Thermomechanical, Dynamic Mechanical and Dielectric Methods

The dimensional and mechanical stability of materials is of paramount importance to their use in the everyday world where they may encounter a wide variation in temperature through design or by accident. Many polymers are processed at elevated temperatures so as to enable them to flow and be more amenable to fabrication. Food items are cooked, pasteurised or otherwise heated or frozen. Ceramics are fired so as to consolidate their final structure. The relationship between a material s dimensional and mechanical properties and its temperature is studied by the techniques described within this chapter and, due to common concepts, the effect of heat on the electrical properties of materials is also considered. 

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Thermomechanical, Dynamic Mechanical and Dielectric Methods Thermomechanical Analysis... 

Unlike crystalline melting, the glass transition temperature is a relaxation transition. This means that it is dependent on the effective frequency of the measureirtent. This frequency is found by dynamic mechanical (DMA), dielectric relaxation, and pulsed nuclear magnetic methods. Quasi-static methods, such as dilatometry, differential scanning calorimetry (DSC), and thermomechanical analysis (TMA), show that the effective frequency depends on the rate of temperature scan. This is one of the reasons why the glass transition temperatures reported for various amorphous materials appear so diverse. 

Most of the physical properties of the polymer (heat capacity, expansion coefficient, storage modulus, gas permeability, refractive index, etc.) undergo a discontinuous variation at the glass transition. The most frequently used methods to determine Tg are differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical thermal analysis (DMTA). But several other techniques may be also employed, such as the measurement of the complex dielectric permittivity as a function of temperature. The shape of variation of corresponding properties is shown in Fig. 4.1. 

Thermal properties Thermal properties are the properties of materials that change with temperature. They are studied by thermal analysis techniques, which include DSC, thermogravimetric analysis (TGA), differential thermal analysis (DTA), thermomechanical analysis (TMA), dynamic mechanical analysis (DMA)/dynamic mechanical thermal analysis (DMTA), dielectric thermal analysis, etc. As is well known, TGA/DTA and DSC are the two most widely used methods to determine the thermal properties of polymer nanocomposites. TGA can demonstrate the thermal stability, the onset of degradation, and the percentage of silica incorporated in the polymer matrix. DSC can be... 

The methods discussed in this book are differential photocalorimetry, differential scanning calorimetry, dielectric thermal analysis, differential thermal analysis, dynamic mechanical analysis, evcrived gas analysis, gas chromatography, gas chromatography (oml)ined with mass spectrometry, mass spectrometry, microthermal analysis, thermal volalilisalion, Ihermogravimetric analysis and thermomechanical analysis. 

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