Oscillatory shear rheometer

Dynamic mechanical properties are measured to evaluate melt rheology of thermoplastics with and without additives which may modify rheological characteristics of these compositions. " Dynamic oscillatory shear rheometers are used for these purposes. Two geometries of test fixtures are used including parallel plates and cone and plate. Instrument use for these measurements must be capable of measuring forces (stress or strain) and frequency. Temperature must be controlled in a broad range and various modes of temperature sweeps should be available. Sample geometry is not specified but it should be suitable for measurement in particular experimental setup. 

The Weissenbeig Rheogoniometer (49) is a complex dynamic viscometer that can measure elastic behavior as well as viscosity. It was the first rheometer designed to measure both shear and normal stresses and can be used for complete characterization of viscoelastic materials. Its capabilities include measurement of steady-state rotational shear within a viscosity range of 10-1 —13 mPa-s at shear rates of 10-4 — 104 s-1, of normal forces (elastic effect) exhibited by the material being sheared, and of an oscillatory shear range of 5 x 10-6 to 50 Hz, from which the elastic modulus and dynamic viscosity can be determined. A unique feature is its ability to superimpose oscillation on steady shear to provide dynamic measurements under flow conditions all measurements can be made over a wide range of temperatures (—50 to 400°C). 

Experimentally, the dynamic shear moduli are usually measured by applying sinusoidal oscillatory shear in constant stress or constant strain rheometers. This can be in parallel plate, cone-and-plate or concentric cylinder (Couette) geometries. An excellent monograph on rheology, including its application to polymers, is provided by Macosko (1994). 

The most common dynamic method is oscillatory testing, in which the sample is subjected to a sinusoidal oscillatory strain, and the resulting oscillatory stress measured. The more sophisticated rotational viscometers have the additional capability of dynamically testing liquid-like materials using small angle oscillatory shear. A parallel disc viscometer can be set up for testing solid-like materials (e.g., butter), in oscillatory shear. Some UTM-type solids rheometers, in which the moving crosshead can be made to reciprocate sinusoidally, can be used to test solid-like materials in oscillatory deformation in compression, tension or shear. 

Viscoelastic measurements in oscillatory shear were performed using a Rheometrics System Four rheometer in the frequency range 10 3 rad/sec < co < 102 rad/sec. 8 mm diameter parallel plates were used for temperatures below 80°C and 25 mm diameter parallel plates were used for T>80°C, with plate separations of 1.1 0.2 mm. Comparison of... 

Both strain- and stress-controlled rotational rheometers are widely employed to study the flow properties of non-Newtonian fluids. Different measuring geometries can be used, but coaxial cylinder, cone-plate and plate-plate are the most common choices. Using rotational rheometers, two experimental modes are mostly used to study the behavior of semi-dilute pectin solutions steady shear measurements and dynamic measurements. In the former, samples are sheared at a constant direction of shear, whereas in the latter, an oscillatory shear is used. 

Letterpress news inks are pseudoplastic but not viscoelastic at the shear rates shown in Figure 9. Polymer resins present in offset ink formulations render them viscoelastic. This can be measured under oscillatory shear in a rheometer such as the Mechanical Spectrometer ... 

From R D to quality control, rheology measurements for each phase of the product development life cycle involve raw materials, premixes, solutions, dispersions, emulsions, and full formulations. Well-equipped laboratories with stress- and strain-controlled oscillatory/steady shear rheometers and viscometers can generally satisfy most characterization needs. When necessary, customized systems are designed to simulate specific user or process conditions. Rheology measurements are also coupled with optic, thermal, dielectric, and other analytical methods to further probe the internal microstucture of surfactant systems. New commercial and research developments are briefly discussed in the following sections. 

Wasan and his research group focused on the field of interfacial rheology during the past three decades [15]. They developed novel instruments, such as oscillatory deep-channel interfacial viscometer [20,21,28] and biconical bob oscillatory interfacial rheometer [29] for interfacial shear measurement and the maximum bubble-pressure method [15,29,30] and the controlled drop tensiometer [1,31] for interfacial dilatational measurement, to resolve complex interfacial flow behavior in dynamic stress conditions [1,15,27,32-35]. Their research has clearly demonstrated the importance of interfacial rheology in the coalescence process of emulsions and foams. In connection with the maximum bubble-pressure method, it has been used in the BLM system to access the properties of lipid bilayers formed from a variety of surfactants [17,28,36]. 

In a dynamic experiment, a small-amplitude oscillatory shear is imposed to a molten polymer confined in the rheometer. The shear stress response of the polymeric system can be expressed as in Equation 22.14. In this equation, G and G" are dynamic moduli related to the elastic storage energy and dissipated energy of the system, respectively. For a viscoelastic fluid, two independent normal stress differences, namely, first and second normal stress differences can be defined. These quantities are calculated in terms of the differences of the components of the stress tensor, as indicated in Equation 22.15a and 22.15b, and can be obtained, for instance, from the radial pressure distribution in a cone-and-plate rheometer [5]. Some other experiments used in the determination of the normal stress differences can be found elsewhere [9, 22] ... 

Prigogine, Trappeniers, and Mathot pressure-volume-temperature measurements lead zirconate titanate quaternary ammonium salts quasi-two-parameter theory rigid amorphous fraction Rheometrics extensional rheometer Rheometrics elongational rheometer for melts room temperature small-angle neutron scattering small-amplitude oscillatory shear flow small-angle x-ray scattering side-chain LCP... 

Because polymers have viscoelastic properties, rheometry is often employed to simultaneously measure both elastic and viscous parameters. To measure the viscoelastic properties using a rheometer, an oscillatory shear force/shear stress is applied to the material. Because of the viscoelasticity, the sample s deforma-tion/shear strain exhibits a phase lag to the shear force/shear stress. When the applied shear stress is ... 

For example, oscillatory shear experiments can be performed on a rotational rheometer (Fig. 4.17), where the polymer network is placed between two parallel plates in order to obtain the complex shear modulus G ... 

Rheometers capable of performing oscillatory shear are widely available as commercial instruments, in addition to more specialised devices [Te Nijen-huis and van Donselaar, 1985]. An example of the latter is an oscillating plate rheometer [Eggers and Richmann, 1995] which requires a very small liquid sample voliune (<0.3 ml) and has a (potentially) great frequency range (2 Hz to IkHz). This latter feature is imusual as it spans the gap between specialised devices and commercially available oscillatory shear instruments, whose frequency range is typically 10 Hz to ca. 10 Hz. 

Although the dynamic shear rheometer (DSR) can measure dynamic viscosity, its main use is to determine the viscous and elastic behaviour of bituminous binders at medium to high temperatures, particularly to determine the complex shear modulus (G ) and the phase angle (5) of bituminous binders when tested in dynamic (oscillatory) shear, using parallel plate geometry. Details for determining viscosity with DSR can be found in Tredrea (2007). 

Therefore, it is more suitable to measure the shear sensitivity of polypropylene. Capillary rheometers or oscillatory shear flow rheometers are widely used for that purpose. Moreover, an investigation of elongational flow properties of molten PP can be used to check, for example, the presence of long-chain branching in some speciality grades of PP (to study the strain-hardening effect). 

A number of experimental methods have been applied to measure the melt viscosity of polymers (53-55,65), but capillary extrusion techniques probably are generally preferred. Rotational methods are also used, and some permit the measurement of normal stress effects resulting from elasticity as well as of viscosity. Slit rheometers can also be used to measure normal stress (66). Oscillatory shear measurements are useful for measuring the elasticity of poljmier melts (57,58). Controlled stress methods have also been applied (59). Squeeze film flow has also been proposed as a geometry suitable for processibility testing of polymer melts... 

The first main feature of the System 4 RMS is its ability to characterize a wider range of the above systems than any other current rheometer. Several drives and stress transducers are required the wide range of modes, rates and materials to be handled would mean that no single drive/transducer combination would be sufficiently flexible and accurate. Instead, the System 4 RMS uses a turret with four different drive or transducer units one unit is used for steady shear tests, one for oscillatory shear, one for tension-compression and bending and one unit is used for low-viscosity-high-shear-rate tests on fluids. The second main feature of the new machine is its completely automatic microprocessor control coupled with data acquisition and reduction. Without doubt, this new rheometer will result in rheo-metrical tests being carried out more reliably, accurately and quickly than previously. 

Steady and oscillatory shear measurements were performed at a temperature of 20 °C with a Rheo-Stress RSI50 rheometer (Haake) using a Couette cell geometry for the glycerol and Sterocoll FD solutions and a cone-plate geometry (60 mm in diameter, 1° cone angle) for the Sterocoll D. 

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