Advances in atomic oxygen resistant polyimide composite films

The relatively poor atomic-oxygen (AO) resistance of the standard polyimide (PI) films greatly limits the wide applications in low earth orbit (LEO) environments. The introduction of polyhedral oligomeric silsesquioxane (POSS) units into the molecular structures of the PI films has been proven to be an effective procedure for enhancing the AO resistance of the PI films. In the current work, a series of POSS-substituted poly (pyromellitic anhydride-4,4′-oxydianiline) (PMDA-ODA) films (POSS-PI) with different POSS contents were synthesized via a POSS-containing diamine, N-[(heptaisobutyl-POSS)propyl]-3,5-diaminobenzamide (DABA-POSS). Subsequently, the effects of the molecular structures on the thermal, tensile, optical, and especially the AO-erosion behaviors of the POSS-PI films were investigated. The incorporation of the latent POSS substituents decreased the thermal stability and the high-temperature dimensional stability of the pristine PI-0 (PMDA-ODA) film. For instance, the PI-30 film with the DABA-POSS content of 30 wt% in the film exhibited a 5% weight loss temperature (T5%) of 512 °C and a coefficient of linear thermal expansion (CTE) of 54.6 × 10−6/K in the temperature range of 50–250 °C, respectively, which were all inferior to those of the PI-0 film (T5% = 574 °C; CTE = 28.9 × 10−6/K). In addition, the tensile properties of the POSS-containing PI films were also deteriorated, to some extent, due to the incorporation of the DABA-POSS components. The tensile strength (TS) of the POSS-PI films decreased with the order of PI-0 > PI-10 > PI-15 > PI-20 > PI-25 > PI-30, and so did the tensile modulus (TM) and the elongations at break (Eb). PI-30 showed the TS, TM, and Eb values of 75.0 MPa, 1.55 GPa, and 16.1%, respectively, which were all lower than those of the PI-0 film (TS = 131.0 MPa, TM = 1.88 GPa, Eb = 73.2%). Nevertheless, the incorporation of POSS components obviously increased the AO resistance of the PI films. All of the POSS-PI films survived from the AO exposure with the total fluence of 2.16 × 1021 atoms/cm2, while PI-0 was totally eroded under the same circumstance. The PI-30 film showed an AO erosion yield (Es) of 1.1 × 10−25 cm3/atom, which was approximately 3.67% of the PI-0 film (Es = 3.0 × 10−24 cm3/atom). Inert silica or silicate passivation layers were detected on the surface of the POSS-PI films after AO exposure, which efficiently prevented the further erosion of the under-layer materials.

Owing to their outstanding comprehensive performance, polyimide (PI) composite films are widely used on the external surfaces of spacecraft to protect them from the adverse conditions of low Earth orbit (LEO). However, current PI composite films have inadequate mechanical properties and atomic oxygen (AO) resistance. Herein, this work fabricates a new PI-based nanocomposite film with greatly enhanced mechanical properties and AO resistance by integrating mica nanosheets with PI into a unique double-layer nacre-inspired structure with a much higher density of mica nanosheets in the top layer. In addition, the unique microstructure and the intrinsic properties of mica also impart the nanocomposite film with favorable ultraviolet and high-temperature resistance. The comprehensive performance of this material is superior to those of pure PI, single-layer PI-mica, and previously reported PI-based composite films. Thus, the double-layer nanocomposite film displays great potential as an aerospace material for use in LEO.

The environment encountered by space vehicles in very low Earth orbit (VLEO, 180 – 350 km altitude) contains predominantly atomic oxygen (AO) and molecular nitrogen (N2), which collide with ram surfaces at relative velocities of ~7.5 km s-1. Structural, thermal-control, and coating materials containing organic polymers are particularly susceptible to AO attack at these high velocities, resulting in erosion, roughening, and degradation of function. Copolymerization or blending of a polymer with polyhedral oligomeric silsesquioxane (POSS) yields a material that can resist AO attack through the formation of a passivating silicon-oxide layer. Still, these hybrid organic/inorganic polymers become rough through AO reactions as the passivating layer is forming. Surface roughness may enhance satellite drag because it promotes energy transfer and scattering angle randomization during gas-surface collisions. As potential low-drag and AO-resistant materials, we have investigated POSS-containing films of clear and Kapton-like polyimides that have an atomically smooth AO-resistant coating of Al2O3 that is grown by atomic layer deposition (ALD). Coated and uncoated films were exposed to hyperthermal molecular beams containing atomic and molecular oxygen to investigate their AO resistance, and molecular beam-surface scattering studies were conducted to characterize the gas-surface scattering dynamics on pristine and AO-exposed surfaces to inform drag predictions. The AO erosion yield of Al2O3 ALD-coated films is essentially zero. Simulations of drag on a representative satellite structure that are based on the observed scattering dynamics suggest that the use of the Al2O3 ALD-coated POSS-polyimides on external satellite surfaces have the potential to reduce drag to less than half that predicted for diffuse scattering surfaces. These smooth and AO-resistant polymer films thus show promise for use in the extreme oxidizing and high-drag environment in VLEO.

Forty-one different polymers are being exposed to the low Earth orbit (LEO) environment on the exterior of the International Space Station (ISS) for one year as part of MISSE (Materials International Space Station Experiment). MISSE is a materials flight experiment sponsored by the Air Force Research Lab/Materials Lab and the National Aeronautics and Space Administration (NASA). A second set of the same polymers is planned to be flown as part of PEACE (Polymer Erosion And Contamination Experiment), a short duration shuttle flight experiment, and therefore these forty-one polymers on ISS are collectively called the MISSE PEACE Polymers. The objective of the MISSE PEACE Polymers experiment is to accurately determine the atomic oxygen (AO) erosion yield of a wide variety of polymeric materials. The polymers range from those commonly used for spacecraft applications, such as Teflon FEP, to more recently developed polymers, such as high temperature polyimide PMR (polymerization of monomer reactants). Additional polymers were included to explore erosion yield dependence upon chemical composition. Details on the specific polymers being flown, flight sample fabrication, and pre-flight characterization techniques will be discussed. The MISSE PEACE Polymers experiment was placed on the exterior of ISS during a spacewalk on August 16, 2001 and is planned to be retrieved in the fall of 2002. The erosion yield data obtained from this experiment will be compared with data from the future short duration experiment PEACE and with predicted results from models developed by a Canadian group that predicts the AO erosion yield of organic materials based on their chemical structure. Having flight data, and comparing flight data with the predictive model results, will be valuable for spacecraft design purposes.

Very low Earth orbit (VLEO) altitude, defined as altitudes below 350 km, offers significant advantages for high-resolution Earth observation, as well as low-cost launches and communications. However, developing long-lived VLEO satellites remains technically challenging due to the highly dense residual atmosphere, primarily composed of atomic oxygen (O, or AO) and molecular nitrogen (N2). At VLEO altitudes, dense AO collides with the ram face of satellites at a relative velocity of ~ 8 km s−1, causing significant erosion, roughening, and degradation of organic materials used for thermal control, structural components, and coatings. In addition, the highly dense atmosphere may induce drag on satellites, complicating orbit maintenance and affecting the propulsion system. This study investigates a polyimide film coated with a photocurable silsesquioxane (SQ, manufactured by Toagosei Co., Ltd.) as a potential AO-resistant material. Through the reaction with AO, the SQ coating forms a passivating silica layer on the surface, protecting the underlying polyimide film from further AO attack. To investigate the potential use of SQ-coated polyimide films as a material for the external surfaces of VLEO satellites, the films were exposed to AO in low Earth orbit (LEO) via Material Degradation Monitor 2 and Carbon Nanotube missions on the International Space Station, as well as a lab-based laser-detonation AO source. The mass losses and erosion yields of LEO-exposed films were higher than those of the lab-exposed films. The erosion of SQ-coated polyimides exhibited sensitivity to the environmental differences between the LEO and the laser-detonation AO source (e.g., the velocity distributions and compositions of O and O2, UV, and thermal cycling). Regardless of the exposure environment, cracks that penetrated the polyimide layer formed when AO exposure exceeded ~ 1 × 1021 atoms cm−2. The cracks formed were more severe under LEO exposure than in the lab. Similar cracking phenomena were observed at LEO and VLEO altitudes during the Material Degradation Monitor mission on Super-Low-Altitude Test Satellite, as indicated by a marked increase in visible diffusion above ~ 1 × 1021 atoms cm−2. Cross-sectional laser Raman spectroscopy of lab-exposed films revealed that the reaction of the coating with AO induced compressive stress, which was relieved by the formation of cracks. These cracks also affect thermo-optical and mechanical properties and may increase satellite drag through multiple-bounce scattering. To enable long-term VLEO missions, the molecular design of the coating must be improved to maintain nanoscale surface smoothness and prevent crack formation under high-fluence AO exposure.

Investigation of degradation mechanisms in mechanical properties of polyimide films exposed to a low earth orbit environment

To study the effects of low earth orbit (LEO) environment on film surface structure and resistance characteristics, reduced graphene oxide paper (rGOP) was irradiated by atomic oxygen (AO) in ground‐based simulation facility. A rough pits surface, chemical oxidation, and change in lamina thickness were found after AO exposure. The erosion yield under AO attack changed from 3.56 × 10 −25 to 4.04 × 10 −25 cm 3 /atom. The resistance data exhibited a linear relationship between R 0 /R and AO fluence, and the maximum AO detect fluence reached to 5 × 10 19 atom/cm 3 when rGOP thickness was 0.8 μm, which implies that greater thickness is expected to improve space service life of rGOP.

Polyimide (PI) composite films with enhanced mechanical properties were prepared by incorporating modified fumed silica (FS) particles while preserving their optical and thermal characteristics. The PI matrix was synthesized using a fluorinated diamine, a fluorinated dianhydride, and a rigid biphenyl dianhydride via chemical imidization. Commercially available FS particles, including unmodified FS particles (0-FS) and particles modified with dimethyl (2-FS), trimethyl (3-FS), octyl (8-FS), octamethylcyclotetrasiloxane (D4-FS), and polydimethylsiloxane (PDMS-FS) were used. Scanning electron microscope images and nitrogen adsorption-desorption isotherms revealed well-defined porous structures in the FS particles. The water contact angles on the composite films increased compared to those of the pristine PI films, indicating improved water resistance. The PI/0-FS films exhibited a typical trade-off relationship between tensile modulus and elongation at break, as observed in conventional composites. Owing to the poor compatibility and agglomeration of the PDMS-FS particles, the PI/PDMS-FS composite films exhibited poor mechanical performance and diminished optical characteristics. Although the longer-chained FS particles (8- and D4-FS) improved the tensile modulus of the PI film by up to 12%, a reduction of more than 20% in toughness was observed. The PI composite films containing the methylated FS particles (2- and 3-FS) outperformed 8- and D4-FS in terms of mechanical properties, with PI/3-FS films showing an over 10% increased tensile modulus (from 4.07 to 4.42 GPa) and 15% improved toughness (from 6.97 to 8.04 MJ/m3) at 7 wt. % silica loading. Except for the PI/PDMS-FS composites, all composite film samples exhibited more than 86% transmittance at 550 nm. Regarding thermal properties, the glass transition temperature (Tg) and thermal stability remained stable for most composite films. In addition, PI/3-FS films demonstrated enhanced dimensional stability with lower coefficients of thermal expansion (from 47.3 to 34.5 ppm/°C). Overall, this study highlights the potential of incorporating specific modified FS particles to tailor the mechanical, optical, and thermal properties of PI composite films.

Reactive molecular dynamics simulations on the mechanism of the different protection methods of KAPTON during atomic oxygen impact

Atomic oxygen is formed in the low Earth orbital environment (LEO) by photo dissociation of diatomic oxygen by short wavelength (< 243 nm) solar radiation which has sufficient energy to break the 5.12 eV O2 diatomic bond in an environment where the mean free path is sufficiently long (~ 108 meters) that the probability of reassociation or the formation of ozone (O3) is small. As a consequence, between the altitudes of 180 and 650 km, atomic oxygen is the most abundant species. Spacecraft impact the atomic oxygen resident in LEO with sufficient energy to break hydrocarbon polymer bonds, causing oxidation and thinning of the polymers due to loss of volatile oxidation products. Mitigation techniques, such as the development of materials with improved durability to atomic oxygen attack, as well as atomic oxygen protective coatings, have been employed with varying degrees of success to improve durability of polymers in the LEO environment. Atomic oxygen can also oxidize silicones and silicone contamination to produce non-volatile silica deposits. Such contaminants are present on most LEO missions and can be a threat to performance of optical surfaces. The LEO atomic oxygen environment, its interactions with materials, results of space testing, computational modeling, mitigation techniques, and ground laboratory simulation procedures and issues are presented.

This paper discusses plans and initial results of efforts to characterize optical coatings, coating materials and optical substrates planned for use in the Space Based Laser (SBL) Integrated Flight Experiment (IPX) for their endurance in the low earth orbit (LEO) space environment. High energy laser (HEL) systems such as IPX place very challenging requirements on their constituent optics, and the degradation due to the natural LEO environment creates additional risk to system performance. The SBL program has chosen to mitigate this risk by use of ground and flight experiments. The flight experiment is participation in the collaborative experiment, Materials International Space Station Experiment (MISSE), a program funded by NASA and the U.S. Air Force Research Laboratory. The ground experiments are to use laboratory facilities to simulate the atomic oxygen exposure of the LEO environment. This paper begins with a discussion of the LEO environment and the potential effects on IPX optical components and materials. The next section gives an overview of the IPX and the optical components that make up each of the program elements. Next, an overview of the planned ground experimentation and participation in the MISSE collaboration is given. The final section gives an example of laboratory measurements madeon the samples with SBL legacy as a preview to the results expected from the flight experiment. The first flight of MISSE samples to Space Station Alpha is Summer 2001, with a return to Earth in June 2002. The Low Earth Orbit Environment and the Optics Challenge to IPX High energy laser (HEL) systems require extreme performance from their optics in order to meet system goals. Typically, HEL optics are stressing for high damage resistance, high reflectance, low absorptance, scatter and aberration along with predictable spatial uniformity. Producing such optics has been a main concern of the HEL community since the discovery of the laser. Usually, just building the optics is challenge enough. The space based laser (SBL) integrated flight experiment (IPX) program must make these optics and have them meet mission requirements in the challenging environs of low earth orbit (LEO). The LEO environment can be characterized by exposure to the neutral thermosphere (ambient atmosphere), charged plasma, the thermal environment (including diurnal cycles), exposure to solar radiation, debris/micrometeorites and ionizing radiation. The neutral thermosphere, is the atmospheric layer above an altitude of approximately 100 km. At an altitude of 80 kilometers or greater, solar ultra-violet (UV) bombardment of the tenuous atmosphere initiates chemical reactions and breaks down molecular species to their atomic components. In this region of the atmosphere, the concentration of individual species is essentially determined by weight segregation. At the expected altitude of IPX, ~480 km, the predominant species is atomic oxygen (AO). The abundance of neutral species at altitude is shown in Figure 1.

Reactive molecular dynamics simulation was employed to compare the damage mitigation efficacy of pristine and polyimide (PI)-grafted polyoctahedral silsesquioxane (POSS), graphene (Gr), and carbon nanotubes (CNTs) in a PI matrix exposed to atomic oxygen (AO) bombardment. The concentration of POSS and the orientation of Gr and CNT nanoparticles were further investigated. Overall, the mass loss, erosion yield, surface damage, AO penetration depth, and temperature evolution are lower for the PI systems with randomly oriented CNTs and Gr or PI-grafted POSS compared to those of the pristine POSS or aligned CNT and Gr systems at the same nanoparticle concentration. On the basis of experimental early degradation data (before the onset of nanoparticle damage), the amount of exposed PI, which has the highest erosion yield of all material components, on the material surface is the most important parameter affecting the erosion yield of the hybrid material. Our data indicate that the PI systems with randomly oriented Gr and CNT nanoparticles have the lowest amount of exposed PI on the material surface; therefore, a lower erosion yield is obtained for these systems compared to that of the PI systems with aligned Gr and CNT nanoparticles. However, the PI/grafted-POSS system has a significantly lower erosion yield than that of the PI systems with aligned Gr and CNT nanoparticles, again due to a lower amount of exposed PI on the surface. When comparing the PI systems loaded with PI-grafted POSS versus pristine POSS at low and high nanoparticle concentrations, our data indicate that grafting the POSS and increasing the POSS concentration lower the erosion yield by a factor of about 4 and 1.5, respectively. The former is attributed to a better dispersion of PI-grafted POSS versus that of the pristine POSS in the PI matrix, as determined by the radial distribution function.

Effects of atomic oxygen (AO) on mechanical properties of polyimide films were investigated using surface topography observations and tensile tests. The ITO‐coated polyimide films, which are expected to have a high durability against AO attacks, were also evaluated. Surfaces of polyimide films irradiated by AO were deeply eroded, exhibiting a rough texture. The roughness developed as the AO fluence increased. Tensile strength and elongation of the polyimide films degraded after AO irradiation tests. In addition, that degradation became marked concomitantly with increased AO fluence. These results indicate that development of surface roughness can degrade mechanical properties of polyimide films. In contrast, surfaces of ITO‐coated polyimide films remained flat and were hardly affected by AO irradiation. Nevertheless, the tensile strength and elongation of ITO‐coated polyimide films were decreased by AO irradiation. Such degradation resulted from undercut cavities formed by AO erosion at the defects sites of ITO coating.

ABSTRACTAtomic oxygen, formed in Earth's thermosphere, interacts readily with many materials on spacecraft flying in low Earth orbit (LEO). All hydrocarbon based polymers and graphite are easily oxidized upon the impact of ∼4.5 eV atomic oxygen as the spacecraft ram into the residual atmosphere. The resulting interactions can change the morphology and reduce the thickness of these materials. Directed atomic oxygen erosion will result in the development of textured surfaces on all materials with volatile oxidation products. Examples from space flight samples are provided. As a result of the erosive properties of atomic oxygen on polymers and composites, protective coatings have been developed and are used to increase the functional life of polymer films and composites that are exposed to the LEO environment. The atomic oxygen erosion yields for actual and predicted LEO exposure of numerous materials are presented. Results of in-space exposure of vacuum deposited aluminum protective coatings on polyimide Kapton indicate high rates of degradation are associated with aluminum coatings on both surfaces of the Kapton. Computational modeling predictions indicate that less trapping of the atomic oxygen occurs, with less resulting damage, if only the space-exposed surface is coated with vapor deposited aluminum rather than having both surfaces coated.

Satellites in the Low Earth Orbit (LEO) environment face various hazards which threaten the long-term sustainability of the space economy. This article describes the potential dangers to satellites in LEO beyond commonly referenced orbital debris and satellite collisions, including hazards that arise from the LEO environment itself and from other anthropogenic sources such as laser interference and orbital light pollution. As the LEO economy surges in the coming decade, preventative policy such as that proposed in this article could address these risks before they become an issue and ensure that the LEO environment continues to grow sustainably.