First Principles Studies of Interface Dielectric Properties of Polymer/metal-oxide Nanocomposites}

Abstract

This thesis is devoted to studying interface dielectric properties of polymer nanocomposites from first principles. We aim to understand at atomic scale the role of interface effects and the dielectric finite size effects of nanoparticles in determining the effective dielectric properties of polymer nanocomposites.

To study surface effects from first principles, we first investigate the two common methods, namely dipole correction and Coulomb cutoff, used to eliminate the artificial effects introduced by using the supercell approximation. We implement Coulomb cutoff technique in a plane-wave-based density functional theory code and compare it with dipole correction for the same system under the same conditions. By comparison, both methods are shown to be equivalent and able to remove the artificial effects of periodic images very accurately. We also find that a combination of these two methods offers an easy way to distinguish the localized bound states of interest from highly delocalized unoccupied states while using a relatively small supercell, and to ascertain the convergence of the results with respect to supercell size.

To understand the dielectric properties at the atomic scale, we develop a new nanoscale averaging model to connect the macroscopic quantities to the corresponding microscopic ones. This model allows us to compute the spatially resolved local dielectric permittivity, including the critically important ionic contributions, for interfaces and other complex structures. In this model, a simple way of evaluating real-space decay length of the nonlocal dielectric functions is also proposed.

By using the dipole correction and our averaging model in supercells, we calculate the optical and static local dielectric permittivity profiles for polymer (polypropylene) / metal-oxide (PbTiO$_3$ and alumina) nanocomposites. Our {em ab-initio} results show that metal-oxide/polymer interface effects are very localized and are mostly confined to the metal-oxide surface side, and that nanoscale metal-oxide slabs can on average retain the macroscopic value of bulk dielectric permittivity. These findings suggest that classical mixing laws associated with macroscopic composites can be applied to model the overall dielectric constant of a real polymer/metal-oxide nanocomposite system.