Sandra J. Tomczak

Atomic Oxygen Effects on POSS Polyimides in Low Earth Orbit

Kapton polyimde is extensively used in solar arrays, spacecraft thermal blankets, and space inflatable structures. Upon exposure to atomic oxygen in low Earth orbit (LEO), Kapton is severely eroded. An effective approach to prevent this erosion is to incorporate polyhedral oligomeric silsesquioxane (POSS) into the polyimide matrix by copolymerizing POSS monomers with the polyimide precursor. The copolymerization of POSS provides Si and O in the polymer matrix on the nano level. During exposure of POSS polyimide to atomic oxygen, organic material is degraded, and a silica passivation layer is formed. This silica layer protects the underlying polymer from further degradation. Laboratory and space-flight experiments have shown that POSS polyimides are highly resistant to atomic-oxygen attack, with erosion yields that may be as little as 1% those of Kapton. The results of all the studies indicate that POSS polyimide would be a space-survivable replacement for Kapton on spacecraft that operate in the LEO environment.

Properties and improved space survivability of POSS (polyhedral oligomeric silsesquioxane) polyimides

Kapton polyimide (PI) is widely used on the exterior of spacecraft as a thermal insulator. Atomic oxygen (AO) in lower earth orbit (LEO) causes severe degradation in Kapton resulting in reduced spacecraft lifetimes. One solution is to coat the polymer surface with SiO 2 since this coating is known to impart remarkable oxidation resistance. Imperfections in the SiO 2 application process and micrometeoroid / debris impact in orbit damage the SiO 2 coating, leading to erosion of Kapton. A self passivating, self healing silica layer protecting underlying Kapton upon exposure to AO may result from the nanodispersion of silicon and oxygen within the polymer matrix. Polyhedral oligomeric silsesquioxane (POSS) is composed of an inorganic cage structure with a 2:3 Si:O ratio surrounded by tailorable organic groups and is a possible delivery system for nanodispersed silica. A POSS dianiline was copolymerized with pyromellitic dianhydride and 4, 4′-oxydianiline resulting in POSS Kapton Polyimide. The glass transition temperature (Tg) of 5 to 25 weight % POSS Polyimide was determined to be slightly lower, 5 – 10 %, than that of unmodified polyimides (414 °C). Furthermore the room temperature modulus of polyimide is unaffected by POSS, and the modulus at temperatures greater than the Tg of the polyimide is doubled by the incorporation of 20 wt % POSS. To simulate LEO conditions, POSS PI films underwent exposure to a hyperthermal O-atom beam. Surface analysis of exposed and unexposed films conducted with X-ray photoelectron spectroscopy, atomic force microscopy, and surface profilometry support the formation of a SiO 2 self healing passivation layer upon AO exposure. This is exemplified by erosion rates of 10 and 20 weight % POSS PI samples which were 3.7 and 0.98 percent, respectively, of the erosion rate for Kapton H at a fluence of 8.5 × 10 20 O atoms cm -2 . This data corresponds to an erosion yield for 10 wt % POSS PI of 4.8 % of Kapton H. In a separate exposure, at a fluence of 7.33 × 10 20 O atoms cm -2 , 25 wt % POSS Polyimide showed the erosion yield of about 1.1 % of that of Kapton H. Also, recently at a lower fluence of 2.03 × 10 20 O atoms cm -2 , in going from 20 to 25 wt % POSS PI the erosion was decreased by a factor of 2 with an erosion yield too minor to be measured for 25 wt % POSS PI.

Space Survivability of Main‐Chain and Side‐Chain POSS‐Kapton Polyimides

Kapton® polyimde (PI) is extensively used in solar arrays, spacecraft thermal blankets, and space inflatable structures. Upon exposure to atomic oxygen (AO) in low Earth orbit (LEO), Kapton® is severely degraded. An effective approach to prevent this erosion is chemically bonding polyhedral oligomeric silsesquioxane (POSS) into the polyimide matrix by copolymerization of POSS‐diamine with the polyimide monomers. POSS is a silicon and oxygen cage‐like structure surrounded by organic groups and can be polymerizable. The copolymerization of POSS provides Si and O in the polymer matrix on the nano level. During POSS polyimide exposure to atomic oxygen, organic material is degraded and a silica passivation layer is formed. This silica layer protects the underlying polymer from further degradation. Ground‐based studies and MISSE‐1 and MISSE‐5 flight results have shown that POSS polyimides are resistant to atomic‐oxygen attack in LEO. In fact, 3.5 wt% Si8O11 main‐chain POSS polyimide eroded about 2 μm during the...