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Loss modulus, viscoelasticity measurements

Considerable information about elastic and viscoelastic parameters may be derived by measuring the response of a polymer to a small-amplitude cyclic deformation. Molecules perturbed in this way store a portion of the imparted energy elastically, and dissipate a portion in the form of heat (Ferry, 1970 Meares, 1965 Miller, M. L., 1966, pp. 243-253 Nielsen, 1962, Chapter 7 Rosen, 1971 Schultz, 1974, pp. 67-71 Williams, D. J., 1971), the ratio of dissipation to storage depending on the temperature and frequency. In dynamic mechanical spectroscopy experiments, a cyclic stress is applied to a specimen, and two fundamental parameters are measured the storage modulus E a measure of the energy stored elastically, and the loss modulus a measure of the energy dissipated. The loss modulus E" may be calculated as follows ... [Pg.27]

A viscoelastic material also possesses a complex dynamic viscosity, rj = rj - - iv( and it can be shown that r = G jiuj-, rj = G juj and rj = G ju), where CO is the angular frequency. The parameter Tj is useful for many viscoelastic fluids in that a plot of its absolute value Tj vs angular frequency in radians/s is often numerically similar to a plot of shear viscosity Tj vs shear rate. This correspondence is known as the Cox-Merz empirical relationship. The parameter Tj is called the dynamic viscosity and is related to G the loss modulus the parameter Tj does not deal with viscosity, but is a measure of elasticity. [Pg.178]

Gels are viscoelastic bodies, the rheological properties of which can be described by two parameters, the storage modulus (G, which is a measure of its elasticity) and the loss modulus (G", which is a measure of its viscous nature). The combined viscoelastic modulus (G ) is a measure of the overall resistance of a gel to deformation. These moduli are often highly dependent on the time-scale of deformation. Another important parameter of a food gel is its yield stress. [Pg.374]

The physical properties of barrier dressings were evaluated using the Seiko Model DMS 210 Dynamic Mechanical Analyzer Instrument (see Fig. 2.45). Referring to Fig. 2.46, dynamic mechanical analysis consists of oscillating (1 Hz) tensile force of a material in an environmentally (37°C) controlled chamber (see Fig. 2.47) to measure loss modulus (E") and stored modulus (E ). Many materials including polymers and tissue are viscoelastic, meaning that they deform (stretch or pull) with applied force and return to their original shape with time. The effect is a function of the viscous property (E") within the material that resists deformation and the elastic property (E )... [Pg.53]

In a rheomety experiment the two plates or cylinders are moved back and forth relative to one another in an oscillating fashion. The elastic storage modulus (G - The contribution of elastic, i.e. solid-like behaviour to the complex dynamic modulus) and elastic loss modulus (G" - The contribution of viscous, i.e. liquid-like behaviour to the complex modulus) which have units of Pascals are measured as a function of applied stress or oscillation frequency. For purely elastic materials the stress and strain are in phase and hence there is an immediate stress response to the applied strain. In contrast, for purely viscous materials, the strain follows stress by a 90 degree phase lag. For viscoelastic materials the behaviour is somewhere in between and the strain lag is not zero but less than 90 degrees. The complex dynamic modulus ( ) is used to describe the stress-strain relationship (equation 14.1 i is the imaginary number square root of-1). [Pg.895]

The viscoelastic properties such as storage modulus, loss modulus, and tan 8 are measured using a dynamic mechanical analyzer (DMA) and/or a rheometer. [Pg.264]

Viscoelastic properties of polypeptides in the solid state have been popular subjects of study however, they appear not to have been studied in the l t of liquid crystals so far, except in a few cases 90, 91). The d matnic loss modulus (measured... [Pg.102]

Viscoelastic measurements of ionomers have been used to indirectly characterize the microstructure and to establish property structure relationships. Forming an ionomer results in three important changes in the viscoelastic properties of a polymer. First, T usually increases with increasing ionization. This is a conseqi nce of the reduced mobility of the polymer backbone as a result of the formation of physical, ionic crosslinks. Second, an extended rubber plateau is observed in the modulus above T, again as a result of the ionic network. Third, a high temperaturi mechanical loss is observed above T, which is due to motion in the ion-rich phase. The dynamic mechan cal curves for SPS ionomers shown in Fig. 9 clearly demonstrate these three characteristics. [Pg.25]


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See also in sourсe #XX -- [ Pg.2 , Pg.959 , Pg.960 ]




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