Normal stress difference unified plots

Normal stress difference data are not as extensively avaUable as the shear viscosity data. The plots, therefore, do not contain the abundance of data as in the unified curves for shear viscosity. The approach here was to establish the curve profile more than prove the validity of the unifying technique which, in the case of shear viscosity, has been conclusively shown to hold and the logic for the development of the unified curve in the case of the elastic material function runs on parallel lines. The unified curves provide the easiest way of getting an estimate of the elasticity of polymer melts simply through knowWge of their MFl. 

It can be seen from Table 6.10 that the damping constant m lies between 0.13 and 0.2, in line with the findings of Wagner [15] for polymer melts. This fact can then be used to generate the unified normal stress difference curves for other polymers [16-18] whose parameter values for the modified Carreau model fit of unified viscosity data are given in Table 6.1. Figures 6.2-6.13 show die predicted normal stress difference curves for different polymer types. The solid line shows the plot generated for a median value of m = 0.16 and the band... 

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