Role of Charge of Micellar Worms in Modulating Structure and Rheological Properties of Their Mixtures with Nonionic Polymer

Abstract

The paper describes phase behavior, microstructure, and rheological properties of the mixtures of surfactant micelles of varying negative charge with nonionic hydrophilic polymer poly(vinyl alcohol) (PVA) weakly interacting with the micelles. To reduce the charge of micelles, a cationic surfactant n-octyltrimethylammonium bromide was gradually added to the anionic surfactant potassium oleate. No appreciable interactions between the micelles and polymer were revealed by NMR spectroscopy. At the same time, the system shows a broad range of compatibility, which may be due to surfactant counterions preventing the demixing. The higher the concentration of added polymer, the higher the charge density of micelles necessary to prevent macrophase separation. However, SANS data demonstrate that upon the addition of polymer the intermicellar distances decrease, which suggests that the micelles are expelled from some area filled by PVA macromolecules, indicating a local demixing. A decrease of micellar charge density induces the growth of micelles from short nonoverlapping cylinders to long entangled wormlike chains both in the presence and in the absence of polymer, which is accompanied by an increase of viscosity by up to 5 orders of magnitude. Despite very weak interactions with the micelles, PVA makes their growth in length much more pronounced and forms the entanglements with micellar chains, thereby enhancing significantly the viscoelastic properties. The effect of polymer on micellar length was attributed to polymer-surfactant microsegregation increasing the local surfactant concentration and consequently the length of micelles. These data demonstrate the effectiveness of the addition of weakly interacting polymer for the modulation of rheology of wormlike surfactant micelles, thus opening a new way for manipulating the rheological properties of viscoelastic surfactants.