Shear elastic moduli glass transition temperature

Elastomers are cross-linked macromolecules above the glass transition temperature. The cross-link density is the fundamental engineering quantity which, for instance, determines the modulus of elasticity. Usually, it is measured during vulcanization of well-defined rubber samples in a vulcameter by the moment necessary to perform a given torsional shear of the test sample. Swelling experiments can be performed alternatively, but are problematic for filled elastomers. Such measurements are based on the assumption that the measured quantity does not vary over the sample volume. Inhomogeneous cross-link densities can be determined from the surface hardness, but volumetric resolution is achieved by conventional methods only after cutting the sample. 

At low temperature, an amorphous polymer is glassy, hard, and brittle, but as the temperature increases, it becomes rubbery, soft, and elastic. There is a smooth transition in the polymer s properties from the solid to the melt, as discussed above, so no melting temperature is defined. At the glass transition temperature, marking the onset of segmental mobility, properties like specific volume, enthalpy, shear modulus, and permeability show significant changes, as illustrated in Fig. 3.43. 

Production of SA having high glass transition temperature but which maintain shear strength and modulus of elasticity comparable to those of existing commercial products that exhibit good creep behaviour... 

The Poisson s ratio can be determined provided the adhesive displacement is measured in the longitudinal and transversal directions. This property is very difficult to measure experimentally, especially in the elastic range. For polymers, the Poisson s ratio varies between 0.3 and 0.5. For temperatures below the glass transition temperature (Tg), the value is close to 0.3. When the adhesive is above Tg or in the plastic region, the Poisson s ratio is about 0.5. Table 19.1 gives the values for several adhesives. One way to determine the Poisson s ratio is to deduce it from the measurement of the Young s modulus and the shear modulus (see Sect. 19.5). 

The temperature range between the glass transition range and the flow or fusion (melt) range, in which the shear modulus remains nearly constant, becomes wider as the molar mass grows. The plastic is then described as entropy or mbber elastic, or even thermoelastic (as opposed to thermoplastic). 

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