Electronic Structure of Polymer Dielectrics: The Role of Chemical and Morphological Complexity

The electronic structure of polymers contains signatures that correlate with their short-term and long-term integrity when subjected to large electric stresses. A detailed picture of the electronic structure of realistic models of polymers has been difficult to obtain, mainly due to the chemical and morphological complexity encountered in polymers. In this work, we have undertaken a comprehensive analysis of the electronic structure of six model polymers displaying chemical and morphological diversity, namely, polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(methyl methacrylate) (PMMA), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT), using first-principles density functional theory computations and classical molecular dynamics simulations. In particular, we have studied the role of monomer chemistry, tacticity, and large-scale morphological disorders in shaping the electronic structure of these polymers. We find that monomer chemistry and morphological disorder cooperate to create localized energy states and the formation of shallow/deep trap depths near the band edges, but tacticity has little effect on the band structure. Appropriate connections and comparisons between the computed results (e.g., band gap and trap depths) and the available experimental data have also been provided. Critical insights on physicochemical and electronic structure relationships are revealed, providing a pathway for understanding the factors that control electrical conduction and degradation of polymers (i.e., charge transport mechanisms, ionization processes, and carrier injections from electrodes).