Space charge characteristics of low density polyethylene oxyfluorinated with high concentrations of oxygen

Two kinds of oxygen/fluorine/nitrogen mixtures with large concentration of oxygen have been used to oxyfluorinate low density polyethylene (LDPE). The O₂/F₂/N₂ volume fraction in the mixtures was 1/1/4 and 2/1/4, respectively. Both the original LDPE and oxyfluorinated samples are plate of 0.25~0.30 mm in thickness. The space charge characteristics of the original LDPE and the oxyfluorinated samples were tested by using the pulsed electro acoustic (PEA) method. Scanning electron microscope (SEM) and attenuated total reflection infrared (ATR-IR) as well as X-ray photoelectron spectroscopy (XPS) were employed to characterize the morphology and the chemical composition of the oxyfluorinated layer, respectively. It was discussed that the effects of the high concentration of oxygen on the space charge characteristics of the LDPE. The relationship between it and the chemical composition evolution of the oxyfluorinated layer was studied, too. The results show that space charge distribution at the surface layer of the LDPE plate changes largely after the oxyfluorination. Positive heterocharge appears near the cathode in the oxyfluorinated samples, which was different from the situation in the original LDPE with only homocharge. The space charge amount increases with the enhancement of the oxyfluorination condition. The reason is considered that oxyfluorination alters the chemical components and microstructure of the oxyfluorinated surface layer, which lowers the trap level of hole and deepens that of electron. Therefore, hole injection from the anode increases and electron injection from the cathode decreases. It results in the accumulation of positive heterocharge near the cathode. Besides, large concentration of oxygen leads to obvious introduction of O atom at the initial oxyfluoriantion stage. However, the formation rate of C=O groups is obviously restrained by the enhancement of oxyfluorination degree. The results obtained are helpful for deepen understanding about the effect of oxyfluoriantion on dielectric properties of LDPE and the mechanism.