Models Elongational viscosity data

The sample is subjected to compression by moving the crosshead downwards at a constant speed. The sample is extruded from between the two discs, undergoing elongational or biaxial flow the sample is stretched radially and azimuthally as it flows outwards between the approaching discs. Lubrication ensures that shear flow cannot occur. Elongational viscosity is calculated directly from the measured force-distance data, disc radius and crosshead speed no rheological model is required (Campanella and Peleg, 2002). 

Figure 6 shows the predictions of the Wagner model compared to experimental data for elongational viscosity, shear viscosity and first normal stress difference of LD. These have been calculated according to ... 

This is in conflict with the expenmental data, which show a strong shear-rate dependence. Also, this model leads to an infinite elongational viscosity at a finite... 

The temporary network model predicts many qualitative features of viscoelastic stresses, including a positive first normal stress difference in shear, gradual stress relaxation after cessation of flow, and elastic recovery of strain after removal of stress. It predicts that the time-dependent extensional viscosity rj rises steeply whenever the elongation rate, s, exceeds 1/2ti, where x is the longest relaxation time. This prediction is accurate for some melts, namely ones with multiple long side branches (see Fig. 3-10). (For melts composed of unbranched molecules, the rise in rj is much less dramatic, as shown in Fig. 3-39.) However, even for branched melts, the temporary network model is unrealistic in that it predicts that rj rises to infinity, whereas the data must level eventually off. A hint of this leveling off can be seen in the data of Fig. 3-10. A more realistic version of the temporary network model... 

Relaxation spectrum has been calculated from oscillatory measurements (Figure 1) whereas the nonlinear parameters of all models were identified on the steady uniaxial elongational data only (Figure 2). All model parameters for corresponding materials are summarized in Tables 1-2. The model predictions in comparison with experimental data (shear viscosity, first q/i, and second normal sttess coefficients, y/2, uniaxial extensional viscosity, qE,u) are depicted in Figures 2-5 for all materials. 

---