Novel Robust Superhydrophobic Coating with Self-Cleaning

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Novel Robust Superhydrophobic Coating with Self-Cleaning Properties in Air and Oil Based on Rare Earth Metal Oxide Liji Xiao, Min Deng, Weiguo Zeng, Boxiao Zhang, Zushun Xu, Changfeng Yi, and Guangfu Liao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03131 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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Industrial & Engineering Chemistry Research

Novel Robust Superhydrophobic Coating with Self-Cleaning Properties in Air and Oil Based on Rare Earth Metal Oxide Liji Xiao†1, Min Deng†1, Weiguo Zeng†, Boxiao Zhang†, Zushun Xu*,†， Changfeng Yi†, Guangfu Liao*,†,‡ †

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory for The Green Preparation and Application of Functional Materials, Hubei University, Wuhan, Hubei, 430062, PR China

‡

School of Materials Science and Engineering, PCFM Lab, Sun Yat-sen University, Guangzhou 510275, China

ABSTRACT:

Herein, a simple spraying method was applied to prepare a novel

robust superhydrophobic coating containing fluorinated hyperbranched polyurethane (F-HPU) resin and lanthanum oxide (La2O3). The composite coating had the maximum water contact angle (WCA) of 157° and low sliding angle (SA) of 5.5° when the content of La2O3 was 17%. Moreover, the composite coating can resist abrasion with sand paper for 80 cycles, which might be the reason that La2O3 particles were adhered strongly on the substrate by the F-HPU resin with strong adhesion. Furthermore, the novel robust superhydrophobic coating can be applied in various substrates, resist water stream impact and keep clean after fouled by contamination in air and oil. Accordingly, the novel robust superhydrophobic coating has potential application in protection-needed materials. Meanwhile, we also provide a new application field for rare earth metal oxides and an alternative strategy for robust self-cleaning coatings.

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Key words: Spraying method, Superhydrophobicity, Robustness, Self-cleaning, La2O3

1. INTRODUCTION Inspired by plants and animals in nature which possess superhydrophobic properties due to their special microstructures, such as lotus leaf surfaces,1 butterfly wings,2 fish scales3

and

water

strider

legs,4

researchers

prepared

various

biomimetic

superhydrophobic surfaces.5-7 Superhydrophobic surfaces exhibiting static water contact angle higher than 150° and sliding angle lower than 10° show significantly important in academic research and practical application.8-10 Generally, two categories, modifying the rough surface with low surface energy materials and constructing micro/nano structures on the low surface energy substrates, are suggested to fabricate superhydrophobic surfaces.11-13 Researchers have been reported that the water contact angle can reach 120° at maximum when the surface is only coated by fluorinated compounds.14 Therefore, it is quite necessary to rough the surface by constructing micro/nano structure to fabricate the superhydrophobic surfaces. Generally, employing inorganic particles is the main strategy for the construction of micro/nano structure.15-16 As an example, Hang’s group reported a superhydrophobic coating composed of caterpillar-like Cu/Ni-Co hierarchical structure by combined electroless and electrodeposition method. The superhydrophobic coating has a contact angle of 165.5° and a low sliding angle of 3.5°.17 However, the micro/nano structure constructing by inorganic particles is too fragile to resist any mechanical damages, such as, finger scraping, knife scratching, and sand paper abrasion and so on. Consequently, it is very

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imperative to fabricate robust superhydrophobic surfaces that can withstand various mechanical damages for a long period. Very recently, many research groups have been made strenuous efforts to improve the robustness of superhydrophobic surface by using adhesives,18-19 endowing self-healing or intermolecular cross-linking. Among them, endowing self-healing ability to superhydrophobic surfaces for protection superhydrophobic properties was popular.20-21 For example, Wang’s group fabricated self-healing superhydrophobic fabrics through immersing in a coating solution containing hexadecyl trimethoxysilane and polydopamine, the self-healing ability was attributed to long chain in hexadecyl trimethoxysilane twisting and migrating to the surface after heating, which reduced surface energy and restored the superhydrophobicity.22 Nevertheless, the self-healing ability will not retain owing to the loss of healing agent after mechanical damages. Also Wong et al designed a durable superhydrophobic coating via spraying a colloid of PU-PMMA

interpenetrated

polymer

perfluorooctyl-dimethylchlorosilane

networks

modified

SiO2,

and

1H,

1H,

successively.

2H,

The

2H-

durable

superhydrophobic coating exhibited excellent antiabrasion resistance with sliding angle of below 10° after 120 continuous abrasion cycles.23 However, the intermolecular cross-linking still exist some problems, such as, the combination between the cross-linking polymer and inorganic particles was poor, even though it could improve the robustness of superhydrophobic coating in a extent. Thus avoiding the loss of superhydrophobicity, researchers designed another approach to improve the durability,

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which is adding adhesives. Wu et al synthesized superhydrophobic emulsions composed of SiO2/TiO2 nanoparticles and epoxy resin, the obtained superhydrophobic coatings were able to maintain water repellency performance after finger wiping, tape peeling and sandpaper abrasion and boiling in water at 100 °C owing to the strong adhesion of the epoxy resin.24 And the used method was green and facile. In addition, Zhi et al prepared a superhydrophobic coating using hydrophobic silica nano-particles and polyurethane as materials, which could keep superhydrophobicity after multiple damages such as knife scratch, folding, kneading, sandpaper abrasion, UV radiation and corrosive liquid contacting.25 Zhang et al. prepared a robust supehydrophobic coating by a“adhesive + coating”method using a polyurethane resin and TiO2 as raw materials post treated by 1H, 1H, 2H, 2H- perfluorooctyltriethoxysilane, and the superhydrophobic coating resisted abrasion with sand paper for 1000 cm.26 Although the above reported works were conductive to the development of the durable superhydrophobic coating, there still existed some drawbacks, such as complicated modification procedure, time-consuming post treatment and expansive equipments, during the preparation process. Therefore, using a facile and cost-efficient method to prepare durable superhydrophobic surface is highly worth considering. As we all known, spraying method is a simple, low-cost route without special instruments and applicable for a variety of substrates and not limited to small area substrates.27-28 Thus, we chose the spraying method to prepare the superhydrophobic coating. In our work, in order to improve the superhydrophobic coating’s durability,

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fluorinated hyperbranched polyurethane resin was used as the adhesive. To the best of our knowledge, polyurethane have been used extensively and one of the most versatile materials due to their excellent chemical and solvent resistance, toughness, flexibility, durability, strong adhesion and easy miscible with other materials during the past few decades.29-30 Moreover, the hyperbranched polyurethane has obtained much attention due to their special properties compared with linear polymers.31 It can be modified for desired functional materials and form membrane easily because of their high-density living end groups and low viscosity.32 Additionally, the hyperbranched polyurethane is also high solubility, miscibility and reactivity due to their high segment density within volume of a molecule and absence of intermolecular entanglement.33 However, the adhesion performance of hyperbranched polyurethane was greatly affected by its intrinsic hydrophilicity.34-36 Therefore, we utilized the fluorinated hyperbranched polyurethane to reduce its surface energy so as to transform the hydrophilic coating to superhydrophobic coating easily. Apart from using fluorinated hyperbranched polyurethane, La2O3 can also improve the superhydrophobic coating’s robustness owing to its intrinsic hydrophobic and high abrasion resistance. And there are few reports using rare earth metal oxides to prepare superhydrophobic coating. It has been reported that rare earth metal oxides are intrinsically hydrophobic due to their unique electronic structure that inhibits the hydrogen binding with interfacial water molecule.37 Therefore, it requires no modification procedure by using La2O3 particles. Moreover, La2O3, a low cost,

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wearability, excellent chemical and physical properties inorganic rare earth metal oxide, has wide applications in the area of fuel cell, catalyst, automobile exhaust-gas converters, water treatment and biomedicine.38-40 In this paper, we synthesized the fluorinated hyperbranched polyurethane (F-HPU) to reduce the coating’s surface energy and improve the durability via a facile stepwise polymerization method in a mild experiment condition. In the meantime, La2O3, a kind of rare earth metal oxide, was used to increase roughness and robustness of the novel superhydrophobic coating. What’s more, the novel robust superhydrophobic coating was fabricated by a versatile and cost efficient spraying method. Furthermore, the prepared novel robust superhydrohphobic coating can be sprayed on different kinds of substrates, which has potential for application in anti-fouling materials and production in large scale.

2. EXPERIMENT 2.1. Materials Polycarbonate diols (PCDL) with functionality of 2 and number-average molecular weight of 1000 (g/mol) were purchased from Xuhuacheng Fine Chemistry Co., Ltd, China. Toluene-2, 4-diisocyanate (TDI) was supplied by Wuhan Jiangbei Chemistry Reagent Factory, China. Pentaerythritol (PE, 98 %) was analytical pure and provided by Aladdin Industrial Corporation (China). Dibutyltin dilaurate (DBTL, 95 %) was analytical pure and supplied from Aladdin Industrial Corporation. Lanthanum oxide (La2O3)

(hexagonal,

200nm-1µm),

FeCl3,

SiO2,

n-caprylic

6


alcohol

and

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1-Methyl-2-pyrrolidinone (NMP) were analytical pure and all purchased from the National Medicine Group Chemical Reagent Co., LTD, China. 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 8-Tridecafluro-1-octanol (F13-OH, 98 %) was supplied by Macklin. Acetone was analytical pure and provided by Jiangsu Qiangsheng functional chemical Incorporated Company, China. CuSO4 with analytical pure was purchased from Tianjin Guangcheng Chemical Reagent Incorporated Company. NMP and acetone were dried by molecular sieve A4 for two days before used. And PCDL was dried by vacuum for 2 h at 120 °C. Other reagents were used without further purification. Glass slide, iron sheet, cotton fabric, sponge and sand paper (800meshes) were purchased from local super market and ultrasonicated with deionized water. Brick was picked up from roadside and washed with water. 2.2. Preparation of Fluorinated HPU (F-HPU) Resin Figure 1 illustrated that the F-HPU resin was prepared according to the following procedures. In details, 5 g of PCDL (5 mmol) dissolved in 40 ml acetone was poured into a four-necked, round-bottomed flask equipped with a stirrer and condenser under nitrogen atmosphere and stirred for a few minutes, then 3.485 g (20 mmol) of TDI and 40 ml acetone were added into the above solution, and 10 drops of DBTL were added into the flask to catalyze the reaction. This reaction was kept at 70 °C for 2 h. Then 0.510 g (3.75 mmol) of PE dissolved in 10 ml NMP was dropped into the mixture, the isocyanate terminated HPU was obtained after the polymerization for 2 h. After that, the solution of 5.462 g (15 mmol) F13-OH in (20 ml) acetone was dropped into the HPU.

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Finally, the fluorinated HPU was obtained after the reaction between F13-OH and isocyanate terminated HPU for 2 h at 80 °C. The obtained resin was stored in a container for usage in next procedure. The hyperbranched polyurethane (HPU) without fluorine was prepared as control to acknowledge the importance of fluorine for the superhydrophobicity. The preparation process of hyperbranched polyurethane resin was the same as the F-HPU resin except substituting the F13-OH by n-caprylic alcohol.

Figure 1. Synthesis route of the F-HPU resin. 2.3. Preparation of the Novel Robust Super-hydrophobic Coatings The preparation process of the robust superhydrophobic coating through a mixture of F-HPU and La2O3 particles was shown Figure 2. A certain amount of fluorinated HPU was mixed with La2O3 in spraying bottles (the distance between the bottle and the substrate was about 10 cm and the used pressure was 2 MPa), the mass ratios of La2O3 to the fluorinated HPU were 11%, 13%, 15%, 17%, 19%，respectively. And then the

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mixture was sprayed on various substrates. The freshly made coatings on various substrates were dried in the oven at the temperature of 60 °C for 12 h. For simplicity, the samples were named F-HPU-0, F-HPU-11, F-HPU-13, F-HPU-15, F-HPU-17 and F-HPU-19 in which the number stands for the ratio of La2O3 to the F-HPU. Furthermore, the coating surface without F-HPU or HPU, HPU with 17% of La2O3 and F-HPU with 17% of SiO2 were prepared as control, which were simplified to La2O3-no-F-HPU, HPU-La2O3 and F-HPU-SiO2, respectively. The preparation process of coating surface without F-HPU or HPU, HPU with La2O3 and F-HPU-SiO2 were presented in Supporting Information.

Figure 2. Schematic presentation of preparation process of robust superhydrophobic coatings. 2.4. CHARACTERIZATION 2.4.1. The Characterization of Fluorinated HPU (F-HPU) Resin Fourier Transform Infrared Spectroscopy (FTIR, Nicolet IS50 Thermofisher USA) was used to characterize the components of F-HPU. The thermal properties and glass transition temperature of F-HPU were recorded by thermo-gravimetric analysis (TGA,

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METTLER, STAReSW13) under nitrogen atmosphere at a rate of 20 °C/min and differential scanning calorimetry (DSC, Q2000 V24.11 Build 124) under nitrogen atmosphere, respectively. 2.4.2. The Characterization of composite Coatings The surface morphologies of the pure polymer and composite coatings were recorded by field electron scanning microscope (FESEM, JSM7100F, Japan). And the scanning electron microscopy (SEM, JEOL, JSM6510LV, Japan) was used to characterize the surface morphologies of the coating coated by F-HPU-17 before or after abrasion, various substrates with or without F-HPU-17, such as filter paper, cotton fabric, sponge and glass, SiO2, La2O3-no-F-HPU, HPU-La2O3 and F-HPU-SiO2. X-ray Photoelectron Spectra (XPS) was used to characterize the chemical composition of the samples of F-HPU, F-HPU-11, F-HPU-17 and F-HPU-19. In addition, the surface wettabilities of the composite coatings were evaluated through measuring the contact angles and sliding angle. The static contact angles and sliding angle of the coated glass surface were measured by the contact angle apparatus (OCA20, Dataphysics, Germany) through the sessile drop method with a water droplet of 2.0 µL at room temperature. The listed angles were average of five measurements at least on the coatings surfaces. 2.4.3. Anti-fouling Ability Test Nestle Coffee solution (a bag of 15 g dissolved in 150 ml water), milk solution (a bag of 13 g dissolved in 150 ml water), CuSO4 particles, CuSO4 solution and FeCl3 solution were used as artificial contamination to test the anti-fouling ability of the coating.

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2.4.4. Abrasion Durability The abrasion durability test was carried on according to the following method: a weight of 50 g was adhered on the coated glass, and then the coated glass was faced on the sand paper, the glass was pushed back and forth for 20 cm, which was one cycle of the abrasion durability test (Figure 8a).

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of F-HPU and F-HPU-17 The components of the F-HPU and F-HPU-17 were demonstrated by the FTIR. As we can see from the Figure 3a, the absorption peak located in 3340 cm-1 corresponded to the –N-H stretching in –NHCO-. And the -CH2 stretching vibration occurred at 2933 and 2855 cm-1. The C=O stretching vibration of carbonate on PCDL and carbamate group located at 1750 and 1600 cm-1, respectively. And the peak of 2273 cm-1 corresponding to the -N=C=O antisymmetric stretching appeared in TDI but disappeared in F-HPU and F-HPU-17. These results proved the complete consuming of TDI and its reaction with the chemical group of –OH on PCDL and F13-OH. Besides, the peaks of 1050 and 1260 cm-1 were designated to the stretching vibration of -CF and the anti-symmetric stretching of –CF3, respectively, which indicated that the F13-OH was grafted on the hyperbranched polyurethane successfully. Compared with F-HPU, there were two new peaks appearing in F-HPU-17, which corresponded to the characteristic peaks of lanthanum hydroxide at 3610 and 640 cm-1 owing to the La2O3 reacting with water/moisture in air to form lanthanum hydroxide.

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As shown in Figure 3b, the first degradation of F-HPU started at 225 and 350 °C as a result of the breaking of hard the segments and the soft segments in polyurethane linkages, respectively. The breaking of C-F in polyurethane occurred at 459 °C, which further confirmed that the fluorinated HPU was synthesized successfully. The weight of F-HPU was 99.5% while the weight of F-HPU-17 was 95% when the temperature reached at 178 °C, which indicated that the La2O3 increased the onset temperature for degradation. What’s more, seen from the curves between the temperature 50 and 250 °C in Figure 3b, we could find that the F-HPU-17 showed slower degradation rates and better thermal stability than that of the F-HPU, demonstrating that the addition of the La2O3 could improve the thermal stability of the coating. Figure 3c shows that the glass transition temperature of the F-HPU appeared at -14.27 °C and there were no exothermic peaks of crystallization and absorption peaks of melting in the DSC curve of F-HPU, both of which demonstrated that the hyperbranched polyurethane was amorphous. In contrast to the F-HPU pure polymer, the glass transition temperature of the F-HPU-17 increased to -10.86 °C, which was caused by the addition of La2O3 and the La2O3 limited the movement of polyurethane linkages.41-43 Based on these analyses, we can conclude that the F-HPU was synthesized successfully and the composite exhibited better thermal stability than pure polymer.

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Figure 3. (a) FTIR spectra of the TDI, F-HPU resin and F-HPU-17, (b) TGA and (c) DSC curves of the F-HPU and F-HPU-17, respectively. 3.2. Surface Wettability The surface wettabilities of the coatings were testified by the contact angle and sliding angle. In Figure 4g, the water contact angle of the coating increased from 146° to 157° when the content of La2O3 improved from 11% to 17%, and then the water contact angle decreased as the La2O3 content was increased to 19%. It could be explained that the fluorinated hyperbranched polyurethane occupied the majority while the La2O3 accounted for the minority when the content was less. As a result, the La2O3 could only distribute on the substrate sparsely so that the distance between particles was large and the water droplet came into the gap once contacting the surface. Just as the Wenzel

44

model described, the surface would become more hydrophobic as the

roughness increased when the surface was hydrophobic. The roughness of the coating

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surface increased as the La2O3 content become higher. Consequently, the higher the content of La2O3 particles, the higher the water contact angle of the coating. However, when the La2O3 content was too high, the La2O3 particles agglomerated easily and the gap should have been trapped air was replaced by La2O3 particles. To the best of our knowledge, the superhydrophobic property was mainly induced by the trapped air between the gaps caused by micro/nano structure. In addition, the coating F-HPU-17 exhibited a low sliding angle about 5.5° and the pictures taken from Movie S1 showed a water droplet rolling down the continuous inclining coating surface at some moment (Figure 4h-i). And the low sliding angle of the F-HPU-17 was a result of the transition of the Wenzel state to the Cassie state. What’s more, The contact angles of different liquids on different substrates. In order to confirm the roles of F-HPU and La2O3 particles played in superhydrophobic coating. The contact angles of HPU-La2O3 and F-HPU-SiO2 were also measured, which were 116.75° and 136.5° (Figure S4), respectively. The results revealed that the F-HPU was important for reducing the surface energy to fabricate superhydrophobic surface. The La2O3 particles contributed to construct micro-nano structures and hydrophobicity compared with SiO2. 3.3. Surface Morphology The surface morphologies of the membrane and composite coating were observed through FESEM. Figure 4a shows a smooth surface of the pure hyper-branched polyurethane membrane, which displayed a contact angle of 110°. As we can see from the Figure 4b-f, La2O3 particles were covered by the hyper-branched polyurethane so

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that the La2O3 could be kept on the substrate firmly. In Figure 4b-c, obviously, the fluorinated polyurethane resin occupied the majority on the surface of F-HPU-11 and F-HPU-13, which made the two composite coating surfaces lower contact angle than others. From Figure 4b to Figure 4e, there was an increasing tendency for La2O3 particles occupying the substrate surfaces, as we discussed in the part of the surface wettabilities of the coatings, the increasing La2O3 particles resulted in the contact angle increasing from 146° of F-HPU-11 to 157° of F-HPU-17. Comparing with F-HPU-19, we can see from the Figure 4e and Figure 4f that the surface of F-HPU-17 had much more pores than that of F-HPU-19, which lead to the higher water contact angle. This discussion was in accordance with the results of the water contact angles stated above. Therefore, we could hypothesize that the reducing water contact angle and low sliding angle might be the reasons stated above. Consequently, the coating F-HPU-17 was the main

research

object

in

the

following

experiment

due

to

its

excellent

superhydrophobicity. And the surface morphologies of various substrates with or without F-HPU-17 were showed in Figure S2. From it, we can see that all the substrates coated by F-HPU-17 exhibited micro-nano structures compared with the substrates without coating. The results were consistent with the contact angles stated in Figure S1.

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Figure 4. FESEM images of the coatings of F-HPU (a), F-HPU-11 (b), F-HPU-13 (c), F-HPU-15 (d), F-HPU-17 (e) and F-HPU-19 (f), the inset is the photo of the water contact angle (110°) on the membrane of F-HPU surface, (g) water contact angle of the coating changes with La2O3 content, (h, i) photos taken during the water droplet rolls along the titling glass coated by F-HPU-17. 3.4. Surface Chemical Composition of the Coatings The geometric structure and chemical composition are two factors in fabricating superhydrophobic surface. It’s necessary to make clear the influence of the chemical composition on the wettability of the coating. The chemical composition and surface atomic concentration of the sample F-HPU-11, F-HPU-17 and F-HPU-19 were characterized by XPS. As shown in Figure 5a, the F1s appearing on three sample surfaces suggested that the fluorinated hyperbranched polyurethane was on the coating surface. Additionally, the high-resolution C1s XPS spectra showed in Figure 5b was

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deconvoluted into five components with binding energy at 291.9 eV (CF2), 294.2 eV (CF3), 290.5 eV (C=O), 285.8 eV (C-N/C-O) and 284.1 eV (C-C/C-H), which further confirmed the fluorinated hyperbranched polyurethane was synthesized successfully. Figure 5c depicts the high-resolution O1s spectra that was deconvoluted into two components at 532.5 eV (O2-) and 534.39 eV (OH-), the existence of O2-, La3d5/2 and La3d3/2 on the right side of Figure 5a confirmed that the inorganic substance was La2O3 rather than other materials

45

, and which also demonstrated that the La2O3 was

composited with polyurethane successfully. From the Table 1, we can see that the F content increased from 9.04% on the pure polyurethane membrane to 49.29% on F-HPU-17 surface, a 5.45 times enhancement, which is attributed to the fluorine containing segment transformation to the coating surface at the temperature over the glass transition temperature. Furthermore, the maximum F content as the low surface energy composition on the F-HPU-17 surface contributed to the contact angle increasing. Aside from the F element, the La and O elements also played an important part in the high contact angle on the F-HPU-17 surface. Both of them accounted for the superhydrophobic property and the higher contact angle of the F-HPU-17 than that of other samples. And the analyses agreed with the water contact angle results very well.

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Figure 5. (a) XPS spectra of the sample F-HPU-11 (blue line)，F-HPU-17 (red line) and F-HPU-19 (black line), the right image is the deconvolution XPS spectra of La, (b) typical high-resolution C1s XPS spectra of F-HPU-11, F-HPU-17 and F-HPU-19, (c) typical high-resolution O1s XPS spectra of F-HPU-11, F-HPU-17 and F-HPU-19. Element Name 0

Atomic Percentage in Samples% 11 17

19

C1s 66.3 47.96 30.85 38.22 N1s 2.04 5.4 6.83 5.82 O1s 22.62 9.52 11.6 16.25 F1s 9.04 36.98 49. 29 38.14 0 0.14 1.43 1.57 La3d Table 1 Surface atomic percentage in the sample F-HPU-11, F-HPU-17, F-HPU-19 and on the bulk membrane of F-HPU. 3.5 Super-water-repellency Behavior As we all known, the water strider can stand on the water because of the superhydrophobic property ascribing to its special microstructure 7. And in this paper, the coated filter paper can’t also be damped by the water even under the outside pressure (for details, see Movie S2). Figure 6g shows the different wettabilities of the uncoated and coated filter paper. It was obvious that the uncoated paper was hydrophilic

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and easily defouled by the coffee solution, while the coated paper showed superhydrophobic property and the coffee could form a spherical water droplet so that the paper can keep clean. Figure 6h demonstrated the super-water-repellency behavior of the coated filter paper. The uncoated paper got wetted immediately and dropped into the bottom, while the coated paper kept dry and floated on the water surface. Additionally, the coated paper can stay on the water surface for a month or even longer, and the water impact resistance of the coated filter paper was also carried on, which was adhered on the glass glide (Movie S3). The coated filter paper still can keep dry after being impacted for several minutes. Meanwhile, the floating time of the coated F-HPU-17 filter paper on the water surface was also tested. Results showed the floating time can reach a long time for one month. In order to expand its application fields, the F-HPU-17 was sprayed on the brick (Figure 6a), filter paper (Figure 6b), cotton fabric (Figure 6c), glass (Figure 6d), iron sheet (Figure 6e) and sponge (Figure 6f). And the coffee, milk, and CuSO4 solution acted as the contamination were dropped on all the substrates surfaces, which transformed from hydrophilic to ultrahydrophobic after coated by F-HPU-17. It could be concluded that the coating was water-repellent and water-impacted resistance, which can be used on various substrates surface and in damped environment without any influence. Moreover, the contact angles of different liquids on different substrates were present in Figure S1. All the substrates coated by F-HPU-17 were showed hydrophobicity in a certain extent, which demonstrated that the coating could be used in many substrates.

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Figure 6. Photo shots of water droplet obtained from coffee , milk and CuSO4 solution show spherical shape on brick (a), filter paper (b), cotton fabric (c), glass slide (d), iron sheet (e) and sponge (f), respectively, (g) the coffee droplets stand on the coated filter paper (right) and spreads on the uncoated filter paper (left), (h) the uncoated filter paper (left) dipped below surface of the water and the coated filter paper (right) floats on the water surface. 3.6 Self-cleaning Properties in Air and Oil The self-cleaning properties of filter paper coated by F-HPU-17 in air and oil were investigated using CuSO4 particles as the artificial dirt and cleaned by water droplets. Additionally, the superhydrophobicity was confirmed by its bounce ability on the coated glass surface. In Movie S4, we can see that the water droplet of 5 µL bounced on the coating surface for 5 times, which showed excellent superhydrophobicity. First, the coated paper was contaminated by CuSO4 particles in air (Figure 7a). Then a water droplet rolled off the sloping paper and removed the CuSO4 particles along the path in

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air (Figure 7b), which displayed the self-cleaning ability in air owing to the micro/nano structure formed by hydrophobic La2O3 and low energy material F-HPU on the coating surface. Finally, almost all the CuSO4 particles were removed by water droplets, and the filter paper returned neat (Figure 7c). Not only the self-cleaning property in air was investigated but also the self-cleaning property in oil-petroleum ether was investigated. And when the stained superhydrophobic coating immersed in petroleum ether, the coating was wetted by petroleum ether immediately, which demonstrated that the super-hydrophobic coating was also superoleophilic (Figure 7d). Similarly, the coating in oil also showed self-cleaning property after washed by water droplets and water sustained spherical shape in oil (Figure 7e-f), which could be explained by the lubricated fluid played by oil and the inherent superhydrophobic of coating 46.

Figure 7. The contaminated filter paper by CuSO4 particles in air (a) and oil-petroleum ether (d), the contaminated paper was cleaned by a water droplet in air (b-c) and oil-petroleum ether (e-f), the filter paper is adhered on the glass by dual adhesive tape. 3.7 Mechanical Durability What’s more, the low durability is the largest bottle-neck for superhydrophobic surfaces to be produced in large-scale and used in practical application. The mechanical

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durability of superhydrophobic surfaces is the main limitation for practical application. Therefore, improving it is quite urgent relatively. In this work, the composite coating based on fluorinated hyperbranched polyurethane and La2O3 particles exhibited strong adhesion resistance and can keep superhydrophobic properties after abrading for 80 cycles with sand paper. In Figure 8b-c, water droplets with different colors stand on the coated glass before and after abrasion for 80 cycles and the water droplets showed globular shape, declaring both the unabrasive and abrasive coated glass were superhydrophobic. And the water contact angle changes with the abrasion cycles showed in Figure 8c. The curve fluctuated between the 150° and 160° during the abrasion process and the water contact angles were 157° and 156° when the abrasion cycles were 0 and 80 cycles, respectively, which strongly demonstrated that the composite coating can withstand mechanical abrasion for 80 cycles. For proving the influence of the F-HPU on the robustness of the coating, we also prepared the coating surface without F-HPU or HPU and tested its robustness. The result shown in Figure S3 revealed that the La2O3 particles on coating surface without F-HPU or HPU could be easily removed by finger, which demonstrated that the F-HPU was quite important for improving the robustness of the superhydrophobic coating. What’s more, surface morphologies of the coating before and after abrasion for 80 cycles were characterized by SEM for making clear the mechanism of the mechanical resistance. Obviously, micro/nano structure and pores piled up by La2O3 particles are observed in Figure 8e. And the abrasive mark by sand paper is showed in Figure 8f, but

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it didn’t impair the superhydrophobic property of the coating because of the exposure of the rest La2O3 particles and remained nano structure even though the upper La2O3 particles were removed during the abrasion.

Figure 8. (a) Illustration of the mechanical durability test; the pictures of the transparent, brownish yellow and blue water droplets made from pure water, CuSO4 and FeCl3 solution stand on F-HPU-17 coated glass surface before abrasion (b) and after abrasion for 80 cycles (c), (d) the water contact angle of the coating F-HPU-17 changes with the abrasion cycles; SEM images of the coating F-HPU-17 before abrasion (e) and after abrasion for 80 cycles (f).

4. CONCLUSION In summary, a series of composite coatings with durable superhydrophobic properties based on fluorinated hyperbranched polyurethane synthesized through stepwise polymerization and rare earth metal oxide La2O3 were prepared by a simple spraying method. The La2O3 can be firmly adhered on the substrate owing to the strong adhesive

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of the hyperbranched polyurethane. When the content of La2O3 was 17%, the coating exhibited a maximum contact angle of 157° with excellent superhydrophobic property and resisted a sand paper abrasion of 80 cycles. Additionally, the robust superhydropobic coating can also float on water surface for one month and resist the impact of strong water flow from water tap. Furthermore, the composite can be sprayed on many substrate surfaces such as glass, bricks, cotton fabrics, iron sheets and sponges, all of which exhibits ultrahydrophobic properties. The robust superhydrophobic coating shows broad prospects in anti-fouling, protecting coatings because of its simple, common, durable and efficient. ASSOCIATED CONTTENT Supporting Information

Sliding behavior on the composite coating surface of F-HPU-17 (Movie S1); coated filter paper by F-HPU-17 floated on the water surface without being damped (Movie S2); The composite coating of F-HPU-17 kept dry after being impacted by strong water flow (Movie S3); The water droplet of 5µL bounced back and forth on the coating surface for 5 times (Movie S4). The contact angles of different liquids on different substrates coated by F-HPU-17. The photos of coating surface without F-HPU and the damaged coating surface without F-HPU. The surface morphologies of various substrates without coating and with coating. The coating surface morphologies and their contact angles of La2O3-no-F-HPU, HPU-La2O3 and F-HPU-SiO2. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (Zushun Xu)

E-mail: [email protected] (Guangfu Liao) Author Contributions 1

These two authors contributed equally to this project.

Note

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The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China. (Grant No. 51573039, 51273058, 8571734, 81372712, 81372369). Thank Prof. Hao Yang for kindly help in CA and SA measurement.

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