Because polyimide films like Kapton are used extensively in space applications such as low Earth orbit (LEO), their performance properties in space conditions are well characterized. Polyimide foams, on the other hand, have not been particularly studied and characterized in this environment. Recent advancements in high-temperature polymeric materials at NASA Langley Research Center have led to the development of new polyimide foam systems with attractive properties. These new polyimide foam systems have potential space applications because of their light weight, their relatively high operating temperature, and their cryogenic properties.

Before utilizing polyimide foams in the aggressive environment of LEO, it is important to understand and predict performance characteristics and the mechanisms of degradation. This information is also important to the protective measures that might be required in the utilization of these materials.

The atmosphere at LEO altitudes has a composition that is essentially the reverse of that in the troposphere, 20-percent nitrogen and 80-percent oxygen. Without the overlying atmosphere to filter short-wavelength ultraviolet (UV) radiation (less than 243 nanometers), the molecular oxygen present is largely photo-dissociated to atomic oxygen (AO). Atomic oxygen is highly reactive and thus is prone to rapidly oxidize materials exposed to it. Making the situation more extreme is the fact that structures in LEO are typically moving rapidly, as fast as 8 kilometers per second, to maintain the orbit. Moving at that speed, it is typical for structures to collide with atomic oxygen with energy of as much as 5 electronvolts and to encounter 10¹⁵ oxygen atoms per square centimeter of surface area per second. In this study, an oxygen plasma generator was utilized to produce an atmosphere of atomic oxygen that would simulate the atmosphere of LEO. The oxygen plasma was generated with an SP1 Plasma Prep II plasma etcher. The effective atomic oxygen flux was determined using ASTM E2089-00, Standard Practices for Ground Laboratory Atomic Oxygen Interaction Evaluation of Materials for Space Applications.

Comparative surface analyses of samples seen in figure 1 (the first letter after TEEK indicates the series and the second letter indicates the density) were performed with a Kratos XSAM X-ray photoelectron spectrometer (XPS). XPS is a surface analysis technique that looks at the upper atomic layers of a solid surface. In XPS, electrons are ejected from a sample surface with a particular binding energy characteristic of the elements present. Shifts in binding energy can be related to oxidation or chemical states.

The mass loss data indicate that chemical structure, then density effects, followed by surface area appear to have the greatest influence on atomic oxygen resistance for the HH, HL, LL, and CL series, with resistance in decreasing order CL>LL>HH>HL (figure 2). The XPS data indicate an overall oxidation of the foams. The data presented on the HL, LL, and CL foams showing an increase in carbonyl after atomic oxygen exposure correlate with the data previously reported on polyimide films. The higher-density HH series showed a decrease in the carbonyl group. This seems to indicate that the plasma is reacting with this group preferentially over atoms in the ring structure, resulting in some volatile products.

Key accomplishments:

• First surface chemistry study to evaluate the new polyimide foams systems performance characteristics with atomic oxygen for space applications.
• A technical paper was accepted for presentation and publication at the National American Institute of Aeronautics and Astronautics (AIAA) Meeting 2002.