The effects of γ radiation on the gas yields and crystallinity of polytetrafluoroethylene

Abstract

Polytetrafluoroethylene (PTFE) has been γ-irradiated in the presence of gaseous radical scavengers. Irradiations in a vacuum resulted in small yields of low-molecular-weight fluorocarbons. The presence of methane during irradiation led to G(CH₃F) = 0.06. Although hydrogen is a major product from the radiolysis of ethylene, none could be detected when PTFE-ethylene mixtures were irradiated. G(H₂) from PTFE-methanethiol was significantly reduced from that expected from methanethiol alone. Both ethylene and methanethiol scavenged PTFE radicals and it is suggested that hydrogen atoms react rapidly with the polymer radicals. Oxygen caused a marked increase in G(gaseous products), the additional gas being due almost entirely to the formation of carbonyl fluoride. The crystallinity of PTFE is shown to increase with radiation dose in the order oxygen > vacuum > methanethiol > ethylene. Initiation of a polymerization reaction of ethylene by PTFE radicals is considered to explain the low rate of crystallinity increase observed in ethylene. The results are shown to be consistent with the view that permanent scission is a predominant reaction in irradiated PTFE. However, the extent of the degradation appears to depend on recombination of fragments resulting from chain scission. Oxygen and methanethiol, by interfering with the recombination processes, accelerate the rate of degradation in relation to PTFE irradiated in vacuum.