Polymeric Microcapsules with Sustainable Core and Hierarchical

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Materials and Interfaces

Polymeric Microcapsules with Sustainable Core and Hierarchical Shell towards Superhydrophobicity and Sunlight-Induced Self-Healing Performance Na Yang, Zi-Sheng Wang, Zhao-Yan Zhu, Si-Chong Chen, and Gang Wu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03374 • Publication Date (Web): 04 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

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Industrial & Engineering Chemistry Research

Polymeric Microcapsules with Sustainable Core and Hierarchical Shell towards Superhydrophobicity and Sunlight-Induced Self-Healing Performance Na Yang,a Zi-Sheng Wang,b Zhao-Yan Zhu,a Si-Chong Chen,a Gang Wu*a a

National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key

Laboratory of Polymer Materials Engineering, College of Chemistry, Sichuan University, Chengdu, 610064, China. E-mail: [email protected]; Fax: +86-28-85410259; Tel: +86-28-85410755 b

College of Letters & Science, University of California Santa Barbara, Santa Barbara, CA,

93111, United States. KEYWORDS: Epoxidized soybean oil, polymeric microcapsules, photopolymerization, superhydrophobicity, self-healing

ACS Paragon Plus Environment

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ABSTRACT: Photopolymerization is considered as one of most promising candidates for microcapsules-embedded self-healing system due to its significant advantages, e.g. fast, energy efficient and commonly economical. However, many microencapsulated photopolymerizable healing agents are usually expensive, unsustainable, or even environmentally hazardous. Herein, a polymeric microcapsule with hierarchical shell and low-cost, sustainable epoxidized soybean oil (ESO) as main core material was prepared through in-situ polymerization in oil-in-water emulsion. Mean diameter of microcapsules versus agitation rate is exponential decay and linear in double logarithm coordinates, indicating their structure can be controlled effectively. ESObased core materials are photopolymerizable under air upon sunlight or xenon lamp irradiation. Microcapsules-embedded epoxy coatings possess sunlight-induced self-healing anticorrosion. Besides, microcapsules have a good thermal stability, a long-term storage stability and a superior tolerance for attacks of light and water. Thanks to micro/nano-hierarchical structure on their surface resembled lotus leaf, microcapsules have a superhydrophobicity and microcapsulesbound coatings exhibit a self-cleaning property.


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INTRODUCTION Since the first autonomically healable/repairable system was presented by Jud et al.,1 selfhealing materials have drawn increasing attention and been extensively developed due to the concerns of service life and safety of materials.2-4 Up to now, two categories of self-healing materials have been reported according to the approaches of healing: (i) intrinsic ones involving to the reconstruction of chemical structures of the materials themselves by varieties of dynamic chemical bonds;5-8 (ii) extrinsic ones relying on the polymerizable/solidifiable healing agent which was released from the ruptured pre-embedded containers such as hollow fibers,9,10 microvasculature11-13 or capsules14-16 to fill the microcracks in materials.17 Compared with the intrinsic self-healing system, the pre-embedded approach has stimulated a particular interest attributing to the capable of healing large-volume cracks and no complicated or tedious chemical modifications for the original chemical structure of materials.18, 19 On behalf of the containers, microcapsules with a core-shell structure, micrometer size and spherical/irregular shape have been employed widely to endow the materials, especially coatings with the self-healing function.20 The first generation of microcapsules-type self-healing materials, i.e. so-called two-component system, consisted mainly of microencapsulated healing agent and catalyst particles or curing agent embedded directly into matrix.16, 21-25 However, a potential inactivation of the catalyst or curing agent due to chemical reactions with the materials matrix could lead to a repair failure in this system. Despite that microencapsulating catalyst or curing agent can effectively solve this issue,26, 27 the two-component system still have several restrictions, such as excessive cost, complicated process route and inhomogeneous distributions of two components in the materials,28, 29 which impedes its wide applications.


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Alternatively, a one-component self-healing system, in which a single microcapsule possesses the healing agent or a mixture of healing agent and latent catalyst/hardener, has been developed as a highly desirable candidate for practical application. Once the healing agent or its mixture with latent catalyst/hardener is released from the ruptured microcapsules, a polymerization is triggered for repairing the cracks by various stimuli, e.g., oxygen,30, 31 moisture,32-35 light,28, 36-38 and other.39 In particular, the ultraviolet (UV)/sunlight-induced self-healing system is fascinating because of its significant advantages, such as remote, cheap, and clean. However, the vast majority of microencapsulated healing agents are usually from petroleum products which are unsustainable, expensive, or even environmentally unsafe. Fortunately, some limited studies on microencapsulated low-cost and renewable plant oils as healing agents, e.g. Tung oil,40 linseed oil,30, 41 neem oil,42 have been reported over the last few years. Overall, there are many reports on the self-healing systems based on micro/nano containers,43, 44

such as polymeric micro/nanocapsules, pipelines and microvascular networks containing

various healing agents. Particularly, the polymeric microcapsules due to their easy preparation and storage, have been employed widely to endow the materials, especially coatings with the self-healing function. However, most of these microcapsules with ordinary shell as protective layer only have a single function of self-healing. More importantly, the microencapsulated healing agents are usually from unsustainable, expensive, or environmentally unfriendly petroleum products. Accordingly, it is quite desirable and a significant challenge to microencapsulate novel healing agents, especially from low-cost and eco-friendly sustainable sources, for the multifunctional applications.


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Figure 1. Schematic diagram of sunlight-induced self-healing of polymeric microcapsules with ESO-based core materials embedded into epoxy coatings. Herein, by in-situ polymerization in oil-in-water emulsion, we prepared a polymeric microcapsule (MC) containing epoxidized soybean oil (ESO) as main core materials. The ESObased core materials are rapidly photopolymerizable under sunlight irradiation in air. The structure of microcapsules (MCs) can be controlled effectively by adjusting the agitation rate. The MCs also possess a long-term storage stability and a superior tolerance for the attacks of heating, light and water. The micro/nano-hierarchical structure on the surface endowed MCs with a superhydrophobicity, which offers a self-cleaning capacity of the microcapsules-bound coatings. As illustrated in Fig. 1, a sunlight-induced self-healing coating was obtained via dispersing the MCs into epoxy matrix and presented a corrosion protection characteristic for iron substrate.

EXPERIMENTAL SECTION Materials and chemicals. Phenyl bis(2,4,6-trimethyl benzoyl) phosphine oxide (BAPO), tris(trimethylsilyl) silane (TTMSS), urea and resorcinol were purchased from J&K. Diphenyl


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iodonium hexafluorophosphate (Ph2I+) and 1,6-Hexamethylene diisocyanate (HDI) were supplied by TCI (Shanghai) Development Co., Ltd. Aqueous formaldehyde solution (37 wt%) and Ethylene-maleic anhydride (EMA) copolymer were purchased from Sigma-Aldrich. Hydrochloric acid, sodium hydroxide, sodium chloride, ammonium chloride, gum arabic (GA) and citric acid were provided by Kelong Reagent Corp. Dimethylformamide (DMF) and acetone were purchased from Zhiyuan Reagent Corporation. Epoxidized soybean oil (ESO), 1-octanol and o-cresyl glycidyl ether (o-CGE) were purchased from Aladdin. Epoxy resin (EPOLAM 5015) and hardener (EPOLAM 5015) were purchased from Axson. Natural dusts were obtained from the Sichuan University. Gypsum powder was bought from Taobao (China). Glass slides (25 mm×76 mm, Sail Boat Lab Co., Ltd.) and iron panels (50 mm×50 mm, the local market in Chengdu) were used as substrates. All the chemicals and materials were used without further purification. Preparation of microcapsules. Microcapsules were synthesized by in situ polymerization in oil-in-water emulsion. The main experimental procedures were as follows: firstly, 6.33 g of formaldehyde aqueous solution (37 wt%) was adjusted to pH 8.0 by using 1 mol L-1 NaOH aqueous solution. After adding 2.5 g of urea, the resulting solution was heated to 70 ℃ for a fixed time under magnetic stirring, and urea-formaldehyde pre-polymer was produced in this step. Secondly, 45.6 g of deionized water, 16.4 of EMA solution (2.5 wt%) and 0.25 g of resorcinol were added to a 250 ml beaker. Then the urea-formaldehyde pre-polymer solution was added to above mixture at stirring rate of 200 rpm at room temperature. The pH of the solution was adjusted to 1.7 with citric acid solution (pH=0.9). Thirdly, a certain amount of core materials was added to this system under various stirring rates at room temperature. Simultaneously, 3-5 droplets of 1-octanol were added to prevent the emergence of bubbles. The stirring rate was


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decreased to 200 rpm after 20 min, and the system kept this rate for 40 min at room temperature. Finally, microcapsules were obtained with continuous stirring rate for 1.5 h at 55 ℃. And then the system was cooled to room temperature. The obtained microcapsules were washed with deionized water, filtered and dried at room temperature. Instruments. The viscosity of core materials was measured by using viscometer (DV-Ⅱ, BROOKFIELD, USA) equipped with the rotor (No. 64) at the rate of 100 rpm. The light intensity was measured by using an optical power meter (CEL-NP2000, CEAULIGHT, China). Scanning electron microscopy (SEM) (JSM-5900LV, JEOL, Japan) was used to image microcapsules and the fracture surfaces of coatings. Energy-dispersive X-ray spectroscopy (EDS) was introduced to acquire the elemental mapping images by using SEM (SU3500, HITACHI, Japan). The chemical structure of shell, core materials, and microcapsules were characterized by Fourier transform infrared spectrophotometer (FTIR, Thermo Nicolet 6700, USA). The photopolymerization of core materials and coatings was conducted with a xenon lamp (PLS-SXE 300 UV, China; λ>320 nm) or natural sunlight (27 August 2018, Chengdu, China; light intensity is 24-31 mW cm-2), respectively. The thermal stability of ureaformaldehyde pre-polymer, shell, core materials, and microcapsules was measured by using thermogravimetric analysis (TGA) (TG 209 F1, NETZSCH, Germany) with a heating rate of 10 ℃ min-1 in the nitrogen atmosphere. Differential scanning calorimetry (DSC) (TA Instruments Q200, USA) was used to detect the thermal property of urea-formaldehyde pre-polymer with a scanning rate of 10 ℃ min-1. Mean diameter and size distribution of the microcapsules were measured by optical microscopy (CSW-3230A, China). The water static contact angles of microcapsules well-dispersed and adhered to the smooth glass slide using double-sided adhesive


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tape were obtained by contact angle testing system (Model DSA-100 TC40-MK1, Germany) with a droplet of 5 μL. Photopolymerization conversion of core materials. The photopolymerization conversion of core materials was calculated by using the following formula: Conversion (%)=(Wafter/Wbefore)×100 where Wbefore is the mass of pristine core materials before photopolymerization. Wafter is the mass of dry solid product which was obtained from core materials by separation, washing, and drying after photopolymerization. Swelling test of photopolymerized products of core materials. Core materials was polymerized to obtain cross-linked products under air upon natural sunlight or xenon lamp irradiation for 4 sec. After washed with acetone, products were dipped in DMF for 48h, and then dried in an oven. The gel content (G) and the degree of swelling (S) were calculated by using the following equations: G (%)=100×m2/ m0

S (%)=100×m1/ m2

where m0, m1, m2 are the masses of original sample, swollem sample and dried sample, respectively. Yield of microcapsules. The yield of microcapsules can be calculated by using the following equation: Yield (%)=100×Wm/(Wu+Wf+Wbefore) where Wm, Wu, Wf are the masses of microcapsules, urea and formaldehyde, respectively. And Wbefore is the mass of core materials before microencapsulation. Core content of microcapsules. Microcapsules were grinded by using a pestle in a mortar. Then, the broken microcapsules were washed with acetone several times and dried at room


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temperature. Accordingly, the core content of microcapsules was calculated as the following equation: Core content (%)=1-100×Wbm/Wpm where Wbm is the mass of the broken microcapsules, and Wpm is the mass of pristine microcapsules before the grind. Preparation of coatings and the accelerated corrosion tests. Specimens of coatings embedded with different diameters and amounts of microcapsules were prepared according to the following steps: firstly, a certain number of microcapsules prepared at a stirring rate of 1200 rpm, 1 g of epoxy resin, and 0.3 g of hardrner were mixed at RT. Then, the mixtures were degassed under vacuum for 15 min. The iron panels with dimensions of 50 mm×50 mm were polished by sand paper and rinsed with deionized water. After drying, the iron panels were coated using the above mixtures with different amounts of microcapsules to an average thickness of 0.59±0.06 mm (nearly 5 wt% of microcapsules), of 0.62±0.13 mm (nearly 10 wt% of microcapsules), and 0.64±0.04 mm (nearly 15 wt% of microcapsules), respectively. After curing for 2 days, uncoated areas of the iron panels were overlaid with waterproof adhesive tape. For comparison, a control specimen of pure epoxy coatings with the approximately thickness and size was fabricated and treated in the same manner. To evaluate self-healing anticorrosion, all of specimens were scratched manually on the surface of coatings by a razor blade, and then irradiated under sunlight or xenon lamp for 10 min. Subsequently, these specimens were immersed in 10 wt% NaCl solution for 48 hours at room temperature. Finally, the corrosion status was recorded by a digital camera. Self-cleaning experiments of coatings. MCs-3-bound epoxy coatings were prepared as follows. Gypsum powder and DI water were well mixed (mass ratio: 3/1), then the mixture was


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poured into mold (35 mm×25 mm×5 mm) and cured at room temperature for 24 hours to obtain plasterboards. The mixture of epoxy resin and hardrner was coated on the plasterboards and cured for 20 sec at 60℃. Afterwards, MCs-3 were well-dispersed and bound onto the coatings. Finally, the specimens were cured for 24 hours. The self-clean ability of epoxy coatings with or without MCs was tested by pouring dusts on their surfaces and then dropping water droplets on top portion with a tilted angle of about 10°. The self-cleaning process was recorded by a digital camera.

RESULTS AND DISCUSSION Thanks to the silyl radical (R3Si.) based photoinitiating chemistry which avoids an oxygen inhibition via the oxygen consumption, the photopolymerization of renewable epoxy monomers such as epoxidized soybean oil (ESO) under sunlight exposure in air, has been developed for the demand of low-energy and eco-friendly green chemistry.45, 46 Such the photopolymerization, i.e. so-called free radical promoted cationic polymerization (FRPCP), needs specific photoinitiating system which typically involves with three components: a light-absorbing molecule (LAM) as a radical source, a silane to supply silyl radicals, and an iodonium salt to oxidize the silyl radical into a silylium cation. For instance, with phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO)

as

the

LAM,

tris-(trimethylsilyl)silane

(TTMSS),

and

diphenyl

iodonium

hexafluorophosphate (Ph2I+) (1/3/1% w/w), tack free coatings of ESO can be obtained after 45 min of sunlight exposure in air.46 However, a direct microencapsulation of the pure ESO is difficult and still a challenge due to a high viscosity of 1070.8±16.6 mpa.s. In this study, core materials consist of ESO, hexamethylene diisocyanate (HDI), o-cresyl glycidyl

ether

(CGE),

and

the

photoinitiating

system


with

a

fixed

component

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(BAPO/TTMSS/Ph2I+=1/3/1% w/w). Among of them, CGE and HDI could be served as reactive diluents to reduce the viscosity of ESO as well as improve the polymerization efficiency. Moreover, HDI can react with the hydrophilic groups of shell, which could endow microcapsules with a hydrophobicity (vide infra).47 In view of flowability and healing efficiency, an optimum mass ratio of core materials was confirmed by comparison of the photopolymerization conversion and viscosity of core materials. As shown in Table 1, the optimum formula of ESO/CGE/HDI was a mass ratio of 6/3/1. This formula of the core materials showed the highest photopolymerization conversion up to 50.1%, and had the desirable viscosity (140.4±2.4 mpa.s) which was beneficial for facilely encapsulating the core materials into microcapsules with suitable sizes. As expected, the photopolymerization conversion and the viscosity of core materials were dramatically regulated by adding CGE and HDI. Table 1. The conversion of different mass ratio of core materials upon sunlight exposure for 100 seconds (light intensity: 24-31 mW cm-2).

Sample

ESO/CGE/HDI (w/w/w)

η (%)

Viscosity (mpa.s)

1

10/0/0

0

1070.8±16.6

2

9/0/1

0

626.3±4.4

3

8/0/2

0

292.7±2.4

4

7/0/3

0

119.1±3.0

5

7/1/2

9.4

166.2±2.9

6

7/2/1

35.1

275.0±8.0

7

6/0/4

0

72.6±2.9

8

6/1/3

3.3

90.0±3,.2

9

6/2/2

23.2

100.2±1.5

10

6/3/1

50.1

140.4±2.4


11


11

5/4/1

18.7

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75.1±0.9

FTIR spectroscopy is used to monitor the photopolymerization of core materials with the optimum formula. As shown in Figure 2a and Figure S1, with increasing the exposure time, a characteristic signal of the epoxy absorption at 841 cm-1 gradually weakened because of the consumption of epoxy groups by ring-opening reaction, and an enhanced signal at 1245 cm-1 assigned to C-O bonds was observed due to the formation of polyether which reflected the efficient

polymerization

of

ESO

by

the

sunlight

induction.

The

corresponding

photopolymerization curves were given in Figure 2b. Upon sunlight exposure for only 6 min, the conversion of approximate 70% of core materials to obtain cross-linked polymer was achieved, which was remarkably faster for reaching the same conversion than some present photopolymerizable healing agents.36, 38 In addition, under xenon lamp, the photopolymerization of core materials was obviously more efficient due to the high light intensity of this irradiation device. As shown in Figure 2b, the conversion of core materials was up to over 60% when only irradiated for 30 sec. Moreover, we investigated the effect of the irradiation intensities of xenon lamp on the photopolymerization conversion of core materials under a fixed irradiation time of 20 sec. As presented in Figure S2, with increasing the irradiation intensity from 55 to 240 mW cm-2, the photopolymerization conversion was increased sharply from 37% to 70.8%, while to further enhance the irradiation intensity did not lead to the obvious increase of the conversion. A swelling test of cross-linked polymers was carried out to investigate the influence of the light intensity on the cross-linking degree of the photopolymerized products. The corresponding data were listed in Table S1. It can be found, with increasing the light intensity of both sunlight and xenon lamp, that gel content (G) increased and degree of swelling (S) decreased, respectively,


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which could indicate that the high light intensity resulted in the high cross-linking degree of photopolymerized products.

Figure 2. (a) FTIR spectra of the core materials (ESO/CGE/HDI=6/3/1) after different sunlight exposure times in air, (b) curves of core materials conversion versus irradiation time upon sunlight (light intensity: 24-31 mW cm-2) and a xenon lamp (light intensity: 55 mW cm-2) exposure in air.

Table 2. The influence of stirring rate on the resultant microcapsules Agitation rate

Core content

Mean diameter

Yield

(rpm)

(%)

(μm)

(%)

MCs-1

600

75.0

257.7±78.2

74.1

MCs-2

800

73.1

121.2±30.0

64.7

MCs-3

1000

72.6

79.3±37.4

58.6

MCs-4

1200

67.0

63.8±18.7

43.9

Sample

As depicted in Table 2, a series of poly(urea-formaldehyde) (PUF) microcapsules were prepared by adjusting agitation rate from 600 to 1200 rpm. The results show that both core


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content and yield decreased gradually with the increase of agitation rate. The reasons could be as follow: smaller oil droplets were produced at high stirring rates, which resulted in an easier destruction of microcapsules; some tiny microcapsules did not be collected completely in the process of filtration and washing, leading to the low yield. By scanning electron microscopy (SEM), the morphology of microcapsules was observed (Figure 3). The results show that the microcapsules obtained at all agitation rates have nearly spherical shape (Figure 3a1-d1), scraggly and rough hierarchical surface caused by aggregation and deposition of PUF particles (Figure 3a2-d2 and 3a3-d3), and core-shell structure (Figure 3a4-d4). The mean diameter of the resultant microcapsules at 600 (MCs-1), 800 (MCs-2), 1000 (MCs-3) and 1200 rpm (MCs-4) were 257.7±78.2, 121.2±30.0, 79.3±37.4 and 63.8±18.7 μm, respectively, and diameter distribution narrowed with increasing the agitation rate (Figure 3e). In addition, the relationship between mean diameter and agitation rate was exponential decay and linear in the double logarithm coordinates (Figure 3f), according well with the previous reports.37 To identify the chemical structure, FTIR tests of broken microcapsules (MCs), shell materials, pristine core materials, and core materials extracted from microcapsules were performed and the spectra are provided in Figure S3. The absorption peaks of shell materials at 3770-3070 cm-1, 1641 cm-1, and 1548 cm-1 are assigned to N-H and O-H, C=O, and C-N, respectively, which supports the PUF chemical composition. In addition, the spectrum of the collected core materials is nearly identical with the pristine core materials, reflecting that the core materials still possess a certain activity after microencapsulation. Overall, the main absorption peaks of shell materials and photopolymerizable core materials were observed in the spectrum of the broken MCs, indicating that the mixture of ESO, CGE, and HDI was successfully encapsulated into PUF microcapsules.


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Figure 3. SEM images of microcapsules prepared at different stirring rates: (a1-a4) 600 rpm, (b1-b3) 800 rpm, (c1-c2) 1000 rpm, and (d1-d3) 1200 rpm. (e) Diameter distributions of microcapsules and (f) curve of their mean diameter vs agitation rate in the double logarithm coordinates.


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Figure 4a presents the TGA curves of pristine core materials, collected core materials from broken microcapsules, MCs, and shell materials. The weight loss of shell and MCs under 100 °C is mainly attributed to the release of entrapped residual water and formaldehyde.48 The decomposition of the PUF could contribute to the weight reduction of MCs and shell between 200 °C and 340 °C. And the weight loss of shell in the range of 370-500 °C is likely to indicate the presence of a cross-linked structure with the higher thermal stability. For the core and MCs, in the ranges of 100-215 °C and 320-480 °C, the weight losses are mainly due to the evaporation of HDI and the removal of epoxy monomer, respectively. The temperature of 5% weight loss of the MCs is about 138 °C, and basically, there is no obvious pyrolysis below 200 °C, indicating that the prepared MCs have a good thermal stability. In addition, after keeping the MCs in a covered transparent glass vial at ambient temperature for 40 days, the TGA curve is almost identical with that of the initial MCs, demonstrating the long shelf life of MCs (Figure S4). In order to further investigate the stability of MCs, the MCs-2 were treated by various methods, i.e. irradiation under a xenon lamp for 20 min (denoted as Irradiated-MCs-2), immersion in DI water for 4 hours (denoted as Water-soaked-MCs-2), and storage at ambient temperature for 6 months (denoted as Aging MCs-2). According to the results of optical microscopy (Figure 4b1-b4), it is distinctly visible that a lot of liquid were released from the ruptured MCs which were treated at different conditions. Indeed, comparing with the pristine MCs-2, the core contents of the treated MCs were just reduced slightly as shown in Table S2. In addition, the spectrum of the core materials for the treated MCs were similar to that of pristine MCs-2 (Figure 4c and S5), reflecting that the core materials of them should still possess the chemical activity. Consequently, the above results reveal that MCs are high tolerant for the attack of light and water and have a long-term storage stability.


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Figure 4. (a) TGA curves of pristine core, core, MCs, and shell. Optical microscopy images of ruptured MCs: (b1) Pristine MCs-2, (b2) Aging MCs-2, (b3) Irradiated-MCs-2; and (b4) Watersoaked-MCs-2. (c) FTIR spectra of core materials of ruptured MCs.


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Figure 5. Photos of corrosion test results for iron panels covered with various coatings: coatings after immersion, (a1) 5 wt% of MCs-3, (b1) 10 wt% of MCs-3, (c1) 15 wt% of MCs-3, (d1) 15 wt% of MCs-1, (e1) 15 wt% of MCs-2; iron panel after immersion, (a2) 5 wt% of MCs-3, (b2) 10 wt% of MCs-3, (c2) 15 wt% of MCs-3, (d2) 15 wt% of MCs-1, (e2) 15 wt% of MCs-2. SEM images of scratched region of coatings surface: (a3) 5 wt% of MCs-3, (b3) 10 wt% of MCs-3, (c3) 15 wt% of MCs-3, (d3) 15 wt% of MCs-1, (e3) 15 wt% of MCs-2. The self-healing anticorrosion performance of epoxy coatings was evaluated through corrosion test. Before immersed in 10 wt% NaCl solution for 48 h, epoxy coating were scratched by a razor blade and then exposed to xenon lamp for 10 min in air (Figure S6). The specimens with 10 wt% and 15 wt% of MCs-3 were free from rust (Figure 5b1 and c1), and no obvious corrosion was found on the corresponding steel panel after peeling off the self-healing coatings (Figure 5b2 and c2). However, a little of rust can be observed on the scribed regions of the coatings with 5 wt% of MCs-3 (Figure 5a1 and a2). A conceivable mechanism of anticorrosion should be that the released core materials from the ruptured MCs fill and heal the crack automatically by the


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photopolymerization, which retard the corrosion of the steel panel. Indeed, according to the SEM images, newly formed materials were observed on the crack, and accumulated compactly with the increase of MCs from 5 wt% to 15 wt% (Figure 5a3, b3, and c3). The results also reveal the reasons why the coatings with 15 wt% of MCs possessed the best corrosion resistance. In contrast, for a control specimen after immersion (vide infra, Figure 7a1 and 7a2), several rusts can be found at scratch areas, and severe corrosion on the surface of iron panel was visible. Subsequently, the influence of MCs’ size on self-healing anticorrosion performance of the coatings was investigated. The slight rust was observed on the scribed regions of the coatings containing MCs-1 (Figure 5d1 and S6d), while the rust was almost invisible for the coatings embedded with the MCs-3 (Figure 5c1) and MCs-2 (Figure 5e1 and S6e), respectively. Although there was no obvious corrosion on the steel panel after peeling off the coatings of three specimens (Figure 5c2, d2, and e2). Thus, we infer that the small MCs was beneficial to completely repair the cracks of coatings because of their larger amounts in unit volume comparing with the small MCs, supporting by SEM (Figure 5c3, d3, and e3). As previously stated (Figure 4), the MCs have a superior stability under various harsh conditions, such as heating, light irradiation, and the attack of water. Practically, after suffering from xenon lamp irradiation for 20 min (denoted as irradiated-MCs-3) and water immersion for 4 hours (denoted as water-soaked-MCs-3), respectively, MCs still retained highly healing activity for the cracks of coatings. Thanks to the repair of cracks by the photopolymerization of the released core materials (Figure 6a3 and b3), it can be found that both of specimens after the accelerated corrosion test were nearly rustless on the coatings and no corrosion on the steel panel (Figure 6a1-a2, b1-b2, S7a, and S7b). In addition, the undamaged specimen after irradiation under xenon lamp for 20 min (denoted as irradiated-unbroken-coatings) showed a good corrosion


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resistance yet (Figure 6c1-c3, and S7c), suggesting that the endowed self-healing performance of the MCs-embedded coatings was high enduring for the light exposure.

Figure 6. Photos of corrosion test results for iron panels covered with various coatings: coatings after immersion, (a1) irradiated-MCs-3, (b1) water-soaked-MCs-3, (c1) irradiated-unbrokencoatings; iron panel after immersion, (a2) irradiated-MCs-3, (b2) water-soaked-MCs-3, (c2) irradiated-unbroken-coatings. SEM images of coatings’ scratched region: (a3) irradiated-MCs-3, (b3) water-soaked-MCs-3, (c3) irradiated-unbroken-coatings. The content of MCs-3 was 15 wt%. In terms of the practical application, it is more important to evaluate the self-healing anticorrosion of the MCs-embedded coatings under sunlight exposure. As shown in Figure S8, the coatings were scratched by a razor blade, and exposed to sunlight in the open air for 10 min. Then, all of specimens were immersed in 10 wt% NaCl aqueous solution at room temperature for 48 hours. After immersion, several rusts can be found at the scratch areas of the control specimen


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(Figure 7a1), and there was severe corrosion on the surface of iron panel (Figure 7a2). In contrast, the rust was almost invisible for both of the specimens with 10 wt% and 15 wt% of MCs-3 (Figure 7b1 and c1), and no obvious corrosion was found on the corresponding steel panel peeled off the self-healing coatings (Figure 7b2 and c2).

Figure 7. Photos of corrosion test results for iron panels covered with various coatings: coatings after immersion, (a1) control specimen, (b1) specimen with 10 wt% MCs-3, (c1) specimen with 15 wt% MCs-3; iron panel after immersion, (a2) control specimen, (b2) specimen with 10 wt% MCs-3, (c2) specimen with 15 wt% MCs-3. SEM and EDS elemental mapping images of the scratched regions of control coatings (d1-d4) and 15 wt% MCs-embedded (e1-e4) coatings. The SEM images showed that the cracks of the control and MCs-embedded coatings seem to be stuffed with some objects (Figure 7d1 and e1). However, the EDS elemental mapping images of the scribed regions of coatings reveal some obvious differences between control and MCsembedded specimens. For the control specimen (Figure 7d2-d4), the coating was mainly composed of carbon, iron, and oxygen elements, and the intensively distributed areas of both O and Fe elements with a highly coincident profile overlaid on the scribed regions, confirming the presence of the rust. In regard to the MCs-embedded specimen, there were the abundant C and O


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elements on the entire surface of the coating (Figure 7e2 and e3) deriving from the epoxy matrix and the photopolymerized core materials, while the Fe element was almost unobservable (Figure 7e4). The results demonstrate that the MCs-embedded coatings possessed the satisfyingly sunlight-induced self-healing feature, and thus can endow iron panel with a good corrosion protection.

Figure 8. Photos of water droplet on microcapsules with different sizes: (a) MCs-1, (b) MCs-2, (c) MCs-3, and (d) MCs-4. (e1-e3) Photos of self-cleaning experiment of MCs-4. In addition, as illustrated in Figure 8a-d (Note: the well-dispersed microcapsules were adhered to the smooth glass slide using double-sided adhesive tape. So the substrate in Figure 8a-d looks to be flat.), the water static contact angle (CA) measurements show that the resultant MCs with different sizes obtained at different stirring rates are hydrophobic and have the CA of over 140°, while the MCs without hierarchical shell (Figure S9) or HDI (Figure S10) showed a lower CA, revealing that both of the hierarchical structure and the HDI are important for enhancing the hydrophobic property of microcapsules. In particular, the CA of the MCs-4 is 154.5±3.5° (Figure


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8d), indicating a good superhydrophobicity and proposing a latent self-cleaning capacity for dirt.47, 49 Indeed, as deionized water was dropped gradually at the higher end of a tilted glass slide with a tilt angle of about 10° which was covered with MCs, the dusts as dirt overlying on the surface of MCs were efficiently and completely rolled down from the surface of the tilted glass slide together with the water droplets (Figure 8e1-e2). Finally, the surface returned to the original state (Figure 8e3). Considering the self-cleaning buildings application, we prepared a self-cleaning plasterboard covered with a MCs-3-bound epoxy coating. As shown in Figure S11, the CA of the self-cleaning plasterboard is 152.0±2.1° and far higher than that of a control plasterboard covered with a pure epoxy coating (75.3±2.0°). Furthermore, the self-cleaning ability was assessed. As presented in Figure 9, when water was dropped at higher end of plasterboards which tilted about 10°, the dusts were efﬁciently and completely rolled down together with the water droplets from the MCs-3-bound epoxy coating surface (Figure 9a1-a3), while the dirt particles on the surface of the control coatings cannot be removed completely (Figure 9b1-b3), indicating that the superhydrophobicity of MCs is a merit for constructing the self-cleaning surfaces.


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Figure 9. Photos of self-cleaning experiment of (a1-a3) MCs-3-bound epoxy coating and (b1-b3) pure epoxy coating covering on the plasterboards, respectively.

CONCLUSIONS In summary, through the facile one-pot strategy, i.e. in situ polymerization in oil-in-water emulsion, we successfully microencapsulated the sunlight-induced polymerizable ESO-based core materials by using the PUF as shell materials. The results show that the mean diameter, core content, and yield of MCs versus can be controlled effectively by adjusting the agitation rate. Thanks to the silyl radical promoted cationic polymerization, it was the approximate 70% yield to convert the liquid core materials into cross-linked polymer after 6 min of sunlight exposure under air or up to over 60% yield after 30 sec of xenon lamp exposure under air. The initial evaporation temperature of MCs (near 5% of weight loss) was about 138°C, and no obvious pyrolysis below 200 °C was observed, indicating a good thermal stability. Importantly, MCs which retained highly healing activity showed a long-term storage stability (6 months) and a superior tolerance for the attacks of light (20 min of xenon lamp irradiation with the light intensity of 850 mW cm-2) and water (4 hours of DI water immersion). The photopolymerization-induced self-healing anticorrosion coatings were prepared by dispersing the MCs into the epoxy matrix. The accelerated corrosion experiment in 10 wt% NaCl solution at ambient temperature reveals that the scratched areas of the iron panel coated with MCsembedded epoxy coatings after exposing to the xenon lamp or sunlight were almost completely rust free for 48 h immersion, clearly demonstrating the excellent corrosion protection. In addition, due to the micro/nano-hierarchical structure on the surface resembled a lotus leaf, the MCs possessed the satisfying superhydrophobicity, and the MCs-bound epoxy coatings exhibited the


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self-cleaning property. We anticipate that our microcapsules provide a promising candidate for convenient self-healing system.

ASSOCIATED CONTENT Supporting Information. Including FTIR spectra of the core materials after different sunlight exposure times in air, TGA curves of MCs, and FTIR spectra of core materials of ruptured MCs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants: 51503026) and the Fundamental Research Funds for the Central Universities (Grants: 2017SCU04A14; YJ201821). REFERENCES (1) Jud, K.; Kausch, H. H.; Williams J. G. Fracture-mechanics studies of crack healing and welding of polymers. J. Mater. Sci. 1981, 16, 204. (2) Wu, D. Y.; Meure, S.; Solomon, D. Self-healing polymeric materials: a review of recent developments. Prog. Polym. Sci. 2008, 33, 479. (3) Wool, R. P. Self-healing materials: a review. Soft Matter. 2008, 4, 400.


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Polymeric Microcapsules with Sustainable Core and Hierarchical

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