The Air Force Research Laboratory, in partnership with several universities, has investigated the effects of space weather damage to polyimides, materials used extensively in spacecraft construction due to their high heat resistance.

The researchers determined the previously unknown chemical and physical effects of electron bombardment in Earth's magnetosphere on these polymers. Electrons trapped in the Earth's magnetic field are the most damaging components of weather in the geosynchronous Earth orbit.

Polyimide films, such as Kapton, are used to construct spacecraft components, including flexible printed circuits, electronics, electronic packaging, wiring and thermal blankets. This material must endure the extreme and variable radiation conditions present in the operational environment for each spacecraft.

Understanding the processes of radiation damage is a critical part of predicting the long-term behaviors of these products and improving their performance and operational longevity.

According to researchers, the stability of the polymer during and after radiation damage is a serious concern. While Kapton is extremely radiation resistant, it suffers serious performance degradation when exposed to the space environment.

Normally flexible within a very broad temperature range that bridges -100 C to 250 C upon radiation exposure, the material turns brittle and loses its superior mechanical properties.

A team of AFRL scientists at the Materials and Manufacturing Directorate and the Space Vehicles Directorate collaborated with a number of academic partners from Johns Hopkins University, Assurance Technology Corporation, Hunter College of the City University of New York and Pennsylvania State University to understand the damage caused by radiation.

The team discovered that when a Kapton sample is irradiated, it changes color from its normal orange-amber to red. This color change is indicative of electron-induced chemical changes in the material. After several hours of exposure to the atmosphere, the sample turns back to its original color.

This "self-healing" effect is pronounced in the atmosphere and led the team to investigate what chemical alterations space-like radiation causes in Kapton.

After thorough testing and modeling, it was discovered that while chemical bonds are broken throughout the material, the damage was localized on a few types of bonds. In other words, space radiation dose not break every chemical bond in the material and selectively leaves large pieces of the polymer unscathed.

This also implies that Kapton is not self-healing after irradiation as was first suggested when the color changed back to normal, but rather forms a new material with the pieces left behind after the damage.

The team captured the effects of radiation damage using a modeling system called Reactive Force Field molecular dynamics known as Reaxff. The modeling system allows them to simulate the process in near real-world conditions. The researchers then correlate these modeling results to experimental characterization, including spectroscopy, thermo-mechanical testing and x-ray diffraction and scattering.

The interpretation of the modeling work combined with the experimental findings led to insights on how to improve the chemical structure of polyimides and create better radiation-hardened materials.

Reaxff with its reactive force fields proves to be an efficient technique to predict the behavior of materials in extreme environments and provides a cost-effective screening tool for the most operational use of materials for extreme applications.

These studies generated insight into improving the finest materials currently available for specialized applications. Preventing brittle behavior after irradiation may be avoided by adding additional flexible units into the polymer backbone and pathways that lead to better recombination and self-healing mechanisms.

This makes the backbone more rubber-like while retaining the high-temperature capabilities of Kapton after electron bombardment. Chemical bonds that are flexible enough could improve absorption of the incoming energy and turn it into heat rather than rupturing the bonds.

Over the course of the collaboration, the team assembled all of the required modeling and evaluation expertise to move these goals closer to reality.

"The modeling tool could be applied as a predictive tool for other systems that may be used in space or aerospace applications being irradiated by x-rays, gamma rays or electrons," said Daniel Engelhart of Assurance Technology Corporation. "We are currently working on modeling more complex composite materials under extreme radiation environments."

"We are also looking at materials which are currently explored as high temperature thermoplastic resins for advanced manufacturing processes," said Hilmar Koerner of AFRL. These materials can be processed through an extruder and turned into a part, similar to 3D printing. Such materials have the potential to eventually replace Kapton, but are currently limited due to their high price.

The team will continue to collaborate on improving the consistency and accuracy of these new theoretical models. The key enabler to solving this difficult problem is the large collaborative effort that was pulled together by AFRL and its partners.

Related Links
Air Force Research Laboratory
Space Technology News - Applications and Research

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