Viscoelastic synergy in aqueous mixtures of wormlike micelles and model amphiphilic triblock copolymers

We explored the rheological consequences of adding low concentrations of model hydrophobically end-capped poly(ethylene oxide)s to aqueous solutions containing 1 wt % cetyltrimethylammonium tosylate, a surfactant well recognized to form wormlike micelles with hydrophobic alkyl cores. We describe the synthesis and characterization of a variety of hydrophobically modified polymers and report the linear viscoelastic moduli and steady shear viscosity for combinations of 1 wt % cetyltrimethylammonium tosylate with the various hydrophobically modified polymers. Addition of as little as 0.1 wt % hydrophobically modified poly(ethylene oxide) leads to a massive enhancement in viscoelastic response that is strongly promoted by increasing hydrophobe length at fixed polymer molecular weight and concentration and also by increasing the poly(ethylene oxide) molecular weight at fixed concentration. From the detailed examination of how polymer concentration, length of hydrophobic sticker, polymer molecular weight, and architecture of the hydrophobically modified polymers influence the rheological properties of these admixtures, we posit a molecular mechanism to explain the enhancement in the viscoelastic response that implicates intercalation of the hydrophobic "stickers" into the wormlike micelles and thus "physically cross-linking" the system.