Shear-thinning phenomenon

The shear-thinning phenomenon in Fig. 6-34 can readily be attributed to the shpping of layers past each other, as described above. In fact, the slope of the viscosity-shear rate curve in Fig. 6-34 for the two more concentrated samples is around — 1 at low shear rates, implying that a yield stress must be exceeded to induce the layers to move over one another (see also Chen et al. 1994). Figure 6-35 shows the scattering pattern in the plane normal to the shear... 

The molecular dynamics theories need to make a proper combination to describe the rheological behaviors of polymer melt in various regions of shear rates (Bent et al. 2003). Above 1/t the convective constraint release dominates the rheological behaviors of polymers in shear flow, and thus explains the shear-thinning phenomenon. Beyond 1/t, the extensional deformation reaches saturation, and the shear flow becomes stable, entering the second Newtonian-fluid region, as demonstrated in Fig. 7.6. 

What are the advantages of the shear-thinning phenomenon in polymer melt... 

Because viseosity is noneonstant, it must be measured across a range of shear rates to fully characterize a material. The equations used to calculate the apparent viscosity are not exaetly correct due to the shear-thinning phenomenon. A correction has been developed (by Rabinowitch [1]) that uses the slope of the OapCyap) curve,... 

Tjccy of the apparent viseosity t) on the strain rate A increases from 0.68 at 400 V/mm to 0.93 at 1000 V/mm [75J. This shear thinning phenomenon can be explained qualitatively with the model that the roughly prolate spheroidal droplets are assumed to form in the condensed FR phase [761- The shear flow will rotate the ellipsoidal droplets and thus the long axis will deviate from the direction of the electric field, leading to the weak... 

As we can see, both are independent of the strain rate 7. Hence, as a first conclusion. Lodge s equation of state cannot describe the shear thinning phenomenon. Equation (7.147) is in fact identical with Eq. (5.107) derived in the framework of linear response theory. The new result contributed by Lodge s formula is the expression Eq. (7.148) for the primary normal stress difference. It is interesting to note that the right-hand side of this equation has already appeared in Eq. (5.108) of the linear theory, formulating the relationship between G t) and the recoverable shear compliance. If we take the latter equation, we realize that the three basic parameters of the Lodge s rubber-like liquid, rjo, and 1,0 are indeed related, by... 

Flow behaviour of polymer melts is still difficult to predict in detail. Here, we only mention two aspects. The viscosity of a polymer melt decreases with increasing shear rate. This phenomenon is called shear thinning [48]. Another particularity of the flow of non-Newtonian liquids is the appearance of stress nonnal to the shear direction [48]. This type of stress is responsible for the expansion of a polymer melt at the exit of a tube that it was forced tlirough. Shear thinning and nonnal stress are both due to the change of the chain confonnation under large shear. On the one hand, the compressed coil cross section leads to a smaller viscosity. On the other hand, when the stress is released, as for example at the exit of a tube, the coils fold back to their isotropic confonnation and, thus, give rise to the lateral expansion of the melt. 

In general, for shear-thinning pseudoplastic fluids the apparent viscosity will gradually decrease with time if there is a step increase in its rate of shear. This phenomenon is known as thixotropy. Similarly, with a shear-thickening fluid the apparent viscosity increases under these circumstances and the fluid exhibits rheopexy or negative-thixotropy. 

Proposed Theories for the Drag Reduction Phenomenon 4.1 Wall Effects — Shear Thinning... 

The term pseudoplastic is somewhat outdated because there is nothing pseudo in the flow behavior of polymers. In this book we use the term shear thinning, which well describes the phenomenon. 

We have already discussed one aspect of non-linear behavior in polymer melts, namely shear thinning. A second aspect manifests itself when we examine the flow of polymer melts through small diameter tubes or capillaries. This is the phenomenon of jet or die swelling, where a polymer forced into a narrow tube, diameter d0, swells when... 

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