Polymer rheology zero-shear-rate viscosity

If values of rja are now plotted as a function of strain rate (Figure 13-62), it can be seen that although there is a dramatic decrease in apparent viscosity at high values of y, at low strain rates the apparent viscosity is essentially constant. Obviously, if you want to compare the rheological properties of different types of polymers, it is this strain rate-independent parameter that would be most useful, as it would presumably be a characteristic property of the polymer. This limiting value is called the zero shear-rate viscosity, rjm. 

Zero-shear-rate viscosity Infinite-shear-rate viscosity High shear relative viscosity Intrinsic viscosity, polymer rheology,... 

The cone-and-plate and parallel-plate rheometers are rotational devices used to characterize the viscosity of molten polymers. The type of information obtained from these two types of rheometers is very similar. Both types of rheometers can be used to evaluate the shear rate-viscosity behavior at relatively low vales of shear rate therefore, allowing the experimental determination of the first region of the curve shown in Figure 22.6 and thus the determination of the zero-shear-rate viscosity. The rheological behavior observed in this region of the shear rate-viscosity curve cannot be described by the power-law model. On the other hand, besides describing the polymer viscosity at low shear rates, the cone-and-plate and parallel-plate rheometers are also useful as dynamic rheometers and they can yield more information about the stmcture/flow behavior of liquid polymeric materials, especially molten polymers. 

Some modifications of the melt flow behavior of thermoplastics that can be observed depending on filler concentration are a yield-like behavior (i.e., in these cases, there is no flow until a finite value of the stress is reached), a reduction in die swell, a decrease of the shear rate value where nonlinear flow takes place, and wall slip or nearwall slip flow behavior [14, 27, 46]. Other reported effects of flllers on the rheology of molten polymers are an increase of both the shear thinning behavior and the zero-shear-rate viscosity with the filler loading and a decrease in the dependence of the filler on viscosity near the glass transition temperature [18, 47-49]. 

Fundamental rheological quantities such as shear viscosity, primary and secondary normal stress coefficients, elongational viscosity and complex viscosity are introduced. Most molten polymers show a reduction in shear viscosity with increasing shear rate (pseudoplasticity). At low shear rates, however, the shear viscosity is practically independent of shear rate. This viscosity value is called the zero-shear-rate viscosity (rjo)- At higher shear rates, the viscosity ( ) decreases with shear rate (y) according to a power law expression t] = where K and n are... 

The rheological properties of polymer melts and solutions are highly dependent on temperature. This is clearly illustrated by the data presented in Figure 2.5 (p. 12), where the zero shear rate viscosity, rio, drops by two orders of magnitude as the temperature is raised from 115 °C to 240 °C. In general, as illustrated in this same figure, the shape of the curves remains nearly unchanged at each temperature. Because of this similarity in the shape of the flow curves, it is possible to... 

Figure 26 Dependence of zero-shear viscosity and critical shear rate concentration on salt concentration for polyacrylamide aqueous solutions. (From KC Tam and C Tiu, Water-soluble polymers (rheological properties) in Polymeric Materials Encyclopedia, JC Salamone, ed. Boca Raton, FL CRC Press, 1996, p. 8655.)...

It is now important to calculate the stress exerted by the particles. This stress is equal to aApgfZ. For polystyrene latex particles with radius 1.55 pm and density 1.05 g cm , this stress is equal to 1.6 x 10 Pa. Such stress is lower than the critical stress for most EH EC solutions. In this case, one would expect a correlation between the settling velocity and the zero shear viscosity. This is illustrated in Chapter 7, whereby v/a is plotted versus 7(0). A linear relationship between log( /a ) and log 7(0) is obtained, with a slope of —1, over three decades of viscosity. This indicated that the settling rate is proportional to [7(0)] . Thus, the settling rate of isolated spheres in non-Newtonian (pseudo-plastic) polymer solutions is determined by the zero shear viscosity in which the particles are suspended. As discussed in Chapter 7, on rheological measurements, determination of the zero shear viscosity is not straightforward and requires the use of constant stress rheometers. 

Fig. 6 Melt rheological behaviOT of polyethylenes polymerized with 4/MAO varying the ethylene concentration and polymerization time. Single points shown at the of the graph represent theoretical j/o values [102] expected for a linear polymer of corresponding M . A decrease in the Ce results in the strongly elevated p at low shear rates and significant shear thinning. Note that for the samples with very high viscosity, the true zero-shear viscosity value >/o would be much higher than the experimentally measured >/ (

Entanglement has a significant impact on the rheological behaviour and mechanical properties of the polymer. Observed from the perspective of polymer melt viscosity, when the shear rate is at zero the polymer viscosity is proportional to the molecular weight of the polymer. When... 

In the melt flow curve (viscosity versus shear rate), a Newtonian plateau at very low shear rate is usually considered as the ZSV. For polymers of very low molecular weight, this ZSV can be obtained directly from the shear rheology experiment (Dealy and Wissburn 1990). However, this is really difficult to obtain for high molecular weight polymers as well for LCPs. From the experimental data presented in Fig. 4.5, it was impossible to determine the ZSV, since there was no distinct plateau detected at a lower shear rate up to 0.01 s . Therefore, a modified Cross model (4.6) was used to determine the zero shear viscosity of the LCPs. Data from low and high shear rates were combined to fit into the Cross model predictions (Fig. 4.5). The model prediction showed good agreement with experimental data. 

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