Remote and efficient infrared induced self-healable stretchable substrate for wearable electronics

Journal Pre-proofs Remote and efficient infrared induced self-healable stretchable substrate for wearable electronics Han Jia, Shu-Ying Gu PII: DOI: Reference:

S0014-3057(19)32591-1 https://doi.org/10.1016/j.eurpolymj.2020.109542 EPJ 109542

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

21 December 2019 15 January 2020 31 January 2020

Please cite this article as: Jia, H., Gu, S-Y., Remote and efficient infrared induced self-healable stretchable substrate for wearable electronics, European Polymer Journal (2020), doi: https://doi.org/10.1016/j.eurpolymj. 2020.109542

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Remote and efficient infrared induced self-healable stretchable substrate for wearable electronics

Han Jia1, Shu-Ying Gu *1,2 1Department

of Polymeric Materials, School of Materials Science and Engineering, Tongji

University, Shanghai, China 2Key

Laboratory of Advanced Civil Engineering Materials, Ministry of Education, Tongji

University, Shanghai, China

*To whom correspondence should be addressed: Shu-Ying Gu, Department of Polymeric Materials, School of Materials Science and Engineering, Tongji University, Shanghai, China. Email: [email protected]

1

ABSTRACT: Flexible and wearable electronics as candidates for the next-generation electric devices have attracted wide attention and been actively investigated recently. A remote, fast and efficient self-healable stretchable polyurethane nanocomposite for the substrate of flexible wearable electronics is reported in this paper. Carbon nanotubes (CNTs) were embedded in a self-healing polyurethane based on disulfide bonds. The tensile strength at break of the composites was improved significantly from 1.01 MPa to 3.64 MPa when the content of CNTs was 5 wt%. The elongation at break decreased as the addition of CNTs, but still remained at the high level above 300%, indicating the composites had enough flexibility for stretchable wearable electronics. The damages of the composites could be healed by near-infrared (NIR) irradiation remotely and quickly due to the photothermal effect of CNTs. The healing efficiencies were over 80% after 1 min of NIR irradiation. The nanocomposites exhibited volume resistivity values in the range of 1.31105 to 1.65105 Ω cm as a result of the incorporation of CNTs which could meet the requirements for electrostatic charge dissipation (ESD) materials requiring conductivities in the range of 105 to 109Ω cm. The conductivity of the broken electric circuit fabricated by silver paste drop-cast on the composite strip was completely recovered after 1 min of NIR irradiation. These results demonstrate that the nanocomposites in this work can be used as substrates of electrical devices, especially for flexible wearable electronics. The electrical devices can be healed by remote NIR irradiation quickly and precisely. Keywords: polyurethane; self-healing; dynamic disulfide bonds; healing efficiency; NIR irradiation

2

1. Introduction Wearable electronic devices demand stretchable and deformable materials to ensure the favorable accommodation to arbitrary surfaces and human motions [1-3]. Flexible and wearable electronics as candidates for the next-generation electric devices have attracted wide attention and been actively explored for their applications in the intelligent motion monitoring system, smart electronic skin and wearable electronic devices [2,4-12]. Wearable electronics might get damaged during unintentional destructive motions, ranging from external mechanical stimuli to internal mechanical friction [6]. Hence, self-healing stretchable substrates have potential applications for wearable or implant electronics to elongate their lifespan and ensure their performance [2,4]. Microcracks in self-healing polymers could be healed by the encapsulated healants in microcapsules, microvascular networks or microballons [13,14] and the reversible covalent bonds or non-covalent interactions under various stimuli such as light [4,5,15], pH value, heat [16,17,18] and etc. Most of the self-healing materials for wearable electronics are based on hydrogen bonding, which is weaker with lower healing efficiency and longer healing time [19,20]. However, polymers based on dynamic covalent bonds possess special multifunction such as healing, recycling and reprocessing abilities [21]. For example, chain exchange reaction of disulfide bonds can be initiated under exposure to heat, UV light and redox conditions without any catalysts or initiators [18,22-24]. Michal et al. developed a reversible shape-memory adhesive with two-level reversible adhesion containing dynamic disulfide bonds [25]. AbdolahZadeh et al. investigated the healing kinetics of healing process. The reversible bonds were responsible for the flow and the interface restoration, while the irreversible crosslinks control the required mechanical integrity during the healing process [26]. Self-healing of polymers based on dynamic exchange bonds can be triggered by direct heating 3

[27] and indirect heating by magnetic fields [28,29] or photothermal effect of nanoparticles [30]. Compared with direct heating or non-contact healing by electromagnetic fields, light is instantaneous, remote, local and targeted healing which has been used widely as triggering resource for self-healing coatings [31,33]. Meanwhile, for light triggered self-healing process, the samples can maintain their original shape without observable deformation after irradiation. The damaged samples can be locally repaired with high precision and efficiency without any obvious influence on other non-damaged parts [15]. Ultrafast self-healing polyurethane nanocomposite based on covalent Diels-Alder chemistry bonds with breaking strength healing efficiency more than 96% after 1 min irradiation time by 980 nm IR laser was reported by Wu et al. The nanocomposite can be used as self-healing flexible substrates for the next generation of intelligent flexible electronics [5]. Epoxy resin-based polysulfide elastomer was used as healing-assisting layer of triboelectric nanogenerator for potential implantable electronics. The near-infrared (NIR) irradiation induced remote and efficient self-healing of the substrate and was undoubtedly an ideal non-invasive choice of triggering methods in biological tissues [4]. Among the thermoplastic polymers, polyurethanes (PU) have attracted a great deal of attention due to their combination of properties such as flexibility, stretchability, good abrasion and chemical resistance, good wear and weather resistance and good mechanical properties [34]. Therefore, polyurethanes are widely used in flexible displays, smart clothing, electronic textiles and etc [35,36].

The purpose of this work is to present a remote, fast and efficient self-healable

stretchable polyurethane nanocomposite for the substrate of flexible wearable electronics. CNTs were embedded in the polyurethane based on disulfide bonds. Ultrafast and high efficient self-healing of damages could be realized by NIR irradiation in minutes. The composite has potential applications for the substrates of flexible displays, smart clothing, electronic textiles and 4

etc. 2. Experimental 2.1. Materials Stannous(II) octoate (Sn(Oct)2, 95%,), hexamethylene diisocyanate (HDI, 99%) were purchased from Aladdin Reagents (Shanghai) Co., Ltd, and used as received. 2,2-Dithiodiethanol (HEDS, 90% ） was purchased from Alfa Aesar (Shanghai) Chemical Technology Co., Ltd. and used as received. Poly(ethylene glycol) (PEG, Mn  2000 g mol-1) was supplied by Meryer (Shanghai) Chemical Technology Co., Ltd. Poly(tetramethylene glycol) (PTMEG, Mn  2,000 g mol-1) was purchased from Aladdin Reagents (Shanghai) Co., Ltd. PTMEG and PEG were used after 6 h of drying under vacuum at 80 C. N,N-dimethylformamide (DMF, 99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd and dried with anhydrous sodium sulfate for 24 h and filtrated prior to use. -OH functionalized multi-walled carbon nanotubes (MWCNTs, purity of 98wt%) with length of 10-30 μm and diameter of 10-20 nm were supplied by Chengdu Organic Chemicals Co., Ltd, Chinese Academy of Sciences. 2.2. Preparation of the self-healing polyurethane nanocomposite The self-healing polyurethane was synthesized according to the procedures as presented in Scheme 1. 3.768 g PEG (1.884 mmol) was dissolved in 10 mL DMF and heated to 80 C for 30 min in a 50 mL glass flask. 0.732 g HDI (4.353 mmol) and 50 μL Sn(Oct)2 (0.001 mol mL-1) were added into the flask after the solution was cooled to room temperature. Then the mixture was re-heated to 80 C and stirred for 10 min. 0.5 g PTMEG (0.25 mmol, 10 wt% to the pre-polymer) was added to the above mixture and kept stirring at 80 C for 6 h (-NCO : -OH = 2.04 : 1). All the procedures were protected under nitrogen. PEG-PTMEG polyurethane (named as PU) was obtained. To prepare self-healing polyurethane (PUSS), 0.51 g 2,2-dithiodiethanol (2.976 mmol) 5

was introduced into the above product as the chain-extender and the mixture was stirred under nitrogen at 80 C for 16 h. The molar ratio of -OH to -NCO was 1.39 to 1. 2 g as-prepared PUSS was dissolved in 20 mL DMF, appropriate CNTs dispersed DMF suspension was added and stirred for 12 h. Then the solution was poured into a Teflon mold and volatilized solution at 80 C for 48 h to obtain polyurethane nanocomposites with 1 wt%, 3 wt% and 5 wt% CNTs named as PUSS1, PUSS3 and PUSS5, respectively. To investigate the effect of CNTs on the NIR triggering healing properties of the polyurethane, films without CNTs (PUSS) were used as the control sample.

Scheme 1. The synthesis procedures of the polyurethane and the self-healing illustration of disulfide bonds.

2.3. Materials characterization FTIR spectra were obtained by a spectrometer (Bruker Equinox 55, Germany) at room 6

temperature in attenuated total reflectance mode in the range from 4000 to 400 cm-1 with a resolution of 4 cm-1. Raman spectra were recorded by a Lab Ram HR Evolution (Horiba Scientific, France) using a laser excitation wavelength of 532 nm. The Raman shift axis was calibrated by silicon. The molecular weight and molecular weight distribution of PU and PUSS were obtained by gel permeation chromatographer (GPC, 1260 Infinity II, Agilent (Shanghai) Technol. Co. Ltd, China) using DMF as the eluent. X-ray diffraction measurements (XRD) were performed on a X'Pert PRO MPD diffractometer (Holland Panalytical, Holland) using Cu K radiation ( = 0.154 nm) at a scanning rate of 5  per minute from 5 to 50 . Differential scanning calorimetry (DSC) analyses were carried out on a thermal analyzer (TA Q100, USA). The samples about 3 mg were heated to 180 C at a rate of 40 C min-1 and cooled down to -20 C at a rate of 10 C min-1 after equilibrium for 10 min. Then the samples were re-heated to 180 C at a rate of 10 C min-1. The transition temperatures and heat flow were recorded from the cooling scans and the second heating traces. Thermogravimetric analysis (TGA) analyses were conducted using a thermal analyzer (TA Q 600, USA) via scanning from 30 C to 800 C at the heating rate of 10 C min-1 and in a nitrogen flow of 100 mL min-1. The dispersion of CNTs in the polymer matrix was observed by a field emission scanning electronic microscopy (FEI Nova NanoSEM 450, USA) at an accelerating voltage of 20 kV. Prior to observation, the cryogenically fractured surfaces were coated with Au. 2.4. Healing properties PUSS, PUSS1, PUSS3 and PUSS5 strips with a dimension of 25 mm  5 mm  0.5 mm were cut from the poured films, respectively. A scratch (cut) perpendicular to the tensile axis in the middle of the strip was performed on each sample surface by a razor blade to an approximate 50% thickness of the strip. Then the strips were healed at 80 C for 1 min, 12 h and 24 h in an oven, respectively. To investigate the healing process under light irradiation, NIR of 808 nm was 7

generated by a LSR808NL II laser (Shanghai Forward Optoelectronics Co. Ltd., China) and irradiated on the samples for 1 min. The output power of NIR laser was 310 mW and the distance between the sample and the laser pointer was 20 cm. Infrared thermal images were recorded by a FLIR ONE PRO infrared camera (FLIR, USA). Tensile tests were carried out using a universal testing machine (M2500, China) at a strain rate of 10 mm min-1. The self-healing efficiencies were calculated according to the following equation. Healing efficiency (%) =

Tensile strength  100% Original tensile strength

Where the original tensile strength, tensile strength are the tensile strengths of the original samples and the healed samples, respectively. Digital images of the tensile tests of the scratched PUSS and PUSS3 after being healed at 80 C for 24 h and 1 min NIR irradiation were recorded. Cracks (made by the blade) on the sample surfaces were observed by a polarizing optical microscope (POM, Leica, DM LP, Germany). The crack surfaces of samples were also observed using field emission scanning electron microscope (FEI Nova NanoSEM 450, USA) at an accelerating voltage of 20 kV after Au sputtering. The electrical resistance was measured by 2-point probe resistance measurement system using PM8233A Digit Precision Multimeter (Peakmeter Instruments Co. Ltd, China). To investigate the potential applications in flexible electronics, the silver paste was drop-cast on a piece of precleaned PUSS3 film and then dried at 60 C for 1 h in a convection oven to form a piece of uniform and conductive film. The electronic circuit was cut and healed, and the conductivity of the circuit was checked by a MP3020D DC power supply (Shanghai Maisheng Industrial Co. Ltd., China). 3. Result and discussion 3.1. Synthesis of the self-healing polyurethane

8

PU PUSS

-1 2274 cm

2500

2400

2300

2200

2100

-1

Wave number(cm ) 4000

3500

3000

2500

2000

1500

1000

500

-1

Wave number(cm )

Fig. 1. FTIR spectra of PU and PUSS.

PU PUSS 510cm-1 640cm-1

500

600

700

-1

Raman shift (cm )

Fig. 2. Raman spectra of PU and PUSS. FTIR and Raman spectroscopy confirmed that the disulfide bonds were introduced into the polyurethane by chain-extending successfully as shown in Fig. 1 and Fig. 2. The absence of characteristic peak of free -NCO groups at 2274 cm-1 in Fig. 1 confirmed that all the -NCO groups were totally expended after the introduction of 2,2-dithiodiethanol. The typical absorption bands of -NH stretching vibration (3500-3200 cm-1), -NH bending vibration (1539-1531 cm-1), -C=O 9

stretching vibration (1760-1690 cm-1), the C-O stretching vibration (1243-1241 cm-1), and C-O-C stretching vibration (1162-1159 and 1046-1044 cm-1) bands proved the formation of polyurethane [37]. Raman spectra were obtained to confirm the existence of disulfide bonds in PUSS. As shown in Fig. 2, intensive band at 510 cm-1 ((S-S)) and 640 cm-1 ((C-S)) indicated the existence of disulfide bonds in PUSS [38]. The spectra analysis above confirmed that the polyurethanes with anticipated structures were synthesized successfully. According to GPC results, the weight average molecular weights of PU and PUSS were 6.36  104 and 2.31  105 g mol-1 with the polydispersity of 1.51 and 1.95, respectively. 3.2. Structures and thermal properties PU PUSS

23.1

5

6.0x10

Intensity (A.U.)

19.1

5

4.0x10

21.5 5

2.0x10

0.0 20

40 

2



Fig. 3. XRD curves of PU and PUSS. The effect of the introduction of PTMEG chain segments and disulfide bonds on the crystalline of polyurethane by XRD was presented in Fig. 3. As shown in Fig. 3, peaks at 19.1  and 23.1  are characteristic peaks of 2/7 helix structure of PEG crystals [39]. No intensive peaks of 20  and 24  of the typical triclinic structure (-form) of PTMEG [40] were observed due to the low content of PTMEG in the polyurethane. The peaks at 19.1  and 23.1  were broadened due to the incorporation of small amount of PTMEG chain segments as shown in Fig. 3. The halo around 21.5  of PUSS revealed an amorphous structure. The XRD results confirmed that the introduction of 10

PTMEG chain segments and disulfide bonds inhibited the crystallization of PEG chain segments. No new crystalline peaks appeared in the XRD curves of PU and PUSS, indicating that the crystal type of PEG was possibly not affected by the incorporation of PTMEG or the disulfide bonds, despite the disturbing effect of the regularity of PEG crystal. (a)

17.0 C -1 65.9 J g

0

23.3 C -1 68.9 J g

52.3 J g-1

16.5 C 2

0

16.0 C

59.4 J g-1

-1 56.3 J g

0

PU PUSS PUSS1 PUSS3 PUSS5 10

20

30

Temperature (C)

Fig. 4.

-2

PU PUSS PUSS1 PUSS3 PUSS5 43.8 C -1 64.5 J g

39.5 C 59.9 J g-1

40.0 C -1 57.4 J g

39.8 C -1 64.8 J g -4

40

(b) 36.4 C -1 52.5 J g

-1

-1

Heat flow(W g )

13.7 C

Heat Flow (W g )

4

20

30

40

Temperature (C)

50

60

DSC curves of PU, PUSS, PUSS1, PUSS3 and PUSS5 during (a) cooling and (b) re-heating cycles.

Fig. 4 presents the DSC curves of PU, PUSS, PUSS1, PUSS3 and PUSS5 during cooling and re-heating cycles. During the cooling cycle (Fig. 4(a)), PU exhibited exothermic peak around 23.3 C with the crystallization enthalpy about 68.9 J g-1. As reported in our previous work, PEG used in our work exhibited exothermic peak around 36.7 C with the crystallization enthalpy about 214.8 J g-1 [27]. That is to say, the exothermic peak and crystallization enthalpy decreased significantly after the incorporation of PTMEG and HDI. The decrease of crystallization of PEG segments might be caused by the suppression effect of hard segment of polyurethane [41] and the disturbance of PTMEG segments. Furthermore, due to the introduction of disulfide bonds, the crystallinity of PUSS was further suppressed. The enthalpy decreased to 52.3 J g-1, indicating that PUSS had a very low crystallinity. The results were also confirmed by the XRD analysis above. The nucleation effects of CNTs caused a little higher crystallinity of PUSS1, PUSS3 and PUSS5 with enthalpy of 65.9 J g-1, 56.3 J g-1 and 59.4 J g-1, respectively. The suppression of the crystallinity could be further 11

confirmed by the DSC curves of the re-heating cycle as shown in Fig. 4(b). PU, PUSS, PUSS1, PUSS3 and PUSS5 exhibited endothermic peaks around 43.8 C, 36.4 C, 39.8 C, 39.5 C and 40.0 C with melting enthalpy of 64.5 J g-1, 52.5 J g-1, 64.8 J g-1, 59.5 J g-1 and 57.4 J g-1, respectively. The polymer chains of amorphous PEG and PTMEG were flexible and had great mobility with low glass transition temperatures about -55 C and -75C, respectively [42,43]. Therefore, the polyurethane nanocomposites with low crystallinity in this work were flexible at room temperature. The thermal properties of PUSS and its nanocomposites were investigated via TGA under a nitrogen atmosphere from 30 C to 800 C. As shown in Fig. 5 and table 1, the thermal degradation of PUSS and its composites consisted of two steps. The onset degradation temperature (5% weight-loss temperature) of PUSS was 224.5 C, while the introduction of CNTs increased the onset degradation temperatures to nearly 260.0 C. In the same way, 10% weight-loss temperatures of nanocomposites were also increased from 240.0 C to around 300.0 C. The amount of char residue at 800 C increased slight with the addition of CNTs. The TGA results showed that the composites had better thermal stability.

12

100

PUSS PUSS1 PUSS3 PUSS5

90

70 60 50 40 30

100 Weight (%)

Weight (%)

80

95

20

90 220

10 0

80

240 260 280 300 Temperature (C)

160 240 320 400 480 560 640 720 800

Temperature(C)

Fig. 5.

TGA curves of PUSS, PUSS1, PUSS3 and PUSS5.

Table 1. Thermal properties of PUSS, PUSS1, PUSS3 and PUSS5. Sample

T5 *(C)

T10* (C)

Char yield at 800 C (%)

PUSS

224.5

240.0

0.5

PUSS1

259.9

300.0

4.0

PUSS3

259.8

304.6

4.6

PUSS5

254.4

300.1

6.9

* T5 is 5% weight-loss temperature, T10 is 10% weight-loss temperature.

Fig. 6. SEM images of the cryogenically fracture surfaces of (a) PUSS1, (b) PUSS3 and (c) PUSS5. 13

The dispersion of CNTs in the polyurethane nanocomposites affected mechanical, conductive and self-healing properties of the as-prepared nanocomposites. Therefore, the dispersion of CNTs was investigated by SEM on the cross section. As revealed in Fig. 6, no agglomeration of CNTs was observed in PUSS1, PUSS3 or PUSS5. A homogeneous distribution of CNTs in the polyurethane matrix was obtained. 3.3. Mechanical properties CNTs have been widely used as efficient fillers for the improvement of mechanical properties at low loading [44,45]. The stress-strain curves of PUSS and its composites are presented in Fig. 7. As shown in Fig. 7, the tensile strength at break was improved significantly from 1.01 MPa to 3.64 MPa when the addition content of carbon nanotubes was 5 wt%. The elongation at break decreased as the addition of CNTs, but still remained at the high level above 300%, indicating the composites had enough flexibility for stretchable wearable electronics.

4.0

3.64MPa

3.5 2.72MPa

Stress (MPa)

3.0

PUSS PUSS1 PUSS3 PUSS5

2.5 1.47MPa

2.0

1.01MPa

1.5 1.0 0.5 0.0

0

200

400

600

800

1000

Strain (%)

Fig. 7.

Mechanical properties of PUSS, PUSS1, PUSS3 and PUSS5.

3.4. Photothermal effect 14

Samples

PUSS

PUSS1

PUSS3

PUSS5

30 s

1 min

2 min

5 min

Fig. 8. Thermographic images under NIR irradiation for different time. 140 130

Temperature (C)

120 110 100 90

PUSS PUSS1 PUSS3 PUSS5

80 70 60 50 40 30

0

1

2

3

4

5

Time (min)

Fig. 9. The maximum temperature as a function of irradiation time. Nanoparticles, such as gold nanoparticles [46,47], Fe3O4 particles [48], graphene oxide [49], 15

graphene nanosheets [5], CNTs [4,15] and even carbon black [32] were reported to have photothermal effect and could in situ convert NIR light into heat and therefore led to a relatively high energy efficiency. Amino-functionalized CNTs were incorporated into epoxy resin cross-linked by dynamic covalent Diels-Alder (DA) bonds. The photothermal effect of CNTs led to a remote, precise and fast NIR irradiation triggering of self-healing composites [15]. A remote, fast and efficient self-healabe triboelectric nanogenerator for potential implantable electronics was realized by NIR irradiation by embedding CNTs in an epoxy resin based on polysulfide elastomers [4]. It was reported that the content of photothermal converters, light intensity and irradiation time were adjustable variables, which could influence the local temperature [15]. The surface temperature increased rapidly in several minutes and reached equilibrium. The maximum equilibrium temperature increased with the light intensity and the loading content of CNTs [15]. The heat energy from the photothermal conversion of CNTs played the key role for determining the healing efficiency of NIR light. Therefore, the surface temperature of samples irradiated by NIR laser were recorded as shown in Fig. 8 and the maximum temperatures were summarized in Fig. 9. The colors in the images showed the temperature distribution on the surface of the samples. As shown in Fig.8 and Fig. 9, after 5 min of NIR irradiation, no obvious increase in temperature for PUSS was detected. For PUSS composites containing CNTs, the surface temperature increased rapidly within the first 2 minutes and then flattened in the following 3 minutes and reached equilibrium which is consistent with the result reported by Li et al. [15]. The equilibrium maximum temperature increased with the content of CNTs. For example, maximum surface temperatures of PUSS1, PUSS3, PUSS5 increased to 98.1 C, 119 C and 129 C with the increase of CNTs content from 1 wt% to 5 wt%, respectively. After 1 min NIR irradiation, the surface temperature of PUSS1, PUSS3 and PUSS5 reached 84.5 C, 95.3 C and 115 C, indicating that the temperature of the 16

nanocomposites could reach the healing temperature of disulfide bonds after only1 min of NIR irradiation. 3.5. Self-healing properties (a)

3.5 3.0

Stress (MPa)

4.0

Pristine Scratched Heating-1min Heating-12h Heating-24h IR-1min

2.5

3.0

2.0 1.5

0.32 MPa 0.47 MPa

1.0 0.5 0.0

0.92 MPa 1.01 MPa 0.75 MPa

2.5 1.01 MPa

2.0

1.21 MPa

1.5

0.44 MPa

0.5 0.0

100 200 300 400 500 600 700 800 900 1000

0.36 MPa 0

100 200 300 400 500 600 700 800 900 1000

Strain (%)

Strain (%)

(c)

3.5

Stress (MPa)

3.0 2.31 MPa 2.22 MPa 2.5

2.72 MPa

2.03 MPa

Pristine Scratched Heating-1min Heating-12h Heating-24h IR-1min

2.0 1.5

4.0

(d)

2.41 MPa 3.64 MPa Pristine Scratched Heating-1min Heating-12h Heating-24h IR-1min

2.97 MPa

3.5 3.0

Stress (MPa)

4.0

1.47 MPa

0.81 MPa

1.0

0.43 MPa 0

Pristine Scratched Heating-1min Heating-12h Heating-24h IR- 1 min

(b)

3.5

Stress (MPa)

4.0

2.5 2.0 1.52 MPa 1.5

1.0

0.56 MPa

1.0

0.65 MPa

0.5

0.31 MPa

0.5

0.26 MPa

0.0

0

100 200 300 400 500 600 700 800 900 1000

0.0

0

100 200 300 400 500 600 700 800 900 1000

Strain (%)

Strain (%)

Fig. 10. Typical stress-strain curves of the original, scratched and healed specimens of (a) PUSS, (b) PUSS1, (c) PUSS3 and PUSS5. The reversible disulfide exchange is a radical-mediated reaction. As reported in our previous work, the heating efficiency and speed increased with the increase of healing temperature. The healing efficiency could reach over 90% under a healing temperature of 80 C for 24 h [50]. The direct heating and light induced self-healing properties of PUSS and its composites were investigated in the work. Tensile tests of the original and healed samples were performed and the healing efficiencies were calculated. According to the photothermal effect discussed above (Fig.8 and Fig. 9), the maximum surface temperature of all PUSS composites in the study reached over 80 17

C. Too high temperature would cause the softening and deformation of the overall samples. Therefore, the NIR healing was carried out for 1 min irradiation. Fig. 10 shows the typical stress-strain curves of the original, scratched and healed specimens at 80 C for different time and NIR irradiation for 1 min. The healing efficiencies were summarized in table 2. Table 2. Healing efficiencies of PUSS and its composites at 80 C for different healing time and NIR irradiation for 1 min (%). Healing at 80 C for different time Samples

IR light emission

1 min

12 h

24 h

1 min

PUSS

41.3

73.3

91.1

46.5

PUSS1

29.5

54.4

68.7

82.3

PUSS3

20.2

73.9

81.6

84.9

PUSS5

17.9

41.8

66.2

81.6

As shown in Fig. 10 and table 2, poor healing efficiencies less than 42% were obtained for all samples after 1 min direct heating. Complete healing of PUSS (91.1%) was achieved after 24 h of directing heating at 80 C. The healing efficiencies of PUSS1, PUSS3 and PUSS5 reached 68.7%, 81.6% and 66.2% after 24 h of direct heating at 80 C, respectively. The CNTs dispersed in polyurethane might decrease the kinetics of diffusion and impede the touching of polymer-polymer interfaces, resulting the decreasing of dynamic exchange of disulfide bonds. Therefore, the self-healing efficiency decreased after addition of CNTs, which was consistent with the results reported in the self-healing waterborne polyurethane/graphene oxide nanocomposites [51]. However, after 1 min of NIR irradiation, the healing efficiencies of PUSS composites were all over 80%, while the healing efficiency of PUSS was only 46.5%. The poor healing efficiency of PUSS 18

was owing to the lower surface temperature of PUSS after NIR irradiation as shown in Fig. 8 and Fig. 9. For PUSS, no photothermal effect generated after NIR irradiation. The low temperature could not accelerate the dynamic covalent exchanging reaction of disulfide bonds, resulting in poor healing efficiency. The poor self-healing of PUSS under 1 min of NIR irradiation could only be attributed to the movement of the polymer chains to bring the crack surface closer together and the formation of hydrogen bonds [50]. It was obvious that CNTs in PUSS composites played a decisive role in the self-healing process of PUSS under NIR irradiation. CNTs absorbed NIR light and transformed the NIR light into the heat energy to raise the temperature of PUSS matrix and heal the scratches excellently.

Fig. 11. Digital photos of the tensile tests of scratched PUSS (a, b, c) and PUSS3 (d, e, f) after heating healed at 80 C for 24 h and NIR irradiation for 1 min.

Samples

PUSS

PUSS1

PUSS3

19

PUSS5

Pristine

Heating 1 min

Heating 12 h

Heating 24 h

IR 1min

Fig. 12. Healing processes of PUSS and its composites under various stimuli. (200 ) To detect the fracture initiation of the samples, tensile tests of scratched PUSS and PUSS3 after heating healed at 80 C for 24 h and IR irradiation for 1 min were recorded. As presented in the red frames in Fig. 11, visible crack initiated at the scratch line on the original scratched specimen (in the red frame) when the elongation reached 20% (Fig. 11(a)), while no crack was 20

observed in the healed PUSS even the elongation reached about 500% (Fig. 11(b)). Crack could also be observed for the tensile test of PUSS after 1 min of NIR irradiation (Fig. 11(c)). For PUSS3, no crack before break was observed for either heating healing at 80 C for 24 h or NIR irradiation for 1 min due to the excellent healing efficiencies (Fig. 11(e), Fig. 11(f)). The result confirmed that the PUSS3 had excellent healing efficiency after 1 min of NIR irradiation. As shown in Fig. 12, POM was used to record the scratches of PUSS and its composites before and after being healed. The scratch on PUSS was nearly invisible after 24 h of direct heating, while no obvious change was observed after 1 min of NIR irradiation. For PUSS composites, the scratches came closer and obvious healing effects were achieved after both 24 h of direct heat healing and 1 min of NIR irradiation. But the scratches on all the samples kept unchanged after 1 min of direct heating. For PUSS5 with high content of CNTs, the surface around the scratch seemed being melted due to high surface temperature (115 C) caused by photothermal effect of too much CNTs. Therefore, PUSS3 with 3 wt% CNTs was more suitable for 1 min of NIR irradiation triggering. To further confirm the results of NIR irradiation healing from microscale, SEM was employed. Fig. 13 illustrated the SEM images of the surfaces of PUSS and PUSS3 before and after 1 min of NIR irradiation. As shown in Fig. 13, no obvious change of the crack on PUSS surface was observed, while the scratch on PUSS3 surface almost disappeared and only a trace line of the scratch was found. The results are consistent with those observed by POM and the results from tensile tests. PUSS composites had excellent NIR irradiation healing properties.

21

Fig. 13. Microscale images of scratches on PUSS and PUSS3 surfaces before and after 1 min of NIR irradiation. (a) PUSS and (b) PUSS3 before 1 min of NIR irradiation, (c) PUSS and (d) PUSS3 after 1 min of NIR irradiation. 3.6. Electrical performance Electrostatics is one of the oldest topics of electrical phenomenon and the undesirable effects of static charge are around us everywhere. Even the slightest of electrostatic can destroy or damage the equipment and due care should be taken during their handling especially for electronic devices [52]. Polymer composites with electrostatic charge dissipation (ESD) properties have wide application in wearable electronics areas. Typically, fillers such as metal particles, graphite, carbon fibers, carbon black, graphene and carbon nanotubes have been widely used to increase the electrical conductivity of polymers and improve their static charge dissipation properties [53]. The electrical resistance of PUSS nanocomposite strips (20 mm  5 mm  0.5 mm) before and after 1 min of NIR irradiation was measured by a 2-point probe multimeter and the volume resistivity was calculated as shown in table 3. Due to the incorporation of CNTs, the nanocomposites exhibited volume resistivity values in the range of 1.31105 to 1.65105 Ω cm, which were much lower than the value of polyurethane without CNTs (1.9×1012 Ω cm) reported [5]. The volume resistivity values of the scratched strips increased but returned nearly to the original values after 1 min of NIR irradiation. But the resistance couldn’t recover to the original values as reported [49,54]. That is to say the PUSS composites in this work could be proposed for ESD materials requiring conductivities of in the range of 105 to 109 Ω cm [52]. To investigate the electrical output performance, PUSS3 was adopted to fabricate a flexible electronics by drop-casting silver paste on it. A battery-powered circuit was constructed to demonstrate the potential of NIR laser self-healing for the electronic circuits. As presented in Fig. 14, the LED light (Fig. 14(a)) was immediately extinguished after the 22

strip was cut by a razor (Fig. 14(b)). However, the conductivity of the broken electric circuit was completely recovered and the LED light lit up again after 1 min of NIR irradiation (Fig. 14(c)). Learning from this healing process, PUSS nanocomposites in this work could be used as the substrates of electrical devices, especially for flexible wearable electronics. The electrical devices could be healed by remote NIR irradiation quickly and precisely. Table 3. The volume resistivities of PUSS nanocomposites. ( cm) Pristine

Scratched

Healed

PUSS1

1.65105

2.13105

1.75105

PUSS3

1.31105

1.53105

1.48105

PUSS5

1.38105

2.44105

1.70105

Fig. 14. A circuit schematic with a LED light in series with PUSS3 strip and the healing process. (a) original, (b) after cutting, (c) after 1 min of NIR irradiation. 4. Conclusions A remote, fast and efficient self-healable stretchable polyurethane nanocomposite for the substrate of flexible wearable electronics is reported in this paper. Carbon nanotubes (CNTs) were embedded in a self-healing polyurethane based on disulfide bonds. The tensile 23

strength at break of the composites was improved significantly from 1.01 MPa to 3.64 MPa when the addition content of carbon nanotubes was 5 wt%. The elongation at break decreased as the addition of carbon nanotubes, but still remained at the high level above 300%, indicating the composites had enough flexibility for stretchable wearable electronics. The dynamic disulfide exchange reaction could be stimulated by near infrared (NIR) irradiation and the damages of the composite could be healed in minutes due to the photothermal effect of CNTs. The healing efficiencies were over 80% after 1 min of NIR irradiation. NIR irradiation could hit the sample locally and precisely. Therefore, the damaged samples can be locally repaired with high precision and efficiency without an obvious influence on those undamaged parts. The nanocomposites had volume resistivity values in the range of 1.31105 to 1.65105 Ω cm as a result of the incorporation of CNTs. That is to say the composites in this work could be proposed for electrostatic charge dissipation (ESD) materials requiring conductivities of in the range of 105 to 109 Ω cm. The conductivity of the broken electric circuit which was fabricated by silver paste drop-cast on the composite was completely recovered after 1 min of NIR irradiation. These results demonstrate that the nanocomposites in this work can be used as substrates of electrical devices, especially for flexible wearable electronics. The damages of the electrical devices can be healed by remote NIR irradiation quickly and precisely.

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 A remote and efficient self-healing flexible polyurethane nanocomposite is reported.  The damages could be healed by NIR due to the photothermal effect of CNTs.  The composites had resistivity which could meet the requirements for ESD materials.  The nanocomposites can be used as substrates for flexible wearable electronics.

Remote and efficient infrared induced self-healable stretchable substrate for wearable electronics Han Jia, Shu-Ying Gu *

32

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

on behalf of all the authors of the paper.

Author Statement The work was contributed by Han Jia and Shu-Ying Gu. Shu-Ying Gu: Supervision, Conceptualization, Methodology, Reviewing and Revision. Han Jia: Experimental, Data analysis, Original draft preparation.

33