Viscoelasticity tests

Rossi et al. [30] evaluated rheologically mucins of different origin with polyacrylic acid and sodium carboxymethyl cellulose. The same group also reported a novel rheological approach based on a stationary viscoelastic test (creep test) to describe the interaction between mucoadhesive polymers and mucins [31,32]. Jabbari et al. [33] used attenuated total-reflection infrared spectroscopy to investigate the ehain interpenetration of polyaciylic acid in the mucin interface. 

Butter and milk fat exhibit viscoelastic behavior at small stresses (Chwiej, 1969 Pijanowski et al., 1969 Shama and Sherman, 1970 Sherman 1976 Shukla and Rizvi, 1995). To probe this behavior, a very small stress or deformation is applied to a sample and the relationships between stress, strain and time are monitored. Viscoelastic testing is performed in the linear viscoelastic region (LVR) where a linear relationship between stress and strain exists and where the sample remains intact. Depending on the material, this region lies at a strain of less than 1.0% (Mulder and Walstra, 1974) or even less than 0.1% (Rohm and Weidinger, 1993). Figure 7.10 shows the small deformation test results for milk fat at 5°C. 

In a dynamic viscoelastic test the following results are obtained ... 

Finally, we propose some useful experiments which have not been extensively exploited. Large-amplitude oscillatory motion constitutes another nonlinear viscoelastic test. Although this dynamic experiment can be run in a steady state (i.e., with strain rate cycling in a steady manner), the fluid structure is supposedly changing with time within each cycle, a characteristic of transient experiments in the general sense. This provides a promising avenue to utilize an "undesirable" artifact—amplitude dependence of oscillatory properties—as a source of new information. 

The viscoelastic behavior of polymer fibers is complex. Experimentally, it is important to perform simple laboratory tests from which information relevant to actual in-use conditions can be obtained. The viscoelastic characterization of polymer fibers often consists of condncting mechanical tests that are similar to those discussed in Chapter 15, but are modified so as to enable the observation of the time dependency of fibers response. Three most important viscoelastic tests are creep, stress relaxation, and dynamic mechanical testing. 

These thermally relevant characteristic values can be obtained by dynamic mechanical analysis (DMA). This analysis returns information about the way mechanical properties change under slight, usually sinusoidal dynamic loading as a function of temperature, time, and/or frequency. The periodic force (input signal) causes a corresponding response signal, namely deformation or strain in the viscoelastic test specimen, which can be evaluated for dynamic-elastic characteristic values in terms of amplitude and phase offset [36]. 

In this paper, we employ the encapsulation method and mixing method to produce SWNT composites. Excess surfactant was extracted and the polymer remelted to form films for characterization. Mechanical, electrical, thermal and linear viscoelastic tests were used to characterize the films. Results from the encapsulation method are not presented because the approach was unsuccessful since the introduction of the reacting emulsion caused migration of surfactant from the nanotubes to the reacting emulsion leading to a loss of dispersion of the nanotubes. 

In the simplest nonlinear viscoelastic test, a large step shear strain y is applied at time 0 to a material and the resulting shear stress Os is measured at time t>0. The nonlinearity of the material deteaed by this test is characterized by... 

The paper discusses the application of dynamic indentation method and apparatus for the evaluation of viscoelastic properties of polymeric materials. The three-element model of viscoelastic material has been used to calculate the rigidity and the viscosity. Using a measurements of the indentation as a function of a current velocity change on impact with the material under test, the contact force and the displacement diagrams as a function of time are plotted. Experimental results of the testing of polyvinyl chloride cable coating by dynamic indentation method and data of the static tensile test are presented. 

The radiation and temperature dependent mechanical properties of viscoelastic materials (modulus and loss) are of great interest throughout the plastics, polymer, and rubber from initial design to routine production. There are a number of laboratory research instruments are available to determine these properties. All these hardness tests conducted on polymeric materials involve the penetration of the sample under consideration by loaded spheres or other geometric shapes [1]. Most of these tests are to some extent arbitrary because the penetration of an indenter into viscoelastic material increases with time. For example, standard durometer test (the "Shore A") is widely used to measure the static "hardness" or resistance to indentation. However, it does not measure basic material properties, and its results depend on the specimen geometry (it is difficult to make available the identity of the initial position of the devices on cylinder or spherical surfaces while measuring) and test conditions, and some arbitrary time must be selected to compare different materials. 

International Rubber Hardness. The International mbber hardness test (ASTM D1415) (2) for elastomers is similar to the Rockwell test ia that the measured property is the difference ia penetration of a standard steel ball between minor and major loads. The viscoelastic properties of elastomers require that a load appHcation time, usually 30 seconds, be a part of the test procedure. The hardness number is read directly on a scale of 0 to 100 upon return to the minor load. International mbber hardness numbers are often considered equivalent to Durometer hardness numbers but differences ia iadenters, loads, and test time preclude such a relationship. 

Piston Cylinder (Extrusion). Pressure-driven piston cylinder capillary viscometers, ie, extmsion rheometers (Fig. 25), are used primarily to measure the melt viscosity of polymers and other viscous materials (21,47,49,50). A reservoir is connected to a capillary tube, and molten polymer or another material is extmded through the capillary by means of a piston to which a constant force is appHed. Viscosity can be determined from the volumetric flow rate and the pressure drop along the capillary. The basic method and test conditions for a number of thermoplastics are described in ASTM D1238. Melt viscoelasticity can influence the results (160). 

Penetration—Indentation. Penetration and indentation tests have long been used to characterize viscoelastic materials such as asphalt, mbber, plastics, and coatings. The basic test consists of pressing an indentor of prescribed geometry against the test surface. Most instmments have an indenting tip, eg, cone, needle, or hemisphere, attached to a short rod that is held vertically. The load is controlled at some constant value, and the time of indentation is specified the size or depth of the indentation is measured. Instmments have been built which allow loads as low as 10 N with penetration depths less than mm. The entire experiment is carried out in the vacuum chamber of a scanning electron microscope with which the penetration is monitored (248). 

Tensile Testing. The most widely used instmment for measuring the viscoelastic properties of soHds is the tensile tester or stress—strain instmment, which extends a sample at constant rate and records the stress. Creep and stress—relaxation can also be measured. Numerous commercial instmments of various sizes and capacities are available. They vary greatiy in terms of automation, from manually operated to completely computer controlled. Some have temperature chambers, which allow measurements over a range of temperatures. Manufacturers include Instron, MTS, Tinius Olsen, Apphed Test Systems, Thwing-Albert, Shimadzu, GRC Instmments, SATEC Systems, Inc., and Monsanto. 

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. 

The energy release rate (G) represents adherence and is attributed to a multiplicative combination of interfacial and bulk effects. The interface contributions to the overall adherence are captured by the adhesion energy (Go), which is assumed to be rate-independent and equal to the thermodynamic work of adhesion (IVa)-Additional dissipation occurring within the elastomer is contained in the bulk viscoelastic loss function 0, which is dependent on the crack growth velocity (v) and on temperature (T). The function 0 is therefore substrate surface independent, but test geometry dependent. 

Strength and Stiffness. Thermoplastic materials are viscoelastic which means that their mechanical properties reflect the characteristics of both viscous liquids and elastic solids. Thus when a thermoplastic is stressed it responds by exhibiting viscous flow (which dissipates energy) and by elastic displacement (which stores energy). The properties of viscoelastic materials are time, temperature and strain rate dependent. Nevertheless the conventional stress-strain test is frequently used to describe the (short-term) mechanical properties of plastics. It must be remembered, however, that as described in detail in Chapter 2 the information obtained from such tests may only be used for an initial sorting of materials. It is not suitable, or intended, to provide design data which must usually be obtained from long term tests. 

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