Extensional Flow Properties and Their Measurement

Abstract

For deformations that are either very small or very slow, the theory of linear viscoelasticity is a unifying concept that provides relationships between the material functions that are determined using various types of deformations. For example, this theory tells us that in start-up flow, the shear stress growth coefficient, η ⁺(t), is independent of shear rate. Furthermore, it provides a simple relationship between this material function and the tensile stress growth function, $\eta _E^ + \left( t \right)$, that is measured at the start-up of steady simple extension:

$$\eta _E^ + \left( t \right) = 3{\eta ^ + }\left( t \right)$$

(6-1)

Thus, as long as the total strain or the maximum strain rate that occurs during a particular deformation is very small, no new information is obtained from the use of an extensional flow, once the linear viscoelastic behavior has been established by use of a shearing deformation. A corollary of this statement is that the response of a melt to any small or slow extensional flow can be calculated from a material function determined using a small or slow shearing experiment.