Rheological determination

Rheological determinations are destructive of the structures they measure for this reason they do not portray the actual structure of the dispersion at rest. Accordingly, various methods have been devised for extrapolating to zero the results of measurements at various shear rates. The most useful one has been the conversion of viscosities to fluidities at various shear rates and the extrapolation of the resulting nearly linear relationship to zero shear, as shown in Figure 7. Sometimes a power of the shear rate, D, provides a better distinction between a sol (essentially a liquid) and a gel (essentially a solid), as shown in the figure, but the difference between a finite intercept (sol) and zero fluidity (gel) is largely fictitious because of the dependence of the intercept on the exponent n. 

Polymers are viscoelastic materials meaning they can act as liquids, the visco portion, and as solids, the elastic portion. Descriptions of the viscoelastic properties of materials generally falls within the area called rheology. Determination of the viscoelastic behavior of materials generally occurs through stress-strain and related measurements. Whether a material behaves as a viscous or elastic material depends on temperature, the particular polymer and its prior treatment, polymer structure, and the particular measurement or conditions applied to the material. The particular property demonstrated by a material under given conditions allows polymers to act as solid or viscous liquids, as plastics, elastomers, or fibers, etc. This chapter deals with the viscoelastic properties of polymers. 

Shah RD, Kabadi M, Pope DG, Augsburger LL. Physico-mechanical characterization of the extrusion-spheronization process. Part II rheological determinants for successful extrusion and spheronization. Pharm Res 1995 12(4) 496-507. 

Figure 7.18. Microstructural analysis Rheological determination of fractal dimension (D) and exponential term (A) for milk fat (A) slowly (0. l°C/min) or (B) rapidly cooled (5.0°C/min) to 5°C.

The rheology determines a distribution of residence time in the barrel, and the resultant heat transfer characteristics. Even with simple flow behaviour, finite-element modelling predicts greater shear rates and heating at the walls. This explains the observations by Richmond, that homogeneous melt structures first form at the wall, and further implies that on exit from the die, the melt stream may not be homogeneous with regard to its shear and/or temperature history. 

TABLE 2. Rheologically Determined Fractal Dimensions (Dr) and Pre-Exponential Terms X) for Anhydrous Milkfat Crystallized at Various Rates of Cooling and Storage Times at 5°C. 

Figure 5-7 shows the frequency dependences of the storage and loss moduli at various times during the reaction, from 6 minutes before G to 6 minutes after it. Note that at tc (labeled Gel Point in Fig. 5-7), G and G" follow power laws over the entire frequency range For times less than this (labeled —2 and —6 in Fig. 5-7), the curves slope downward at low frequencies, which is indicative of fluid-like behavior, while at times after the gel point (labeled -t-2 and - -6), G flattens at low frequency—a characteristic of solidlike behavior. Thus, the intermediate state with a power-law frequency dependence over the whole frequency range is the transitional state between liquid-like and solid-like behavior, and therefore it defines the gel point. This rheologically determined gel point coincides with the conventional value, namely the maximum degree of cure at which...

Rheology. Determination of mechanical properties can be done in various deformation modes (shearing, compression, extension), leading to different results. Of the parameters to be determined, the modulus or stiffness of materials has been studied and explained best results can give information about structure. 

The yield stress can be obtained by the rotational viscometry technique as shown in Chapter 4. Thus from two sets of rheological determinations, one the capillary flow technique and the other rotational viscometry, enough information can be obtained to treat the problems associated with the flow of grease in pipes. 

Gharachorlou, A. and Goharpey, F. (2008) Rheologically determined phase behavior of LCST blends in the presence of spherical nanoparticles. Macromdlecules, 41 (9), 3276-3283. 

We also noted that the alignment persisted through thermal cycles even 14°C above ODT and in the absence of an electric field. This suggests that ordered structure persists well above the rheologically determined ODT, presumably in a metastable state. This surprising result could be explained by the possibility that the rheological experiments used to measure the order-disorder transition temperature destroy metastable structure that persists in our quiescent experiments. A theory by Fredrickson and Binder [74] predicts that lamellar microstructure will persist as metastable up to 9°C above ODT-... 

Stamhuis, J.E. and Loppe, J.P.A. (1982) Rheological determination of polymer-filler affinity, Rheol Acta, 21,103-5. 

For dip coating, the substrate is immersed into the precursor solution for wetting and is then removed from the solution bath to obtain a gel layer after drainage and solvent evaporation. The velocity rate of the substrate withdrawal and the solution rheology determine the film thickness (Figure 27.4b). 

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