Reactions of atomic oxygen [O(3P)] with polymer films

The reactions of polymer films with oxygen atoms are reviewed. Emphasis is on work from this laboratory on polybutadienes having different amounts of 1,4 or 1,2 double bonds and their polyalkenamer homologues, on a polyimide (Kapton), and on a series of polyolefins with increasing fluorine content ranging from polyethylene (PE) to polytetrafluoroethylene (PTFE). Interest in this topic stems from a need to understand the mechanism(s) of surface erosion, or etching, that occurs in organic polymers when exposed to the low Earth orbital (LEO) environment, where ground-state atomic oxygen [O(³P)] is the most abundant constituent. The major findings for O atom reactions with the polyalkenamers were: (1) etch rates increase with decrease in -CH=CH-unsaturation, starting with 1,4-polybutadiene and reaching the maximum rate with PE or ethylene-propylene rubber; (2) in polybutadienes having both 1,4 and 1,2 double bonds, the rate of O(³P)-induced etching is lower the higher the 1,2 content; and (3) the reactions are confined to the polymer surface. Relative etch rates for various commercial polymer films (e.g., Kapton, Teflon, PE) exposed to O(³P) downstream from a low-pressure, radio-frequency O₂ plasma were compared with corresponding literature data for films exposed to O(³P) either during a Space Shuttle flight or in the plasma glow (where many species besides O(³P) are present). The use of O₂ discharge reactors – whether exposures are conducted “out of the glow” or “in the glow” – cannot serve as a routine, ground-based simulation of exposures of a polymer to the LEO environment. ESCA analysis of the surfaces of Kapton and other polymers, before and after reaction with O(³P), showed that there is a steady-state competition between surface recession and oxidation. In a series of fluorine-containing polyolefins exposed to O(³P), the maximum oxygen uptake increased regularly with decrease in fluorine content, ranging from a minimum in PTFE to a maximum in PE.