Proton Irrdadiation Induced Damage Effects And Free Radicals Evolution Behaviours In PTFE

PTFE is a m aterial with good electrical and mechanical properties, which is widely used as a polymer in spacecraft. Harsh space environments, for example, thermal cycles, solar winds and Van Allen radiation belt, will change the structure and properties of PTFE. Degradation would occur as a result of exposure to radiation environment, and the interaction between PTFE and low energy protons is one of the most significant inducement of degradation. Radical reactions during radiation and post-radiation in PTFE is complex and degradation or evolution model is still unknown. Thus changes of structure and properties were measured by transmittance spectra of UV-Vis, mechanical tests, XPS, XRD, Raman and so on. EPR was used to observe the behavior of radicals and XPS was used to detect the evolution of composition. Based on these data, the mechanisms of degradation of PTFE by low energy protons were revealed.Protons radiation leads to the darkening of PTFE. The absorption peak at 220 nm is formed firstly, and with increase of fluence, the absorption peak at 280 nm appears. The variation is more associated with displacement damage dose. Further increase of fluence could cause the red shift of absorption peak. Irradiation could also lead to changes of mechanical properties. The decrease of elongation at break and the slight increase of yield point happens during the radiation. The tension strength increases firstly, and a drop flows, which may due to the crosslinking to chain scission conversion. Roughness test by AFM indicated a growth followed by a drop, which is probably related to crosslinking/ chain scission behavior.EPR measurement shows chemical bonds break of PTFE, generating F-containing radicals(g=1.9907) and pyrolysis carbon radicals(g=2.0025). The type of radicals changes with fluence. The F-containing radicals increase firstly and then gradually turn to pyrolysis carbon radicals. The density of pyrolysis carbon radicals slowly increases and become dominated at the fluence higher than 5×1015cm-2.Modern analysis method were used to analysis the composition and structure changes of PTFE during and post radiation. During radiation, the breakage of C-F and the loss of F lead to the formation of C-C and C=C. Formation of cumulative C=C generates a large fluorescent background in Raman spectra. Radical reaction models was built based on EPR, and radical formation, recombination and variation were discussed. The change of structure of PTFE was also studied. Data suggest a linear-exponential decrease of density radiation. The lifetime of F-containing radicals is longer than pyrolysis carbon radicals. The evolution model of radicals post radiation is coincident with experiment data.