Shear measurements

Brown, W Schillen, K Hvidt, S, Triblock Copolymers in Aqueous Solution Studied by Static and Dynamic Light Scattering and Oscillatory Shear Measurements. Influence of Relative Block Sizes, Journal of Physical Chemistry 96, 038, 1992. 

Small an jlitude oscillatory shear measurements have been widely used to follow the gelation process of biopolymers [9] because of their non-destroying character and the great variability of procedures, which allow simultaneous investigations of various features of a single sample. 

Dynamic oscillatory shear measurements of polymeric materials are generally performed by applying a time dependent strain of y(t) = y0sin(cot) and the resultant shear stress is a(t) = y0[G sin(a)t) + G"cos(cot)], with G and G" being the storage and loss modulus, respectively. 

As in PP-based nanocomposite systems, the extended Trouton rule, 3r 0 (y t) = r E (so t), also does not hold for PLANC melts, in contrast to the melt of pure polymers. These results indicate that in the case of P LANC, the flow induced internal structural changes also occur in elongation flow [48], but the changes are quite different in shear flow. The strong rheopexy observed in the shear measurements for the PLA-based nanocomposite at very slow shear rate reflects the fact that the shear-induced structural change involved a process with an extremely long relaxation time. 

If a disklike specimen is sheared between two end plates by rotation of one over the other to obtain the shear modulus, then at any moderate twist angle the strain (and strain rate) vary along the radius, so only an effective shear modulus is obtained. For better results the upper plate is replaced with a cone of very small angle. Figure 4 shows fche cone-and-plate and two other possible test geometries for making shear measurements. 

Fernandez et al. (2007) have characterized the rheological behavior of the mashed potatoes with added biopolymers using steady shear measurements. Fresh and frozen/thawed mashed potatoes present shear thinning with yield stress (Canet et al., 2005a), and dynamic shear data reveal weak gel-like behavior in potato purees (Alvarez et al., 2004). The effects are strongly... 

Mills,N.J., Nevin,A. Oscillatory shear measurements on polystyrene melts in the terminal region. J. Polymer Sci. Pt. A-2 9,267-281 (1971). 

Other techniques can only be mentioned here. The most important are the "delay rod technique" and the "multiple echo techniques". In the first method the sample is placed between two quartz rods which are directly bonded (by a silicone liquid) to the transducers. Longitudinal and shear measurements are made separately with different sets of quartz transducers (X-cut crystal for longitudinal, Y-cut crystal for shear waves). 

The usual shear measurements on polymer melts are performed as steady-state experiments in which a stationary state of shear deformation is maintained. A steady-state experiment on tensile deformation, however, means an imitation of a melt spinning process. This type of experiment has several disadvantages ... 

Samples and Measurements. A scheme for shear measurements is shown in figure 2. Two identical polymer samples of length L, thickness h, and width W are bonded to two rigid metal mounts. The inner mount is attached to an impedance head (a combination force and acceleration sensor), which in turn is attached to a shaker. An accelerometer is attached to the top center of the outer mount. 

The linear viscoelastic properties of all samples were characterized by dynamic shear measurements in the parallel-plate geometry. Experimental details have been previously published [9]. Using time-temperature equivalence, master curves for the storage and loss moduli were obtained. Fig. 1 shows the master curves at 140°C for the relaxation spectra and Table 3 gives the values of zero-shear viscosities, steady-state compliances and weight-average relaxation times at the same temperature. 

Differences between solid-like and liquid-bke complex fluids show up in all three of the shearing measurements discussed thus far the shear start-up viscosity t), the steady-state viscosity rj(y), and the linear viscoelastic moduli G co) and G (o). The start-up stresses a = y/ +()>, t) of prototypical liquid-like and solid-like complex fluids are depicted in Fig. 1-6. For the liquid-like fluid the viscosity instantaneously reaches a steady-state value after inception of shear, while for the solid-like fluid the stress grows linearly with strain up to a critical shear strain, above which the material yields, or flows, at constant shear stress. 

Wang, Y. and J.G. Williams, Corrections for mode II fracture toughness specimens of composite materials. Composites Science and Technology, 1992. 43 p. 251-256. O Brien, T.K., Composite interlaminar shear fracture toughness, Gnc Shear measurement or sheer myth in Composite Materials Fatigue and Fracture 7. 

Entanglement networks and weak gels may exhibit thixotropy, or time-dependent flow, in addition to exhibiting either pseudoplastic or plastic flow. Thixotropy, which is noted in a rheogram as a hysteresis loop, occurs because the gel requires a finite time to rebuild its original structure that breaks down during continuous shear measurements. The degree of thixotropy depends on gel type, sample history, and experimental conditions. 

The ring or annular shear cell, was developed by Carr and Walker as early as 1968. In recent years this tester has undergone a number of modifications. Peschl has developed an annular shear cell in which the sample and shear cell consists of a full circle. This contrasts to the earlier cells that have a band of sample on the outer portion of the circle. This was done to eliminate wall friction. It is also rotated very slowly, since at low speed, velocity variability becomes more negligible in the shear measurement. In this way a full ring can be utilized and speed differences in the outside and inside of the ring become negligible. 

In order to investigate the influence of chemical structure on the melt viscosity, polyesters A1.A2.B1 and B2 were compared in the steady mode in plate and plate-geometry at 300°C. A2 and B2 should be in the isotropic region, whereas Al, B1 would be in the nematic state. Prior to steady shear measurements a transient step from... 

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. 

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