The Effects of High-Molecular-Weight Components on the Viscoelastic Properties of Polystyrene

Abstract

The effects of molecular weight distribution (MWD) on the steady-state shear and the dynamic viscoelastic properties of high-molecular-weight (MW>97000) polystyrene have been examined with a Weissenberg rheogoniometer. Synthetic MWDs were produced by blending polystyrene components of defined MWs and narrow MWDs. Experiments were designed to examine the effects of small amounts of a high-MW component on the viscous and elastic properties of the melt.The frequency response of the storage and loss moduli, G′ and G″, and the storage compliance J′, are compared with the shear rate dependence of the shear stress τ12, the primary normal stress difference P11−P22, and the shear modulus G, as a function of the composition of the blend. At low deformation rates the dynamic and the steady-state shear properties are equal; G″=τ12, G′=(P11−P22/2, and G=1/J′. At higher deformation rates the steady-state shear properties are primarily controlled by the high-MW component while the dynamic properties reflect the response of each component. The zero shear viscosities of the blends and the narrow-MWD components exhibit the same dependence on the weight-average MW. Four predictions for the MWD dependence of the steady-state shear compliance Je° are compared with the experimental results. Although overestimating the magnitude in the maximum in Je°, only Graessley’s theory accounts for the observed behavior on either side of the maximum.The data is used to discuss the previously reported dependence of the characteristic relaxation time, τ=η°Je°, on the shape of the non-Newtonian flow response. The τ is found to be a measure of the time scale of the relaxation processes of the high-MW component.