b. State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China;
c. Department of Chemistry and Biochemistry, Montana State University, 103 Chemistry and Biochemistry Bldg., Bozeman, Montana 59717, United States of America;
d. Department of Physics, School of Science, East China University of Science and Technology, Shanghai 200237, China;
e. Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn VIC 3122, Australia;
f. Beijing Institute of Spacecraft Environment Engineering, Beijing 100094, China
While polymers could feasibly be important materials for use on the outer surfaces of spacecraft in low Earth orbit (LEO) [1, 2], the harsh environment of atomic oxygen (AO), ultraviolet (UV) and vacuum ultraviolet (VUV) radiation, extreme thermal fluctuations, space debris, ionizing radiation, and high vacuum, presents very challenging conditions for polymers. Polymeric materials flowed in the LEO environment have been shown to succumb to accelerated surface erosion and cracking, oxidation, material degradation, surface out gassing, and contamination [3-5]. For polymers, AO is arguably the most destructive one of these environmental hazards due to its relatively high concentration and powerful capacity for oxidation. The neutral atmosphere at 300 km altitude consists of
Two possible ways of protecting polymeric materials against AO erosion have been used, including coatings on the surfaces of exposed polymers and copolymerization or blending of an inorganic component to the polymer. Coatings typically consist of inorganic materials that can act as a protective layer, shielding the polymer from exposure to AO. Atomic layer deposition (ALD) of a
Like coatings, the addition of an inorganic component to a polymer by copolymerization or blending can also reduce the inherent reactivity of the material, while at the same time having the additional benefit of being less susceptible to permanent damage by cracking and debris impacts. Unlike coatings, the inorganic component in copolymer or blend can conceivably be uniformly distributed within the polymer, effectively forming an inorganic oxide protective layer upon AO exposure that can be resistant against further AO attack while also having the ability to potentially "self-heal" if damaged during flight. Copolymerization of a polyimide matrix (which has the same chemical structure with Kapton-H) with polyhedral oligomeric silsesquioxane (POSS) or siloxane has shown promising resistance against AO exposure [11, 12]. Depending on the weight percentage of the POSS cage, the erosion yield of the POSS polyimides may be as little as
While copolymerization can undoubtedly be an effective approach to increasing resistance of polymer materials to AO, production of these materials is complicated and costly. What's more, the optical property of polymer might be changed by copolymerization of higher content reinforcement materials. As a less costly and simpler approach, addition of nanoclay particles to polymers has recently been introduced as a way to enhance polymer performance [13-15]. Nanoclay powder is mainly composed of O and Si particles (containing small amounts of C, Al, Mg, Na, and N) that are 20-200 nm in size [16-18]. Nanoclay reinforcement, in the range of 1 wt%-4 wt%, in polymers has been shown to improve the mechanical and thermal performance of polymers [14, 19-21]. The addition of nanoclay to polymers has also been shown to increase resistance to an AO plasma. Upon exposure to AO plasmas, nanoclay reinforced epoxy (NCRE) produced via the Quickstep process and nanoclay reinforced melt extruded nylon 6 showed surface erosion thickness can be significantly reduced after the passivation silica layer was formed .
While the resistance of the nanoclay reinforced polymers (NCRPs) to oxidation shows promise in a plasma environment, their durability in the space environment has not been assessed. In order to understand the susceptibility of NCRPs to hyperthermal AO in the LEO environment, we exposed NCRE composites to a directed beam of
The NCRE composites were synthesized by using the method we developed [15, 23]. Epoxy resin, Araldite GY 251 (Diglycidylether of bisphenol A, DGEBA), and hardener HY 956 (Triethylenetetramine) from CIBA-GEIGY, were used in the ratio of 5:1 by weight to form the base polymer materials . Nanoclay particles, SiO
A detailed description of the NCRE preparation has been described elsewhere . Briefly, the predetermined weight content of nanoclay was added into the epoxy resin. The nanoclay/epoxy resin was put into a bell jar on a rotating platform under vacuum for 55 min to extract gas bubbles. In order to ensure the nanoclay to be as uniformly dispersed as possible, the following process was repeated for 5 min: rotating the platform from 0 to 930 r/min for 2 s, rotating the platform from 930 r/min to 0 r/min for 1 s, and allowing the mixture to rest on a stationary platform for 7 s. The hardener was then added, and the new mixture underwent the same spinning and resting process as described above for 1 h under vacuum until the viscosity increased to avoid any movement of nanoclay inside the resin and the semi-cured uniformly-dispersed nanoclay/epoxy sample. The semi-cured sample was then removed from the bell jar and stored in a vacuum chamber for 12 h at 40 ℃ to form the cured sample.
Four types of sample with different loadings of nanoclay were prepared in this study: pure epoxy (0 wt% nanoclay content), NCRE composites with 1 wt%, 2 wt%, and 4 wt% nanoclay content.B. Hyperthermal AO exposures
AO exposures of the NCRE samples were conducted using a hyperthermal laser-detonation source of atomic oxygen . Details on the functioning of the source have been described elsewhere . In these exposures, pulses of pure oxygen gas (99.9%) with a backing pressure of 550 psi, were introduced into the source nozzle through a pulsed valve. 200 μs after the pulsed valve was triggered, a pulse of 10.6 μm light (7.5 J/pulse) from a CO
Samples were placed in a sample mount located at 40 cm from the apex of the conical nozzle and were exposed to beams of hyperthermal O/O
All samples (
All samples were kept in ambient air after exposure, and the impact of AO exposure on the materials was investigated using several surface analysis techniques, including etch depth measurements, surface chemistry analysis via XPS, and morphology measurements via SEM.
Etch depths were measured by a DekTak surface profiler from Veeco Instruments Inc. Forty steps were measured at different positions for each sample in order to obtain an average etch depth and standard deviation. XPS analysis was conducted before and after exposure with AO fluence 6.31
Kapton-H is a commonly used polymer for spacecraft. It can be severely eroded by AO and has a erosion yield of 3.0
|$ h=\sigma F $||(1)|
Etch depths of pure epoxy and NCRE composites and corresponding etch depth ratios of them to Kapton-H reference sample were obtained and shown in FIG. 3. For all three AO exposures, an increase in nanoclay content was correlated with a decrease in average etch depth and increase in standard deviation of the etch depth measurements (FIG. 3(a)). An increase in standard deviation of the etch depth measurements was also observed in samples exposed to higher AO fluence. The ratio of pure epoxy and NCRE composites to Kapton-H etch depths decreased as nanoclay content increased (FIG. 3(b)). But the lowest ratio was still above 80%, indicating that the etch depth of NCRE composites was relatively high.
Etch depth of NCRE composites was also converted to erosion yield according to Eq.(1). By substituting the etch depth value and the corresponding AO fluence into Eq.(1), the average erosion yields of pure epoxy and NCRE composites with nanoclay contents of 1 wt%, 2 wt%, and 4 wt% were obtained to be (3.92
Surface chemistry was investigated using XPS for all samples before and after AO exposure. First, XPS survey scans were conducted to get a comprehensive picture of all elements and their atomic concentrations on the surfaces. Then high resolution XPS spectra of C 1s were collected to obtain further information on chemical bonding.
For both control and exposed samples, the main elemental constituents of epoxy and NCRE composites were identified based on their corresponding principal photoelectron peaks: carbon (C 1s, 284.6 eV), oxygen (O 1s, 533 eV), nitrogen (N 1s, 400 eV), silicon (Si 2p, 103 eV) and aluminum (Al 2p, 76 eV). Besides, trace components from calcium (Ca 2p, 345.9 eV), fluorine (F 1s, 685.7 eV), sodium (Na 1s, 1072.0 eV), and tin (Sn 3d5, 495 eV), were also detected for all the samples. But the atomic concentrations of these elements were quite low and would not affect the whole picture of this research.
Table Ⅰ presents the relative atomic concentrations of pure epoxy and NCRE composites before and after hyperthermal AO exposure to a fluence of 6.31
High-resolution C 1s XPS spectra of control and exposed samples (FIG. 4) were acquired and show that after AO exposure, all spectra broadened mainly to the high binding energy direction which suggests that the surface was oxidized. The binding energies with peaks at 284.5 eV (peak 1), 286.01 eV (peak 2), 288 eV (peak 3), 282.48 eV (peak 4) were attributed to C-C/C-H, C-O, ketones, and carbides respectively. Furthermore, following AO exposure for the nanoclay containing samples, a new peak arose at 290.87 eV in the spectra (labeled as peak 5 in FIG. 4(b)) and was attributed to carbonates. An analysis of the relative area of C-related bonds from the high-resolution C 1s XPS spectra in FIG. 4 before and after AO exposure (Table Ⅱ) shows that decrease in overall carbon atomic concentration is accompanied by the increase of oxygen-containing carbon species C-O, ketones, and carbonates, and decrease of C-C/C-H, indicating that all samples were eroded and oxidized by hyperthermal AO. In more detail, for pure epoxy, 1 wt%, 2 wt%, and 4 wt% NCRE composites, the decreases of C-C/C-H component are 34.1% (86.3% to 52.2%), 29.6% (78.4% to 48.8%), 33.9% (80.6% to 46.7%), and 37.2% (78.1% to 40.9%), the increases of C-O component are 32.3% (6.9% to 39.2%), 32.5% (8.0% to 40.5%), 25.1% (5.7% to 30.8%), and 19.1% (17.2% to 36.3%), the increases of ketone component are 2.5% (2.7% to 5.2%), 2.0% (2.9% to 4.9%), 6.3% (3.5% to 9.8%), and 9.2% (1.8% to 11.0%), the increases of carbonates are 0%, 1.6%, 3.5%, and 3.7%, the sums of the increase of oxygen-relating C component are 34.8%, 36.1%, 34.9%, and 32.0%, respectively.C. Surface morphology
Surface morphologies of control and samples exposed to AO fluence of 5.46
High-magnification SEM images (FIG. 6) show that the roughness of the control NCRE samples increases with increasing nanoclay content. Following AO exposure, the pure epoxy sample surfaces were relatively smooth with some etch pits forming. These etch pits were distributed on the surface randomly. Like the low magnification SEM images results, as the nanoclay content increased, aggregates were observed on the sample surfaces. In addition, following the increasing fluence of AO exposure, more and more aggregates formed on the NCRE samples surface and these aggregates were more exposed with longer exposure. Furthermore, it is obviously from these high magnification SEM images that the size of aggregates increased as the increasing nanoclay content and increasing AO fluence.Ⅳ. DISCUSSION
In this study, etch depth was observed for both pure epoxy and nanoclay containing NCRE composites after exposure to AO, which indicates that both pure epoxy and nanoclay reinforced epoxy samples were etched during exposure to hyperthermal AO attack. During exposure, AO attacked the surface and eroded samples by releasing volatile products, which resulted in the etch depth. Pure epoxy did not show favorable AO resistance when compared to the Kapton-H reference. Etch depth ratios of pure epoxy to Kapton-H of three exposures increased from 1.13, 1.19, to 1.60, with increasing AO fluence. This can also be seen from the calculated average erosion yield of pure epoxy (3.92±0.76)
Variations in etch depth became more apparent with increasing content of nanoclay reinforcement and AO fluence, indicating increased surface roughness, which commensurate with SEM images. The decreased etch depth and increased surface roughness reveal that NCRE composites experienced milder erosion, with less mass taken away by volatile reaction products and relatively more non-volatile oxidized products staying on the surface.
XPS information reveals chemical changes on surfaces and provides insight into reaction mechanism of pure epoxy and NCRE composites with AO. The changes in surface element concentrations show that for all the samples atomic concentration of C decreased while that of O increased after AO exposure, indicating that sample surfaces were eroded and oxidized, which has also been observed in previous studies on polymers exposed to AO [11, 13, 31, 32]. This is because of the high energy and the strong oxidizing property of the hyperthermal AO. Energetic collisions of hyperthermal AO with sample surfaces can degrade materials by erosion and oxidation reaction, through breaking weak bonds and creating new chemical bonds. During hyperthermal AO bombardment, volatile molecules CO, CO
The increase in N concentration after AO exposure is consistent with surface erosion and was particularly believed to be more redistribution of eroded material at the surface . The increase in Si and Al atom concentration after AO bombardment is the result of that during exposure more Si and Al in nanoclay clusters were exposed on surfaces. High resolution spectra of C 1s indicated that, as the samples were exposed, carbon species were gradually oxidized, which resulted in a broadening of C peak and occurrence of a new peak (peak 5, FIG. 4(b)) of carbonates near 290.7 eV. The broadened carbon peak was resulted from the decrease in C-C bonds and concomitant increase in C-O bonds, ketones, and carbonates. It appears that the surface oxidation extent got increased for both pure epoxy and NCRE composites by reaction with hyperthermal AO. This is due to that components C-C/C-H with low binding energy are susceptible to the formation of C-O, ketones and carbonates components with high binding energy through breaking of C-C/C-H bonds by hyperthermal atomic oxygen. Specifically, pure epoxy exhibited most C-O component increase, while 4 wt% NCRE composite exhibited most ketones and carbonates increase. Furthermore, nanoclay containing composites favors the formation of the new component carbonates. It appears that addition of nanoclay makes the reaction between surface and energetic AO more prone to produce high oxide components (ketone, carbonates). This is probably related to the interfacial bonding formed between nanoclay and epoxy . From the total increase in oxygen-relating C component, it can be seen that 4 wt% NCRE component got least oxidized, consistent with the smallest erosion depth. Although pure epoxy, 1 wt% NCRE, and 2 wt% NCRE showed similar oxidation extent, it is probably because of the different erosion extent that caused the different erosion depths. Unfortunately, current study does not permit a more detailed understanding of the reaction mechanism through which how nanoclay act to enhance the AO resistance of epoxy.
Surface morphology changed significantly after AO exposure, as can be seen in FIG. 5 and FIG. 6. The surface morphology of exposed samples was the combined results of oxidation and erosion caused by hyperthermal AO . Compared with control sample, except the erosion pits, the exposed pure epoxy showed a smoother surface morphology, which can be seen obviously in high-magnification SEM images. From the surface morphology evolution of pure epoxy, it can be deduced that the reaction between hyperthermal AO and pure epoxy was mainly erosion. As a comparison, the NCRE samples showed a different surface morphology evolution process with the increase of AO fluence. Due to erosion, epoxy part of the NCRE samples showed relative smooth surface morphology. Besides that, nanoclay clusters evolved to aggregates, which were clearly seen on the surface of NCRE samples. The size and distribution densities of the aggregates increased with the increasing nanoclay content and AO fluence, which lead to the observed increase in surface roughness in SEM images. Evidently, the reaction between hyperthermal AO and NCRE samples was more complex than that with pure epoxy. The exposed NCRE samples appear to be visibly covered with aggregations after AO exposure. The produced aggregates potentially act as a "physical shield" and protected epoxy below from being attacked by hyperthermal AO. Unfortunately, no direct correlation was found between surface morphologies and changes in surface chemistry. The aggregation layers were imperfect as gaps formed in areas where epoxy unprotected by the nanoclay was eroded by AO (FIG. 6). Therefore, compared to reference Kapton-H samples, the 4 wt% NCRE samples still showed significant erosion and consequently relatively high etch depth ratios to Kapton-H.Ⅴ. CONCLUSION
Epoxy resin composites with different content of nanoclay reinforcement have been evaluated under ground simulated LEO atomic oxygen conditions by use of laser detonation AO source. The addition of nanoclay enhanced the resistance of NCRE composites to AO attack through reduced erosion depth. Occurrence of chemical reactions such as erosion and oxidation resulting from degradation by the AO was apparent from XPS data of all the tested samples. The degree of surface oxidation was related to the nanoclay weight percentage. The 4 wt% NCRE composite suffered least surface oxidation after AO bombardment. Erosion feature was observed in surface morphology of both pure epoxy and NCRE composites. Nanoclay clusters etched at a slower rate than the epoxy resin and the produced aggregates occurred on NCRE composite surfaces. The aggregates potentially act as a "physical shield" and partly protect epoxy below from being attacked by hyperthermal AO.Ⅵ. ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (No.21473015 and No.41574101), and the Fundamental Research Funds for the Central Universities (No.3132018233).
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b. 中国科学院大连化学物理研究所分子反应动力学国家重点实验室，大连 116023;
c. 美国蒙大拿州立大学化学与生物化学系，蒙大拿 59717;
d. 华东理工大学理学院物理系，上海 200237;
e. 澳大利亚斯威本科技大学科学、工程和技术学院，维多利亚 3122;
f. 北京卫星环境工程研究所，北京 100094