Highly Stretchable and Conductive Superhydrophobic Coating for

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Surfaces, Interfaces, and Applications

A Highly Stretchable and Conductive Superhydrophobic Coating for Flexible Electronics Xiaojing Su, Hongqiang Li, Xuejun Lai, Zhonghua Chen, and Xingrong Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01382 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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ACS Applied Materials & Interfaces

A

Highly

Stretchable

and

Conductive

Superhydrophobic Coating for Flexible Electronics Xiaojing Su,a Hongqiang Li,*a,b Xuejun Lai,a,b Zhonghua Chena and Xingrong Zeng*a,b a

College of Materials Science and Engineering, South China University of Technology,

Guangzhou 510640, China b

Key Lab of Guangdong Province for High Property and Functional Polymer Materials,

Guangzhou 510640, China KEYWORDS: superhydrophobic coating, stretchability, conductivity, durability, flexible electronics

ABSTRACT: Superhydrophobic materials integrating stretchability with conductivity have huge potential in the emerging application horizons such as wearable electronic sensors, flexible power storage apparatus and corrosion-resistant circuits. Herein, a facile spraying method is reported to fabricate a durable superhydrophobic coating with excellent stretchable and electrical performance by combing 1-octadecanethiol modified silver nanoparticles (M-AgNPs) with polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) on a pre-stretched natural rubber (NR) substrate. The embedding M-AgNPs in elastic SEBS matrix and relaxation of prestretched NR substrate construct hierarchical rough architecture and endow the coating with dense charge-transport pathways. The fabricated coating exhibits superhydrophobicity with

ACS Paragon Plus Environment

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water contact angle larger than 160o and high conductivity with resistance of about 10 Ω. The coating not only maintains superhydrophobicity at low/high stretch ratio for the newly generated small/large protuberances, but also responds to stretching and bending with good sensitivity, broad sensing range and stable response cycles. Moreover, the coating exhibits excellent durability to heat, strong acid/alkali and mechanical forces including droplet impact, kneading, torsion and repetitive stretching-relaxation. The findings conceivably stand out as a new tool to fabricate multifunctional superhydrophobic materials with excellent stretchability and conductivity for flexible electronics under wet or corrosive environments.

Introduction Taking advantage of micro-nano rough structure and low-surface-energy substance to trap air between water droplets and micro-pockets, superhydrophobic surfaces possess high contact angles above 150o and have been of immense interest in scientific and technological field for their broad applications.1-4 In the last decade, many important approaches including electrospinning technique,5,

6

self-assembly,7,

8

sol-gel process9,

10

and chemical vapor

deposition,11, 12 have been put forward for preparing superhydrophobic materials and surfaces. However, with the rapid development of modern electronics industry, the requirements for superhydrophobic materials under extreme working conditions become more urgent. Especially, the water-repellency preservation of superhydrophobic coatings under stretching is a big challenge for that the vulnerable hierarchical roughness is easily destroyed when suffering great flexural strain or tensile deformation. Recently, several stretchable superhydrophobic coatings have been prepared for flexible electronics, sensing devices, or functional textiles to resist


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physical deformation and keep water-repellent.13-18 Ju et al.16 achieved a stretchable fluorine-free superhydrophobic coating with strong robustness on a pre-stretched silicone elastomer with hydrophilic micro-sized silica particles. Whereas, limited by the relatively low elastic modulus of silicone substrate, the superhydrophobic surface was broken at stretched states of beyond 500% strain. Using electrospinning to prepare polyurethane fiber as backbone, dilute polymerization technique to form polyaniline hairy nanostructure and dip-coating in polytetrafluoroethyelene to achieve low surface energy, Cho et al.17 fabricated a rubberlike superhydrophobic surface with stretchability and gas breathability. Nevertheless, the fabrication process was complicated and time-consuming, and the tensile deformation of superhydrophobic coating maintaining antiwetting was only below 350% strain. The electrical performance is also critical for superhydrophobic materials when being applied in electronics, and the combination of superhydrophobicity and electricity can not only endow the materials with multifunctionality in practical applications (e.g., anti-icing,19, 20 anti-frost,21 repairing,22 water-droplet manipulation,23, 24 electronic skin25), but also prolong the lifetime of electronics under wet or corrosive environment. Matsubayashi et al.26 embedded conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) nanoparticles in ethyl cyanoacrylate to develop a conductive superhydrophobic coating against freezing rain with its water-repellent and electrothermogenic properties. By means of hydrophobic polyvinylidene fluoride and mesoporous carbon capsules, Mittal et al.27 prepared a versatile electrical superhydrophobic coating with exceptional thermal, mechanical, corrosive and environmental stability. Heng et al.28 demonstrated a conductive superhydrophobic surface with switchable water-droplet adhesion through three steps including breath figure procedure, oxygen plasma treatment and heptadecafluorodecyl-trimethoxysilane

hydrophobization.

However,

despite

of

tedious


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fabrication process and utilization of fluorine-containing chemicals, these materials are fragile and easily damaged when suffering a tensile, compressive or torsional deformation, and consequently lose their superhydrophobic and electrical characteristics. Therefore, it is a significant value to efficiently integrate stretchability with conductivity on superhydrophobic coatings for further expanding their application horizons, such as wearable electronic sensors, flexible power storage apparatus and corrosion-resistant circuits.29, 30 Up to date, the work about the fabrication of stretchable and conductive superhydrophobic materials was seldom reported. Herein, we propose a simple spraying method to fabricate a highly stretchable and conductive superhydrophobic coating by combing 1-octadecanethiol modified AgNPs (M-AgNPs) with polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) on a pre-stretched natural rubber (NR) substrate. M-AgNPs embedding in SEBS can construct rough structure on coating surface and charge-conducting pathways in coating layer. As a kind of excellent thermoplastic elastomer, SEBS is used to endow the coating with reversible tensile deformation. By adjusting the pre-stretched strain of NR substrate and the mass ratio of M-AgNPs/SEBS, a highly stretchable and conductive superhydrophobic coating was successfully achieved with hierarchical roughness and intact charge-conducting pathways after relaxing. Interestingly, the coating maintained superhydrophobicity in Cassie state under tensile deformation even at stretch ratio up to 9, and the mechanism between mechanical deformation and wetting behavior were investigated. Furthermore, the superhydrophobic coating exhibited high conductivity under stretching and bending, and presented good sensitivity and stable response to large stretching strain and wide bending angle. Importantly, with the superhydrophobic coating as one of components, the electric circuit kept great conductivity after compressing, stretching and bending, and the electrical performance of circuit was not affected by continuous water droplets


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impacting on coating surface. Additionally, the superhydrophobic coating also owned excellent durability to heat, strong acid/alkali, water droplet impact, kneading and torsion, and repetitive stretching-relaxation. Our findings illustrate a new strategy to fabricate a highly stretchable and conductive superhydrophobic coating for potential applications, such as wearable sensors to realtimely detect human activities and flexible electric circuit to operate under humid condition. Experimental Section Materials. Silver nitrite (AgNO3, A.R., 99.8%), potassium iodide (KI, AR, ≥ 99.0%), and 1octadecanethiol (97%) were purchased from Aladdin reagent Co., Ltd. (China). Sodium citrate tribasic dihydrate (ACS reagent, ≥ 99.0%) and L-ascorbic acid were obtained from SigmaAldrich Co. (USA). Polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS, S1611) was supplied by Asahi Kasei Co. (Japan). Toluene (A.R.) and anhydrous ethanol (A.R.) were provided by Guangzhou Chemical Reagent Factory (China). Natural rubber (NR) strip with a thickness of 0.6 mm was bought from Nanjing Tonghui Rubber and Plastic Co., Ltd. (China). All chemicals were used as received without further purification, and deionized (DI) water was used for all the experiments and tests. Synthesis of AgNPs and Their Surface Modification. Aqueous solutions of sodium citrate tribasic dihydrate (2 wt%, 100 mL), AgNO3 (2 wt%, 25 mL) and KI (24 mM, 0.5 mL) were added in turn to 150 mL DI water in a beaker with gentle stirring for 4 min at room temperature to form a uniform solution A. Next, aqueous L-ascorbic acid solution (0.2 M, 5 mL) was added into 450 mL of boiled DI water in a 1000 mL three-necked flask and stirred at 100 oC for 3 min. Then the above-configured solution A was quickly poured into the flask and reacted at 100 oC for 90 min. After that, the AgNPs were separated by filtering the cooled solution and washing with DI water for three times, and then dried in vacuum oven at 60 oC for 12 h. To achieve


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surface modification, 1 g of the dried AgNPs was added into 100 mL of alcoholic solution of 1octadecanethiol (1 wt%), and then stirred dramatically for 24 h at room temperature. After filtering the solution, washing with anhydrous ethanol for three times and drying under vacuum at 60 oC for 12 h, the AgNPs grafted with long-chain alkane were obtained and denoted as MAgNPs. Fabrication of Highly Stretchable and Conductive Superhydrophobic Coatings. First, a 2 wt% stock solution of SEBS in toluene was prepared by heating the sealed SEBS and toluene at 60 oC under magnetic stirring until SEBS completely dissolved in toluene. Secondly, 1.05 g of M-AgNPs was dispersed into 17.5 g of the resultant solution with an ultrasonic treatment (Branson, S-450D) for 30 min to form a homogenous dispersion (the mass ratio of M-AgNPs to SEBS was 3). Subsequently, the NR strip (0.8 cm × 3.5 cm) was stretched at a uniaxial tensile strain (ε) of 200% (ε was defined as the change of length over the initial length). Then the above M-AgNPs/SEBS dispersion was sprayed on the pre-stretched NR strip using a spray gun equipped with an air compressor (DUN-30L, China) at a pressure of about 0.5 MPa and a distance of 15 cm. The pre-stretched NR strip was placed for 20 min to ensure the complete evaporation of solvent, and then relaxed to recover its initial length. To explore the effect of tensile strain of NR substrate on electrical and water-repellent properties, the samples were also prepared with ε at 100% and 0%, respectively. Besides, the as-fabricated sample obtained at ε=200% was further stretched at different stretch ratios (λ, the ratio of final length to initial length, 1≤λ≤9) to observe the change of the electric resistance and contact angle (CA) under tensile deformation. Characterization. X-ray diffraction (XRD) pattern was recorded using a powder diffractometer (PANalytical, Holland) with Cu-Kα radiation. Fourier transform infrared


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spectroscopy (FI-IR) was performed on a Bruker Tensor 27 spectrometer (Bruker Optics, Germany) from 4000 to 400 cm-1 with a resolution of 4 cm-1 and scanning times of 16. Particle size distribution was determined using a dynamic light scattering (DLS) detector (90 plus, Brookhaven instruments Corporation, USA). Thermogravimetric (TG) analysis was carried out by a thermogravimeter (TG209, Germany) at a heating rate of 20 oC·min-1 from 30 to 900 oC under N2 atmosphere. Morphology of M-AgNPs and M-AgNPs/SEBS coating surface were characterized using a EV0 18 scanning electron microscope (SEM, Carl Zeiss Jena, Germany) with an acceleration voltage at 10.0 kV, and the cross-section morphology was used to estimate the thickness (t) of coating. To test resistance (R, Ω), two copper wires were tightly fixed on the two ends of M-AgNPs/SEBS coating (2 cm × 0.8 cm) with adhesive tape to serve as contact points for the microohmmeter (TEGAM 1740, USA). Contact angles (CAs) and sliding angles (SAs) were measured with a contact angle meter (DSA100, Germany) equipped with a video capture using 6 µL and 10 µL of water as probe liquid at room temperature, respectively. Water droplet with volume of 6 µL was as probe liquid for CA tests and 10 µL for SA tests. The CA and SA were the average values obtained from at least three different positions. Results and Discussion Fabrication of Highly Stretchable and Conductive Superhydrophobic Coatings. To achieve highly stretchable and conductive superhydrohpobic coatings, thermoplastic SEBS was selected to endow the coating with hydrophobicity and great stretchability, the synthesised hydrophobic M-AgNPs (Figure S1) were utilized to construct roughness and conductive pathways, and a pre-strained NR strip was acted as an elastomeric substrate to further increase roughness and conductive pathways after relaxing. The schematic illustration for fabricating highly stretchable and conductive superhydrophobic coating on NR strip is presented in Figure 1.


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A suspension of M-AgNPs and SEBS with mass ratio of 3 was sprayed on a pre-strained NR strip at ε=200%, and the SEM images are presented in Figure 2a and 2d. Clearly, the rough surface on NR strip appeared superhydrophobic state with CA of 162o. In comparison, the coating fabricated with ε at 100% exhibited a CA of 158o (Figure 2b), and that at 0% presented a CA of 154o (Figure 2c). The roughness of the coating surfaces became lower, and the average thickness decreased to 54 and 40 µm from 84 µm, respectively (Figure S2). Meanwhile, the superhydrophobic coatings were highly conductive with a gradual increasing electrical resistance of 10, 58.5 and 165 Ω for the coating fabricated at ε of 200%, 100% and 0%, respectively. Obviously, the M-AgNPs were easily aggregated in SEBS matrix to form a large amount of small microscale mountain-like protuberances in spraying process. Furthermore, the innumerable small M-AgNPs/SEBS protuberances were passively compressed to construct closely accumulated clusters with bigger scales by the contraction force of SEBS matrix after the relaxation of pre-strained NR strip, thus leading to the rougher structure and denser chargetransport pathways. Besides, the mass ratio of M-AgNPs/SEBS also largely affected the electrical performance of coating on NR substrate. When the mass ratio was adjusted to 2 at the tensile strain of 200%, although the coating possessed rough surface (Figure S3) and superhydrophobicity with CA of 152o, it became nonconductive, indicating that the fewer M-AgNPs were difficult to construct an intact percolation pathway in SEBS matrix. Wetting Behavior and Mechanism of Superhydrophobic Coatings under Stretching. Generally, the rough structure of superhydrophobic materials is easily destroyed under large mechanical deformation, which results in the transition of Cassie to Wenzel state and even loss of water repellency.31, 32 Accordingly, the construction of an extremely stable superhydrophobic


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surface at large tensile deformation has a significant value for flexible materials.17, 30 To study the wetting behavior under different stretching states, the superhydrophobic coating fabricated on NR strip with 200% of pre-stretched strain and 3 of M-AgNPs/SEBS mass ratio was stretched to different λ (1≤λ≤9), and the changes of CAs and SAs are presented in Figure 3. Notably, the CAs and SAs of the superhydrophobic coating changed little with λ increasing from 1 to 9. Even at a high λ of 9, the CA and SA still remained 154.5o and 8.9o, respectively, exhibiting an outstanding capability to maintain superhydrophobicity under extreme tensile deformation. To the best of our knowledge, it was much far superior to most of flexible superhydrophobic coatings reported before (Figure 3b).13, 16, 17, 29, 33-35 To further study the inherent mechanism of this phenomenon, the surface morphologies at different λ were investigated (Figure 4 and S4). From the cross-section topologies, it was found that the coating layer gradually became thin along uniaxial stretching direction from 84 µm at λ=1 to 21 µm at λ=9 (Figure S5). To ensure the accuracy,29 the average thickness value (t) was calculated using the power law curve fitted with the obtained average thickness values at different λ (Figure S6). According to the equation (1) of power law curve,

t = 84.7 ⋅ λ −0.58

(1)

the average values of t at λ=1, 3 and 9 were calculated to be approximately 85, 45 and 24 µm, respectively. The superhydrophobic coating at λ=1 (without stretching) was covered with a thick bottom layer and a dense top layer concluding plenty of closely packed large protuberances with average diameter of about 65 µm (Figure S7a). At relatively low tensile deformation (1

Highly Stretchable and Conductive Superhydrophobic Coating for

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