UV-Triggered Self-Healing of a Single Robust SiO2 Microcapsule

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UV-triggered Self-healing of a Single Robust SiO2 Microcapsule Based on Cationic Polymerization for Potential Application in Aerospace Coatings Wanchun Guo, Yin Jia, Kesong Tian, Zhaopeng Xu, Jiao Jiao, Ruifei Li, Yuehao Wu, Ling Cao, and Haiyan Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06091 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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ACS Applied Materials & Interfaces

UV-triggered Self-healing of a Single Robust SiO2 Microcapsule Based on Cationic Polymerization for Potential Application in Aerospace Coatings Wanchun Guo a,*, Yin Jia a, Kesong Tian a, Zhaopeng Xu b, Jiao Jiao a, Ruifei Li a, Yuehao Wu a, Ling Cao a, Haiyan Wang a a

Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, Hebei, 066004, China b

Key Laboratory for Special Fiber and Fiber Sensor of Hebei Province, College of

Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, Hebei, 066004, China

ACS Paragon Plus Environment

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ABSTRACT UV-triggered self-healing of single microcapsules has been a good candidate to enhance the life of polymer-based aerospace coatings because of rapid healing process and healing chemistry based on an accurate stoichiometric ratio. However, free radical photoinitiators used in single microcapsules commonly suffer from possible deactivation due to the presence of oxygen in space environment. Moreover, entrapment of polymeric microcapsules into coatings often involves elevated temperature and/or a strong solvent, probably leading to swelling and/or degradation of polymer shell, and ultimately the loss of active healing species into the host matrix. We herein describe the first single robust SiO2 microcapsule self-healing system based on UV-triggered cationic polymerization for potential application in aerospace coatings. Based on the similarity solubility parameters of the active healing species and the SiO2 precursor, the epoxy resin and cationic photoinitiator are successfully encapsulated into a single SiO2 microcapsule via a combined interfacial/in-situ polymerization. The single SiO2 microcapsule shows solvent-resistance and thermal stability, especially a strong resistance for thermal cycling in a simulated space environment. In addition, the up to 89% curing efficiency of the epoxy resin in 30 min, and the obvious filling of scratches in the epoxy matrix demonstrate the excellent UVinduced healing performance of SiO2 microcapsules, attributed to a high load of healing species within the capsule (up to 87 wt%) and healing chemistry based on an accurate stoichiometric ratio of the photoinitiator and epoxy resin at 9/100. More importantly, healing chemistry based on UV-triggered cationic polymerization mechanism is not sensitive to oxygen, extremely facilitating future embedment of this single SiO2 microcapsule in spacecraft coatings to achieve self-healing in a space environment with abundant UV-radiation and oxygen.


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KEYWORDS: self-healing coating, single SiO2 microcapsule, photo-triggered, cationic polymerization, aerospace application


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INTRODUCTION Advanced polymer composite coatings onto spacecraft, as the protective barrier, have drawn great attention due to their many advantages such as light weight, tunable flexibility, and easy process ability.1-4 Unfortunately, the harsh space environment factors, such as atomic oxygen, thermal cycling, charged ions, UV radiation and low pressure, commonly induce unavoidable formation and propagation of microcracks in spacecraft coatings during long-term flight, thus reducing the coating life. Introducing self-healing strategies into coatings5 opens up a great opportunity to significantly enhance the coating lifetime of a spacecraft in orbit. Up to now, selfhealing strategies6,7 could be mainly divided into intrinsic ones8, 9 based on reversible bonds inherent in materials and extrinsic ones10-13 based on external healing agents pre-embedded into matrices. In particular, microencapsulated self-healing systems, a typical extrinsic strategy, have attracted considerable attention due to their general usability in most polymer coatings, their capacity for healing microcracks on different size scales by tuning the microcapsule size, and their minor influence on the performance of the coatings.14-17 The early extrinsic microcapsule-based self-healing systems consisted mainly of healing agent-containing microcapsules and catalyst particles dispersed directly into matrices.10,18-21 The healing agent, when released from the broken microcapsules along the formation of cracks, would flow into the cracks and subsequently repair them via polymerization with the catalyst. However, catalysts embedded directly into the coatings always suffer from a fatal issue: the possibility of deactivation due to chemical reactions with the coating matrix. Isolating the healing agent and catalyst into discrete microcapsules effectively addresses this issue.22 However, inhomogeneous distributions of dual-capsules in the coating creates the problem of the healing chemistry occurring in an uncontrollable stoichiometric ratio, as well as necessitating a


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complicated process route. It therefore limits the availability of healing agent and catalyst, and limits their wide applications. The rational design of a single microcapsule self-healing system, in which the healing agent or the mixture of healing agent and latent curing agent are encapsulated into a single polymer microcapsule provides important advantages: both agents can be released simultaneously from the ruptured microcapsules, repairing cracks by polymerization triggered by either the residual functional groups in the host matrix, or by an environment stimulus such as the moisture,23 oxygen,24 light (e.g. UV radiation or sunlight),25-27; it enhances the utility of the healing agent because of its basis in healing chemistry in an accurate stoichiometric ratio; and it simplifies the synthesis process of self-healing system. With respect to a self-healing system for an aerospace coating, the sufficient UV radiation in space can be a strong extrinsic stimulus triggering single capsule self-healing materials. However, free radical photoinitiators used in the photo-induced single microcapsule self-healing systems mentioned above are not well suitable for aerospace coating, because of their possible deactivation due to the presence of oxygen in the extremely harsh space environment.28 In addition, processing polymeric microcapsules into coatings often involves elevated temperature and/or strong solvents, probably leading to swelling and/or degradation of polymer shell and ultimately to the loss of active healing species in the host matrix.29 Accordingly, it is still highly desirable and a significant challenge to design a photoinduced single robust capsule self-healing system for potential aerospace coatings. In this paper, we describe the first single SiO2 microcapsule self-healing system based on UVinduced cationic polymerization for potential application in aerospace coatings. Based on the similar solubility parameters of the active healing species and the silica precursor, an accurate stoichiometric ratio of epoxy resin and a cationic photoinitiator solution in propylene carbonate


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were successfully encapsulated into a single robust SiO2 microcapsule via a combined interfacial/in-situ polymerization strategy. The SiO2 microcapsules exhibit not only solvent resistance and thermal stability, especially a strong resistance for thermal cycling in simulated space environment, but also high repairing efficiency, attributed to a high load of healing species in the microcapsule, and the accurate stoichiometric ratio of the UV-triggered healing chemistry. Furthermore, healing chemistry based on a cationic polymerization mechanism is not sensitive to oxygen, thereby greatly facilitating the potential self-healing application of the single SiO2 microcapsule in aerospace coating. As depicted in Scheme 1, monodispersed SiO2 microcapsules are embedded into epoxy resin matrices in advance. Upon rupture of the microcapsules by the formed and extended microcracks, the epoxy resin and photoinitiator are simultaneously released due to the capillary effect and fill in the newly formed microcracks. Subsequently, the microcracks are self-repaired to a large degree after UV radiation initiates the curing of the epoxy resins.


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Scheme 1. Illustration of UV-triggered self-healing of SiO2 microcapsules embedded into epoxy resin coating. EXPERIMENTAL Materials. Bisphenol A diglycidyl ether (Epoxy E-51), Epoxidized Hydrogenated Bisphenol A (Epoxy A1815) and amine curing agent (1670T) were purchased from Qingda-Qs Materials Co., Ltd. Mixed triarylsulfonium hexafluorphosphate salts solution in propylene carbonate (30 wt%, PI6992) were purchased from Jiasheng Chemical Co., Ltd. Tetraethyl orthosilicate (TEOS) (>99%), N,N-Dimethylformamide (DMF), acetone, carbon tetrachloride (CCl4) were purchased from Aladdin. Poly (ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymer (PEO100-PPO65-PEO100, Pluronic F127) and 2-Anilino-3-methyl-6-dibutylaminofluorane


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(ODB-2) were purchased from Sigma and Jianxin Chemical Co., Ltd., respectively. An HCl solution (2 mol/L) and deionized water were used in the experiments. Synthesis of a single SiO2 microcapsule. The SiO2 microcapsules were synthesized through a combined interfacial/in-situ polymerization strategy based on the hydrolysis and condensation of TEOS catalyzed by HCl in an O/W emulsion. The synthetic procedure is described as follows: the homogeneous mixture of E-51 (1.0 g), A1815 (1.0 g), PI6992 (0.6 g) and TEOS (1.0 g) as organic phase were added into aqueous F127 solution (40 mL, 2 wt%) and then emulsified at 8500 rpm for 10 min at 25 oC, forming the stable oil-in-water (O/W) emulsion. Subsequently, in the first step designated as interfacial polymerization, an aqueous HCl solution (600 uL, 2.0 mol/L) was added dropwise into the resultant emulsion, and then reaction mixture was heated to 50 oC and stirred mechanically at 300 rpm for 3 h. The subsequent second step was designated as in-situ polymerization. TEOS (1.5 g) was added into the agitated dispersion, followed by the dripped addition of aqueous HCl solution (1800 uL, 2.0 mol/L) in three equal portions at 4-hour intervals while stirring. After 12h, the reaction mixture was cooled to room temperature, and the final products were isolated, rinsed with deionized water several times and dried at 60 oC for 24 h. Preparation and performance of self-healing coating based on single SiO2 microcapsule. SiO2 microcapsules were placed in our self-made aging equipment, followed by setting the environmental pressure at 5000 ± 500 Pa and the temperature range from 110 oC to -50 oC. After being cycled for 5 days, the aged microcapsules and other pristine microcapsules (control microcapsules) were embedded into the epoxy-amine coating to evaluate their respective selfhealing performances.


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The self-healing coatings were prepared by mixing E-51 epoxy resin, 1670T amine as a curing agent, and SiO2 microcapsules with a mass ratio of 5: 2: 3. Subsequently, the mixture was degassed, coated on the tinplate sheet, and cured for 24 h at 40 °C. Measurement of photo-polymerization Conversion. The conversion of epoxy resin was determined by real-time infrared spectra (RTIR). The mixtures of epoxy resin and cationic photoinitiator on different ratios were placed between two KBr crystals and then irradiated with a UV lamp (UVEC-4, Lamplic, China) at a wavelength of 365 nm and radiation intensity of 1.2 mW/cm2. The time-dependent conversion of polymerization could be recorded by monitoring the decay of the epoxy peak in the range from 895 to 922 cm-1 under UV irradiation. Thus the conversion could be calculated by measuring in situ the peak area with prolonged reaction time according to the following formula: Conversion (%) =(A0-At )/At×100，A0 and At correspond to the peak areas of epoxy group at 0 s and at t time under irradiation, respectively. Characterization. Morphology and surface features of the SiO2 microcapsules were obtained by optical microscopy (OM) (Canon XSZ-G), scanning electron microscopy (SEM) (Zeiss SUPRA 55), and transmission electron microscopy (TEM) (HITACHI HT 7700). The UV absorption spectra of the cationic photoinitiators were analyzed with a Shimadzu UV 2550 spectrophotometer at room temperature in the wavelength range of 200-600 nm. FTIR spectra were analyzed on a Nicolet IS10 infrared spectrometer using KBr pellets. The conversions of epoxy resin were measured with real-time infrared spectroscopy (Nicolet 5700 FTIR) with a UV LED lamp (UVEC-4, Lamplic, China) (RTIR). TGA data were obtained with a NETZSCHSTA449 C instrument from 25 oC to 1000 oC in argon. The particle size distributions of SiO2


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microcapsules were determined by counting one hundred particles in the low magnification SEM images. SiO2 microcapsules were embedded into Spurr’s epoxy resin. Subsequently, the epoxy resin containing the microcapsules was cured for 8 h at 70 oC, and then sliced into 60-100 nm sections using a LKB V microtome, and the slices were deposited on formvar film-coated Cu grids for TEM.RESULTS AND DISCUSSION

Figure 1. UV absorption spectra of cationic photoinitiators (PI6992) in propylene carbonate solution. The mixture of Epoxy resins E51) and A1815 were used as healing agents due to their advantages of high thermal stability, approximate mobility and good adhesion to many polymers.30,31 In contrast to free radical photoinitiators susceptible to oxygen, mixed triarylsulfonium hexafluorphosphate salts (PI6992), one of the most important cationic photoinitiators, is strongly sensitive in the wide UV-radiated region from 200 nm to 350 nm (Figure 1) and not sensitive to atomic oxygen, thus appropriate for use as a photoinitiator in the harsh space environment with abundant UV-radiation and atomic oxygen.32 In addition, inorganic materials such as SiO2 are good candidates for the shells of microcapsules in a single


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microcapsule self-healing system design due to their high thermal and chemical stability, and strong solvent-resistance.33 Synthesis and characterization of single SiO2 microcapsule

Scheme 2. Synthetic illustration of the final SiO2 microcapsules via a combined interfacial and in-situ polymerization route.


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Figure 2. Optical images of (a) the initial emulsion droplets; (b) the intermediate SiO2 microcapsules with the thin shell obtained by the interfacial polymerization step (the inset is the SEM image of one intermediate microcapsule); (c) the final SiO2 microcapsules with the thick shell synthesized by the subsequent in-situ polymerization step (the inset is the SEM image of one final microcapsule); SEM images of (d) the intermediate microcapsules, (e) the surface morphology of the intermediate collapsed microcapsules and (f) one final broken microcapsule. As depicted in Scheme 2, the SiO2 microcapsules containing epoxy and cationic photoinitiator were synthesized via a single-batch process, an interfacial/in-situ polymerization strategy. On the basis of the similar solubility parameters of epoxy resin, propylene carbonate, and TEOS that are 23.2,34 27,35 and 16.4 MPa1/2,

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respectively, the homogeneous oil phase containing

photoinitiator could be emulsified in aqueous PEO-PPO-PEO (F127) solution to form a stable oil/water emulsion (Figure 2a). The central PPO segment locates onto hydrophobic interfaces,


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whereas the two lateral PEO blocks extend into the hydrophilic phase to enhance the steric hindrance of emulsion droplets against their coalescence upon collision. In the first step designated as interfacial polymerization, upon the collision of protons as catalysts in the aqueous phase with the TEOS molecules in hydrophobic phase, the hydrolysis and subsequent polycondensation of TEOS molecules (Figure S1) at oil/water interface results in the intermediate SiO2 microcapsules (Figure 2b).37 These SiO2 microcapsules always suffer from undesired collapse during the drying process, even at temperatures as low as 60 oC (Figure 2d and Figure S2), because of their extremely thin shell thickness of approximately 40 nm determined by surface morphology of the collapsed microcapsules (Figure 2e). However, a thicker silica shell is unachievable via a single interfacial polymerization route, possibly because the thin, highly cross-linked silica shells isolate unreactive TEOS monomers in oil droplets from the aqueous phase, and consequently prevent hydrolysis and the subsequent deposition of inner TEOS monomers onto the inner wall of silica shells. Thus, in the second step designated as insitu polymerization, additional TEOS monomers were added into the above dispersion for their in-situ hydrolysis and subsequent condensed polymerization onto the outer surface of the thin silica shells with the protons as catalysts (Figure S1).38 In this step, no more additional protons results in no further increase in the shell thickness of the final SiO2 microcapsules due to insufficient protons onto the intermediate microcapsules available for capture of the primary SiO2 particles in the reaction system (Figure S3a). The addition of HCl solution (1800 uL, 2.0 mol/L) in one portion also leads to a similar result (Figure S3b), because too many protons endow both the intermediate SiO2 microcapsules and the primary SiO2 particles with more positive charges, leading to strong electrostatic repulsion between them. Additional protons in three equal portions at 4-hour intervals exactly supply sufficient catalytic species adsorbed onto


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the ever-increasing SiO2 shell, with more surface area to capture the primary SiO2 particles generated in the reaction system to deposit onto these microcapsules through condensation reaction, forming the final SiO2 microcapsules (Figure 2c) with the shell thickness of about 3 µm (Figure 2f). This is because additional protons in three equal portions can compensate for proton consumption in the in-situ polymerization step, corresponding to the pH increase of the emulsion from 0.91 to 1.04, from 0.87 to 0.86 and from 0.73 to 1.22 (Table S1) after every addition of protons and before the next addition of protons.

Figure 3. The as-synthesized SiO2 microcapsules: (a) SEM image; (b) SEM image of one broken microcapsule; (c) TEM image of microcapsule slice; (d) Particle size distribution plot of the microcapsules; (e) SEM image of one complete microcapsule; (f-h) the EDX elemental maps of C (f), Si (g) and O (h), respectively; (i) Color-change images of the ODB-2 solution in DMF with milled SiO2 microcapsules containing only epoxy (control sample, sample 1), and with our milled SiO2 microcapsules (sample 2), before and after exposure to UV radiation.


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The SEM image (Figure 3a) reveals that the as-synthesized SiO2 microcapsules are relatively uniform, with a size of about 20-30 µm, highly spherical with a rough surface. The rough surfaces may be because the fast diffusion of the protons accelerates the hydrolysis and polycondensation of TEOS monomers onto the oil droplets, forming many small secondarilynucleated silica particles on the silica shell. Both the broken microcapsule in the SEM image (Figure 3b) and the thin microcapsule section in the TEM image (Figure 3c) show its welldefined hollow structure, with an uniform shell thickness of approximately 3 µm, indicating successful encapsulation of the liquid core by the SiO2 microcapsules. In addition, particle size analysis (Figure 3d) indicates that the as-synthesized SiO2 microcapsules have a relatively narrow distribution, with a mean size of 26.2 µm. Compared to the EDX mapping images (Figure 3e-3h) of one complete microcapsule, those of broken microcapsules (Figure S4) show the presence of additional elements such as F, P, S, in addition to C, Si, O elements with uniform distributions, which conforms encapsulation of the epoxy resin and F, P, S elements-containing photoinitiator in a dense silica shell, and successful formation of single-component SiO2 microcapsules. On the basis of UV-induced color change of ODB-2 dye in the presence of the photoinitiator,39 a simple reactivity test was designed to further verify encapsulation of the photoinitiator in SiO2 microcapsules. The ODB-2 solution in DMF mixed with milled microcapsules containing epoxy only (control sample) exhibits no any obvious color change after UV irradiation for 0.5 h (Figure 3i-1). In sharp contrast, the similar solution with our milled SiO2 microcapsules shows distinct change in color after similar UV irradiation (Figure 3i-2). The above results indicate that the active photoinitiator was successfully encapsulated in our SiO2 microcapsules, in favor for potential healing application in space.


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FTIR spectroscopy was also utilized to further clarify the fine composition and structure of the SiO2 microcapsules. Compared with the FTIR spectra of the oil mixture and SiO2 (Figure S5a, S5b), that of the microcapsules shows characteristic peaks of both the oil mixture and SiO2. As depicted in Figure S5c, the peaks at around 920 cm-1, 840 and 564 cm-1 could be assigned to the epoxy group

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and the P-F functional groups of cationic photoinitiator.42 In addition, the

absorption peaks at 475, 798, 1100, 956 and 3385 cm-1 are attributed to the bending vibration of Si-O bonds, the symmetric and asymmetric bending vibrations of Si-O-Si bonds, and the bending and stretching vibrations of Si-OH groups, respectively.43,44 The above results confirm that the successful encapsulation of epoxy resin and cationic photoinitiator within the silica shell.

Figure 4. TG curves of (a) pure SiO2, (b) the as-synthesized SiO2 microcapsules, and (c) the oil mixture of the epoxy resin and photoinitiator. The thermal stability of microcapsules is of great importance for their storage and selfhealing application. As a result, the thermal performances of pure SiO2, the as-synthesized SiO2 microcapsules, and the oil mixture of the epoxy resin and photoinitiator were studied by TG measurements. As seen in Figure 4a, the weight-loss of pure SiO2 is extremely low, approximately 6 wt% ranging from room temperature to 1000 oC (which corresponds to the


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removal of absorbed water), exhibiting robust thermal stability. The TG curve of the assynthesized SiO2 microcapsules (Figure 4b) is very similar to that of the oil mixture of epoxy resin and photo initiator (Figure 4c) in the entire temperature range. No obvious weight losses were observed in these two samples at temperature below 254 oC, indicating relatively high thermal stability for the liquid inner core, i.e. the oil mixture of epoxy resin and photoinitiator. The weight losses in the two TGA curves, 62 wt% in Figure 4b and 78 wt% in Figure 4c, could be attributed to thermal decomposition of the epoxy resin and photoinitiator at temperatures ranging from 254-410 oC. With an increase in temperature to 1000 oC, about 26 wt% of the residual weight in microcapsules consists of pyrolyzed product of the liquid core (13 wt%, Figure 4c) and the silica shell, demonstrating that the encapsulation loading of the liquid healing agent in our SiO2 microcapsules is up to 87 wt%. This high encapsulation loading is just one requirement underlying high self-healing efficiency of microcapsules.45 The SiO2 microcapsules were immersed into different solvents including H2O, DMF, acetone and CCl4 from polarity to nonpolar, to evaluate the solvent resistance of our SiO2 microcapsules at room temperature. According to optical images in Figure S6, all the microcapsules preserve their initial core-shell structure and spherical morphology after their immersion in the various solvents for 20 days, indicating good solvent stability in favor of their processing. Effect of the HCl concentration on the shell thickness of SiO2 microcapsules


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Figure 5. Representative SEM images (a-d), particle size distributions (e-h) of SiO2 microcapsules and TEM images of ultrathin slices of SiO2 microcapsules (i-l) synthesized with the total HCl concentrations at (a), (e), (i) 0.08 mol/L; (b), (f), (j) 0.10 mol/L; (c), (g), (k) 0.12 mol/L; and (d), (h), (l) 0.14 mol/L, achieved by adding 2 mol/L HCl solutions (1600, 2000, 2400, and 2800 uL) in four equal portions, respectively. Microcapsules with appropriate shell thickness are of great importance for self-healing applications. Microcapsules with excessively thin shells commonly suffer from damage during processing, and those with excessively thick shell may not be ruptured in response to cracks in the matrix, preventing the release of the healing agent.46 Considering that the rate of acid-


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catalyzed hydrolysis and condensed polymerization of TEOS monomers depends strongly on the number of the protons during the preparation of microcapsules via a combined interfacial/in-situ polymerization, the effect of the total HCl concentration on the shell thickness of SiO2 microcapsules was investigated. As shown in Figure 5a-5d, the size of silica particles on the microcapsules becomes much smaller with an increase in the total HCl concentration. By increasing the total HCl concentration from 0.08, 0.10, 0.12, to 0.14 mol/L, the mean size of SiO2 microcapsules increases from about 25.0, 25.6, 26.2 to 28.2 µm (Figure 5e-5h) in combination with low-magnification SEM images in Figure S7, and the thickness of the silica shell increases from about 2, 2.6, 3 to 3.2 µm, respectively (Figure 5i-5l). This tendency could be explained by saying that the higher proton concentration promotes hydrolysis of TEOS and thus the generation of the more silica oligomers33,47 and that their subsequent condensation constructs the thicker SiO2 shell. Meanwhile, the more silica oligomers possibly result in many smaller primary SiO2 particles deposited onto SiO2 shell, in agreement with the aggregation of smaller particles on the surface of the final SiO2 microcapsules. UV-induced self-healing performance of the single SiO2 microcapsule.

Figure 6. (a) Time-dependent UV-vis absorption spectra of epoxy resin photo-cured by cationic photoinitiator (9 wt%), (b) the plots of the conversion (C0-Ct/C0) against the curing time (t) for


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the photopolymerization of epoxy resin with different concentrations of the cationic photoinitiator from 3 wt% to 15 wt%. To evaluate the photo-cured performance of the epoxy resin, UV-vis spectroscopy was used to in-situ monitor the time-dependent absorption of epoxy groups during the photo-cured process. As shown in Figure 6a, with the concentration of cationic photoinitiator at 9 wt%, the epoxy absorption ranging from 895 to 922 cm-1 decreases gradually over time under UV irradiation, consistent with the consumption of epoxy groups due to their ring-opening reaction during the photo-curing process.48 Considering that the photo-cured efficiency of epoxy resin usually relies on the concentration of the cationic photoinitiator (PI6992), photo-cured performances of epoxy resins with different concentrations of the cationic photoinitiator were also measured to determine the optimum proportion of photoinitiator in our single-component self-healing system. According to Figure 6b, the epoxy conversion increases from 60%, 81% to 89% by tuning the concentration of the photoinitiator from 3 wt%, 6 wt% to 9 wt% within 1800 s. This is attributed to the additional protons generated by increased photoinitiator species under UV irradiation accelerating the ring-opening reaction rate of the epoxy group to reach its higher conversion. However, with a continued increase in the concentration of the photoinitiator from 9 wt%, 12 wt% to 15 wt%, the epoxy conversion decreases instead, from 89%, 84%, to 81%. This tendency may be explained by the following: after the absorption of incident UV radiation, the numerous photoinitiator molecules in the liquid surface layer generate sufficient protons to drastically accelerate drastically the photo-cured process, such that a thin coating of cured epoxy resin is formed, hampering the absorption of incident UV radiation by the remaining unreacted photoinitiator molecules below. In view of the results mentioned above, the concentration of the


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photoinitiator at 9 wt% is the optimum choice for our single-capsule self-healing system based on photo-curable epoxy resin.

Figure 7. SEM images of the pristine microcapsules (a), the microcapsules after thermal cycling in the temperature range from -50 oC to 110 oC for 5 days (d), the scribe region of the selfhealing coating containing the pristine microcapsules before (b) and after (c) UV irradiation for 30 min, and the scribe region of the self-healing coating containing the thermally cycled microcapsules before (e) and after (f) UV irradiation for 30 min. The robustness of a self-healing system’s resistance to long-term thermal cycling is extremely significant for aerospace self-healing coatings undergoing the harshly thermal cycled space environment. Thus we simulated the high/low temperature of thermal cycling and the low atmospheric pressure of space environment to assess the self-healing performance of our single SiO2 microcapsules. Compared with the pristine SiO2 microcapsules (Figure 7a), the microcapsules after thermal cycling in the temperature range from -50 oC to 110 oC for 5 days show similar spherical morphology (Figure 7d). The pristine microcapsules and the thermally


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cycled microcapsules were embedded into the amine-cured epoxy coatings and the two selfhealing coatings were subsequently scribed by a razor blade. The SEM images in Figure 7b and Figure 7e reveal the scribe region to have a width of about 20-40 µm. After being exposed to UV irradiation for 30 min, the scratches both in the pristine microcapsule-based self-healing coatings and in the thermally cycled microcapsule-based self-healing coatings were almost completely filled as expected (Figure 7c and 7f), this being ascribed to the simultaneous flow of epoxy resin and cationic photo initiator released from the ruptured microcapsules towards cracks and subsequent UV-triggered curing to achieve the self-healing effect. This result confirms that the single SiO2 microcapsule with epoxy resin and cationic photoinitiator is strongly resistant to the extremely harsh thermal cycling environment and serves especially as the highly efficient UV-triggered self-healing materials used in future aerospace coatings in view of the abundant UV irradiation in the space environment. CONCLUSIONS In summary, we have developed the first robust single SiO2 microcapsule self-healing system based on UV-triggered cationic polymerization for potential application in aerospace coatings. Epoxy resin and cationic photoinitiator solution in propylene carbonate in an accurate stoichiometric ratio were successfully encapsulated into a robust single SiO2 microcapsule by a novel single-batch route, a combined interfacial/in-situ polymerization strategy. SiO2 microcapsules exhibit solvent resistance and thermal stability due to their inorganic shell. In particular, after thermal cycling for 5 days SiO2 microcapsules maintain high self-healing performance for scratches in the epoxy matrix. This is attributed to the high encapsulation load of active healing species in the capsule, to the accurate stoichiometric ratio of the healing chemistry, and to the lack of oxygen sensitivity of the UV-triggered cationic polymerization


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mechanism. This single SiO2 microcapsule would be well suited for potential self-healing application in spacecraft coatings in the harsh space environment with abundant UV-radiation and oxygen, and a combined interfacial/in-situ polymerization strategy could be used to synthesize more inorganic microcapsules for single-component self-healing application. ASSOCIATED CONTENT Supporting Information Additional characterization data, including OM images, SEM images and EDX elemental maps. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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This study has received grants from National Natural Science Foundation of China (Grant No. 51503178), Natural Science Foundation of Hebei Province (No. E2015203114), China Postdoctoral Science Foundation (No. 2015M571278), Postdoctoral Science Foundation of Hebei Province (B2014003009), Science and Technology Project of Higher Education in Hebei Province (QN2016001), Independent Research Project of Yanshan University for Young Teachers (14LGB019) and the Doctor Foundation of Yanshan University (B789). REFERENCES (1) Park, C.; Onuaies, Z.; Watson, K. A.; Pawlowski, K.; Lowther, S. E.; Connell, J. W; Siochi, E. J.; Harrison, J. S.; Clair, T. L. St. Polymer-Single Wall Carbon Nanotube Composites for Potential Spacecraft Applications. Mater. Res. Soc. Symp. Proc. 2001, 706, Z3.30.1-Z3.30.6. (2) Jra, J. G. S.; Deloziera, D. M.; Connella, J. W.; Watsonb, K. A. Carbon NanotubeConductive Additive-Space Durable Polymer Nanocomposite Films for Electrostatic Charge Dissipation. Polymer 2004, 45, 6133-6142. (3) Delozier, D. M.; Watson, K. A.; Smith, J. G.; Connell, J. W. Preparation and Characterization of Space Durable Polymer Nanocomposite Films. Compos. Sci. Technol. 2005, 65, 749-755. (4) Lei, X. F.; Chen, Y.; Zhang, H. P.; Li, X. J.; Yao, P.; Zhang, Q. Y. Space Survivable Polyimides with Excellent Optical Transparency and Self-Healing Properties Derived from Hyperbranched Polysiloxane. ACS Appl. Mater. Interfaces 2013, 5, 10207-10220. (5) Périchaud, A. A.; Iskakov, R. M.; Kurbatov, A.; Akhmetov, T. Z.; Prokohdko, O. Y.; Razumovskaya, I. V.; Bazhenov, S. L.; Apel, P. Y.; Voytekunas, V. Y.; Abadie, M. J. M. In High Performance Polymers-Polyimides Based-From Chemistry to Applications. INTECH Open Access Publisher, 2012; Chapter 11, pp 216-244.


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Table of Contents Graphic


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Table of Contents Graphic 83x34mm (300 x 300 DPI)


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Scheme 1. Illustration of UV-triggered self-healing of SiO2 microcapsules embedded into epoxy resin coating. 149x99mm (300 x 300 DPI)



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Scheme 2. Synthetic illustration of the final SiO2 microcapsules via a combined interfacial and in-situ polymerization route. 152x74mm (300 x 300 DPI)


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Figure 1. UV absorption spectra of cationic photoinitiators (PI6992) in propylene carbonate solution. 199x148mm (300 x 300 DPI)



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Figure 2. Optical images of (a) the initial emulsion droplets; (b) the intermediate SiO2 microcapsules with the thin shell obtained by the interfacial polymerization step (the inset is the SEM image of one intermediate microcapsule); (c) the final SiO2 microcapsules with the thick shell synthesized by the subsequent in-situ polymerization step (the inset is the SEM image of one final microcapsule); SEM images of (d) the intermediate microcapsules, (e) the surface morphology of the intermediate collapsed microcapsules and (f) one final broken microcapsule. 150x99mm (300 x 300 DPI)


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Figure 3. The as-synthesized SiO2 microcapsules: (a) SEM image; (b) SEM image of one broken microcapsule; (c) TEM image of microcapsule slice; (d) Particle size distribution plot of the microcapsules; (e) SEM image of one complete microcapsule; (f-h) the EDX elemental maps of C (f), Si (g) and O (h), respectively; (i) Color-change images of the ODB-2 solution in DMF with milled SiO2 microcapsules containing only epoxy (control sample, sample 1), and with our milled SiO2 microcapsules (sample 2), before and after exposure to UV radiation. 124x82mm (300 x 300 DPI)



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Figure 4. TG curves of (a) pure SiO2, (b) the as-synthesized SiO2 microcapsules, and (c) the oil mixture of the epoxy resin and photoinitiator. 286x202mm (300 x 300 DPI)


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Figure 5. Representative SEM images (a-d), particle size distributions (e-h) of SiO2 microcapsules and TEM images of ultrathin slices of SiO2 microcapsules (i-l) synthesized with the total HCl concentrations at (a), (e), (i) 0.08 mol/L; (b), (f), (j) 0.10 mol/L; (c), (g), (k) 0.12 mol/L; and (d), (h), (l) 0.14 mol/L, achieved by adding 2 mol/L HCl solutions (1600, 2000, 2400, and 2800 uL) in four equal portions, respectively. 166x124mm (300 x 300 DPI)



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Figure 6. (a) Time-dependent UV-vis absorption spectra of epoxy resin photo-cured by cationic photoinitiator (9 wt%), (b) the plots of the conversion (C0-Ct/C0) against the curing time (t) for the photopolymerization of epoxy resin with different concentrations of the cationic photoinitiator from 3 wt% to 15 wt%. 174x65mm (300 x 300 DPI)


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Figure 7. SEM images of the pristine microcapsules (a), the microcapsules after thermal cycling in the temperature range from -50 oC to 110 oC for 5 days (d), the scribe region of the self-healing coating containing the pristine microcapsules before (b) and after (c) UV irradiation for 30 min, and the scribe region of the self-healing coating containing the thermally cycled microcapsules before (e) and after (f) UV irradiation for 30 min. 122x81mm (300 x 300 DPI)


UV-Triggered Self-Healing of a Single Robust SiO2 Microcapsule

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