Mechanistic Studies of Atomic Oxygen Reactions with Polymers and Combined Effects with Vacuum Ultraviolet Light
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Masahito Tagawa and Timothy K. Minton Abstract This article focuses on mechanistic aspects of hyperthermal atomic oxygen reactions with polymers, which are the major contributor to material degradation in low Earth orbit. Due to the importance of well-controlled experiments in the understanding of the reaction mechanisms, ground-based experimental results obtained by a hyperthermal atomic oxygen beam generated by laser detonation facilities are mainly surveyed. Combined effects of atomic oxygen and vacuum ultraviolet (VUV) light on fluorinated polymers are also described. Such combined effects of hyperthermal atomic oxygen and VUV light are important not only from a fundamental point of view but also for engineering purposes (i.e., methodology for ground-based space environmental simulation). The VUV-sensitive polymers, poly(methyl methacrylate), and Teflonfluorinated ethylene-propylene do not show significant synergistic effects. Instead, the effect of combining atomic oxygen and VUV light produces erosion of the polymer that is the sum of the erosion caused by atomic oxygen and UV light acting individually. The experimental results suggest that material erosion in a complicated space environment may be quantitatively predicted if the erosion yields caused by the individual action of atomic oxygen and VUV light are known.
Introduction In the low Earth orbit (LEO) environment, hyperthermal reactions of atomic oxygen (AO), acting alone or perhaps in conjunction with vacuum ultraviolet (VUV) light with wavelengths between 100 and 200 nm, cause oxidation and erosion of materials, particularly hydrocarbon-based polymers. A typical number density of AO at space shuttle altitudes (300 km) is 109 atoms/cm3. At orbital altitudes of 300–400
km, the LEO environment subjects materials on the ram side of a spacecraft to collisions with ambient AO that have an average impact velocity of 7.4 km/s, corresponding to O atoms with a mean translational energy of 4.5 eV striking the surfaces. The combination of impact velocity and density yields an AO flux of approximately 1015 atoms cm−2 s−1 on ram surfaces. Due to its high chemical reactivity and high colli-
MRS BULLETIN • VOLUME 35 • JANUARY 2010 • www.mrs.org/bulletin
sion energy, AO erodes many polymeric materials used in LEO.1–3 The AO-induced erosion phenomena were recognized in early space shuttle missions. Many flight and ground-based experiments have been conducted to discover the reactivity of high-energy collisions of AO with various materials.4–7 Since direct collisions with AO are restricted to the materials used on the exterior surface of spacecraft, AO reactions with the materials that are used in thermal control systems, solar panels, and other exposed facilities have been extensively studied.8,9 The hydrocarbon polymer, polyimide, has been widely used in many space applications because of its superior physical and chemical properties, such as thermal stability, heat capacity, thermal conductivity, and heat resistance. It is well known that polyimide is eroded by
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