Shear Viscosity of Polymeric Melts

Melt viscosity of polymeric materials is a measure of resistance to deformation. It is useful for material characterization, quality control, equipment design, and process simulation and optimization. For all polymers, under conditions of constant temperature and pressure, the viscosity is constant at low shear rates, but it reduces by orders of magnitude as the shear rate is progressively increased. For a specific polymer, the low-shear-rate viscosity as well as the shear-thinning behavior depends on the polymer molecular weight, molecular weight distribution, long-chain branching, and the presence of additives. This dependence on molecular structure can be explained, at least qualitatively, by theories of polymer dynamics. Furthermore, data obtained at different temperatures and pressures can often be superposed by using shift factors, and the resulting master curves can be described by simple equations that are useful for analytical or numerical work. This knowledge affords the possibility of tailoring polymer melt behavior to the needs of a given application of interest.