Number-average molecular weight of radiation-degraded poly(tetrafluoroethylene). An end group analysis based on solid-state NMR and IR spectroscopy

Changes in the chemical structure of poly(tetrafluoroethylene) (PTFE) induced by electron beam irradiation at room temperature in air have been studied by 19F solid-state NMR and IR spectroscopy. Chain scission associated with the formation of trifluoromethyl (–CF3), carboxylic acid fluoride (–COF) and carboxylic acid (–COOH) end groups was confirmed to be the predominant process under these conditions. The number-average molecular weight of radiation-degraded PTFE as a function of irradiation dose was calculated based on quantitative end group analysis. It strongly decreases with increasing dose to approximately 4000 g/mol at a irradiation dose of 4 MGy. The molar ratio of CF3 end groups to oxygen-containing (–COF, –COOH) end groups was found to depend on the irradiation dose and the irradiation conditions. For low dose level, the molar ratio determined experimentally coincides well with the mechanism proposed for radiation-induced degradation of PTFE.