Polymer rheology pseudoplastic

Polymer rheology can respond nonllnearly to shear rates, as shown in Fig. 3.4. As discussed above, a Newtonian material has a linear relationship between shear stress and shear rate, and the slope of the response Is the shear viscosity. Many polymers at very low shear rates approach a Newtonian response. As the shear rate is increased most commercial polymers have a decrease in the rate of stress increase. That is, the extension of the shear stress function tends to have a lower local slope as the shear rate is increased. This Is an example of a pseudoplastic material, also known as a shear-thinning material. Pseudoplastic materials show a decrease in shear viscosity as the shear rate increases. Dilatant materials Increase in shear viscosity as the shear rate increases. Finally, a Bingham plastic requires an initial shear stress, to, before it will flow, and then it reacts to shear rate in the same manner as a Newtonian polymer. It thus appears as an elastic material until it begins to flow and then responds like a viscous fluid. All of these viscous responses may be observed when dealing with commercial and experimental polymers. 

The results of a number of studies on polymer rheology in porous media are then reviewed. Firstly, results for pseudoplastic fluids (mainly xanthan) are discussed and then results are reviewed for fluids showing some viscoelastic/extensional viscosity behaviour (e.g. HPAM, PEO). In all of these studies, flow is purely single-phase, and most experiments have been performed at 100% water saturation in the porous pack or core, although a few have been done at residual oil. 

POLYMER RHEOLOGY IN POROUS MEDIA 6.3.2 Xanthan in-situ rheology pseudoplastic behaviour... 

Shellflo-S, succinoglycan is an interesting new polymer for use in the oilfield, in many ways complementary to xanthan. It is particularly appropriate for use in well completion fluids at moderate temperatures, where a more pseudoplastic rheology than that provided by HEC, and a more rapidly breaking polymer than xanthan is required. 

In terms of the overall rheology profile of acrylic polymers when used in finished formulations, the behavior of the nonassociative thickeners is relatively easy to predict, as there is little interaction from a rheological point of view between the thickener and the matrix. Significantly higher polymer levels will be required if electrolytes are present, but the overall formulation rheology (e.g., pseudoplasticity, yield development) will remain similar. In most circumstances, though,... 

The rheological behavior of aqueous pAm solutions is typically pseudoplastic. An example of the use of pAm polymer to thicken an acidic composition is disclosed in patent application WO 9419443 A1 [16]. 

Block polymers, owing to the tendency for formation of regular strucmres tailored by molecular design, are ideal models for compatibilized, two-phase polymer blends or alloys. Blends do show similar rheological behavior, e.g., yield, pseudoplasticity, thixotropy, structural rearrangements, but since the morphology is more difficult to control, the interpretation of data could present serious difficulties. 

A further complication to understanding the rheology of hydrophobically associating polymers is their unique response to shear rate and solvent quality (e.g., salt content). As shown in Figures 5 and 6, the viscosity can be independent (Newtonian), decrease (pseudoplastic), or even increase (dilatant)... 

Abstract This chapter focuses on dental biomaterials designed for permanent placement in the mouth. The development of flowahle polymer-ceramic composites is traced and their rheological properties, such as pseudoplasticity and thixotropy, discussed. Also considered are some materials that are being developed for root canal therapy, including calcium phosphate cements. There is vast scope for research into materials development, clinical applications and fundamental mechanisms. 

Xanthan is a water-soluble polymer with interesting rheological properties. Aqueous solutions are exceptionally pseudoplastic, but have only a little thixotropy. Turbulent flow is considerably reduced on adding very small quantities of xanthan. Salt concentrations of below 0.01 % reduce the solution viscosity, but those of over 1% increase it. Bivalent cations precipitate xanthan at pH values above 9, and trivalent cations precipitate xanthans at lower pH s. 

With nonadsorbing polymer, significant rheological transitions correlate with the phase boundaries mentioned above (55). The slowness of the macroscopic phase separation permits rheological characterization of a metastable structure that changes little over time for samples formulated within the two-phase region. The systems respond pseudoplastically, but the microstructure recovers to a reproducible rest state after shear. 

Thus soluble polymer, interacting in a controlled fashion with colloidal particles, can transform both the equilibrium state and the mechanical properties of dispersions. The possibilities range from equilibrium, low viscosity fluids to nonequilibrium, pseudoplastic pastes with high yield stresses. However, substantial ga( still exist in the ability to, for example, (i) create high viscosity equilibrium fluids with prescribed relaxation spectra, (ii) impart a sol-gel transition at prescribed conditions, or (iii) connect explicitly macromdecular structure with rheological behavior. 

Many water soluble polymers show a solution behavior which is non-Newtonian. When flow curves are experimentally obtained that plot the fluid shear stress versus shear rate, the curves show that in most cases the solution rheology is pseudoplastic. For pseudo-... 

Two separate theoretical analyses of polymer molecular behavior in solution have been carried out by Bueche (12) and Graessley (13). Both analyses predict a pseudoplastic rheology for polymer solutions in simple shear. However, Bueche s explanation is based on the response of independent polymer molecules to a shearing flow field and Graessley s explanation is based on the response of entangled molecules when placed in a shearing flow field. 

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