Doi‐Edwards theory evaluation in double‐step strain flows

It is widely accepted that for reversing double‐step strain deformations, predictions based on the Doi‐Edwards (DE) molecular theory without the independent alignment approximation (IAA) are superior to predictions obtained with the IAA, or equivalently, the Kaye‐Bernstein‐Kearsley‐Zapas (K‐BKZ) theory. This summation, however, is based on data obtained over limited ranges of strain and time: the time both between the step strains (t₁) and following the second step strain (t–t₁). In this study, a thorough evaluation of the DE theory is carried out using a comprehensive double‐step strain flow data set. The results of this study indicate that the DE theory is an improvement over the K‐BKZ theory in flows with strain reversal but only for cases when the criteria t₁, t–t₁ ≫ τ_k is satisfied. The constant τ_k, defined as the time beyond which the stress relaxation modulus is factorable: G(γ, t) = h(γ)G(t), is believed to represent the end of the chain retraction process in the DE theory. It appears that the dynamics of chain retraction have an important influence on double‐step strain behavior and, therefore, should be accounted for in molecular‐based theories devised to have general validity in this important deformation history. © 1994 John Wiley & Sons, Inc.