Nanoparticle Dynamics in Polymer Matrices

The addition of nanoparticles to a polymer matrix can significantly improve the thermal, mechanical, and optical properties of the composite. Experimentally it has been found that the viscoelastic response, which is relevant for processing, of the composite can increase or decrease depending on the size of the nanoparticle and the nature of the interaction between the nanoparticle and the surrounding polymer matrix. Using equilibrium and non-equilibrium molecular dynamics simulations, the diffusivity of nanoparticles in weakly interacting mixtures of nanoparticles and polymer melts and viscosity of the mixture are determined as a function of the size of the nanoparticles in mixtures with unentangled and entangled polymers. These simulations clearly show that the dynamics of these composites have two very different classes of behavior depending on size of the nanoparticle. For nanoparticles smaller than the polymer entanglement mesh size, relaxation times and their diffusivities are completely described by the local, Rouse dynamics of the polymer chains. Analogous to solvent molecules, small nanoparticles act akin to plasticizers and reduce the viscosity of a polymer melt. However for larger nanoparticles, which are comparable to the entanglement mesh size, the nanoparticles are significantly slowed by chain entanglements, and is not describable by the Stokes-Einstein relationship. For entangled polymer melts there is an additional effect of the nanoparticles due to the release of topological constraints.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.