Radiation hardening of polytetrafluoroethylene against chemical etching

Exposure of polytetrafluoroethylene (PTFE) to MgKα x‐rays (1253.6 eV) for times as short as 10 min is observed to lead to permanent radiation hardening toward chemical etching. X‐ray‐dosed surfaces are visibly less etched, and the loss of F is over a factor of 2 smaller. For PTFE itself, the x‐ray photoelectron spectrum shows only slight changes for x‐ray exposures up to several hours, but thermal desorption spectroscopy yields a direct correlation between the desorption of low‐molecular‐weight fluorocarbons and x‐ray exposures down to the order of minutes. Irradiation of PTFE is know to lead to molecular weight degradation and branching and/or crosslinking through free radical reactions. The degradation reactions are the source of the low‐molecular‐weight fluorocarbons which desorb at elevated temperatures. Branching and/or crosslinking, which produce a more rigid, thermally stable structure, are proposed as the mechanism for resistance to chemical etching. Similar resistance to chemical etching, in the absence of x‐rays, is observed for the fluorinated ethylene‐propylene copolymer which contains intrinsic fluorinated methyl group side chains.