Viscoelastic Synergy and Microstructure Formation in Aqueous Mixtures of Nonionic Hydrophilic Polymer and Charged Wormlike Surfactant Micelles Andrey V. Shibaev, † Ksenia A. Abrashitova, † Alexander I. Kuklin, ‡ Anton S. Orekhov, § Alexander L. Vasiliev, § Ilias Iliopoulos, ∥ and Olga E. Philippova* ,† † Physics Department, Moscow State University, 119991 Moscow, Russia ‡ Joint Institute for Nuclear Research, 141980 Dubna, Russia § National Research Centre “Kurchatov Institute”, 123182 Moscow, Russia ∥ Arkema France, 92700 Colombes, France * S Supporting Information ABSTRACT: We studied the effect of neutral polymer poly(vinyl alcohol) on the rheological properties and microstructure of highly charged mixed wormlike micelles of anionic and cationic surfactants, potassium oleate and n-octyltrimethylammonium bromide, without adding salt. It was shown that the polymer induces a hundredfold increase of viscosity and of longest relaxation time and the appearance of well-defined plateau modulus, which was assigned to interlacing of polymer and micellar chains. When the amount of added polymer exceeds 2 wt %, the rheological characteristics (the viscosity, the longest relaxation time, and the plateau modulus) level off because of microphase separation appearing as a result of the interplay of the segregation on the microscopic scale triggered by the energetic repulsion between polymer and surfactant components, on the one hand, and the translational entropy of counterions preventing the macroscopic phase separation, on the other hand. The formation of surfactant-rich and polymer-rich microphases was evidenced by small-angle neutron scattering and cryogenic transmission electron microscopy data. The results obtained open a new way to modify the rheological properties and the microstructure of wormlike micellar solutions. ■ INTRODUCTION Ionic surfactants are able to self-assemble into very long wormlike micelles, which can interlace thereby imparting viscoelastic properties to solutions. 1−6 Because of noncovalent links between surfactant molecules in micellar chains, these properties are very sensitive to many factors including temperature, shear, and different additives. 5−7 Responsive viscoelasticity of wormlike micellar solutions is currently exploited in a variety of applications including cosmetics, heating and cooling systems, oil recovery, etc. 4,8 Wormlike micelles are usually obtained by adding salt to ionic surfactant solution. Salt screening the electrostatic repulsion between charged surfactant head groups allows their tighter packing, thus favoring the transition from spherical to cylindrical micelles and further growth of cylindrical micelles in length in order to reduce the number of thermodynamically unfavorable spherical end-caps. Another way to get wormlike micelles consists in the addition of oppositely charged surfactant, which permits to obtain long micellar chains with no added salt. In this system, the electrostatic screening triggering the growth of micelles in length proceeds in two ways: (i) by ion pairing of the oppositely charged head groups, which reduces the charge density of the micellar surface, and (ii) by the release of counterions, which increase the ionic strength of the solution. 9 The efficiency of oppositely charged surfactant to induce micellar growth depends crucially on the length of its hydrophobic tail and on its content in the surfactant mixture. On an example of the mixtures of sodium oleate and alkyltrimethylammonium bromide with different n- alkyl groups, it was demonstrated 9 that cationic surfactant with short alkyl tail (C6) induces only weak growth of sodium oleate micelles. By contrast, cationic surfactants with long alkyl tail (C10−C12) provide too strong attraction with the anionic surfactant, which induces not only a dramatic growth of micelles but also the phase separation. The most promising results were obtained for n-octyltrimethylammonium bromide (C 8 TAB). 9 In this case, the attractive interactions are strong enough to produce a pronounced micellar growth, but not so strong to induce phase separation. 9 At a given size of surfactant tail there is an optimum content of oppositely charged surfactant, which provides the maximum effect on rheological properties. 9,10 In particular, in sodium oleate/C 8 TAB solution Received: November 3, 2016 Revised: December 8, 2016 Published: December 20, 2016 Article pubs.acs.org/Macromolecules © 2016 American Chemical Society 339 DOI: 10.1021/acs.macromol.6b02385 Macromolecules 2017, 50, 339−348 Downloaded via IMMANUEL KANT BALTIC FEDERAL UNIV on October 11, 2018 at 09:53:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.