Title: Molecular Modeling for Rational Design of Polymer Dielectrics
Description: The state-of-the-art in high voltage and high energy density capacitors is dominated by biaxially oriented polypropylene (BOPP), a linear dielectric with electronic polarizability but low dielectric constant (2.2). BOPP provides an energy density of 5 J/cm3 at the breakdown, which occurs at 720 MV/m for films 10 micrometer thick. While there are many approaches to increase the energy, they either offer solutions to specific applications or suffer from fundamental limitations. The principal focus of the dissertation will be centered on rational design for the development of such materials. We study all three verticals of dielectric properties, namely: dielectric permittivity; dielectric loss; and breakdown strength. We then use the information obtained to design a copolymer with enhanced dielectric properties. We start by using simulations and experiments to delineate the mechanism by which the addition of a small number of polar --OH groups to a nonpolar polymer increases the static relative permittivity (or dielectric constant) by a factor of 2.

However, the dielectric loss in the frequency regime of interest to power electronics is less than 1%. We observe that a small amount of adsorbed water plays a critical role in this attenuated loss. Further, we study the effect of other polar pendant groups on dielectric properties of polyethylene. By systematically comparing the static relative permittivity of crystalline and semi-crystalline samples we find amorphous phase as the dominant player in these types of material. The constraints provided by the surrounding chains significantly impede dipolar relaxations in the crystalline regions, whereas amorphous chains must sample all configurations to attain their fully isotropic spatial distributions. We also explore the use of the time--temperature superposition (tTS) principle for calculating the dielectric loss of the dielectric materials. This approach helps us explore time scales in simulations which were previously inaccessible using classical MD. We find that the tTS method performed well in determining dielectric losses in the system as long as unrelaxed components are not included in the calculation.

This methodology, which provides us with a significantly faster and reliable pathway for calculation of dielectric loss, allows us to identify the role of polar sidegroups on the dielectric loss of common non-polar polymeric dielectrics. Further, we explore the dielectric breakdown mechanism in polymer dielectrics by introducing external electric fields in the materials. Conventionally the prediction of dielectric strength has focused on ground state energy calculation, thus restricting the analysis of the breakdown process to purely electronic in nature. While this provides reasonable predictions for low-temperature systems, we observe that electromechanical breakdown plays a crucial role in the high-temperature regimes. Our simulation results suggest that fracture mechanics drive electromechanical breakdown, which dominates over electronic breakdown at relevant operating temperatures. Finally, we utilized these fundamental insights into dielectric properties for designing copolymer with enhanced dielectric properties.