(413c) Structure, Rheology and Optical Properties of Self-Assembled Micelle-Nanoparticle Plasmonic Nanogels: Experiments and Molecular Dynamics Simulations

Cationic surfactants such as cetyl-trimethylammonium bromide (CTAB) have the ability to self assemble with metallic nanoparticles to form a corona or a double-layer vesicular structure [1]. Such structures, upon further interaction with wormlike micelle fragments, are hypothesized to form micelle-nanoparticle elastic networks [2, 3]. Recently, we have shown that self-assembly of metallic nanoparticles with wormlike micelles in surfactant solutions is a robust route for producing stable multicomponent nanogels with remarkable color uniformity [4, 5]. The optical properties of the gels can be easily tuned by varying the type, shape and/or concentration of nanoparticles. In particular, multicomponent gels capable of broadband absorption of visible light can be robustly manufactured via the self-assembly route. Small angle X-ray scattering and rheological studies suggest that the nanoparticles materially participate in the wormlike micelle network to form a more compact double network. Such plasmonic gels exhibit rich rheological behavior depending on the nanoparticle concentration and the salt/surfactant ratio. Specifically, non-monotonic dependence of zero shear viscosity on nanoparticle concentration, rheopexy, shear thickening, shear banding and shear thinning are observed. The nanogels exhibit enhanced viscoelasticity upon the addition of more nanoparticles and are thermoreversible. The mechanisms contributing to the rheological and structural properties of the plasmonic nanogels are studied by coarse-grained Molecular Dynamics simulations based on the MARTINI force field framework [6, 7]. The results suggest that vesicular nanoparticle-surfactant structure can form stable bridges with cylindrical micelles through two mechanisms: end-cap opening and lateral attachment. The energetics of bridge formation, the effect of flow shear on the stability of the end-cap and lateral bridges and potential applications to efficient light trapping in photovoltaic cells and optofluidic devices will be discussed.

Acknowledgements: We acknowledge National Science Foundation grant CBET-1049454 for partial support of this research. Syracuse University has filed US and PCT patent applications based on the findings of tis work.

References:

1. B. Nikoobakht and M.A. El-Sayed, Langmuir, 17, 6368 (2001)
2. F. Nettesheim, M.W. Liberatore, T.K. Hodgdon, N.J. Wagner, E.W. Kaler, and M. Vethamuthu, Langmuir, 24, 7718 (2008).
3. M.E. Helgeson, T.K. Hodgdon, E.W. Kaler, and N.J. Wagner, Langmuir, 26, 8049 (2010).
4. T. Cong, S.N. Wani, P.A. Paynter, and R. Sureshkumar, Applied Physics Letters, 99, 043112 (2011).
5. T. Cong, S.N. Wani, G. Zhou, E. Baszczuk, and R. Sureshkumar, Proc. SPIE, 8097, 80970L (2011).
6. A.V. Sangwai and R. Sureshkumar, Langmuir, 27, 6628 (2011).
7. A.V. Sangwai and R. Sureshkumar, Langmuir, 28, 1127, (2012).