Transmission Electron Microscopy Simulation of Van Allen Belt Electrons for Hardening Electronic Polymers

Polymeric materials properties relative to their mass make them ideally suited for space applications, where reduced weight pays dividends in cost and service life, since less fuel is needed to maneuver the object. Materials in space are subject to myriad environmental hazards not present on Earth. Among these hazards are the Van Allen belts, which surround the planet in two bands of energetic charged particles, primarily electrons and protons, which affect all object beyond low Earth orbit, and extend beyond geostationary orbit. The electrons trapped in these belts range in energies from 10s to 100s of keV, and approach energies as high as 10 MeV in the outer belt. These energies can be achieved on the ground in dedicated electron chambers as well as in variable energy transmission electron microscopes (TEMs), enabling characterization of the response of polymeric materials to ionizing radiation. In particular, structural changes as a function of electron dose can be tracked in TEM by quantifying changes in mass loss, electron energy loss spectroscopy, and diffraction intensity in a series of scans at increasing dose. This procedure has enabled us to determine the critical dose of several semiconducting organic materials, and explore methods of hardening by incorporating low-dimensional materials such as graphene. In the case of graphene interfaced directly with a semiconducting polymer, PBTTT, we observed an orientation-based effect, indicating that the graphene was able to mitigate charge buildup due to secondary damage processes, suggesting an increase in polymer stability is possible using this approach. Raman and photoluminescence as a function of radiation dose also demonstates changes to the polymer chemistry resulting from damage. Additionally, high spatial and time resolution HREM images were captured from pentacene and copper pthalocyanine demonstrating defect-mediated response to ionizing radiation damage. We have shown this method can be used to determine mechanisms of improving the electron radiation hardness of polymeric materials as well as determine the specific structural features that change leading up to failure.