Shear Viscosity of Polymer Solutions

As with polymer melts, polymer solution viscosity is a measure of resistance to deformation, and it is useful for the same reasons. As might be expected, under conditions of constant temperature and pressure, polymer solution viscosity is constant at low shear rates, but it reduces by orders of magnitude as the shear rate is progressively increased; the limit of the high shear viscosity, though, is the solvent viscosity. A new variable now is polymer concentration c , and it is customary to examine viscosity behavior at infinite dilution. This leads to the definition of intrinsic viscosity, a quantity that reveals information on the size of the polymer molecule in solution. The intrinsic viscosity can, in turn, be related to the polymer molecular weight. There is interplay between the polymer concentration and the polymer molecular weight, and cM appears as a single variable in theories of polymer solution viscosity. Indeed, for flexible chain polymers, the zero-shear viscosity is found to be a unique function of cM , and this can be predicted by theory. Also, the influence of molecular weight distribution, chain branching, temperature, and pressure on the zero-shear viscosity of polymer solutions is like the effect of these variables on the viscosity of polymer melts. Similarly, one is able to superpose flow curves to obtain a master curve that is independent of concentration, molecular weight, solvent viscosity, and temperature. The chapter concludes with a discussion of rheology modifiers; these behave in a manner similar to polymers in solution, and, as the name suggests, their use allows one to engineer desired rheological behavior for practical applications of interest.