(224e) Relating Particle Microstructure to Mechanical Reinforcement of Polymer Nanocomposites

The microstructure of polymer-grafted nanoparticles in polymer melts that are chemically identical to the brush was investigated using ultra-small angle x-ray scattering (USAXS). The dispersions were thermodynamically stable. Particle size and melt molecular weight were varied such that the particle softness, L*, or ratio of the brush thickness to the particle radius, ranged from L* = 0.02 ? 0.29. At particle concentrations above the jamming transition, the experimentally extracted structure factors and radial distribution functions from USAXS show stronger nanoparticle interactions in lower molecular weight melts due to brush stretching. Good agreement was found between the measured structure factor and that predicted using Percus-Yevick (PY) closure. Stronger nanoparticle interactions detected by USAXS correspond to enhancements in the elastic shear modulus measured by rheology. Further, the mechanical enhancements can be predicted through the Zwanzig-Mountain relation and Monte Carlo simulations. Overall, the use of USAXS and rheology enables the connection between experiment and theory to predict, and ultimately control the interactions of polymer-grafted nanoparticles in polymer melts.