Fluorine-free Superhydrophobic and Conductive Rubber Composite

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Applications of Polymer, Composite, and Coating Materials

Fluorine-free Superhydrophobic and Conductive Rubber Composite with Outstanding Deicing Performance for Highly Sensitive and Stretchable Strain Sensors Ling Wang, Junchen Luo, Yang Chen, Liwei Lin, Xuewu Huang, Huaiguo Xue, and Jie-feng Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03545 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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ACS Applied Materials & Interfaces

Fluorine-free Superhydrophobic and Conductive Rubber Composite with Outstanding Deicing Performance for Highly Sensitive and Stretchable Strain Sensors Ling Wang a, Junchen Luo a, Yang Chen a, Liwei Lin a, Xuewu Huang a, Huaiguo Xue a, Jiefeng Gao*a,b a

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou,

Jiangsu, 225002, China b

State Key Laboratory of Polymer Materials Engineering, Sichuan University,

Chengdu, Sichuan 610065, China *Corresponding authors: [email protected];

Keywords: Superhydrophobic; Conductive; Strain sensor; Joule heating; Deicing Abstract Conductive polymer composite (CPC) based strain sensor due to its light weight, tunable electrical conductivity and easy processing has promising application in wearable electronics. However, it is still challenging to develop the CPC strain sensors with excellent stretchability, sensitivity, durability, anti-corrosion and deicing performance. Herein, a facile method is proposed to prepare fluorine-free superhydrophobic and highly conductive rubber composite. Ag nanoparticles (AgNPs) are first decorated on the RB (rubber band) surface, forming a conductive shell. Then, the RB/AgNPs experiences PDMS (polydimethylsiloxane) modification, which could not only endow the composite with superhydrphobicity and hence excellent corrosion resistance, but also improve the interfacial adhesion between the AgNPs. The RB composite possesses a good self-cleaning performance and keeps superhydrophobic even after experiencing cyclic abrasion or stretching-releasing test. Also, the high conductivity and superhydrophobicity endow the RB composite with excellent Joule heating performance and water repellence, broadening its application in deicing and

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water removal. Moreover, the obtained RB composite exhibits both large stretchability with a break elongation larger than 900% and high sensitivity with a response intensity as high as 3.6×108 at a strain of 60%. In addition, the RB composite strain sensor can be used to detect full-range human motions including large and subtle body movement. The flexible, durable and anti-corrosive RB composite has potential applications in flexible electronic, health monitoring, physical therapy, and so on.

1 Introduction Wearable strain sensors have promising applications in wearable electronics, and can be used for body motion detection and health monitoring.1-8 Generally, large deformation is indispensable for the strain sensors. To this end, conductive polymer composites (CPCs) are promising as strain sensing devices, thanks to their light weight, flexibility and controllable conductivity, etc..9-16 The polymers usually include rubber and elastomers that are served as the matrix 12,13, 17-19 or encapsulation materials 14,15 for the strain sensors. Boland et al. prepared an electrically conductive composite by infusing the graphene onto the surface region of the natural rubber band (RB) which is pre-swollen by the toluene. When the composite was used as a strain sensor, it could be stretched with a strain larger than 800%, and exhibited a gauge factor of 35.12 Similar rubber composite strain sensors were prepared by anchoring CNTs into the RB through combination of swelling-ultrasonication treatment and polydopamine (PDA) modification.13 Although these rubber composite had super-elasticity, the conductivity were relatively low (the maximum electrical conductivity was only 0.1 S·m-1 and 0.19 S·m-1 for the above mentioned graphene/RB and PDA/CNTs/RB, respectively). In addition, the sensitivity is not high enough to achieve the full range and precise detection for human body motions. Another point that should be taken into consideration especially in the practical application is the corrosion resistance of the strain sensors, because it can determine whether the sensors can work under harsh environment such as moist or corrosive conditions. When the CPC is exposed to the acid or alkaline conditions, the polymer may be partially degraded and conductive pathways are also easily damaged, giving rise to the unreliability of the sensing


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behavior. Furthermore, it is still challenging that the CPC strain sensors keep their flexibility and stretchability at low temperatures, because the elastic polymer would lose its deformability and become “rigid” when the temperature is approaching its glass transition temperature. As a result, the CPC is frozen, and ice may be formed on the material surface. It is therefore desirable the CPC possesses good “heating effect” and deicing performance, and thus can be used in cold regions.20-22 In our previous work, we proposed a bioinspired method for fabrication of conductive rubber composites for the wearable strain sensor. The PDA was first decorated onto the RB surface and the Ag precursor was then adsorbed on the PDA modified surface. Then the Ag nanoparticles (AgNPs) were produced once the Ag precursor was reduced completely. After fluorination, the superhydrophobic and conductive composite was obtained.23 The composite strain sensor displayed large stretchability, high sensitivity and good anti-corrosive properties. However, the preparation process is complicated and environmentally-unfriendly agent containing fluorine was used. Moreover, the interfacial adhesion between the AgNPs and between the AgNPs and the RB surface was relatively weak, although the PDA layer could, to a certain degree, promote the interfacial interaction. As a result, the AgNPs easily fall off the RB especially at a high AgNPs content, leading to the decline of surface conductivity and thus unstable sensing performance when the materials underwent surface abrasion or repeated stretching. Herein, we develop a simple and environmentally-friendly protocol to fabricate fluorine-free and conductive rubber composite with superior superhydrophobicity. RB is immersed into the Ag precursor solution, and the AgNPs shell wrapping the RB is then created once the precursor is reduced. Then, the conductive AgNPs/RB is modified with one thin layer of PDMS. Note that the PDMS is served as a binder that could not only stick the AgNPs together but also adhere the AgNPs with the RB tightly. As a result, the strong interfacial adhesion endows the RB composite with excellent durability, even when the composite suffers from abrasion and multiple stretching. Benefiting from the PDMS binding layer, a high AgNPs concentration and thus a very high conductivity (as high as 241 S·m-1) for the RB composite is achieved. Furthermore,



the PDMS could significantly decrease the surface energy of the AgNPs, making the composite superhydrophobic. Compared with fluorine containing molecules on the AgNPs, 23 the robust PDMS is more effective and stable as a protective layer that can prevent the AgNPs from oxidation. Also, water and even corrosive solution (salt, acid and base solution) are prohibited to diffuse inside the composite. In addition, the highly conductive composite possesses good Joule heating and hence deicing performance. The RB composite based strain sensor displays an extremely large stretchability with a break elongation of around 930% and response intensity as high as 3.6×108 under a strain of 60%. It can be used to realize real time and full range monitoring of various human motions from large joint bending to subtle vocal cord movement even under harsh conditions, exhibiting potential applications in wearable and flexible electronics.

2 Experimental 2.1 Materials The starting material, namely elastic rubber band (RB), was purchased from a store (cross sectional area: 3.1×1.5 mm2, perimeter: 161 mm). Silver trifluoroacetate (STA, AgCF3COO, 98%), anhydrous alcohol and hexane were provided by Sinopharm Chemical Reagent Co., Ltd. Hydrazine monohydrate (N2H4·H2O, 80%) was obtained from the Energy Chemical. Polydimethylsiloxane (PDMS) was obtained from Dow Corning Corporation Midland – Michigan USA. Silver paste was provided by Shenzhen Sinwe New Material Co., Ltd. 2.2 Preparation of the RB/AgNPs The RB experienced abrasion treatment on a sandpaper and was then washed with the ethanol for a few times. 15 wt% Ag precursor solution was prepared by dissolving a certain number of STA powders in ethanol. Then the pre-treated RB was dipped in the STA for some time and dried at room temperature. Subsequently, the rubber bands containing STA were dipped into 50 wt% hydrazine hydrate solution (ethanol: water=1:1 v/v) for 40 min to completely reduce the STA to AgNPs. Finally, the RB decorated with Ag nanoparticles was obtained after washed with deionized water for several times. For convenience, the obtained RB composite is represented by


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RB/AgNPs-x, where x refers to the immersion time (min) of RB in the STA solution. 2.3 Fabrication of superhydrophobic RB/AgNPs/PDMS The superhydrophobic RB/AgNPs/PDMS was prepared as follows: the PDMS base and curing agent with the mass ratio of 10:1 was mixed together in hexane, and then stirred under magnetic stirring for half an hour. Secondly, the as prepared composite (RB/AgNPs) was dipped in the PDMS solution (1 wt%) for a certain time. Then, the composites adsorbed with PDMS were transferred into an oven and heated at 80 °C for 2 hours to complete the curing of PDMS. The finally obtained composite is denoted as RB/AgNPs-x/PDMS-y, where y represents the immersion time (min) in PDMS solution. 2.4 Characterization X-ray diffraction (XRD, D8 Advance, Bruker AXS, Germany) was used to characterize the crystalline structure of the AgNPs with a 2θ from 10 to 80° and scanning rate of 10° min-1. Fourier transform infrared spectra (FT-IR) measurement was carried out by using a Cary 670-FTIR + 610-FTIR Microscope instrument ranging from 400 to 4000 cm−1 under an attenuated total reflection (ATR) mode. The thermal stability of the RB composite was investigated by a thermogravimetric analysis system (TGA/ Pyris 1 TGA, PerkinElmer Co. Ltd, USA). The sample was heated from room temperature to 800 °C in a nitrogen flow with a heating rate of 10 °C min-1.

Field emission scanning

electron microscopy (FE-SEM, Zeiss Supra55, Germany) was used to observe the morphology of the RB composite. Also, the cross-sectional morphology of the composite after brittle fracture at low temperature is observed. High resolution transmission electron microscopy (HRTEM) was conducted to reveal the detailed morphological information about the PDMS modified AgNPs. Unless otherwise specified, RB/AgNPs-40/PDMS-60 is chosen to do characterization and strain sensing test. 2.6 Contact angle measurement Video optical contact angle measuring instrument (OCA40, Germany) was used to measure the static contact angles (CAs) of water on the RB composite. Deionized water with a volume of 5 μL was dripped on the RB composite. For accuracy, three measurements were made on specimen surface in different positions for the



determination of the average CA values. 2.7 Mechanical properties measurement The mechanical properties of the RB composites were tested by using a universal testing machine (Instron Co., Ltd., U.S.A.). The specimen with a length of 5 cm was tested at a crosshead speed of 100 mm/min under ambient temperature. At least three specimens were tested and the mechanical properties were obtained by averaging the measured values. 2.8 Strain sensing test The two ends of the composite strips were wrapped by two copper wires that are served as the electrodes, and then the silver paste was used to eliminate the contact resistance and ensure their good connection. The strain sensing performance of the composite was tested by combination of a universal testing machine and a resistivity meter (Changzhou Tonghui Electronics Co., Ltd. TH2684A). The universal testing machine was used to conduct the cyclic loading and unloading of samples, while the real time resistance of the samples was in-situ recorded by the resistivity meter. The sensing intensity was denoted by R/R0, where R0 is the initial resistance of RB composite and R represents the real-time resistance of the sensor during the test.

3 Results and discussion 3.1 Fabrication of RB based strain sensor The preparation of conductive and superhydrophobic RB/AgNPs/PDMS is schematically illustrated in Figure 1a. The RB is firstly immersed in a STA solution to adsorb the Ag precursor. Then, the STA is reduced to AgNPs, forming a conductive shell on the RB surface. After PDMS modification, the conductive and superhydrophobic RB composite is finally obtained. The surface morphology of the RB is unveiled by the SEM image shown in Figure S1a. It’s obvious that latex fragments scatter on the RB surface, which is not beneficial for the construction of the conductive network, because the insulating large fragment protrusion may, to a great degree, block the interconnection of the reduced AgNPs. Thus, the RB is polished using a sand paper to remove the latex fragments, and grooves appear on the RB surface (see Figure S1b).


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These grooves increase the interfacial area for the STA adsorption and thus the AgNPs concentration. After the RB has swelled in Ag precursor solution, its color changes from light yellow to dark brown (Figure S1c), suggesting the successful adsorption of Ag precursor. The adsorbed Ag+ is then reduced by hydrazine hydrate to AgNPs that could be observed from the SEM image in Figure S1d. After modification by PDMS, the AgNPs distribution on the composite surface varies little (Figure 1g), compared with that in Figure S1d. The conductive AgNPs shell with a thickness of a few micrometers can be clearly observed from the cross sectional SEM image as shown in Figure 1h. Some AgNPs were also present inside the RB matrix. The thin PDMS layer is revealed from the HRTEM image in Figure 1b. Clearly, the AgNPs stick together with the assistance of PDMS, and the thickness of the PDMS wrapping the AgNPs is a few nanometers (see the yellow dotted curves in Figure 1b). The uniform distribution of Ag, C, O and Si on AgNPs is verified from the elemental mapping of the composite (Figure 1c-1f). Therefore, the cured PDMS could be served as a “glue”, and significantly improves the interfacial adhesion between AgNPs and RB and also between the AgNPs. By combination of the rough surface protrusion of AgNPs stacking and the low surface energy of PDMS, the RB composite becomes superhydrophobic. The superhydrophobicity can be even maintained under a large strain of 200%, and the water droplets on the composite surface keep their spherical shape under different strain (Figure 1i). Although the insulating PDMS layer could, to a certain extent, decrease the composite conductivity, the surface resistance of the RB/AgNPs-40/PDMS-60 is still as low as 44.6 Ω (Figure 1j).



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Figure 1 (a) Schematic illustration for the preparation of the conductive and superhydrophobic composite. (b) HRTEM image of AgNPs/PDMS. (c-f) Element mapping images for Ag, C, O, and Si, respectively. SEM image of (g) the surface and (h) cross-sectional morphology of RB/AgNPs-40/PDMS-60. (i) Photos demonstrating the good water repellent property of RB composite. (j) Resistance of the RB/AgNPs40/PDMS-60 with a length of 5 cm. The crystalline structure of the synthesized AgNPs on the RB surface is investigated by using the XRD technique, as shown in Figure S2a. Four distinct diffraction peaks at 2θ values of 38°, 44°, 64° and 77° are present for both RB/AgNPs and RB/AgNPs/PDMS and they are assigned to the (111), (200), (220), and (311) planes of the face centred cubic silver, respectively. Clearly, the PDMS introduction doesn’t change the microstructure of the AgNPs.24-26 The functional groups of the RB and its composite are examined by the FT-IR spectra as shown in Figure S2b. The principal peaks at 2960 cm-1, 2916 cm-1, 2848 cm-1, 1660 cm-1, 1446 cm-1 and 1375 cm-1 are assigned to the isoprene functional groups of natural rubber13, component of the RB.

27

that is the main

After the RB/AgNPs is modified by PDMS, the peaks at 1082

cm-1 and 968 cm-1 are attributed to the asymmetric and symmetric stretching of two neighbor siloxane bond, and the peaks at 1257 cm-1 and 800 cm-1 are assigned to the in-


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plane bending or scissoring and out-plane oscillations of Si-CH3 bonding, respectively.28 The thermogravimetric analysis (TGA) is used to investigate the thermal stability and calculate the content of Ag nanoparticles in the RB composite. Based on the residue weight percentage of the materials as shown from the TGA curves in Figure S2c, the AgNPs is calculated to be 5.42 wt%. The detailed information about the calculation can be found in the Supporting Information. Also, the maximum decomposition temperature (Figure S2d) for pure RB is about 375.7 °C, and it slightly downshifts to 374.3 °C after introduction of AgNPs onto the RB. But the temperature upshifts to 383.9 °C for the RB/AgNPs-40/PDMS-60. The good adhesion interactions between RB, AgNPs and PDMS can, to a certain extent, limit the macromolecular movement, leading to the enhancement of the thermal stability.29, 30 The conductivity of the RB composite is determined by AgNPs density that is controlled by the immersion time of the RB in STA solution. As shown in Figure S3, the conductivity of the RB composite dramatically increases with the immersion time (less than 10 min). As the immersion time increases from 10 to 40 min, the conductivity slightly increased, and it fluctuates at around 200 S·m-1 when the immersion time is larger than 40 min. For Joule heating and deicing performance, low resistance (high conductivity) is preferable, and hence RB/AgNPs-40 is chosen for investigating the superhydrophobicity, Joule heating and strain sensing behavior of the RB composite. As mentioned, the PDMS layer can decrease the surface energy of AgNPs, and thus its thickness that is controlled by the immersion time in PDMS solution has great influence on the water repellent property of the material. Figure S4a shows the CA of the composite as a function of PDMS modification time. The CA for RB/AgNPs-40 is around 136°, and the hydrophobicity originates from the high surface roughness caused by the Ag nanoparticles stacking (see in Fig S1d). The CA increases to around 150° and 155° for the RB/AgNPs-40/PDMS-10 and RB/AgNPs-40/PDMS-30, respectively, displaying superhydrophobicity. The interface between the AgNPs becomes vague after the PDMS modification (Figure S4b and S4c). With further increasing the immersion time to 60 min, the CA of the RB composite reaches as high as 156°, and the PDMS can be regarded as a “glue”, sticking the AgNPs together (Figure 1b and 1g). The PDMS



becomes more evident for the RB/AgNPs-40/PDMS-90, and the AgNPs seem to be embedded into the PDMS layer (Figure S4d), indicating more strong adhesion. Once the AgNPs are tightly wrapped by the PDMS, the CA varies little. As seen from the SEM images in Figure S4c and S4d, the conductive network is maintained after the PDMS introduction, although the conductivity may be, to a certain degree, reduced. 3.2 Mechanical, electrical and superhydrophobic performance Figure 2a shows the typical stress-strain curves of the RB and it composites. Clearly, the introduction of AgNPs and AgNPs/PDMS enhances both the Young’s modulus and tensile strength of the RB based composite, while the materials maintain the excellent stretchability. Table S1 summarizes the mechanical properties of the RB and its composites. The introduction of AgNPs decreases the elongation at break of the RB from 933.6% to 881.6%, while the PDMS modification could enhance the value to around 901.5%. The RB/AgNPs/PDMS has a much higher Young’s modulus and tensile strength of 9.0 MPa and 42.8 MPa than pure RB (5.4 MPa and 24.2 MPa, respectively). The uniform distribution of AgNPs embedded onto the RB surface and good inter-adhesion between AgNPs and the RB strip effectively improve the stress transfer, resulting in enhanced Young’s modulus and tensile strength. 31, 32 After the PDMS modification, the interfacial adhesion between AgNPs and the RB becomes stronger and the interfacial contact area is also enhanced, which could further transfer the stress and consume more energy during the stretching and thus improve the mechanical properties.33 It is desired that the conductivity and CA can keep stable when the materials suffer from the long-term external force. Figure 2b shows the CA variation of the RB composite with the applied strain. Interestingly, the CA firstly drops from initial 156° to 150° with strain from 0% to 60%, and then it jumps to 155° by increasing the strain from 60% to 200%. In general, the mechanical stretching would damage the hierarchical structure of the material and thus the decrease of the surface roughness, resulting in decline of the CAs.34 In this study, the PDMS modified AgNPs are distributed on the RB surface, and the stretching would induce the separation of the


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nanoparticles, thus producing some cracks. The stretching could, to a certain extent, flat the material surface by reducing the surface concave-convex, hence decreasing the CA. On the other hand, the generated cracks could be considered as grooves that is beneficial to the enhancement of the surface roughness particularly when the crack is developed to a certain stage at a relatively large strain. The detailed morphology evolution for the RB composite with the strain will be revealed in the following section. To verify the reusability and durability, the RB composite experiences cyclic stretching-releasing test under 50% strain. As displayed in Figure 2c, the resistance change (Ra/R0, where Ra represents the resistance of the RB composite after the stretching is released) of the RB composite slightly increases during the first 30 cyclic test and then fluctuates at around 2.0, and the CAs are kept at larger than 150° during the whole test. The excellent surface stability ensures the reliability and durability of the RB composite based strain sensor in practical applications. Apart from cyclic mechanical deformation, materials inevitably undergo surface abrasion especially in practical applications. Figure 2d and Figure 2e demonstrate the variation of CA and resistance of RB/AgNPs-40 and RB/AgNPs-40/PDMS-60, respectively, with the abrasion times. As displayed in Movie S1, the RB composite is rubbed with a sandpaper under a weight of 20 g, and the sample is then dragged forward 3.5 cm as one abrasion test. For RB/AgNPs-40/PDMS-60, the R/R0 slightly increases to 6 at the 10th test, and then it fluctuates at this value during the following cyclic test, displaying good stability of the conductivity (the blue lines in Figure 2d). By comparison, the R/R0 for RB/AgNPs-40 shows a continuous increase in the whole test (the orange lines in Figure 2d), and it arrives at about 34 after 60 times of abrasion. In addition, the CA of RB/AgNPs-40/PDMS-60 keeps almost unchanged during the abrasion test (see the blue curve in Figure 2e), and it is still as high as 154°, exhibiting outstanding durability of the surface superhydrophobicity. However, there is a sustained decline for the CA of RB/AgNPs-40, as shown from the orange curve in Figure 2e. The CA is only 137° after 60 times of abrasion. In practical applications, stable electrical conductivity and water repellence are very



important. The superhydrophobic surface of the RB composite could effectively prevent the corrosive solutions (like acidic, basic and salty solution) from diffusing inside the composite, thus guaranteeing the durability and reliability of the materials used in a harsh environment. As shown in Figure S5, the CAs of the RB composites are around 154° regardless of the pH, exhibiting good anti-corrosive property. Besides the high CAs, the RB/AgNPs/PDMS shows extremely low sliding angles (SAs) and thus excellent water repellence, which is demonstrated in Movie S2. It’s evident that water droplets could easily roll off the surface of RB/AgNPs/PDMS, while water droplets adhere well on the surface of RB/AgNPs. The extremely low SAs endow the RB composites with excellent self-cleaning performance. As demonstrated in Movie S3, the model contaminant (carbon black powders) can be easily removed from the composite surface under the assistance of the water droplets. The RB composite is also immersed in an acid solution (pH=1) to further verify its anti-corrosion property. As shown in the insets of Figure 2f, the material floats on the surface of the acidic solution (left corner), due to its superhydrophobicity. The composite is compelled to stay in the acid solution with the help of a piece of slide (right corner in Fig. 2f), which can increase the contact area between the material and the acid solution. It can be seen from Figure 2f that the CA is kept around 154° and R/R0 fluctuates around 1.5 after acid solution treatment for 8 h.

Figure 2 (a) Stress-strain curves of RB, RB/AgNPs and RB/AgNPs/PDMS. (b) CAs of


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the composite under different strain. (c) Ra/R0 and CAs of the composite during cyclic stretching-releasing under a strain of 50%. The vibration of (d) R/R0 and (e) CAs of RB/AgNPs and RB/AgNPs/PDMS as a function of abrasion times. (f) The CAs and R/R0 of the RB composite after immersed in an acid solution (pH=1) for different time (insets are the pictures of the material floating on the solution surface and immersed in the solution). 3.3 Joule heating and deicing performance of the RB composite In addition to the excellent water repellent property, the flexible, superhydrophobic and electrically conductive RB composite also exhibits good electrothermal performance. The Joule heating performance of the composite under different applied voltages is explored, as shown in Figure 3. The insets in Figure 3a are the corresponding temperature distribution images detected by an infrared camera. Apparently, no obvious temperature increase is observed at a voltage of 1 V. As the input voltage is improved to 2 V, the composite yields a saturated temperature of about 21.5 °C, which is further enhanced to 35 °C and 53 °C at a voltage of 3 V and 4 V, respectively. The excellent repeatibility of electrothermal conversion performance is assessed by cyclic applying and removing the voltage (Figure 3b). Figure 3c shows the Joule heating performance of the material under different strain. The saturated temperature for the RB/AgNPs/PDMS experiencing no stretching is 53 °C, while it downshifts to 34.1 °C and 28.6 °C for the material at a strain of 5% and 10%, respectively. It’s believed that the heat generated by Joule heating at a fixed voltage is inversely proportional to the material resistance. Thus, when a strain is applied on the RB composite, its resistance increases, producing a reduced heat at a fixed voltage. Additionally, the combination of superhydrophobicity and Joule heating effect entitles the RB/AgNPs/PDMS to be used in water removal and efficient deicing. As shown in Figure 3d and Figure 3e, a water droplet with the volume of 5 μL is dripped on the surface of the unheated material and heated one with an input voltage of 4 V, respectively. The volume of the water droplet on the heated RB/AgNPs/PDMS becomes smaller with time, while there is no obvious volume reduction for the water droplet on the unheated sample. The deicing efficiency of the composite is demonstrated in Figure 3f and Figure 3g, respectively. It can be observed that the ice placed on the unheated RB based sample barely melts and



it still stands on the surface of the RB based sample even after 5 min at ambient temperature (Figure 3f). By contrast, when a voltage of 4 V is applied on the composite, the ice starts to melt and rolls off the material surface within 68 s (Figure 3g, Movie S4). Due to the electrothermal performance, the surface temperature of the RB composite increases under an applied voltage, which effectively accelerates the process of water removal and deicing.

Figure 3 (a) Transient temperature evolution of RB/AgNPs/PDMS under different applied voltage. (Insets are temperature field captured by an IR camera at each applied voltage.) (b) Temperature change of the RB/AgNPs/PDMS under cyclic heatingcooling process at a voltage of 4 V. (c) Transient temperature evolution of the composite under different strains at a fixed voltage of 4 V. The volume evolution of one water droplet over time on the superhydrophobic surface of the RB composite (d) without applied voltage and (e) with a voltage of 4 V. Deicing performance of RB/AgNPs/PDMS (f) without applied voltage and (g) with a voltage of 4 V. 3.4 Strain sensing performance The sensing performance of the RB compostie under differetn strain (ε=1%, 3%, 5%, 10%, 30%, 50%, and 60%) are displayed in Figure 4a-4e. It is found that the sensing curve for each cyclic test is almost identical at a fixed strain (either small or large), that is, the R/R0 increases during the stretching and then decreases during the releasing. The resistance tends to return to its original value at the end of each cyclic test, displaying excellent reliability and repeatability. The resistance response is determined by the


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evolution of the conductive network that is gradually damaged during stretching while recovered during releasing. The larger the strain is, the more severely the conductive network is damaged and thus the higher the resistance becomes. For example, the resistance response intensity (RI) defined as the maximum resistance change (Rmax/R0) varies from 1.005 at the strain of 1% to as high 3.6×108 at the strain of 60%, exhibiting extremely high sensitivity while low detection limit. To examine the durability, the RB composite strain sensor experiences stretching-releasing at the strain of 30% for 2000 cycles, and the sensing signals are shown in Figure 4c. The R/R0 shows repeatable and periodic variations duirng the multiple cycle tests, and the good recoverbility and longterm durability come from the superb visoelasticity of natural rubber. It is worth noting that the RI significantly increases from 64 at ε=50% to 3.6×108 at ε=60%, which suggests that the conductive network at this strain has been completely damaged. Even so, the resistance of the strain sensor could go back to original value after the strain is totally released, indicating the strong reconstruction ability of conductive network in the strain sensor. The extremely high sensitivity with a RI of 3.6×108 together with the large stretchability makes the RB composite a prospective candidate in wearable strain sensor. Figure 4f shows the R/R0 of the RB/AgNPs/PDMS based strain sensor as a function of the strain ranging from 0% to 60%. Apparently, the R/R0 displays a synchronous change with the strain, i.e., R/R0 would gradually grow with the stretching of the sample and decrease with the releasing of the sample (ε≤60%). The relative resistance change with the strain (ε < 55%) can be divided to three linear regions, as shown in Figure S6. The three regions correspond to three gauge factors (GFs, where GF=ΔR/εR0 (ΔR=Rmax R0)). The GFs are 36.7 for area 1 (ε=0-32%), 264.9 for area 2 (ε=32.1%-47.9%) and 1153.0 for area 3 (ε=48%-54.2%), respectively. When the strain increases from 54% to 60%, the resistance shows more than five orders of magnitude increase (Fig.4f), corresponding an extremely high response intensity of ~ 108. The maximum workable strain range and maximum gauge factor of the RB composite in our work and some elastomer based strain sensors reported in the literature are summarized in Table S2. It can be found that the RB composite in this study displays a larger gauge factor than



most of the stretchable strain sensors in the literatures. Apart from the strain influence, the effect of the tensile speed on the sensing response of the strain sensor is investigated, as shown in Figure S7. It can be found that the responsivity varies little with the strain rate from 20 mm/min-150 mm/min, and the strain frequency independence of the sensing performance is, in many cases, desirable, especially for the application of wearable electronics, since the disturbance of motion rate on the sensing signals can be eliminated. Also, the sensitivity of the RB composite to external strain is demonstrated by connecting the RB composite with a LED lamp and a battery. As shown in Movie S5, when the RB based composite is at its original state, the LED lamp is brightly illuminated at a voltage of 3V, due to the high conductivity of the composite. The lamp is gradually dimmed and finally extinguished with the incremental strain applied on the RB based composite, while it lights again and turns from dark to light when the strain is gradually released, which is based on the conductivity variation originating from the disconnection and reconnection of the conductive network during the stretchingreleasing process.

Figure 4 (a) The resistance response of the RB composite under relatively small strains at a tensile speed of 20mm/min. Strain sensing behavior of the composite at a strain rate of 50 mm/min under different strain (b) ε=10%, (c) ε=30% (2000 cyclic tests), (d) ε=50%, (e) ε=60%. (f) The sensing performance of the RB composite under different applied strain during the stretching-releasing. Thanks to the outstanding flexibility, stretchability and durability of the RB composite strain sensor, it can achieve real-time detection of full-range human body


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motions including vigorous joint movements, subtle muscle actions and tiny vocal cord movement, which is shown in Figure 5. As displayed in the inset of Figure 5a, the sensor is attached on the throat of the volunteer to monitor her vocal cord movement when she says “YZU”. Obviously, the regular sensing signals are observed as the testee starts speaking. Besides, the sensor device can be used to monitor the masseter muscle actions of the tester when she eats food. The elastic RB composite is sensitive to the small movement of the masseter muscle and the ups and downs of output signals are in accordance with the rhythm of the muscle movement (see Figure 5b). Figure 5c exhibits the detection of clicking the mouse by adhering the sensor material on the joint of the index finger. The intermittent clicking causes the bending and stretching of the finger joint, leading to the regular changes in the resistance. Apart from detection for subtle body movements, the sensor is also used to monitor relatively large body movements. The sensor is attached on the neck of the tester to monitor her sitting position and the resistance varies as the testee nods periodically (Figure 5d). The sensing signals of finger bending and wrist movement are also studied and shown in Figure 5e and Figure 5f, respectively. The large body joint movements like elbow and knee bending are tested and exhibited in Figure 5g-5i. With increase of the bending angle, the conductive network is damaged more severely, thus resulting in a larger response signal. The full range monitoring of the human motions originates from the excellent stretchability, low detection limit and high sensitivity of the RB composite.



Figure 5 Transient resistance change (R/R0) of the RB composite with response to different human body motions. (a) The vocal cord movement (the testee speaks “YZU”). (b) The cheek movement. (c) Clicking mouse. (d) Cyclic nodding-looking up movement. (e) Cyclic finger bending. (f) Wrist movements. (g) The arm joint motion. knee-joint movements: (h) walking, (i) squating. The insets in all the figures are the photographs showing the corresponding body movement. To reveal the strain sensing mechanism, the conductive network evolution during stretching and releasing is investigated by observation of the microstructure of the RB based composite under different strain. As shown in Figure 6a, for the pristine composite, the AgNPs contact with each other and are evenly embedded onto the RB composite, creating perfect conductive pathways. A relatively small deformation is present at a low strain of 10% (Figure 6b). This deformation can induce tiny separation between the conductive nanoparticles, leading to a slight increase of the resistance. With the increase of the strain to 30%, evident separation between the AgNPs occurs, producing visible micro-cracks on the conductive pathways (see the yellow dotted line in Figure 6c). When the strain is further increased to 50% and 60%, more and large cracks are generated (marked with yellow dotted rectangle) and the gaps between the cracks become larger as shown in Figure 6d and Figure 6e, cutting off the conductive pathways and hence leading to a rapid growth of electrical resistance. Reversibly, as the applied strain is released to 50% and 30%, the number of cracks is reduced and the gaps between the cracks get narrower (Figure 6f and Figure 6g). The cracks become


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vague when the strain is further released to 10% (Figure 6h). As a consequence, the resistance of the sensor can return to its original value after the strain is completed released (ε=0). In addition, the surface morphology of the composite under different strain is also observed using an optical microscope (Figure S8). Cracks appear and become increasingly large with the extension of the strain. Reversely, the damaged conductive network can gradually heal during releasing of the stretching, which is in high accordance with the morphology evolution as shown in the SEM images. The outstanding resilience of the RB drives the quick reconstruction of the conductive network during the releasing, guaranteeing the excellent reproducibility and durability of the strain sensing performance.

Figure 6 SEM images of the composite at (a) ε=0%, (b) ε=10%, (c) ε=30%, (d) ε=50%, (e) ε=60% during stretching and (f) ε=50%, (g) ε=30%, (h) ε=10% during the releasing.

4. Conclusion In conclusion, a superhydrophobic and highly conductive RB/AgNPs/PDMS composite was prepared. The RB was first soaked in Ag precursor for a certain time, and AgNPs were formed after the Ag precursor adsorbed on the RB was completely reduced. The RB decorated with AgNPs was then modified by the PDMS. The PDMS served as a glue could not only endow the material with superhydrophobicity, but also stick the nanoparticles and RB tightly, thus enhancing the interface adhesion. The introduction of AgNPs/PDMS improved both the Young’s modulus and tensile strength while without sacrificing the super-elasticity of the RB. The superhydrophobic material possessed excellent self-cleaning and anti-corrosive performance, and the



suerperhydrophobicity can be maintained after multiple abrasion and stretching tests. Furthermore, the RB composite exhibited good Joule heating effect and thus excellent deicing performance. The RB composite strain senor displayed both broad workable strain range (ε=1%-60%) and superior sensitivity (RI ≈3.6ⅹ108), and was capable to monitor a full-range body motions in real time such as body joint motions, masseter muscle movement, etc. The strain sensing mechanism was revealed based on the conductive network evolution during the stretching and releasing. The strain induced damage to the conductive network led to the increase of the resistance, while the releasing could cause the reconstruction of the conductive network and thus decrease of the resistance.

Supporting Information The surface morphologies of pure RB, pure RB after abrasion, photograph of RB after adsorption of Ag precursor, SEM image of RB/AgNPs-40 (Figure S1); XRD pattern and FT-IR spectra for RB, RB/AgNPs and RB/AgNPs/PDMS respectively, TGA and differential thermogravimetric curves for RB and its composites (Figure S2); The electrical conductivity of the RB with immersion time in silver trifluoracetate solution (Figure S3), the variation of water CAs for the RB/AgNPs-40 as a function of the PDMS modification time and the corresponding SEM images of RB/AgNPs-40/PDMS10, RB/AgNPs-40/PDMS-30, RB/AgNPs-40/PDMS-90 (Figure S4); contact angles for the different aqueous solution on the sample (Figure S5); ΔR/R0 of the sensor as a function of strain (Figure S6); the strain sensing curves of RB/AgNPs-40/PDMS-60 at different tensile speed under 30% strain (Figure S7); optical images of the sensor under different strain (Figure S8); mechanical properties of the different RB based samples (Table S1); Comparison between the sensing performance for flexible strain sensors in literatures and our work (Table S2) (PDF) Abrasion test for RB/AgNPs-40/PDMS-60 (AVI) Water droplet repellent ability of RB/AgNPs-40 and RB/AgNPs-40/PDMS-60 (AVI) Self-cleaning performance of RB/AgNPs-40/PDMS-60 (AVI) Joule heating effect of RB/AgNPs-40/PDMS-60 applied in deicing (AVI)


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RB/AgNPs-40/PDMS-60 used as a stretchable electronic (AVI)

Acknowledgements This work was financially supported by Natural Science Foundation of China (No. 51873178, 51503179, No.21673203), the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (No. sklpme2018-4-31), Qing Lan Project of Jiangsu province, the China Postdoctoral Science Foundation (No. 2016M600446), the Jiangsu Province Postdoctoral Science Foundation (No. 1601024A), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Innovation Program for Graduate Students in Universities of Jiangsu Province (No. KYCX18_2364).

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Fluorine-free Superhydrophobic and Conductive Rubber Composite

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