Comparison of polyimide film surface properties exposed to real and simulated space environments: Relevance of atomic oxygen effects to wettability in space - Polyimides and Other High Temperature Polymers - page 426

3.2. Ground-based experimental results
(a) Effect of air exposure on atomic oxygen-exposed polyimide surfaces
An atomic oxygen-exposed polyimide film with an atomic oxygen fluence of 7.6
x 10 17 atoms/cm 2 was stored in a Teflon container under a clean air environment
of class 10,000, and the XPS spectrum was measured periodically in order to de-
termine the stability of the surface chemistry of the atomic oxygen-exposed poly-
imide. The results are shown in Table 2. It was observed that N was relatively
stable, whereas the atomic concentration of C increased and that of O decreased
with air exposure. Namely, the carbon and oxygen concentrations before air ex-
posure were 62.4% and 28.5%, respectively, but, with an air exposure for 220
days, the carbon and oxygen concentrations changed to 69.2% and 23.3%. These
values are close to the analytical results on STS-8 and STS-46 flight samples [11,
12].
Figure 6 shows the XPS C1s core level spectra of the atomic oxygen-exposed
polyimide films with an atomic oxygen fluence of 7.6 10 17 atoms/cm 2 (a) with-

out air exposure and (b) with air exposure of 220 days. The XPS spectrum of
control sample (not exposed to atomic oxygen) is shown in Figure 4 (a). Note
that Figures 6 (a) and (b) were obtained from the same sample. It is obvious from
a comparison of Figure 6 (b) and Figure 4 that both flight and laboratory samples
exposed to ambient air gave similar C1s XPS spectra. However, before ambient
air exposure, the atomic oxygen-exposed surfaces showed the C1s spectrum con-
taining a larger amount of carbonyl (287.7 eV) and ketone (288.9 eV) groups,
i.e., a highly oxidized state. These XPS results indicate that the decrease in O
shown in Table 2 was due to the decrease of carbonyl and ketone groups at the
atomic oxygen-exposed polyimide surfaces. From the experimental results
shown in Figure 6, it is suggested that the highly oxidized surfaces produced on
exposure to atomic oxygen were deoxidized by ambient air exposure, and the in-
fluence of atomic oxygen exposure was lost [23]. Deoxidation process involves
removal of volatile products (maybe carbon monoxide and/or carbon dioxide).
Diffusion of the high-energy surface functional groups into the bulk is also a
possibility.

×

Table 2.
Effect of air exposure on the surface composition of atomic oxygen-exposed polyimide. Atomic
oxygen fluence was 7.6 × 10 17 atoms/cm 2

Air exposure (day) Atomic concentration (%)

C O N

0 62.4 28.5 9.1

4 64.1 26.6 9.3

10 68.7 25.1 6.2

220 69.2 23.3 7.5