Correlation of viscoelastic properties with the molar mass distribution for aqueous polyacrylamide solutions

Abstract

AbstractThe dependence of viscous and elastic rheological functions on molar mass and molar mass distribution was investigated under shear flow conditions using concentrated solutions of polyacrylamides and polyacrylamide mixtures. The zero‐shear viscosity η0 correlates with the weight‐average molar mass M̄w; with increasing shear rate the corresponding controlling molar mass average tends to change to the number‐average molar mass M̄n. It is shown that the analysis of the shape of the whole flow curve is a simple way to obtain qualitative information about molar mass distribution. Elastic forces are influenced mainly by the high‐molar‐mass tail of a molar mass distribution. The zero normal stress coefficient ψ0 depends on a molar mass average between M̄w and z‐average molar mass M̄z·ψ0 increases with the 7,4th power of the molar mass for the polyacrylamides (not for the mixtures). The steady‐state compliance Je(0) is, for the investigated system, independent of the absolute molar mass, but increases with the 5,5th power of the ratio M̄z/M̄w. The elasticity modulus G0(A) can be correlated with M̄n of the polyacrylamide samples and their mixtures. These investigations show that the analysis of various rheological functions can lead to a wealth of information about the molar mass distribution of a polymer.