Degradation of γ-irradiated linear perfluoroalkanes at high dosage

The effects of the carbon backbone chain length on the EPR spectra of linear perfluoro-n-alkanes (PFAs) γ-irradiated at 77 K was studied for the short chain n-C6F14, n-C8F16, n-C12F26, and n-C16F34 molecules as well as the polymer polytetrafluoroethylene (PTFE). The experimental data show that the processes occurring during radiolysis of perfluoro-n-alkanes and polytetrafluoroethylene are very similar. EPR spectra of irradiated perfluoro-n-alkanes at low radiation dose show superimposed signals from three radicals: –F2CCFCF2–, –CF2CF2 and F3C. The signal intensity decreases with perfluoro-n-alkanes chain length. At doses above 2.0 MGy, a constant increase in concentration of the radicals –F2CCFCF2– and –CF2CF2 is observed with decreasing chain length. The concentration of these radicals formed during radiolysis of PFA is described by the ratio: [–CF2CF2]/[–F2CCFCF2–] ≈3/(n − 2), where n is the number of carbon atoms in the linear perfluoroalkanes. Density functional theory was used to calculate the structures of the radicals and C–F bond energies in model perfluoro-n-alkanes as well as the EPR spectra of the associated radicals. This data is used to provide further insight into the radiation stability of PTFE. Four topographical structures of polytetrafluoroethylene, one amorphous and three crystalline, were identified by thermomechanical analysis. In the crystal phase, γ-irradiation results in their transformation to the amorphous form. The helical structure of individual perfluroalkanes readily distorts on removal of a fluorine and this will have an impact on the overall structure of the material. Such structural reorganization can lead to loss of the mechanical stability of polytetrafluoroethylene.

A combined computational and experimental study of the irradiation of perfluoroalkanes and polytetrafluoroethylene shows that individual chains of the perfluoropolymer show less radiation damage than do polyethylene chains. The helical structure of individual perfluroalkanes readily distorts on removal of a fluorine leading to loss of the mechanical stability of bulk polytetrafluoroethylene as compared to polyethylene.Figure optionsDownload as PowerPoint slide