Abstract
A brief account is given for the development of studies in nonlinear viscoelasticity of polymer melts and concentrated solutions. The first stage of rapid progress starting around 1950 was led by the discovery of the normal stress effect. In this period, the rheological properties in steady shear flow were extensively studied and the concept of entanglement network was introduced as the origin of stress in polymeric liquids. The second stage which was partly induced by the development of rheological apparatuses in steady shear, was concerned with the nonsteady as well as nonlinear stress-strain relations. About the middle of 1970s, the strain-dependent constitutive equation was recognized as an appropriate equation to describe the stress for a wide group of flow histories. At the third stage of progress the tube model theory of Doi and Edwards was published in 1978 and it has been proved capable of describing most of the nonlinear viscoelastic behavior so far investigated. In the present paper a special emphasis is put on the birefringence induced by the strain. This phenomenon, if studied in appropriate nonsteady flows, may well give a clue to prove the molecular motion assumed in the tube model theory.
