Thermal analysis, degradation measurement

Measurements of thermal analysis are conducted for the purpose of evaluating the physical and chemical changes that may take place in a heated sample. This requires that the operator interpret the observed events in a thermogram in terms of plausible reaction processes. The reactions normally monitored can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

In DSC the measured energy differential corresponds to the heat content (enthalpy) or the specific heat of the sample. DSC is often used in conjunction with TA to determine if a reaction is endothermic, such as melting, vaporization and sublimation, or exothermic, such as oxidative degradation. It is also used to determine the glass transition temperature of polymers. Liquids and solids can be analyzed by both methods of thermal analysis. The sample size is usually limited to 10-20 mg. 

Thermal analysis is a group of techniques in which a physical property of a substance is measured as a function of temperature when the sample is subjected to a controlled temperature program. Single techniques, such as thermogravimetry (TG), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), dielectric thermal analysis, etc., provide important information on the thermal behaviour of materials. However, for polymer characterisation, for instance in case of degradation, further analysis is required, particularly because all of the techniques listed above mainly describe materials only from a physical point of view. A hyphenated thermal analyser is a powerful tool to yield the much-needed additional chemical information. In this paper we will concentrate on simultaneous thermogravimetric techniques. 

Thermogravimetry is a technique that measures the weight change of a sample as a function of temperature or time (time is suitable only when thermal analysis is performed at specified constant temperature increments). The solid or liquid sample is heated or cooled at a selected rate or isothermally maintained at a fixed temperature. TG is used to measure degradation, oxidation, reduction, evaporation, sublimation, and other heat-related changes occurring in polymers. 

Thermal analysis is capable of providing accurate information on the phase transition temperatures, degradation temperatures, heat capacity, and enthalpy of transition of polymers using comparatively simple DTA, DSC, and TG instruments. The measurement time is short compared with other techniques, such as viscoelastic measurement and nuclear magnetic resonance spectroscopy. Moreover, any kind of material, e.g., powders, flakes, films, fibers, and liquids, may be used. The required amount of sample is small, normally in the range of several milligrams. 

Measurement of the heat effect associated with the degradation can be performed by differential thermal analysis (DTA). The degraded sample... 

DSC is a thermal analysis technique that is used to measure the temperatures and energy flows related to transitions in materials as a function of time and temperature.These measurements provide qualitative and quantitative information about physical and chemical changes that involve endothermic or exothermic processes or changes in heat capacity. Any event, such as loss of solvent, phase transitions, crystallization temperature, melting point, and degradation temperature of the plastic sample, result in a change in the temperature of the sample. The systems available cover a wide temperature range, e g., -60°Cto>l,500°C. 

DDM = Diamino diphenyl methane For the mixtures of epoxy monomers, 1 1 mol ratio was used. Stoichiometric amounts were used in all cases. From differential scanning calorimetry method measurements (10 °C/min). Maximum of the loss modulus from dynamic mechanical thermal analysis measurements. a relaxation peak of the loss factor. Reproduced with permission from M. Sponton, L.A. Mercado, J.C. Ronda, M. Galia and V. Cadiz, Polymer Degradation and Stability, 2008, 93, 2025. 2008, Elsevier [22] ... 

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