Dynamic stress rheometer

Several researchers reported viscoelastic behavior of yeast suspensions. Labuza et al. [9] reported shear-thinning behavior of baker s yeast (S. cerevisiae) in the range of 1 to 100 reciprocal seconds at yeast concentrations above 10.5% (w/w). The power law model was successfully applied. More recently, Mancini and Moresi [10] also measured the rheological properties of baker s yeast using different rheometers in the concentration range of 25 to 200 g dm. While the Haake rotational viscometer confirmed Labuza s results on the pseudoplastic character of yeast suspension, the dynamic stress rheometer revealed definitive Newtonian behavior. This discrepancy was attributed to the lower sensitivity of Haake viscometer in the range of viscosity tested (1.5 to 12 mPa s). Speers et al. [11] used a controlled shear-rate rheometer with a cone-and-plate system to measure viscosity of... 

In our laboratory, we have applied biconical bob rheom-etry to highly elastic films formed at asphal-tene-containing, model oil-water interfaces (80). Instead of using a torsion wire, however, we have utilized a commercially available, highly sensitive dynamic stress rheometer in conjunction with a serrated edge stainless steel bob. As we will show, the results of the rheometry method compare well with measures of emulsion stability under comparable conditions. Thus, this method provides useful insights into the... 

The thixotropy is illustrated for one material in Fig. 9.5 (Owen and Combe, unpublished results). Using a Rheometrics Dynamic Stress Rheometer, materials... 

The rheological measurements were made using a controlled-stress Rheometrics dynamic stress rheometer (SR 500) with a data module and computer acquisition system. The measuring head was a parallel-plate system with diameter 2.5 and 4 cm and a gap of 1 mm. The steady-state viscosity curves, the creep curves and the components of the complex modulus and the complex viscosity were determined in the temperature region between 170 and 230 °C in a dry nitrogen atmosphere. Samples were produced in the form of injection-moulded disks. Orchestrator 6.5.5 software was used to analyse the data. The characteristics of materials investigated are summarized in Table 1. 

Two copolymers, a poly(styrene-6-isoprene-7>-styrene) (SIS) triblock (60 wt% S Mn=100,000, Mw/Mn=1.04) and a poly(styrene-Wsoprene) (SI) diblock (70 wt% S Mn=50,000, Mw/Mn=1.05), were synthesized by anionic polymerization. The selective solvent used here was an aliphatic white mineral oil (MO) produced by Witco (380PO). Specific masses of each copolymer and MO weae dissolved in cyclohexane and cast into molds. Upon solvent evaporation, the resultant films were vacuum-dried for up to 7 h at 120 C. Steady-shear tests were performed on a Rheometrics dynamic stress rheometer (DSR) as a function of shear stress (x) to measure the solution viscosity (q), while dynamic tests were performed here to discern G and G" as functions of x, oo and temperature. 

When using small deformation rheology there are several useful parameters that may be obtained to describe a material the complex modulus (G ), storage modulus (G ), loss modulus (G") and the tangent of the phase shift or phase angle (tan 5). These values must be taken from within the LVR, and are obtained using a dynamic oscillatory rheometer (Rao 1999). Outside the LVR, important information may be obtained such as the yield stress and yield strain. 

While the dynamic experiments described above are to be conducted in the linear viscoelastic range, another experiment can be conducted in which the results obtained in the non-linear range are useful. With a controlled-stress rheometer, one can conduct an experiment in which the stress is increased continuously at a constant oscillatory frequency, say 1 Hz. Results obtained in such an experiment are shown schematically in Figure 3-40. As the stress is increased continuously, initially, G and G" remain relatively constant until at a critical value of stress, Oc, the magnitude of G decreases sharply and that of G" also decreases not as sharply after a slight inerease. One may also use the value of the applied stress at which the curves of G and G" intersect... 

Small deformation rheometry refers to testing procedures that do not cause structural damage to the sample. Constant stress rheometers, such as dynamic mechanical analyzers or oscillatory constant stress rheometers, are often used. 

Neat polymers and their blends were studied In dynamic shear field (using RMS) and In constant shear stress field using Rheometrlc Stress Rheometer, RSR. The molecular parameters of polymers and blends were determined by Size Exclusion Chromatography in trichlorobenzene at 14O C. The morphology of freeze-fractured specimens was characterized In Scanning Electron Microscope, SEM, Jeol JSM-35CF. 

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]. 

Rheological measurements were performed in a stress rheometer fixture with a 2-cm cone and plate having a 1° cone angle and gap of 27 pm. Dynamic shear moduli were measured at 0.5% strain between 0.1 and 100 rad/s. Creep compliance was measured with a constant applied stress in the range of 0.1 to 5 kPa. Both measurements were performed over a series of temperatures to obtain data for time-temperature superposition. 

During rheometer and dynamic mechanical analyses instruments impose a deformation on a material and measure the material s response that gives a wealth of very important information about structure and performance of the basic polymer. As an example stress rheometers are used for testing melts in various temperature ranges. Strain controlled rheology is the ultimate in materials characterization with the ability to handle anything from light fluids to solid bars, films, and fibers. 

Measuring yield stress of concentrated suspensions can be carried out using various rheological techniques that can be broadly classified under two categories the controlled rate rheometry and the controlled stress rheometry. A controlled rate rheometer deforms a specimen at a constant shear rate and measures the shear stress. On the other hand, a controlled stress rheometer imposes a constant shear stress on a specimen and then measures the corresponding strain. The latter approach involves a more sophisticated control system and is only introduced in the last ten years. These techniques can be further classified as direct (or static) or indirect methods (or dynamic). The indirect determination of yield stress involves the extrapolation of experimental shear stress - shear rate data to obtain a yield stress, which is the shear stress at zero shear rate. This is illustrated in Figure 9. It is evident that the choice of the model or methods yield differing values of yield stress. 

Rheological properties of the gels were performed with a constant stress rheometer (Rheolyst ARIOOO N, TA Instruments) with a cone and plate geometry (cone diameter 40 mm and 2° angle). Rheological experiments were cmied out in steady flow, creep and dynamic oscillation modes over a wide range of shear rates, frequencies, temperatures and time. To prevent evaporation of the hydrocarbon a solvent trap (as supplied by TA Instruments) was installed for all experimrats. 

The Weissenberg 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 characteri2ation of viscoelastic materials. Its capabiUties include measurement of steady-state rotational shear within a viscosity range of 10 — mPa-s at shear rates of, of normal forces (elastic... 

The shear modulus of a material can be determined by a static torsion test or by a dynamic test employing a torsional pendulum or an oscillatory rheometer. The maximum short-term shear stress (strength) of a material can also be determined from a punch shear test. 

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). 

Steady shear viscosities can be measured with two different instruments. The System IV can measure polymer viscosities from about 0.001 to 10 sec 1 while the Gottfert Capillary Rheometer is capable of obtaining viscosities from 0.1 to 100,0001/s. In steady shear, the strains are very large as opposed to the dynamic measurements that impose small strains. In the capillary rheometer, the polymer is forced through a capillary die at a continuously faster rate. The resulting stress and viscosity are measured by a transducer mounted adjacent to the die. A schematic of the system is illustrated in Figure 5. 

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