Synthesis of UV-Responsive Self-Healing Microcapsules and Their

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Synthesis of UV-Responsive Self-Healing Microcapsules and Their Potential Application in Aerospace Coatings Yuye Zhu, Kangli Cao, Min Chen, and Limin Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10737 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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ACS Applied Materials & Interfaces

Synthesis of UV-Responsive Self-Healing Microcapsules and Their Potential Application in Aerospace Coatings Yuye Zhu, † Kangli Cao, ‡ Min Chen, *, † and Limin Wu† †Department

of Materials Science and State Key Laboratory of Molecular Engineering

of Polymers, Advanced Coatings Research Center of Ministry of Education of China, Fudan University, Shanghai 200433, China ‡Shanghai

Institute of Spacecraft Equipment, Shanghai 200240, China

* Corresponding

author, email: [email protected]

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ABSTRACT: Advanced polymer composite coatings in spacecraft are threatened by harsh space environment factors, such as strong UV radiation, atomic oxygen, thermal cycles, space debris, etc. Their service life can be drastically shortened by the unavoidable formation of cracks caused by these factors (especially strong and abundant UV radiation) during long-term flight. Herein, a UV-responsive microcapsule-based coating is developed for in-orbit damage repairing. UV-responsive microcapsules, of which the inner polymeric shell can be degraded rapidly by the outer pure TiO2 shell under UV radiation, are produced by UV-initiated polymerization of Pickering emulsions and subsequently embedded into silicon resin matrices. When damaged, some microcapsules will be ruptured under the stimulus of external force, afterwards the unbroken ones around the scratched areas will be degraded by UV radiation, as a result, encapsulated healing agents can be released and finally repair cracks. In this system, UV-responsive microcapsules can release more agents more effectively due to the dual release mode, compared with traditional crack-repairing system. Moreover, the damage of UV radiation in space can be transferred into the favorable ones, which makes it have a potential application in aerospace coatings. KEYWORDS: UV-responsive microcapsules, self-healing, aerospace coatings

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INTRODUCTION Advanced polymer composite coatings, as the protective barrier, are widely used in spacecraft due to their excellent flexibility, light weight, outstanding mechanical and thermal stability and ease of processing.1-4 For example, thermal-control coatings, an important part of thermal control system, are designed to maintain the temperature in and out of spacecraft within appropriate range. By using their own thermal properties (solar absorptivity and thermal emissivity), they can adjust the absorbed solar radiation energy and the emitted radiation energy on the composite surface, which makes spacecraft operate normally. Unfortunately, harsh environment factors in space, such as strong UV radiation, atomic oxygen, space debris, thermal cycling, charged ions and low pressure, commonly lead to mechanical damage of these coatings.5-13 For instance, unavoidable collision caused by space debris can produce openings and cracks on coatings.5 When the spacecraft fly toward or opposite to the sun, their surface temperature can range from 100 to -100 °C, which maybe also cause cracking of coatings.6-7 Atomic oxygen can create physical and chemical erosions.8-11 High-energy UV radiation can gradually degrade covalent bonds and molecular chains of polymer materials, or can crosslink polymer chains and result in fracture and softening of materials.12-13 In low earth orbit (LEO), polymer coatings are mainly threatened by strong UV radiation and atomic oxygen. While at a higher altitude, atomic oxygen becomes less prevalent, but stronger UV radiation is of major concern.1 In general, harsh space environment factors, especially strong and abundant UV radiation, usually induce unavoidable formation and 3



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propagation of cracks in spacecraft coatings during long-term flight, drastically decreasing their mechanical stability and shortening their service life, thus seriously threaten their flight safety.14-15 One of the effectively employed approaches to solve this problem is to develop a new resin matrix, which is more resistant to UV radiation or atomic oxygen, but this needs a long development cycle and high cost. In recent years, smart coatings with self-healing property, which are capable of repairing cracks and/or restoring their original functions when damaged, have drawn great attention.16-19 Introducing self-healing strategies into spacecraft coatings have opened up a new opportunity to repair in-orbit damage. Self-healing strategies can be divided into intrinsic ones, which based on dynamic covalent bonds20-24 and noncovalent interactions25-30, and extrinsic ones in which healing agents are pre-embedded. Compared with the intrinsic ones, the extrinsic approach is particularly interesting because of their no change in original chemical structures of polymers, ease of fabrication, wide range of application in most polymer matrices and capability of non-contact self-healing. In brief, the extrinsic strategies can be divided into microcapsule-based system31-51 and microvascular network system.52-56 In early 2001, the first microcapsule-based system was reported.31 This system can be subdivided into function-restoring and crack-repairing systems, respectively. When a mechanical damage occurs in a functionrestoring system, healing agents wrapped in microcapsules will be released under the stimulus of external force or other stimuli, such as pH,32-35 light,35-37 temperature38 and reduction.38-39 However, this system can only restore their original functions, but cannot 4


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repair cracks. The early crack-repairing system31, 40-43 consists mainly of healing agentcontaining microcapsules and catalyst particles dispersed in the microcapsules or matrices. Healing agents released in this system will polymerize on touch with catalysts to repair cracks. Due to the limitations including high cost, low stability, environmental toxicity of catalysts and materials processing, catalyst-free crack-repairing system44-51 has also been studied, where healing agents polymerize triggered by atmospheric moisture44-47, light48-51 and other natural conditions. However, the release of healing agents in this system occurs only in the damage plane. Developing a stimuli-responsive crack-repairing system may solve the problems existing in traditional crack-repairing system, such as single release mode and low microcapsule utilization rate. In this study, we report a UV-responsive microcapsule-based system to repair cracks of spacecraft coatings. As depicted in Scheme 1, UV-responsive microcapsules with TiO2 nanoparticles and polymers as shell are produced by UV-initiated polymerization of Pickering emulsions and subsequently embedded into silicon resin matrices. When damaged, some microcapsules will be ruptured and release healing agents. While the unbroken ones around the scratched areas will be degraded by UV-radiation, which is strong and abundant in outer space, realizing the collaborative release. Epoxy silicon oil and cationic photoinitiator, as healing agents, can be released before curing thanks to the rapid degradation rate of polymeric shell. In this system, the release of healing agents can be realized under the stimuli of external force and UV radiation, which means more agents can be released due to this dual release mode and finally microcapsule utilization rate in crack-repairing system can be improved. Moreover, the 5



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damage of UV radiation in space can be transferred into favorable ones to realize effective crack repairs. Scheme 1. Schematic Illustration of UV-Responsible Microcapsule-Based Systemα

α(a)

Self-healing coating on substrate. (b) Some microcapsules are ruptured under the stimulus of

external force. (c) Unbroken ones around the crack are degraded by UV-radiation. (d) The crack is repaired by healing agents.

EXPERIMENTAL SECTION Materials (3-isocyanatopropyl)triethoxysilane (IPTS, ≥ 95%), dibutyltin dilaurate (DBTDL, ≥ 95%) and t-octylphenoxypolyethoxyethanol (Triton X-100, biochemical grade) were purchased from Aladdin. n-butyl acrylate (BA) and 1,6-hexanediol diacrylate (HDDA, ≥ 98%) were purchased from Sinopharm Chemical Reagent Co., Ltd and Polynaisse Resources Chemicals Co., Ltd. 1,3-Bis(glycidoxypropyl)tetramethyldisiloxane (B3001, 6


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≥98%) was purchased from Zhengzhou Gecko Scientific Inc. Benzoin isobutyl ether (BIE, >94%) was purchased from TCI. Mixed triarylsulfonium hexafluorphosphate salts solution in propylene carbonate (30 wt %, PI 6992) were received from Jiasheng Chemical Co., Ltd. A nano-titania aqueous dispersion (ACTiV™ S5-300B, 18 wt%, pH:9-10, diameter: 50 nm) and a silica aqueous dispersion (25 wt%, pH:6-8, diameter: 20 nm) were purchased from Cristal and Shanghai Seebio biological Technology Co., Ltd. Silicon resin (CSM1) and Zinc Oxide (ZnO) were supplied by Shanghai Institute of Spacecraft Equipment. The main chain of this resin is a network structure of Si-OSi, containing cross-linkable double bonds. Deionized water was used throughout the experiments. Syntheses of UV-responsive microcapsules SiO2 and TiO2 nanoparticles were modified according to our previously reported method.35-36 Briefly, 2.5 g of IPTS, 6.5 g of Triton X-100 and 0.03 g of DBTDL were added to a three-neck flask, stirred mechanically at 40 °C for 4 h to synthesize Triton X-100-IPTS (T-IPTS). 5.55 g of original TiO2 sols or 4 g of original SiO2 sols, 0.25 g of T-IPTS and 50 g of deionized water were added into a three-neck flask and stirred mechanically for 20 h at 65 °C. Then, a certain quality of modified TiO2 sols (1 wt%, aqueous dispersion), modified SiO2 sols (1 wt%, aqueous dispersion) and 35 g of deionized water were added to a brown three-neck flask containing BA, HDDA, BIE, B3001 and PI 6992. The mixture was emulsified at 1000 rpm for 40 min at 5 °C, forming the stable O/W emulsion. The resulting emulsion was transferred to a 150 mm crystallizing dish and polymerized under a UV lamp (395-400 nm, 609 W/m2) for 40 7



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min in ice bath. The microcapsules received were centrifuged at 8000 rpm for 5 min and then washed with water and ethanol two times. Table 1 summarizes the syntheses of UV-responsive microcapsules. According to the amounts of healing agents, the microcapsules fabricated as runs 2, 3, 4, 5 were defined as B0-, B0.6-, B0.7-, B0.8-, capsules respectively. While microcapsules synthesized as runs 6, 7, 8 were defined as T0.02-, T0.04-, T0.08-capsules according to the ratio of TiO2 nanoparticles added mass to oil phase mass (mT/mo). One from the original TiO2 sols was named as OT-capsules. Table 1. The Formulations for Syntheses of UV-Responsive Microcapsules

Water phase

Polymeric shell

Healing agents

Runs

Original TiO2 sol (g)

Modified TiO2 sol (g)

Modified SiO2 sol (g)

n-BA (g)

HDDA (g)

BIE (g)

B3001 (g)

PI 6992 (g)

1

2.5

-

-

0.7

0.3

0.04

0.7

0.056

2

-

45

-

0.7

0.3

0.04

0

0

3

-

45

-

0.7

0.3

0.04

0.6

0.048

4

-

45

-

0.7

0.3

0.04

0.7

0.056

5

-

45

-

0.7

0.3

0.04

0.8

0.064

6

-

3.5

41.5

0.7

0.3

0.04

0.7

0.056

7

-

7

38

0.7

0.3

0.04

0.7

0.056

8

-

14

31

0.7

0.3

0.04

0.7

0.056

Preparation of self-healing coatings based on UV-responsive microcapsules In order to verify the self-healing properties, UV-responsive microcapsules (B0.7capsules) were embedded into Silicon resin coatings. The silicon resin matrices were prepared by mixing silicon resin and ZnO with a mass ratio of 5:2. Subsequently, the 8


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microcapsules were added into these matrices with a definite mass ratio to silicon resin. The mixture was dispersed with ultrasonication for 2 min with a sonotrode (SCIENTZII, 25 kHz, 200 W, 6 mm tip-diameter, Ningbo Scientz Biotechnology Co., Ltd, China) and then cast on glass slides, cured for 24 h at 75 °C. For the sake of comparison, blank and control coatings were also fabricated with microcapsules (B0-capsules and T0.02capsules). Characterization The UV absorption spectra of PI 6992 and BIE were obtained by a U-4100 spectrophotometer (HITACHI) in the wavelength range of 250-420 nm at room temperature. The average sizes and size distribution of nano-TiO2 before and after modification were measured by dynamic light scattering (DLS, Nano-ZS90, Malvern Instruments Ltd). The morphology of emulsion was observed by optical microscopy (OM, KH-7700, HIROX). The microcapsules synthesized were centrifuged at 8000 rpm for 5 min, washed with water and ethanol two times (washed microcapsules) and then re-dispersed in ethanol. The morphology and surface features of microcapsules were determined by scanning electron microscopy (SEM, Ultra 55, ZEISS) and transmission electron microscopy (TEM, Tecnai G2 20 TWIN, FEI). The size distribution of microcapsules was measured by counting 300 particles in lowmagnification SEM images. Elemental composition of microcapsules was analyzed by energy dispersive spectroscopy (EDS) conducted on SEM and Fourier transform infrared spectroscopy (FT-IR, Nicolet Nexus 470, Thermo Fisher Corp.) with a resolution of 0.5 cm-1 for 32 scans (Transmission mode), in the wavenumber range of 9



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400-4000 cm-1. Washed microcapsules mentioned above were dried in the vacuum oven at 40 °C for 24 h (dried microcapsules) and then mixed with KBr to prepare FT-IR sample plates. The amount of healing agents in microcapsules were determined by Thermal gravity analysis (TGA, Pyris 1, PE) from 30 to 800 °C in nitrogen at a heating rate of 10 °C min-1. The TGA samples were prepared with dried microcapsules described above. UVresponsive property of microcapsules is determined by SEM, TEM and FT-IR. Washed microcapsules were exposed under a UV LED lamp (310 nm, 582 W/m2) for a certain time. Exposed microcapsules were soaked in ethanol. SEM and TEM samples were prepared with ethanol solution of exposed microcapsules. FT-IR measurements were carried out by the method mentioned above with samples prepared by the extracted ethanol solution of these microcapsules. The self-healing, blank and control coatings were scribed with a scalpel, and then exposed to UV radiation (310 nm, 582 W/m2) for 12 h. The final self-healing properties were determined by SEM. The anti-aging performances of self-healing coatings under long-term radiation were analyzed with a QUV accelerated weathering tester (QUV/se, Q-Panel Co., Ltd, USA). Coating samples were exposed to UV-radiation (310 nm, 0.71 W/m2) for 4 h at 60 °C, followed by condensation for 4 h at 50 °C. Surface morphology of coatings before and after accelerated weathering test was determined by SEM. Elemental composition of coatings was conducted by FT-IR measurements mentioned above (Attenuated Total Reflectance mode, ATR). Coatings before and after test were used directly for FT-IR measurements. 10


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Self-healing properties of coatings after accelerated weathering treatment were determined by SEM. The self-healing coatings after tests were scribed with a scalpel, and then exposed to UV radiation (310 nm, 582 W/m2) for 12 h.

RESULTS AND DISCUSSION Syntheses and characterization of UV-responsive microcapsules Scheme 2. Schematic Illustration of the Synthesis of UV-Responsive Microcapsules

Scheme 2 briefly shows the synthetic process of the UV-responsive microcapsules. These microcapsules were produced by UV-initiated polymerization of Pickering 11



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emulsions, in which TiO2 nanoparticles acting as Pickering emulsifiers can directly assemble onto the interfaces to form a hierarchical structure. BA and HDDA initiated by BIE, one of the most important free radical photoinitiators with a wide UV-sensitive region up to 400 nm,48 were chosen as the inner polymeric shell. The mixture of B3001 and its cationic photoinitiator, PI 6992, were used as healing agents. In contrast to free radical photoinitiators, PI 6992 is not sensitive to atomic oxygen, making it a more appropriate choice to use in harsh space environment. These stable milky emulsions formed by mechanical stirring were then initiated by a UV lamp with a wavelength range from 395 nm to 400 nm, in which only the monomers could be initiated by BIE. As shown in Figure 1, BIE has an obvious UV-sensitive region up to 400 nm, while PI 6992 is not sensitive in the UV-radiated region from 380 nm to 400 nm, which are in accordance with previous works.48, 50 Consequently, the UV-responsive microcapsules with healing agents enwrapped inside could be produced successfully and quickly.

12


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Figure 1. UV absorption spectra of photoinitiators (BIE and PI 6992) in propylene carbonate solution.

The comparison between emulsions stabilized by original and modified TiO2 sols were observed by OM. As shown in Figure 2, the average size of nano-TiO2 increased from 50 nm to 62 nm after modification (Figure 2a). The tiny emulsion droplets prepared with modified TiO2 sols are isolated and homogeneous, showing no obvious stratification (Figure 2b), while droplets achieved by original ones are slightly poorly dispersed (Figure 2c). After polymerization, a homogeneous and milky emulsion was maintained for the modified TiO2-stabilized emulsion. However, a complete phase separation was obtained for emulsion stabilized by original TiO2 nanoparticles because most of droplets were destabilized during polymerization (digital photographs in Figure 13



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2b-c, Figure S2).

Figure 2. (a) Particle size of nano-TiO2 before and after modification measured by DLS. (b and c) Typical optical photographs of emulsions stabilized with (b) modified TiO2 sols (Run 4), (c) original TiO2 sols (Run 1). Insets: the corresponding digital photographs of emulsions after polymerization.

The electron images reveal that these capsules (B0.7-capsules) were covered by a dense layer of TiO2 nanoparticles (Figure 3a-b). The morphology and surface features of microcapsules can be confirmed by an EDS spectrum (Figure 3c). Nanoparticles on the surface can be proved to be TiO2 by the appearance of Ti elements in EDS spectrum. Particle size analysis further demonstrates that these capsules have a relatively narrow distribution, with an average size of 3.87 μm (Figure 3d). The yield of microcapsules (mass ratio of the dried microcapsules to the total mass of fed oil phase and modified TiO2) is high to 83.7%. The composition of microcapsules was also confirmed by FTIR spectra. As depicted in Figure 3e, healing agents show absorption peaks at 910 cm14


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1 due

to the vibration of epoxy group. Monomers (a mixture of BA and HDDA with a

mass ratio of 7:3) show absorption peaks of C=C stretching vibration at 1639 cm−1and C=O stretching vibration at 1726 cm−1. The composition of capsules can be proved by the appearance of the two absorption peaks at 1726 and 910 cm-1 ascribed to C=O in the polymeric shell and epoxy group in the encapsulated healing agents. The absorption peak at 1639 cm−1 is not obvious but it still exists in the spectrum of microcapsules, which means there may be still some carbon-carbon double bonds in monomers remaining after UV initiation. The other capsules with different amount of healing agents (B0-, B0.6-, B0.8-capsules) were also obtained (see Figure S3).

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Figure 3. Characterization of microcapsules (Run 4). (a and b) Typical SEM and TEM images of microcapsules. (c) EDS spectrum of microcapsules. (d) Particle size distribution plot of

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microcapsules. (e) FT-IR spectra of B3001, a mixture of shell monomers and dried microcapsules.

The maximum loading capacities of healing agents (B3001) in the capsules were determined by TGA measurements. As shown in Figure 4, the capsules experienced significant weight loss from approximately 150 to 300 °C, which belongs to the loss of healing agents encapsulated. Another obvious weight loss from 350 to 600 °C is attributed to the thermal decomposition of polymeric shell. A residual weight of B0.6capsules at 300 °C reached a plateau of 75.0 wt% (Figure 4a), indicating that the content of loaded healing agents is 25.0 wt%. The B0.8-capsules had a weight residual plateau of 74.0 wt% at 300 °C, demonstrating that the encapsulation loading of healing agents is up to 26.0 wt% (Figure 4c). The B0.7-capsules have the maximum loading capacity in comparison with the B0.6-capsules and B0.8-capsules, which reaches to 29.0 wt% (Figure 4b) and is close to the theoretical loading of 31.8 wt% according to the feed m(B3001)

ratio. (theoretical loading = m(oil) + m(particle))

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Figure 4. TGA curves of B3001, B0-capsules and different capsules. (a) B0.6-capsules. (b) B0.7capsules. (c) B0.8-capsules.

UV-responsive property of the Microcapsules Since these microcapsules are covered by a dense layer of TiO2 nanoparticles, which are one of the most effective photocatalysts, the inner polymeric shell can be degraded and subsequently, the healing agents can be released upon UV radiation. In order to analyze the UV-responsive properties of the microcapsules, selected samples with different amount of TiO2 nanoparticles (T0.02-, T0.04-, T0.08-, B0.7-capsules, Table 1) were compared. As illustrated in Figure S5 and 5b, with the increase of TiO2 content in hybrid shell, the degradation speeds of microcapsules increase. Time for completely degradation of 18


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microcapsules remarkably decreases from 72 h (T0.02-capsules, mT/mo = 0.02) to 4 h (B0.7-capsules, mT/mo = 0.25). The TEM image (Figure 5c) proves the partial release of healing agents since a certain hollow structure is observed. The FT-IR spectrum of the extracted ethanol solution of the broken B0.7-capsules (Figure 5d) displays absorption peaks of Si-O-Si at 803 cm-1 and epoxy group at 910 cm-1, which belong to the healing agents. No peak is observed within these wavelengths for control sample before UV radiation, further confirming the UV-responsive release of healing agents from the microcapsules.

Figure 5. (a) Schematic illustration of the microcapsules (Run 4) responsive to UV radiation. (b) Typical SEM and (c) TEM images of B0.7-capsules after UV radiation under a UV LED lamp (310 nm, 582 W/m2) for 4 h. (d) FT-IR spectra of B3001, the extracted ethanol of microcapsules before (control sample) and after UV radiation for 4 h.

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However, not all microcapsules can successfully release healing agents recall that the cationic photoinitiator, PI 6992, is also sensitive to the UV radiation in 310 nm, which means the healing agents should be released from the microcapsules before curing. As shown in Figure S5d, no peak at 803 cm-1 and 910 cm-1 is revealed in the spectra of T0.02-, T0.04- and T0.08-capsules, indicating that the time for the complete degradation of these capsules were too long and the healing agents were cured before the degradation of polymeric shell. As discussed above, only the B0.7-capsules can be used for further formation of self-healing coatings because the other ones are failed in agent release. Performance of self-healing coatings based on UV-responsive microcapsules The preparation procedure of self-healing coatings was described in the above experimental part. UV-responsive microcapsules (B0.7-capsules) were embedded into the pre-mixed silicon resin matrices. Three different kinds of coatings were prepared with the content of microcapsules ranging from 50 wt% to 60 wt%. These coatings were defined as SH-50, SH-55 and SH-60 according to the content of microcapsules, respectively. The blank samples were produced with microcapsules without healing agents inside (B0-capsules) in one-to-one corresponding contents, named as BS-50, BS-55 and BS-60. Another control samples were prepared with microcapsules costabilized by SiO2 and TiO2 nanoparticles (T0.02-capsules), which failed in agent release under UV radiation as proved above, with the same contents, named as CS-50, CS-55 and CS-60. The thickness of these coatings is appropriately 12 μm controlled by 20


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QXG (see Figure S6). Scratches were applied on these coatings manually by scalpels, giving cracks with a width of about 3-4 μm confirmed by SEM images (Figure 6). Then, the scribed coatings were exposed to a UV LED lamp (310 nm, 582 W/m2) for 12 h to analyze their self-healing performances. The UV-responsive self-healing property of these coatings was confirmed by SEM (Figure S7 and 6). As shown in the images (Figure S7a, 6a and 6d), cracks in BS-50, BS-55 and BS-60 are not repaired as excepted, because there are no healing agents in these microcapsules. The control samples, CS-50, CS-55 and CS-60, also show no evident crack-repairing effect (Figure S7b, 6b and 6e). As illustrated above, microcapsules used in control samples were failed in agent release under UV radiation, which means agent-release of these microcapsules can only be triggered by external force. According to these results, agent release of these microcapsules by single release mode is too low to repair cracks. Compared with control samples, self-healing samples with the same content of microcapsules show different results. SH-55 displays an evident self-healing effect (Figure 6c). The cracks are mostly filled by cured agents but not completely, which can be ascribed to the successful but not enough flow of healing agents released from capsules toward cracks. With the increasing content of microcapsules, the cracks in SH-60 are almost completely healed after UV radiation (Figure 6f). However, SH-50 could only partially heal the cracks (see Figure S7) and the coatings with lower content of microcapsules had no obvious self-healing property due to the relatively low loading capacity. UV-responsive microcapsules can release more healing agents more effectively due to dual release mode, recalling that control 21



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samples show no evident effect in the same content of microcapsules. Compared with the traditional system which can only release healing agent in damage plane, UVresponsive crack-repairing system can achieve the same self-healing effect in a lower content of microcapsules.

Figure 6. SEM images of the scribe region of (a) BS-55, (b) CS-55, (c) SH-55, (d) BS-60, (e) CS60 and (f) SH-60 after UV radiation for 12 h.

The long-term anti-aging performances of self-healing coatings under UV-radiation were analyzed by accelerated weathering tests. When exposed in a UV accelerated weathering tester for 216 h, the surface morphology of coatings remains (Figure 7a). Elemental composition of coatings does not change according to the FT-IR spectrum. After accelerated weathering treatment for 72 h, 144 h and even for 216 h, cracks in sample SH-60 could be still healed after exposure to the UV light for 12 h, as shown in 22


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the images (Figure 7c). Owing to the addition of UV-blocking ZnO powders in coatings, the loss of microcapsules caused by long-term UV-radiation can be considerably reduced, which helps these coatings remain self-healing properties after long-term UVradiation tests.

Figure 7. (a) SEM images of SH-60 before and after accelerated weathering treatment for 216 h. (b) FT-IR spectra of SH-60 before and after accelerated weathering treatment for 72 h, 144 h and 216 h. SEM images of the scribe region of (c) BS-60 and (d) crack-repairing samples of SH-60 after accelerated weathering treatment.

The UV-responsive self-healing process can be described as follows: with the formation and extension of cracks, part of microcapsules will be ruptured directly. Healing agents enwrapped in these microcapsules will flow out first to the cracks. When exposed to UV radiation, the first outflows will be initiated and solidified, while the unbroken microcapsules around the cracks will be degraded by the outer TiO2 shell simultaneously. Healing agents from the unbroken ones can be released subsequently 23



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due to the rapid degradation rate of polymeric shell and further fill the cracks together with the first outflows. In this system, more agents can be released because of the dual release mode. By now, the damage of UV-radiation in space has been transferred into the favorable ones successfully to realize efficient crack repairs. Cracks caused by strong UV radiation, space debris and other harsh space environment factors may be dramatically healed by abundant UV radiation resources in space.

CONCLUSIONS In this study, we report a UV-responsive microcapsule-based system for crack repairing of spacecraft coatings. Healing agent-containing UV-responsive microcapsules were synthesized by UV-initiated polymerization of Pickering emulsions under a certain wavelength, embedded into the silicon resin matrices and then the self-healing coatings were prepared. When damaged, microcapsules in this system can release agents under the stimuli of external force and UV radiation. Moreover, cracks of spacecraft might be dramatically healed by abundant UV resources in space, which transferred the damage of UV radiation into favorable ones. However, the content of microcapsules was high in these coatings to receive an effective repair due to the relatively low loading capacity, which limited its further application. More effort need to be made to improve the loading capacity of these stimuli-responsive microcapsules.

ASSOCIATED CONTENT Supporting Information 24


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The Supporting Information is available free of charge on : UV absorption spectra of photoinitiators (BIE and PI 6992) in propylene carbonate solution (250-420 nm). SEM images of OT-capsules. SEM and TEM images of B0, B0.6- and B0.8-capsules. FT-IR spectra of B3001, a mixture of shell monomers, dried microcapsules and the extracted ethanol of microcapsules (400-4000 cm-1). SEM images of T0.02-, T0.04-, T0.08capsules before and after UV radiation for a certain time and corresponding FT-IR spectra of the extracted ethanol. SEM images of side surface of coatings. SEM images of the scribe region of BS-50, CS-50 and SH-50 after UV radiation for 12 h.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements Financial supports of this research from National Key Research and Development Program of China (2017YFA0204600) and the National Natural Science Foundation of China (51673041, 51721002, 51673045) and the Development Fund for Shanghai Talents (201643) and Equipment Pre-Research Fund (61400040403) are appreciated.

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Synthesis of UV-Responsive Self-Healing Microcapsules and Their

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