Rheometers comparative analysis

It should be emphasized that many constitutive models have been proposed especially for polymeric solutions and melts, and there is a great deal of current research that is aimed at both new models25 and numerical analysis of fluid motions by use of the existing models 26 The problem is that few have been carefully compared with the behavior of real fluids outside the highly simplistic flows of conventional rheometers, and then mainly under flow conditions in which the perturbations in material structure are weak. Thus there is currently no model that is known to provide quantitatively accurate or even qualitatively reliable descriptions of real complex fluids for a wide spectrum of flows. 

If the capillary rheometer is used to compare different polymers, it is not necessary to go through the various correction procedures. However, if one wants to know the absolute values of the viscosity, it is important to apply the various correction factors. The most important corrections are the correction of the shear rate for non-Newtonian fluid behavior (often referred to as Rabinowitsch correction) and the correction of the shear stress for entrance effects (often referred to as Bagley correction). These are the most common corrections applied to capillary rheometers. Other corrections that are sometimes considered are corrections for viscous heating, corrections for the effect of pressure on viscosity, corrections for compressibility, correction for time effects, etc. If many corrections are applied to the data, the whole measurement and data analysis procedure can become very complex and time consuming. 

The observed shift of the glass transition was further analyzed by thermal analysis. Differential scanning calorimetry (DSC) was used and the results are shown in Figure 4. A similar shift to that found in the rheometer was observed. Without the frequency dependence however, the Tg was -27°C for the starting PTMC as compared to -16°C for the PC ionomer. 

The viscosity of a 5,000-ppm solution of partially hydrolyzed polyacrylamide was measured at several shear rates with a cone-and-plate rheometer. Table 5.59 gives experimental data as a function of shear rate. Plot the viscosity vs. shear rate on log-log paper and determine the power-law exponent, n, and the power-law constant, K, from the experimental data using least-squares analysis and correlate the viscosity as a function of shear rate. Compare values of the viscosity from the correlation with experimental data. 

---