(450i) Nanoparticle-Organic Hybrid Materials: The Particle That Carries Its Fluid On Its Back

Nanoparticle-organic hybrid materials (NOHMs) consist of 10 nm diameter spherical inorganic core particles functionalized with oligomeric organic molecules. Although these systems contain no added solvent, they exhibit fluid behavior with the fluidity provided by the attached oligomers. We will present a theory for the equilibrium structure and transport properties of these materials based on an assumption that the intercore forces are mediated by entropic effects associated with the conformations of the oligomers. The compression of oligomer brushes when two particles approach one another leads to steric repulsion as is typically found in hairy particles. However, the oligomers also lead to entropic attractive forces. The oligomeric fluid is assumed to be incompressible and oligomers (modeled as bead-spring systems) must stretch to fill the space between the cores. This effect prevents the formation of large regions of free volume between the core particles. Because each particle carries its share of the fluid phase, the structure factor at zero wave number is equal to zero, a value indicative of a deficit of one neighboring core particle in the vicinity of any chosen particle. We determine the long-time diffusivity, low shear rate viscosity and high frequency shear modulus for NOHMs in the limiting case where the radius of gyration of the oligomers is large compared with the core radius. In this limiting case, each core experiences weak interactions with the many other cores residing in its neighborhood (within a radius of gyration). Exploiting this limit, the transport properties can be determined in a quasi-analytical manner based on a solution of the non-equilibrium probability density for pairs of particles experiencing an intercore potential of mean force derived from density-functional theory.