Growth of wormlike micelles of surfactant induced by embedded polymer: role of polymer chain length

Abstract

Incorporation of polymer chains into wormlike surfactant micelles, which find a large range of applications, offers the opportunity to modify their structure and properties. In this paper, using spectroscopic, scattering and rheological techniques and computer simulations, we study the incorporation of poly(4-vinylpyridine) of two different molecular weights (MWs) into entangled networks of wormlike surfactant micelles of potassium oleate. Using NMR-spectroscopy we show that, independent of its MW, the polymer incorporates into the core–corona interface of the surfactant micelles. According to SANS data, the polymer does not alter the micelle structure or the micelle radius, but diminishes the packing density of the surfactant. At the same time, rheology reveals a stark difference between the surfactant networks with embedded polymers of different MWs. Networks with the higher-MW polymer possess larger viscosity and a longer relaxation time, which we attribute to the larger length of the hybrid micelles. Moreover, we demonstrate that in an intermediate concentration range the higher-MW polymer is able to link neighbouring surfactant micelles together, which has never been previously observed. However, with a further increase in polymer content the micelles become smaller due to the high breaking susceptibility of the boundaries of polymer-containing sections, leading to the stabilization of micellar end-caps by the embedded macromolecules. This process is more prominent in the case of the shorter polymer. Our finding that an increased MW of macromolecules permits the formation of longer hybrid micelles and enhances their rheological properties is of obvious importance for the fundamental understanding of polymer–surfactant interactions and the development of new industrial formulations based on hybrid polymer–wormlike surfactant micelles.