Small-angle neutron scattering of silica nanoparticles templated in PEO-PPO-PEO cubic crystals

Small-angle neutron scattering (SANS) is used to characterize self-assembled nanocomposites. Thermo-reversible block-copolymer cubic crystals (Pluronic^® F127) are used as three-dimensional templates to control the spatial arrangement of silica nanoparticles. The particles are dispersed in the block-copolymer phase and are organized by excluded volume interactions as they are driven into the interstitial cavities of the micelle cubic crystal. The technique of contrast variation is used to evaluate the structure of the templated nanoparticle array and the micelle crystal as a function of relative size, relative concentration and temperature. We show that shearing the samples results in macroscopic alignment of both the micelle cubic crystal and the templated particles. We use this highly ordered state to verify the feasibility of this templating approach. Through a study of control parameters, we find that templating is most effective at silica concentrations below the stoichiometric ratio of one particle per available template site. Higher polymer concentrations in the matrix and temperature above, but near, the order-disorder transition are also found to be favorable conditions for nanoparticle templating. The level of order of the cubic template is also found to be affected by the addition of the particles. At low silica concentrations, particle addition increases the long-range cubic order while at higher concentrations particle addition promotes disorganization. At a high polymer concentrations (30 w%), the addition of silica nanoparticles is also shown to trigger a change from a face-centered cubic (FCC) to a body-centered cubic (BCC) lattice of the micelle crystal. Through this work, we demonstrate the feasibility of this templating approach and also provide design criteria for developing structured thermo-reversible nanocomposites.