DYNAMICS OF NANOPARTICLES IN POLYMER NANOCOMPOSITES

Abstract

Polymer nanocomposites are materials comprising of nanometer-sized particles embedded inside polymers. The study of dispersion and dynamics of nanoparticles in polymer matrices has important implications in the processing and long-term stability of polymer nanocomposite materials, which have found application in areas such as packaging, energy storage, and biomedical devices, among others. Another factor guiding our interest in such studies is the occurrence of noncontinuum dynamical behaviors as particle sizes become comparable or smaller than the radius of gyration (Rg) of the host polymers. In simple fluids such as polymer melts and solutions where the particle size is much greater than the radius of gyration of the host polymer, the long-time particle dynamics obeys random-walk statistics, and the bulk zero-shear viscosity of the material determines the drag experienced by the particle. In contrast, in polymer liquids (melts or solutions) where the particle size is smaller than the radius of gyration of the host, the effective frictional resistance to probe motion arises from the localized motion of polymer chain segments with size similar to those of the nanoparticle probes. We utilize a combination of theoretical and experimental techniques to examine the noncontinuum diffusion of nanoparticles in polymer nanocomposites. Through fluorescence microscopy we track the motion of nanoprobes in aqueous solutions of linear polyethylene oxide and probe particle dynamics in different regimes of particle diameter (d), with respect to characteristic polymer length scales, viz., the correlation length (ξ), the entanglement mesh size (a), and the radius of gyration (Rg). We show that in entangled polymer solutions when particle size becomes greater than the entanglement tube diameter, nanoparticle dynamics transition from diffusive to subdiffusive, reminiscent of particle transport in a field with obstructions. This last finding is in stark contrast to the nanoparticle dynamics observed in entangled polymer melts where X-ray photon correlation spectroscopy measurements reveal hyperdiffusive dynamics. The conventional wisdom surrounding hyperdiffusive processes calls for the presence of residual stresses or force fields that can bias particle motion. While this can explain the occurrence of hyperdiffusion in aggregating particles or materials close to their glass transition temperatures, this argument cannot explain the occurrence of hyperdiffusion in non-aging and uniform nanoparticle-laden polymer melts or solutions. We design a simple theoretical framework to understand the origins of diffusive and subdiffusive dynamics observed in entangled polymer solutions against the hyperdiffusive dynamics observed in entangled polymer melts and study the role of solvents in setting structural correlations in polymer nanocomposite systems.