Normal stress and shear stress in a viscoelastic liquid under steady shear flow: Effect of molecular weight heterogeneity

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Abstract
Under steady shear flow, the normal stress and the shear stress in both dilute and concentrated solutions of monodisperse poly‐α‐methylstyrenes and their blends were measured. It was confirmed that the molecular theories of Rouse and Zimm extended to concentrated solutions can explain the relation between the zero‐shear normal stress coefficient and the zero‐shear steady‐flow viscosity for both monodisperse and polydisperse systems. Shear‐rate dependence of steady‐flow viscosity can be understood fairly well by the molecular entanglement concept proposed by Graessley so long as the polymer is monodisperse or the amount of the higher molecular weight component is high. However, zero‐shear viscosity of blended systems cannot be explained quantitatively by the theory of Graessley. The shear‐rate dependence of steady‐state compliance of blended systems was also observed, and it can well be explained by the theory of Tanaka, Yamamoto, and Takano which interpreted the shear rate‐dependent steady‐state compliance in terms of the relaxation time spectrum and its variation with shear rate.