| Summary: | Optically transparent polyimide (PI) films with good dielectric properties and long-term sustainability in atomic-oxygen (AO) environments have been highly desired as antenna substrates in low earth orbit (LEO) aerospace applications. However, PI substrates with low dielectric constant (low-<i>D</i><sub>k</sub>), low dielectric dissipation factor (low-<i>D</i><sub>f</sub>) and high AO resistance have rarely been reported due to the difficulties in achieving both high AO survivability and good dielectric parameters simultaneously. In the present work, an intrinsically low-<i>D</i><sub>k</sub> and low-<i>D</i><sub>f</sub> optically transparent PI film matrix, poly[4,4′-(hexafluoroisopropylidene)diphthalic anhydride-<i>co</i>-2,2-bis(4-(4-aminophenoxy)phenyl)hexafluoropropane] (6FPI) was combined with a nanocage trisilanolphenyl polyhedral oligomeric silsesquioxane (TSP-POSS) additive in order to afford novel organic–inorganic nanocomposite films with enhanced AO-resistant properties and reduced dielectric parameters. The derived 6FPI/POSS films exhibited the <i>D</i><sub>k</sub> and <i>D</i><sub>f</sub> values as low as 2.52 and 0.006 at the frequency of 1 MHz, respectively. Meanwhile, the composite films showed good AO resistance with the erosion yield as low as 4.0 × 10<sup>−25</sup> cm<sup>3</sup>/atom at the exposure flux of 4.02 × 10<sup>20</sup> atom/cm<sup>2</sup>, which decreased by nearly one order of magnitude compared with the value of 3.0 × 10<sup>−24</sup> cm<sup>3</sup>/atom of the standard PI-ref Kapton<sup>®</sup> film.
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