Self-Assembled Photoresponsive and Thermoresponsive Nanostructures
Raghavan, Srinivasa R.
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Responsive complex fluids based on nanostructures (e.g., micelles, vesicles and nanoparticles) have received considerable attention recently. The ability of these materials to be tuned by light or heat can have many potential applications in the areas of drug delivery, coatings, sensors, or microfluidic valves and dampers. However, most current photoresponsive and thermoresponsive formulations require the synthesis of complex organic molecules, and this prevents them from being used widely for commercial applications. In this dissertation, we seek to develop new classes of photoresponsive (PR) and thermoresponsive (TR) nanostructures based on commercially available, inexpensive precursors. In the first part of this study, we report a new PR fluid based on light-activated nanoparticle assembly. Our system consists of disk-like nanoparticles of laponite along with a surfactant stabilizer (Pluronic F127) and the photoacid generator (PAG), diphenyliodonium-2-carboxylate monohydrate. Initially, the nanoparticles are sterically stabilized by the surfactant and the result is a stable, low-viscosity dispersion. Upon UV irradiation, the PAG gets photolyzed, lowering the pH by about 3 units. In turn, the stabilizing surfactant is displaced from the negatively charged faces of the nanoparticle disks while the edges of the disks become positively charged. The particles are thereby induced to assemble into a 3 dimensional "house-of-cards" network that extends through the sample volume. The net result is a light-induced <italic>sol to gel transition</italic>, i.e., from a low, water-like viscosity to an <italic>infinite</italic> viscosity and yield stress. The yield stress of the photogel is sufficiently high to support the weight of small objects. The gel can be converted back to a sol by either increasing the pH or the surfactant content. Evidence for the above mechanism is provided from a variety of techniques, including small-angle neutron scattering (SANS). In the second part of this study, we demonstrate that laponite/PF127 mixtures also show thermogelling, i.e., the fluids transform from low viscosity sols to stiff gels upon heating above a critical temperature. This phenomenon is reversible and it requires the presence of sufficient amounts of both components. At room temperature, PF127 adsorbs onto laponite disks and stabilizes them by steric repulsion. Upon heating, the PF127 layer on the disks becomes thicker, and more importantly, PF127 micelles in the bulk solution grow significantly. Evidence for the growth of micelles is presented from SANS modeling and from transmission electron microscopy (TEM). At a distinct temperature, we believe the micelles induce <italic>depletion flocculation</italic> of the laponite particles into a gel network. Interestingly, if the PF127 concentration is increased further, the thermogelling is eliminated - this is suggested to be due to the micelles providing depletion stabilization of the particles.