First-principle simulations of electronic structure in semicrystalline polyethylene

Abstract

In order to increase our fundamental knowledge about high-voltage cable insulation materials, realistic polyethylene (PE) structures, generated with a novel molecular modeling strategy, have been analyzed using first principle electronic structure simulations. The PE structures were constructed by first generating atomistic PE configurations with an off-lattice Monte Carlo method and then equilibrating the structures at the desired temperature and pressure using molecular dynamics simulations. Semicrystalline, fully crystalline and fully amorphous PE, in some cases including crosslinks and short-chain branches, were analyzed. The modeled PE had a structure in agreement with established experimental data. Linear-scaling density functional theory (LS-DFT) was used to examine the electronic structure (e.g., spatial distribution of molecular orbitals, bandgaps and mobility edges) on all the materials, whereas conventional DFT was used to validate the LS-DFT results on small systems. When hybrid functionals were used, the simulated bandgaps were close to the experimental values. The localization of valence and conduction band states was demonstrated. The localized states in the conduction band were primarily found in the free volume (result of gauche conformations) present in the amorphous regions. For branched and crosslinked structures, the localized electronic states closest to the valence band edge were positioned at branches and crosslinks, respectively. At 0 K, the activation energy for transport was lower for holes than for electrons. However, at room temperature, the effective activation energy was very low (∼0.1 eV) for both holes and electrons, which indicates that the mobility will be relatively high even below the mobility edges and suggests that charge carriers can be hot carriers above the mobility edges in the presence of a high electrical field.

The Role of Interfaces in Polyethylene/Metal‐Oxide Nanocomposites for Ultrahigh‐Voltage Insulating Materials

“…DFT/MD reveals that the pure crystalline phase has a slightly larger bandgap than the pure amorphous phase (Figure , upper right). The calculated bandgaps for amorphous, crystalline and semicrystalline structures are, respectively, 5.89, 6.00, and 5.82 eV, and their calculated charge mobilities at room temperature are, respectively, 0.007, 0.008, and 0.0002 cm 2 V −1 s −1 . The valence and conduction band edges move closer together when the crystalline and amorphous phases are considered as parts of an overall semicrystalline phase.…”

“…The electronic band structure of semicrystalline polyethylene is shown at the upper right. Reproduced under the terms of the CC‐BY 4.0 license . Copyright 2017 The Authors; published by American Institute of Physics.…”

“…Despite its smaller bandgap, the charge mobility of the semicrystalline phase is at least one order of magnitude lower than that of the pure crystalline and amorphous phases, which means that the semicrystalline phase has the lowest apparent conductivity of these three structures. These calculations were however made on a periodic boundary conditions (simulation box was set as an orthorhombic (4 a , 6 b , 8 c ) polyethylene crystal vertically aligned with a 2.462 nm thick amorphous layer) and a physically relaxed/equilibrated semicrystalline structure without any applied electric field . The local conformational disorder of polyethylene chain segments in the amorphous phase may also produce localized states or shallow traps at the edge of both conduction and valance bands, with a depth in the range 0.15–0.3 eV and a residence time of ≈10 −12 s .…”

Influence of degree of crystallinity on DC-conduction of polypropylene

“…In the recent work of Moyassari et al [21] a realistic semicrystalline structure of polyethylene (PE) was created with MD (molecular dynamics)-simulations. DFT (Density Functional Theory) was applied to the modelled amorphous, crystalline and semicrystalline structures of PE in order to obtain information about electronic structure.…”

Electrical Properties of Polyethylene/Polypropylene Compounds for High-Voltage Insulation

“…The hole activation energy of the amorphous structure is lower than that of the crystalline region, but the electron activation energy of the amorphous structure is higher than that of the crystalline structure. This means that the amorphous phase has a strong influence on the electron and hole activation and, hence, on the breakdown phenomena of polymers [14].…”