Thermal transitions, sensitive recording

Dilatometry and thermomechanical analysis (TMA) are also techniques used to monitor the thermal behavior of fibers. They both employ a sensitive probe in contact with the surface of the sample, and the thermal transitions are detected either by a change in volume or modulus of the sample, respectively. In the latter case, the probe necessarily penetrates the sample surface. A variable transformer records the voltage output that is directly proportional to the degree of displacement of the probe during a thermally induced transition. TMA is a more sensitive technique than either DTA or DSC for detecting thermal transitions. 

Preliminary TGA and differential thermal analysis (DTA) curves were obtained on individual oxides and on the mixed transition metal-uranium oxides to ascertain the reaction characteristics of the individual systems. The TGA data were obtained on an Ainsworth BR balance equipped with an AU recorder. Samples of 1 gram each were heated to 1100°G. at 10°G. per minute. DTA information was obtained on a Tempres Research Model DT-4A instrument. Samples weighing approximately 50 mg. were heated at 5°G. per minute to 1250°G. Differential temperatures were measured with a platinum-platinum 10% rhodium thermocouple at a recorder sensitivity of 20 microvolts per inch. 

For TRXRD measurements to be meaningful, special attention must be devoted to the questions posed, to the manner in which the experiments designed to answer these questions are performed, and to the proper choice of materials and complete documentation of experimental protocols and results. As an example, in the case of a T-jump experiment, documentation should include direction, rate and magnitude of the jump, initial and final temperatures, and the thermal history of the sample. Ideally, in-sample temperature should be recorded at or close to the interrogating X-ray beam throughout the course of the transition. The manner in which the characteristic transition time is determined must be adequately described and, where possible, an estimate made of the number of molecules transforming per unit of time and volume. A measure of the sensitivity of the detection system to the presence of intermediates or minor and disordered phases would enormously benefit data evaluation. 

Circular dichroism employs standard dispersive or interferometric instrumentation, but uses a thermal source that is rapidly modulated between circular polarization states using a photoelastic or electro-optic modulator. Using phase-sensitive detection, a difference signal proportional to the absorption difference between left- and right-polarized light, A A = AL — AR, is recorded as a function of wavenumber. Relative differential absorptions (dissymmetry factors) AA/A at absorption maxima are typically 0.1—0.01 for uv—vis electronic transitions and 10-4 —10-5 for vibrational modes in the ir. 

Often, differential thermal analyses are employed to determine phase diagrams. These analyses can be viewed both positively and negatively. A positive view is that thermal behavior, such as phase transitions, can be viewed directly on charts and this method is usually rapid. If samples are heated or cooled too slowly or too rapidly, phase transitions can be passed over without causing changes in a temperature curve. If heated or cooled too slowly, the thermal activity was spread over too large a time span to be recorded. If heated or cooled too rapidly, a transition of the solid itself can be skipped. This is independent of the sensitivity of the instrument. 

The number of phenomena which can be directly studied by thermal analysis (DSC (DTA), TG, TMA, DMTA, TOA and DETA) is impressive. Typical of these methods is that only small amounts of sample (a few milligrams) are required for the analysis. Calorimetric methods record exo- and endothermic processes, e.g. melting, crystallization, liquid-crystalline phase transitions, and chemical reactions, e.g. polymerization, curing, depolymerization and degradation. Second-order transitions, e.g. glass transitions, are readily revealed by the calorimetric methods. Thermodynamic quantities, e.g. specific heat, are sensitively determined. TG is a valuable tool for the determination of the content of volatile species and fillers in polymeric materials and also for studies of polymer degradation. The majority of the aforementioned physical transitions can also be monitored by TMA (dilatometry). DMTA and DETA... 

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