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Ultrafastly Self-healing Nanocomposites via Infrared Laser and Its Application in Flexible Electronics Shuwen Wu, Jinhui Li, Guoping Zhang, Yimin Yao, Gang Li, Rong Sun, and Ching-Ping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15476 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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ACS Applied Materials & Interfaces

Ultrafastly Self-healing Nanocomposites via Infrared Laser and Its Application in Flexible Electronics Shuwen Wu†,‡, Jinhui Li†, Guoping Zhang*,†,§, Yimin Yao†, Gang Li†, Rong Sun,†, Chingping Wong §,ǁ

†

Guangdong Provincial Key Laboratory of Materials for High Density Electronic Packaging,

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China.

‡

Nano Science and Technology Institute, University of Science and Technology of China

(USTC), Suzhou 215123, China.

§

ǁ

School of Materials Science and Engineering, Georgia Institute of Technology, USA.

Faculty of Engineering, the Chinese University of Hong Kong 999077, Hong Kong, China.

KEYWORDS: flexible electronics, self-healing, substrate, Diels-Alder chemistry, graphene

ABSTRACT: The continuous evolution toward flexible electronics with mechanical robust property and restoring structure simultaneously places high demand on a set of polymeric material substrate. Herein, we describe a composite material composed of a polyurethane based

on

Diels-Alder

chemistry

(PU-DA)

covalently

linked

with

func

ACS Paragon Plus Environment

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tionalized graphene nanosheets (FGNS), which shows mechanical robust and infrared (IR) laser self-healing properties at ambient conditions, and therefore suitable for flexible substrate applications. The mechanical strength can be tuned by varying the amount of FGNS and breaking strength can reach value as high as 36 MPa with only 0.5 wt% FGNS loading. On rupture, the initial mechanical properties are restored with more than 96% healing efficiency after 1 min irradiation time by 980 nm IR laser. Especially, this is the highest value of healing efficiency reported in the self-healable materials based on DA chemistry systems until now. And the composite exhibits a high volume resistivity up to 5.6 × 1011 Ω·cm even the loading of FGNS increased to 1.0 wt%. Moreover, the conductivity of the broken electric circuit which was fabricated by silver paste drop-casted on the healable composite substrate was completely recovered via IR laser irradiating bottom substrate mimicking human skin. These results demonstrate that the FGNS-PU-DA nanocomposite can be used as self-healing flexible substrate for the next generation of intelligent flexible electronics.

INTRODUCTION

Flexible electronics have been extensively investigated in hopes of realizing system-onplastic (SoP) applications as the next generation technology in various areas, ranging from consumer electronics to bio-integrated medical devices.1-3 An ideal flexible electronic device should possess some characteristics, such as bendable (or twistable), stretchable, stable electrical performance, and safe operation. The flexible electronic is usually fabricated by integrating electroactive materials, conductive materials or other functional materials and


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flexible substrate using spin-coating, roll-to-roll, and other processes.

4-7

The flexible

substrate as a support is often necessary to develop a lightweight, thin, elastic, ductile and efficient electronic device. Therefore, to fabricate high-performance portable/wearable electronics, numerous polymeric materials have been used as flexible substrate, such as textile-type

materials,8

polyimide

(PI),9,

10

polyethylenenaphthalate

(PEN),11

polydimethylsiloxane (PDMS)12-16 and polyethylene terephthalate (PET).17-20 However, the conventional polymeric flexible substrates may possibly undergo mechanical fractures caused by deformation over time or accidental fracture in practical applications. These failures would seriously degrade the functionality and lifetime of the flexible electronic, resulting in the entire component breakdown of the electronic devices, abundant electronic waste and safety hazards.21-26 Thus, an ideal flexible electronic device should not only possess high electrical performance, but also be endowed with impressive healability to prevent the structural fractures, or to restore the configuration integrity of devices after mechanical damage.

Self-healing materials as one kind of intelligent materials inspired by the powerful biological healing function, to improve the safety, lifetime, energy efficiency, and environmental impact of man-made materials, have attracted more and more attention in recent years.27 Self-healing materials have the capability of repairing or recovering themselves once or multiple times upon encountering damages autonomously or non-autonomously.28-30 For autonomic selfhealing materials, the harnessed chemical potential is automatically released into the crack planes and facilitates the healing in response to damage by bonding the crack surfaces closed


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. On the other hand, non-autonomic self-healing materials require a modest external stimulus (heat,31 light,32 pH,33 etc.34,

35

), to drive the healing process. Introducing self-healing

properties to flexible devices can compensate for incidental scratches and/or mechanical cuts that may destroy the performance of the device, and effectively solve the problems of waste and pollution of device failure. At present, self-healing property has been decorated to numerously functional materials, such as hydrogel,36 biomedical material,37 shape memory material,38 but the research of healable substrate for flexible electronics are scarcely studied. Therefore, exploring healable substrate material is of significant importance for developing smart flexible electronics.

The Diels-Alder (DA) reaction and its retro-Diels-Alder (rDA) analogue as promising routes introducing self-healing properties to polymeric systems can be performed under mild condition without any catalyst or healing agent.39-41 However, the self-healing polymers based on DA chemistry are repaired always by heating directly and overall. By this means, it will not only waste energy, but also be possible to cause interference of other parts in the system which is susceptive to heat and often results in the deterioration of device. And the healing efficiency via heating directly based on DA chemistry is also needed to be improved. In our previous work, the recovering of mechanical performance of the repaired sample (graphene oxide/polyurethane nanocomposite) based on DA chemistry is not well via heating directly.31 And the highest value of healing efficiency reached 92.4% at 130 oC for about 5 min followed by 1 day at 55 oC in term of breaking strength so far.42 In contrast to heat, light is particularly advantageous in terms of its intensity and being able to exclusively expose the


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damaged region without interference of other parts. Especially, the specific mode (asymmetric C–O–C stretch) of reduced graphene oxide (rGO) structure has characteristic of absorbing IR light, which convert it into heat rapidly and efficiently, resulting in a rapid temperature increasing locally.43, 44 Furthermore, the rGO has been widely used as efficient filler to enhance the electronic conductivity, thermal resistance, thermal conductivity and ultrahigh mechanical strength of the resultant composites.36, 45, 46 Therefore, these properties prompt us to believe that combining rGO and the polymer based on DA chemistry might generate a novel self-healing composites, which not only have enhanced integrated properties but also can be healed rapidly and precisely by IR light. To the best of our knowledge, no such study has been reported so far.

In this work, we reported a novel self-healing composite composed of functionalized graphene nanosheets and polyurethane based on DA chemistry with covalent linking, which can be used as flexible substrate material with enhanced mechanical properties and also can be precisely healed under the irradiation of IR laser. With 0.5 wt% loading of FGNS in the as-prepared nanocomposite, the Young’s module increase to 127 MPa, the tensile strength achieve 36 MPa, the break elongation still be able to beyond 1100%. Different from the conventional self-healing

materials

based on

DA chemistry,

the

FGNS-PU-DA

nanocomposites can be healed ultrafastly and precisely via IR laser with excellent healing efficiency higher than 96% after 1 min irradiation. So far this is the highest values of healing efficiency reported in the self-healable materials based on DA chemistry. Furthermore, the nanocomposite exhibits a high volume resistivity up to 5.6 × 1011 Ω·cm with loading of 1.0


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wt% FGNS which means it can be used as promising flexible substrate. Lastly, the asprepared flexible substrate was adopted to fabricate flexible electronics by drop-cast silver paste on it. The conductivity of the broken electric circuit was completely recovered via IR laser irradiating bottom substrate.

RESULTS AND DISCUSSION

Graphene nanosheets (GNS) exfoliated from pristine graphite are widely introduced into polymer matrix to prepare polymeric nanocomposites due to its unique combination of thermal conduction, electrical conductivity, high mechanical strength, and photothermal conversion. However, pure GNS is difficult to disperse uniformly and tended to agglomerate in polymer matrix. Chemically functionalized graphene improved dispersity in solvents and polymer matrix so as to further broaden the available application range of graphene. As described in Scheme 1, surface modification of graphene involves hydramine-functionalized graphene oxide (FGO) and reduction of FGO to afford hydramine-functionalized graphene nanosheets that is hydroxylated-GNS (denoted as FGNS).


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Scheme 1. Schematic procedure for the preparation of FGNS. The transformations of the surface functional groups of graphene have been successfully monitored by using Fourier transform infrared spectroscopy (FTIR, Figure 1a). Firstly, graphene oxide (GO) was synthesized from raw graphite by modified Hummers method.47 GO exhibits representative oxidized group (-OH, C=O, C-O-C) absorption peaks at 3170, 1734 and 1220 cm1

. The resultant FGNS shows two new absorption peaks at 1577 cm-1 and 1377 cm-1, respectively

corresponding to N-H and C-OH stretching vibration. UV-vis shows that the main absorption peak at 230 nm for GO red-shifts to 250 nm for FGNS (Figure 1b), indicating the recovery of electronic conjugation within FGNS upon ethanolamine reduction. X-ray diffraction (XRD) patterns (Figure 1c) confirmed the efficient de-oxygenation of GO to form graphene framework of FGNS upon ethanolamine reduction. The interlayer distance of FGNS is calculated to be 3.81 Å, which is much lower than that of GO precursor (8.26 Å) while slightly higher than that of well-ordered graphite (3.35 Å) due to the presence of functionalized organic groups on the surface of graphene sheets. The broad XRD peak of the FGNS indicates the disordered crystal structure of graphene sheets and reflects that graphene sheets obtained here are not pure but


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chemically modified to some extents. The surface functionalization of graphene was further confirmed using atomic force microscopy (AFM) characterization. As shown in Figure 1d, AFM images show a clear height variation between single-layer GO (0.9 nm) and FGNS (1.5 nm). Considering the height of reduced GO is a little lower than that of GO due to de-oxygenation48 and the substituted organic groups contribute ~0.3 nm in heights,49 we believe both sides of FGNS are grafted by ethanolamine groups via covalent linking successfully.

Figure 1. (a) FTIR spectra of GO, FGNS and FGNS-PU-DA. (b) UV-vis spectra of aqueous solutions of GO and FGNS. (c) XRD patterns of GO and FGNS. (d) AFM images of single-layer GO and FGNS sheets.


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The IR laser triggered self-healing polymer composites were formed via the introduction of FGNS into the polyurethane based on DA chemistry, which was prepared from the condensation of NCO-terminated PU with a resultant of furfuryl alcohol and bismaleimide. Schematic procedure for the preparation of PU-DA described in Figure S1. The characteristic peaks of the polymer was found in the FTIR spectra with the formation of the polymer nanocomposites, e.g., the appearance of a stronger absorption at 1776 cm-1 was specific to C=C of the DA adduct, 1715 cm-1 of a peak can be ascribed to the C=O band of bismaleimide and -NH of secondary amide group at 3303 cm-1 indicated the formation of PU pre-polymer, suggesting the formation of polyurethane based on DA chemistry in the nanocomposites (Figure 1a). Applying the DA chemistry in the novel composite successfully is also verified by the characteristic peak of the rDA reaction. All the final composites with different FGNS contents (0.1 wt%, 0.5 wt%, 1.0 wt%) were labeled as FGNS-PU-DA-1, FGNS-PU-DA-2 and FGNS-PU-DA-3, respectively. The DSC curves of as-prepared nanocomposites exhibited a significant rDA reaction endothermic peaks at 140-160 oC in agreement with pure PU-DA (Figure 2) indicating that introduction of FGNS into the composites does not have a significant effect on the endothermic peak. And the significant endothermic peak of rDA reaction further indicates the successful synthesis of the FGNS-PU-DA composites based on the DA reaction, provides the thermal healing properties of all the nanocomposites based on the thermal reversibility of the DA chemistry.50,

51

The

dispersion of FGNS sheets in the composites significantly affects mechanical properties and selfhealing performance of the as-prepared nanocomposites. So the investigation of distribution was carried using scanning electron microscopic (SEM) on the cross-sectional surface. As revealed in the cross-sectional SEM images (Figure 3), surface-functionalized graphene are well dispersed


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in the polymer matrix uniformly which were predominately attributed to the urethane bonds (NH-CO-) resulting from the reaction between the hydroxyl groups on surface of the FGNS and NCO end groups of PU chains in terms of the interface. The thermal properties of the PU-DA and FGNS-PU-DA nanocomposites were investigated via TGA under a nitrogen atmosphere between 30 oC and 800 oC. As shown in Figure 4 (Table 1), the amount of char residue at 800 o

C increased slightly with the addition of FGNS indicating the successful introduction of the

graphene. Meanwhile, PU-DA and FGNS-PU-DA showed 5% weight loss at around 330 oC suggesting that all these composites possess excellent thermal stability.

Figure 2. DSC measurements of PU-DA and FGNS-PU-DAs.


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Figure 3. The SEM images of the cryogenically fractured surfaces of films of (a) PU-DA, (b) FGNS-PU-DA-1, (c) FGNS-PU-DA-2, (d) FGNS-PU-DA-3.

Figure 4.TGA measurements of PU-DA and FGNS-PU-DAs.


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Table 1. Thermal Properties of PU-DA and FGNS-PU-DAs Weight-loss temperature (T5) (oC)

Weight-loss temperature (T10) (oC)

Tmax (oC)

Char yield at 800 oC (%)

PU-DA

334

357

418

10.85

FGNS-PU-DA-1

340

365

422

12.78

FGNS-PU-DA-2

327

356

423

13.36

FGNS-PU-DA-3

335

359

418

14.57

Sample

T5: 5% weight-loss temperature, T10: 10% weight-loss temperature, Tmax: maximum weight loss temperature Graphene have been widely used as efficient filler that can dramatically improve mechanical properties of the resultant nanocomposites at lower loading due to the large π-conjugated network.52,

53

The mechanical properties of the PU-DA and FGNS-PU-DAs nanocomposites

including Young’s modulus, elongation at break and tensile strength are shown in Figure 5 and summarized in Table 2. It can be clearly seen that a dramatic improvement in mechanical strength, e.g., about one order of magnitude increase of Young’s modulus from the pure PU polymer network, has been observed after the incorporation of FGNS into the network which can be attributed to efficient load transfer between the FGNS and the PU matrix resulting from the chemical bonding, i.e., 47 to 100 MPa of the nanocomposite with only 0.1 wt% FGNS. With the addition of FGNS, the Young’s modulus of FGNS-PU-DA-2, FGNS-PU-DA-3 increased to 126, 130 MPa, respectively. The breaking strength is also increased from 16 MPa of pure PU to 25, 36 MPa of FGNS-PU-DA-1, FGNS-PU-DA-2 nanocomposites, respectively. But compared with


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FGNS-PU-DA-2, the tensile strength of FGNS-PU-DA-3 decreased to 29 MPa, which may due to the formation of aggregates at this higher loading. While compared with the pure PU-DA, the addition of FGNS both enhanced the tensile modulus and tensile strength of the FGNS-PU-DA nanocomposites. The elongation at the break decreased from 1553% to 1189% with the incorporation of 1.0 wt% FGNS indicating that the elastic deformation is reduced because of the crosslinking of PU chains by the FGNS. The enhanced mechanical property of an optimized weight fraction of FGNS in the PU-DA matrix partially highlight the potential of FGNS-PU-DA composites for practical applications compared with compromised or poor mechanical strength of conventional self-healing materials due to the limitation of their healing mechanisms. 54-56

Figure 5. Mechanical properties of PU-DA and FGNS-PU-DAs.

Table 2. Summary of the mechanical properties of PU-DA and FGNS-PU-DAs. The average values were obtained from more than 3 samples Sample

Young's modulus

Strain-at-break

Stress-at-break [

[MPa]

[%]

MPa]


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PU-DA

47.49 ± 5.70

1553 ± 36

16.85 ± 1.21

FGNS-PU-DA-1

100.63 ± 10.34

1288 ± 54

27.23 ± 3.75

FGNS-PU-DA-2

126.97 ± 13.15

1212 ± 29

36.48 ± 4.31

FGNS-PU-DA-3

130.55 ± 10.14

1189 ± 81

29.43 ± 2.17

Figure 6. Illustration of self-healing process of FGNS-PU-DA nanocomposite. The mechanism of self-healing of polymer networks has been studied based on temperaturedependent reversible covalent cross-linking of polymers without any catalyst or healing agent.57 The DA adducts perform retro-DA reaction at higher temperature, the disconnected furan and maleimide moieties have enough mobility to heal the fractured place of the sample through network reconnection during cooling down to room temperature by natural convection. The self-


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healing performance of the FGNS-PU-DA nanocomposite was realized via graphene network combined with photothermal conversion (Figure 6). The temperature curve of the irradiated spot on the FGNS-PU-DA nanocomposites under continuous IR laser irradiation (980 nm, 200 mW, 3 mm spot diameter) was recorded by FLIR T335 infrared camera as shown in Figure S2. Graphene has good IR absorbing capacity,58,

59

and the addition of FGNS could make the

nanocomposites show strong IR absorption, which is totally different from the original IR transparent PU-DA matrix. As shown in Figure 7a, the temperature of the nanocomposites at the spot is increased from 30.2 oC to maximum temperature within 20 s. And the higher the FGNS content, the faster the temperature increase, the higher maximum temperature which is consistent with the result reported by Zhang et al..60 For the FGNS-PU-DA nanocomposites, the temperature increases from 30.2 oC to 150 oC over within 20 s under the same condition, indicating that the temperature of the nanocomposites can reach the healing temperature of rDA chemistry happened under IR laser irradiation. However, the healing temperature of the PU-DA cannot be achieved under IR laser irradiation, because the temperature was only 39 oC under IR laser irradiation for more than 130 s. It has provided a powerful evidence to verify photothermal conversion of graphene affecting the DA chemistry to repair the crack. That is to say, the healing temperature of these nanocomposites at around 150 oC can be controlled under the specific irradiation condition respectively.


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Figure 7. (a) Change in temperature over time for the PU-DA and FGNS-PU-DAs upon exposure to IR laser in air. (b) The relationship between light density and FGNS-PU-DAs when the temperature achieved to 150 oC under the irradiation of 980 nm laser. We further investigated the self-healing performance based on IR laser photoirradiation through the variation of mechanical properties of composites at different stage, such as original, break and healed. All samples were shaped into dumbbell-like (the middle strips: 13mm × 2 mm × 0.15±0.05 mm) by mould for the self-healing test. 1 mm width break was cut with a razor blade perpendicular to tensile direction in the middle of dumbbell. And the optimal healing condition was controlled in 1min for all samples at 150 oC, then turn off the light slowly to achieve the best healing efficiency under the given conditions (such as content of FGNS, light strength). The temperature of FGNS-PU-DA nanocomposites with different contents of FGNS was controlled at 150 oC under irradiation of IR laser corresponding to different light density, 2.8 w cm-1, 2.4 w cm-1 and 2.1 w cm-1, respectively (Figure 7b) which verified the higher the FGNS content, the higher photothermal conversion efficiency again. The crack of pure PU-DA cannot be repaired under the irradiation of IR laser because the healing temperature cannot be achieved which was in agreement with above result. The damaged FGNS-PU-DA-1 was repaired in 1min by keeping


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the break in irradiated with IR laser (2.8 w cm-1) under ambient condition. The healing property was evidenced in the strain-stress measurements. As shown in Figure 8I, The mechanical strength of the break samples was dramatically diminished, the healed polymer nanocomposites almost completely gained its original strength after 1 min of light exposure at the specific intensity. The healing efficiencies of FGNS-PU-DA samples with different FGNS loadings are all as high as 96%, e.g. the healing efficiency of FGNS-PU-DA-1 nanocomposite up to more than 96% in terms of Young's modulus (100%), break strength (96%), break elongation (97%) (Figures 8Ia,d). As imaged by using SEM (Figure 8 II), the cracks that were cut across the samples are almost disappeared. Photos recording the cutting and healing process can be found in Figure 8IIIa-e. The further increase of FGNS content (0.5 wt%, 1.0 wt%) in the nanocomposites shows stronger IR absorption, and consequently, the lower light strength to achieved more than 96% healing efficiencies (Figures 8Ib,c, Figure S3). The formation of aggregates at 1.0 wt% loading, have no impact on the healing efficiencies, because few contents of FGNS were enough to achieve high healing efficiency.


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Figure 8. I. The tensile stress–strain measurement of self-healing of (a) FGNS-PU-DA-1, (b) FGNS-PU-DA-2, (c) FGNS-PU-DA-3 of original polymer sample, break sample and healed sample under NIR laser irradiation after 1 min at room temperature. (d) The healing efficiency of FGNS-PU-DA-1 (red) compared with that of break sample (green) in terms of Young’s modulus,


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break strength and break elongation. II. SEM images of healing process of FGNS-PU-DA-1. III. Photographs of the FGNS-PU-DA-1: (a) original, (b) after cutting, (c) contacted and irradiated with a 980 nm laser, (d) after irradiation for 1 min, (e) stretch of healed sample (d) to 960%.

Figure 9. Volume electrical resistivity of PU-DA and FGNS-PU-DAs. Considering the addition of FGNS might endow the electrical conductivity of FGNS-PU-DA substrate, the volume resistivity of FGNS-PU-DA nanocomposite film was measured (Figure 9). The pure PU-DA film possesses a high volume resistivity over 1.9 × 1012 Ω·cm. After incorporation of 1.0 wt% FGNS, the value decreases to 5.6 × 1011 Ω·cm, which is still beyond the critical resistance for electrical insulation (109 Ω·cm) indicating that the FGNS-PU-DA nanocomposites possess high electrical insulation. Human skin is spontaneously repaired through granulation tissue in the dermal layer, serves as a new substrate to facilitate the proliferation and migration of keratinocytes in the epidermal combined with sprouts of capillaries associated with macrophages and fibroblasts from the neighboring undamaged skin.61,

62

Learning from this healing process, doping functional


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materials into the surface of healable polymer films as substrate extends the electrical device for more potential. The functional device can be repaired by healable polymer films to convey the healing function of the bottom layers to the top functional layers.63 Various healable functional materials that can simultaneously heal the damaged structures and restore functions have been fabricated using the healability conveyance method.5, 64-66 The FGNS-PU-DA nanocomposites can be used as substrate based on enhanced mechanical properties, high healing efficiency and electrical insulation to develop functional device. The silver paste was drop-casted on the precleaned substrate and then dried at 60 °C for 1 h in a convection oven to form a uniform and conductive film with tens micrometers thickness. As shown in Figure 10a, after the electronic circuit which was completely cut by razor blade was repaired under the irradiation of IR laser, the resistance of circuit was almost recovered compared with original. A battery-powered circuit was constructed to further demonstrate the potential of IR laser self-healing for electronic circuits. Healed film being flexed to show its mechanical strength and flexibility after healing under the irradiation of IR laser at room temperature (Figure 10b)


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Figure 10. (a) The change of electronic resistance of the simple self-healing circuit coating silver paste before and after healing. (b) Demonstration of the healing process for the nanocomposite with an LED in series with the self-healing electrical conductor, corresponding to undamaged conductor, completely severed conductor (open circuit), electrical healing. CONCLUSIONS In summary, we have demonstrated the fabrication of almost completely healable substrate materials based on Diels-Alder chemistry combining PU polymer elastomer and FGNS. The introduction of FGNS not only improves mechanical properties but also endows the film with photothermal conversion properties to initiate the healing of the substrate film by IR laser, which then successfully conveys this healability to the conductive layer. The nanocomposite with 0.5 wt% loading of FGNS, the Young’s module increase to 127 MPa, the tensile strength achieve 36


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MPa, the break elongation still be able to beyond 1100%. And only in 1 min under the irradiation of IR laser, the healing efficiency of the nanocomposite up to more than 96% in terms of Young's modulus, break strength, break elongation. And after incorporation of 1.0 wt% FGNS, the nanocomposite possessed a high volume resistivity over 5.6 × 1011 Ω·cm, which is still beyond the critical resistance for electrical insulation (109 Ω·cm). Furthermore, the flexible electronics was prepared by coating conductive material on to the healable substrate and the electronic resistance of circuit was almost recovered after the substrate was exposed to IR laser. These results make us believe that our novel material design will open avenues for developing more composites with similar properties as a promising self-healing material for wide applications in intelligent components, construction industries, electronics and so on. EXPERIMENTAL SECTION Materials: 4,4-diphenylmethane diisocyanate, (MDI, 99%), N,N’-(4,40-diphenylmethane) bismaleimide (BMI) were purchased from J&K Chemical. Furfuryl alcohol (FA) and graphite powder were supplied by Aladdin, Poly(tetramethylene glycol) (PTMG, Mn=2000 g mol-1) was purchased from Aladdin and used after 2 hours of drying under vacuum at 110 oC. Dimethyl formamide (DMF) was dried with the molecular sieves for 24 h before use. Potassium permanganate (KMnO4), sodium nitrate (NaNO3), concentrated sulfuric acid (98%), concentrated hydrochloric acid and ethanolamine were all of reagent grade and obtained from Beijing Chemical Reagents Company. Silver paster was obtained from Yick-Vic Chemicals & Pharmaceuticals (Hongkong). Preparation of FGNS: GO were prepared through the chemical exfoliation of graphite powder following Hummers' method. Ethanolamine-functionalized graphene was obtained by a covalent chemical functionalization method. Briefly, 3 g concentrated hydrochloric acid was added into


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300 mg (1 mg/mL) GO aqueous solution to adjust the pH at 1~2. Then, 3 g of ethanolamine was added into the above GO suspension at 60 oC for 12 h, the reaction went on for another 12 h at 90 oC. The resultant solution was then centrifuged at 4 000 rpm for 15 min in order to remove residual large size and unexfoliated graphite powders, and the supernatant was collected. Then the supernatant was subjected to a 30 min centrifugation at 10 000 rpm to precipitate. The FGNS was separated by centrifugation, and washed with double distilled water 3 times, anhydrous dimethylformamide (DMF) 3 times. Finally, the FGNS was redispersed in anhydrous DMF and formed homogenous solution ultrasonically. Fabrication of Self-Healing Polymer Nanocomposites: PTMG/DMF (4 g/15 g) was dropwise added into MDI/DMF (1 g/10 g) slowly at 80 °C for 2 h under intensive magnetic stirring and a nitrogen atmosphere. 0.4 g FA and 0.753 g BMI were mixed in anhydrous DMF at 60 oC for 2 h. The resultant mixture was added above solution at 80 °C for 2 h under intensive magnetic stirring and a nitrogen atmosphere to form NCO-terminated polyurethane. The FGNS dispersion in anhydrous DMF (for 0.1 wt%, 0.5 wt%, and 1.0 wt% loading, respectively) was added stepwise the pre-PU/DMF and stirred for overnight at 80 oC under nitrogen. The resulted mixture was pressed into 1mm thickness Teflon plates and drying at 60 oC in a convection oven for 12 h and 100 oC in a vacuum oven for 1 day to fabricate the films. Instruments and Methods: Fourier transform infrared (FTIR) measurement was recorded using a Bruker Vertex 70 spectrometer (Bruker Optik GmbH, Ettlingen, Germany) in the range of 4000– 400 cm-1. UV-vis spectroscopy was recorded at a wavelength of 665 nm using Shimadzu UV3600. X-ray diffraction (XRD) analysis was studied by use of an Rigaku D/Max 2500 X-ray diffractometer with monochromated Cu Kα radiation (λ=1.54 Å) at a scanning rate of 2 o min-1. Atomic Force Microscopy (AFM) images were recorded with a MFP-3D™ (Asylum Research,


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USA) and operated in air in tapping mode. Differential scanning calorimetry (DSC) was performed using Q20-1173 DSC thermal system (TA instruments, New Castle, USA) in the temperature range of 30-180 oC at 10 oC min-1, nitrogen gas was purged at 50 mL min-1. Thermogravimetric analysis (TGA) was carried out using a TA SDTQ600 thermogravimetric analyzer, the temperature was scanned from 30 to 800 °C under a nitrogen flow of 100 mL min-1. The healing process was carried out by 980 nm infrared laser (MCPL-980-400-1-TI, China). Infrared thermal images were recorded by a FLIR T335 infrared camera. Mechanical tensilestress experiments were conducted on a RGM-4100 (Reger Instrument Co., Ltd., China) at room temperature with a crosshead speed of 40 mm min-1. At least four samples of each loading fraction were tested. The specimens were dumbbell-like (35 mm×6 mm×0.15±0.05 mm). The electrical resistance was measured by 2-point probe resistance measurement system using Tektronix DMM 4050 6-1/2 Digit Precision Multimeter. GPD-3303S DC power supply (Bangya Electronic, Shenzhen, China) was employed to provide DC voltage. ASSOCIATED CONTENT Supporting Information Additional figures as described in the text (Figure S1-S3) are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by NSFC-Guangdong Jointed Funding (U1601202), NSFCShenzhen Robot Jointed Funding (U1613215), Guangdong and Shenzhen Innovative Research Team Program (No.2011D052, KYPT20121228160843692), Key Laboratory of Guangdong Province (2014B030301014) and R&D Funds for basic Research Program of Shenzhen (Grant No. JCYJ20150401145529012, JCYJ20160331191741738, JSGG20160229194437896). REFERENCES (1) Klauk, H., Plastic Electronics: Remotely Powered by Printing. Nat. Mater. 2007, 6, 397-398. (2) Salvatore, G. A.; Münzenrieder, N.; Kinkeldei, T.; Petti, L.; Zysset, C.; Strebel, I.; Büthe, L.; Tröster, G., Wafer-Scale Design of Lightweight and Transparent Electronics that Wraps Around Hairs. Nat. Commun. 2014, 5,1-8 (3) Yao, S.; Zhu, Y., Nanomaterial-Enabled Stretchable Conductors: Strategies, Materials and Devices. Adv. Mater. 2015, 27, 1480-1511. (4) Ko, S. H.; Pan, H.; Grigoropoulos, C. P.; Luscombe, C. K.; Fréchet, J. M.; Poulikakos, D., All-Inkjet-Printed Flexible Electronics Fabrication on a Polymer Substrate by Low-Temperature High-Resolution Selective Laser Sintering of Metal Nanoparticles. Nanotechnology 2007, 18, 345202. (5) Wang, H.; Zhu, B.; Jiang, W.; Yang, Y.; Leow, W. R.; Wang, H.; Chen, X., A Mechanically and Electrically Self-Healing Supercapacitor. Adv. Mater. 2014, 26, 3638-3643. (6) Kim, S.; Son, J. H.; Lee, S. H.; You, B. K.; Park, K. I.; Lee, H. K.; Byun, M.; Lee, K. J., Flexible Crossbar-Structured Resistive Memory Arrays on Plastic Substrates via Inorganicd Based Laser Lift-off. Adv. Mater. 2014, 26, 7480-7487. (7) Carlson, J.; English, J.; Coe, D., A Flexible, Self-Healing Sensor Skin. Smart. Mater. Struct. 2006, 15, N129–N135. (8) Kim, D.-H.; Viventi, J.; Amsden, J. J.; Xiao, J.; Vigeland, L.; Kim, Y.-S.; Blanco, J. A.; Panilaitis, B.; Frechette, E. S.; Contreras, D., Dissolvable Films of Silk Fibroin for Ultrathin Conformal Bio-Integrated Electronics. Nat. Mater. 2010, 9, 511-517. (9) Takei, K.; Takahashi, T.; Ho, J. C.; Ko, H.; Gillies, A. G.; Leu, P. W.; Fearing, R. S.; Javey, A., Nanowire Active-Matrix Circuitry for Low-Voltage Macroscale Artificial Skin. Nat. Mater. 2010, 9, 821-826. (10) Viventi, J.; Kim, D.-H.; Moss, J. D.; Kim, Y.-S.; Blanco, J. A.; Annetta, N.; Hicks, A.; Xiao, J.; Huang, Y.; Callans, D. J., A Conformal, Bio-Interfaced Class of Silicon Electronics for Mapping Cardiac Electrophysiology. Sci. Transl. Med. 2010, 2, 1-9. (11) Someya, T.; Sekitani, T.; Iba, S.; Kato, Y.; Kawaguchi, H.; Sakurai, T., A large-area, Flexible Pressure Sensor Matrix with Organic Field-Effect Transistors for Artificial Skin Applications. Proc. Natl. Acad. Sci. 2004, 101, 9966-9970.


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