ESR study on free radicals trapped in crosslinked polytetrafluoroethylene (PTFE) - II radical formation and reactivity

The radiation crosslinked PTFE(RX-PTFE) was irradiated by Co-60 gamma-ray at 77 K and at room temperature under vacuum or in air. Free radicals trapped in RX-PTFE were studied by an electron spin resonance (ESR) spectroscopy. Radicals trapped in RX-PTFE by gamma-irradiation in air at room temperature are assigned as peroxy radicals. ESR spectra of peroxy radicals trapped in RX-PTFE were more symmetric in the line shape compared with that of virgin material because the molecular motion in amorphous region in RX-PTFE is enhanced at room temperature above the glass transition. Free radicals in RX-PTFE which formed by gamma-irradiation under vacuum react effectively with oxygen to convert peroxy radicals, and the peroxy radicals were rather stable at room temperature. The conversion rate of free radicals into peroxy radicals decreased by increasing crosslinking density for the film specimen (thickness with 0.5 mm). Both alkyl type and chain end radicals convert almost completely to peroxy radicals for an extended period. It is suggested that the diffusion coefficient of oxygen into PTFE decreases with increasing the crosslinking density. The G-value of peroxy radicals formed by irradiation at room temperature in air increases with the crosslinking density up to 3.0 or more. The increment is remarkable for a small change of crosslinking density. A similar trend has been observed for G-values of peroxy radicals transformed from radicals formed under vacuum irradiation, When the specimen containing peroxy radicals is heated in oxygen-free atmosphere, they are being replaced by alkyl radicals together with a small quantity of chain end radicals. Temperature starting the radical conversion decreases with increasing crosslinking density, The transformed radicals are converted to peroxy radicals by introduction of oxygen once more. It means that peroxy radicals abstract fluorine atoms from the surrounding molecules to produce alkyl radicals. (C) 1999 Elsevier Science Ltd. All rights reserved.