Yield viscoelastic behavior

FT rheometry is a powerful technique to document the nonlinear viscoelastic behavior of pure polymers as observed when performing large amplitude oscillatory strain (LAOS) experiments. When implemented on appropriate instmments, this test technique can readUy be applied on complex polymer systems, for instance, filled mbber compounds, in order to yield significant and reliable information. Any simple polymer can exhibit nonlinear viscoelastic properties when submitted to sufficiently large strain in such a case the observed behavior is so-called extrinsic... 

Binary fluorides, methods of preparing noble-gas, 77 335-336 Binary heterogeneous polymer blends compliance of, 20 347-348 moduli of, 20 346-347 nonlinear viscoelastic behavior of, 20 348 yield and/or tensile strength of, 20 348-349... 

Dynamic mechanical experiments yield both the elastic modulus of the material and its mechanical damping, or energy dissipation, characteristics. These properties can be determined as a function of frequency (time) and temperature. Application of the time-temperature equivalence principle [1-3] yields master curves like those in Fig. 23.2. The five regions described in the curve are typical of polymer viscoelastic behavior. 

There have been a number of studies that demonstrate that crystallized AMF and butter exhibit linear (ideal) viscoelastic behavior at low levels of stress or strain (4), where the strain is directly proportional to the applied stress. For most materials, this region occurs when the critical strain (strain where structure breaks down) is less than 1.0%, but for fat networks, the strains typically exceed 0.1% (4, 66). Ideally, within the LVR, mUkfat crystal networks will behave like a Hookean solid where the stress is directly proportional to the strain (i.e., a oc y), as shown in Figure 15 (66, 68). Within the elastic region, stress will increase linearly with strain up to a critical strain. Beyond that critical strain (strain at the limit of linearity), deformation of the network will occur at a point known as the yield point. The elastic limit quickly follows, beyond which permanent deformation and sample fracture occurs. Beyond these points, the structural integrity of the network is compromised and the sample breaks down. 

Although motional averaging might occur in ways other than that envisioned by Cates, temperature-jump experiments have yielded values of Tbr that indicate Tbr < in the region where the relaxation is nearly monoexponential, in agreement with Cates theory. In addition, Cates theory offers distinctive predictions for the concentration-dependencies of the viscoelastic behavior these allow the theory to be tested rather stringently. To obtain these predictions, we note that in the semi-dilute regime, the mean-field reptation time is L 4>, where 0 is the volume fraction of surfactant. Hence, from Eqs. (12-31) and... 

Flow of hexagonal liquid crystalline systems is presumably a function of the alignment of the rod-like aggregates along their long axis in the direction of flow.f " The shear thinning flow process can be accompanied by an apparent yield stress. Viscoelastic behavior has also been reported. ... 

This chapter is an in-depth review on rheology of suspensions. The area covered includes steady shear viscosity, apparent yield stress, viscoelastic behavior, and compression yield stress. The suspensions have been classified by groups hard sphere, soft sphere, monodis-perse, poly disperse, flocculated, and stable systems. The particle shape effects are also discussed. The steady shear rheological behaviors discussed include low- and high-shear limit viscosity, shear thinning, shear thickening, and discontinuity. The steady shear rheology of ternary systems (i.e., oil-water-solid) is also discussed. 

Extensional Flows The yield stress also occurs in extensional flows [Kamal et al., 1984 Utracki, 1988]. Yield stress is apparent in two related dependencies (i) as a vertical displacement in the stress growth function at decreasing strain rates, contrasting with the normal linear viscoelastic behavior of single-phase polymeric melts, and (ii) as a deviation from the theory that Lim tie(e) = 3... 

ABSTRACT A rotational benchtop Rheometer with vane spindles can be used to measure the static yield stress behavior of materials. By running at different rotational speeds, the Rheometer data can be equated with the viscoelastic information determined by an oscillating rheometer. The rotational Rheometer offers a less expensive method suitable for Quality Control needs. 

In this respect, textile polymers can almost be described in the same way as isotropic polymers in terms of their viscoelastic behavior, but not quite. The same relative criteria apply, such as elastic deformation, primary creep, yield, secondary creep, creep recovery, isothermal, or adiabatic deformation. 

The thixotrqiic behavior of suspensions and emulsions is a rather difficult property to measure, and this is further cmnplicated by the contribution of viscoelastic behavior (see next section). The main reason for this difflculty lies in the nature of the phenomenon involved. Thixotropy arises because of the existence of interparticle forces that produce three-dimensional microsmictures called flocculates or aggregates (see Sec. III). Depending on the magnitude of these forces, these microstructures are more or less prone to destruction by shearing, in consequence, any manipulation of a thixotropic sample may induce structural breakdown, thereby changing, most often irreversibly, the viscosity and yield stress of the sample. 

Increasing tb may also increase viscoelastic behavior and the yield stress value. Figure 23 displays values of storage modulus G as a function of clay concentration and pH, for sodium montmorillonite suspensions aged for 200 min (12). It can be observed that the higher the clay content, the higher G. ... 

It does not yield an accurate description of viscoelastic behavior. 

As mentioned above, interfacial films exhibit non-Newtonian flow, which can be treated in the same manner as for dispersions and polymer solutions. The steady-state flow can be described using Bingham plastic models. The viscoelastic behavior can be treated using stress relaxation or strain relaxation (creep) models as well as dynamic (oscillatory) models. The Bingham-fluid model of interfacial rheological behavior (27) assumes the presence of a surface yield stress, cy, i.e.. 

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