Molecular Layer Deposition of Polyurea on Silica Nanoparticles and Its Application in Dielectric Nanocomposites

Polymer nanocomposites (NCs) offer outstanding potentialfor dielectricapplications including insulation materials. The large interfacialarea introduced by the nanoscale fillers plays a major role in improvingthe dielectric properties of NCs. Therefore, an effort to tailor theproperties of these interfaces can lead to substantial improvementof the material's macroscopic dielectric response. Graftingelectrically active functional groups to the surface of nanoparticles(NPs) in a controlled manner can yield reproducible alterations incharge trapping and transport as well as space charge phenomena innanodielectrics. In the present study, fumed silica NPs are surfacemodified with polyurea from phenyl diisocyanate (PDIC) and ethylenediamine(ED) via molecular layer deposition (MLD) in a fluidized bed. Themodified NPs are then incorporated into a polymer blend based on polypropylene(PP)/ethylene-octene-copolymer (EOC), and their morphological anddielectric properties are investigated. We demonstrate the alterationsin the electronic structure of silica upon depositing urea units usingdensity functional theory (DFT) calculations. Subsequently, the effectof urea functionalization on the dielectric properties of NCs is studiedusing thermally stimulated depolarization current (TSDC) and broadbanddielectric spectroscopy (BDS) methods. The DFT calculations revealthe contribution of both shallow and deep traps upon deposition ofurea units onto the NPs. It could be concluded that the depositionof polyurea on NPs results in a bi-modal distribution of trap depthsthat are related to each monomer in the urea units and can lead toa reduction of space charge formation at filler-polymer interfaces.MLD offers a promising tool for tailoring the interfacial interactionsin dielectric NCs.