Unraveling the role of surface molecular structure on vacuum flashover for fluorinated copolymers

To investigate the influence of surface molecular structure on the electrical performance of fluorinated copolymers, ultraviolet (UV) curable resin containing different types of fluorinated monomers (ethers and alkyls) were prepared by photopolymerization. The X-ray photoelectron spectra indicated the presence of fluorinated monomers on the surface after UV radiation. Furthermore, the surface morphology measurement revealed that these monomers cause a slight change to the surface roughness. Subsequently, the electrical properties were examined through a pulsed flashover test in vacuum. The results showed an improvement in the electrical strength of the specimen doped with dodecafluoroheptyl methacrylate (alkyls). In contrast, the sample doped with perfluoropolyether chains (ethers) exhibited no alleviation in flashover performance, despite the fact that its surface F 1s/C 1s atomic ratio is 10 times higher than that of the former. A coarse-grained model based on quantum chemical calculations reveals that electron traps are introduced at the surface because of the grafted fluorinated chains. Further explanation based on the free volume theory indicates that the higher gas absorption arising from the amorphous structure during the electron-induced degassing process accounts for the deterioration of the insulation system, which was verified by surface charge behavior before and after the flashover.