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After 34,000 orbits of earth, polymers analyzed by SPM.

One of the more unusual applications of scanning probe microscopy (SPM) may be the characterization of materials that have been in space for extended periods.

SPM data gathered from materials exposed for years in space are now producing new insights for researchers and practical uses for industry.

In April 1984, NASA's Long Duration Exposure Facility was placed in low Earth orbit by the space shuttle Orbiter Challenger. LDEF carried experiments from government facilities, private industry, and universities, as well as the famous "tomato seeds" experiment conducted by elementary school students across America.

After nearly six years in low Earth orbit, LDEF was retrieved in January 1990 by the shuttle Columbia. The knowledge being derived from materials on this spacecraft during its 34,000-orbit, 750-million-mile flight will set the standard for environmental effects well into the 21st century.

Unique environment. Low Earth orbit comprises an environment of atomic oxygen (AO), ultraviolet (UV), vacuum ultraviolet (VUV), particulate ([e.sup-] and [p.sup.+]) and cosmic radiation, meteoroids and debris, and vacuum.

LDEF was stabilized in such a way that one end of the vehicle faced space, the other Earth. The exposure a specimen received depended in part on where it was located on the vehicle. Some experiment trays (such as Raw 9) faced the ram direction for the duration of the flight, while others (Row 3) faced the wake direction.

STM line plots were made of silvered Teflon thermal blankets located at F2, C5, and C8, along with a control. These mirror-like coatings, with silver vapor deposited on the bottom side of a film of FEP Teflon, are used extensively on spacecraft for thermal control.

The topography seen on the image from the F2 location is a regular egg-crate type pattern indicative of erosion by atomic oxygen. The C5 location saw less AO, but significant vacuum ultraviolet radiation. The pattern is fairly smooth, with evidence of VUV crazing.

The C8 location saw significant AO and VUV. Erosion was extensive. The surface of this specimen was so rough that a small scan area had to be chosen to image in a specific area of the film to avoid the STM tip from "crashing" into the remaining surface.

Kapton, another high-performance material used extensively in space applications, was also scanned by the STM. Line plots of protected and exposed portions of Kapton film flown on Row 3 of the LDEF. The environment for that location would be low AO and high VUV.

The surface of the exposed material appears to be less heavily eroded as compared to the thermal blankets. This appearance has been largely attributed to VUV radiation exposure.

STM surface plots were made for a polyimide-polysiloxane copolymer film flown on Row 9 of LDEF inside a canister that opened in orbit for a 10-month period.

This film experienced not only erosion from atomic oxygen but chemical changes as well. Spectroscopic analysis reveals the formation of an AO-resistant surface on the film. As erosion took place, AO reacted with the polymer to form a silica/silicate (Si[O.sub.x]) coating. This effectively stopped further erosion.

This phenomenon is contributing to the development of a second generation of materials for space applications. These new materials contain chemical functional groups that react with AO such that after a short reaction period, a resistant surface is formed.

AFM scans of polymer AOR-PAEBI demonstrate this phenomenon. This film was exposed to a simulated space environment without UV present. After 60 min of exposure, peaks areas appear from AO erosion. However, by 75 min, fewer and less regular peaks occur as the protective surface forms.

LDEF provided a wealth of information on the effects of exposure of polymeric materials to the low Earth orbit environment. Both scanning tunneling and atomic force microscopy have proved to be highly effective analytical tools for imaging the fine details of surface phenomena associated with space exposure.

New uses. The techniques are also contributing to a new generation of materials for long-term application in the space environment.

In fact, the results of the surface analysis have contributed to a new technology for producing lightweight organic polymer materials that are very resistant to atomic oxygen. These materials have potential applications on spacecraft in low Earth orbit as solar array substrates and multilayer thermal insulating blankets.

Triton Systems Inc., Chelmsford, Mass., licensed the technology last September. The company seeks to commercialize films and fibers of these polymers for potential applications on spacecraft in low Earth orbit.

Chemist Carol Kalil is a task leader with Analytical Services and Materials, an on-site contractor to NASA Langley Research Center, based in Hampton, Va.
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Title Annotation:scanning probe microscopy
Author:Kalil, Carol R.
Publication:R & D
Date:Apr 1, 1995
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