Polymer nanocomposites (NCs) offer outstanding potential
for dielectric
applications including insulation materials. The large interfacial
area introduced by the nanoscale fillers plays a major role in improving
the dielectric properties of NCs. Therefore, an effort to tailor the
properties of these interfaces can lead to substantial improvement
of the material’s macroscopic dielectric response. Grafting
electrically active functional groups to the surface of nanoparticles
(NPs) in a controlled manner can yield reproducible alterations in
charge trapping and transport as well as space charge phenomena in
nanodielectrics. In the present study, fumed silica NPs are surface
modified with polyurea from phenyl diisocyanate (PDIC) and ethylenediamine
(ED) via molecular layer deposition (MLD) in a fluidized bed. The
modified NPs are then incorporated into a polymer blend based on polypropylene
(PP)/ethylene-octene-copolymer (EOC), and their morphological and
dielectric properties are investigated. We demonstrate the alterations
in the electronic structure of silica upon depositing urea units using
density functional theory (DFT) calculations. Subsequently, the effect
of urea functionalization on the dielectric properties of NCs is studied
using thermally stimulated depolarization current (TSDC) and broadband
dielectric spectroscopy (BDS) methods. The DFT calculations reveal
the contribution of both shallow and deep traps upon deposition of
urea units onto the NPs. It could be concluded that the deposition
of polyurea on NPs results in a bi-modal distribution of trap depths
that are related to each monomer in the urea units and can lead to
a reduction of space charge formation at filler-polymer interfaces.
MLD offers a promising tool for tailoring the interfacial interactions
in dielectric NCs.