Space charge of polyethylene and electronic structure analysis of trapping site using common chemical groups

Pulsed electroacoustic (PEA) measurement technology is widely accepted to detect the space charge distribution inside insulating materials all over the world. In this work, we elucidated further the space charge mechanism based on the typical PEA results of low-density polyethylene (LDPE). Clearly, significant positive accumulation and charge packets are formed inside the sample, which is derived from the chemical and physical structures of polyethylene. Usually, some common chemical groups, such as hydroxyl, carbonyl, double bond, conjugated double bond, and oxide center, exist in polyethylene owing to the manufacturing process and complex ambient. By employing an advanced quantum chemical calculation technique, we obtained the electronic structures of a small polyethylene (PE) chain (C₂₄H₅₀) and some PE chains with different chemical groups. After analyzing the electron energy level, molecular orbital, and electric potential, we determined the trapping sites by the chemical groups. These chemical groups with special structures contribute to the narrowing of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gaps (namely, the band gap here), indicating the introduction of trapping levels within the band gap of PE. Additionally, 3D electric potential distributions demonstrate that an obvious potential distortion occurs around these introduced chemical groups. This means that the moving electron or hole carriers can be captured here, leading to the formation of trapped charges. These trapping sites inside the polyethylene are closely related to the space charge accumulation in LDPE.