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 429

Figure 8. Advancing (●) and receding (○) contact angles of water on polyimide films as a function
of surface oxygen concentration.
the order of a few nanometers. Miller et al . have demonstrated that the advancing
and contact angles on vacuum-deposited poly(tetrafluoroethylene) films observed
by the sessile drop method are influenced strongly by nanometer size surface
roughness [27]. In the present study, the advancing and receding angles on the
atomic oxygen-exposed polyimide measured by the sessile drop method were in
fairly agreement with those by the Wilhelmy method, which correspond to the in-
trinsic angles of the low-energy and high-energy regions on the heterogeneous
surface [24, 28]. Such experimental fact seems to show that surface roughness in
the range of a few nanometers does not affect the observed contact angles for the
oxygen-exposed polyimide films.
The Lifshitz-van der Waals (L-W) component and Lewis acid and base pa-
rameters of the surface free energy of polyimide films are plotted on a log scale in
Figure 9 as a function of oxygen concentration. The base parameter increased
with increasing oxygen concentration whereas the L-W component and acid pa-
rameter remained constant. This result may be explained by the introduction of
surface functional groups due to atomic oxygen bombardment. The formation of
carbonyl and carboxyl groups at the atomic oxygen-exposed polyimide surface
was clearly detected by XPS as described earlier. These surface functional groups
are expected to play the role of electron donors. Although the acid parameter ap-
pears to decrease with increasing oxygen concentration, the tendency may not be