Effect of surfactant on viscoelasticity and shear thickening in aqueous solutions of hydrocarbon end-capped poly(ethylene oxide)

Abstract

The effect of the anionic surfactant sodium dodecyl sulfate (SDS) on the linear viscoelastic and shear thickening behavior of aqueous solutions of hydrophobically end-capped poly(ethylene oxide) was examined. At low SDS concentration, the linear viscoelastic response is similar to that in pure water. As SDS concentration increases, especially above the critical micellization concentration, deviations from the behavior of the polymer in pure water occur at high frequencies. However, if the data are corrected for an effective viscosity of the surrounding medium (ηs), the observed behavior of a Maxwellian fluid with a single relaxation time is preserved and it is also possible to superimpose G′ and G″ data at different SDS concentrations. ηs was found to be 0, 0.02, 0.2, and 0.28 Pa s for solutions with 0.0025M, 0.005M, 0.025M, and 0.05M SDS, respectively. Shear thickening was only observed at the highest SDS concentration. It is not conclusive whether shear thickening is present in lower SDS concentrations or not because of the onset of flow instability. Comparing the shear thickening behavior in two systems (model polymers in pure water and with 0.05M SDS), a common location of shear thickening (on a reduced shear rate scale) was observed. This is consistent with the non-Gaussian chain stretching mechanism where the onset of shear thickening depends only on the polymer molecular weight, and not on the viscosity of the surrounding medium.