Polymer rheology shear thickening

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). 

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

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. 

Concentrated dispersions may be shear thickening, as opposed to the shear thinning of dilute polymer solutions. Some materials, such as latex paints, tend to form a structure. As the structure breaks down with shearing action, the viscosity decreases. Such materials are thixotropic. Some fluids have a yield stress. A thorough characterization of the rheology may require a number of different measurements. 

Hydrophobically associating water-soluble polymers are of interest because of their unusual properties in solution. Even small levels of hydrophobic groups on water-soluble polymer backbones can have a profound effect on aqueous solution rheology. For example, solutions of such polymers show enhanced viscosification efficiency, shear-thickening rheology, as well as improved shear and salt stability [1-19]. 

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