Rheology dynamic shear

Rheological Measurements of Bitumens Using Dynamic Shear Rheometers ... 

The rheological behavior of storage XGs was characterized by steady and dynamic shear rheometry [104,266]. Tamarind seed XG [266] showed a marked dependence of zero-shear viscosity on concentration in the semi-dilute region, which was similar to that of other stiff neutral polysaccharides, and ascribed to hyper-entanglements. In a later paper [292], the flow properties of XGs from different plant species, namely, suspension-cultured tobacco cells, apple pomace, and tamarind seed, were compared. The three XGs differed in composition and structural features (as mentioned in the former section) and... 

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... 

Alvarez, M. D., Fernandez, C., Canet, W. (2004). Rheological behaviour of fresh and frozen potato puree in steady and dynamic shear at different temperatures. Fur. Food Res. Techrwl, 218, 544-553. 

With the aid of these relaxation times other rheological properties like normal stresses, flow birefringence and dynamic shear moduli can be calculated. A more detailed discussion of this procedure wiE be given in Chapter 4. 

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 nonlinear viscoelastic models (VE), which utilize continuum mechanics arguments to cast constitutive equations in coordinate frame-invariant form, thus enabling them to describe all flows steady and dynamic shear as well as extensional. The objective of the polymer scientists researching these nonlinear VE empirical models is to develop constitutive equations that predict all the observed rheological phenomena. 

It is well known in polymer rheology that a torsional parallel-plate flow cell develops certain secondary flow and meniscus distortion beyond some stress level [ 14]. For viscoelastic melts, this can happen at an embarrassingly low stress. The critical condition for these instabilities has not been clearly identified in terms of the shear stress, normal stress, and surface tension. It is very plausible that the boundary discontinuity and stress intensification discussed in Sect. 4 is the primary source for the meniscus instability. On the other hand, it is well documented that the first indication of an unstable flow in parallel plates is not a visually observable meniscus distortion or edge fracture, but a measurable decay of stress at a given shear rate [40]. The decay of the average stress can occur in both steady shear and frequency-dependent dynamic shear. 

Tam, K.C. and Tiu, C. 1989. Steady and dynamic shear properties of aqueous polymer solutions. Journal of Rheology 33 257-280. 

The rheological properties of the complex network structure of tomato pastes may be assumed to be made up of two contributions (1) one network structure contributed by the solids phase, in proportion to 0s, and (2) another network structure contributed by the liquid (continuous phase), in proportion to 0i = 1 - 0s- However, the effective continuous phase is not the low-viscosity serum itself, but a highly viscous liquid that is an integral part of the tomato paste. Also implicit is that the solids fraction plays a major role in the structure of the TP samples, that is, it can be considered to be the structuring component. These assumptions are also in line with the weak gel behavior indicated by the dynamic shear data. 

W. Loose and S. Hess, Rheol. Acta, 28, 91 (1989). Rheology of Dense Model Fluids Via Nonequilibrium Molecular Dynamics Shear Thinning and Ordering Transition. 

LDPE, and with polypropylene, PP, was studied In steady state shear, dynamic shear and uniaxial extenslonal fields. Interrelations between diverse rheological functions are discussed In terms of the linear viscoelastic behavior and Its modification by phase separation Into complex morphology. One of the more Important observations Is the difference In elongational flow behavior of LLDPE/PP blends from that of the other blends the strain hardening (Important for e.g. fllm blowing and wire coating) occurs In the latter ones but not In the former. 

A standard commercial film blowing LLDPE resin, LPX-30, was blended at different ratios with either other LLDPE s or a LDPE polymer. The characteristic properties of these materials are listed In Table II. The resins were generously donated to the project by Esso Chem., Canada. Prior to blending the polymers were thoroughly characterized by SEC, SEC/LALLS, solution viscosity, CNMR, Atomic Absorbance, and their rheological behavior was characterized In steady state and dynamic shear flow as well as In the uniaxial extenslonal deformation (44-46). 

Dynamic shear rheology involves measuring the resistance to dynamic oscillatory flows. Dynamic moduli such as the storage (or solid-like) modulus (G ), the loss (or fluid-like) modulus (G"), the loss tangent (tan 8 = G"IG ) and the complex viscosity ( / ) can all be used to characterize deformation resistance to dynamic oscillation of a sinusoidally imposed deformation with a characteristic frequency of oscillation (o). 

Shear rheology itself can be further subdivided into cases of steady shear, dynamic shear and transient shear. 

In dynamic-shear rheology an oscillating or dynamic shearing deformation is applied to the material, and dynamic-shear properties are measured. For the simplest case of an infinite parallel-plate system, Figure 3.68, one can define relevant properties as follows. 

Other parameters that may be defined in dynamic shear rheology are the viscous dynamic viscosity, rj = G"/o, and the solid dynamic viscosity, tj" = G Ico. 

The relationship between steady-shear viscosity and dynamic-shear viscosity is also a common fundamental rheological relationship to be examined. The Cox-Merz empirical rule (Cox, 1958) showed for most materials that the steady-shear-viscosity-shear-rate relationship was numerically identical to the dynamic-viscosity-frequency profile, or r] y ) = r] m). Subsequently, modified Cox-Merz rules have been developed for more complex systems (Gleissle and Hochstein, 2003, Doraiswamy et al., 1991). For example Doriswamy et al. (1991) have shown that a modified Cox-Merz relationship holds for filled polymer systems for which r](y ) = t] (yco), where y is the strain amplitude in dynamic shear. 

Dynamic Shear Rheology of A1203 Filled Composites... 

Three types of flow are mainly used in the rheological measurements steady state shearing, dynamic shearing, and elongation. The three can be classified according to the strain, y, vorticity, as well as uniformity of stress, a, and strain within the measuring space (see Table 7.1). 

Blends of atactic poly(methyl methacrylate) with poly(ethylene glycol), PMMA/PEG, were reported miscible [Colby, 1989]. Their rheology, PMMA/PEG = 50/50 and 80/20 at T = 160-210°C, was studied in a dynamic shear field [Booij and Palmen, 1992]. By contrast with homopolymers, the blends did not follow the time-temperature superposition. The deviation was particularly large at low temperatures. The reason for the deviation is most likely based on the different temperamre dependence of the relaxation functions. The authors concluded that in miscible blends, the temperature dependence of the relaxation times of individual macromolecules depends on composition. This leads to different degrees of mutual entanglement and hence the rubber plateau moduli. 

Smith, J. R., Smith, T. L., and Tschoegl, N. W. (1970). Rheological properties of wheat flour doughs. III. Dynamic shear modulus and its dependence on amplitude, frequency and dough composition. Rheol. Acta 9, 239-252. 

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