THE STRESS JUMP OF A SEMIRIGID MACROMOLECULE AFTER SHEAR - COMPARISON OF THE ELASTIC STRESS TO THE BIREFRINGENCE

In this work it is shown that the amount of stress retained at the instant of shear cessation, denoted as the elastic stress by Doi, and not the total shear stress is proportional to the shear equivalent of the optic birefringence even in the nonlinear shear stress—shear rate region. The system studied was xanthan gum in a viscous solvent, fructose dissolved in water, and may be considered as a system with rigid constraints. Coaxiality of the elastic stress and refractive index ellipsoids has been predicted theoretically for these systems; however, previous studies have only compared the total stress to the birefringence which generally leads to invalidation of the stress-optic relation. The results of this work have three consequences; it reinforces the fact that stress jumps are present, the stress-optic relation is only true when the elastic and not the total stress is used, and two unique types of stress constitute the total stress as predicted by theory for systems with rigid constraints. © 1995, The Society of Rheology. All rights reserved.