Shear and extensional viscosity characteristics of a series of hydrophobically associating polyelectrolytes

Abstract

The shear and extensional viscosity characteristics of aqueous solutions of a series of hydrophobically associating polyelectrolytes have been investigated and compared. The polymers studied consisted of a methacrylic acid/ethyl acrylate copolymer backbone containing varying levels of incorporation (0–2 mol%) of a pendant C₁₈ hydrophobe. The extensional viscosities of all solutions were found to be higher than their corresponding shear viscosities, with the calculated Trouton ratio tending towards the predicted value of three at low strain rates but increasing with increasing extensional strain rate thereafter. In both flow modes, the viscosities of the solutions increased with increasing concentration and hydrophobe content of the polymers. In contrast to the purely shear-thinning characteristics of the polymer solutions observed in shear flow, the extensional viscosity–strain rate profiles of a number of systems were found to contain an initial region of strain-thickening, although strain-thinning behaviour was observed at higher strain rates. For any given system, the strain rate at which the evident maxima in extensional viscosity occurred decreased with increasing polymer concentration. In the presence of additives such as excess surfactant or a miscible alcoholic non-solvent, which are known to disrupt regions of intermolecular hydrophobic associations, the extensional viscosity of a typical solution of an associative polyelectrolyte was found to be significantly reduced and no longer to exhibit a maximum in the viscosity–strain rate profile.