Photoresponsive Self-Healing Polymer Composite with

Subscriber access provided by University of Pennsylvania Libraries

Article

Photo-responsive self-healing polymer composite with photo-absorbing hybrid microcapsules Lei Gao, Jinliang He, Jun Hu, and Chao Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09121 • Publication Date (Web): 28 Oct 2015 Downloaded from http://pubs.acs.org on October 30, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Photo-responsive self-healing polymer composite with photo-absorbing hybrid microcapsules †

†

†

Lei Gao , Jinliang He* , Jun Hu and Chao Wang †

‡

The State Key Lab of Power System, Department of Electrical Engineering, Tsinghua

University, Beijing 100084, P.R.China. ‡

Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA.

KEYWORDS: Photo-responsive, Self-healing, Photo-absorbing, Microcapsule, Coating Abstract Microcapsule-based self-healing polymer materials are highly desirable because it can heal large-volume cracks without changing the original chemical structures of polymers. However, it’s limited by processing difficulties and inhomogeneous distributions of two components. Herein, we report a one-component photo-responsive self-healing polymer composites with photo-absorbing hybrid microcapsules (PAHM), which gives the microcapsules photo-absorbing properties by introducing nano-TiO2 particles as photo-absorbing and emulsified agent in the poly(urea-formaldehyde)/TiO2 hybrid shells. Upon mechanical damages and then exposed to light, the photo-responsive healing agents in the cracks will be solidified to allow for selfhealing, while the healing agents in the unbroken PAHM will be protected and remain unreacted, which endows this photo-responsive microcapsule-based self-healing composite with self-

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 24

healing properties like the conventional two-component microcapsule-based systems do. Given the universality of this hybrid polymerization method, incorporation of the photo-absorbing particles to conventional polymer shells may further broaden the scope of applications of these widely used materials. Introduction The ability to heal the mechanical damages of biological materials is an important survival feature for animals and plants. This self-healing capability is highly desirable for synthetic materials because it can significantly increase their lifetime and safety. There are two major categories of self-healing polymer systems: i) dynamic polymers1-6 ii) polymer composites with microcapsules/vesiculars loaded with healing agents7-10. The dynamic polymers mainly involve thermodynamics11 and varieties of self-healing reactions, i.e., covalent bonding12-13, supramolecular chemistry14, H-bonding15, etc. Since the dynamic polymers need reactive groups to facilitate rebonding to achieve successful self-healing, it usually needs to change the original chemical structures of polymers in despite of their multi-healing properties. Compared with the dynamic polymer system, the composite approach is particularly interesting because it doesn’t need to change the original chemical structures of polymers and can heal large-volume cracks. Typically, the polymer composite healing approach employs composites containing embedded reactive microcapsules and catalyst16-18. After a mechanical damage breaks the microcapsules, the healing agents subsequently flow out of the ruptured capsules, fill the damaged areas and polymerize on touch with the embedded catalysts, leading to the repair of the cracks. However, the two capsule systems have several major limiting factors, such as processing difficulties and inhomogeneous distributions of two components. To tackle these challenges, stimuli-responsive one-component self-healing system is highly desirable19-21. For instance, to


2

Page 3 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


take advantage of moisture around, isocyanates were microencapsulated and embedded in the polymer.22 When the mechanical damages break the microcapsules, the isocyanates flow out and react with the moisture in the cracks. The polymerization can also be triggered by a number of other stimuluses, including heat23, light24-30, etc. Among them, photo-responsive self-healing system by ultraviolet (UV) light or sunlight is particularly attractive because it’s clean, remote, cheap and readily available. Chung et al described the first example of extrinsic photoinduced self-healing system29,

31

. However, there are still a major challenge in design the

photo-

responsive self-healing polymer composite, which is that photo irradiation will also trigger the polymerizations of healing agents in non-ruptured microcapsules, making the materials lose healing capability after one damage.

Scheme 1. (a) Self-healing protective coating on protected materials; (b) microcapsules are ruptured when cracks are generated and the healing agent is released from ruptured microcapsules and fills the cracks; (c) cracks are healed by photo-polymerization of the healing agent induced by light.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 24

Herein, we successfully solved this critical challenge by using a polymer composite containing photo-absorbing hybrid microcapsules (PAHM), which are fabricated through a hybrid approach of combining nano-TiO2 particles (NTP) as photo-absorbing layer and poly(urea–formaldehyde) (PUF) as polymer shells. The polymer composite thus offers a photoresponsive one-component microcapsule-based self-healing system. In the polymer composite, PAHM are embedded in the matrix to endow them with self-healing properties (Scheme 1a). Ruptured by the propagating cracks, the microcapsules release the healing agent and fill the cracks (Scheme 1b). Then exposed to light, the healing agents in the cracks are cured, while the agents in the microcapsules remain unreacted due to the protection of the photo-absorbing shells (Scheme 1c). Therefore, the photo-triggered self-healing system remains useful until all the healing agents flow out the microcapsules.

Experimental Section Materials. Resorcinol, urea, ammonium chloride, aqueous formaldehyde solution (37 wt %), Polyvinyl Acetate (PVA) and hydrochloric acid solution (36 wt %) from were received Sinopharm Chemical Reagent Co., Ltd.. Modified nano-TiO2 particles (Haitai nano materials) were used as emulsifiers. Dglycidyl ether of bisphenol A (EPON 828, Shell Chemicals), the hardener phenol-aldehyde amine (PAA, G.C.Chem) and the diluent Diisobutyl phthalate (DBP, G.C.Chem) were used as polymer matrix. Bisphenol A epoxy acrylate resin (BAEA), trimethylolpropane-triacrylate (TMPTA) from Dow Corning and Irgacure 184 (Ciba, Switzerland) were employed as photo-response healing agents. Synthesis of photo-absorbing hybrid microcapsules. The required amount of NTP (1.20 g) was dispersed in 60 ml PVA aqueous solution (0.001 g/ml) by ultrasonic treatment for 30 min to


4

Page 5 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


get homogenous suspensions. Meanwhile, the mixture of BAEA, TMPTA and photoinitiator 184 (weight ratio, BAEA:TMPTA:184=64:32:4) was stirred for 30 min to get homogeneous photosensitive resin. Then, a small amount of coumarin 6 ( 0.05 wt %) was dispersed into the mixture. Subsequently, the photosensitive resin (20 ml) and suspensions (60 ml) were added into a 250 ml three-neck round-bottomed flask and agitated at 800 rpm by a mechanical stirrer for 30 min. At the same time, under mechanical stirring, the mixture of urea (5.0 g) and formaldehyde (13.51 g) was adjusted to pH = 8.5 by triethanolamine and then kept at 70 °C 1 h to obtain the prepolymer solution of urea–formaldehyde. Then, ammonium chloride (0.5 g) and resorcinol (0.5 g) was added into the prepolymer solution to form Solution B. Solution B was slowly added to Emulsion A, and then the pH value was adjusted to about 3.5 by hydrochloric acid solution. Then, the emulsion was slowly heated to the target temperature of 60 °C, and kept at the temperature for 3 h. Cooled down to ambient temperature, the suspensions of the obtained microcapsules were rinsed with deionized water for 3 times and then filtered. Finally, the filter residue was air-dried for 24 h to obtain the microcapsules. Preparation of Self-Healing Coating. The unfilled coatings were fabricated by mixing 100 parts EPON 828 epoxy with 25 parts curing agent PAA, and 10 parts diluent DBP. The selfhealing coatings were prepared by uniformly mixing a certain amount (15 wt%) of microcapsules with the aforesaid mixture of EPON 828, PAA and DBP. Either the unfilled epoxy or filled version was degassed, casted on clean metal plate, and cured for 24 h at room temperature, followed by 24 h at 40 °C. Characterization. Morphology and surface features of the microcapsules were observed by optical microscopy (OM) (Olympus SZ-16), scanning electron microscopy (SEM) (Carl Zeiss


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 24

EVO 50) and transmission electron microscopy (TEM) (FEI Tecnai G2 20). The yield of microcapsule synthesis was calculated as the weight ratio between the collected microcapsules and the initial substances (PUF prepolymer, photosensitive resin, nano-TiO2 particles). The amount of photosensitive resin into microcapsules was determined by ethanol extraction of core materials. Small amounts of microcapsules were crushed and washed with ethanol for several times. After filtration and drying, pure microcapsule shell was obtained. The percentage of core and shell fraction of microcapsules was calculated. The photo-sensitive resin was coated on a KBr disk and photo-irradiated with UV light. The photoreaction conversion was measured by FT-IR spectroscopy: the ratios of the calculated areas of the two absorption bands (1635 cm-1 for C=C and 1726 cm-1 for C=O as an internal standard) before and after exposure were compared to determine the degree of conversion of cinnamoyl C=C bond using the following formula. Conversion % = 1 −

C = C /C = O after UV − exposure " × 100 C = C /C = O before UV − exposure

UV−vis spectra were collected at room temperature in the wavelength range of 200−600 nm, using an Evolution 600 UV−vis spectrophotometer (Thermo Scientific). A liquid cell of 1 cm path length with quartz windows was used. UV−vis spectra of Irgacure 184 were recorded in acetonitrile solution. Thermogravimetric analysis (TGA) was performed from 0 to 800 °C at a rate of 20 °C/min in air on a Q50 TGA system (TA Instruments). Results and discussion As illustrated in Scheme 2, our photo-absorbing hybrid microcapsules were prepared by using Pickering emulsion polymerization32-34. NTP were involved as photo-absorbing agents and emulsifiers due to its excellent photo-absorbing properties. NTP are covered by Al2O3 and SiO2


6

Page 7 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


first to avoid degradation of the polymer35-36. Then, NTP with different surface modifications were further investigated and it was found that only hydrophobic particles (contact angle >130o) could form stable emulsions34. A small amount of polyvinyl alcohol was dissolved in distilled water to prevent the aggregation of TiO2 particles and promote the emulsification process as coemulsifier. By dispersing the photo-sensitive resin (PSR) in TiO2/PVA--containing water under vigorous mechanical stirring, stable oil-in-water (O/W) emulsions were prepared through the formation of the inner TiO2 shells (Figure 1a-b). Then, polymer shells was introduced by a typical two-step method37. Firstly, the mixture of urea and formaldehyde reacted at a certain pH and temperature to form the pre-polymer solution of urea–formaldehyde (UF). Secondly, the prepolymer solution was added into the emulsions, the crosslinking reaction of which was triggered by adjusting the pH value and temperature, leading to the formation of poly(urea–formaldehyde) (PUF)/TiO2 hybrid shells. As shown in Figure 1c-d, the microcapsules exhibit a diameter of 150±50 µm with a wall thickness of 4.5±1.2 µm. To monitor the distribution of TiO2 in the shell, the cross-section of the microcapsules embedded in epoxy matrix were studied. As shown in Figure 1e-f, the nanoparticles tend to concentrate in the inner side of the shell, which is in accordance with the fabrication process, i.e., that most of particles are attached to the PSR to form stable emulsions before the polymerization. It was concluded that the yield of microcapsules is ∼80 %, and the determination of core fraction is ∼85 %.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 24

Scheme 2. Schematic illustration of the fabrication of microcapsules with organic–inorganic hybrid shell using NTP-stabilized Pickering emulsion polymerization

Figure 1. Characterization of microcapsules (a) OM image of O/W emulsion. (b) OM image of broken O/W emulsion. (c) SEM image of microcapsules. (d) SEM image of shell wall profile. (e)


8

Page 9 of 24

SEM images of cross-section microcapsule embedded in polymer matrix. (f) TEM images of cross-section of PAM shell. The thermal properties of PUF/ TiO2 hybrid microcapsule shells were characterized by TGA to get the content of the TiO2 in the hybrid shells (Figure 2). The TiO2 nanoparticles showed a weight loss of about 3.2% between 25 °C and 600 °C due to the removal of physically adsorbed water and the decomposition of coupling agent groups. The PUF/ TiO2 hybrid microcapsule shells showed the subsequent loss of about 80 % due to the decomposition of PUF component. Furthermore, compared with the derive of weight loss of pure PUF, the degradation temperature of PUF in PUF/ TiO2 hybrid microcapsule shells was a little higher, which might be the result of the chemical or physical interaction between TiO2 nanoparticles and PUF during the polymerization process. 1.0

80

PUF PUF/TiO2

248.5

233.9

0.8

TiO2

60 40

0.6 0.4

508.6

20

0.2

511.5 19.62%

0

Deriv. Weight (%/°C)

100

Weight Loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 0

100

200

300 400 500 Temperature ( )

600

700

800

Figure 2. TGA thermograms of NTP, PUF shell and PUF/ TiO2 hybrid shells Notably, the size of the microcapsules can be readily tuned by varying the mechanical stirring speed. As the agitation rate increases from 500 to 1500 rpm, the average diameter of the microcapsules decreases, and the size distribution narrows (Figure 3a). It is possibly because higher stir speed can facilitate the formation of homogeneous emulsion, leading to narrower size distribution. Moreover, the size of the microcapsules has a great influence on the content of TiO2


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 24

and thickness of the shells, which may be the key to the photo-absorbing properties. The relationship of them with the size of the microcapsules is measured and shown in Figure 3b. As expected, the thickness of the shells increases with the increasing mean diameter, while the content of NTP decreases with that. As demonstrated in Figure 1, most of NTP serve as emulsifiers and concentrate at the inner side of the shells, resulting in less TiO2 in the outer shell. Besides, the increase of the shell mainly refers to the increase of the outer shell. Consequently, the content of polymer increases with the increasing outer shell, leading to the decreasing TiO2 content.

Figure 3. Relationship of microcapsule properties with agitation rate (a) Mean microcapsule diameter as a function of agitation rate. (b) Shell thickness and TiO2 content as a function of the mean diameter of the microcapsules.


10

Page 11 of 24

To investigate the photo-absorbing properties, the solidified time for the healing agent in the photo-absorbing microcapsules was measured first (Figure S1). Control group was PUF shelled microcapsules (PSM), prepared through in situ polymerization (Figure S2). It was concluded that the time required to solidify the healing agent in the microcapsules is 180±9 s for PAHM and 30±5 s for PSM, which indicates that large extended solidified time are achieved due to the excellent photo-absorbing properties of the shells of PAHM. The service life of microcapsules with different diameter was also measured (Figure S3), which indicates that the service life increases with the diameter. Specifically, with the increase of the diameters from 87 µm to 520 µm, the service life increases from 151 s to 224 s. b

a PVDF with 20 wt% NTP PVDF with 10 wt% NTP PUF PVDF

0.8 0.4 0.0

d=10 µm d=5 µm d=3 µm

0.8 0.4 0.0

200

c

1.2 Absorbance (a.u) Absorbance (a.u)

Absorbance (a.u) Absorbance (a.u)

1.2

300 400 500 Wavelength (nm)

600

200

d

30 0

Conversion (%)

PVDF with 20 wt% NTP PVDF with 10 wt% NTP PVDF

60

300 400 500 Wavelength (nm)

600

d=10 µm d=5 µm

45

90 Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


d=3 µm

30 15 0

0

100

200 Time (s)

300

0

100 200 Time (s)

300

Figure 4. Photo-absorbing properties of the shells. (a) Photo−vis absorption spectra of PUF and PVDF films with various content of NTP; the thickness of the films is 5 µm. (b) Photo−vis absorption spectra of PVDF films with different thickness; the content of NTP is 10 wt %. (c) Plots of conversion of the cinnamoyl group vs. photo-irradiation time under different cover films, the thickness of the cover films is 5µm. (d) Plots of conversion of the cinnamoyl group vs. photo-irradiation time under different cover films, the content of NTP is 10 wt %.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

Next, to systematically study the photo-absorbing properties of the shells, thin films (PUF and poly(vinylidene fluoride) (PVDF)) with similar content and thickness to the shells (Figure S4) were fabricated, and the photo-vis absorption spectrums of them were measured. The spectrums reveal that pristine polymer films don’t possess photo-absorbing properties, while the films with TiO2 shows excellent photo-absorption properties between 200~400 nm (Figure 4a). Specifically, the photo-absorption spectrum of the films begins at 400 nm and reaches the peak at 320 nm. Subsequently, although the spectrum shows a slight decrease with the decrease of the wavelength, the absorptivity still remains relatively high. To insure the protecting effects of the shells, photoinitiator Irgacure 184 with low absorption peak (< 330 nm) is selected, which shows an absorption peak at 246 nm ( the according absorbance rate of shell is 0.92) (Figure S5). Then, the influence of the content of TiO2 and thickness of the shell is further investigated. When the content of TiO2 varies from 10 wt % to 20 wt %, the peak of the spectrum increases from 0.83 to 0.97, which shows a small increase in absorption rate (85 % to 89 %) (Fig. 4a). Similarly, with the increase of the films’ thickness from 3 µm to 10 µm, the absorbing rate increases from 81% to 90% (Figure 4b). The results indicate that TiO2 composite shells are excellent photo-absorption materials with wide absorbing band and high absorptivity. The photo-cross-linking of PSR, which was covered by different films to simulate the shells, was measured by FT-IR spectroscopy to further investigate the protection properties of the shells. The ratios of the calculated areas of the two absorption bands (1635 cm-1 for C=C and 1726 cm-1 for C=O) before and after exposure were compared to determine the conversion degree of photo-cross-linking of PSR (Figure S6)31. The conversion of PSR has an almost linear relationship with the exposure time at low conversion degree (< 80 %) (Figure 4c). As expected,


12

Page 13 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with the increase of NTP’s content and films’ thickness, the conversion shows a dramatically decrease, which is in accordance with the photo-absorbing results (Figure 4d). In a word, with a proper thickness and content of NTP, the shells can absorb most of the photo-light and effectively protect the healing agent inside.

Figure 5. Optical images after 48 h immersion in salt water. (a) CC1, (b) CC2, and (c) selfhealing coatings. The self-healing capability of the polymer composite coating was evaluated through anticorrosion testing. In comparison, we employed coatings without microcapsules as control coating1 (CC1) and composites with PUF-shelled microcapsule fillers as control coating2 (CC2). Typically, a mechanical damage is induced by hand scribing through the ca. 500 µm thick coatings using a razor blade. And it was further conformed that the scribes are deep enough to reach the steel substrates (Figure S7). Following the scribing procedure, samples was placed in a


13


dark room for 1h before exposed to UV-light for 30 s to heal the cracks. Control coatings were also scribed and photo-exposed. Then, all samples were immersed in 10 wt% aqueous NaCl solution for 48 h. As expected, CC1 corroded, while the self-healing coatings (SHC) and CC2 showed no visual evidence of corrosion (Figure 5a). Then, the above-mentioned steps were repeated to form the subsequent scribing, and all those scribing was created at different locations. As shown in Figure 5b, CC2 also corroded, but the self-healing coatings still showed no visual evidence of corrosion. Before the fifth scribing and healing, all the self-healing coatings showed good anticorrosion properties, thereby indicating that the self-healing samples have self-healing properties as the conventional two-component microcapsule-based systems (Figure 5c). CC2, 1st scribing CC2, 2nd scribing SHC, 1st scribing SHC, 2nd scribing SHC, 3rd scribing SHC, 4th scribing SHC, 5th scribing

60 Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 24

40

1.0

0.5

0.0

20

0

0

50

100 Time (s)

150

200

Figure 6. Current versus time for scribed and healed control and self-healing samples. Table 1 Conductivity of self-healing samples and control samples after scribing and healing. Scribing healing order Conductivity self-healing samples (mA)

and 1st

2nd

3rd

4th

5th

of 0.11 0.23 0.41 0.78 6.91

Conductivity of 0.10 8.23 control samples

-

-


14

Page 15 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(mA) Conductivity was measured at 150 s after the scribing and healing procedure. Each conductivity is the average value of 3 samples.

The self-healing properties were further monitored by the electrochemical testing, in which the coated metal substrate serves as one electrode in a conventional three-electrode electrochemical cell (Figure S8). The steady-state conduction between the coated metal substrate and a counter electrode held at 3 V through an aqueous electrolyte (1 M NaCl) was measured (Figure 6 and Table 1). The current passing through the control and self-healing polymer coatings before scribing are nearly identical, 8.2 µA. After the first damage, the current passing through the CC1 increased immediately, while the CC2 and self-healing samples show dramatically reduced currents, which are 0.10 mA and 0.11 mA, respectively. After the second scribing and healing, the CC2 samples show a large current (8.23 mA) after the second scribing and healing, while the self-healing samples still shows a small current (0.23 mA). Before the fifth scribing and healing, the current only shows a small increase with more scribing on the samples.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

Figure 7. (a) SEM image of scribe region. (a) CC1, (b) CC2, and (c) self-healing coatings. The photo-responsive self-healing were finally confirmed by SEM. Figure 7a is the morphology of the film after first scribing. A crack with a width of ~10 µm can be observed. In CC2, after the healing process, the first scribed crack was healed (Figure 7b). However, photo irradiation triggered the polymerizations of healing agents in both scribing region and nonruptured microcapsules. As expected, there is no available fluxible healing agent for the following scribing, leading to the unhealed second scribing (Figure 7b). Figure 7c shows the images of self-healing samples after scribing and healing. When exposed to light, the healing agent in the cracks are solidified, while the healing agent in the non-ruptured microcapsules cannot be solidified due to the protection of the photo-absorbing shells. Consequently, the following scribing, which is created somewhere else, can also be successfully healed. However, since the photo-absorbing shells cannot absorb all the light, the healing agents were also


16

Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


gradually solidified with the increase of the accumulative exposure time. Consequently, after scribed for the fifth time, the self-healing samples cannot be healed any more. Conclusions In conclusion, we successfully realized a one-component photo-responsive microcapsule-based self-healing system consisting of photo-absorbing hybrid microcapsules. By endowing the microcapsules with excellent photo-absorbing shells to protect the healing agents inside from being solidified by light, our one-component photo-responsive microcapsule-based system achieves self-healing properties like the conventional two-component microcapsule-based systems do. The microcapsules can be fabricated through a hybrid polymerization method, employing photo-absorbing nanoparticles as emulsifying agent and PUF polymer as shells. Further studies to improve the photo-absorbing properties of the shells are underway, and we anticipate that these photo-responsive microcapsule-based materials may eventually be widely used in outdoor applications, such as protection coatings.

Supporting Information Measurement of the solidified time of microcapsules (Figure S1), Fabrication of PUF microcapsules (Figure S2), Service life as a function of diameter (Figure S3), Schematic diagram of the photoreaction conversion test (Figure S4), UV−vis absorption spectra of Irgacure 184 (Figure S5), Infrared spectra of photo-sensitive resin (Figure S6), OM image of scribed steel substrates (Figure S7) and schematic diagram of electrochemical test (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

*E-mail: [email protected]. Notes The authors declare no competing ﬁnancial interest. Acknowledgements The authors gratefully acknowledge the financial support from the National Basic Research Program of China (973 Program) under grant 2014CB239505

Notes and references (1) Herbst, F.; Döhler, D.; Michael, P.; Binder, W. H. Self-Healing Polymers via Supramolecular Forces Macromol. Rapid Commun. 2013, 34, 203-220. (2) Liu, Y.; Chuo, T. Self-healing Polymers based on Thermally Reversible Diels–Alder Chemistry Polym. Chem. 2013, 4, 2194-2205. (3) Kuroki, H.; Tokarev, I.; Nykypanchuk, D.; Zhulina, E.; Minko, S. Stimuli-Responsive Materials with Self ‐ Healing Antifouling Surface via 3D Polymer Grafting Adv. Funct. Mater. 2013, 23, 4593-4600. (4) Imato, K.; Nishihara, M.; Kanehara, T.; Amamoto, Y.; Takahara, A.; Otsuka, H. SelfHealing of Chemical Gels Cross-Linked by Diarylbibenzofuranone-Based Trigger-Free Dynamic Covalent Bonds at Room Temperature Angew. Chem., Int. Ed. 2012, 51, 1138-1142. (5) Ying, H.; Zhang, Y.; Cheng, J. Dynamic Urea Bond for the Design of Reversible and Self-healing Polymers Nat. Commun. 2014, 5, 3218.


18

Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Nakahata, M.; Takashima, Y.; Yamaguchi, H.; Harada, A. Redox-responsive Selfhealing Materials formed from Host-guest Polymers Nat. Commun. 2011, 2, 511. (7) White, S.; Sottos, N.; Geubelle, P.; Moore, J.; Kessler, M.; Sriram, S.; Brown, E.; Viswanathan, S. Autonomous Healing of Polymer Composites Nature 2001, 409, 794−797. (8) Park, J.; Braun, P. Coaxial Electrospinning of Self‐Healing Coatings Adv. Mater. 2010, 22, 496-499. (9) Blaiszik, B. J.; Baginska, M.; White, S. R.; Sottos, N. R. Autonomic Recovery of Fiber/matrix Interfacial Bond Strength in a Model Composite Adv. Funct. Mater. 2010, 20, 3547-3554. (10)

Mookhoek, S. D.; Blaiszik, B. J.; Fischer, H. R.; Sottos, N. R.; White, S. R.; van

der Zwaag, S. Peripherally Decorated Binary Microcapsules Containing two Liquids J. Mater. Chem. 2008, 18, 5390-5394. (11)

Wang, P. P.; Lee, S.; Harmon, J. P. Ethanol-induced Crack Healing in Poly

(methyl methacrylate) J. Polym. Sci.,Part B: Polym. Phys. 1994, 32, 1217-1227. (12)

Chen, X.; Dam, M. A.; Ono, K.; Mal, A.; Shen, H.; Nutt, S. R.; Wudl, F. A

Thermally Re-mendable Cross-linked Polymeric Material Science 2002, 295, 16981702. (13)

Chen, X.; Wudl, F.; Mal, A. K.; Shen, H.;

Nutt, S. R. New Thermally

Remendable highly Cross-linked Polymeric Materials Macromolecules 2003, 36, 18021807.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(14)

Page 20 of 24

Aida, T.; Meijer, E. W.; Stupp, S. I. Functional Supramolecular Polymers Science

2012, 335, 813-817. (15)

Cui, J.; del Campo, A. Multivalent H-bonds for Self-healing Hydrogels Chem.

Commun. 2012, 48, 9302-9304. (16)

Rule, J. D.; Moore, J. S. ROMP Reactivity of e ndo-and e xo-

Dicyclopentadiene Macromolecules 2002, 35, 7878-7882. (17)

Liu, X.; Sheng, X.; Lee, J. K.; Kessler; M. R.; Kim, J. S. Rheokinetic Evaluation

of Self-healing Agents Polymerized by Grubbs Catalyst Embedded in Various Thermosetting Systems Compos. Sci. Technol. 2009, 69, 2102-2107. (18)

Cho, S. H.; Andersson, H. M.; White, S. R.; Sottos; N. R.; Braun, P. V. ;

Polydimethylsiloxane-Based Self-Healing Materials Adv. Mater. 2006, 18, 997-1000. (19)

Meng, L. M.; Yuan, Y. C.; Rong, M. Z.; Zhang, M. Q. A dual mechanism single-

component self-healing strategy for polymers J. Mater. Chem. 2010, 20, 6030-6038. (20)

Caruso, M. M.; Delafuente, D. A.; Ho, V., Sottos; N. R., Moore; J. S.; White, S.

R. Solvent-promoted self-healing epoxy materials Macromolecules 2007, 40, 88308832. (21)

Dong Y. Z.; Wetzel B.; Noll A.; Min Z. R.; Ming Q. Z. Thermo-molded Self-

healing Thermoplastics Containing Multilayer Microreactors J. Mater. Chem. A 2013, 1, 7191-7198. (22)

Huang, M.; Yang, J. Facile Microencapsulation of HDI for Self-healing

Anticorrosion Coatings J. Mater. Chem. 2011, 21, 11123-11130.


20

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23)

Klukovich, H. M.; Kean, Z. S.; Iacono, S. T.; Craig, S. L. Mechanically Induced

Scission and Subsequent Thermal Remending of Perfluorocyclobutane Polymers J. Am. Chem. Soc. 2011, 133, 17882-17888. (24)

Chung, C. M.; Roh, Y. S.; Cho, S. Y.; Kim, J. G. Crack healing in Polymeric

Materials via Photochemical [2+ 2] Cycloaddition Chem. Mater. 2004, 16, 3982-3984. (25)

Jun L.; Min Z. R.; Ming Q. Z. Photo-stimulated Self-healing Polyurethane

Containing Dihydroxyl Coumarin Derivatives Polymer 2012, 53, 2691. (26)

Froimowicz, P.; Frey, H.; Landfester, K. Towards the Generation of Self-Healing

Materials by Means of a Reversible Photo-induced Approach Macromol. Rapid Commun. 2011, 32, 468-473. (27)

Kiskan, B.; Yagci, Y. Self-healing of Poly (propylene oxide)-polybenzoxazine

Thermosets by Photoinduced Coumarine Dimerization J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2911-2918. (28)

Young-Kyu S.; Chan-Moon. C. Repeatable Self-healing of a Microcapsule-type

Protective Coating Polym. Chem. 2012, 4, 4940. (29)

Song, Y. K.; Jo, Y. H.; Lim, Y. J.; Cho, S. Y.; Yu, H. C.; Ryu, B. C.; Chung, C.

M. Sunlight-induced Self-healing of a Microcapsule-type Protective Coating ACS Appl. Mater. Interfaces 2013, 5, 1378-1384. (30)

Ghosh, B.; Urban, M. W. Self-repairing Oxetane-substituted Chitosan

Polyurethane Networks Science 2009, 323, 1458-1460.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(31)

Page 22 of 24

Song, Y. K.; Chung, C. M. Repeatable Self-healing of a Microcapsule-type

Protective Coating Polym. Chem. 2013, 4, 4940-4947. (32)

Stiller, S.; Gers-Barlag, H.; Lergenmueller, M.; Pflücker, F.; Schulz, J.; Wittern,

K. P.; Daniels, R. Investigation of the Stability in Emulsions Stabilized with Different Surface Modified Titanium Dioxides Colloids Surf., A 2004, 232, 261-267. (33)

Chen T.; Colver P. J.; Bon S. A. F. Organic–Inorganic Hybrid Hollow Spheres

Prepared from TiO2-Stabilized Pickering Emulsion Polymerization Adv. Mater. 2007, 19, 2286-2289. (34)

Chevalier, Y.; Bolzinger, M. A. Emulsions Stabilized with Solid Nanoparticles:

Pickering Emulsions Colloids Surf., A 2013, 439, 23-34. (35)

Wei S.; Hu X.; Liu H.; Wang Q.; He C. Rapid Degradation of Congo Red by

Molecularly Imprinted Polypyrrole-coated Magnetic TiO2 Nanoparticles in Dark at Ambient Conditions J. Hazard. Mater. 2015, 294, 168-176. (36)

Khade G. V.; Suwarnkar M. B.; Gavade N. L.; Garadkar K. Green Synthesis of

TiO2 and its Photocatalytic Activity M. J. Mater. Sci.: Mater. Electron. 2015, 26, 33093315. (37)

Li, H.; Wang, R.; Hu, H.; Liu, W. Surface Modification of Self-healing Poly

(urea-formaldehyde) Microcapsules using Silane-coupling Agent. Appl. Surf. Sci. 2008, 255, 1894-1900.


22

Page 23 of 24


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

Table of Contents Graphic


24

Photoresponsive Self-Healing Polymer Composite with

Recommend Documents