PTFE degradation under heavy ion irradiation

Polytetrafluoroethylene (PTFE) shows an outstanding combination of chemical and physical properties such as excellent resistance to chemical reagents, thermal stability in a wide temperature range, high electric resistance, and low friction coefficient. However, PTFE is known to be extremely sensitive to high energy radiation undergoing scission of the main chain. As a result of the reduction of the molecular weight, the mechanical properties are deteriorated. In the nineties, it was found that PTFE crosslinks under electron irradiation under specific conditions, in particular in oxygen free atmosphere and at temperatures just above the melting point (~ 340 °C). A recent study points out that this cross linking can be obtained even at room temperature irradiation using -particles [1]. In the present work we studied the degradation of PTFE under heavy ion beam irradiation at room and cryo temperatures. PTFE films (25 and 50 μm thick, Enflo Canada Ltd.) were irradiated with different ions (C, Xe, and U) up to energies of 1.5 GeV and fluences of 1x10 ions/cm at the UNILAC. The M3-beam line of the new materials research M-Branch allows cryo-irradiations (T = 23-300 K) and online residual gas analysis of volatile degradation fragments. The irradiated PTFE foils were also analysed by offline FTIR spectroscopy. Mass spectra recorded during room temperature ion irradiation show outgassing of several fragments (figure 1), with CF and CF3 being the most dominant species. Almost no fragments are observed during the cryoirradiation at 23 K. However, subsequent sample heating to room temperature leads to outgassing of the fragments starting above 150 K. This result indicates that at low irradiation temperatures small fragments are frozen in and accumulated in the sample.