BK9780854046102-00094

BK9780854046102-00094 - C hapter 4 Thermomechanical Dynamic...

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Chapter 4 Thermomechanical, Dynamic Mechanical and Dielectric Methods D. M. Price IPTME, Loughborough University, UK INTRODUCTION AND PRINCIPLES 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 con- cepts, the effect of heat on the electrical properties of materials is also considered. Thermomechanical Analysis and Thermodilatometry Thermomechanical Analysis (TMA) can be defined as the measurement of a specimen’s dimensions (length or volume) as a function of tempera- ture whilst it is subjected to a constant mechanical stress. In this way thermal expansion coefficients can be determined and changes in this property with temperature (and/or time) monitored. Many materials will deform under the applied stress at a particular temperature which is often connected with the material melting or undergoing a glass-rubber transi- tion. Alternatively, the specimen may possess residual stresses which have 94
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Thermomechanical, Dynamic Mechanical and Dielectric Methods 95 Figure 1 cant i lever Common mechunicul deformation modes: compression, tension, shear, torsion, bending (single cantilever, dual cantilever, three point bending) been “frozen-in” during preparation. On heating, dimensional changes will occur as a consequence of the relaxation of these stresses. Stress (a) is defined as the ratio of the mechanical force applied (F) divided by the area over which it acts (A): The stress is usually applied in compression or tension, but may also be applied in shear, torsion, or some other bending mode as shown in Figure 1. The units of stress are N m-2 or Pa. If the applied stress is negligible then the technique becomes that of thermodilatometry. This technique is used to determine the coefficient of thermal expansion of the specimen from the relationship: where a is the coefficient of thermal expansion (ppm OC-l or pm m-l OC-’), lo is the original sample length (m) and dl/dTis the rate of change of sample length with temperature (pm OC-’).
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96 Chapter 4 Dynamic Mechanical Analysis Dynamic Mechanical Analysis (DMA) is concerned with the measure- ment of the mechanical properties (mechanical modulus or stiffness and damping) of a specimen as a function of temperature. DMA is a sensitive probe of molecular mobility within materials and is most commonly used to measure the glass transition temperature and other transitions in macromolecules, or to follow changes in mechanical properties brought about by chemical reactions.
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BK9780854046102-00094 - C hapter 4 Thermomechanical Dynamic...

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