Microcapsule-Type Organogel-Based Self-Healing System Having

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Microcapsule-type organogel-based self-healing system having secondary damage preventing capability Hye-In Yang, Dong-Min Kim, Hwan-Chul Yu, and Chan-Moon Chung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02118 • Publication Date (Web): 12 Apr 2016 Downloaded from http://pubs.acs.org on April 16, 2016

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ACS Applied Materials & Interfaces

Microcapsule-type organogel-based self-healing system having secondary damage preventing capability Hye-In Yang, Dong-Min Kim, Hwan-Chul Yu* and Chan-Moon Chung* Department of Chemistry, Yonsei University, Wonju, Gangwon-do 220-710, Republic of Korea KEYWORDS. organogel, viscoelasticity, microcapsule, self-healing coating, prevention of secondary damage

ABSTRACT. We have developed a novel microcapsule-type organogel-based self-healing system in which secondary damage does not occur in the healed region. A mixture of an organogelator, poor and good solvents for the gelator is used as the healing agent; when the good solvent evaporates from this agent, a viscoelastic organogel forms. The healing agent is microencapsulated with urea-formaldehyde polymer, and the resultant microcapsules are integrated into a polymer coating to prepare self-healing coatings. When the coatings are scratched, they self-heal, as demonstrated by means of corrosion testing, electrochemical testing, optical microscopy and scanning electron microscopy (SEM). After the healed coatings are subjected to vigorous vibration, it is demonstrated that no secondary damage occurs in the healed region. The secondary damage preventing capability of the self-healing coating is attributable to

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the viscoelasticity of the organogel. The result can give insight into the development of “ permanent” self-healing system.

Introduction

Recently, self-healing materials have attracted great attention because the ability to selfrepair can improve public safety, extend lifetime of materials, enhance energy efficiency and reduce maintenance cost.1-8 Microcapsule-type self-healing systems have been extensively studied because of their ease of preparation and applicability in a variety of matrix materials. Notably, many self-healing protective coatings that have been reported so far are microcapsulebased systems.9-13 When this type of coating is damaged, a healing agent flows out from broken microcapsules and fills the damaged region. Conventional microcapsule-type self-healing coatings incorporate healing agents that solidify through polymerization during such events. Basically, these systems are limited to single local self-healing event:1 when damage occurs again in the healed region, the self-healing cannot be repeated, not only because the microcapsules are depleted but also because the solidified healing agent is a hard solid with no additional self-healing ability. Self-healing protective coatings that can be potentially used in bridges, tunnels, buildings and vehicles may be exposed to various vibrations induced by heavy traffic, rock blasting, machine operation, vibratory soil compactors, and so forth. It is well known that vibrations can cause damage such as microcracking and crazing.14 Under the mechanical stress, secondary damage can occur in healed regions where a healing agent is converted into a hard material. The


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secondary damage may occur in the solidified healing agent or in boundary between the healing agent and matrix. To address this issue, we have previously developed repeatable self-healing microcapsule-type protective coatings.15 In the present study we developed a more advanced microcapsule-type self-healing system that is highly resistant to secondary damage in the healed region. This approach can give an insight in “permanent self-healing” of a damaged material. When our self-healing coating is damaged, a healing agent (an organogel precursor material) flows out from broken microcapsules, fills the damaged region and turns into an organogel (i.e., the first damage is healed). In this system, the healing agent forms a viscoelastic organogel rather than a hard solid. An organogel is typically comprised of an organogelator and an organic liquid, 16-25 and able to maintain their shapes under limited directional stress due to its viscoelasticity.26 Therefore, we anticipated that an organogel would not be damaged by stress such as vibration (i.e., secondary damage does not occur). Furthermore, even if scratched, it may self-heal due to their viscoelasticity.15 The organogel-based self-healing system can be applied in protective coatings for metals, ceramics and polymers. In the case of self-healing protective coating, the recovery of barrier property would be more important than that of mechanical strength.

Results and Discussion

In most cases, conventional organogels have been prepared by heat-treatment: heating produces a homogenous solution of gel precursor, which then undergoes gelation upon cooling. 16-24,26

But in the present work, we designed a gel precursor material that can form a gel when

one of its constituents evaporates. This type of gelation system would be more suitable for use as


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a self-healing agent than a conventional heating–cooling gelation system because the new system has the advantage of autonomous healing at ambient temperature without heat treatment. Our healing agent is composed of an organogelator (hexatriacontane, C36) (Figure S1), poor and good solvents for C36. We thought that, when the good solvent evaporates, the organogelator would gel the remaining poor solvent. C36 was chosen because it can gel a variety of liquids. 27 In order to select proper solvents, we tested various poor and good solvents for C36 and measured their critical gel concentrations (CGCs) (Table 1). Cyclohexane was selected among the good solvents because it is volatile (boiling point = 81°C) and had the highest CGC (3.5 wt%) among the good solvents tested. The use of a high-CGC solvent can prevent the gel precursor material (i.e., the core material) from gelation during microencapsulation because a certain amount of solvent can evaporate during the encapsulation. Tetramethyl tetraphenyl trisiloxane (MPS) (Figure S1) was selected among the poor solvents because it is a nonvolatile liquid with a high boiling point (215°C), and had the lowest CGC (1.5 wt%) among the poor solvents tested. A poor solvent with the lowest CGC would undergo gelation most easily when the good solvent evaporates. Figure 1b shows an organogel obtained through the evaporation of cyclohexane from a solution composed of MPS, cyclohexane and C36 (Figure 1a). To find a proper composition of healing agent, we studied the gelation properties of healing agent candidates of different composition; Table 2 lists the results. A solution of 0.5 wt% C36 in cyclohexane was mixed with solutions of 0.5, 1, 1.5 and 2 wt% C36 in MPS in a 50/50 volume ratio to obtain four different C36-containing mixtures (samples A, B, C and D, respectively). When the solutions are just mixed, samples A and B are homogeneous solutions (Figure 1a) whereas samples C and D are partial gels. Upon cyclohexane evaporation, sample A underwent partial gelation and was still able to flow (Figure 1c and Figure S2a), whereas sample


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B turned into a turbid gel that did not flow (Figure 1b and Figure S2b). Because samples C and D partially gelled upon mixing the solutions, they are unsuitable for use as core material (i.e., healing agent): these samples would likely undergo gelation through the evaporation of a certain amount of cyclohexane during microencapsulation, and thus would be unlikely to flow out of ruptured microcapsules. Based on these results, in the present study we used the sample B composition as the core composition. The organogel (sample B gel in Figure 1b) formed from sample B solution through cyclohexane evaporation was observed by means of optical microscopy (Figure 1d); its rough appearance was consistent with previous findings that C36 gelator molecules in other organogels crystallize into thin microplatelets with irregular edges.28 It is known that junctions between the plate edges allow a rigid three-dimensional superstructure to assemble, preventing solvent from flowing. Figure S3 shows the mechanical spectrum at 25 °C for sample B gel, which reveals that the organogel is viscoelastic.29-32 For this system, the loss modulus G" values are higher than the storage modulus G' values over the frequency window used in the measurement. It has been shown that civil engineering structures such as bridges are usually exposed to traffic-induced vibration of 2 to 10 Hz.14 When the sample B gel was subjected to more vigorous vibration of 10 to 30 Hz for 1 h using a vibration tester, no damage was observed in the sample. The result of the vibration test implies that microcracking or crazing does not occur in the organogel due to its viscoelasticity under the vibration condition. On the other hand, the sample B gel was scratched with a razor blade to investigate its scratch self-healing ability. Micrographs of the organogel show that the scratch was self-healed in 10 min (Figure S4). The sample B solution, as a core material, was microencapsulated using ureaformaldehyde (UF) polymer as a shell material. The core/shell ratio strongly influenced the


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morphology of the resulting microcapsules (Figure 2). Using a core/shell ratio of 5.2:1 yielded a large amount of UF particles, which conglutinated with the microcapsules (Figure 2a). That is to say, UF particle formation was favored over UF encapsulation of the core material; this was probably due to the small surface area of the core droplets. The use of a core/shell ratio of 6.5:1 yielded spherical microcapsules, and no significant UF particle was observed (Figure 2b). The use of a higher core/shell ratio of 8.5:1 also yielded microcapsules (Figure 2c), but their shells were so thin that they easily collapsed; this arose because of the excessive surface area of the core droplets. Based on these results, in this study we decided to use the core/shell ratio of 6.5:1. Scanning electron microscopy (SEM) observations of the resulting spherical microcapsules showed that their surfaces were rough (Figure 2d). The size distribution of the microcapsules was investigated; the average capsule diameter was measured to be 350 µm (Figure 3). After storage of the microcapsules for 12 months in the temperature range of 10-25℃, the core still flows out of ruptured capsules and undergoes gelation. This might mean that the capsules can operate in the temperature range for at least 12 months. The microcapsules were integrated into a commercial epoxy coating matrix to prepare self-healing coatings. When the self-healing coating was scratched with a razor blade, the core material was released and filled the scratched region (Figure S5). The evaporation behavior of cyclohexane from the released core was investigated by using the precision balance of a thermogravimetric analysis (TGA) instrument (Figure 4a). Because MPS and C36 have practically no volatility at room temperature, it was assumed that cyclohexane would only evaporate from the core. The selective evaporation of cyclohexane was confirmed by 1H NMR spectroscopy (Figure S6). After rupture of the microcapsules, most of cyclohexane evaporated within 2 min as shown in Figure 4a.


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It was observed by optical microscopy that gelation occurred within 2 min after scratching of the organogel-based self-healing coating. The healing agent that flowed out of the ruptured microcapsules was observed: just after scratching, the healing agent looked like a liquid (Figure 4b) and after 2 min it turned into a material having many microplatelets (Figure 4c).28 The appearance of this material was similar to that of the gel obtained from the unencapsulated healing agent (Figure 1d, sample B gel). This indicates that, after the healing agent filled the scratched region, it turned into a gel through solvent evaporation. The self-healing and secondary damage prevention functions of the organogel-based self-healing coating system were evaluated by means of steel corrosion testing (Figure 5). The microcapsules filled with the gel precursor material were integrated into a commercial epoxy coating matrix to prepare self-healing coatings on steel panels. For comparison, we employed methacryloxypropyl-terminated polydimethylsiloxane (MAT-PDMS), a healing agent that generates a hard solid through photocrosslinking;33 MAT-PDMS was microencapsulated to prepare comparison self-healing coatings. The self-healing coating samples were damaged by scratching using a razor blade; it was confirmed that each cut was deep enough to reach the surface of the steel substrate. Control coatings without microcapsules were also scratched. The scratched samples were healed and then immersed in 5 wt% aqueous NaCl solution for 72 h. All the control samples corroded, but the self-healing coatings showed no visual evidence of corrosion. The scratched, healed and immersed self-healing coatings were then subjected to vibration (10 to 30 Hz) for 1 h, and immersed in the NaCl solution again. All of the MAT-PDMS self-healing samples corroded, but the organogel-based self-healing coatings showed no visual evidence of corrosion. Comparison tests were conducted without the first immersion, and the same results were obtained. Repeated corrosion tests demonstrated good reproducibility.


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Electrochemical testing was conducted to provide further evidence of the secondary damage prevention capability of the organogel-based self-healing coatings (Figure 6). The tests were performed in a conventional three-electrode electrochemical cell; the coated steel panel served as the working electrode (Figure S7). The steady-state conduction between the coated metal substrate and a counter electrode held at 3 V through an aqueous electrolyte (1 mol/L NaCl) was measured. The scratched control coating showed a current flow of 12 mA, whereas no current flowed through the organogel-based and MAT-PDMS-based samples after scratching and healing. These results indicate that efficient self-healing occurred in both organogel- and MATPDMS-based coatings. After scratching, healing and subsequent vibration, however, the current flow through the MAT-PDMS-based coating increased to 0.6 mA, whereas the organogel-based coating still did not pass current. These results imply that the vibration caused secondary damage in the MAT-PDMS-based coating, and the damage could not self-heal; contrastingly, vibrationinduced secondary damage was evidently prevented in the organogel-based coating. Repeated electrochemical tests exhibited good reproducibility. SEM imaging of the scratched regions in the coated samples was carried out (Figure 7). The organogel-based self-healing coating was scratched and stored at room temperature for 1 day to induce gelation of the healing agent that flowed out of the broken capsules; the MATPDMS-based self-healing coating was also scratched and photoirradiated to induce photocrosslinking. SEM observations of the scratched area in the self-healing coatings clearly confirmed that the healing agent filled the scratched area, indicating effective healing. As expected, the damaged area in the control coating remained unfilled. After applying vibration to the scratched and healed self-healing coatings, secondary damage occurred in the healed region of the MAT-PDMS-based coating (Figure 7). It was


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concluded that the solidified MAT-PDMS polymer filling the scratched region was broken, and some fragments of the polymer fell out upon vibration. In contrast, the healed region of the organogel-based coating did not undergo secondary damage. This was probably due to the viscoelasticity of the gel filling the scratched region. As mentioned above, the vibration test for the organogel itself showed that no damage occurred in the sample. This result would preclude the possibility of secondary damage occurring in the healed region and subsequent self-healing of the damage. On the other hand, the SEM observation indicates that entire region of the organogel-based coatings were not damaged under the vibration conditions.

Conclusion

We have developed a new microcapsule-type organogel-based self-healing system that is highly resistant to secondary damage in the healed region. A combination of C36, MPS and cyclohexane was used as the healing agent; when the cyclohexane evaporates from this agent, a viscoelastic organogel forms. The healing agent was microencapsulated with UF polymer, and the resultant microcapsules were integrated into a polymer coating to prepare a self-healing coating. When the coatings were scratched, they self-healed, as demonstrated by means of corrosion testing, electrochemical testing and SEM observations. After the healed coatings were subjected to vigorous vibration, no secondary damage was observed. The secondary damage preventing capability of the organogel-based self-healing coating is attributable to the viscoelasticity of the organogel. The results can give insight into the development of “permanent” self-healing system. Our new self-healing system can be applied to self-healing protective coatings for the protection of bridges, tunnels, transportation systems and machinery that


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experience vigorous and steady vibrations.

Experimental Section

Materials: Urea, aqueous formaldehyde solution (37 wt%), poly(ethylene-alt-maleic anhydride) (EMA), resorcinol, 1-octanol, benzoin isobutyl ether (BIE) and hexatriacontane (C36) were purchased from Sigma-Aldrich. Methacryloxypropyl-terminated polydimethylsiloxane (MAT-PDMS) (molecular weight 380–550 g/mol, viscosity 4–6 cSt) was purchased from Gelest. Ammonium chloride was purchased from Duksan Pharmaceutical. Tetramethyl tetraphenyl trisiloxane (MPS) (RUI 704) was purchased from RUI Chem. An enamel paint (KCl 7200, white) was purchased from Kunsul Chemical Industrial Co. An epoxy coating agent (KU420K40) was purchased from Kukdo Chemical Co. 2 x 5 cm steel panels were used in corrosion testing and electrochemical testing. Instruments: Photoirradiation was conducted by using an exposure system (NEXSAL, Hantech) equipped with a xenon short-arc lamp (light intensity: 22.7 mW/cm2) in conjunction with a UV cutoff filter (>305 nm; Edmund Optics Co.). A mechanical stirrer (NZ-1000, Eyela) equipped with a propeller-type impeller was used in microencapsulation. Proton nuclear magnetic resonance (1H NMR) spectra were taken on a Bruker AVANCE II 400 MHz spectrometer. A microscope (BX-51, Olympus) was used to take pictures of scratched surfaces and microcapsules. Microcapsule size was analyzed by using a CCD camera (CC-12, Olympus) attached to the microscope and image analysis software (analySIS TS, Olympus). Mean diameter and standard deviation were determined from data sets of at least 400 measurements. Scanning electron microscopes (Hitachi SU-70 and FEI FEG 250) were used to examine surface and shell


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morphologies of the microcapsules and coating surfaces. Samples were sputtered with a thin layer (~10 nm) of osmium to reduce charging. The balance of a TGA instrument (Shimadzu TGA-50) was used to measure the remaining amount of healing material during solvent evaporation after microcapsule rupture. Vibration tests were performed by using a BR-50UD vibration tester (IDEX). Gelation testing: The solubility behavior of C36 in various solvents was investigated as follows. For each solvent, 0.05 g of C36 and 9.95 g of the solvent were placed into a sealed vial and the mixture was vortexed at room temperature for 30 min. After storing at room temperature for 1 h, the vial was inverted for observation. The gelation properties of the C36-containing solvents upon solvent evaporation were investigated as follows. Equal volumes of 0.5 wt% C36 in cyclohexane and C36 in MPS (of 0.5, 1, 1.5 or 2 wt%) were mixed in a sealed vial at room temperature. The state of each such mixture was observed just after finishing the mixing process. Then the vial was unsealed to induce solvent evaporation at room temperature for 1 d. The state of the mixtures was then observed again. Microencapsulation: In a 100-mL beaker, urea (0.503 g, 0.0083 mol) followed by resorcinol (0.050 g) and ammonium chloride (0.050 g) were dissolved in water (20 mL). The beaker was suspended in a temperature-controlled water bath maintained at 25 °C. A 2.5 wt% aqueous solution of EMA (5 mL) was added to the reaction mixture and the pH of the reaction mixture was adjusted to 3.5 by using 10% NaOH solution. The reaction mixture was agitated at 800 rpm, and to the stirred solution was added 10 mL of a core material (a mixture of C36, MPS and cyclohexane). One or two drops of 1-octanol were added to eliminate surface bubbles. To the agitated emulsion was added 37 wt% formaldehyde (1.456 g, 0.0179 mol) solution; the reaction mixture was then heated to 60 °C and held at this temperature for 4.5 h. The reaction


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mixture was cooled to room temperature and the microcapsules were isolated by vacuum filtration. The microcapsules were washed with water and acetone and then air dried. MATPDMS-containing microcapsules were prepared according to a previously reported method. 33 Measurement of remaining material after crushing of microcapsules: Organogelbased self-healing microcapsules were crushed by pressing them between a TGA aluminum sample pan and cover. The amount of remaining material in the crushed microcapsules was then measured by using the precision balance of a TGA instrument for 30 min, at intervals of 2 min. Corrosion testing: The microcapsules filled with the gel precursor material or MATPDMS were prepared as described in Supporting Information. The microcapsules and a commercial epoxy coating formulation were mixed with a mass ratio of 1:4 to give self-healing coating formulations. The gel- and MAT-PDMS-based self-healing coating formulations were each applied to one side of a steel panel; the other side of each panel was coated with an enamel paint. The coatings were dried for 3 d under ambient conditions. Control coatings were also prepared in a similar fashion using the epoxy coating formulation without microcapsules. A razor blade was used to manually apply cross scratches on the prepared self-healing and control coatings. The scratched gel-based self-healing coatings were stored at room temperature for 1 day to induce gelation of the core material that flowed out of ruptured microcapsules. The scratched MAT-PDMS-based coatings were photoirradiated with a xenon lamp for 1.5 h. The scratched specimens were then immersed in 5 wt% sodium chloride solution for 72 h as an accelerated corrosion test. After washing with distilled water and removal of residual water, the coatings were observed by optical photography. The scratched and healed self-healing coatings were subjected to vibration (10 to 30 Hz) for 1 h, and then the immersion and photography were


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carried out again. For each kind of sample, at least four specimens were used for the corrosion test. Electrochemical testing: Scratched and healed self-healing coatings and scratched control coatings were prepared by means of the method applied for the corrosion testing. The application of vibration was also conducted in a similar fashion to corrosion testing. Electrochemical tests were conducted by using a computer-controlled potentiostat/galvanostat (model 263A, Advanced Measurement Technology) in a conventional three-electrode electrochemical cell equipped with a platinum counter electrode, an Ag/AgCl electrode in a saturated NaCl aqueous solution as the reference electrode, and the coated steel panel as the working electrode (Figure S7). The steady-state conduction between the coated metal substrate and a platinum electrode held at 3 V through an aqueous electrolyte (1 mol/L NaCl) was measured. The current passing through the specimen was recorded by using Electrochemistry PowerSuite software. For each kind of sample, at least four specimens were used and an average of the measured values was taken.

ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge on the ACS Publications website at DOI:. Chemical structures of organogel constituents, photographs of sample A partial gel and sample B gel, storage (G’) and loss (G”) moduli dependence on frequency for sample B gel, optical micrographs showing the scratch healing ability of sample B gel, optical micrographs showing a scratched region being filled with the healing agent, 1H NMR spectra showing the evaporation of cyclohexane from the core, schematic diagram of electrochemical test.


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AUTHOR INFORMATION Corresponding Authors *Tel./fax: +82 033 7602266. E-mail: [email protected] *Tel./fax: +82 033 7602389. 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.

ACKNOWLEDGEMENT This research was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Education (grant number: NRF2013R1A1A2065172).

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(15) Song, Y. K.; Chung, C. M. Repeatable Self-Healing of a Microcapsule-Type Protective Coating. Polym. Chem. 2013, 4, 4940-4947. (16) Sahoo, S.; Kumar, N.; Bhattacharya, C.; Sagiri, S. S.; Jain, K.; Pal, K.; Ray, S. S.; Nayak, B. Organogels: Properties and Applications in Drug Delivery. Des. Monomers Polym. 2011, 14, 95108. (17) Liu, S.; Yu, W.; Zhou, C. Solvents Effects in the Formation and Viscoelasticity of DBS Organogels. Soft Matter 2013, 9, 864-874. (18) Terech, P.; Weiss, R. G. Low Molecular Mass Gelators of Organic Liquids and the Properties of their Gels. Chem. Rev. 1997, 97, 3133-3160. (19) Mallia, V. A.; George, M.; Blair, D. L.; Weiss, R. G. Robust Organogels from NitrogenContaining Derivatives of (R)-12-Hydroxystearic Acid as Gelators: Comparisons with Gels from Stearic Acid Derivatives. Langmuir 2009, 25, 8615-8625. (20) Shikata, T.; Ogata, D.; Hanabusa, K. Viscoelastic Behavior of Supramolecular Polymeric Systems Consisting of N, N', N''-tris (3, 7-dimethyloctyl) benzene-1, 3, 5-tricarboxamide and NAlkanes. J. Phys. Chem. B 2004, 108, 508-514. (21) Lescanne, M.; Colin, A.; Mondain-Monval, O.; Fages, F.; Pozzo, J.-L. Structural Aspects of the Gelation Process Observed with Low Molecular Mass Organogelators. Langmuir 2003, 19, 2013-2020. (22) Wright, A. J.; Marangoni, A. G. Formation, Structure, and Rheological Properties of Ricinelaidic Acid-Vegetable Oil Organogels. J. Am. Oil Chem. Soc. 2006, 83, 497-503. (23) Pal, A.; Chhikara, B. S.; Govindaraj, A.; Bhattacharya, S.; Rao, C. N. R. Synthesis and Properties of Novel Nanocomposites Made of Single-Walled Carbon Nanotubes and Low


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(31) Marangoci, N.; Ardeleanu, R.; Ursu, L.; Ibanescu, C.; Danu, M.; Pinteala, M.; Simionescu, B. C. Polysiloxane Ionic Liquids as Good Solvents for β-Cyclodextrin-polydimethylsiloxane Polyrotaxane Structures. Beilstein J. Org. Chem. 2012, 8, 1610-1618. (32) Newman, S. A.; Forgacs, G.; Hinner, B.; Maier, C. W.; Sackmann, E. Phase Transformations in a Model Mesenchymal Tissue. Phys. Biol. 2004, 1, 100-109. (33) Song, Y. K.; Jo, Y. H.; Lim, Y. J.; Cho, S. Y.; Yu, H. C.; Ryu, B. C.; Lee, S. I.; Chung, C. M. Sunlight-Induced Self-Healing of a Microcapsule-Type Protective Coating. ACS Appl. Mater. Interfaces 2013, 5, 1378-1384.


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Figure 1. (a-c) Photographs of (a) sample B solution, (b) sample B gel and (c) sample A partial gel. (d) Optical micrograph of sample B gel.


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Figure 2. (a–c) Optical micrographs of the organogel precursor-containing microcapsules synthesized by using core/shell mass ratios of (a) 5.2:1, (b) 6.5:1 and (c) 8.5:1. (d) SEM micrograph showing the morphology of microcapsules prepared by using the core/shell mass ratio of 6.5:1.


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Figure 3. Size distribution of the organogel precursor-containing microcapsules prepared by using the core/shell ratio of 6.5:1.


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Figure 4. (a) Amount of remaining materials versus time at room temperature after crushing the microcapsules. (b, c) Optical micrographs of a damaged region in an organogel-based selfhealing coating: (b) just after scratching and (c) 2 min after scratching.


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Figure 5. Photographs of scratched control and self-healing coating samples after immersion for 72 h in 5 wt% sodium chloride aqueous solution.


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Figure 6. Electrochemical evaluation of scratched control and self-healing coatings: current versus time for (1) control sample, (2) organogel-based sample, (3) MAT-PDMS-based sample, (4) organogel-based sample after vibration and (5) MAT-PDMS-based sample after vibration.


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Figure 7. SEM images of scratched control and self-healing coating samples.


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Table 1. Solubility behavior of C36 in various solvents and critical gel concentration (CGC) of C36-containing solutions Solventa

Boiling point [°C]

Dissolution at RT b

CGCc [wt%]

Dodecane

215

I

4.9

Toluene

111

I

2

Benzene

80

I

1.8

Ethyl acetate

77

I

1.5

MPS

215

I

1.5

Cyclohexane

81

S

3.5

Hexane

69

S

2

THF

66

S

1.3

Chloroform

61

S

1

MPS: tetramethyl tetraphenyl trisiloxane, THF: tetrahydrofuran; bDissolution behavior of 0.5 wt% C36 at room temperature: I = insoluble, S = soluble; cCritical gel concentration upon cooling to room temperature after dissolution by heating. a


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Table 2. Gelation properties of C36-containing solvents Sample

C36 in MPS [wt%]

State of mixturea

State of mixture after cyclohexane evaporationb

A

0.5

S

PG

B

1

S

G

C

1.5

PG

G

D

2

PG

G

a

State of mixture at room temperature when equal volumes of C36 in MPS and 0.5 wt% C36 in cyclohexane were mixed: S = solution, PG = partial gel; bstate of mixture after cyclohexane evaporation: G = gel.


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Table of Contents Graphic and Synopsis

This paper describes a new microcapsule-type organogel-based self-healing system that strongly resists secondary damage in the healed region due to the viscoelasticity of the gel.


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Microcapsule-Type Organogel-Based Self-Healing System Having

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