Self-cleaning, chemically stable, reshapeable, highly conductive

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Applications of Polymer, Composite, and Coating Materials

Self-cleaning, chemically stable, reshapeable, highly conductive nanocomposites for electrical circuits and flexible electronic devices Ximing Zhong, Hengfeng Hu, and Heqing Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07575 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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ACS Applied Materials & Interfaces

Self-cleaning, chemically stable, reshapeable, highly conductive nanocomposites for electrical circuits and flexible electronic devices Ximing Zhong, Hengfeng Hu, Heqing Fu * (School of Chemistry and Chemical Engineering, Guangdong Provincial Key Lab of Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, P.R. China)

KEYWORDS: multifunctional nanocomposites, reshapeable, highly conductive, electrical healing, self-cleaning, chemically stable, flexible electronics

ABSTRACT：： Materials with multiple functions are highly desirable in practical applications. Developing multifunctional nanocomposites by a straightforward process is still a challenge. Here, a versatile nanocomposite has been developed by simple blending and pressing of multi-walled carbon nanotubes (MWCNTs) and modified polydimethylsiloxane (MPDMS). Due to the synergistic effect of MWCNTs and MPDMS, this nanocomposite exhibits outstanding hydrophobic property, striking self-cleaning capability, and excellent chemical stability against strong acid and strong base, which makes it possible to work under wet and even extreme chemical conditions. Besides, because of flexibility, this nanocomposite can be reshaped, bended, twisted and molded into on-demand patterns for special applications. Owing to the well distribution of MWCNTs, the nanocomposite shows high conductivity

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(with sheet resistance of 86.33 Ω sq-1) and high healing efficiency (above 96.53 %) in electrical filed, and it also exhibits outstanding performance in various electrical circuits and flexible electroluminescent devices. Furthermore, inherent portability, recyclability and reusability of this nanocomposite make it more convenient and environmentally-friendly for practical applications. Thus, our work provides a new strategy to develop a multifunctional nanocomposite and it shows tremendous potential in flexible electronics.

INTRODUCTION Flexible conductive nanocomposites, generally composed of electrically conductive materials and flexible polymer matrices, are of importance for their tremendous potential in electronic devices,1,2 sensors,3-7 actuators,8,9 and energy storage devices.10,11 A substantial amount of effort has been made on the research of conductive components.12 To render high electrical conductivity, used frequently are metal nanoparticles/nanowires,13,14 ionic liquids,15 conductive polymers,16,17 and carbon-based fillers,18,19 such as carbon nanotubes and graphene. Although exhibiting striking conductivity, silver nanowires, silver flakes,20 and ionic liquids are costly. While for organic conductive polymers, low conductivity and aging problem limit their further applications.

Instead, carbon nanotubes (CNTs) exhibit not only high conductivity but also


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superlative mechanical and thermal properties, which make them an ideal candidate for high performance and multifunctional nanocomposites,21 and the resultant nanocomposites inherently perform favorable characteristics, such as light weight, cost-effectiveness, facile processability and scalability.22 Generally, in insulating polymer matrices, by increasing the contents of conducting nanofillers, the transition from insulator to conductor occurs in nanocomposites, and the critical nanofiller content for conductive behavior is referred as percolation threshold. While the CNTs content for conductive nanocomposite in most polymer matrices is below 5 wt% depending on their aspect ratio and dispersion state.23,24 Furthermore, the excellent chemical stability provided by CNTs endows nanocomposites with high corrosion resistance against various climatic actions as well as aggressive media.25 Aside from above merits, the hydrophobicity and roughness of CNTs make them potential for superhydrophobic functional nanocomposites. To combine the

superhydrophobicity

and conductivity of multi-walled carbon nanotubes (MWCNTs), Zhang et al fabricated a smart coating with superhydrophobic property against water and acid/alkali stress for wearable sensing electronics by spray-coating MWCNTs dispersed in a thermoplastic elastomer (TPE) solution, followed by ethanol treatment.26 To the best of our knowledge, superhydrophobic surfaces, generally created by etching,27 templating,28 electrospinning,29 layer-by-layer growth,30 and lithographing,31 followed by post-treatment with low-surface-energy materials, are typically characterized by a high water static contact angle (> 150°) and a low sliding



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angle (< 10°),32 possessing huge potential applications in waterproofing, self-cleaning, anti-fogging, oil/water separation and drag reduction.33-36 However, the fragile textures weaken their mechanical strength,37 limiting their practical applications. Instead, by previous investigations, we note smooth surfaces, which overcome the challenges that plague the rough ones, can also be used for waterproofing, self-cleaning and corrosion resistance.38-40 Thus, smooth nanocomposites are more suitable with regard to practical applications.

On the other side, materials with reshapeable property show potential application in electronic devices. Su et al reported a reshapeable and conductive green hydrogen bonded networks (GHBN) fabricated by amylopectin/water/sodium salt for wearable sensor, flexible electronic devices and circuit repair.41 However, the GHBN required external sealant in case the evaporation of water when it was used. Reshapable and highly conductive nanocomposites with striking hydrophobicity, chemical stability and self-cleaning capability have not yet been reported, nanocomposites that not only address the problem of water evaporation but also exhibit more functions for various applications.

As

we

know,

among

many

flexible

polymer

matrices,42

polydimethylsiloxane (PDMS) is widely used for its attractive properties, including chemical

inertness,

hydrophobicity,

biocompatibility,

non-toxicity,

optical

transparency, and high flexibility.43,44 To prepare a suitable polymer matrix for multifunctional nanocomposites in this work, PDMS necessitated little crosslinking to achieve reshapability, while isophorone diisocyanate (IPDI) was used to modify


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PDMS (modified PDMS denoted as MPDMS) to offer proper flexibility.

To combine the properties of reshapeability, high conductivity, hydrophobicity, chemical stability and self-cleaning capability, instead of sophisticated processing for special structures,45,46 MWCNTs were uniformly distributed in MPDMS matrix by a simple method of blending and pressing to form a compact MWCNT/MPDMS nanocomposite. Due to the synergistic effect of MWCNTs and MPDMS, the smooth nanocomposite was reshapeable, twistable, bendable, moldable, and exhibited excellent hydrophobic performance, self-cleaning property and chemical stability against strong acid and strong base. Moreover, owing to the well distribution of MWCNTs, the nanocomposite was endowed with high conductivity (with sheet resistance of 86.33 Ω sq-1), and showed no anisotropic conductivity behavior but an outstanding healing efficiency (above 96.53%) in electrical field. Thus, this nanocomposite, even under wet and harsh chemical conditions, could be applied as materials for electrical circuits and flexible electronic devices, such as electroluminescent devices, with on-demand patterns for special applications.

EXPERIMENTAL SECTION Materials. Isophorone diisocyanate (IPDI) was purchased from Aldrich. Hydroxyl-terminated poly (dimethysiloxane) (PDMS, Mn = 3150 g mol-1) was supplied from Shin-Etsu Chemical Co., Ltd.. Multiple-walled carbon nanotubes



(MWCNTs) with an average length of 0.5-2.0 µm and an average diameter of 30-50 nm (>95 wt% purity) were purchased from Nanjing XFNANO Materials Tech Co., Ltd.. Methoxy-2-propyl acetate (99.0%, Aladdin) was distilled under vacuum before use. Sylgard 184 was obtained from Dow Corning Corporation. Absolute ethanol, dibutyltin dilaurate, methylene blue, and methyl red were of analytical reagent grade and provided by Aladdin. Biaxially oriented polypropylene (BOPP) film with 28 µm thickness was kindly offered by Guanhua Film Industry Co., Ltd.. Blue LEDs (3.0 V, wavelength of 460 nm) and green LEDs (3.0 V, wavelength of 530 nm) were purchased from Shenzhen Looking Long Tech Co., Ltd.. Solar panel (53 × 29 mm2) was provided by local science education apparatus department. Electroluminescent ZnS:Cu powders and corresponding input power device were obtained from Shanghai KPT Company.

Synthesis of modified PDMS (MPDMS). IPDI (1.00 g), PDMS (13.09 g) were mixed thoroughly in a 100 mL flask under magnetic stirring. The flask was then heated to 85 ℃, and dibutyltin dilaurate (0.02 g) acted as catalyst was added into the above mixture. After 10 min, 43.00 g Methoxy-2-propyl acetate was added. The reaction proceeded at 85 ℃ for 4 h. Then, the resultant solution was cooled down, poured into a Teflon mold and dried at room temperature for one day followed by drying at 80 ℃ for 12 h under vacuum. The gel-like sticky MPDMS was obtained for further use.

Preparation of MWCNT/MPDMS nanocomposites. MWCNTs (0.50 g) were


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dispersed with ethanol solution (10.00 g) in a 100 mL beaker, and MPDMS (1.50 g) was subsequently added. Above mixture was heated to 60 ℃ with continuous stirring for 15 min, followed by sonication with a KH2200DE sonicator at 100 W for another 15 min. Then the mixture was kept at 60 ℃ for 24 h to let ethanol evaporate and reach a stable weight. The dried nanocomposite was further placed between two smooth BOPP films, pressed under 1.0 MPa by a compressing machine, folded and pressed again for 10 cycles to make a compacted nanocomposite film with a smooth surface. The nanocomposite film could be reshaped into various on-demand patterns for further tests, and all nanocomposite electrical wires and patterns were fabricated by hand except the molded patterns with the assistance of Teflon molds.

Preparation of the electroluminescent device. The electroluminescent device consisted of three layers: two nanocomposite films served as the top/bottom electrodes and an emissive layer in the middle. The preparation of the middle emissive layer was as follow. First, 1.0 g Sylgard 184 (Dow Corning) was mixed with 0.1 g curing agent, and 11.0 g electroluminescent ZnS:Cu powders was added and mixed thoroughly. Then the mixture was scattered onto a BOPP film, and a 1.0 kg roller with a diameter of 75.0 mm under an external force of 400 N was employed to compact the mixture before it was fully solidified under 50 ℃ for 24 h. Subsequently, it was peeled off and used. Copper wires were used to electrically connect the top and bottom nanocomposite layer. The input power device provided by Shanghai KPT Company was used to supply alternating voltage.



Characterization and Measurement. A digital camera (Canon EOS 700D) was used to take the optical photographs. A ZEISS Merlin scanning electron microscope (SEM) was used to characterize the morphology of nanocomposite surface and cross-section at vertical and horizontal dimension. Especially, nanocomposite film was immersed in liquid nitrogen for 10 min and broken for SEM cross-section analysis. A Bruker atomic force microscopy (AFM) was adopted to further characterize the nanocomposite morphology and measure surface roughness. Chemical composition was investigated by a Thermo Fisher Scientific ESCALAB 250Xi X-ray photoelectron spectrum (XPS). Fourier transform infrared (FT-IR) spectroscopy was conducted using a Bruker VERTEX 70 spectrometer and scanning from 4000 cm-1 to 400 cm-1 with KBr as background. Thermogravimetric analysis (TGA) was performed to confirm the content of MWCNTs by adopting a NETZSCH STA449 F3 simultaneous thermal analyzer using nitrogen as purging gas with temperature ranging from 30 ℃ to 800 ℃ at a heating rate of 10 ℃ min-1. The crystal structures of MWCNTs, MPDMS and nanocomposite were examined by Bruker 8D ADVANCE X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.154 nm) and 2θ ranging from 5° to 80°. A RTS-9 four-probe detector from Guangzhou 4 PROBS TECH was employed to measure the sheet resistance, electrical resistivity and conductivity of the nanocomposite. And the IV curves were recorded by a CHI 660E electrochemical workstation with a maximum voltage of 10.0 V and a maximum current of 0.25 A. Silver pastes were used as electrodes. Except the nanocomposite


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with a dimension of 10 ×10 × 10 mm3 was used for the IV curve measurement to demonstrate isotropic conductivity, other nanocomposites employed for IV curve measurement were with a dimension of 15 × 4 × 4 mm3. And the nanocomposites immersed in solvents for 24 h were rinsed with deionized water for 5 min, and then was placed at 50 ℃ overnight before IV curve measurement. Mechanical properties of the nanocomposite were measured via a CMT-series microcomputer electronic universal testing machine (MST), and tensile tests were carried out at ambient temperature at a constant speed of 1 mm/min. The nanocomposites with a dimension of 55 × 10 × 4 mm3 were used for tensile tests. A two-probe digital multi-meter (VICTOR 86E) linked with a laptop was used to record the resistance changes and current variations. And it also used to measure the voltage generated from solar panel. Accurate pH value was determined with a PHS-3C pH meter, and visual color changes caused by strong acid and strong alkali were demonstrated by pH-indicator papers. A Dataphysics OCA40 Micro instrument was used to measure the contact angles and sliding angles at room temperature. The droplet for contact angle measurement was 5.0 µL in volume, while 40.0 µL for sliding angle measurement. All of reported contact angle and sliding angle values represented the average of five measurements.

RESULTS AND DISCUSSION



Insert Figure 1

Figure 1a illustrates the fabrication procedure of MWCNT/MPDMS nanocomposites, and it briefly includes three sections. First, PDMS reacted with IPDI to achieve transparent MPDMS (the complete consumption of isocyanate groups was confirmed by FT-IR in Figure S1). Second, the as-prepared MPDMS mixed thoroughly with MWCNTs under stirring with the aid of ethanol. At last, after the complete evaporation of ethanol, resultant mixture was pressed to form a compact nanocomposite. Before mixing with MWCNTs, the gel-like MPDMS was sticky and readily attached on which it contacted, which made it impossible for practical applications. Considering the enhancement effect of MWCNTs on polymers, different weight ratios of MWCNTs to MPDMS were investigated. As shown in Figure S2, before MWCNTs content reached 25.0 wt%, the resultant MWCNT/MPDMS nanocomposites remained sticky, black traces left after the removal of nanocomposites (Figure S2a-c). When MWCNTs content exceeded 25.0 wt%, though no mark retained, nanocomposite became brittle and tend to fracture (Figure S2e). Besides, the electrical resistivity of nanocomposite, after measurements, varied slightly from 25.0 wt% to 30.0 wt% (Figure S3a), and the nanocomposite also showed a linear Ohmic response (Figure S3b).Thus, nanocomposite with 25.0 wt% MWCNTs was suitable for further investigations, and the exact feed ratio of MWCNTs was confirmed via thermogravimetric analysis as seen in Figure S4.

After pressed by external force, the nanocomposite became compact, and a


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smooth nanocomposite film was successfully achieved (Figure 1b). Similar to previous work,38,39 according to the SEM image (Figure 1c), this PDMS-based film exhibited a fairly smooth surface. In addition, AFM characterization revealed the root-mean-square roughness of the surface was only 2.58 nm (Figure S5), which further confirmed the smoothness of nanocomposite surface. In XRD spectra, a sharp and strong diffraction peak at 2θ = 25.8° and a shorter band at 2θ = 42.7° were observed, corresponding to the (002) and (100) Bragg reflection of hexagonal graphite structure of MWCNTs.47 As for MWCNT/MPDMS nanocomposite, expected MWCNT diffraction peaks, though decreased, were also found, indicating the presence of MWCNTs in nanocomposite. Besides, according to the XPS survey spectrum (Figure 2a), a high content of Si (21.86%) on nanocomposite surface not only indicated the introduction of MPDMS, but also was believed to provide surface with low surface tension.

Insert Figure 2

Because the MPDMS and MWCNTs we used are water-repellent, the resultant nanocomposites are supposed to be hydrophobic. To verify our speculation, the wetting properties of this nanocomposite were evaluated by determining the static contact angle and the sliding angle on nanocomposite surface of water with surface tension of ~72 mN m-1.39 As we know, the maximum achievable static contact angle of water never exceeds ~120° on a smooth surface.48 Strikingly, owing to the synergic effect of MPDMS and MWCNTs, the static contact angle and sliding angle of water



on nanocomposite surface turned out to be 112.6° ± 0.9° and 49.0 ± 1.2°, respectively, indicative of high hydrophobicity of the nanocomposite. Furthermore, liquids with different pH values ranging from 0 to 14 were used to evaluate the chemical stability of the nanocomposite. As seen in Figure 2b, as for the droplets with pH from 1 to 13, their contact angles were stable. While at extreme chemical conditions, pH = 0 and pH = 14 (that is, strong acid and strong base), the contact angles on nanocomposite surface slightly decreased, demonstrating the superb chemical stability of the nanocomposite. To further demonstrate hydrophobicity and chemical stability, a half-coated glass plate was laid horizontally, and a home-made bibulous tissue strip with a pH-indicator paper inserted was used to absorb the test droplets (water, strong acid and strong base, 0.5 mL in volume, respectively) sitting on the surface of nanocomposite film. As showed in Figure 2c, when in contact with droplets, tissue strips were quickly wetted and the pH-indicator papers inserted simultaneously exhibited characteristic color to indicate the nature of droplets. At the same time, droplets gradually shrank into small ones instead of pinned on the surface and were completely absorbed within 4 s, leaving behind a clean nanocomposite surface, indicating of high hydrophobicity and chemical stability of the nanocomposite.

Due to excellent hydrophobicity and chemical stability, the nanocomposite was also endowed with self-cleaning property against water, strong acid and strong base. As seen in Figure 2d, a half-coated glass plate was tilted at 52.0° to ensure the sliding of droplets, and water with a volume of 40.0 µL cleanly glided down the


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nanocomposite surface without leaving noticeable traces along its path. More strikingly, as for strong acid and strong base droplets, the nanocomposite also exhibited similar self-cleaning property, no trace left along the path of droplet movement on nanocomposite surface. However, it was not applicable for concentrated sulfuric acid (98 wt%) because of its potent corrosive property.

Apart from fabulous repellency against strong acid and strong base, resistance of the nanocomposite against ethanol was also investigated. A nanocomposite film with a dimension of 10 × 10 × 1 mm3 was immersed into an absolute ethanol solution for 12 h, and water contact angle was measured after the removal of nanocomposite film from solution and the complete evaporation of ethanol at room temperature. Instead of being thoroughly dissolved or disintegrated, the nanocomposite film maintained its original shape but a slightly rough surface occurred. Different from highly crosslinked PDMS-based

coatings

with

excellent

ethanol-resistant

performance,38

little

crosslinking in this nanocomposite caused the loss of surficial MPDMS by ethanol and a slightly rough surface created by the uncovered MWCNTs (Figure S6a). Owing to the compact structure and sufficient prevention of insoluble MWCNTs against further permeation of ethanol, immersed nanocomposite film was able to retained pristine shape. To the best of our knowledge, when the intrinsic contact angle is below 65°, an increase in roughness will bring about a decrease in contact angle, whereas the contact angle will increase in response to greater roughness as the intrinsic contact angle is above 65°.49 Consequently, it is conceivable that water contact angle on



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ethanol-treated nanocomposite surface is supposed to be more than 112.6° because of a greater roughness. Consistent with our speculation, water contact angle on ethanol-treated surface turned out to be 123.2° (inset in Figure S6a), which was unachievable for a smooth surface, further demonstrating the existence of roughness. However, on this treated surface, water droplets pinned tightly instead of slid down, which could be potentially used for liquids transportation. As for pristine MWCNTs (scattered onto 3M double-sided tape in case the movement of MWCNTs by water) (Figure S6b), rougher surface resulted in a water contact angle of 152.2°, and water droplets

readily rolled

off

without

wetting

surface,

corresponding

to a

superhydrophobic surface. Thus, different morphologies of surface bring about diverse wetting properties, and both of them show tremendous potential in practical applications.

Insert Figure 3

Aside from hydrophobic property, self-cleaning performance and chemical stability, this nanocomposite also exhibited outstanding electrical conductivity because of sufficient incorporation of MWCNTs. After measurement, the sheet resistance of the nanocomposite reached 86.33 Ω sq-1. Due to high conductivity, this nanocomposite was able to be utilized as conductive materials for diverse electrical circuits. As seen in Figure 3a-c, series circuit, parallel circuit, and series-parallel circuit were achieved by connecting three LEDs using nanocomposite wires, respectively. When direct voltage was applied, LEDs were lighted up simultaneously.


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Besides, it also could be employed to connect a solar-powered device. Basking in natural sunlight, the voltage of a solar panel (53.0 × 29.0 mm2) reached up to 2.252 V (Figure S7a), and a fan worked when the circuit was connected by the nanocomposite (Figure S7b). However, the fan stopped operating when the circuit was disconnected (Figure S7c). Furthermore, this nanocomposite could also serve as a single-pole double-throw switch to dominate switching and interconnecting functions. As shown in Figure 3d, by gently relocating the switch, LED in different pathway could be interconnected and switched to light. Thus, the nanocomposite could be employed as an emergency circuit repair material for diverse electrical circuits.

Insert Figure 4 Different from aligned structures constructed for anisotropic properties,45,50 in view of the preparation method we employed, MWCNTs should be well distributed, and the nanocomposite would show electrically isotropic property. Figure 4 represents the vertical and horizontal cross-section SEM images of the nanocomposite containing 25.0 wt% MWCNTs at different magnifications, demonstrating the well distribution of MWCNTs in nanocomposite. Thus, this nanocomposite is supposed to show electrical conductivity at all directions. To verify our speculation, a rectangle-shape nanocomposite was used for investigation. Owing to the uniform structure inherent in nanocomposite, LED was lighted up when nanocomposite wires contacted the rectangle-shape nanocomposite positioned at different directions (A to B, C to D, E to F, B to A, see Figure S8), offering compelling evidence of isotropic



conductivity in this system. For further verification, electrical measurement was conducted on the nanocomposite with a dimension of 10 ×10 × 10 mm3 from different directions. And the IV curves recorded from X axis, Y axis and Z axis overlapped and showed a linear Ohmic response (Figure S9), indicating a same electrical resistivity at all directions, confirming the isotropic conductivity of the nanocomposite.

Insert Figure 5

Furthermore, healing process in electrical field was investigated by separating nanocomposite completely with a blade and subsequently bringing together the resultant two detached parts. As seen in Figure 5a-c, LED was off when the nanocomposite was separated, while LED worked as two fragments were reconnected. Thus, the nanocomposite showed healing capability in electrical field, and the proposed electrical healing mechanism is represented in Figure 5d. As demonstrated in Figure 4, MWCNTs were densely distributed in polymer matrix. Consequently, MWCNTs were also densely distributed on the fresh cut surfaces. Thus, the separation of nanocomposite naturally blocked the transference of electrons due to the breakdown of conductive network, while the reconnection of two detached parts resulted in the re-contact of MWCNTs, leading to the recovery of MWCNTs network on the contact interface, bringing about the re-formation of continuous electrical conductive paths and finally causing the recovery of conductive network. Owing to the highly dense distribution of MWCNTs in the nanocomposite, the electrical healing efficiency, defined as the ratio of nanocomposite conductivities after/before healing,


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was above 96.53% even after 50 separating-reconnecting cycles (Figure S10). While some voids inevitably existing in the contact interface and the imperfect reconnection of MWCNTs resulted in the decrease of conductivity compared with the intact one.

Insert Figure 6

Besides, we found the nanocomposite could be reshaped into various patterns. As aforementioned, because the as-prepared MPDMS was sticky, MPDMS could be reintegrated under external forces due to the mobility of MPDMS chains. The addition of MWCNTs enhanced mechanical strength of MPDMS and then brought about the non-sticky nanocomposite suitable for practical application. However, although the mechanical enhancement of MWCNTs to MPDMS was effective, there was

little

crosslinking

between

MWCNTs

and

MPDMS.

Therefore,

the

nanocomposite could be reshaped because the mobility and reintegration of MPDMS chains could be achieved under external forces, leading to the reshapeability of the nanocomposite. As showed in Figure 6a, a Halloween pumpkin was shaped by hand kneading using a nanocomposite piece, and then it was reshaped into a methane model (Figure 6b).

The mechanical properties of pristine nanocomposite, reshaped nanocomposite and reconnected nanocomposite (Figure 5c) were measured, and the stress-strain curves were shown in Figure S11. As for pristine nanocomposite, the maximum stress was 0.21 MPa, and the maximum strain was 42.71 %. Obviously, when the maximum



stress was achieved (30.90 % strain), crack occurred on the nanocomposite, and the crack was extended under constant stretching, leading to the rapid decrease of stress. And the reshaped nanocomposite showed similar stress-strain curve, indicating the reshaped nanocomposite maintained the mechanical property of the pristine one. As for reconnected nanocomposite, because the two cut surfaces were only directly contacted, only a part of nanocomposite was reintegrated, leading to less stress (0.08 MPa) and strain (2.52%) compared with the pristine one. Besides, the resistance change along with the stretching of intact nanocomposite was recorded in Figure S12. Combined with Figure S11, the result indicated that, before maximum stress, the resistance of nanocomposite increased slightly and steadily with the increase of strain. However, when the stress exceeded the maximum one, due to the continuous extension of the crack on nanocomposite caused by constant stretching, the resistance increased sharply because of the loss of conductive network.

Aside from reshapeability, the nanocomposite was also endowed with bendable, moldable and twistable properties. Because of flexibility, nanocomposite wires could be bent into different patterns locating on 3D spherical surface (Figure 6c, Figure S13) or hanging on the columns of a sand glass (Figure 6d). As seen in Figure 6e, alphabetic patterns (SCUT) were molded and peeled off from different Teflon molds, and subsequently connected by three LEDs to form a conductive path. In addition, molded strips with 5.0 mm thickness were bent to construct a two-flying-seagulls pattern (Figure 6f). To demonstrate twistability, a 2D nanocomposite ladder laid


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horizontally was readily twisted into a 3D DNA model by hands (Figure 6g,h). Because of high conductivity, LEDs used to connect nanocomposite patterns were lighted when voltage was applied. It is worth noting that this nanocomposite is easy to be removed, recycled, and reused. Above characteristics make it become more economical and potential for practical applications.

Insert Figure 7

Furthermore, a comprehensive test about hydrophobicity, chemical stability and electrical conductivity was implemented on the reshapeable nanocomposite. As demonstrated in the schematic illustration (Figure 7a), nanocomposite was shaped into two box-shape boats with blue solution (solution of water and methylene blue) and red solution (solution of ethanol and methyl red) within, floating on strong acid and strong base solution, respectively. And two boats were linked by nanocomposite wires and a LED to form a series circuit. With an average thickness of 0.8 mm, if the nanocomposite shows weak resistance against water, ethanol, strong acid or strong base, the boats would be corroded and permeated, and the clear strong acid and strong alkali solution would be colored. However, after 24 h of immersion, solution retained clear and colorless, and the LED kept lighting as well, confirming the durability of nanocomposite against water and extreme chemical conditions. Furthermore, the IV curves of the nanocomposites after immersion in water, ethanol, strong acid, and strong base for 24 h were shown in Figure S14a, and all the nanocomposites showed a linear Ohmic response. Except the resistivity of ethanol-treated nanocomposite



decreased slightly mainly due to the loss of insulting MPDMS on nanocomposite surface (Figure S6a), other treated nanocomposites showed little variation in resistivity (Figure S14b), indicating that the nanocomposite could be applied under various chemical conditions.

Insert Figure 8

At last, we found this nanocomposite could be served as electrodes to fabricate electroluminescent devices with patterned display. As seen in Figure 8a, the electroluminescent device was comprised of top and bottom nanocomposite layers with sandwiched a ZnS:Cu/Sylgard 184 (Dow Corning) emissive layer. When a high-frequency alternating voltage was applied, the emissive layer afforded most of the voltage, and blue luminescence was emitted simultaneously. Because of the opacity of nanocomposite, the profile of nanocomposite pattern was visualized by the emitting blue luminescence (Figure 8c). Thus, by designing and shaping the nanocomposite electrodes into on-demand patterns, pattern display can be readily achieved, and such multifunctional nanocomposite exhibits huge potential in flexible electronic devices.

CONCLUSIONS In summary, we have developed a flexible multifunctional MWCNT/MPDMS nanocomposite that can be reshaped, bended, molded and twisted into various patterns.


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This nanocomposite exhibits striking hydrophobicity, excellent self-cleaning performance, and outstanding chemical stability against strong acid and strong base. Thus, it can be applied on wet and extreme chemical conditions. In addition, due to the well distribution of MWCNTs, this nanocomposite is highly conductive and shows no evidence of anisotropic conductive behavior, and it also demonstrates a high electrical healing efficiency above 96.53%. As a result, it can be served as conductive materials for diverse electrical circuits and applied as switch to act switching and reconnecting functions. Moreover, it can be used to construct electroluminescent devices and shows tremendous potential in flexible electronics. With the advantages of portability, recyclability and reusability, this nanocomposite makes it more potential in practical applications.

ASSOCIATED CONTENT Supporting Information FT-IR spectrum of MPDMS; photographs of nanocomposites with different weight ratios of MWCNTs; electrical measurements of the nanocomposites with different contents of MWCNTs; thermogravimetric analysis; AFM image of nanocomposite surface; SEM image of ethanol-treated nanocomposite surface and pure MWCNTs; photographs of a fan driven by a solar panel with nanocomposite connected; photographs showing isotropic conductivity behavior of nanocomposite at all



directions; IV curves of the nanocomposite recorded from different directions; healing efficiency of nanocomposite in electrical field at different separating/reconnecting cycles; stress-strain curves of the pristine nanocomposite, reshaped nanocomposite, and reconnected nanocomposite; resistance change of the nanocomposite as a function of strain; photographs of on-demand patterns bent by nanocomposite wires on 3D spherical surfaces; IV curves and corresponding resistivity of the nanocomposites before and after immersed in various solvents for 24 h.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Phone: +86 020 87114919

ORCID：0000-0002-1298-8306 Heqing Fu Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We wish to appreciate the Science Foundation of State Key Laboratory of


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Structural Chemistry for financial support under grant of No. 20160027.

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Figure Captions Figure 1. Schematic illustration and essential characterizations of MWCNT/MPDMS nanocomposite. (a) Fabrication procedure of the nanocomposite. (b) Optical image of a nanocomposite film. (c) SEM image showing the morphology of nanocomposite surface.

(d)

Comparison

of

XRD

spectra

of

MWCNTs,

MPDMS

and

MWCNT/MPDMS nanocomposite. Figure 2. Hydrophobicity, chemical stability and self-cleaning capability performed on nanocomposite. (a) XPS analysis of the nanocomposite. (b) Variation of contact angle as a function of droplets (5.0 µL in volume) with different pH. Acid solutions correspond to H2SO4 aqueous solution with various concentrations, while base solutions represent NaOH aqueous solution with various concentrations. Inset images are the corresponding droplets on nanocomposite surface. (c) Time-lapse photographs of water (pH = 7), strong acid (H2SO4 aqueous solution, pH = 1) and strong base (NaOH aqueous solution, pH = 14) droplets (0.5 mL in volume, respectively) absorbed by homemade tissue strip with a pH-indicator paper inserted. Droplets gradually shrank and finally were absorbed, leaving no trace on nanocomposite surface. (d) Snapshots showing the movement of water, strong acid and strong base droplets (40.0 µL in volume, respectively) on nanocomposite surface at various times. When pH-indicator paper is wetted by water, strong acid and strong base, its color will become yellow, red and dark violet, respectively, as seen in (c) and (d). (scale bar,



10 mm) Figure 3. Nanocomposite wires for different electric circuits and served as switching and interconnecting functions. (a) Series circuit. (b) Parallel circuit. (c) Series-parallel circuit. (d) A single-pole double-throw switch showing switching and interconnecting functions. By relocating the nanocomposite switch, electrical circuit can be reconnected and the LED in connected path will be switched to light. Direct voltage applied for (a), (b), (c) was 6.0 V, while 3.0 V was applied for (d). (scale bar, 10 mm) Figure 4. Cross-section SEM images of MWCNT/MPDMS nanocomposite at vertical and horizontal direction. (a1-a3) Vertical cross-section images at different magnifications. (b1-b3) Horizontal cross-section images at different magnifications. Cross-section SEM images confirmed the well distribution of MWCNTs in the nanocomposite. Figure 5. Healing process in electrical field. (a) Intact nanocomposite wire with LED lighting. (b) Damaged nanocomposite wire left LED unlit. (c) LED working once two fractured parts were pushed together. (d) Schematic illustration for electrical healing process. (scale bar, 10 mm) Figure 6. Reshapeable, bendable, moldable and twistable properties of the nanocomposite. (a) A Halloween pumpkin lantern shaped by the nanocomposite. (b) A reshaped methane model. (c) A nanocomposite wire bent into a heart-shape pattern placing upon 3D spherical surface. (d) A heart-shape pattern hanging on the columns of a sand glass. (e) LED-connected alphabetic string (SCUT patterns) molded from different Teflon molds. (f) LED-connected seagull-like patterns constructed by several bent strips that molded from a same Teflon mold. (g) A ladder constructed by several nanocomposite wires. (h) A DNA model twisted by the nanocomposite ladder from (g). Because of high conductivity of the nanocomposite, LEDs connected in series were lighted up once direct voltage was applied. And direct voltage applied for (a-h) was 3.0 V except 6.0 V for (f). (scale bar, 10 mm)


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Figure 7. A comprehensive test on hydrophobicity, chemical stability, electrical conductivity of the reshapeable nanocomposite as a function of time. (a) Schematic illustration of the test. (b) At the beginning of the test (0 h). (c) After 12 h of testing. Because of darkness, photograph was taken with the help of a spotlight. (d) After 24 h of testing. Due to excellent properties, even after 24 h of immersion in water, ethanol, strong acid and strong base, the box-shape nanocomposite boats with an average thickness of 0.8 mm showed no evidence of leaking, and the LED kept working. Red solution and blue solution were replenished to original volume every 8 hour because the evaporation of water and ethanol. Direct voltage applied was 4.5 V. (scale bar, 10 mm) Figure 8. Nanocomposite based electroluminescent device. (a) Schematic illustration of a nanocomposite based electroluminescent device. The device was composed of a bottom and a top nanocomposite layer with sandwiched a ZnS:Cu emissive layer. When a high-frequency alternating voltage was applied, the electroluminescent region sandwiched between the top and the bottom layer. However, because of the opacity of nanocomposite, only the profile of top layer was exhibited. (b) Photograph of an electroluminescent device with a star-shape top layer without applied voltage. (c) Clear profile of top nanocomposite layer exhibited when alternating voltage was applied. (scale bar, 5 mm)



Figure 1. Schematic illustration and essential characterizations of MWCNT/MPDMS nanocomposite. (a) Fabrication procedure of the nanocomposite. (b) Optical image of a nanocomposite film. (c) SEM image showing the morphology of nanocomposite surface. (d) Comparison of XRD spectra of MWCNTs, MPDMS and MWCNT/MPDMS nanocomposite. 92x70mm (600 x 600 DPI)


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Figure 2. Hydrophobicity, chemical stability and self-cleaning capability performed on nanocomposite. (a) XPS analysis of the nanocomposite. (b) Variation of contact angle as a function of droplets (5.0 µL in volume) with different pH. Acid solutions correspond to H2SO4 aqueous solution with various concentrations, while base solutions represent NaOH aqueous solution with various concentrations. Inset images are the corresponding droplets on nanocomposite surface. (c) Time-lapse photographs of water (pH = 7), strong acid (H2SO4 aqueous solution, pH = 1) and strong base (NaOH aqueous solution, pH = 14) droplets (0.5 mL in volume, respectively) absorbed by homemade tissue strip with a pH-indicator paper inserted. Droplets gradually shrank and finally were absorbed, leaving no trace on nanocomposite surface. (d) Snapshots showing the movement of water, strong acid and strong base droplets (40.0 µL in volume, respectively) on nanocomposite surface at various times. When pH-indicator paper is wetted by water, strong acid and strong base, its color will become yellow, red and dark violet, respectively, as seen in (c) and (d). (scale bar, 10 mm) 104x84mm (300 x 300 DPI)



igure 3. Nanocomposite wires for different electric circuits and served as switching and interconnecting functions. (a) Series circuit. (b) Parallel circuit. (c) Series-parallel circuit. (d) A single-pole double-throw switch showing switching and interconnecting functions. By relocating the nanocomposite switch, electrical circuit can be reconnected and the LED in connected path will be switched to light. Direct voltage applied for (a), (b), (c) was 6.0 V, while 3.0 V was applied for (d). (scale bar, 10 mm) 62x32mm (600 x 600 DPI)


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Figure 4. Cross-section SEM images of MWCNT/MPDMS nanocomposite at vertical and horizontal direction. (a1-a3) Vertical cross-section images at different magnifications. (b1-b3) Horizontal cross-section images at different magnifications. Cross-section SEM images confirmed the well distribution of MWCNTs in the nanocomposite. 52x24mm (300 x 300 DPI)



Figure 5. Healing process in electrical field. (a) Intact nanocomposite wire with LED lighting. (b) Damaged nanocomposite wire left LED unlit. (c) LED working once two fractured parts were pushed together. (d) Schematic illustration for electrical healing process. (scale bar, 10 mm) 65x38mm (600 x 600 DPI)


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Figure 6. Reshapeable, bendable, moldable and twistable properties of the nanocomposite. (a) A Halloween pumpkin lantern shaped by the nanocomposite. (b) A reshaped methane model. (c) A nanocomposite wire bent into a heart-shape pattern placing upon 3D spherical surface. (d) A heart-shape pattern hanging on the columns of a sand glass. (e) LED-connected alphabetic string (SCUT patterns) molded from different Teflon molds. (f) LED-connected seagull-like patterns constructed by several bent strips that molded from a same Teflon mold. (g) A ladder constructed by several nanocomposite wires. (h) A DNA model twisted by the nanocomposite ladder from (g). Because of high conductivity of the nanocomposite, LEDs connected in series were lighted up once direct voltage was applied. And direct voltage applied for (a-h) was 3.0 V except 6.0 V for (f). (scale bar, 10 mm) 81x52mm (300 x 300 DPI)



Figure 7. A comprehensive test on hydrophobicity, chemical stability, electrical conductivity of the reshapeable nanocomposite as a function of time. (a) Schematic illustration of the test. (b) At the beginning of the test (0 h). (c) After 12 h of testing. Because of darkness, photograph was taken with the help of a spotlight. (d) After 24 h of testing. Due to excellent properties, even after 24 h of immersion in water, ethanol, strong acid and strong base, the box-shape nanocomposite boats with an average thickness of 0.8 mm showed no evidence of leaking, and the LED kept working. Red solution and blue solution were replenished to original volume every 8 hour because the evaporation of water and ethanol. Direct voltage applied was 4.5 V. (scale bar, 10 mm) 72x47mm (600 x 600 DPI)


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Figure 8. Nanocomposite based electroluminescent device. (a) Schematic illustration of a nanocomposite based electroluminescent device. The device was composed of a bottom and a top nanocomposite layer with sandwiched a ZnS:Cu emissive layer. When a high-frequency alternating voltage was applied, the electroluminescent region sandwiched between the top and the bottom layer. However, because of the opacity of nanocomposite, only the profile of top layer was exhibited. (b) Photograph of an electroluminescent device with a star-shape top layer without applied voltage. (c) Clear profile of top nanocomposite layer exhibited when alternating voltage was applied. (scale bar, 5 mm)

78x58mm (600 x 600 DPI)


Self-cleaning, chemically stable, reshapeable, highly conductive

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