2,l-6][l,3,4]oxadiazines <93AX(C)1786> and hexahydropyrido[2,l-e][l,2,5]oxadiazines <92AP(325)157> were described on the basis of x-ray and NMR studies, respectively. 

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