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Figure 10.
Advancing (●) and receding (○) contact angles of water on polyimide films as a function
of surface oxygen concentration (ground-based data). Data from flight samples are added; STS-8
(advancing: ▲, receding:
∆
), STS-46 (advancing: ■, receding: □), and SFU (advancing:
♦
, reced-
ing:
). The data points for the SFU sample before cleaning are shown in brackets.
same line as the ground-based data. In contrast, the data points for the SFU flight
sample, which are marked by diamonds in Figure 10, show large contact angles.
From the XPS and FT-IR measurements, it was suggested that the surface was
covered by a silicone-based contamination, i.e., talc-like material (see Figure 5).
The large contact angle and the high oxygen concentration of the SFU sample is
due to the silicone-based contamination adsorbed during the space flight. In order
to remove the contamination layer from the polyimide surface, the SFU sample
was cleaned by ultrasonication in ethyl ether. After the cleaning procedure, ad-
vancing and receding contact angles on the SFU sample decreased to 53
,
respectively. The surface oxygen concentration also decreased to 36.7%. The de-
crease in oxygen concentration is due to the removal of silicone-based contamina-
tion consisting of SiOx which may be formed by the atomic oxygen-induced oxi-
dation of adsorbed silicone contamination. These values gave the data plot for the
SFU sample on the same line as determined for the ground-based simulated sam-
ples. The finding that the original polyimide surface of the SFU sample with the
contamination layer showed a high oxygen concentration of 36.7% and small con-
tact angles (53
°
and 19
°
) suggested that the polyimide surface was modified by
atomic oxygen exposure before being contaminated. Namely, the contamination
adsorption on the SFU spacecraft did not occur in the beginning of the mission
which is contradictory to the scenario of the vaporization of a silicone-based ad-
°
and 19
°
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