Dielectric breakdown strength of epoxy bimodal-polymer-brush-grafted core functionalized silica nanocomposites

The central goal of dielectric nanocomposite design is to create a large interfacial area between the matrix polymer and nanofillers and to use it to tailor the properties of the composite. The interface can create sites for trapping electrons leading to increased dielectric breakdown strength (DBS). Nanoparticles with a bimodal population of covalently anchored molecules were created using ligand engineering. Electrically active short molecules (oligothiophene or ferrocene) and matrix compatible long poly(glycidyl methacrylate) (PGMA) chains comprise the bimodal brush. The dielectric breakdown strength was evaluated from recessed samples and dielectric spectroscopy was used to study the dielectric constant and loss as a function of frequency. The dielectric breakdown strength and permittivity increased considerably with only 2 wt% filler loading while the dielectric loss remained comparable to the reference epoxy.

I. Introduction

NANODIELECTRICS, or dielectric polymer nanocomposites, can exhibit significant improvements in endurance strength and dielectric breakdown strength compared to the unfilled polymer [1]–[4]. There are experimental results suggesting that in addition to controlling the dispersion of particles, controlling the relative polar or nonpolar nature of the particle surface will allow for property optimization [5]. In addition, directly bonding the particle to the polymer matrix has been shown to prevent conductive percolation across particle surfaces resulting in reduced interfacial polarization within the composite and increased dielectric breakdown strength [6]. Furthermore, significant reduction in leakage currents and dielectric losses and improvement in dielectric breakdown strengths have resulted when phenyl rings with electron-withdrawing functional groups were grafted to the particle surface [7]. While it is clear that the nanofiller/matrix interface is critical in controlling the dielectric properties, the mechanisms leading to these properties are not fully understood. This lack of understanding limits our ability to optimize the dielectric response. Therefore, in order to realize the promise of polymer nanodielectrics and create insulating materials that reach into a new property space, a more fundamental understanding of the role of the nanoparticle interface in controlling properties is needed.