(354h) Brownian Dynamics Simulations of Self-Suspended Hairy Nanoparticles

Nanoparticle-organic hybrid materials (NOHMs) are a new class of complex materials which consists of inorganic core particles with covalently grafted oligomers on the surface. Due to the lack of any other solvent, the cores are ?self-suspended' in a fluid phase of the attached oligomers. The oligomer mediated interactions between the cores prevent the particles from agglomerating, thus, providing a convenient framework for mixing inorganic cores in organic solvents. In previous work, a density functional theory has been developed for the equilibrium structure of such a fluid in the limit where the radius of gyration of the oligomers is large compared with the interparticle spacing. A principle result of that theory is that the static structure factor approaches zero as the wave number is reduced to zero indicating that each particle and its incompressible oligomers displaces exactly one neighbor. Here we develop a simulation method to explore the non-equilibrium properties of NOHMs and also relax the constraint on the ratio of oligomer to particle size.

The space filling constraint of the oligomers leads to interactions between the cores which are not pair-wise-additive. In this work, we have developed effective potentials between the cores which depend on all the nearest neighbors. The constraint that each particle occupies its fair share of the volume is applied by requiring the volumes of the Voronoi cells surrounding each particle to be equal. This constraint would normally be satisfied in a crystalline solid, but here the suspension is disordered aside from the volume constraint due to the attached hairs. To assess the energy penalty for volume conserving deformations of Voronoi cells, we have solved for the configurational free energy of oligomers that are constrained to fill the space between particles in periodic arrays of cores as the initial FCC structure undergoes extensional and simple shear deformations. The oligomers are modeled as bead-springs tethered to the cores and their free energy for each particle configurations subject to the constraint that the bead concentration is constant. This detailed calculation is used to specify the parameters of a Tersoff potential model where the energy depends on the bond lengths and bond angles between nearest neighboring cores. In this talk, we will present the results for equilibrium structure and long-time diffusivity of the cores.