Timothy M. Krentz

Ligand Engineering of Polymer Nanocomposites: From the Simple to the Complex

One key to optimizing the performance of polymer nanocomposites for high-tech applications is surface ligand engineering of the nanofiller, which has been used to either tune the nanofiller morphology or introduce additional functionalities. Ligand engineering can be relatively simple such as a single population of short molecules on the nanoparticle surface designed for matrix compatibility. It can also have complexity that includes bimodal (or multimodal) populations of ligands that enable relatively independent control of enthalpic and entropic interactions between the nanofiller and matrix as well as introduce additional functionality and dynamic control. In this Spotlight on Applications, we provide a brief review into the use of brush ligands to tune the thermodynamic interactions between nanofiller and matrix and then focus on the potential for surface ligand engineering to create exciting nanocomposites properties for optoelectronic and dielectric applications.

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.

Investigation of dielectric breakdown in silica-epoxy nanocomposites using designed interfaces

Adding nano-sized fillers to epoxy has proven to be an effective method for improving dielectric breakdown strength (DBS). Evidence suggests that dispersion state, as well as chemistry at the filler-matrix interface can play a crucial role in property enhancement. Herein we investigate the contribution of both filler dispersion and surface chemistry on the AC dielectric breakdown strength of silica-epoxy nanocomposites. Ligand engineering was used to synthesize bimodal ligands onto 15nm silica nanoparticles consisting of long epoxy compatible, poly(glycidyl methacrylate) (PGMA) chains, and short, π-conjugated, electroactive surface ligands. Surface initiated RAFT polymerization was used to synthesize multiple graft densities of PGMA chains, ultimately controlling the dispersion of the filler. Thiophene, anthracene, and terthiophene were employed as π-conjugated surface ligands that act as electron traps to mitigate avalanche breakdown. Investigation of the synthesized multifunctional nanoparticles was effective in defining the maximum particle spacing or free space length (Lf) that still leads to property enhancement, as well as giving insight into the effects of varying the electronic nature of the molecules at the interface on breakdown strength. Optimization of the investigated variables was shown to increase the AC dielectric breakdown strength of epoxy composites as much as 34% with only 2wt% silica loading.

Suppression of space charge in crosslinked polyethylene filled with poly(stearyl methacrylate)-grafted SiO2 nanoparticles

Incorporating inorganic nanoparticles (NPs) into polymer matrices provides a promising solution for suppressing space charge effects that can lead to premature failure of electrical insulation used in high voltage direct current engineering. However, realizing homogeneous NP dispersion is a great challenge especially in high-molecular-weight polymers. Here, we address this issue in crosslinked polyethylene by grafting matrix-compatible polymer brushes onto spherical colloidal SiO2 NPs (10–15 nm diameter) to obtain a uniform NP dispersion, thus achieving enhanced space charge suppression, improved DC breakdown strength, and restricted internal field distortion (≤10.6%) over a wide range of external DC fields from −30 kV/mm to −100 kV/mm at room temperature. The NP dispersion state is the key to ensuring an optimized distribution of deep trapping sites. A well-dispersed system provides sufficient charge trapping sites and shows better performance compared to ones with large aggregates. This surface ligand s...

Enhanced charge trapping in bimodal brush functionalized silica-epoxy nanocomposite dielectrics

This manuscript details the processing, and investigates the dielectric properties, of surface ligand engineered epoxy nanocomposites. They display significant improvements in dielectric breakdown strength (DBS). Thermally stimulated depolarization current (TSDC) measurements and pulsed electroacoustic analysis (PEA) results are used to investigate space charge evolution and trapping. These techniques reveal the potential underlying phenomena behind the DBS enhancement.

Prediction of interface dielectric relaxations in bimodal brush functionalized epoxy nanodielectrics by finite element analysis method

Finite element 2-D analysis was implemented to simulate the dielectric spectra of nanodielectrics. As a test case, silica modified with a high graft density of short molecules and a low graft density of epoxy compatible chains were incorporated into epoxy. TEM images of the composites filler distribution were used to construct the model geometry with the interfacial area specifically included. The interfacial area was found to have dielectric relaxation behavior different from that of the matrix, as described by additional fitting parameters. This modeling method has the potential to improve our understanding of the impact of interface properties on the dielectric properties of composites.

Free volume in nanodielectrics

It is now well established that the incorporation of nanoparticulates into a polymer matrix to form a nanodielectric can bring about useful improvements in electric strength provided that the processing results in acceptable particle dispersion. One of the theories of electrical breakdown in polymers seeks to associate breakdown with the polymer free volume, and the well-documented substantial changes in electric strength occurring at glass transition provide some credence to that theory. While it is clear that interactions at the large internal interfacial area in a nanodielectric facilitate the augmentation of dielectric strength, the exact mechanism has not been clarified. Very early work using a compression methodology to estimate free volume in an epoxy nanodielectric indicated that the particles did little to affect free volume. However, this contribution seeks to use positron annihilation lifetime spectroscopy to estimate free volume in an epoxy and several of its SiO2-based nanocomposites to estimate the changes in free volume brought about both through the incorporation of nanoparticulates and also through their functionalization. This study broadly confirms the earlier work and provides good evidence that the free volume is little affected, and thus is not an explanation for the observed change in dielectric strength.