Aging Effects in Nanoparticle Composites

We have studied the aging behavior of compatible mixtures of nanoparticles and polymers, and in a parallel study polymer grafted nanoparticles cast from solvent. Small angle x-ray scattering and x-ray photon correlation spectrocopy show that these materials restructure through NP motion, even though the initial state corresponds to a random dispersion of the NPs. We conjecture that this reordering is driven by the charge on the NPs which leads to a long range repulsion. We thus propose that mixtures of nanoparticles and polymers are frequently out of equilibrium and that the preperation method may sometimes critically affect the properties that result.