Steady Shear Elastic Properties

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... 

The generated data may appear confusing at the outset if looked at cursorily. The presence of fillers in the pol)nners seems to show decreases as well as increases in elasticity and the reports may appear contradictory at times. However, a careful look at the elastic properties data of filled pol5mtier systems shows that it is the method of representation and certain categories of fillers which show unexpected trends. The effects of various parameters on the steady sheelastic properties are discussed in various subsections as was done for the steady shear viscous properties in the preceding chapter. However, all subcategories may not be covered and the extent of discussion is at times concise due to limited information. 

The experimental studies of the influence of fillers on the rheological properties of polymer melts by White et al. [29] best illustrate the effect of filler type. The steady shear elastic data were generated in terms of the first normal stress difference using the cone and plate arrangement of the Rheometrics Mechanical Spectrometer at a fixed temperature of 180°C. 

A typical evolution of equilibrium mechanical properties during reaction is shown in Fig. 6.1. The initial reactive system has a steady shear viscosity that grows with reaction time as the mass-average molar mass, Mw, increases and it reaches to infinity at the gel point. Elastic properties, characterized by nonzero values of the equilibrium modulus, appear beyond the gel point. These quantities describe only either the liquid (pregel) or the solid (postgel) state of the material. Determination of the gel point requires extrapolation of viscosity to infinity or of the equilibrium modulus to zero. 

The viscous and elastic properties of orientable particles, especially of long, rod-like particles, are sensitive to particle orientation. Rods that are small enough to be Brownian are usually stiff molecules true particles or fibers are typically many microns long, and hence non-Brownian. The steady-state viscosity of a suspension of Brownian rods is very shear-rate- and concentration-dependent, much more so than non-Brownian fiber suspensions. The existence of significant normal stress differences in non-Brownian fiber suspensions is not yet well understood. 

This relation between y from recoil and the zero-shear-rate properties j0 and 60 from steady shear flow has also been obtained by Lodge [(46), p. 141] for his elastic liquid model which is derivable from a network entanglement theory his model does not, however, give the higher... 

In marked contrast to measurements of shear rheological properties, such as apparent viscosity in steady shear, or of complex viscosity in small amplitude oscillatory shear, extensional viscosity measurements are far from straightforward. This is particularly so in the case of mobile elastic liquids whose rheology can mitigate against the generation of well-defined extensional flow fields. 

Correlation Between Steady-Shear and Oscillatory Data. The viscosity function is by far the most widely used and the easiest viscometric function determined experimentally. For dilute polymer solutions dynamic measurements are often preferred over steady-shear normal stress measurements for the determination of fluid elasticity at low deformation rates. The relationship between viscous and elastic properties of polymer liquids is of great interest to polymer rheologists. In recent years, several models have been proposed to predict fluid elasticity from shear viscosity data. 

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]. 

Traditionally, tack properties of a PSA have been correlated to their linear rheological behavior, such as elastic and loss modulus [1-3]. While this type of phenomenological analysis provides many clues for the practical design of PSA, it is intrinsically limited by the fact that a tack experiment involves large strains and transient behaviors of the PSA, which cannot be easily predicted by either viscosity (shear, elongational) or any other small strain steady-state dynamical property. The simple observation of the debonding of a PSA tape from a solid... 

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