Polymer rheology Newtonian liquids

A Newtonian liquid of viscosity 0.1 N s/m2 is flowing through a pipe of 25 mm diameter and 20 m in lenglh, and the pressure drop is 105 N/m2. As a result of a process change a small quantity of polymer is added to the liquid and this causes the liquid to exhibit non-Newtonian characteristics its rheology is described adequately by the power-law model and the flow index is 0.33. The apparent viscosity of the modified fluid is equal to ihc viscosity of the original liquid at a shear rate of 1000 s L... 

Molecularly motivated empiricisms, such as the solubility parameter concept, have been valuable in dealing with mixtures of weakly interacting small molecules where surface forces are small. However, they are completely inadequate for mixtures that involve macromolecules, associating entities like surfactants, and rod-like or plate-like species that can form ordered phases. New theories and models are needed to describe and understand these systems. This is an active research area where advances could lead to better understanding of the dynamics of polymers and colloids in solution, the rheological and mechanical properties of these solutions, and, more generally, the fluid mechaiucs of non-Newtonian liquids. 

Rheology deals with the deformation and flow of any material under the influence of an applied stress. In practical apphcations, it is related with flow, transport, and handling any simple and complex fluids [1], It deals with a variety of materials from elastic Hookean solids to viscous Newtonian liquid. In general, rheology is concerned with the deformation of solid materials including metals, plastics, and mbbers, and hquids such as polymer melts, slurries, and polymer solutions. 

The Bingham Fluid. The Bingham fluid is an empirical model that represents the rheological behavior of materials that exhibit a no flow region below certain yield stresses, tv, such as polymer emulsions and slurries. Since the material flows like a Newtonian liquid above the yield stress, the Bingham model can be represented by... 

There are several aspects of rheological behavior exhibited by polymeric liquids that set these materials apart from Newtonian fluids. An excellent summary of the differences in fluid response between Newtonian liquids and non-Newtonian polymeric liquids under various scenarios has been given by Bird and Curtis [3]. Two very well-known atypical phenomena exhibited by polymeric liquids are the Weissenberg effect (a polymer melt or solution tends to climb a rotating rod) and extrudate swelling, which are illustrated in Figure 22.4. 

When the polymer concentration (or molecular mass) is sufficiently high and viscosity of solution exceeds essentially that of solvent, the rheological effects in bubble dynamics become much more pronounced. In Figure 7.2.7, data are presented for the relative damping decrement of free oscillations of air bubble with Ro = 2.8 mm in aqueous solution of POE via concentration. The dashed line represents theoretical values of the decrement, corresponding to Newtonian liquid with 13 =T)p. The actual energy losses, characterized by experimental points, remained almost unchanged, despite the sharp rise in the Newtonian decrement, Ap, with c. This result correlates well with the above theoretical predictions and it is explained by viscoelastic properties of the solution. The same explanation has the phe-... 

Molten polymers are viscoelastic materials, and so study of their behaviour can be complex. Polymers are also non-ideal in behaviour, i.e. they do not follow the Newtonian liquid relationship of simple liquids like water, where shear-stress is proportional to shear strain rate. Unlike Newtonian liquids, polymers show viscosity changes with shear rate, mainly in a pseudoplastic manner. As shear rate increases there is a reduction in melt viscosity. This is true of both heat-softened plastics and rubbers. Other time-dependent effects will also arise with polymer compounds to complicate the rheological process behaviour. These may be viscosity reductions due to molecular-mass breakdown or physical effects due to thixotropic behaviour, or viscosity increases due to crosslinking/branching reactions or degradation. Generally these effects will be studied in rotational-type rheometers and the extrusion-type capillary rheometer. 

In an ideal liquid, the rate of flow is proportional to the force inducing that flow. Mobile liquids approximate to this ideal behaviour and are known as Newtonian liquids. For most commercial emulsion polymers the rate of flow increases mote rapidly than the rise in inducing force. This is known as pseudoplasticity. A simple illustration of different rheological behaviour is shown in Figure 7-14. 

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