A study of shear‐stress relaxation anomalies in binary mixtures of monodisperse polystyrenes

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We report measurements of the nonlinear relaxation moduli after a step‐shear strain of polystyrene solutions with nearly monodisperse and with bidisperse distributions of molecular weight. We find, as have others, that for monodisperse solutions with M/Me > 60, there are anomalies, such as an unusually low nonlinear modulus and a kink in a plot of shear stress versus time after the step strain. Here M is the polymer molecular weight and Me is the entanglement molecular weight. We find that in the bidisperse solutions the anomalies persist as long as Mw/Me > 60, where Mw is the weight‐averaged molecular weight of the bidisperse solution. The persistence of the anomalies in bidisperse solutions disagrees with a theory of Marrucci and Grizzuti that attributes the anomalies to strain inhomogeneities similar to shear banding. The Marrucci‐Grizzuti theory predicts that as little as 10% short chains in the bidisperse mix should eliminate the anomalies, whereas in the experiments reported here at least 30% is required. Nevertheless the way in which the anomalies disappear at high strains when one increases the fraction of low‐molecular‐weight component is qualitatively similar to the theoretical predictions and supports the notion that strain inhomogeneities occur in these systems. © 1992 John Wiley & Sons, Inc. Copyright © 1992 John Wiley & Sons, Inc.

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Journal of Polymer Science Part B: Polymer Physics