Shear Thickening in Aqueous Solutions of Hydrocarbon End-Capped Poly(ethylene oxide)

Abstract

The shear thickening behavior in aqueous solutions of model hydrophobically end-capped poly(ethylene oxide) was examined. Poly(ethylene oxide) fully end-capped with C12, C16, or C18 alkanes was prepared by direct addition of monoisocyanate to poly(ethylene glycol). The response of these polymers in aqueous solution to both steady shear and oscillatory shear was determined. The effects of polymer concentration, temperature, association strength, and polymer chain length on shear thickening have been investigated. PEO with higher molecular weight (35 000) shows only shear thinning behavior whereas PEOs with lower molecular weights (10 000 and 20 000) show mild shear thickening over a range of concentrations. The magnitude of shear thickening was found to increase with association strength and to decrease with temperature. The critical shear rate at which onset of shear thickening occurs shifted to lower shear rate as the association strength or concentration increases or as temperature decreases. The scaling factors of the magnitude of shear thickening, the critical shear rate, and the critical shear stress have been interpreted in terms of the free path model proposed by Marrucci et al.,1 which suggests that shear thickening is due to non-Gaussian chain stretching and fast recapture of dissociated end groups.