ECAMP 15

Organic polymers have wide applications in the aereospace industry (e.g. in satellites and spacecrafts). However, when such materials are used in spacecraft external surfaces in Low Earth Orbit (LEO - up to 2000 km above Earth’s surface), they are subject to erosion due to exposure to the main atmospheric components at such altitudes, i.e. atomic oxygen (AO) and O$^+$ ion. Current strategies to limit polymer deg-radation (e.g. coatings and self-modifying materials) are largely based on a trial-and-error approach, due to a lack of understanding of the reaction mechanisms responsible for erosion.

Atomic oxygen in ionized form (O$^+$) is the dominant cation in the Earth’s thermosphere, where it is known to exist not only in its ground state ($^4$S) but also in its excited states $^2$D and $^2$P. Due to their long lifetimes (3.6 hours for $^2$D, 4.6 s for $^2$P) they have sufficient time to react before de-excitation under the low-pressure conditions of LEO. Hence, performing single collision gas-phase experiments between state selected O+ ions and polymers will permit to gain insight into the erosion mechanisms at an atomistic level, by identifying molecular sites most vulnerable to O$^+$ attack. Due to the low volatility of the monomers, we identify key molecular moieties of space-grade polymers that are volatile enough to be brought to the gas phase, while retaining the relevant functional groups and molecular structures of the whole polymers. In particular we choose:
a) benzene (C$_6$H$_6$), naphthalene (C$_{10}$H$_{8}$) and phenanthrene (C$_{14}$H$_{10}$), as templates for graphite;
b) ethylbenzene (C$_{6}$H$_{5}$CH$_{2}$CH$_{3}$), styrene (C$_{6}$H$_{5}$CHCH$_{2}$) and 1,3-diphenylpropane (C$_{6}$H$_{5}$(CH$_{2}$)$_{3}$C$_{6}$H$_{5}$), as templates for polystyrene
c) diphenylether (C$_{6}$H$_{5}$OC$_{6}$H$_{5}$), N-methylmaleimide (CH$_{3}$-NCOCHCHCO) and N-vynilformamide (HCONHCHCH$_{2}$) as templates for Kapton. In addition, aniline and phenol are used in substitution of 4-aminophenol.

Experiments are conducted using the CERISES setup: a guided ion beam tandem mass spectrometer that couples ion generation via tunable VUV light (from the DESIRS beamline of the SOLEIL synchrotron radiation facility) with octupolar RF guiding and trapping of ions to measure absolute integral reactive cross sections and product branching ratios as a function of collision energy (ranging from meV to tens of eV).
The selective generation of O$^+$ ions in the ground ($^4$S) is achieved via dissociative photoionization of O$_2$ at 19.4 eV photon energy, while a mixture of ground and first excited ($^2$D) state is produced at 23.05 eV photon energy. In the latter case, a “titration” method is put in place to estimate the relative yield of $^4$S and $^2$D states by comparing results on the reactivity of O$^+$ with CD$_4$ with reference data obtained using a state-selective method (PEPICO) [1].

Absolute cross sections and branching ratios for the reactions of state selected O$^+$ ions will be presented for a selection of the above mentioned molecules. In most cases the main reactivity is due to dissociative and non dissociative charge exchange. This study will improve our understanding of polymer erosion in space and aid the design of more resilient materials for satellites, space stations, and spacecraft.

[1] B. Cunha de Miranda, C. Romanzin, S. Chefdeville et al., J. Phys. Chem. A, 2015, 119(23): 6082–98

Acknowledgments

The research is carried out within the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 1409 (14.9.2022) by the Italian Ministry of University and Research (MUR), funded by the European Union – NextGenerationEU – Project Title P20223H8CK "Degradation of space-technology polymers by thermospheric oxygen atoms and ions: an exploration of the reaction mechanisms at an atomistic level" - CUP E53D23015560001. Thanks to Laurent Nahon & DESIRS team @SOLEIL SRF for assistance under Proposal No. 20240422.