Shear thickening, polymers

Laun, H. M. Bung, R., and Schmidt, F. 1991. Rheology of extremely shear thickening polymer dispersions passively viscosity switching fluids. / Rheol. 35 999-1034. 

Hilliou L, Vlassopoulos D (2002) Time-periodic structures and instabilities in shear-thickening polymer solutions. Ind Eng Chem Res 41(25) 6246-6255... 

Shearing causes polymer chains to break, therefore a decrease in molecular weight and, consequently, in thickening power. It has been shown that the higher its molecular weight, the more the polymer is sensitive to mechanical shearing (Brlant et al., 1985). 

Shape-memory polymers, 207 Shear, 124, 261 Shear-thickening fluids, 125 Shear-thinning fluids, 124-125 Shirakawa, Hideki, 74 Shivers, Joseph, 149 Shrink-wrap film, 207 Siemans, 72... 

The grafted layer also affects two other features of the rheology. First, thicker polymer layers enhance the elasticity due to the longer range of the repulsion relative to the hard core size. Thus, samples formulated at 4>cff mo possess easily measurable static elastic moduli. Second, the softer repulsion apparently suppresses the shear thickening observed at high volume fractions for the harder particles, in accordance with earlier measurements by Willey and Macosko (1978). 

If)/ is independent of shear history, the material is said to be time independent. Such liquids can exhibit different behavior patterns, however, if, as is frequently the case with polymers, )/ varies with shear rate. A material whose viscosity is independent of shear rate, e.g., water, is a Newtonian fluid. Figure 11-26 illustrates shear-thickening, Newtonian and shear-thinning rj-y relations. Most polymer melts and solutions are shear-thinning. (Low-molecular-weight polymers and dilute solutions often exhibit Newtonian characteristics.) Wet sand is a familiar example of a shear-thickening substance. It feels hard if you run on it, but you can sink down while standing still. 

Dintzis, F. R. and Bagley, E. B. 1995. Shear-thickening and transient flow effects in starch solutions. J. Appl. Polymer Sci. 56 637-640. 

Figure5.21 Viscosity versus shear rate for 1.0 wt% HEUR =51,000Mt /M — 1.7) telechelic polymers with hexadecanol end caps at 22°C. The illustrations show the structural transitions that are thought to occur as the shear rate is increased. First, the bridging chains are stretched, producing shear thickening. Then, many bridging chains are pulled out at one end from the micelles to which they were attached, and shear thinning occurs. (Reprinted with permission from Yekta et al.. Macromolecules 28 956. Copyright 1995 American Chemical Society.)...

Shear thickening in polymers with multiple stickers is thought to be caused by a shear-induced change in the balance between intramolecular and intermolecular associations (Witten and Cohen 1985). According to this idea, at low shear rates, many of the associations are intramolecular and therefore contribute little or nothing to the viscosity. Shearing flow stretches the molecules, and thus it makes intermolecular associations more probable. The result is an increase in viscosity. The increase in viscosity causes the chain to stretch even more, and this promotes even more interchain associations. The result can be a runaway increase in the viscosity, or shear-induced gel formation. 

Severe shear thickening is most likely to occur for multisticker, entangled chains. For such chains, relaxation after a sticker is released is slower than reassociation, so that chains reassociate while still in a stretched state, and very high viscosities can then build up. In short unentangled telechelic polymers, on the other hand, chain relaxation is expected to be fast enough that chains are unstretched when they reassociate, and the shear thickening is then modest.------------------------------------------------------------------------------... 

Thermotropic LCPs have high melt elasticity, but exhibit little extrudate swell. The latter has been attributed to a yield stress and to long relaxation times (60). The relaxation times for LCPs are normally much longer than for conventional polymers. Anomalous behavior such as negative first normal stress differences, shear-thickening behavior and time-dependent effects have also been observed in the. rheology of LCPs (56). Several of these phenomena are discussed for poly(benzylglutamate) solutions in the chapter by Moldenaers et al. 

This shear reversibihty is very beneficial in enhanced oil recovery field applications. It improves the well injectivity because of the shear thinning effect at the perforation and near the wellbore. Far away from the wellbore, flow velocity is reduced and viscosity is restored. For the hydrophobically associating polymers, both shear thickening and shear thinning were observed by Bock et al. (1988). It has also been reported that the viscosity after shearing was stopped was higher than that before shearing (McCormick et al., 1988). 

When describing dilatant behavior, the maximum stretch rate, e, in the converging flow at the contraction is a better parameter, but more difficult to be calculated. Instead of the term stretch rate, other authors also used deformation rate (e.g., Chauveteau, 1981) or elongational rate (e.g.. Sorbic, 1991). The shear-thickening viscosity is also called elongational viscosity (often referred to as the Trouton viscosity Sorbie, 1991) or extensional viscosity in the literature. James and McLaren (1975) reported that for a solution of polyethylene oxide (a flexible coil, water-soluble polymer physically similar to HPAM), the onset of elastic behavior at maximum stretch rates was of the order of 100 s and shear rates of the order of 1000 s. In this instance, the stretch rate is about 10 times lower than the shear rate. However, some authors use shear rate instead of stretch rate in defining the Deborah number—for example, Delshad et al. (2008). 

In the shear-thickening regime, K increases with polymer concentration and molecular weight, but it is not sensitive to permeability. In this equation, n could be 1.311 to 1.437. 

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