Radiation-induced free radicals and their behaviour in crosslinked polytetrafluoroethylene (PTFE)

Abstract

Free radicals formed in radiation crosslinked PTFE (RX-PTFE) by 60 Co γ-rays at 77 K and at room temperature (297 K) were studied by electron spin resonance (ESR) spectroscopy. ESR spectra observed in RX-PTFE were very different from those of non-crosslinked virgin PTFE (V-PTFE); a broad singlet component assigned to alkyl type radicals was trapped in the amorphous region of RX-PTFE and increased with progress of crosslinking density. The G value of trapped free radicals in RX-PTFE was about 30 times higher than that of V-PTFE. The increase in G value observed upon increasing the crosslinking density was remarkable in a dose range below about 1 MGy. Trapped free radicals decayed around 383K, the α relaxation temperature for RX-PTFE. Hence free radicals trapped in the crosslinked amorphous region of RX-PTFE could be stabilized by reduction of the molecular motion in the crosslinked polymer matrix. Free radicals formed in RX-PTFE by γ-irradiation were converted effectively to peroxy radicals by reaction with oxygen around 297 K, and the peroxy radicals were also stable at those temperatures. The conversion rate of free radicals to peroxy radicals decreased upon increasing the crosslinking density, because the oxygen diffusion was depressed by the crosslinking. When the specimen containing peroxy radicals was heated to 373K under vacuum, alkyl radicals appeared via the decay of peroxy radicals; the radicals were reconverted to peroxy radicals by the introduction of oxygen. This means that peroxy radicals abstract F atoms from the main chain of surrounding PTFE molecules, resulting in the formation of alkyl radicals.