Three-Dimensional Graphene Structure for Healable Flexible

Subscriber access provided by UNIV OF DURHAM

Article

Three-Dimensional Graphene Structure for Healable Flexible Electronics based on Diels-Alder Chemistry Jinhui Li, Qiang Liu, Derek Ho, Songfang Zhao, Shuwen Wu, Lei Ling, Fei Han, Xinxiu Wu, Guoping Zhang, Rong Sun, and Ching-Ping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19649 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018

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

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

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

ACS Applied Materials & Interfaces

Three-Dimensional Graphene Structure for Healable Flexible Electronics based on Diels-Alder Chemistry ║

Jinhui Li†,‡, Qiang Liu†, Derek Ho‡, Songfang Zhao , Shuwen Wu†, Lei Ling†, Fei Han†, Xinxiu Wu†, Guoping Zhang†*, Rong Sun†* and Ching-Ping Wong§ †

‡

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

Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee

Avenue, Kowloon, Hong Kong ║

School of Material Science and Engineering, University of Jinan, Jinan 250022, China

§

Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong

ABSTRACT: Wearable electronics with excellent stretchability and sensitivity have emerged as a very promising field with wide applications such as e-skin and human motion detection. Although three-dimensional (3D) graphene structures (GS) have been reported for high performance strain sensors, challenges still remain such as the high cost of GS preparation, low stretchability, as well as the lack of ability to heal itself. In this paper, we reported a novel self-healing flexible electronic with 3D GS based on Diels-Alder (DA) chemistry. Furfurylamine was employed as a reducing as well as modifying agent, forming FAGS/DAPU with the “infiltrate-gel-dry” process. The as-prepared composite exhibited excellent stretchability (200%)

ACS Paragon Plus Environment 1

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

Page 2 of 25

and intrinsic conductivity with low incorporation of graphene (about 2 wt%), which could be directly employed for the flexible electronics to detect human motions. Besides, the FAGS/DAPU composite exhibited lower temperature retro-DA response for the continuous graphene networks. Highly effective healing of the composites by heat and microwave has been demonstrated successfully.

KEYWORDS: Diels-Alder chemistry, self-healing, flexible electronics, 3D graphene structure, assembly INTRODUCTION Three-dimensional (3D) graphene structures (GS), also known as sponges, foams or aerogels, have drawn much attention during the past few years for their excellent conductivity, high porosity, large specific surface area and low density1-5, showing great applications such as energy storage, catalysis, absorbents, and flexible electronics, and so forth6-11. Especially, for flexible electronics, 3D GS have been employed for flexible conductors or stretchable sensors due to their high conductivity and porous structure, which is highly suitable for the filling or combination of the elastic polymers12-16. Although graphene possesses excellent mechanical property which is the world's thinnest, strongest, and stiffest material, 3D graphene-only materials usually exhibit poor mechanical strength17. In fact, most of the reported 3D GS would be damaged after bending or stretching18-19. To obtain flexible electronics with excellent flexibility and stretchability, reinforcement using various polymers are essential for 3D GS. For example, Cheng’ group20 reported the 3D graphene foam-based flexible conductors for the first time by template-directed chemical vapor deposition and the combination of poly(dimethyl siloxane) (PDMS), which exhibited stable and high electrical conductivity. Yu Pang et al.21 prepared a similar composite for flexible strain sensor,


Page 3 of 25 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


which could recognize the walking states, the degree of finger bending and the wrist blood pressure. However, the elastomer mentioned above cannot be healed after damage, leading to the failure of the entire electronics10, 12-16, 22-23. In 2015, Eduardo Saiz et al.24 reported a self-healing 3D graphene-based flexible sensor with polyborosiloxane (PBS) that served as a self-healing material. The composite was able to detect the pressure and flexion angle and healed automatically for 6-8 cycles. However, the mechanical strength of PBS was poor, which rendered it not suitable for usage in harsh environments. Self-healing materials based on thermally reversible Diels-Alder (DA) chemistry has developed rapidly during the past decade25-26, which has been employed in healable flexible electronics. Qibing Pei et al.27 reported a healable, semitransparent silver nanowire-polymer composites conductor that could be healed by heating to 110 °C, upon which 97% of the surface conductivity could be recovered in 5 min. After that, a healable stretchable transparent electrode was demonstrated

in

their

group28.

The

as-prepared

film

was

comprised

of

poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT) and silver nanowires (AgNWs) hybrid layer as well as a DA elastomer substrate, and the damages could be healed for three times at the same location. Besides, Youngmin Kim and Jong-Woong Kimet al.29-31 prepared the healable transparent electrodes by DA chemistry and AgNWs, which could be healed with intense pulsed light in less than 1 ms. Our group also had reported self-healing graphene-based flexible electronics based on DA chemistry healed by microwave32 and infrared laser33 with high resultant mechanical strength. However, to our best knowledge, there were no report on 3D graphene-based healable composite based on DA chemistry. In this paper, we reported a self-healing composite based on 3D GS and DA chemistry. The GS was prepared with furfurylamine (named as FAGS), which served as not only a reducing



Page 4 of 25

agent but also a modifying agent of graphene oxide, forming the 3D FAGS with free furan groups. Then, the furfuryl-contained linear polyurethane and bismaleimide were filled in the 3D FAGS and crosslinked to form a gel, after which the composites were dried, resulting in the 3D FAGS/DAPU composite. The as-prepared composite exhibited excellent stretchability and intrinsic conductivity with low concentration of graphene (about 2 wt%), which could be directly employed for the flexible sensor to detect human motion. In addition, the FAGS/DAPU composite exhibited lower temperature retro-DA response for the continuous graphene networks. Lastly, the successful demonstration of healing of the composites by heat and microwave with high efficiency were discussed.

MATERIALS AND METHODS Materials. Graphite powder was supplied by Aladdin (Shanghai, China) and used as received. The chemicals included potassium permanganate (KMnO4), sodium nitrate (NaNO3), concentrated sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl) were all reagent grade purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Diglycidyl ether of bisphenol A (DGEBA) was obtained from Sigma-Aldrich Co. 4,4-diphenylmethane diisocyanate, (MDI, 99%), furfurylamine (FA) and N,N’-(4,4’-diphenylmethane)bismaleimide (BMI) were supplied by Aladdin and used as received, poly(tetramethylene glycol) (PTMEG, M̅n = 2000 g/mol) was purchased from Aladdin and used after 2 h drying under vacuum at 110 oC. Dimethyl formamide (DMF) was dried with the molecular sieves for more than 24 h and freshly distillated before use. The preparation of 3D graphene structure by furfurylamine (FAGS). Graphene oxide (GO) was prepared by the modified Hummers’ method34. In a typical process, 2 mL of GO solution (5 mg mL-1) and 30 mg FA were mixed and sealed in a square quartz mold (40 mm × 40 mm) and


Page 5 of 25 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


heated at 90 oC for 5 h. Then the bottle was cooled down naturally to the room temperature resulting in the graphene hydrogel by furfurylamine (named as FAGH). The FAGH was washed by ethanol and deionized water for several times. Finally, the 3D FAGS was obtained after freeze-drying. The preparation of the 3D FAGS/DAPU healable strain sensor. Furfuryl-contained linear polyurethane (FLPU) and DAPU were prepared according to our previous work32 and the synthesis processes were shown in Figure S1. And in order to prepare the 3D FAGS based healable flexible electronics, a piece of FAGS with a size of 35 mm × 35 mm resulting from the assembly process was employed and 2 mL of 25 wt% DAPU solution (FLPU and BMI were mixed, without crosslinking) was dropped into the FAGS and degassed in a vacuum oven for 1 h. Then, the composites were placed in a sealed vessel and heated at 65 oC for 5 h to form a gel. After that, the composite was dried in the oven to obtain the FAGS/DAPU composite. As comparison, the dried composite without the gel process was named as FAGS/DAPU’. Characterization. Atomic force micrographs (AFM) were recorded with a Dimension Icon (Bruker, USA) instrument and operated in air in AC mode. Scanning electronic micrographs (SEM) were recorded with a Nova NanoSEM 450. Powder X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (RiGSku D/Max 2500) with monochromated Cu Ka radiation (λ=1.54 Å). Raman spectra were measured by LabRAM HR Raman Spectrometer (HORIBA Jobin-Yvon, France) with a laser at the excitation wavelength of 632.8 nm and 15.7 mW power irradiation. Fourier transform infrared (FTIR) spectra were recorded with Bruker Vertex 70 spectrometer (Bruker Optik GmbH, Ettlingen, Germany) in the range of 4000-400 cm–1. X-ray photoelectron spectroscopy (XPS) analyses were conducted with a XSAM800 system, where an Al Kα excitation source was used. Thermal gravity analysis (TGA) was made on a TA SDTQ600



Page 6 of 25

thermo-gravimetric analyzer; the microbalance has a precision of ±0.1 µg. Samples of 8-10 mg were placed into 70 µL alumina pans. The samples were heated from 30 oC to 800 oC under a nitrogen flow of 100 mL min-1. Differential scanning calorimetry (DSC) was performed on the Q20-1173 DSC thermal system (TA instruments, New Castle, USA) with a heating rate of 10 °C min-1 and ranging from 15 °C to 180 °C. Nitrogen gas was purged at a flow rate of about 50 mL min-1. Tensile test was carried out on a stretching machine (AG-X Plus 100N) at room temperature with a speed of 5-10 mm min-1. The electrical properties were recorded by a digital source meter (Keithley 2000).

RESULTS and DISCUSSION Preparation of 3D FAGS The 3D FAGS/DAPU was prepared in the following steps as shown in Figure 1 and Figure S2. Firstly, FAGS was obtained by the reaction of GO (AFM images shown in Figure S3) and FA. Organic amine, i.e. ethylenediamine, has been reported for the reduction and modification of GO for the existence of amines35-36. In this case, FA was employed here for the first time to reduce and modify GO, which resulted in 3D FAGS after freeze-drying. Also, furfuryl groups were introduced into the graphene sheets at the same time by the reaction of amino groups and epoxide groups of GO, which would react with maleimide in the following steps to form a six-membered ring structure based on DA chemistry. After that, the DMF solution of FLPU and BMI with a molar ratio of 1:1 was dropped in to FAGS, gelled for the DA reaction of FLPU and BMI and dried to finally form the FAGS/DAPU composite. The dynamic DA reaction provides the self-healing property of the composite. And the 3D FAGS offers the thermal and electronic conductivity which would benefit the healing process for the thermal response DA reaction and enable the


Page 7 of 25 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


mechanical-resistance response for the strain sensor. It should be noticed that the gel process is quite important for the formation of the high-performance FAGS/DAPU composites.

Figure 1. Schematic of the fabrication process of FAGS/DAPU self-healing composites In order to prepare 3D FAGS with strong mechanical property and high porosity, which enable the infiltration of the self-healing polymer, the concentration and the volume of the GO solutions were discussed firstly. As shown in Figure S4, GO solutions with concentrations of 1 mg/mL, 2 mg/mL, 3 mg/mL and 5 mg/mL were employed. It was found that, with the increase of the concentration, the mechanical strength increased significantly. After the assembly process, FAGH-1 was still a solution because of the low concentration, whereas FAGH-2, FAGH-3 and FAGH-5 were all cylinders with the similar size (Figure S4a). Besides, FAGH-3 and FAGH-5 could be taken out unbroken while some defects could be found in FAGH-2 after it is taken out (Figure S4b). In addition, FAGH-5 could survive a weight of 200 g but FAGH-3 could not, as shown in Figure S4c. In this case, the 5 mg/mL GO solution was selected for the subsequent experiments.



Page 8 of 25

Figure 2. SEM images of FAGS prepared with different amount of GO: (a, c and e) The top view SEM images of FAGS prepared by 1 mL, 2 mL and 3 mL of GO separately; (b, d and f) cross-sectional SEM images of FAGS prepared by 1 mL, 2 mL and 3 mL of GO separately One of the most important aspects for flexible electronic is thickness, which affects the flexibility and stretchability significantly. Another key factor of 3D FAGS is porosity, which would influence the infiltration of the polymers. In this work, we employed 1-3 mL GO solution with the concentration of 5 mg/mL sealed in a square quartz mold for the assembly process. The


Page 9 of 25 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


inner structures after freeze-drying were shown in Figure 2. It can be seen that all the samples exhibited porous structure with the pore size ranging from a few micrometers to hundreds of micrometers. From the top-view images (Figures 2a, c and e), it can be observed that with the increase of GO solution, the pore size increased at first and then decreased, which may be due to the excess usage of GO. The high magnification inset images showed the increase of the thickness of the side wall composed of the reduced graphene oxide sheets. Figures 2b, d and f showed the change of the thickness of the as-prepared 3D FAGS, which increased from 500 um to 1000 um and then 1400 um with the increase of the usage of GO solution with excellent controllability. Besides, the porosity of FAGS reduced with an increase of GO, which was consistent with our previous results18. In order to get the FAGS with abundant conductive network as well as high porosity, FAGS with a thickness of 1000 um was selected for the preparation of FAGS/DAPU composites. Figure 3 showed the chemical components and microstructure of FAGS by FTIR, XRD, Raman, TGA and XPS. The FTIR spectra is shown in Figure 3a, with a peak at 1726 cm-1, which can be attributed to the C=O, which disappeared after reduction by FA. Besides, the peak at 1579 cm-1 confirmed the presence of the –NH– group and the peak at 1072 cm-1, which confirmed the presence of C–O–C of furfuryl of furfurylamine. All results suggest that GO was reduced and modified by furfurylamine resulting the FAGS containing with furfuryl. The Raman spectra of GO and FAGS exhibited two remarkable bands at around 1334 and 1594 cm-1, which can be assigned to the D- and G-bands of carbon. The ID/IG ratio of GO was 1.08 whereas the corresponding value of FAGS increased to 1.12, which indicated that the GO sheets were reduced to rGO and their conjugated structures were partly restored during the reduction process with furfurylamine37-39. In the XRD patterns, the diffraction peak for GO appears was around



2θ=10.60

o

Page 10 of 25

(d-spacing 8.33 Å). After reduction and freeze-drying, a new broadened diffraction

peak at 2θ=25.34

o

(d-spacing 3.51 Å) appeared in the aerogel, which is close to the d-spacing

3.36 Å of the graphite, which suggested that chemical reduction happened successfully during the process18, 40. Besides, the TGA curve of FAGS (Figure 3d) displayed a slowly downward sloping line compared with that of GO, which indicated the enhanced thermal stability due to the removal of oxygen-containing groups.

Figure 3. The structure characterization of FAGS: (a) FTIR spectra of GO and FAGS. (b) Raman spectra of GO and FAGS. (c) XRD patterns of GO and FAGS. (d) TGA spectra of GO and FAGS.


Page 11 of 25 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


(e) XPS survey spectra of FAGS and (f) high-resolution XPS spectra of the C 1s region. The XPS spectra in Figure 3e exhibited three characteristic peaks located at 285, 399 and 532 eV, corresponding respectively to C 1s, N 1s, and O 1s, which indicate that N atoms were successfully introduced into the FAGS. The elemental contents of C, N and O in RFGO were 87.53%, 6.28% and 6.19%, respectively. The high-resolution spectrum of C 1s in FAGS could be fitted by several peaks corresponding to C–C (284.8 eV), C-O (286.7 eV), C=O (288.0 eV), while the particular peak located at 285.6 eV suggests the presence of C–N (Figure 3f). The results of XPS spectra suggested that furfurylamine served as the reducing as well as modifying agent of GO, forming the 3D FAGS with free furan groups, which participated in the DA reaction in the subsequent infiltration of FLPU and BMI.

Preparation of 3D FAGS/DAPU self-healing composite According to the analysis, FAGS was successfully prepared by the reduction and modification of furfurylamine. The thickness and porosity could be controlled facilely. In order to obtain the FAGS/DAPU self-healing composite, a solution with 25 wt% FLPU and the same moles of BMI were added into the FAGS, gelled at 65 oC, and then dried at last, which was known as the “infiltrate-gel-dry” process. The SEM images were shown in Figure 4. Figure 4a-c exhibited the surface images of the FAGS-DAPU from which it is evident that the 3D porous structures were maintained. The results suggest that the “infiltrate-gel-dry” process successfully protected the 3D graphene structure and the conductive path, which is suitable for application in flexible electronics. Besides, the cross-section images of the as-prepared FAGS/DAPU films were collected as well and shown in Figures 4d-f. The composites film exhibited a significant decrease of the thickness from about 1000 um of FAGS to about 220 um after drying, which can be



Page 12 of 25

attributed to the evaporation of DMF. In addition, Figure 4d shows the uniformity infiltration of the DAPU. The enlarged images in Figure 4e and f confirm the integrity of the pore wall made up of graphene sheets and the cell walls were well bonded by the DAPU matrix indicating the strong interaction of FAGS and DAPU.

Figure 4. SEM images of FAGS/DAPU: (a-c) top view of FAGS/DAPU at different magnifications; (d-f) cross-sectional view of FAGS/DAPU at different magnifications

It should be noticed that the “infiltrate-dry” process compared with the “infiltrate-gel-dry” process exhibited much difference and decrease in mechanical properties, as the SEM images of FAGS/DAPU’ show in Figure S5. The surface image of FAGS/DAPU’ was quite smooth and the 3D porous structure could not be recognized, which mean the polymer of DAPU was concentrated at the surface because of the capillarity formed during the drying process. Besides, the cross-section images in Figure S5d-f showed that the distribution of the polymer was disordered. Bubbles and cracks could be found in the composites, which reduce its mechanical properties and conductivity.


Page 13 of 25 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


The mechanical property and thermal effect of 3D FAGS/DAPU The typical strain-stress curves of FAGS/DAPU and FAGS/DAPU’ were shown in Figure 5 and the data of mechanical strength were collected in Table 1. The mechanical properties of FAGS/DAPU were much better than that of the FAGS/DAPU’ in terms of Young’ Modulus, strain-at-break, and stress-at-break, which was due to the uniformity of the 3D graphene structure and the polymer. Besides, compared with DAPU, FAGS/DAPU exhibited a much higher Young’ Modulus whereas the strain-at-break and stress-at-break reduced. It should be attributed to the existence of 3D graphene structure, which increased the strength and decrease the flexibility. Even though the strain-at-break decreased, a large strain of more than 200% could be obtained, which achieves the high stretchability suitable for flexible electronics41-42.

Figure 5. The typical stress–strain curves of FAGS/DAPU and FAGS/DAPU’ (25 oC)



Page 14 of 25

Table 1 Summary of the mechanical properties of the FAGS/DAPU and FAGS/DAPU’ (The average values were obtained from more than 10 samples) Sample

Young’ Modulus

Strain-at-break

Stress-at-break

(MPa)

(%)

(MPa)

DAPU

29.27±2.44

358±12

19.92±1.22

FAGS/DAPU

76.31±5.19

210.26±18.59

11.96±0.95

FAGS/DAPU’

43.55±7.45

84.26±22.34

5.78±0.90

*The experimental data of DAPU were collected from our previous research32.

Successful DA chemistry was confirmed by DSC, which was verified by the characteristic peak of retro-DA (rDA) reaction as shown in Figure 6a. The DSC curves of DAPU and FAGS/DAPU exhibited significant endothermic peaks at 120-150 oC, which can be attributed to the rDA reaction

43-45

. It should be noticed that due to the existence of 3D graphene structure in

FAGS/DAPU, the rDA reaction occurred at lower temperature (120 oC) compared to that of DAPU (135 oC) that could enable the healing process due to the thermal reversibility of the DA chemistry. Subsequently, the thermal stability of FAGS/DAPU and DAPU were assessed by TGA under a nitrogen atmosphere between 30 oC and 800 oC as shown in Figure 6b. It can be seen that the decomposition of FAGS/DAPU and DAPU took place in a single step, which suggests a high uniformity of as-prepared materials. It can be observed that the temperature at 5% weight loss (T5) reduced with an increase of FAGS. The T5 of DAPU was 328 oC whereas the T5 of FAGS/DAPU was 296 oC, which may be due to the limited reducing ability of FA resulted in the residual organic group in the FAGS. Besides, the TGA curve of FAGS/DAPU was similar to that


Page 15 of 25 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


of FAGS, which exhibited a significant weight loss at about 200 oC. In addition, it can be seen that with the addition of FAGS the char yields of FAGS/DAPUs at 800 oC (18.25 wt%) increased slightly compared to that of DAPU, which was 10.87 wt%.

Figure 6. (a) DSC curves and (b) TGA curves of DAPU and FAGS/DAPU The flexibility and sensing property of 3D FAGS/DAPU Importantly, the unique structure of 3D FAGS/DAPU composite endows it to be highly bendable and foldable while maintaining its original conductivity. The resistance variations upon tension bending has been investigated. Figure 7a showed the results of R/R0 as a function of bending radius (r). With the decrease of bending radius, R/R0 exhibited a slight increase, which gradually increases to 1.021 at a bending radius of 8.0 mm during the first bending cycle. After the first cycle, the 3D FAGS/DAPU composite can almost recover the original value after straightening. Besides, Figure 7b exhibited the stability of the resistance of FAGS/DAPUs after 1000 cycles with a bending radius of 8.0 mm which were very stable. Bending tests demonstrated that bending processes could not destroy conductive networks of the as-prepared FAGS/DAPU composite leading to a stable resistance during bending cycles.



Page 16 of 25

Figure 7. (a) R/R0 change of FAGS/DAUs at a bend radius up to 8.0 mm in the first bending cycle. The inset pictures describe the bending process. (b) R/R0 of FAGS/DAPUs as a function of cycles for a bend radius of 8.0 mm. (c) Resistance-time curve of FAGS/DAPUs for the application of the flexible strain sensor during the finger bending cycles. (d) Durability test of the FAGS/DAPU as a strain sensor under 10% strain for 200 cycles. In addition, the 3D graphene networks in the as-prepared FAGS/DAPU composite provided highly effective conductive path that could be exploited to realize flexible strain sensors. In order to assess the ability of the proposed composite in serving a practical application, a prototype was fabricated. As shown in Figure 7c, the FAGS/DAPU composite was cut into slice with the width of 2 mm and connected by the copper wires. The as-prepared flexible strain sensor was tied on the surface of the glove to detect the movements of the finger. It can be seen that the starting resistance was about 80 kΩ, which is much larger than that of graphene sponge prepared by (NH4)2S according to our previous research18. This can be attributed to the limited reducing


Page 17 of 25 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


ability of FA as well. However, the as-prepared strain sensor was able to detect the signals of the finger bending for cycles, which means the FAGS/DAPU was suitable for the application of flexible sensors. Also, the durability of the FAGS/DAPU composites were investigated with a speed of 5 mm/min. As shown in Figure 7d, after 200 cycles of stretching-relaxing under 10% strain, the composites remained excellent stability. Healing property of FAGS/DAPU composite According to the results, FAGS/DAPU exhibited excellent stretchability and detection of human movements as well as a lower temperature rDA response. In this case, the healing process of FAGS/DAPU composite was investigated. In addition, DSC analysis showed a significant endothermic peak at 120 oC of FAGS/DAPU, which was due to the 3D graphene network. In this case, 120 oC was set as the healing temperature for the composites. Initially, the FAGS/DAPU composite was healed with heat. As shown in Figure 8a-c and Figure S6, the composites were cut by a blade with a signal of “×”, then the composites were heated at 120 oC for 30 min for the rDA reaction and subsequently at 65 oC for 5 h for the DA reaction. Figures 8b and S6d exhibited the different places of the wound (Figure 8b depicted the area around the cross position whereas Figure S6d showed locations far away from the cross). It can be seen that the width of the crack was about 13 um and then, after healing, the crack almost disappeared (Figure S6b and S6e). Even under high magnification (Figures 8c and S6f), no obvious cracks could be found. The crack healing ability of FAGS/DAPU composite based on DA reaction was clearly demonstrated. Microwave has been found to be a very useful tool for the healing of the graphene-based composite based on DA chemistry with high efficiency32. The healing effect of FAGS/DAPU composite with microwave was investigated in the following parts as well. Similar to Figure 8a-c, the FAGS/DAPU composite film was cut and shown in Figure 8d-f and Figure S7. The cracks



Page 18 of 25

was healed with microwave for 5 min and heated in the oven at 65 oC for 5 h. Figures S7a and d exhibited similar cracks shown in Figures S6a and d, and after healing for 5 min, the cracks could no longer be found (Figures S7b and e). The enlarged pictures of Figures S7c and f did not show any cracks as well. The effective absorption of the microwave can be attributed to the high uniformity of the 3D graphene network in the composites.

Figure 8. The healing process of FAGS/DAPU with heat and microwave: (a) Schematic diagram of the healing process of FAGS/DAPU with heat. (b) fissures cut by a knife, (c) surface of the sample after healing with heat. (d) Schematic diagram of the healing process of FAGS/DAPU with microwave. (e) fissures cut by a knife, (f) surface of the sample after healing by microwave.

In addition, the healing of the conductivity of FAGS/DAPU composites using microwaves was investigated and shown in Figure 9 as well. At first, the initial resistance of FAGS/DAPU was about 64 KΩ as shown in Figure 9a and the SEM image exhibited a connected structure of the composite. Then the FAGS/DAPU composite was cut off by a clean blade. Figure 9b showed that the digital source meter was not able to read the resistance and the SEM image exhibited a


Page 19 of 25 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


clear crack in the composite. After that, the FAGS/DAPU composite was pressed together and then put into the microwave oven for 5 min. After the measurement by the digital source meter, it was found that the resistance was recovered which was about 85 KΩ. The sight increase in the resistance may due to the misconnection of the broken surfaces of the composite. And in this way, the FAGS/DAPU composite exhibited its potential application in flexible electronics as well as its healing ability.

Figure 9. The healing process of as-prepared flexible electronics. (a) The original sample before cutting off; (b) the sample after cutting off; (c) the cutting off sample after healing by microwave.

CONCLUSION In summary, a novel self-healing flexible 3D FAGS/DAPU composite based on DA chemistry was prepared. Furfurylamine was employed as the reducing as well as modifying agent to form FAGS/DAPU by the “infiltrate-gel-dry” process. The as-prepared composite exhibited excellent stretchability (more than 200%) and intrinsic conductivity with low cooperation of graphene (2 wt%), which could be directly employed as a flexible strain sensor to detect human motion. The



Page 20 of 25

FAGS/DAPU composite exhibited a lower temperature retro-DA response for the continuous graphene networks. In addition, effective healing of the composites by heat and microwave were demonstrated. The proposed FAGS/DAPU composite exhibited great application potential in applications such as sensors, e-skin and human-machine interfaces.

■ASSOCIATED CONTENT Supporting Information Additional figures as described in the text (Figure S1-S7) are available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by NSFC-Guangdong Jointed Funding (U1601202), NSFC-Shenzhen Robot Jointed Funding (U1613215), Guangdong and Shenzhen Innovative Research Team Program (No.2011D052, KYPT20121228160843692), Key Laboratory of Guangdong Province (2014B030301014), R&D Funds for Basic Research Program of Shenzhen (Grant No. JCYJ20150401145529012, JCYJ20160331191741738, JSGG20160229194437896), Research Grants Council (No. 11213515), National Natural Science Foundation of China


Page 21 of 25 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


(21601065) and SIAT Innovation Program for Excellent Young Researchers (Y6G015).

REFERENCES 1. Cong, H.-P.; Chen, J.-F.; Yu, S.-H., Graphene-Based Macroscopic Assemblies and Architectures: An Emerging Material System. Chem. Soc. Rev. 2014, 43, 7295-7325. 2. Li, C.; Shi, G., Three-Dimensional Graphene Architectures. Nanoscale 2012, 4, 5549-5563. 3. Qiu, L.; Bulut Coskun, M.; Tang, Y.; Liu, J. Z.; Alan, T.; Ding, J.; Truong, V. T.; Li, D., Ultrafast Dynamic Piezoresistive Response of Graphene‐Based Cellular Elastomers. Adv. Mater. 2016, 28, 194-200. 4. Gao, H.-L.; Zhu, Y.-B.; Mao, L.-B.; Wang, F.-C.; Luo, X.-S.; Liu, Y.-Y.; Lu, Y.; Pan, Z.; Ge, J.; Shen, W., Super-Elastic and Fatigue Resistant Carbon Material with Lamellar Multi-Arch Microstructure. Nat. Commun. 2016, 7, 12920. 5. Wu, Y.; Yi, N.; Huang, L.; Zhang, T.; Fang, S.; Chang, H.; Li, N.; Oh, J.; Lee, J. A.; Kozlov, M., Three-Dimensionally Bonded Spongy Graphene Material with Super Compressive Elasticity and near-Zero Poisson’s Ratio. Nat. Commun. 2015, 6, 6141. 6. Zheng, Q.; Cai, Z.; Ma, Z.; Gong, S., Cellulose Nanofibril/Reduced Graphene Oxide/Carbon Nanotube Hybrid Aerogels for Highly Flexible and All-Solid-State Supercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 3263-3271. 7. El-Kady, M. F.; Shao, Y.; Kaner, R. B., Graphene for Batteries, Supercapacitors and Beyond. Nat. Rev. Mater. 2016, 1, 16033. 8. Shehzad, K.; Xu, Y.; Gao, C.; Duan, X., Three-Dimensional Macro-Structures of Two-Dimensional Nanomaterials. Chem. Soc. Rev. 2016, 45, 5541-5588. 9. Wang, M.; Duan, X.; Xu, Y.; Duan, X., Functional Three-Dimensional Graphene/Polymer Composites. ACS Nano 2016, 10, 7231-7247. 10. Li, J.; Zhao, S.; Zeng, X.; Huang, W.; Gong, Z.; Zhang, G.; Sun, R.; Wong, C.-P., Highly Stretchable and Sensitive Strain Sensor Based on Facilely Prepared Three-Dimensional Graphene Foam Composite. ACS Appl. Mater. Interfaces 2016, 8, 18954-18961. 11. Shao, J. J.; Lv, W.; Yang, Q. H., Self‐Assembly of Graphene Oxide at Interfaces. Adv. Mater. 2014, 26, 5586-5612.



Page 22 of 25

12. Wu, S.; Ladani, R.; Zhang, J.; Ghorbani, K.; Zhang, X.; Mouritz, A. P.; Kinloch, A. J.; Wang, C. H., Strain Sensors with Adjustable Sensitivity by Tailoring the Microstructure of Graphene Aerogel/Pdms Nanocomposites. ACS Appl. Mater. Interfaces 2016, 8, 24853-24861. 13. Qin, Y.; Peng, Q.; Ding, Y.; Lin, Z.; Wang, C.; Li, Y.; Xu, F.; Li, J.; Yuan, Y.; He, X., Lightweight, Superelastic, and Mechanically Flexible Graphene/Polyimide Nanocomposite Foam for Strain Sensor Application. ACS Nano 2015, 9, 8933-8941. 14. Xu, R.; Lu, Y.; Jiang, C.; Chen, J.; Mao, P.; Gao, G.; Zhang, L.; Wu, S., Facile Fabrication of Three-Dimensional Graphene Foam/Poly (Dimethylsiloxane) Composites and Their Potential Application as Strain Sensor. ACS Appl. Mater. Interfaces 2014, 6, 13455-13460. 15. Li, Y.-Q.; Zhu, W.-B.; Yu, X.-G.; Huang, P.; Fu, S.-Y.; Hu, N.; Liao, K., Multifunctional Wearable Device Based on Flexible and Conductive Carbon Sponge/Polydimethylsiloxane Composite. ACS Appl. Mater. Interfaces 2016, 8, 33189-33196. 16. Samad, Y. A.; Li, Y.; Alhassan, S. M.; Liao, K., Novel Graphene Foam Composite with Adjustable Sensitivity for Sensor Applications. ACS Appl. Mater. Interfaces 2015, 7, 9195-9202. 17. Li, J.; Zhang, G.; Deng, L.; Zhao, S.; Gao, Y.; Jiang, K.; Sun, R.; Wong, C., In Situ Polymerization of Mechanically Reinforced, Thermally Healable Graphene Oxide/Polyurethane Composites Based on Diels–Alder Chemistry. J. Mater. Chem. A 2014, 2, 20642-20649. 18. Li, J.; Zhao, S.; Zhang, G.; Gao, Y.; Deng, L.; Sun, R.; Wong, C.-P., A Facile Method to Prepare Highly Compressible Three-Dimensional Graphene-Only Sponge. J. Mater. Chem. A 2015, 3, 15482-15488. 19. Han, Z.; Tang, Z.; Li, P.; Yang, G.; Zheng, Q.; Yang, J., Ammonia Solution Strengthened Three-Dimensional Macro-Porous Graphene Aerogel. Nanoscale 2013, 5, 5462-5467. 20. Chen, Z.; Ren, W.; Gao, L.; Liu, B.; Pei, S.; Cheng, H.-M., Three-Dimensional Flexible and Conductive Interconnected Graphene Networks Grown by Chemical Vapour Deposition. Nat. Mater. 2011, 10, 424-428. 21. Pang, Y.; Tian, H.; Tao, L.; Li, Y.; Wang, X.; Deng, N.; Yang, Y.; Ren, T.-L., Flexible, Highly Sensitive, and Wearable Pressure and Strain Sensors with Graphene Porous Network Structure. ACS Appl. Mater. Interfaces 2016, 8, 26458-26462. 22. Wu, S.; Ladani, R. B.; Zhang, J.; Ghorbani, K.; Zhang, X.; Mouritz, A. P.; Kinloch, A. J.; Wang, C. H., Strain


Page 23 of 25 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


Sensors with Adjustable Sensitivity by Tailoring the Microstructure of Graphene Aerogel/Pdms Nanocomposites. ACS Appl. Mater. Interfaces 2016, 8, 24853-24861. 23. Liu, H.; Dong, M.; Huang, W.; Gao, J.; Dai, K.; Guo, J.; Zheng, G.; Liu, C.; Shen, C.; Guo, Z., Lightweight Conductive Graphene/Thermoplastic Polyurethane Foams with Ultrahigh Compressibility for Piezoresistive Sensing. J. Mater. Chem. C 2017, 5, 73-83. 24. D'Elia, E.; Barg, S.; Ni, N.; Rocha, V. G.; Saiz, E., Self‐Healing Graphene‐Based Composites with Sensing Capabilities. Adv. Mater. 2015, 27, 4788-4794. 25. Liu, Y.-L.; Chuo, T.-W., Self-Healing Polymers Based on Thermally Reversible Diels–Alder Chemistry. Polym. Chem. 2013, 4, 2194-2205. 26. Chen, X.; Dam, M. A.; Ono, K.; Mal, A.; Shen, H.; Nutt, S. R.; Sheran, K.; Wudl, F., A Thermally Re-Mendable Cross-Linked Polymeric Material. Science 2002, 295, 1698-1702. 27. Gong, C.; Liang, J.; Hu, W.; Niu, X.; Ma, S.; Hahn, H. T.; Pei, Q., A Healable, Semitransparent Silver Nanowire‐Polymer Composite Conductor. Adv. Mater. 2013, 25, 4186-4191. 28. Li, J.; Qi, S.; Liang, J.; Li, L.; Xiong, Y.; Hu, W.; Pei, Q., Synthesizing a Healable Stretchable Transparent Conductor. ACS Appl. Mater. Interfaces 2015, 7, 14140-14149. 29. Pyo, K.-h.; Lee, D. H.; Kim, Y.; Kim, J.-W., Extremely Rapid and Simple Healing of a Transparent Conductor Based on Ag Nanowires and Polyurethane with a Diels–Alder Network. J. Mater. Chem. C 2016, 4, 972-977. 30. Heo, G.; Pyo, K.-h.; Da Hee Lee, Y. K.; Kim, J.-W., Critical Role of Diels–Adler Adducts to Realise Stretchable Transparent Electrodes Based on Silver Nanowires and Silicone Elastomer. Sci. Rep. 2016, 6, 25358. 31. Lee, D. H.; Heo, G.; Pyo, K.-h.; Kim, Y.; Kim, J.-W., Mechanically Robust and Healable Transparent Electrode Fabricated Via Vapor-Assisted Solution Process. ACS Appl. Mater. Interfaces 2016, 8, 8129-8136. 32. Li, J.; Zhang, G.; Sun, R.; Wong, C.-P., A Covalently Cross-Linked Reduced Functionalized Graphene Oxide/Polyurethane Composite Based on Diels–Alder Chemistry and Its Potential Application in Healable Flexible Electronics. J. Mater. Chem. C 2017, 5, 220-228. 33. Wu, S.; Li, J.; Zhang, G.; Yao, Y.; Li, G.; Sun, R.; Wong, C.-P., Ultrafastly Self-Healing Nanocomposites Via Infrared Laser and Its Application in Flexible Electronics. ACS Appl. Mater. Interfaces 2017, 3040–3049. 34. Li, J.; Zhang, G.; Fu, C.; Deng, L.; Sun, R.; Wong, C.-P., Facile Preparation of Nitrogen/Sulfur Co-Doped and



Page 24 of 25

Hierarchical Porous Graphene Hydrogel for High-Performance Electrochemical Capacitor. J. Power Sources 2017, 345, 146-155. 35. Hu, H.; Zhao, Z.; Wan, W.; Gogotsi, Y.; Qiu, J., Ultralight and Highly Compressible Graphene Aerogels. Adv. Mater. 2013, 25, 2219-2223. 36. Li, J.; Li, J.; Meng, H.; Xie, S.; Zhang, B.; Li, L.; Ma, H.; Zhang, J.; Yu, M., Ultra-Light, Compressible and Fire-Resistant Graphene Aerogel as a Highly Efficient and Recyclable Absorbent for Organic Liquids. J. Mater. Chem. A 2014, 2, 2934-2941. 37. Fan, Z.-J.; Kai, W.; Yan, J.; Wei, T.; Zhi, L.-J.; Feng, J.; Ren, Y.-m.; Song, L.-P.; Wei, F., Facile Synthesis of Graphene Nanosheets Via Fe Reduction of Exfoliated Graphite Oxide. ACS Nano 2010, 5, 191-198. 38. Kuila, T.; Bose, S.; Khanra, P.; Mishra, A. K.; Kim, N. H.; Lee, J. H., A Green Approach for the Reduction of Graphene Oxide by Wild Carrot Root. Carbon 2012, 50, 914-921. 39. Liu, Y.; Qi, G.-Q.; Liang, C.-L.; Bao, R.-Y.; Yang, W.; Xie, B.-H.; Yang, M.-B., Effect of Graphite Oxide Structure on the Formation of Stable Self-Assembled Conductive Reduced Graphite Oxide Hydrogel. J. Mater. Chem. C 2014, 2, 3846-3854. 40. Li, R.; Chen, C.; Li, J.; Xu, L.; Xiao, G.; Yan, D., A Facile Approach to Superhydrophobic and Superoleophilic Graphene/Polymer Aerogels. J. Mater. Chem. A 2014, 2, 3057-3064. 41. Yang, J.; Ran, Q.; Wei, D.; Sun, T.; Yu, L.; Song, X.; Pu, L.; Shi, H.; Du, C., Three-Dimensional Conformal Graphene Microstructure for Flexible and Highly Sensitive Electronic Skin. Nanotechnology 2017, 28, 115501. 42. Kim, T.; Park, J.; Sohn, J.; Cho, D.; Jeon, S., Bioinspired, Highly Stretchable, and Conductive Dry Adhesives Based on 1d–2d Hybrid Carbon Nanocomposites for All-in-One Ecg Electrodes. ACS Nano 2016, 10, 4770-4778. 43. Yu, S.; Zhang, R.; Wu, Q.; Chen, T.; Sun, P., Bio‐Inspired High‐Performance and Recyclable Cross‐Linked Polymers. Adv. Mater. 2013, 25, 4912-4917. 44. Zhang, Y.; Broekhuis, A. A.; Picchioni, F., Thermally Self-Healing Polymeric Materials: The Next Step to Recycling Thermoset Polymers? Macromolecules 2009, 42, 1906-1912. 45. Li, J.; Zhang, G.; Deng, L.; Jiang, K.; Zhao, S.; Gao, Y.; Sun, R.; Wong, C., Thermally Reversible and Self‐ Healing Novolac Epoxy Resins Based on Diels–Alder Chemistry. J. Appl. Polym. Sci. 2015, 132, 42167.


Page 25 of 25 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


Table of Contents Graphic


Three-Dimensional Graphene Structure for Healable Flexible

Recommend Documents