Viscoelastic Synergy and Microstructure Formation in Aqueous Mixtures of Nonionic Hydrophilic Polymer and Charged Wormlike Surfactant Micelles

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 <i>n</i>-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.