Shear modification of polymers

Time dependence of recovery from shear history effects in polymer liquids can be used advantageously to modify the properties of macromolecules. The net result of shearing is a reduction in elasticity and possibly in viscosity of the polymer fluid. The material may be processed subsequently in a transient, less elastic, and less viscous state if the diffusion-related rate of recovery of a more fully entangled equilibrium state is not too rapid. Shear-induced changes in entanglement density are manifested in variations in die swell, die entrance pressure losses, normal stresses, melt strength, flow defects such as melt fracture and sharkskin surfaces, haze of extruded films, application properties of coatings liquids and, other properties. Evidence for shear-modification effects is reviewed in this article. The polymer species that are most amenable to this treatment are identified in general terms; shear modification processes that have been used are summarized. Methods for augmenting the efficiency of mechanical shear modification include addition of plasticizers, fillers, and incompletely miscible polymers. The expedient mentioned last can be particularly effective. Assessments of the degree of shear modification are reviewed and applications to coatings mixtures are also summarized. The time-dependent behavior of the type reviewed in this article limits the success of theoretical approaches, as in the application of constitutive equations to describe polymer-liquid behavior. Attempts to relate Newtonian viscosities and other rheological parameters to polymer characteristics are similarly hindered when shear history effects are not taken into account.