Influence of Oxygen Impurity in Fluorinating Mixtures on Charge Blocking Properties of Fluorinated Surface Layer of Polyethylene

Polyethylene samples were surface fluorinated by two kinds of F-2/N-2 mixture, containing 0.04% and 0.2% oxygen by volume, respectively. Infrared analyses do not clearly indicate the presence of oxygen in the surface layer fluorinated by the former, but clearly show that oxygen has been incorporated into the surface layers fluorinated by the latter. Moreover, the amount of fluorine introduced in the fluorinated layer is found to increase with the fluorine partial pressure or fluorination time. Thicknesses of the fluorinated layers are determined by SEM cross section images. Space charge measurements show that the thin fluorinated layer (0.24 mu m) without appreciable oxygen can block the charge injection from electrodes very effectively, while the thick fluorinated layer (0.39 mu m) with oxygen does not show the effectiveness. In this case, a further thick fluorinated layer is needed to achieve the blocking effect. The permittivity of the fluorinated layers is evaluated by the polar component of surface energy, and the results suggest an increase of the permittivity because of the incorporation of polar groups. Thermally stimulated discharge current measurements reveal different charge trapping properties between the fluorinated layers and a negative influence of oxygen on the decrease in free volume of the surface layer. The decrease in free volume is more important in suppressing the charge injection than the increase in surface layer permittivity and the change in surface trap.