Photocured materials with self-healing function through ionic

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Applications of Polymer, Composite, and Coating Materials

Photocured materials with self-healing function through ionic interactions for flexible electronics Haoran Gong, Yanjing Gao, Shengling Jiang, and Fang Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08884 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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ACS Applied Materials & Interfaces

Photocured materials with self-healing function through ionic interactions for flexible electronics Haoran Gong,1,2 Yanjing Gao,2 Shengling Jiang, 3 Fang Sun*1,2 1

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical

Technology, Beijing 100029, People’s Republic of China 2

College of Science, Beijing University of Chemical Technology, Beijing 100029, People’s

Republic of China 3

College of Materials Science and Engineering, Beijing University of Chemical Technology,

Beijing 100029, People’s Republic of China ABSTRACT Photocured materials with self-healing function have the merit of long life time and environmentally-benign preparation process, and thus find potential applications in various fields. Herein, a novel imidazolium-containing photocurable monomer (IM-A) was designed and synthesized. Self-healing polymers were prepared by fast photocuring with IM-A, isobornyl acrylate, 2-(2-ethoxyethoxy) ethyl acrylate, and 2-hydroxyethyl acrylate as the monomers. The mechanical and self-healing properties of the polymers were tuned by varying the contents of IM-A and other monomers. The as-prepared self-healing polymer IB7-IM5 exhibited a tensile strength of 3.1 MPa, elongation at break of 205%, healing efficiency of 93%, and wide healing temperature range from room temperature to 120°C. The self-healing polymer was also employed as a flexible substrate to fabricate flexible electronic device, which could be healed and completely restore its conductivity after the device was damaged. KEYWORDS: photocuring, self-healing, ionic interactions, imidazolium, flexible electronics 1. INTRODUCTION Photocuring is an emerging technology which leads to the cross-linking of liquid monomers or oligomers to form solid polymers in the presence of light (UV or visible light) 1. Photocuring has aroused

wide

interest

thanks

environmental-friendly nature

2-4

to

its

efficient,

economical,

energy-saving,

and

. However, photocuring technology has some shortcomings, such

5

as oxygen inhibition , limitation in coating thickness 6, high volumetric shrinkage 7. Because of

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

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the fast curing rate and high volumetric shrinkage, photocured materials are generally non-uniform in microstructures, which causes structural defects and may readily lead to material deterioration or failure 8. As a result, the service life of photocured materials is limited. In addition, the damaged photocured materials are hardly recycled via melting and dissolving 9, leading to huge waste of resources and serious environmental problems. Therefore, development of durable, safe and reliable photocured materials is desirable. Self-healing materials have the characteristics of long lifetime, intrinsic safety, and mitigated environmental impact as a result of allowing the autonomous restoration of their original properties after damages over a long period of use

10-12

.

Consequently, the introduction of the self-healing function into photocured materials can extend the service life of the materials 13. The rapid development of self-healing materials based on supramolecular interactions, including pi–pi stacking 14, hydrogen bonds 15, and ionic interactions 16, has attracted considerable attention in recent years. Of all the supramolecular interactions, ionic interactions are very interesting because of its high aggregation strength that results in low healing agent content for self-healing materials

17-19

. A large number of thermocured self-healing materials based on ionic

interactions have been synthesized. Ko et al. 20 developed a self-healing dielectric material based on catechol-functionalized polymer and imidazole ionic liquid. This self-healing polymer showed a self-healing capability under mild self-healing conditions at 55 °C for 30 min. Madsen et al.

21

demonstrated that an ionically cross-linked silicone elastomer could self-heal after mechanical damage and electrical breakdown. He et al.

22

obtained a poly(ionic liquid)-containing hybrid

hydrogel with self-healing nature, good toughness, and high conductivity. The reported self-healing materials based on ionic interactions are all thermocured materials. To the best of our knowledge, there is no report on photocured materials with self-healing function through ionic interactions. Flexible electronics as candidates for the next-generation electrical devices have attracted wide attention and been actively explored for their applications in the intelligent motion monitoring system, smart electronic skin, and wearable/portable electronic devices flexible electronics have the disadvantages of limited service life 27

26

23–25

. However, current

and poor stretch performance

. In addition, they are fabricated by coating conductive materials on thermocured flexible

substrate, which involves a complicated and long production process and causes environmental pollution from the emission of organic solvents 28-30. Therefore, it poses a challenge to develop


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flexible electronic materials with environmentally-benign production process and self-healing capability to extend their service life and improve the safety. The above background motivated us to design photocured materials with self-healing function through ionic interactions for flexible electronics. In this work, we designed and synthesized a novel

imidazolium-containing

photocurable

monomer,

6-(3-(3(2-hydroxyethyl)-1H-imidazol-3-ium bromide)propanoyloxy) hexyl acrylate (IM-A), based on 1,6-hexanediol diacrylate (HDDA) and 1H-imidazole and 2-bromoethanol. Then, we prepared self-healing flexible materials by fast photocuring with IM-A, isobornyl acrylate (IBOA), 2-(2-ethoxyethoxy) ethyl acrylate (EOEOEA), and 2-hydroxyethyl acrylate (HEA). Herein, IM-A containing imidazolium plays a self-healing role in the materials; IBOA with rigid ring as a hard monomer can improve the mechanical strength of the materials; EOEOEA with soft ether chain as a soft monomer can enhance the flexibility of the materials, and HEA can improve the compatibility of the components in the materials. We investigated the photopolymerization characteristics of the self-healing flexible materials by real-time Fourier transform infrared spectroscopy (FTIR), which indicated that it took only 20-60s to obtain the self-healing materials. By carefully adjusting the content of monomers, the self-healing materials exhibited good tensile strength, elasticity, and self-healing ability. Finally, a flexible electronic device was fabricated by casting commercial silver paste on the photocured substrate with self-healing function. The damaged electronic device can self-heal and completely restore their flexibility and conductivity. 2. EXPERIMENTAL SECTION 2.1. Materials CR-grade 1,6-hexanediol diacrylate (HDDA), isobornyl acrylate (IBOA), 2-(2-ethoxyethoxy) ethyl acrylate (EOEOEA), and 2-hydroxyethyl acrylate (HEA) were purchased from Eternal Specialty

Chemical

Co.,

Ltd.

AR-grade

2-hydroxy-2-methylpropiophenone

(1173),

azobis(isobutyronitrile) (AIBN), and 1H-imidazole (98%) were acquired from Energy Chemical. 2-Bromorthanol (98%) was supplied by Beijing HWRK Chemical Co., Ltd. Silver paste (KD-2) was obtained from Beijing Gangtuo Metallurgy Technology Research Institute. AR-grade sodium chloride, anhydrous magnesium sulfate, p-benzoquinone, and phosphorus pentoxide were



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procured from Beijing Chemical Works. Petroleum ether, ethyl acetate, dichloromethane, acetonitrile,

alcohol,

tetrahydrofuran

(THF),

deuterated

dimethylsulfoxide

(d6-DMSO),

deuterochloroform (d1-CDCl3) and triethylamine were purchased from Sinopharm Chemical Reagent Co., Ltd. Dichloromethane and acetonitrile were dried by refluxing with P2O5 prior to use. The synthesis and characterizations of IM-A are provided in the Supporting Information and the synthesis route is displayed in Scheme 1.

Scheme 1. Synthesis route of IM-A. 2.2. Fabrication of Self-healing Polymers Containing IM-A Fabrication of photocured materials: A series of formulas were designed according to the mass ratio of components, as listed in Table 1. 2-Hydroxy-2-methylpropiophenone (1173) (2 wt% relative to resins) as a photoinitiator was added in each formula. The obtained samples were mechanically agitated to obtain homogenous solutions, degassed under reduced pressure, and then cast in the Teflon mold (55 mm×6 mm×1 mm) to form liquid film with a thickness of 1 mm. Subsequently, the liquid film was exposed to UV light (incident light intensity 40 mW cm-2 and wavelength 365 nm, recorded by a UV radiometer [Photoelectric Instrument Factory, Beijing Normal University, Beijing, China]) for 60 s to obtain the self-healing polymer films. The basic concept in our approach is illustrated in Scheme 2.


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Scheme 2. (a) Schematic of the preparation of photocured films; (b) Synthesis reaction of the photocured films. The self-healing polymer films were afterwards annealed by slowly cooling from 120 °C to room temperature for over 12 h to minimize the residual stress in the materials 31. Fabrication of thermocured materials: To examine the effect of molecular weight on the self-healing properties, we synthesized three self-healing thermocured materials with different molecular weights by using the conventional free radical polymerization and characterized their self-healing properties. The procedure is as follows. According to the formula of IB7-IM5 (shown in Table 1), monomers (10 g) and thermoinitiator AIBN (1wt%) were dissolved in anhydrous acetonitrile (40 mL). The mixture was degassed under reduced pressure and then heated at 80 °C for 24 h. After the reaction, the solution was poured into a beaker with a large amount of ethyl acetate to precipitate the polymer, which was subsequently dissolved in alcohol (5 mL), cast in the Teflon mold, and then dried in a vacuum



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oven at 80 °C for 48 h to obtain the self-healing polymer film with a certain molecular weight (Sample T1-IB7-IM5)

32

. Samples T2-IB7-IM5 and T3-IB7-IM5 were prepared following the

above process with AIBN content of 2 wt% and 3wt% respectively and heating time of 36 h and 48 h respectively. Table 1. Formula of the photocured sample containing IM-A HEA:IM-A: IBOA: EOEOEA

IM-A content

IBOA content

(Mass ratio)

(mol%)

(mol%)

IB10-IM5

10:5:10:0

8

33

IB9-IM5

10:5:9:1

8

29

IB8-IM5

10:5:8:2

8

27

IB7-IM5

10:5:7:3

8

23

IB6-IM5

10:5:6:4

8

22

IB7-IM0

10:0:7:3

0

25

IB7-IM2

10:2:7:3

3

24

IB7-IM4

10:4:7:3

7

23

IB7-IM5

10:5:7:3

8

23

IB7-IM6

10:6:7:3

10

22

Sample

2.3. Characterization The 1H NMR and 13C NMR spectra were recorded with an AVANCE III Spectrometer (Bruker, Germany) operating at 400 MHz by using d6-DMSO or d1-CDCl3 as the solvent and tetramethylsilane as an internal reference. Electrospray ionization-mass spectrometry (ESI-MS) was performed with a Xevo G2 Qtof MS Spectrometer (Waters, America) equipped with an ESI source and was operated in positive ionization mode. The mobile phase was acetonitrile with a flow rate of 250 µL min−1, and the injection volume was 10 µL.


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Photopolymerization kinetics was investigated by the real-time FTIR on a Nicolet 50 XC Spectrometer (Thermo Scientific, USA). The samples were irradiated by an EfosLite mercury vapor curing lamp equipped with a fiber optic light guide for 200 s at an intensity of 40 mW cm−2 under nitrogen atmosphere. The real-time conversion rate of acrylate monomer was calculated by integrating the change in peak area of 6250 - 6100 cm−1 in the near-infrared region of the C=C bond 33. Dynamic mechanical thermal analysis was performed on a NETZSCH DMA 242 rheometer (Netzsch, Germany) equipped with a temperature-controlled test chamber and adopting the 10 mm parallel plate geometry. Tests were performed on the samples at a frequency of 1 Hz. The heating rate was 5 °C min−1, and the testing temperature ranged from -20 °C to 120 °C. Infrared spectra were recorded using a 70 V FTIR instrument (Bruker, Germany) and scanned between 500 and 4000 cm−1. Gel permeation chromatography analyses were carried out by an Agilent LC 1100 Series HPLC system (Agilent, USA) equipped with a Polymer Laboratory’s PLgel 5 µm mixed-C column at 25 °C by using THF as the eluent and polystyrenes of known molecular weight and dispersity as the standards at a flow rate of 1.0 mL min−1 to determine the average molecular weights (Mn) of the polymers. Rectangular specimens (55 mm×6 mm×1 mm) of all the samples in Table 1 were prepared to investigate the mechanical and self-healing properties of the photocured polymers. Each specimen was severed from the middle with a sharp knife. Then the two fractured surfaces were pressed together with hands before the damaged specimen was heated at 80 °C for 24 h in an oven. The original and healed specimens were tested by a Model 5966 tensile tester (Instron, USA) at a stretching rate of 200 mm min−1. Five parallel samples for each formula in Table 1 were prepared and tested, and the average value of the five samples was employed to assess the tensile stress and tensile strain of the samples. Finally, the healing efficiency of each formula was calculated by the ratio of the average strain (or stress) at break of the healed sample to that of the original sample. Volume electrical resistivities of the photocured materials were measured by a 2-point probe



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resistance measurement system using the Tektronix DMM 4050 6−1/2 Digit Precision Multimeter (Tektronix, USA). Change of electrical resistance of the electronic device was measured by a digital multimeter (Peak meter, China). 3. RESULTS AND DISCUSSION 3.1. Photopolymerization Kinetics The influences of IM-A content and ratio of IBOA to EOEOEA on the photopolymerization kinetics of the self-healing photocurable systems were investigated. As shown in Figure 1, the final conversions and photopolymerization rates of the self-healing photocurable systems did not vary significantly when the ratio of IBOA to EOEOEA and the content of IM-A changed. The photopolymerization rates of the photocurable systems increased rapidly after UV irradiation for 4 s and reached the maximum after UV irradiation for 10 s. Meanwhile, the final conversion of double bonds reached 95% in 20 s. These results indicate that the IM-A monomer has the same photopolymerization capability as the common photocurable monomers

34,35

, and the addition of

IM-A and the ratio of IBOA to EOEOEA exert insignificant influence on the photopolymerization kinetics of the self-healing photocurable systems.


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Figure 1. Photopolymerization kinetics of self-healing photocurable systems: (a) double-bond conversion and (b) polymerization rate of samples with different ratio of IBOA to EOEOEA; (c) double-bond conversion and (d) polymerization rate of samples with different content of IM-A. 3.2 Dynamic Mechanical Thermal Analysis As shown in Figure 2, for all samples except IB10-IM5 and IB9-IM5, their modulus plots exhibited several states (i.e., from glassy region to matrix-glass transition and then to rubbery plateau) with increasing temperature. As for IB10-IM5 and IB9-IM5, the moduli showed an additional glassy region. Unlike the common ionomers 36–38, our samples did not exhibit the ionic plateau and cluster glass transition, suggesting that IM-A did not form large ionic clusters or microphase separation in the solid state. As shown in Figure 2a, for the systems with varying ratio of IBOA to EOEOEA, the glassy region became narrow with the decrease in IBOA content, and the moduli drop also shifted to low temperature region. However, all samples showed the same modulus at the rubbery plateau. Figure 2b shows that the log E′ of the photocured materials was similar for all the samples irrespective of IM-A content. Nevertheless, the temperature range for the glassy state decreased with the increase in IM-A content. Thus, the position of the modulus drop related to the matrix glass transition also shifted to low temperature region. Moreover, all samples exhibited the same modulus at the rubbery plateau.



Figure 2. Storage modulus (E′) plots as a function of temperature for photocured materials: (a) different ratio of IBOA to EOEOEA; (b) different IM-A content. As depicted in Figure 3a, sample IB10-IM5 exhibited two tan δ peaks, which was attributed to the microphase separation caused by the great polarity difference between IBOA and other monomers. With the decrease in IBOA content, the second tan δ peak shifted to low temperature region and gradually integrated with the first tan δ peak. When the IBOA content was lower than 27 mol%, the second tan δ peak disappeared, and the first tan δ peak shifted slowly from 26 °C to 11 °C, which could be associated with the peak height that increased from 0.61 to 1.20. These results indicate that the decrease in IBOA content enhanced the compatibility and flexibility of the copolymer. Figure 3b shows that the samples with different contents of IM-A had one tan δ peak even if IM-A content reached 10 mol%, demonstrating that IM-A has good compatibility with acrylate monomers. With the increase in IM-A content, the position of the tan δ peak shifted from 33 °C to 20 °C, which could be associated with the peak height that decreased from 1.65 to 1.01, implying that the mobility of the polymer chain was enhanced.


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Figure 3. Tan δ plots as a function of temperature for photocured materials: (a) different ratio of IBOA to EOEOEA; (b) different IM-A content. 3.3 Mechanical Properties of Photocured Films Before Cut and After Self-Healing The mechanical properties of the original and healed photocured materials with different ratios of IBOA/EOEOEA and different contents of IM-A were investigated. The self-healing process was recorded in Movie S1, and some photos taken during the process are shown in Figure 4.

Figure 4. Pictures of self-healing (For the convenience of observation, the samples were colored by orange and black dyes). The tensile stress and strain of the original and healed materials are displayed in Figures 5 and S5. Young’s moduli were obtained from the tangent of the stress-strain curves (Table S1). As shown in Figures 5a and 5b, with the mass ratio of IBOA to EOEOEA varying from 10:0 to 6:4, the average tensile stress decreased from 10 MPa to 1 MPa, but the tensile strain, restoration of break strain, and restoration of break stress increased from 4% to 235%, 68% to 93%, and 30% to 98% respectively. These results can be attributed to the decrease in the content of the hard monomer IBOA and the increase in the content of the soft monomer EOEOEA. When IBOA content is high, the rigid six-membered ring of IBOA in the polymeric matrix restricts the free movement of the polymer chains 39, whereas the 2-(2-ethoxyethoxy)ethyl side chains of EOEOEA can weaken the cohesion of the polymeric matrix by reducing the van der Waals force between the polymer main chains

40,41

. Therefore, decreasing IBOA content and increasing EOEOEA content



boost the mobility of the polymer chains, thereby enhancing the contact between dissociative IM+ and Br− at the fractured surfaces, which results in the recombination of the surfaces via ionic interactions to achieve self-healing of the materials. Figures 5c and 5d show that as IM-A content increased from 0 mol% to 10 mol%, IBOA content reduced from 25 mol% to 22 mol%. When the ionic monomer content was lower than 7 mol%, the ionic interactions was too weak to offset the decrease in strength induced by the reduction in IBOA content. When the ionic monomer content exceeded 7 mol%, the ionic interactions played a decisive role in the mechanical properties of the materials. Thus, the tensile strength initially decreased from 2.82 MPa to 2.30 MPa and then increased to 3.29 MPa. By contrast, the elongation at break initially increased from 244% to 260% and then decreased to 179%. Nevertheless, the restoration of tensile stress and tensile strain gradually increased with the increase of IM-A content; tensile stress increased from 40% to 93%, and elongation at break increased from 53% to 95%. The results indicate that the dynamic reversible property of ionic interactions enables the polymer network to reorganize and heal successfully.


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Figure 5. Mechanical properties of the photocured films before cut and after self-healing: (a) stress with different ratios of EOEOEA to IBOA; (b) strain with different ratios of EOEOEA to IBOA; (c) stress with different IM-A contents; and (d) strain with different IM-A contents. The control sample IB7-IM0 without IM-A also showed limited self-healing ability under the same heating program possibly due to hydrogen bonds between the hydroxyl groups of HEA. In fact, hydrogen bonds also play an important role in the self-healing of materials. All samples except IB7-IM0 have two possible hydrogen bonding interactions. One is the interaction between ions and hydroxyl groups, and the other is the interaction between hydroxyl groups 17. As shown in the FTIR spectra of the samples (Figure 6), the O-H stretching vibration band at 3600–3300 cm–1 slightly shifted with the increase of IM-A content because of the hydrogen bonding interactions between ions and hydroxyl groups. Therefore, the increase in IM-A content not only enhanced the ionic interactions, but also increased the hydrogen bonding interactions. The self-healing process of the photocured polymer is shown in Scheme 3.



Figure 6. FTIR spectra of photocured polymers with different IM-A contents

Scheme 3. Illustration of self-healing process of the photocured polymer. 3.4. Self-Healing Conditions As shown in Figure 5, among the nine investigated samples, sample IB7-IM5 (mass ratio of HEA: IM-A: IBOA: EOEOEA=10:5:7:3, 1173: 2 wt%) with 3.1 MPa tensile strength and 205%


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elongation at break has balanced mechanical properties and self-healing ability. After heated at 80 °C for 24 h, the damaged sample IB7-IM5 exhibited a healing efficiency of ~93%, and its stress and strain at break were similar to those of the original material. Therefore, the self-healing behavior of sample IB7-IM5 at different healing temperatures and times was investigated as an example. As shown in Figure 7a and Table S2, the healing efficiency increased with the extension of the healing time. After healed at room temperature (25 °C) for 1 day, the sample exhibited an elongation at break of 159%. After healed for 7 days, the sample withstood a strain of 182% (healing efficiency of ~89%) and a stress of 1.39 MPa (healing efficiency of ~80%), indicating that a certain degree of dynamic movement already existed at room temperature, which allowed the polymer chains and ionic associates to rearrange. As shown in Figure 7b and Table S2, the self-healing efficiency reached ~96% for tensile strain and ~92% for tensile stress as the temperature increased to 120 °C for 12 h. This phenomenon is attributed to the reduced internal cohesion of the ionic interactions and hydrogen bonds, and good chain mobility at high temperature 42. The damage-healing cycles were repeated for three times for sample IB7-IM5, and the stress-strain curves and healing efficiency are shown in Figure 7c and Table 2. Sample IB7-IM5 showed a nearly constant healing efficiency in the three damage-healing cycles, and the stress healing efficiencies of stress were 93.7%, 91.7%, and 90.2% for the first, second and third cycle, respectively. The results manifest that self-healing could be repeated for several times. The healing efficiency slightly dropped for each damage-healing cycle, which may be attributed to the unhealable bonds that were damaged 43,44.



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Figure 7. Self-healing characteristics of IB7-IM5: (a) stress-strain curves for different healing times at room temperature (25 °C); (b) stress-strain curves for different healing times at 120 °C; and (c) stress-strain curves for three damage-healing cycles (healing at 80 °C for 24h). Table 2. Healing efficiency of IB7-IM5 Healing efficiency

1st cycle

2st cycle

3st cycle

Estress (%)

93.7

91.7

90.2

Estrain (%)

91.0

89.7

87.7

3.5. Effect of Molecular Weight on Self-Healing Molecular weight is an important parameter that affects the performance of polymers significantly. In general, high molecular weight corresponds to good strength, toughness, thermal properties, and chemical resistance 45,46. However, high molecular weight may play a negative role in the self-healing of materials. To examine the effect of molecular weight on the self-healing properties of IB7-IM5, we synthesized three thermocured polymers with different molecular weight by using conventional free radical polymerization according to the formula of IB7-IM5 (T1-IB7-IM5, Mn=1.99×104 g mol−1; T2-IB7-IM5, Mn=3.32×104 g mol−1; T3-IB7-IM5, Mn=5.35×104 g mol−1). As shown in Figure 8, when the molecular weight of the thermocured polymers increased from 1.99×104 g mol−1 to 5.35×104 g mol−1, the elongation at break decreased from 309% to 237%, while the tensile strength increased from 0.43 MPa to 0.56 MPa. Moreover, the healing time


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increased from 30 min to 120 min. These results are attributed to the fact that low molecular weight leads to a high mobility of polymer chains

47

. The elongation at break (309%) of the

thermocured T1-IB7-IM5 was 1.5 times of that of the photocured IB7-IM5 (205%), but the tensile strength of T1-IB7-IM5 (0.43 MPa) was approximately 7 times lower than that of the photocured IB7-IM5 (3.1 MPa).

Figure 8. Self-healing characteristics of thermocured T1-IB7-IM5, T2-IB7-IM5 and T3-IB7-IM5: healing temperature for the three thermocured polymers was 80 °C, healing time for T1-IB7-IM5, T2-IB7-IM5 and T3-IB7-IM5 was 30 min, 60 min and 120 min, respectively. Figure 9 shows the dissolution behavior of thermocured T1-IB7-IM5 and photocured IB7-IM5. When the samples were immersed in methanol and subjected to ultrasonic treatment for 10 min, a transparent solution formed due to the dissolution of thermocured T1-IB7-IM5, whereas photocured IB7-IM5 only swelled. Photocured and thermocured polymers were synthesized via free radical polymerization as shown in Scheme 4. The main difference in their polymerization mechanism is that the photoinitiator generates radicals under UV-light irradiation, while the thermoinitiator produces radicals under heating. In addition, compared with the thermocuring process, more chain transfer reactions to macromolecules occur in the photocuring process to form polymers with cross-linked structure (c in Scheme 4) because of fast photopolymerization rate 48. Therefore, photocured IB7-IM5 exhibits better mechanical properties, but needs longer healing time compared with thermocured T1-IB7-IM5.



Figure 9. Dissolution behavior of thermocured T1-IB7-IM5 and photocured IB7-IM5.

Scheme 4. Polymerization mechanism of thermocured and photocured polymers. 3.6. Self-healing Flexible Electronic Device


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As mentioned previously, self-healing flexible electronics have been attracting considerable attention recently 49,50. As the support of flexible electronics, flexible substrate is often required to have light weight, electrical insulation, mechanical flexibility, and self-healing characteristics. To investigate the potential of our photocured films as the substrate of flexible electronics, we measured the electrical conductivity of sample IB7-IM5. The result shows that the IB7-IM5 film displayed a volume resistivity over 2.3×1010 Ω·cm, which is beyond the critical resistance for electrical insulation (109 Ω·cm)

28

. This finding indicates that the IB7-IM5 substrate is a good

electrical insulator. Afterward, the commercial silver paste was spin-coated on the newly prepared IB7-IM5 substrate and then dried at 40 °C for 5 h in an oven to form a conductive film with a thickness in 20 µm. Then, the silver-functionalized flexible electronic device was connected in series with a light-emitting diode (LED) and a battery by electric wires to construct a circuit, and the resistance was measured by a digital multimeter (Figure S6). As shown in Figure 10, Figure S6, and Movie S2, this flexible electronic device can be folded and twisted without the change in its resistance (R=7.1±0.1 Ω). After the flexible electronic device was severed by a sharp knife and then healed at 80 °C for 24 h, its resistance (R=7.2±0.1 Ω) completely recovered. The flexible electronic device was restored by a healable substrate to convey the healing function of the polymer layer to the unhealable silver layer 51-53.

Figure 10. Healing of the flexible electronic device connected in series with a LED and a battery. 4. CONCLUSION



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A novel imidazole ionic-based photocurable monomer (IM-A) was successfully synthesized, and self-healing photocured materials were fabricated using a green photopolymerization process in only 20–60 s by carefully adjusting the content of HEA, IM-A, IBOA, and EOEOEA. The IM-A monomer showed superior photopolymerization performance and good compatibility with acrylate monomers. The as-prepared self-healing material IB7-IM5 exhibited good mechanical properties, and its elongation at break and tensile strength reached 205% and 3.10 MPa, respectively. The material also demonstrated a high healing efficiency that reached up to ~93% for the stress and strain after healing at 80 °C for 24 h as well as a wide healing temperature window (i.e. from room temperature to 120 °C). The increase in healing temperature can greatly stimulate free movement of the polymer chains to accelerate the healing process. Importantly, the self-healing material exhibits the characteristics of light weight, electrical insulation, and flexibility necessary for the substrate of flexible electronics. Thus, the self-healing flexible electronic device prepared by coating commercial silver paste onto the photocured substrate can be folded, twisted, and self-healed without change in its resistance. The photocured self-healing materials based on ionic interactions demonstrate a great potential application in durable, reliable, and flexible electronics.

ASSOCIATED CONTENT

Supporting Information Additional experimental details, material characterization data (1H NMR,

13

C NMR), and

strain-stress curves for all samples (PDF) Movie of the self-healing process of the photocured material shown in Movie S1 (AVI) Movie of the self-healing process of the flexible electronic device prepared using IB7-IM5 and commercial silver paste shown in Movie S2 (AVI)

AUTHOR INFORMATION

Corresponding Author


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*E-mail: [email protected] ORCID Fang Sun: 0000-0003-4064-1921 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

We gratefully acknowledge the financial support from the National Key R&D Plan (2017YFB0307800), the Special Funds of Jiangsu Province Key Research and Development (BE2015058) and National Natural Science Foundation of China (Grant no. 51273014).

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TOC Graphic 324x165mm (300 x 300 DPI)


Photocured materials with self-healing function through ionic

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