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Photothermal Conversion Triggered Precisely Targeted Healing of Epoxy Resin Based on Thermo-reversible DielsAlder Network and Amino-functionalized Carbon Nanotubes Qiu-Tong Li, Miao-Jie Jiang, Gang Wu, Li Chen, Si-Chong Chen, Yu-Xiao Cao, and Yu-Zhong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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ACS Applied Materials & Interfaces

Photothermal Conversion Triggered Precisely Targeted Healing of Epoxy Resin Based on Thermoreversible Diels-Alder Network and Aminofunctionalized Carbon Nanotubes Qiu-Tong Li, Miao-Jie Jiang, Gang Wu, Li Chen, Si-Chong Chen,* Yu-Xiao Cao, Yu-Zhong Wang* National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, College of Chemistry, Sichuan University, Chengdu, 610064, China. E-mail: [email protected], [email protected] KEYWORDS: epoxy resin, Diels-Alder reaction, photothermal conversion, recycle, self-healing ABSTRACT: In the present work, we demonstrated the recyclability and precisely targeted reparability of amino functionalized multi-wall carbon nanotubes/epoxy resin based on dynamic covalent Diels-Alder (DA) network (NH2-MWCNTs/DA-epoxy) by exploring the photothermal conversion of CNTs to trigger the reactions of dynamic chemical bonds. The covalent crosslinked networks of NH2-MWCNTs/DA-epoxy resin change their topology to linear polymer

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by thermally activated reverse Diels-Alder (r-DA) reactions at high temperatures, which endues the resin with almost 100% recyclability. The self-healing property of the epoxy resin was confirmed by complete elimination of cracks after the reconstruction of DA network induced by heating or near-infrared (NIR) irradiation. For heat triggered self-healing process, heat energy may also act on those uninjured parts of the resin and cause the dissociation of whole DAnetwork. Therefore, redundant r-DA/DA reactions, which have no contribution to self-healing, are also triggered during thermal treatment, resulting in not only waste of energy, but also deformation of the sample under external force. While for NIR triggered self-healing process, the samples can well maintain their original shape without observable deformation after irradiation. The NIR triggered healing process, which using MWCNTs as the photothermal convertor, have very good regional controllability by simply tuning the MWCNTs content, the distance from NIR laser source to sample, and the laser power. The injured samples can be locally repaired with high precision and efficiency, without obvious influence on those uninjured parts. 1. INTRODUCTION Epoxy resin, owing to its excellent thermal stability and solvent resistance, satisfactory electrical and mechanical properties, remarkable adhesive strength, ease of curing and processing, have long been considered as one of the most extensively applied thermosetting polymers in a wide range of fields, including printed circuit boards, potting materials for electronics, coatings and adhesives, etc.1-6 However, epoxy resins are brittle and likely to initiate minor defects during service. The formed defect may gradually propagate into large damage within a short time, and cause failure of the whole system. What’s more, epoxy systems are difficult to recycle at the end of their life because of its covalently cross-linked network.7-9 The increase in epoxy resins wastes has resulted in


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healthy problems and environmental pollution. There is thus an urgent need to recycle or reuse epoxy resins. Several technologies have emerged for the self-healing of epoxy-based systems over the past several years, included polymers containing microcapsules,5,10-13 vascular systems14, 15 and dynamic covalent bonds16-31 and so on. The self-healing properties of microcapsule-based composites were achieved by incorporating microcapsules which contained reactive chemicals into the epoxy matrix. Upon crack damage, the microcapsules can release its chemicals and recover the defect. However, the resin can only be healed for only once through this method, which limits its application. Moreover, its synthetic process of vascular systems is too complex to produce in mass. In alternative approach, self-healing is realized by the modification of polymers with functional group based on dynamic bond. Dynamic bond, which can unlock the cross-linked network under external stimuli, can impart the properties of both self-healing and recycling to resins. Such as ester exchange,16-18 disulfide exchange,19-21 Diels-Alder (DA) bond,22-30,39 are generally used for reversible network formation. Among all healing systems based on dynamic bonds to date, DA bond as result of its high efficient reversibility and moderate sensitivity to temperature,22 is supposed to be one of the most reliable systems. The Wudl group23, 24 demonstrated self-healing of covalent bond polymeric networks by using the thermoreversible nature of the DA reaction between furfuryl and maleimide groups. Following Wudl’s approach, some researchers developed thermo-healable epoxy resin by modifying the epoxy monomer with DA group. Liu and Hseih25 set epoxy compounds as precursors to simply synthesize thermally mendable cross-linked polymers. Subsequently, epoxy


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resin with furan groups was made by Tian et. al.26,

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and reacted with bifunctional

maleimide to form a cross-linked epoxy which can heal the cracks resulting from reverse DA (r-DA) and DA reactions. However, this method of modifying the epoxy monomer with DA group is too expensive to widely implement and not economic for such common, commercially available epoxy monomers. So in subsequent researches, DA group was incorporated into the diamine cross-linker rather than epoxy monomer as healing building blocks to establish a reversible network in the epoxy matrix, which can be used for a diverse range of different commercial epoxy monomers in industry. Bai N et al.28,

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synthesized a new diamine cross-linker with DA adducts and used to cure epoxy monomers to yield a new self-healing epoxy resin. However, unwanted side-reaction between maleimides and free amines may also occur during the cross-linking process. Kuang et al.30 synthesized a newly diamine DA adduct cross-linker to cure common epoxy monomers, but the synthesis process is too complex to avoid the side-reaction that amino groups were protected by Boc group. Subsequently, Turkenburg et al.31 developed a twostep process to minimize the side-reaction, the first step is a prepolymerization of furfuryl amine with epoxy monomers, then the prepolymer was cross-linked with BMI to obtain epoxy resin with self-healing property. However, the key limitation for self-healing of dynamic covalent cross-linking system is that using heat as stimuli to repair damaged resin is difficult for operation when the materials are in service. The heat energy may also act on those uninjured parts of the resin. Therefore, redundant r-DA/DA reactions, which have no contribution to selfhealing, are also triggered during thermal treatment, resulting in not only waste of energy, but also deformation or pyrolysis of the materials because of strong molecular chain


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motion at high temperature. Therefore, it is impossible for achieving precisely targeted repair of thermoset resin when using heat as the stimuli. Here we presented a novel approach to overcome the abovementioned problems simultaneously by using photothermal conversion to trigger the r-DA/DA reactions. Epoxy resin based on thermo-reversible Diels-Alder network (DA-epoxy) were prepared, and DA bond, as a switch to “lock” or “unlock” the cross-linked network in system, endowed resins with the properties of both recycling and self-healing. The amino-functionalized carbon nanotubes (NH2-MWCNTs) were introduced to the DA-epoxy resin for in-situ converting near-infrared light (NIR) to thermal energy because of its efficient conversion performance32, 33 and well dispersion in epoxy resin.34 Comparing with heat, light, as a remote stimuli, is more controllable and can be switch on or off immediately among applications.35 Therefore, without additional tuning and processing, the obtained NH2MWCNTs/DA-epoxy resins exhibited very good recyclability and heat/light dual-stimuli self-healing properties. Moreover, targeted repair of the resin could be successfully and easily achieved without obvious influence on those uninjured part owing to the precise location of NIR laser and high efficient in-situ photothermal conversion of well-dispersed NH2-MWCNTs, which not only extend the service life of materials, but also reduce waste of energy. 2. EXPERIMENTAL SECTION 2.1.

Materials. The bisphenol A diglycidyl ether type epoxy resin E-51 was supplied by

Bluestar New Chemical Materials Co. China. 2-Furylmethylamine (FA), 1,2-diaminoethane, 1,4diaminobutane, 1,6-diaminohexane and 1,8-diaminooctane were supplied by Aldrich. Maleic anhydride, ethylenediamine, dimethylformamide, dichloromethane, sodium bicarbonate were


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provided by Kelong Reagent Corp (China). All the chemicals were used without further purification. Two types of multi-wall carbon nanotubes, amino modified (NH2-MWCNTs) and unmodified (MWCNTs), were purchase from Chengdu Organic Chemicals Co. Ltd. (Purity >95wt %, outer diameter 8~15 nm, as-produced length ~50 µm) 2.2. Preparation of E-51/FA/NH2-MWCNTs prepolymer. The reaction of E-51 with amino groups of furylamine (FA) and NH2-MWCNTs was performed as described in the literature with minor modifications.31 Briefly, E-51 (20g, 0.051 mol) and NH2-MWCNTs with different contents (= 0.2%, 0.5%, 1% and 2% of E-51 in weight) were mixed directly with a magnetic stir and under ultrasound for 30min. Then, 3.94 g of FA (0.041 mol) was added, and the mixture was gradually heated to 125oC in 30min and kept for 1.5h under magnetic stirring. The reaction product obtained as black viscous state and cooled to 25oC resulting in a polymeric black brittle material (yield of 90%) and expressed as “prepolymer”. The control samples with pristine MWCNTs or without CNTs was synthesized for the same heating process. Then grinding and sieving reserved. 2.3. Synthesis of bismaleimide (BMI). Bismaleimide (BMI) was prepared as the method reported36 in two step, took 1,8-bis(maleimido)octane (BMO) for example: first step involved the addition of 1 equivalent of 1,8-diaminooctane to 2.1 equivalents of maleic anhydride to formed bisamic acid in DMF. The second step induced the dehydration of this amic acid end-group and cyclization through the importation of sodium acetate and acetic anhydride. The 1H-NMR spectrum of BMO was shown in Figure S2. 1H NMR (400 MHz, DMSO-d6, δ) 7.00 (s, 4H), 3.403.35 (t, 4H), 1.50-1.41(m, 4H), 1.26-1.16 (m, 8H). 2.4. Synthesis of NH2-MWCNTs/DA-epoxy resins via batch extrusion. The reaction of BMO and prepolymer was carried out in the melt using a HAKKE mini lab II double screw extruder at


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150oC for 10min with the rotation speed was 100 rpm. The melt was circulated through a feedback loop for 5 min to ensure homogeneous distribution. Finally, black solid was obtained and expressed as NH2-MWCNTs/DA-epoxy resin (or pristine-MWCNTs/DA-epoxy resin). 2.5. Solvent exposure test. Typically, 0.1 g of resin was submersed in a solvent in a closed flask and kept at room temperature under static conditions. After 1 day, samples were taken out to determine the mass of the residue. The swelling ratio was calculated as: Swelling ratio =

Wa × 100% W

W: Weight of NH2-MWCNTs/epoxy resin before reaction; Wa: Weight of NH2-MWCNTs/epoxy resin after immersing without drying. Subsequently, samples were dried to constant weight in a vacuum oven at 60oC to remove its adhering solvent for 24 h before further characterization, the gel ratio was calculated as: Gel ratio =

Wb × 100% W

Wb: Weight of immersed epoxy resin after drying. 2.6. Recovery of NH2-MWCNTs/DA-epoxy resins. Recovery process was carried out in a 150mL single-necked round-bottomed flask, equipped with a reflux condenser. Typically, the resin was added in DMF at 120oC. After 2 days, the resin was de-crosslinked and dissolved in solvent, the insoluble substances, included that the reaction product of NH2-MWCNTs and E-51, were separated from solution by filtration. Then deionized water was added to the liquid products and suspension was obtained. Finally, recycled epoxy monomer was received by centrifuging and drying to a constant weight. The recovery ratio was calculated as: Recovery ratio = Wc + Wr × 100% W

W: Weight of NH2-MWCNTs/epoxy resin before reaction;


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Wc: Weight of the production of NH2-MWCNTs and E-51 separated from dissolving solvent; Wr: Weight of recycled epoxy monomers. 2.7. Characterization and Measurements. A NICOLET 6700 apparatus (Thermo Electron Corporation, USA) has been used to measure the IR absorbance of E-51, FA, BMO, prepolymers and their Diels-Alder adducts in transmission. 1

H-NMR measurement was conducted with a Bruker AC400 Fourier transform spectrometer

operating at 400 MHz. Unless noted otherwise spectra were recorded in DMF and in reference to a TMS standard. Differential scanning calorimetry (DSC) was performed on a DSC Q200 instrument (TA, USA) over the temperature range from 0 to 200oC at a heating (or cooling) rate of 10 °C·min−1 under a steady flow of ultra-high-purity nitrogen purge and an empty aluminum as the reference. Gel permeation chromatography (GPC) of prepolymers were carried out with a Waters 1525 GPC system (Waters Corporation, USA) equipped with a refractive index detector. Dimethylformamide (DMF) acted as elution solvent with two columns (PLgel 5 µm MIXED-C) operated at a flow rate of 1 mL·min−1. The column temperature was set at 35 °C, and polystyrene as standard calibrators. GPC of r-DA products were carried out with a Tosoh Ecosec HLC-8320 GPC system (Tosoh Corporation, Japan) equipped with a refractive index detector. Dimethylformamide (DMF) acted as elution solvent with two columns (TSK gel Super AWM-H) operated at a flow rate of 0.4 mL·min−1. The column temperature was set at 40 °C, and polymethylmethacrylate as standard calibrators. A razor blade was used to make cracks on the sample surface, and the cracks were about 30 micrometers in width. Self-healing was processed by heating at 120oC or irradiating under an NIR laser light. Images of cracks on sample surface before and after healing were obtained with


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an optical microscope (ECLIPSE LV100 POL, NIKON) equipped with a digital camera. Fracture morphology of samples were observed by a field emission scanning electron microscopy (JSM5900LV, JEOL, Japan). The samples were irradiated by an NIR laser (LSR808H-FC-7W, Lasever) vertically for 5min, the temperature variation was recorded by Thermal infrared imaging FLTR T460 during this period. The tensile properties of the samples were measured on an Instron Universal Testing Machine (Model 4302, Instron Engineering Corporation) at a crosshead speed of 5 mm/min. The testing temperature and humidity were 25 °C and 50%, respectively. 3. RESULTS AND DISCUSSION 3.1. Synthesis, preparation and microstructure analysis of NH2-MWCNTs/DA-epoxy resins. The structure of bisphenol A diglycidyl ether type epoxy resin E-51 was shown in Figure 1(a). The epoxy values (n) of E-51 is approximately equal to 0.13, and its average molecular weight was calculated to be 392.16. NH2-MWCNTs/DA-epoxy resins based on thermo-reversible DA-bonds were synthesized with different NH2-MWCNTs contents. As showed in Figure 1(a), the first step involved the of epoxy monomer (E-51) with amino groups of NH2MWCNTs and FA. Since one primary amine could react with two epoxide groups forming tertiary amine, both of the NH2-MWCNTs and FA can be incorporated into epoxy prepolymer. The ring-opening reaction of epoxy groups was confirmed by FT-IR and shown in Figure 1(b). Comparing the FT-IR spectrum of E-51, FA and prepolymer, the absorption peaks at approximately 917 cm-1 and 764 cm-1 of the prepolymer, which corresponding to the stretching vibration of the epoxy groups, were weaker than those of E-51. The absorption peak at approximately 3400 cm-1 of the prepolymer, which was assigned to the stretching


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vibration of O-H groups, was much wider and stronger than that of E-51. It should be due to the addition of FA, which caused the ring-opening of epoxy groups resulting in the production of O-H groups. The monosubstituted furan signal at 737cm-1 was observed after prepolymerization, proved that furan groups had been introduced into the system. In addition, the molecular weight of prepolymers after removing MWCNTs were determined by gel permeation chromatography (GPC) and listed in Table 1. The GPC traces of the prepolymer in Figure S1 showed a distinct peak, while the negative, inverted signal belonged to the DMF mobile phase. All samples had similar number average molecular weight of 2.6~2.9×103 g/mol and polydispersity of 2.12~2.18, which were in good accordance with the feed ratio of BMO and E-51, indicated that the prepolymer had been successfully synthesized with a desired chain length. Table 1. GPC data of prepolymer with different content of NH2-MWCNTs. Content of

Mn

Mw

NH2-MWCNTs (%)

(103 g/mol)

(103 g/mol)

0.2

2.9

6.4

2.18

0.5

2.8

5.9

2.12

1

2.8

5.9

2.15

2

2.6

5.6

2.12

PDI

Bismaleimide (BMI) can react with furan rings of prepolymer to yield cured DA-epoxy resins. Among four different BMI, 1,8-bis(maleimido)octane (BMO, Figure S2) was chosen for preparation of DA-epoxy resins because it had relatively low melting point (112oC, Figure S3), which is in favor of mixing, processing and reconstruction of DA network. The NH2-MWCNTs, BMO and prepolymer have been mixed and processed at


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150oC to ensure homogeneous distribution of NH2-MWCNTs, then DA bonds were formed during cooling to room temperature (Figure 1(a)).

Figure 1. (a) The synthesis route of prepolymer and NH2-MWCNTs/DA-epoxy resin. (b) FT-IR spectra of E-51, FA and prepolymer. (c) FT-IR spectra of BMO, prepolymer and NH2MWCNTs/DA-epoxy resin. (d) DSC curves of the NH2-MWCNTs/DA-epoxy resin with different NH2-MWCNTs contents. The FT-IR spectrum of BMO, prepolymer and NH2-MWCNTs/DA-epoxy resin were also recorded and compared to illustrate the DA-reaction (Figure 1(c)). The signal at 1710 cm-1 in FT-IR spectrum of the epoxy resin was observed which corresponding to the carbonyl group of BMO residues. The formation of cross-linked network in resin was demonstrated by the absorption peaks at 1770 cm-1, which was a characteristic absorption peak standing for DA addition product.37, 38 In addition, the absorption peak at 737 cm-1


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of the NH2-MWCNTs/DA-epoxy resin, which was assigned to the monosubstituted furan groups, was much weaker than that of prepolymer due to the DA addition of FA rings and BMO. Moreover, DSC was performed for further confirmation of the DA bond in the resin as illustrating in Figure 1(d). Tg peak was emerged at 60oC, while at temperatures above 100oC there was a broad endothermic peak corresponding to the r-DA reaction of the resin. Dispersion of carbon nanotube in polymer matrix is an unneglectable issue for its application. To demonstrate the role of NH2-groups in the dispersion of MWCNTs, pristine-MWCNTs/DA-epoxy resin were prepared as control samples. Images (Figure 2(a) and (b)) of the fracture surface of resins with different MWCNTs type and content were photographed using a scanning electron microscope (SEM), while the high resolution SEM images of NH2-MWCNTs/epoxy resins were also presented in Figure S4. As shown in Figure 2(a), 2(b) and S4, the dispersion of carbon nanotubes in NH2MWCNTs/epoxy resin were much better than those of pristine-MWCNTs/epoxy resin with same MWCNTs contents. Obvious aggregation of MWCNTs was observed for those pristine-MWCNTs/epoxy resins especially with high MWCNTs content, as highlighted by red cycle in Figure 2(b). The amino groups of NH2-MWCNTs can react with the epoxy monomers to form epoxy modified MWCNTs, and thus improved the compatibility between MWCNTs and resin matrix. To verify the formation of epoxy modified MWCNTs, we conducted the reaction of E51 with NH2-MWCNTs at 125°C. After removing the unreacted epoxy monomers by solvent extraction, the epoxy modified MWCNTs was obtained and characterized by FTIR and TG (as shown in Figure 2(c) and 2(d)). Characteristic peak of epoxy group at


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about 910cm-1 was observed in the FT-IR spectrum of epoxy modified MWCNTs. Meanwhile, the absorption peak at 1298 cm-1 stood for tertiary amine, which indicated that epoxy rings were opened by the amino groups of NH2-MWCNTs. In addition, the absorption peaks at 1500 cm-1, 1249 cm-1 and 1180 cm-1 were assigned to E-51 chemically bonded to MWCNTs. The TG curve of epoxy modified MWCNTs showed about 30% of mass loss after pyrolysis, which was obviously higher than that of NH2-MWCNTs, also indicating the formation of epoxy modified MWCNTs.

Figure 2. SEM images of resins with different content of (a) NH2-MWCNTs; (b) MWCNTs. Scale bars: 10 µm. FT-IR spectra (c) and TG analysis (d) of NH2-MWCNTs and NH2-MWCNTs/E-51.


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3.2. Solvent exposure and recyclability. Solvent exposure tests were carried out to evaluate the crosslink density of NH2-MWCNTs/DA-epoxy resins. As listed in Table 2, the swelling ratio and gel ratio of all NH2-MWCNTs/DA-epoxy resins were about 140 wt% and 90 wt%, respectively, suggested that the resins had high cross-link density after reacting with BMO. As shown in Figure 3(a), the solvent remain colorless after immersing the NH2-MWCNTs/DA-epoxy resin in DMF for two days, which confirmed the formation of networks. However, after heating at 120oC in DMF for 48 hours, as shown in Figure 3(b), almost all resins were dissolved with only a few of NH2-MWCNTs settling at the bottom of the vials, and the color of the solution changed from colorless to brown. Owning to the r-DA reaction at high temperature, the cross-linked network of resins based on DA bonds were broken, resulting in dissolution of the r-DA products. The covalent crosslinked networks of NH2-MWCNTs/DA-epoxy resin change their topology to linear polymer by thermally activated r-DA reactions at high temperatures, which endue the resin with potential recyclability. Recovery experiment of NH2-MWCNTs/epoxy resin was carried out at 120oC using DMF as the solvent. After cooling to room temperature and being precipitated in excess water, the recycled products were obtained (Figure 3(e), remarked as r-DA products). As shown in Table 2, a recovery ratio higher than 90% was achieved. The r-DA products were soluble in DMF, indicating that the cross-linked network was disconnected by rDA reaction. The r-DA products were further characterized by gel permeation chromatography (GPC), FT-IR and 1H-NMR. The GPC data of r-DA products were also listed in Table 2, it manifested that the resins can be almost all recycled into small linear molecular with low molecular weight after treating in DMF. The GPC traces of r-DA products after removing MWCNTs (Figure 3(d)) suggested that the r-DA products had a bimodal distribution with relatively large PDI. The two peak values at 2.1~3.0×103 g/mol and 0.42~0.50×103 g/mol were


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recorded as Mp1 and Mp2, respectively. This phenomenon could be attributed to the release of BMO, whose molecular weight is 304 g/mol, from de-crosslinked resins together with prepolymer after r-DA reaction. Therefore, the Mn of r-DA product was much smaller than that of corresponding prepolymer. These results confirmed again that r-DA reaction did occur and as a consequence, the prepolymer and BMO were released and dissolved in the DMF. To further investigate the reversibility of the resin, the r-DA products were thermal treated at 80oC for 2 days in order to conduct the reconnection of DA bond. The recovered resin, as shown in Figure 3(c), swelled rather than dissolved in DMF, which further demonstrated the reversibility of DA bond in the resin. FT-IR spectra (Figure 3(f)) also suggested that the DA characteristic peak at 1770cm-1 was not detected after recycling. 1H-NMR spectrum of the r-DA product (Figure S5) showed resonances at 7.5 ppm, 6.26 ppm and 6.36 ppm, which belonged to the furan groups. In addition, the peaks of BMO at 7.0 ppm, 1.55 ppm and 1.25 ppm were also observed. As expected, no resonance was observed at 5.23 ppm, which corresponding to the DA adduct 22, 39. Table 2. The recovery ratio of NH2-MWCNTs/DA-epoxy resins and GPC data after recycling. Content of

Swelling

Recovery

Molecular weight of

ratio

r-DA products (103 g/mol)

Gel ratio NH2-MWCNTs

ratio (%)

(%)

(%)

(%)

Mn

Mw

Mp1

Mp2

0.2

150.1

89.2

93.5

0.63

5.3

3.0

0.50

0.5

139.4

88.1

92.5

0.55

6.8

2.1

0.46

1

139.2

89.0

94.0

0.47

3.1

2.1

0.50

2

140.6

90.5

94.6

0.43

2.8

2.1

0.42


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Figure 3. Solubility of NH2-MWCNTs/DA-epoxy resin in DMF (a) at room temperature; (b) after heating at 120oC for 2 days; and (c) recovered resin at room temperature. (d) GPC traces of r-DA products after removing MWCNTs. The photographs (e) and FT-IR spectra (f) of NH2MWCNTs/DA-epoxy resin before and after recycling. 3.3. Self-healing properties. The thermal- and light-induced self-healing properties of the NH2-MWCNTs/DA-epoxy resins were systematically investigated using an optical microscope equipped with a hot stage and an NIR light source. Figure 4 showed the optical microscope photographs of the injured samples before and after heating. For all


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samples, most part of the cracks could be healed within 1 minute at 120oC. However, complete healing was achieved after at least 5 hours treatment. According to the results of FT-IR (Figure 3(f)), DA bonds were broken and reconnected in the NH2MWCNTs/epoxy resin during heat treatment, which led to repairation of cracks. However, the heating method exhibited relatively poor energy utilization efficiency because a large part of heat energy was consumed to induce reactions and molecular chain motion irrelevant to self-healing during their conduction from the heat source to the injured parts of samples. Therefore, relatively long time was needed for complete selfhealing.

Figure 4. The thermal-triggered recovery of NH2-MWCNTs/DA-epoxy resins under 120oC recorded by optical microscope. The NH2-MWCNTs contents are (a) 0.2%; (b) 0.5%; (c) 1%; (d) 2%, respectively. Scale bar: 100µm.


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Generally, carbon nanotubes can in-situ convert NIR light into heat, and therefore led to a relatively high energy utilization efficiency. To demonstrate the role of CNTs in the light-induced self-healing of NH2-MWCNTs/DA-epoxy resins, we conducted a control experiments by using neat DA-epoxy resin without carbon nanotubes for characterization. As shown in Figure 5(A), the surface of the neat resin was scratched, but there appeared to be no change under NIR laser irradiation, which indicated that CNTs is the key ingredient for light induced self-healing of the resin. For samples containing NH2MWCNTs, as seen from Figure 5(B)-(E), obvious healing of the cracks were observed with only 30 s under irradiation. In other words, self-healing process of the resins can be successfully activated by an NIR laser light.40 The SEM observation on cross-section of the injured samples also indicated that NIR irradiation had comparable repairing effect as that of direct heating (Figure S6). Moreover, the self-healing efficiency of the resins increased obviously with the increase of NH2-MWCNTs contents. For those resins with relatively low NH2-MWCNTs contents (0.2% and 0.5%), slight interface mismatch were observed on samples’ surface even after long time treatment, suggested that the NH2-MWCNTs contents was not enough to produce sufficient photothermal conversion for full r-DA reaction. With further increase of the NH2-MWCNTs contents, the time which required to a complete self-healing was significantly reduced. In detail, 1.5 h for sample with 1% NH2-MWCNTs and only 30 min for sample containing 2% NH2-MWCNTs. That is to say, the process of repair can be accelerated by increasing the content of photothermal convertor, i.e. NH2-MWCNTs.


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Figure 5. The healing process of (A) resin without CNTs; (B-E) NH2-MWCNTs/DAepoxy resins with different NH2-MWCNTs contents; (F) pristine-MWCNTs/DA-epoxy resin with 1% MWCNTs under NIR irradiation recorded by optical microscope. Laser power: 3.00 ± 0.01W; distance from the sample surface to laser: 7cm. While (a-f) are corresponding thermographic images under NIR irradiation for 5min of the resins, respectively. All the thermographic images are in the same scale range.


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Apart from the content, the dispersity of CNTs in resin matrix may also have important influence on the light-induced self-healing behavior of the resin. When using pristineMWCNTs as the nanofiller, agglomeration of CNTs may inevitably occur due to their intrinsic poor dispersibility (Figure 2(b)). The light-induced self-healing efficiency of resin with poor MWCNTs dispersity (Figure 5(F)) is much lower than that of resin with good MWCNTs dispersity (Figure 5(D)). The cracks of NH2-MWCNTs/DA-epoxy resin were completely healed within 1.5 hours. While for pristine-MWCNTs/DA-epoxy resin with same MWCNTs content, an obvious interface mismatch could be still observed even after 2 hours irradiation. Moreover, there is no regular relationship between the contents of pristine MWCNTs and the healing efficiency triggered by NIR light (Figure S7). The heat energy from the photothermal conversion of CNTs plays the key role for determining the healing efficiency of NIR light. Therefore, the surface temperature of samples irradiated by NIR laser were recorded with an infrared imaging device, as shown in Figure 5(a)-(f), while the temperature elevation curves of resins were summarized in Figure 6. The colors in the images showed the temperature distribution on the surface of the resins. After 5 minutes of irradiation, DA-epoxy resin without CNTs reached to only 45.6oC, which is too low to trigger the r-DA/DA reactions. For resins containing CNTs, the temperature increased rapidly within the first 2 minutes, and then became flatten in following 3 minutes, suggested that the conversion and dissipation of heat energy gradually reached equilibrium. For resins containing NH2-MWCNTs, the maximum surface temperature of laser spot increased from 97.3oC to 133.9oC with the increase of NH2-MWCNTs contents from 0.2% to 2%, respectively. Both DA and r-DA reaction may occur at this temperature range, but high temperature is in favor of chain motion and


20

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network reconstruction. Therefore the resins with higher NH2-MWCNTs contents exhibited faster and better self-healing. In the control experiment, the maximum surface temperature of pristine-MWCNTs/DA-epoxy resin after irradiation did not show similar change rule as NH2-MWCNTs/DA-epoxy resins. When small amount of pristineMWCNTs were added (0.2% and 0.5%), the increment behavior of surface temperature of resins was similar to those NH2-MWCNTs/DA-epoxy resins with same CNTs contents. However, when more pristine-MWCNTs were added, the surface temperature showed a decrease trend rather than an increase one, suggesting that the photothermal conversion was inhibited owing to the agglomeration of MWCNTs. The relatively low surface temperature of pristine-MWCNTs/DA-epoxy resins after irradiation is responsible for their low degree and efficiency of self-healing (Figure 5(f) and Figure S6).

Figure 6. The temperature elevation of resins with different content of NH2-MWCNTs (a) and pristine-MWCNTs (b) as a function of irradiation time. Laser power: 3.00 ± 0.01W; distance from the sample surface to laser: 7cm. Owing to the photothermal effect, the light-induced self-healing not only endows DAepoxy resins with higher energy utilization efficiency and faster healing speed, but also


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generates relatively smaller influence on uninjured parts when compared to directly thermal-induced healing process.37 As shown in Figure 7, 1% NH2-MWCNTs/DA-epoxy resin was horizontally placed and partially suspended in the air without support. Photographs were taken before and after different self-healing process. For sample healed by directly heating, it became soft and bent obviously under gravity because the heat energy may broke the whole DA-network, resulting in a deformation of the sample. During the heating process, a large amount of redundant r-DA/DA reactions may also occur at those uninjured parts of the sample, which had no contribution to the self-healing but waste energy and damage the whole crosslink network. While for the sample healed by light, it well maintained its original shape after irradiation, suggested that the lighttriggered healing process have very good regional controllability and small influence on those unexposed parts. Since the DA-network of those unexposed parts was wellpreserved, there was no observable deformation of the sample during irradiation.

Figure 7. The photographs of 1% NH2-MWCNTs/DA-epoxy resin before and after (a) thermal- and (b) light-induced self-healing, (c) Tensile properties of 1% NH2MWCNTs/DA-epoxy resin before and after light-induced self-healing.


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The tensile properties of 1% NH2-MWCNTs/DA-epoxy resin before and after selfhealing were also evaluated for illustrating the healing efficiency. As shown in Figure 7(c), the tensile strength of the original resin was 22.1 MPa; and the injured one decreased to 11.5 MPa, where the testing sample fractured precisely at the scratch trace. After light-induced healing process and without compression stress, the tensile strength of the light-healed one could recover to 17.0MPa, thus the healing efficiency was 77%. Meanwhile, for the same sample healed by heat, it was too fragile to be tested and no valid result was obtained, suggesting that the redundant DA/r-DA reactions during thermal-induced healing process may also deteriorate the mechanical properties of resin. To further verify the location controllability of light-induced self-healing, we performed a targeted healing test for NH2-MWCNTs/DA-epoxy resin. A long crack (about 3cm) across the whole sample was scratched, the sample was then placed under the NIR light, as illustrated in Figure 8(a). Photographs of the crack at different distance from the laser spot center before and after irradiation were recorded by optical microscope and showed in Figure 8(b) (1% NH2MWCNTs/DA-epoxy resin, see also in Figure S8 for other samples). Take 1% NH2MWCNTs/DA-epoxy resin as an example, the part of crack which was directly irradiated by NIR light can be almost completely healed. With the increase of distance from observation point (O) to laser spot center (C), the crack was only partly healed or even remained unchanged. This phenomenon could be attribute to the temperature gradient of the sample under irradiation. Only those area of resin irradiated by the NIR light directly can in-situ convert NIR light to thermal energy and trigger the dissociation and reconstruction of DA network, resulting in self-healing of the resin (Figure 8(c)). Therefore, the center of laser spot shows the highest temperature, while with the increase of distance to the laser spot center, the temperature decreased sharply (see also


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in Figure 6a-f) because of the heat dissipation to room temperature environment. For those area relatively far from the spot of NIR laser, the temperature is too low to induce r-DA/DA reaction. As we expect, the length of repaired section of the crack (Table 3) increased with the NH2MWCNTs content since the temperature gradient of the sample is depend on the content of photothermal converter. The temperature of laser spot of resin with high NH2-MWCNTs content under NIR irradiation is high enough to raise the temperature of adjacent area for triggering sufficient r-DA/DA reaction.

Figure 8. (a) Schematic of targeted healing test; (b) The crack images observed by optical microscope before and after 5 minutes healing. The numbers above images represented the distance from observation point ‘O’ to the laser spot center ‘C’. (c) Schematic of the self-healing process. The diameter of laser spot is 0.6~0.7 cm. Scale bar: 100 µm. Laser power: 2.50 ± 0.01W; distance from the sample surface to laser source: 6cm.


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Table 3. The repair degree of NH2-MWCNTs/DA-epoxy resins with different exposure conditions. Content of

Laser power

The distance from the

The length of repaired

NH2-MWCNTs (%)

(W)

sample surface to laser (cm)

section of crack (cm)

0.2

2.5

6

0.7

0.5

2.5

6

1.0

1

2.5

6

1.4

2

2.5

6

1.7

1

2.5

7

0

1

3

7

0.7

Moreover, the distance of the laser source to sample and power of NIR laser may also have important influence on the repairing. The length of repaired section of crack under different exposure conditions were also listed in Table 3. For samples with NH2-MWCNTs content of 1%, when fixing the laser power at 2.5W and varying the distance of laser source to sample only from 6 to 7 cm, the self-healing effect of NIR laser was almost turn off. Then increased the laser power from 2.5 to 3W and the distance was fixed at 7cm, the crack could be healed again after irradiation, in detail, its length of repaired section was 0.7 cm measured by optical microscope. These results implied that the light-induced self-healing by photothermal conversion have very good location controllability. When the sample is exposed to NIR laser, the heat energy for triggering self-healing was photothermally converted within a limited area essentially determined by MWCNTs content, the size of the laser spot and laser power. Therefore, the light induced


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self-healing of NH2-MWCNTs/DA-epoxy resin could be precisely controlled by those factors mentioned above in consideration of the balance between healing efficiency and localization. 4. CONCLUSIONS A novel strategy to fabricate recyclable and targetedly repairable epoxy resin is well-established by incorporating amino-functionalized CNTs into epoxy resin crosslinked by dynamic covalent DA bonds. From the view of working mechanism, heat energy, directly applied or photothermally

converted

by

NH2-MWCNTs,

initiated

the

healing

of

resin

through

dissociation/reconstruction of DA-network, which also imparts recyclability to the resin. The results of FT-IR, DSC and GPC analysis and solvent exposure test not only revealed the dynamic performance of DA-networks, but also concluded a high recovery rate of 90% of the resin in solvent. It was confirmed by SEM that the dispersion of NH2-MWCNTs in DA-epoxy was much better than that of pristine-MWCNTs with the same contents and therefore responsible for their much better light-induced repairability. NH2-MWCNTs/DA-epoxy resin can be healed in relatively short time under low-power NIR light irradiation when compared with thermalinduced one, while the light-induced self-healing efficiency of the resins can be enhanced by increasing the NH2-MWCNTs content. What’s more, using NIR laser as the remote stimuli for self-healing of epoxy resin have a characteristic of targeted reparation property, allowing to heal at the specific areas as required in practical applications. The targeted reparation of the resins can be well controlled by the NH2-MWCNTs content, distance of the laser source to sample and laser power. The noncontact way of light-induced self-healing of NH2-MWCNTs/DA-epoxy resin is beneficial to broaden the applications of epoxy resins due to its convenience, high-efficiency and localizability.


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AUTHOR INFORMATION Corresponding Author * [email protected] (S. C. Chen) and [email protected] (Y. Z. Wang) ASSOCIATED CONTENT Supporting Information Including GPC traces of prepolymers, NMR spectrum of BMO and r-DA product, DSC curves of BMIs, high resolution SEM images of resins with NH2-MWCNTs, fracture surface of the resins, the healing process of pristine-MWCNTs/DA-epoxy resins, and the crack images of pristine-MWCNTs/DA-epoxy resins taken before and after thermal or light induced self-healing. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21474066 and No. 51421061) and the Foundation for Young Scientists of State Key Laboratory of Polymer Materials Engineering (No. sklpme2014-3-09). The Analytical and Testing Center of Sichuan University provided TEM analysis. REFERENCES 1.

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Table of Contents


33

Photothermal Conversion Triggered Precisely ... - ACS Publications

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