DO POLYMERIC LIQUIDS EXHIBIT STRAIN-HARDENING

The development of new experimental methods in recent years has made possible the study of the response of melts and concentrated polymer solutions to transient deformations in which markedly nonlinear behavior is exhibited. To provide a basis for the interpretation of nonlinear data it is a common practice to compare nonlinear data with the response predicted by the linear theory of viscoelasticity for the same deformation. Such a comparison has led to the use of the term “strain hardening” to describe a positive deviation from linear behavior in the response of some polymeric liquids to the start-up of tensile extension and to exponential shear. Likewise, “strain softening” is defined as a negative departure from the linear prediction. In both cases, it is demonstrated that the usual methods used to calculate the linear behavior are inappropriate. When valid reference cases are used for comparison, all available data indicate that polymeric liquids are strain softening rather than strain hardening. © 1990, The Society of Rheology. All Rights Reserved.