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Colorful Superamphiphobic Coatings with Low Sliding Angles and High Durability Based on Natural Nanorods Jie Dong, Qin Wang, Yujie Zhang, Zhaoqi Zhu, Xianghong Xu, Junping Zhang, and Aiqin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13539 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016

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ACS Applied Materials & Interfaces

Colorful Superamphiphobic Coatings with Low Sliding Angles and High Durability Based on Natural Nanorods Jie Dong,†,┴ Qin Wang,† Yujie Zhang,† Zhaoqi Zhu,‡ Xianghong Xu,§ and Junping Zhang†* and Aiqin Wang† †

Key Laboratory of Clay Mineral Applied Research of Gansu Province, and State Key

Laboratory for Oxo Synthesis & Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 730000, Lanzhou, P.R. China, ‡Department of Chemical Engineering, College of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, P.R. China, §Department of Biotherapy Center, Gansu Provincial Hospital, 730000, Lanzhou, P.R. China, and ┴Graduate University of the Chinese Academy of Sciences, 100049, Beijing, P.R. China. * Address correspondence to [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Superamphiphobic coatings with low sliding angles (SAs) and high durability are very attractive in academic and industrial areas, but are very challenging to invent. Here, inspired by Maya Blue, we report for the first time colorful superamphiphobic coatings with low SAs and high durability by the combination of natural palygorskite (PAL) nanorods and organosilanes. The coatings were characterized using a wide range of electron microscopy and other analytical techniques. Different from the previously reported methods, the micro-/nanostructure of the superamphiphobic coatings were constructed by using the abundant natural PAL nanorods as the building blocks. Superamphiphobicity of the coatings depends on surface morphology and chemical composition of the coatings, which can be regulated by the concentrations of PAL and organosilanes. The colorful superamphiphobic coatings feature high contact angles and low SAs for various liquids including water and n-decane. The coatings also showed high mechanical, environmental, chemical and thermal durability even under harsh conditions. Moreover, the coatings in different color with comparable superamphiphobicity and durability can be prepared using different cationic dyes and applied onto various substrates via the same approach. The colorful superamphiphobic coatings with low SAs and high durability may be useful in various fields, e.g., anti-creeping of oils and restoration of cultural relics.

KEYWORDS: superoleophobic, silanes, Maya Blue, superhydrophobic, attapulgite

INTRODUCTION Wettability is one of the most important physicochemical properties of solid surfaces.1,

2

Inspired by the unique water-repellent surfaces of the lotus leaf and the leg of the water strider in the natural world,3,

4

superhydrophobic surfaces have attracted intensive attention in both


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fundamental research and industrial applications in the past two decades.1,

5, 6

Thousands of

artificial superhydrophobic surfaces have been prepared via different methods, and have applications in various fields such as self-cleaning,7,

8

anti-corrosion,9,

10

anti-icing,11 anti-

fouling,12, 13 and oil/water separation.14-16 Different from superhydrophobic surfaces, superamphiphobic surfaces have contact angles (CAs) higher than 150° for both water and organic liquids of low surface tension, and have more extensive potential applications in oil transportation, microfluidic and anti-creeping of lubrication oils, etc.17-19 However, it is still very challenging to create superamphiphobic surfaces because of the very low surface tension of organic liquids, e.g., n-decane (23.8 mN m-1), compared to that of water (72.8 mN m-1).20-22 The simple combination of a hierarchical rough surface and materials of low surface tension is unlikely to form superamphiphobic surfaces with excellent superamphiphobicity, i.e., low sliding angles (SAs) for organic liquids of low surface tension, and high durability.23, 24 In fact, it is very difficult in theory to create superamphiphobic surfaces that liquids of low surface tension could roll off easily with low SAs because the solidliquid interaction must be very weak.25 Until now, only a few studies have reported superamphiphobic surfaces with high CAs and low SAs for organic liquids by designing some special micro-/nanostructures such as reentrant structures, candle soots, overhang structures and silicone nanofilaments (SNFs) or by using fluoroPOSS with very low surface tension.5, 26-28 30

29,

For example, Tuteja and Cohen et al. created excellent superamphiphobic surfaces based on

reentrant structures and fluoroPOSS.26 Liu and Kim designed a surface superrepellent even to completely wetting liquids based on doubly reentrant structures.31 Although very encouraging results have been achieved, there are some crucial issues remained to be solved in the field. The low durability, especially the low mechanical durability, is still a common problem for most


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superamphiphobic surfaces. Also, the preparation of superamphiphobic surfaces often requires complicated methods, and expensive materials and equipment to generate the surfaces with special artificial micro-/nanostructures. These issues have seriously restricted the large scale preparation and practical applications of superamphiphobic coatings. From the viewpoint of practical applications, colorful superhydrophobic coatings have recently received much attention. Colorful superhydrophobic coatings could be created by using structural color32 and coloring species like pigments and dyes.33, 34 Gu et al. fabricated a colorful superhydrophobic inverse opal film with a nanostructured surface.35 Li et al. prepared colorful superhydrophobic coatings via chemical reactions between inorganic salts and sodium stearate.36 However, colorful superamphiphobic coatings have not yet been invented so far. This is because it is very difficult to give consideration to the color of the coatings in designing the micro/nanostructures and surface chemical composition of superamphiphobic coatings. For colorful coatings, fading and fouling are the frequently encountered problems. The stability of colorful coatings could be enhanced by incorporating the coloring species into rigid matrices such as polymers and inorganic materials, thereby restricting the mobility of the coloring species and the diffusion of oxygen.37, 38 For example, Maya Blue is a hybrid pigment composed of palygorskite (PAL) and indigo from the leaves of añil plant, and was widely used in mural paintings and ceramic pieces in Yucatan by Mayan.39-42 PAL is a natural phyllosilicate clay mineral with a unique nanorod-like microstructure.43 The high aspect ratio, large surface area and high adsorption capacity of PAL are of great benefit for its applications in the acceptance of guest molecules44, 45 and the preparation of organic-inorganic hybrid materials.46, 47 The host-guest interactions between PAL and indigo generate Maya Blue with extraordinarily high stability against acids, alkalis, organic solvents and UV irradiation. On the other hand, the


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stability of colorful coatings may also be enhanced by introducing a superhydrophobic or superamphiphobic coating. This is because wetting of a solid surface by a liquid is essential for the interaction between them. Superhydrophobic or superamphiphobic coatings with liquid droplets in the Cassie-Baxter state can trap a thin layer of air cushion at the solid-liquid interface, which could effectively prevent the direct contact between them, and then enhance the stability of

coatings

against

corrosive

liquids.

Compared

to

superhydrophobic

coatings,

a

superamphiphobic coating could prevent corrosion of a coating by both aqueous solutions and organic liquids. Moreover, a superamphiphobic coating could make colorful coatings antifouling and self-cleaning even encountered with organic liquids, superior to the lotus leaf. Here, inspired by Maya Blue, we report for the first time colorful superamphiphobic coatings with low SAs and high durability by the combination of the Maya Blue-like pigments, 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (PFDTES) and tetraethoxysilane (TEOS). Different from the previously reported superampiphobic coatings, the surface micro-/nanostructure of the coatings in this study was constructed using the natural PAL nanorods by a simple spray-coating method. The colorful superamphiphobic coatings feature high CAs and low SAs for various liquids, and high durability under various harsh conditions.

RESULTS AND DISCUSSION Preparation of Colorful Superamphiphobic Coatings. The colorful superamphiphobic coatings were prepared by the combination of the Maya Blue-like pigments, PFDTES and TEOS (Figure 1). One of the Maya Blue-like pigments was prepared by adsorption of Astrozon Brilliant Red 4G (ABR), a cationic dye, onto PAL. The other pigments were prepared according


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to the same procedure using different cationic dyes. The as-prepared red PAL/ABR pigment (1.96 wt% ABR) was then modified with polymerized perfluoroalkylsilane (fluoroPOS) by hydrolytic condensation of PFDTES and TEOS via a modified Stöber method.48 In an ethanolwater solution with ammonia as the catalyst, PFDTES and TEOS gradually co-condensed onto the surface of PAL/ABR via the Si-O-Si bonds to form the homogeneous suspension of PAL/ABR@fluoroPOS. Subsequently, the colorful superamphiphobic coatings were obtained simply by spray-coating the mixture of the PAL/ABR@fluoroPOS suspension and PFDTES in ethanol onto various substrates including glass, wood and plastic.

Figure

1.

(a)

Schematic

illustration

for

preparation

of

the

superamphiphobic

PAL/ABR@fluoroPOS coatings. SEM and TEM images of (b, e) PAL, (c, f) PAL/ABR and (d, g) PAL/ABR@fluoroPOS coatings. CPAL = 15 g L-1, CPFDTES-i = 22.7 mM and CTEOS = 8.9 mM.


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The microstructures of PAL, PAL/ABR and PAL/ABR@ fluoroPOS were observed using the scanning electron microscopy (SEM) and transmitting electron microscopy (TEM), and are shown in Figure 1. The nanorod-like crystals of PAL developed very well (Figures 1b and 1e). The surface of the PAL nanorods in PAL/ABR became rough compared to PAL according to the TEM image (Figure 1f), which is owing to the adsorption of ABR onto the PAL nanorods. However, this difference cannot be clearly seen via SEM owing the uniform distribution of ABR on the surface of the PAL nanorods. The presence of fluoroPOS on the surface of the PAL/ABR@fluoroPOS nanorods was confirmed by TEM, and some free fluoroPOS could also be seen (Figure 1g). Moreover, some aggregates of the PAL/ABR@fluoroPOS nanorods were observed and the micro-/nanostructure was formed (Figures 1d and 1g), as the PAL/ABR nanorods were linked together by fluoroPOS. Thus, fluoroPOS acts as a crosslinker among the PAL/ABR nanorods. In addition, the thickness of the PAL/ABR@fluoroPOS coating is about 24.47 µm according to the cross-sectional SEM image (Figure S1). The adsorption of ABR onto PAL, and further modification with fluoroPOS were also studied by Fourier transformed infrared (FTIR) spectroscopy (Figure 2a). In the spectrum of PAL, three strong bands attributed to H-O-H (coordinated and zeolitic water) of PAL appear at 3532, 3434 and 1623 cm-1. In the spectrum of PAL/ABR, the band at 3434 cm-1 shifted to 3426 cm-1 and became weak. This is due to the loss of zeolitic water and a part of adsorbed water during adsorption of ABR. In the spectrum of PAL/ABR@fluoroPOS, the absorption bands corresponding to the C-F group at 1241 and 1208 cm-1 can be easily recognized.49 The band at 1208 cm-1 was attributed to both the C-F group and the Si-O group (1196 cm-1). The band at 1150 cm-1 is attributed to the possible silsesquioxane bands stemming from the polycondensation of hydrolyzed PFDTES and TEOS. In the spectrum of PAL/ABR@fluoroPOS, the -OCH2CH3


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bands (956, 820 and 778 cm-1) of PFDTES were not detected, suggesting complete hydrolysis of PFDTES.

Figure 2. (a) FTIR spectra, (b) XPS spectra and (c) C 1s XPS spectra of PAL, PAL/ABR and PAL/ABR@fluoroPOS coatings. CPAL = 15 g L-1, CPFDTES-i = 22.7 mM and CTEOS = 8.9 mM. The surface chemical compositions of the PAL, PAL/ABR and PAL/ABR@fluoroPOS coatings were studied by X-ray photoelectron spectroscopy (XPS, Figure 2b-c). The C, O, Si, Mg and Al elements were detected on the surface of PAL. The adsorption of ABR onto PAL did not result in any obvious change of the XPS spectrum, as ABR is mainly composed of C and O elements. XPS measurements confirmed the presence of C, O, F and Si elements on the surface of the PAL/ABR@fluoroPOS coating. The F 1s peak is very strong, and the CF2 (290.58 eV) can be easily recognized in the C 1s spectrum of the PAL/ABR@fluoroPOS coating (Figure 2c). This means a very high F content (45.14 at.%) on the surface of the coating, which could effectively decrease the surface energy of the coating. The C/O/F atomic ratio on the surface of


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the PAL/ABR@fluoroPOS coating is 1:1.45:2.80. The chemical compositions of the coatings were also analyzed by energy-dispersive X-ray spectroscopy (EDS), and the results are consistent with the XPS analysis (Figure S2). In addition, the PAL/ABR@fluoroPOS coating is very homogeneous as verified by the C, O, F and Si elemental maps (Figure S3). Effects of PFDTES, TEOS and PAL on Superamphiphobicity. As is well known, wettability of a solid surface depends on both chemical composition and topography of the surface. The effects of the concentrations of PFDTES (CPFDTES-i), TEOS (CTEOS) and PAL (CPAL) on superamphiphobicity of the PAL/ABR@fluoroPOS coatings were systematically investigated in this section. Wettability of the PAL/ABR@fluoroPOS coatings strongly depends on the CPFDTES-i in the preparation of the PAL/ABR@fluoroPOS suspension (Figure 3a). This is because the content of the perfluorodecyl groups on the surface of the PAL/ABR@fluoroPOS coatings determines the surface energy. The PAL/ABR coating is superamphiphilic, and the droplets of water and various organic liquids could easily wet and penetrate into the coatings. Once a small amount of PFDTES (CPFDTES-i = 4.5 mM) was used to modify the PAL/ABR nanorods, the coating became superhydrophobic with the water droplets in the Cassie-Baxter state (CAwater = 163.9° and SAwater = 2.8°). However, the effect of the CPFDTES-i is obviously different when n-decane is used as the probe. The coating with a CPFDTES-i of 4.5 mM still can be wetted by n-decane (CAn-decane = 40.2°). The CAn-decane increased significantly to 147.7° with increasing the CPFDTES-i to 13.7 mM, whereas the n-decane droplets adhered strongly on the coating even when the coating was turned upside down. This means the n-decane droplets are in the Wenzel state on the coating. The CAndecane

continuously increased to 149.2° with increasing the CPFDTES-i to 18.2 mM, and remained

almost constant with further increasing the CPFDTES-i to 29.6 mM. The SAn-decane is 20.8°-24.3°


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and the n-decane droplets could roll off the PAL/ABR@fluoroPOS coatings in the range 18.227.3 mM CPFDTES-i, indicating a transition from the Wenzel state to the Cassie-Baxter state. It is surprising that a proper concentration of TEOS is helpful to enhance the superamphiphobicity, as the SAn-decane decreased from 24.2° to 18.6° with increasing the CTEOS from 0 to 8.9 mM (Figure 3b). However, the further increase of the CTEOS to 17.8 mM resulted in increase of the SAn-decane to 22.5°. The surface microstructure and the surface chemical composition of the PAL/ABR@fluoroPOS coatings with different CTEOS were analyzed by SEM (Fgiure S4) and XPS (Figure S5 and Table S1), respectively, in order to investigate the influences of TEOS. It was found that the coatings with higher surface roughness were formed in the presence of TEOS as shown in Fgiure S4. This is because TEOS acts as a coupling agent and could induce the hydrolytic condensation of PFDTES on the surface of the PAL/ABR nanorods. Thus, the PAL/ABR nanorods were linked together by more fluoroPOS, which could cause aggregation of the PAL/ABR nanorods as mentioned above. Consequently, the coatings with higher surface roughness were generated in the presence of TEOS. On the other hand, evident changes in the F content and the C/O/F atomic ratio were detected with increasing the CTEOS from 0 to17.8 mM. The F content increased from 37.75 at.% to 41.52 at.% with increasing the CTEOS from 0 to 8.9 mM, which further confirmed that TEOS could induce the condensation of PFDTES onto the PAL/ABR nanorods. Then, the F content decreased to 36.67% with further increasing the CTEOS to 17.8 mM. This should be attributed to the hydrophilic nature of the hydrolytic product of TEOS. Thus, the change in the SAn-decane with the CTEOS is owing to the simultaneous changes in the surface microstructure and the surface chemical composition.


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Figure 3. Variation of CAn-decane and SAn-decane of the PAL/ABR@fluoroPOS coatings with (a) CPFDTES-i (CPAL = 10 g L-1 and CTEOS = 4.5 mM) and (b) CTEOS (CPAL = 10 g L-1 and CPFDTES-i = 22.7 mM). The superamphiphobicity of the PAL/ABR@fluoroPOS coatings is also highly dependent on the CPAL as shown in Figure 4. In the absence of PAL/ABR, the n-decane droplets showed a CAn-decane of 67.8° on the fluoroPOS coating. The CAn-decane increased observably from 67.8° to 153.9° with increasing the CPAL from 0 to 5 g L-1, and then remained constant with further increasing the CPAL to 20 g L-1. The SAn-decane suddenly decreased to 18° once the PAL/ABR nanorods with a CPAL of 5 g L-1 were introduced. The SAn-decane slightly decreased to 14.1° with increasing the CPAL to 10-15 g L-1, and then increased to 22° with further increasing the CPAL to 20 g L-1. The results indicate that all the n-decane droplets are in the Cassie-Baxter state when


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the CPAL is in the range of 5 to 20 g L-1. Besides, the surface morphology of the PAL/ABR@fluoroPOS coating without any PAL/ABR was also observed by SEM for comparison (Figure S6).

Figure 4. (a) Variation of CAn-decane and SAn-decane of the PAL/ABR@fluoroPOS coatings with CPAL. SEM images of the PAL/ABR@fluoroPOS coatings with a CPAL of (b) 5, (c) 10, (d) 15 and (e) 20 g L-1. CPFDTES-i = 22.7 mM and CTEOS = 8.9 mM. It has been shown that the described changes of the CAn-decane and SAn-decane are due to the introduction

of

PAL/ABR

and

the

different

CPAL

in

the

preparation

of

the


12

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PAL/ABR@fluoroPOS suspensions. The PAL/ABR nanorods act as the building blocks of the micro-/nanostructure of the superamphiphobic PAL/ABR@fluoroPOS coatings. It was found that the behavior of the n-decane droplets on the coatings is closely related to the micro/nanostructure of the coatings, which can be regulated simply by changing the CPAL (Figure 4). With a CPAL of 5 g L-1, a large part of the PAL/ABR nanorods were embedded in fluoroPOS and a rough coating was formed. The surface micro-/nanostructure of the coating obtained under this condition is sufficient to support the n-decane droplets in the Cassie-Baxter state with a SAn-decane of 18°. With increasing the CPAL to 15 g L-1, the PAL/ABR nanorods were loosely linked together by fluoroPOS and the surface roughness was enhanced. Such a surface micro/nanostructure could trap more air beneath the n-decane droplets, which is responsible for the decrease of the SAn-decane to 15°. The further increase of the CPAL to 20 g L-1 did not obviously change the micro-/nanostructure. Thus, the increase of the SAn-decane to 22° at a CPAL of 20 g L-1 should be attributed to the hydrophilic nature of the PAL/ABR nanorods. According to the changes of the CAn-decane and SAn-decane with CPFDTES-i, CTEOS and CPAL, we can conclude that the SAs are more sensitive than the CAs in response to the changes in surface micro-/nanostructure and chemical composition of a superamphiphobic coating. Also, the SAs are closely related to the unique self-cleaning properties of the bioinspired superhydrophobic or superamphiphobic coatings. So, we should pay much attention to the changes in the SAs besides the CAs in evaluating the stability of a superhydrophobic or superamphiphobic coating. In order to further improve superamphiphobicity and durability of the PAL/ABR@fluoroPOS coating, we used the mixture of the PAL/ABR@fluoroPOS suspension and a proper amount of PFDTES (CPFDTES-ii) to fabricate the coatings. The changes of the CAn-decane and the SAn-decane with CPFDTES-ii are shown in Figure 5a. A CPFDTES-ii of 0-7.6 mM has no obvious influence on the


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CAn-decane. Whereas the SAn-decane slightly decreased from 14.1° to 12.6° with increasing the CPFDTES-ii from 0 to 7.6 mM. This is because a proper amount of PFDTES could form additional crosslinking points with fluoroPOS in the coatings and decrease the surafce energy without obviously changing the surface micro-/nanostructure as shown in Figure 4d (CPFDTES-ii = 0 mM) and Figure 5b (CPFDTES-ii = 7.6 mM). The further increase in the CPFDTES-ii to 18.9 mM results in the gradual increase of the SAn-decane to 18.7°, as the excess PFDTES may decrease the surface roughness by covering the nanorods. The effects of the CPFDTES-ii on the CAn-decane and the SAn-decane in the water jetting test are shown in Figure 5c-d. Water jetting test is a commonly used method to evaluate the mechanical durability of a superhydrophobic or superamphiphobic coating based on the existing literatures.50 The CAn-decane of all the coatings decreased from ~157° to ~154° after water jetting at 50 kPa for 1 min, and then slightly decreased to ~153° with further increasing the time to 10 min. The CPFDTES-ii did not show obvious influence on the CAn-decane in the water jetting test (Figure 5c), however, big difference in the SAn-decane was recorded between the coatings (Figure 5d). The coating with a CPFDTES-ii of 0 mM showed the lowest durability, as the SAn-decane increased significantly to 37.5° with increasing the jetting time to 3 min, and then increased gradually to 41° with further increasing the time to 10 min. The introduction of PFDTES into the PAL/ABR@fluoroPOS suspension observably improved mechanical durability of the coating, as the SAn-decane remained lower in the water jetting test compared to the coating with a CPFDTES-ii of 0 mM. The coating with a CPFDTES-ii of 7.6 mM showed the highest durability in the water jetting test. Also, the burst increase of the SAn-decane in the first 3 min was overcome. The improved mechanical durability is owing to the formation of the additional crosslinking points between PFDTES and fluoroPOS.


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Figure 5. (a) Variation of CAn-decane and SAn-decane of the PAL/ABR@fluoroPOS coatings with CPFDTES-ii, (b) SEM image of the PAL/ABR@fluoroPOS coating with a CPFDTES-ii of 7.6 mM, effects of CPFDTES-ii on (c) CAn-decane and (d) SAn-decane in the water jetting test at 50 kPa. CPAL = 15 g L-1, CPFDTES-i = 22.7 mM and CTEOS = 8.9 mM. Superamphiphobicity of PAL/ABR@fluoroPOS Coatings. After verifying the relationship between preparation parameters, surface micro-/nanostructure and chemical composition, the superamphiphobicity of the coatings was further tested by recording the CAs, SAs, and kinetic behaviors of various typical liquids with different surface tensions. The CAs, SAs and bouncing times of the liquids on the PAL/ABR@fluoroPOS coating are shown in Table 1. All the liquids investigated, even n-decane, had high CAs (>154°, Figure 6a) and low SAs (

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