Shear stress response solution process

However, in the solution process, the same two opposing forces reach a different balance. In Figure 83, the shear response of polymer is again shown as a function of the catalyst activation temperature, but in this case the polymers were made in the solution process. Because the LCB level of polymers made in the solution process is much lower, it is the breadth that now dominates the response to shear stress. The data of Figure 83 show that the response to shear stress decreases with rising activation temperature. Again, the comparison is made with polymers produced to have the same three MI values shown in Figure 82. Three parallel lines are produced that decline with activation temperature. 

FIGURE 83 Response to shear stress, shown here as the polymer HLMI/MI ratio, as a function of catalyst activation temperature for polymers made in the solution process with Cr/silica-alumina catalyst. Reaction temperature was varied (125-150 °C) to produce three series of polymers of constant melt index. (Compare with Figure 82.)... 

In contrast, Figure 95 shows a plot of the response to shear stress of polymers made at a high temperature in the solution process [407,521]. Once again, the reaction time was varied to produce polymers in different yields. The reaction temperature was also varied to produce polymers of varying MW. Under these conditions, the catalyst exhibited full activity immediately. The response to shear stress is in essence an approximation of the slope of the melt viscosity curve. A high response to shear stress indicates rheological breadth, which can derive from breadth in the MW distribution, or from LCB. In this case, the MW distribution was quite ordinary and did not vary with time. Therefore, the response to shear stress can serve as a surrogate for the level of LCB. 

FIGURE 95 Response to shear stress of PE made in the solution process at varying polymer yields (e.g., varying reaction times). An increase in the catalyst productivity (yield) lowers the shear-thinning ability, or HLMI/MI ratio, of polymers of a given Ml. 

Behavior of Entangled Polymer Melts and Solutions Transient Response. While the steady-state response of polymers in shear and elongational flows is of much interest, there are also many instances in which the transient response is important because not all processes attain steady state. There are two important transient responses in the nonlinear regime of behavior. These are the stress relaxation response in which the deformation is held constant and the stress evolution with time is followed. This was discussed above for the linear viscoelastic case. In addition, the response to a constant rate of deformation can be an important transient response to study. Also note that creep experiments are sometimes used to characterize the nonlinear response of polymeric fluids and these will also be discussed briefly. 

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