Erosion of FEP Teflon and PMMA by VUV radiation and hyperthermal O or Ar atoms

Formation of Nanoscale Protrusions on Polymer Films after Atomic Oxygen Irradiation: Changes in Morphologies, Masses, and FT-IR Spectra

Atomic oxygen (AO) is the main component of the residual atmosphere in a low Earth orbit. AO with a translational energy of 5 eV colliding with artificial satellites forms nano- and microscale protrusions on polymeric materials. This study investigated the influences of AO (fluence and velocity distribution) and a polymer's chemical structure on such surface morphologies. The correlations between samples' mass losses and positions in the irradiation field of an AO beam were analyzed with polyimide (Kapton) films, a standard reference material for AO fluence measurements. The characterizations of polyethylene (PE), polypropylene (PP), and polystyrene (PS) films were studied using gel permeation chromatography and X-ray diffraction. The sample surfaces were observed using a field emission scanning electron microscope. Nanoscale protrusions were formed on all the samples and were larger but fewer with increasing AO fluence. The numerical density of protrusions formed on PE and PP was lower than that on PS. However, the erosion yields and functional groups of PE, PP, and PS were similar per FT-IR spectra.

Resistance of POSS Polyimide Blends to Hyperthermal Atomic Oxygen Attack

Copolymers of polyhedral oligomeric silsesquioxane (POSS) and polyimide (PI) have shown remarkable resistance to atomic oxygen (AO) attack and have been proposed as replacements for Kapton on the external surfaces of spacecraft in the harsh oxidizing environment of low Earth orbit (LEO). POSS PI blends would be an economical alternative to the copolymers if they also resisted AO attack. Thus, blends of trisilanolphenyl (TSP) POSS and PI with different weight percentages of the Si₇O₉ POSS cage were cast into films and exposed to a hyperthermal AO beam, and they were characterized in terms of their recession, mass loss, surface morphology, and surface chemistry. In order to compare the AO resistance of the blends with POSS PI copolymers, samples of previously studied copolymers were also investigated in parallel with the blends. For all POSS PI materials, the AO resistance increased with increasing AO fluence and with increasing POSS cage loading. At similar POSS cage loadings and exposure conditions, the TSP POSS PI blends showed comparable erosion yields to the POSS PI copolymers, with specific samples of blends and copolymers achieving erosion yields as low as 0.066 × 10^-24 cm³ atom^-1 with an AO fluence of 5.93 × 10²⁰ O atoms cm^-2. SEM and XPS analyses indicated that passivating SiO_x layers were formed on the surfaces of all POSS-containing polymers during AO exposure. Thus, a TSP POSS PI blend is proposed as a low-cost variant of a POSS polyimide for use in extreme oxidizing environments, such as LEO.

3D Graphene-Infused Polyimide with Enhanced Electrothermal Performance for Long-Term Flexible Space Applications

Polyimides (PIs) have been praised for their high thermal stability, high modulus of elasticity and tensile strength, ease of fabrication, and moldability. They are currently the standard choice for both substrates for flexible electronics and space shielding, as they render high temperature and UV stability and toughness. However, their poor thermal conductivity and completely electrically insulating characteristics have caused other limitations, such as thermal management challenges for flexible high-power electronics and spacecraft electrostatic charging. In order to target these issues, a hybrid of PI with 3D-graphene (3D-C), 3D-C/PI, is developed here. This composite renders extraordinary enhancements of thermal conductivity (one order of magnitude) and electrical conductivity (10 orders of magnitude). It withstands and keeps a stable performance throughout various bending and thermal cycles, as well as the oxidative and aggressive environment of ground-based, simulated space environments. This makes this new hybrid film a suitable material for flexible space applications.

Atomic-Oxygen-Durable and Electrically-Conductive CNT-POSS-Polyimide Flexible Films for Space Applications

In low Earth orbit (LEO), hazards such as atomic oxygen (AO) or electrostatic discharge (ESD) degrade polymeric materials, specifically, the extensively used polyimide (PI) Kapton. We prepared PI-based nanocomposite films that show both AO durability and ESD protection by incorporating polyhedral oligomeric silsesquioxane (POSS) and carbon nanotube (CNT) additives. The unique methods that are reported prevent CNT agglomeration and degradation of the CNT properties that are common in dispersion-based processes. The influence of the POSS content on the electrical, mechanical, and thermo-optical properties of the CNT-POSS-PI films was investigated and compared to those of control PI and CNT-PI films. CNT-POSS-PI films with 5 and 15 wt % POSS content exhibited sheet resistivities as low as 200 Ω/□, and these resistivities remained essentially unchanged after exposure to AO with a fluence of ∼2.3 × 10(20) O atoms cm(-2). CNT-POSS-PI films with 15 wt % POSS content exhibited an erosion yield of 4.8 × 10(-25) cm(3) O atom(-1) under 2.3 × 10(20) O atoms cm(-2) AO fluence, roughly one order of magnitude lower than that of pure PI films. The durability of the conductivity of the composite films was demonstrated by rolling film samples with a tight radius up to 300 times. The stability of the films to thermal cycling and ionizing radiation was also demonstrated. These properties make the prepared CNT-POSS-PI films with 15 wt % POSS content excellent candidates for applications where AO durability and electrical conductivity are required for flexible and thermally stable materials. Hence, they are suggested here for LEO applications such as the outer layers of spacecraft thermal blankets.

Space survivable polyimides with excellent optical transparency and self-healing properties derived from hyperbranched polysiloxane

A novel space survivable polyimide with a variety of desirable properties such as excellent thermal stability, high optical transparency, good mechanical strength, satisfactory break elongation, and outstanding atomic oxygen (AO) erosion resistance has been prepared by first synthesizing hyperbranched polysiloxane (HBPSi) and second incorporating HBPSi into polyimide (PI) chains via copolycondensation reactions. The 29Si nuclear magnetic resonance (29Si NMR) spectrum of HBPSi indicated that HBPSi possessed hyperbranched topology. The ground-based simulated AO exposure experiments demonstrated the mass loss of HBPSi polyimides decreased with increasing HBPSi addition and AO fluence, and it reached as low as 7.7% that of pristine polyimide when HBPSi addition was 29.7 wt % after 22 h AO exposure. Surface morphologies confirmed that pristine polyimide was significantly roughened after AO exposure while HBPSi polyimide had even less rough surface topography. During exposure of HBPSi polyimide to AO, the organic polyimide of the surface was first degraded and a silica protective layer eventually formed, which enabled the surface to be "self-healing". It is this passivation layer that prevents the underlying polymer from additional erosion. The whole preparation process of HBPSi polyimide is moderate, low-cost, environmentally friendly, and suitable for industrialized mass production, which contributes this novel material to a "drop-in" replacement for the widely used Kapton on spacecrafts functioning in space environment.

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.

Protecting polymers in space with atomic layer deposition coatings

Polymers in space may be subjected to a barrage of incident atoms, photons, and/or ions. Atomic layer deposition (ALD) techniques can produce films that mitigate many of the current challenges for space polymers. We have studied the efficacy of various ALD coatings to protect Kapton polyimide, FEP Teflon, and poly(methyl methacrylate) films from atomic-oxygen and vacuum ultraviolet (VUV) attack. Atomic-oxygen and VUV studies were conducted with the use of a laser-detonation source for hyperthermal O atoms and a D2 lamp as a source of VUV light. These studies used a quartz crystal microbalance (QCM) to monitor mass loss in situ, as well as surface profilometry and scanning electron microscopy to study the surface recession and morphology changes ex situ. Al2O3 ALD coatings protected the underlying substrates from atomic-oxygen attack, and the addition of TiO2 coatings protected the substrates from VUV-induced damage. The results indicate that ALD coatings can simultaneously protect polymers from oxygen-atom erosion and VUV radiation damage.

Effect of Ultraviolet Radiation from an Oxygen Plasma on the Atomic Oxygen-induced Etching of Fluorinated Polymer

The contribution of extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) from a laser-sustained plasma on mass loss phenomenon of fluorinated polymer in the ground-based laser-detonation atomic oxygen (AO) beam source was evaluated. The AO beam and EUV/VUV from an oxygen plasma were separated by a high-speed chopper wheel installed in the beam source. Mass changes of fluorinated polymer and polyimide were measured from the frequency shift of the quartz crystal microbalances during the beam exposures. It has been made clear that the fluorinated polymer is eroded by EUV/VUV exposure alone. In contrast, no erosion was detected for polyimide by EUV/VUV alone. The AO-induced erosion was measured for both materials even without EUV/VUV exposure. However, no strong synergistic effect was observed for the fluorinated polymer even under the simultaneous exposure condition of AO and EUV/VUV. Similar results were observed even in the simultaneous exposure condition of AO (without EUV/VUV from the laser plasma) and VUV from the 172 nm excimer lamp and D2 lamp. These experimental results suggest that the primary origin of the accelerated erosion of fluorinated polymer observed in the laser detonation AO source is not EUV/VUV from the laser-sustained plasma.