Effect of surfactant and temperature on the rheological properties of aqueous solutions of unmodified and hydrophobically modified polyacrylamide

Abstract

Linear and nonlinear viscoelasticity of semidilute aqueous solutions of an unmodified reference polyacrylamide (PAM) and of a hydrophobically modified polyacrylamide (HM-PAM) in the presence of various amounts of sodium dodecyl sulfate (SDS) and at different temperatures have been examined. Oscillatory shear experiments in the linear viscoelastic domain revealed significant polymer-surfactant interaction for the HM-PAM-SDS system at a surfactant concentration of ca. 8 mmol/kg, whereas practically no surfactant effect is observed for the PAM-SDS system. The rheological features for the HM-PAM-SDS system are found to be strongly dependent upon the level of surfactant addition, with first an increase and then a decrease in the values of parameters such as the dynamic viscosity. While the position of the maximum of the viscosity curve, with regard to the surfactant concentration, is independent of polymer concentration and temperature, the strength of the network is promoted by increasing polymer content and decreasing temperature. At higher surfactant concentrations, a breakdown of chain associations occurred. A non-Newtonian shear thinning behavior, as well as a thixotropy, is detected for HM-PAM solutions in the presence of surfactant. These effects are most pronounced at ca. 8 mmol/kg SDS at the lowest temperature. The rheological features reveal that the hydrophobic associations, induced by HM-PAM-SDS interactions, play an important role for the viscoelastic properties of the system.