Viscoelastic shear

Several studies of high frequency shear viscoelastic liquids have also been reported by Tozaki [68, 69] and Kanazawa [14] with the QCM. 

Barry BW. Continuous shear, viscoelastic and spreading properties of a new topical vehicle, FAPG base. J Pharm Pharmacol 1973 25 131— 137. 

An alternative explanation is based on the dispersion and its dependence on the dynamic variables probed (i.e., rotation versus translation) [172]. The dispersion of rotational diffusion and of the shear viscoelastic response is broader than that of translational diffusion or the mean-square displacement of the molecule. It is a consequence of the cooperative dynamics that the dispersion of different dynamic variables for the same substance can be different and the... 

The zero-shear viscoelastic properties of concentrated polymer solutions or polymer melts are typically defined by two parameters the zero-shear viscosity (f]o) and the zero-shear recovery compliance (/ ). The former is a measure of the dissipation of energy, while the latter is a measure of energy storage. For model polymers, the infiuence of branching is best established for the zero-shear viscosity. When the branch length is short or the concentration of polymer is low (i.e., for solution rheology), it is found that the zero-shear viscosity of the branched polymer is lower than that of the linear. This has been attributed to the smaller mean-square radius of the branched chains and has led to the following relation... 

FIG. 4.20 Stress/strain relationship for oscillatory shear viscoelastic measurements. 

They adapted an interfacial shear rheometer (plate/ rod) to measure the shear viscoelasticity of the system with and without dispersant. At an applied shear stress, creep curves for the system were monitored. There were no instantaneous elasticity and viscosity for the Kuwait and Tia Juana crudes with and wifliout dispersant. They attributed this to a network structure of flocculated asphaltenes in the films. They found that there was some dilatancy in their crude oil films, described as a stick/slip flow in their flow curves. However, fliis flow was attributed to thick films of asphaltene particles building up at the interface. Lfsing creep measurements, they examined a model system of as-phaltenes/n-heptane/toluene. They found a retarded elastic deformation, which was different from the response of the crade oils. This suggested to fliem that there was a different type of interfaeial slrueture formed with the model oil, and this may be attributed to die solveney of the medium and not to die lower asphaltenes eontent in the model system. 

These relations enable one to relate the shear viscoelastic functions to their tensile counterparts. At high compliance levels, rubbers are highly incompressible, and the proportional relation between the tensile and shear moduli and compliances holds. However, at lower compliances approaching Jg, the Poison ratio fi (which in an elongational deformation is -(Mw/dM, where w is the specimen s width and / is its length) is less than Eqs. (28) and (29) are then no longer exact. For a glass ju T. When G(t) = K(t), E t) = 2.25 Gif). 

Besides viscoelastic measurements described above, there are a variety of techniques that can probe the dynamic and viscoelastic properties of amorphous polymers. Their advantages and disadvantages as compared with shear viscoelastic measurements in elucidating the dynamic properties become clear in the following sections. 

The extensional thickening of polymer solutions is one form of viscoelastic behavior. This ability to support a tensile stress can also be demonstrated in a tubeless syphon with dilute aqueous solutions of polsrmers such as polyacrylamide or polyethylene oxide. If you suck up solution with a medicine dropper attached to a water aspirator and then lift the dropper out of the solution, the solution will still be sucked up. In shear, viscoelastic fluids develop normal stresses, which causes rod climbing on a rotating shaft, as opposed to the vortex and depressed surfaces that form with Newtonian liquids. Polsrmer solutions and semiliquid poljnners exhibit other viscoelastic behaviors, where, on short time scales, they behave as elastic solids. Silly putty, a childrens toy, can be formed into a ball and will slowly turn into a puddle if left on a flat surface. But if dropped to the floor it boimces. 

Chapters 6 to 9 discuss the steady shear viscous properties, steady shear elastic properties, unsteady shear viscoelastic properties and extensional flow properties, respectively. The effect of filler type, size, size distribution, concentration, agglomerates, smface treatment as well as the effect of polymer type are elucidated. The tenth chapter has been... 

In the preceding two chapters, various effects on steady shear viscous and elastic properties of filled polymer systems were discussed. The present chapter focuses on the unsteady shear viscoelastic properties of these systems. The unsteady shear characteristics are mainly discussed with respect to small-amplitude oscillations, namely, dynamic rheological data. In some cases, the thixotropic sweep responses and the stress relaxation behavior are also included because they bring out the rheological characteristics in some situations in a much better manner. Tbie extensive literature [1-85] on the rheology of filled polymer systems, however, contains quite limited information on the unsteady shear data [1,8,43-45,54,61,62,64,68,71,72,74,91,92]. 

The effect of various parameters on the unsteady shear viscoelastic properties are discussed at varying lengths in the following subsections. 

The smaller the size of the filler, the greater the particle-particle interaction and this reflects greatly on the unsteady shear viscoelastic properties. In the case of complex viscosity vs. frequency curves, it would be natural to expect yield stress with decreasing particle size as... 

Surface modifiers such as those listed in Table 15 are often used in order to achieve better filler dispersion and reduced agglomeration due to improvement in the wettability of the filler and on accoimt of promotion of filler-polymer contact rather than filler-filler contact. The effect of surface treatment on unsteady shear viscoelastic properties has been studied in highly filled systems [43-4554,92] and the available data do provide some basis for understanding the use of surface modifiers. 

Figure 8 shows a simple but useful analogy to this situation. It compares a squeezed solid rubber cylinder and a sheared viscoelastic liquid cylinder, where in both cases a force emerges at right angles—or normal—to the squeezing direction hence we talk about a normal force. 

The inclusion of values in Table 1 l-III derived from dynamic bulk viscoelastic measurements implies the concept that the relaxation times describing time-de-pendent volume changes also depend on the fractional free volume—consistent with the picture of the glass transition outlined in Section C. In fact, the measurements of dynamic storage and loss bulk compliance of poly(vinyl acetate) shown in Fig. 2-9 are reduced from data at different temperatures and pressures using shift factors calculated from free volume parameters obtained from shear measurements, so it may be concluded that the local molecular motions needed to accomplish volume collapse depend on the magnitude of the free volume in the same manner as the motions which accomplish shear displacements. Moreover, it was pointed out in connection with Fig. 11 -7 that the isothermal contraction following a quench to a temperature near or below Tg has a temperature dependence which can be described by reduced variables with shift factors ay identical with those for shear viscoelastic behavior. These features will be discussed more fully in Chapter 18. 

Molecular motions very similar to some of these may also occur in vitrifying liquids of low molecular weight near and below Tg. Indeed, the bulk viscoelastic properties, as evidenced by the course of volume contraction near Tg illustrated in Fig. 11 -7 and discussed further in Chapter 18, seem to be very similar for both polymers and small molecules (Section B1 of Chapter 18). In shear viscoelastic properties, however, there are some characteristic differences, and it is instructive to examine the behavior of small molecules first. 

Figure 1.12 Stress-strain relationship for a typical oscillatory surface shear viscoelasticity measurement.

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