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characteristics of the atomic oxygen-exposed polyimide surfaces were character-
ized using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared
(FT-IR) spectroscopy and atomic force microscopy (AFM). The effect of atomic
oxygen exposure was discussed with respect to contact angles and surface free
energy in conjunction with surface analytical results.
2. EXPERIMENTAL DETAILS
2.1. Materials
Three flight samples were analyzed in this study. A polyimide sample aboard a
space shuttle flight in 1983 (STS-8) was exposed to atomic oxygen environment
for 41.75 hours [11]. Total atomic oxygen fluence was estimated to be 3.5 x 10
20
atoms/cm
2
. A number of polymeric materials were flown on STS-46 in 1992. In
this flight experiment samples were mounted in a shuttle cargo bay and exposed
to atomic oxygen for 42.25 hours at an altitude of 222 km. Total atomic oxygen
fluence was estimated to be 2.2-2.5 x 10
20
atoms/cm
2
[12]. A longer exposure was
used on the Space Flyer Unit (SFU) mission in 1995-96. The SFU spacecraft was
launched by a Japanese launch vehicle H-2, and was retrieved by the space shuttle
mission (STS-72). Total atomic oxygen fluence predicted by the MSIS-90 atmos-
pheric model was 5.6-6.2 x 10
19
atoms/cm
2
[13]. The high orbital altitude of SFU
(482 km) compared with STS-8 (220 km) is responsible for the small atomic oxy-
gen fluence. The polyimide surface located at ram surface of the spacecraft,
where the incident angle of atomic oxygen is 0 degree with respect to surface
normal, was analyzed in this study.
The polyimide samples used in the ground-based experiments in this study
were pyromellitic dianhydride-4, 4'-oxydianiline (PMDA-ODA), commercially
available from DuPont (Kapton-H). The thickness of the polyimide film was 25
ยต
m. Prior to use, the polyimide films were cleaned by ultrasonication in water,
ethanol, and ethyl ether. Water was deionized and doubly distilled using a boro-
silicate glass apparatus.
2.2. Atomic oxygen exposure
In this study, a laser-detonation atomic oxygen source was used to simulate the
atomic oxygen environment in LEO. Figure 1 shows the schematic drawing of the
fast atomic oxygen beam facility including the beam diagnostic system used in
this study [14]. The atomic oxygen source is based on the laser detonation phe-
nomenon and was originally developed by Caledonia et al. [15]. This type of
atomic oxygen source uses a pulsed CO
2
laser and a pulsed supersonic valve. The
laser light was focused on the nozzle throat with the concave Au mirror located
50 cm away from the nozzle. The pulsed supersonic valve introduced pure oxygen
gas into the nozzle and the laser light was focused on the oxygen gas in the noz-
zle. The energy for the dissociation of oxygen molecules into atomic oxygen and
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