Polymer-Threaded and Polymer-Wrapped Wormlike Micelle Solutions: Molecular Dynamics Simulations

Using united atom and coarse-grained molecular dynamics (MD) simulations, we investigate a new type of polymer-threaded wormlike micelles with polymer localized at the center of wormlike micelle (WLM), forming its backbone. We show that polystyrene (PS) is rapidly encapsulated by potassium oleate WLM in aqueous solutions containing 6% by weight of potassium chloride (KCl) leading to formation of a threaded micelle where PS is located in a highly extended conformation within the hydrophobic core. Similar behavior is observed in CGMD simulations for poly(3,4-ethylenedioxythiophene) (PEDOT) in C12E5 WLM aqueous solution, indicating the potential for conjugated polymers solubilization by wormlike micelles. Polystyrene encapsulation into a potassium oleate WLM is accompanied by surfactant reorganization which results in a slightly thicker WLM diameter for the PS-threaded section of the WLM and a significant increase in the persistence length of the polymer. In contrast, a less hydrophobic polymer polycaprolactone (PCL) or poly(4 vinylpyridine) (P4VP) is found to wrap around the hydrophobic core of potassium oleate WLM in a Gaussian-like conformation forming approximately 0.86 hydrogen bonds with surrounding water per carbonyl of PCL (or 0.88 per repeat unit of P4VP), which contributes to screening the WLM from water and reduces the number of surfactants per nm of WLM. For both threaded (PS) and wrapped (PCL) polymers, the equilibrium properties of the WLM remain practically intact, while the segmental dynamics of the polymers within the WLM is significantly slower than in the corresponding good solvents (benzene and chloroform). Nonequilibrium MD simulations under shear flow revealed that shear induced scission of WLM mostly occurs at the boundary of polymer threaded or polymer-wrapped sections of WLMs, therefore affecting the dynamics of stress relaxation in polymer-containing WLM solutions, as has been experimentally observed for polymer-embedded WLM solutions, and suggesting new ways of controlling viscoelastic behavior and responsiveness of WLMs to external stimuli.