Measurement of viscoelasticity

Newton s law of viscous flow and Hooke s law for solids describe the perfect state for each. In practice however, few if any materials show this ideal behaviour and are more appropriately described as viscoelastic. That is to say, they exhibit both viscous and elastic behaviour. More importantly, the relative contribution of each with regard to a materials response will depend on the time scale of the experiment. If the experiment is relatively slow the material will appear viscous. Conversely, if the experiment is relatively fast the material will appear more elastic. 

Typically the stress wave will have a phase difference (d) between 0° and 90°. The in-phase and out-of-phase components of the stress wave can be separated to give the elastic or storage modulus (G ) and the viscous or loss modulus (G ). These can be calculated from the expression  

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Viscoelastic phenomena always involve the change of properties with time and, therefore, the measurements of viscoelastic properties of solid polymers may be called dynamic mechanical. Dynamic mechanical thermal analysis (DMTA) is a very useful tool for studying... 

Rao, V.N.M. 1992. Classification, description and measurement of viscoelastic properties of solid foods. In Viscoelastic Properties of Foods (M.A. Rao and J.F. Steffe, eds.) pp. 3-47. Elsevier Science Publishers, New York. 

The relative magnitudes of elasticity and viscosity for a viscoelastic material depend on the scale of the observation time. If the rate of a deformation is very slow, a material may behave more like a viscous fluid. The faster the deformation occurs, the more elastic the material appears. Therefore, in the measurement of viscoelasticity, the rate of deformation should be chosen based on a practical situation of interest (e.g., the rate of processing or mastica-... 

The position of the transition point can be estimated (see Section 6.4), due to measurements of viscoelastic properties, as M (4.6-12.0)Me. It corresponds to the above value of transition point, though the empirical evaluation of relaxation times could not be done with great accuracy in these investigations. 

Rao, M. A. 1992. Measurement of Viscoelastic Properties of Fluid and Semisolid Foods, in Viscoelastic Properties of Food, ed. M. A. Rao and J. F. Steffe, pp. 207-232, Elsevier Applied Science Publishers, New York. 

The cone-plate geometry is widely used in rheological measurements of viscoelastic fluids. The fluid is placed between a plate of radius and a cone of the same radius. The angle, a, between the cone and the plate is usually smaller than 3° (see Fig. 13.19). 

R448 L. A. Marky and D. W. Kupke, Enthalpy - Entropy Compensations in Nucleic Acids Contribution of Electrostriction and Structm-al Hydration , Methods Enzymol, 2000,323,419 R449 A. F. Martins, Measurement of Viscoelastic Coefficients for Nematic Mesophases Using Magnetic Resonance , EMIS Datarev. Ser., 2000, 25,405... 

In the model most often used to represent QCM-D measurements of viscoelastic films, probed in either gas or liquid environments, the films are represented by a Voigt element, characterized by a complex shear modulus ... 

Dynamic measurements of viscoelastic behavior are based on a different conception. The nature of a stress-relaxation or a creep experiment is such tiiat no accurate measurements can be made for times leas than 1 sec in some cases 10 sec or more pass before the first measurement is made. Very often interesting phenomena of polymer viscoelasticity occur in a... 

Measurements of viscoelastic properties under severe deformation which may lead ultimately to rupture have considerable technological and theoretical importance. For example, in periodic deformation the lifetimes of a set of specimens from a sample show large scatter. With fibers it is not unusual that a section of a fiber which was broken in a fatigue experiment has a lifetime which is comparable with or higher than the lifetime of the original specimen. 

Smith R. and Keiper D. 1965. Dynamic measurement of viscoelastic properties of bone. Am. J. Med. Elec. 4 156. 

We have used the generalized phenomenological Maxwell model or Boltzmann s superposition principle to obtain the basic equation (Eq. (4.22) or (4.23)) for describing linear viscoelastic behavior. For the kind of polymeric liquid studied in this book, this basic equation has been well tested by experimental measurements of viscoelastic responses to different rate-of-strain histories in the linear region. There are several types of rate-of-strain functions A(t) which have often been used to evaluate the viscoelastic properties of the polymer. These different viscoelastic quantities, obtained from different kinds of measurements, are related through the relaxation modulus G t). In the following sections, we shall show how these different viscoelastic quantities are expressed in terms of G(t) by using Eq. (4.22). 

Scott, W. W. and Bhushan, B. 2003. Use of phase imaging in atomic force microscopy for measurement of viscoelastic contrast in polymer nanocomposites and molecularly thick lubricant films. 

Standard applications of DLS experiments in bulk samples include measurements of viscoelastic coefficients and dispersion relations of orientational fluctuation modes. As the experimental details are given further in the text, let us first discuss the nature of orientational fluctuation modes in nematics. 

Viscoelastic data of relatively high concentrated solutions were measured in oscillating and steady-shear flow at room temperature (ca. 25C) by using Fluid Spectrometers RFS-8500 and RFSn, and a Mechanical Spectrometer RMS-800 of Rheometrics, Inc., with the cone-and-plate and the co-axial cylinder. At relatively low concentrations most of 7 data was measured with capillary viscometers of Maron-Krieger-Sisko type at 2511, and J data at two low concentrations were obtained from flow birefringence measurements[12] for comparison. To examine the degradation of the samples after the measurements of viscoelastic properties, we measured [ 7 ] to confirm that there was no degradation. 

The objective of this study was to make sure that degradation of PE was prevented during the conditioning process. Different techniques were used to examine the stability of PE in the melt blender. Small-strain dynamic oscillatory measurements of viscoelastic properties (r] ) in a mechanical spectrometer as well as and molecular weight distribution from GPC analysis were used to assess the stability of samples of linear low-density polyethylene (LLDPE) and LDPE in the melt blender. 

There has long been interest in making measurements of viscoelastic properties in this instrument. Mooney [M42] described measurements of elastic recoil, and Koopmann and Kramer [K16] of Bayer AG developed a Mooney viscometer allowing stress relaxation after flow. Such an instrument was later discussed by Montes et al. [M37]. Monsanto [G21] and later Alpha Technologies have commercialized a quahty-control instrument similar to that of Koopmann and Kramer. 

Viscoelastic flow behaviour of dilute polymer solutions in porous media is described as a method for characterization of polymer-solvent-temperature systems. Porous media flow tests provide information on the solution state of polymer solutions and the molecular weight of the polymers used. Furthermore, flow-induced and thermally induced degradation effects - frequently observed in polymer solutions - can be characterized by the measurement of viscoelastic effects in flow through porous media. Decrease of molecular weight and changes of the conformation of macromolecules in solution are important parameters in these processes. 

The conclusion is that, for polyethylene gels, the measurement of viscoelasticity is underexploited. 

It will be clear that in the field of block copolymers, fallow aretis are present to a high degree. The difficulty of the measurement of viscoelastic properties is the sensitiven of the gels to (shear) deformations linear viscoelasticity is, in general, only measurable at very small deformations, so that very small stresses have to be measured. Special sensitive instruments are needed for these studies. 

DMA Measurement of viscoelastic properties of solid materials as a function of temperature and time Glass transition, brittleness, anisotropy, crystallinity, residual stress, residual cure, and aging. 

Measurement of viscoelastic and rheological properties of polymers are of increasing importance and recently developed instrumentation for measuring these properties discussed in Chapter 15 includes dynamic mechanical analysis, thermo-mechanical analysis... 

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