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 428

(b) Effect of atomic oxygen exposure on contact angles and surface free energies
Figure 7 shows the contact angles of water as a function of atomic oxygen (AO)
fluence. With increasing atomic oxygen fluence, both the advancing and receding
contact angles decreased, i.e., the wettability increased. The oxygen concentration
at the polyimide surfaces measured by XPS also increased with atomic oxygen
fluence. The contact angles were replotted against the oxygen concentration, and
the results are shown in Figure 8; both advancing and receding contact angles de-
creased linearly with increasing oxygen concentration. The behavior of contact
angles as observed in the present study is consistent with the theoretical calcula-
tions of the contact angles on heterogeneous surfaces as reported by Johnson and
Dettre [24], i.e., both advancing and receding contact angles decrease with in-
creasing percentage of the high-energy region for heterogeneous surfaces such as
polyimide film surfaces.
Surface roughness is another factor influencing the contact angles. The surface
roughness of the pristine and the atomic oxygen-exposed polyimide films were
examined by AFM. It was observed that the roughness of the polyimide surfaces
increased with increasing atomic oxygen fluence. This is considered to be an early
stage of the formation of the “shag carpet-like structure” which is widely known
as a surface texture of the polyimide exposed to LEO space environment (Figure
3). The roughness of the atomic oxygen-exposed polyimide surfaces was small on
Figure 7. Advancing (∆) and receding (□) contact angles of water on atomic oxygen-exposed poly-
imide films as a function of total atomic oxygen fluence. The oxygen concentration at the surface is
also indicated by ● symbol.