Atomic-Oxygen-Induced Microstructures Formed on Hydrocarbon Polymer Films

Atomic oxygen (AO) is a dominant constituent of the residual atmosphere in low Earth orbit (LEO). Spacecraft collides with AO at the relative velocity of 8 km/s, which corresponds to the translational energy of 5 eV. Polymers used for thermal control are oxidized and eroded by such AO collisions, degrading their mechanical and thermo-optical properties. In the erosion processes, AO forms nanoscale protrusions on the polymer surface. If the surface morphologies and physical properties (e.g., wettability and optical property) can be controlled by AO irradiation, it would be useful in modifying the surfaces for various polymers.<br/>This study aims to clarify the factors that determine the characteristics of AO-induced microstructures (protrusions’ size and numerical density). Hydrocarbon polymers with a simple chemical structure were selected for the samples: polyethylene (PE), polypropylene (PP), and polystyrene (PS). The polymer films (23–30 μm in thick) were irradiated with AO by a laser-detonation source at Kobe University (5–7 km/s). The mass losses were measured by an electronic microbalance. The changes in chemical bonds were evaluated by Fourier transform infrared spectroscopy (FT-IR). The surface morphologies were observed by a field emission scanning electron microscope (FE-SEM) and an atomic force microscope (AFM). The changes in depth profiles for the chemical structures and free-volume holes’ sizes were evaluated by positron annihilation lifetime spectroscopy (PALS) at AIST (1.3–10 keV).<br/>The three polymers had similar changes in masses (or erosion yields) and chemical bonds but different surface morphologies. The protrusions formed on PE and PP were larger and fewer than those formed on PS, which was irradiated with comparable AO fluences [Goto, A., et al. Langmuir 2022, 38, 3339-3349.]. The polymers are known to have different higher-order structures, so it might affect the spatial scales where interactions occur with AO, leading to the different morphologies. Further, the formed protrusions became larger and fewer with rising temperature of the polymer irradiated with AO (25–75 °C, controlled by the sample holder with a heater). The PALS evaluations indicated that the polymers have different radii in free-volume holes (PS (0.29 nm) &lt; PP (0.31 nm) &lt; PE (0.32 nm)) and different thicknesses of an oxidized layer formed by AO (PS &lt; PE ≈ PP). Based on these results, the regions where AO interacts with the polymer would depend on the thermal motions of polymer chains and/or sizes in free-volume holes.