Dynamic mechanical measurements tests

Dynamic mechanical measurements for elastomers that cover wide ranges of frequency and temperature are rather scarce. Payne and Scott [12] carried out extensive measurements of /a and /x" for unvulcanized natural mbber as a function of test frequency (Figure 1.8). He showed that the experimental relations at different temperatures could be superposed to yield master curves, as shown in Figure 1.9, using the WLF frequency-temperature equivalence, Equation 1.11. The same shift factors, log Ox. were used for both experimental quantities, /x and /x". Successful superposition in both cases confirms that the dependence of the viscoelastic properties of rubber on frequency and temperature arises from changes in the rate of Brownian motion of molecular segments with temperature. 

Dynamic-Mechanical Measurement. This is a very sensitive tool and has been used intensively by Nielsen (17) and by Takayanagi (18). When the damping curves from a torsion pendulum test are obtained for the parent components and for the polyblend and die results are compared, a compatible polyblend will show a damping maximum between those of the parent polymers whereas the incompatible polyblend gives two damping maxima at temperatures corresponding to those of the parent components. Dynamic mechanical measurement can also give information on the moduli of the parent polymer and the polyblend. It can be shear modulus or tensile modulus. If the modulus-temperature curve of a polyblend locates between those of the two parent polymers, the polyblend is compatible. If the modulus-temperature curve shows multiple transitions, the polyblend is incompatible. 

Dynamic mechanical measurements were performed with a Rheometrics model RMS 7200 mechanical spectrometer at a fixed frequency of 1 rad/s through a temperature range from -100 C to 150 C under dry nitrogen. The test specimens were prepared in rectangular shape about 60 mm in length, 11 mm in width, and 4 mm in thickness. The applied strain was 1%. 

Dynamic mechanical measurements are performed at very small strains in order to ensure that linear viscoelasticity relations can be applied to the data. Stress-strain data involve large strain behavior and are accumulated in the nonlinear region. In other words, the tensile test itself alters the structure of the test specimen, which usually cannot be cycled back to its initial state. (Similarly, dynamic deformations at large strains test the fatigue resistance of the material.)... 

There are other supports used in torsional braid (or perhaps, more strictly termed, supported dynamic mechanical) measurements. These include solid substrates (solids that are free of mechanical transitions under the testing conditions), which have been shown to improve the precision of results over those obtained using conventional glass braids (Wetton, 1986). However, these solid substrates generally require thick coatings. Also wire-mesh... 

The elastic nature of a fluid is characterized by dynamic mechanical or stress relaxation techniques. Dynamic mechanical (oscillatory) testing is a procedure in which a sample is sinusoidally strained and the resultant stress is measured. The shear stress T varies with the same frequency as the shear rate... 

In dynamic mechanical tests, the response of a material to periodie stress is measured. There are many types of dynamic mechanical test instruments. Each has a limited Irequency range, but it is generally possible to cover frequencies from 1(E to 10 cycles per second. A popular instrument for dynamic mechanical measurements is the torsion pendulum (Figure 13.6A). A polymer sample is elamped at one end, and the other end is attaehed to a disk that is free to oscillate. As a result of the damping characteristics of the test sample, the amplitude of oscillation decays with time (Figure 13.6B). 

Dynamic mechanical measurements are not limited to running experiments on samples in air or inert gases. With care, measurements can be carried out with the test specimen immersed in a liquid or on liquid samples themselves as the following examples demonstrate. 

Compatibility of two- and threecomponent systems of poly(vinyl chloride) with homo- and copolymers of methyl methacrylate and butyl methacrylate has been studied. The mixtures were tested for compatibility in the dynamic viscosity of polymer solutions, in microscopic observations in polarized light, as well as by dynamic mechanical measurements. It was found that poly(methyl methacrylate) and methacrylic copolymers were compatible with PVC. Poly(butyl methacrylate) appears to be incompatible with PVC. Estimation of solubility parameter values 6 made it possible to predict the compatibility of polymer pairs. Critical A6 value for compatible polymers has been found to be 0.5 (cal cm 3)1/2. 

Standard Definitions and Descriptions of Terms Relating to Conditioning, E41, Annual book of ASTM Standards, American Society for Testing and Materials Standard Definitions and Descriptions of Terms Relating to Dynamic Mechanical Measurements on plastics, D4092, Annual book of ASTM Standards, American Society for Testing and Materials... 

Storage modulus n. In dynamic mechanical measurements, the part of the complex modulus that is in phase with the strain, with the symbol G if the testing mode is shear, E if it is tension or compression. 

ASTM D4092-01 (ISO 6721) is entitled Standard Terminology for Dynamic Mechanical Measurements on Plastics, DoD Adopted. It contains descriptions of the technical terms for dynamic mechanical property measurements on plastics including solids, melts, and solutions. It is also relatable to ISO 472 Definitions and certain items in ISO 6721-01, Plastics—Determination of Dynamic Mechanical Properties, Part 1, General Principles. Other ASTM/ISO standard test methods for dynamic mechanical analyses are... 

Perhaps it is more understandable if we correlate adhesive performance to viscoelastic properties determined at the adhesive testing temperature. The bond is formed and destroyed during testing using conditions which differ in stress level, deformation rate, and extent of deformation at room temperature. Therefore, the measurement of viscoelastic properties at room temperature may be pertinent. Viscoelastic properties at constant temperature are determined by dynamic mechanical measurement over a range of frequencies, for example, from 0.1 to 100 rad/sec. We see that the frequency scan curves are approximately a reciprocal... 

Generally, two different types of measurement are applied to determine the linear viscoelastic behavior, namely static (or equilibrium) and dynamic mechanical measurements. Static tests involve the imposition of a step change in stress and the observation of any subsequent development in time of the strain, whereas dynamic tests involve the application of a harmonically varying strain. In ordinary thermoplastic polymer systems, test conditions such as strain or frequency must be in the linear range otherwise, the results will be dependent on the experimental details rather than on the material under test. 

Dynamic mechanical relaxation tests measurement of molecular mobility in different polymer phases as a function of temperature (24). 

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

Measurement of dynamic mechanical properties was carried out under tension mode using a viscoelasto-meter, (Rheovibron DDV-III-EP, M/s, Orientec Corp., Tokyo, Japan). Sample size was 3.5 cm x 6.5 mm x 2 mm. Testing was carried out at a low amplitude, 0.025 mm, over a temperature range of - 100°C to +200°C. Heating rate was TC/min and frequency of oscillation was 3.5 Hz or 110 Hz. 

Dynamic mechanical tests measure the response or deformation of a material to periodic or varying forces. Generally an applied force and its resulting deformation both vary sinusoidally with time. From such tests it is possible to obtain simultaneously an elastic modulus and mechanical damping, the latter of which gives the amount of energy dissipated as heat during the deformation of the material. 

This second group of tests is designed to measure the mechanical response of a substance to applied vibrational loads or strains. Both temperature and frequency can be varied, and thus contribute to the information that these tests can provide. There are a number of such tests, of which the major ones are probably the torsion pendulum and dynamic mechanical thermal analysis (DMTA). The underlying principles of these dynamic tests have been covered earlier. Such tests are used as relatively rapid methods of characterisation and evaluation of viscoelastic polymers, including the measurement of T, the study of the curing characteristics of thermosets, and the study of polymer blends and their compatibility. They can be used in essentially non-destructive modes and, unlike the majority of measurements made in non-dynamic tests, they yield data on continuous properties of polymeric materials, rather than discontinuous ones, as are any of the types of strength which are measured routinely. 

The authors have characterized the graft polymer by solvent extraction, transmission electron microscopy, dynamic mechanical analysis, mechanical testing (including measurement of tensile, tear, and impact strength), and morphology by SEM. The reaction scheme is given in Figure 11.25. 

The B-series of silica samples were also blended with rubber and the compound formulation is shown in Table 17.6. The uncured gums were then tested according to ISO 5794-2 1998. The uncured samples were tested using a Mooney viscometer and an RPA, which measures the dynamic mechanical properties as the samples cure. Figure 17.7 shows the results of these two tests for the Mooney viscosity at 100°C, storage modulus, loss modulus, and tan 8. 

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