Smooth Water-Based Antismudge Coatings for Various Substrates

Research Article pubs.acs.org/journal/ascecg

Smooth Water-Based Antismudge Coatings for Various Substrates Xu Wu,*,† Minhuan Liu,† Ximing Zhong,† Guojun Liu,*,‡ Ian Wyman,‡ Zhengping Wang,*,† Yaqian Wu,§ Hui Yang,§ and Jinben Wang§ †

Department of Chemistry and Chemical Engineering, Guangzhou University, 230 Outer Ring West Road, Guangzhou, Guangdong 510006, China ‡ Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada § Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, 2 Zhongguancun North Road, Beijing 100190, China S Supporting Information *

ABSTRACT: Smooth particle-free antismudge coatings show potential for various applications because they are not prone to the limitations that plague rough self-cleaning surfaces such as poor durability and transparency. These smooth coatings are typically prepared from solvent-based precursors due to their requirement for amphiphobic moieties. We report herein a facile strategy to prepare from water-based precursors smooth antismudge coatings that can be readily applied onto various substrates including metal, wood, paper, and glass. These novel coatings exhibit unprecedented antismudge properties even with a thickness of only 5.0 ± 0.5 μm and contain only 0.7744 wt % of fluorinated polymer. In addition, these transparent coatings retain their antismudge properties even after they are subject to bending, impact, scratching, abrasion, corrosion, UV irradiation, and thermal shock tests. KEYWORDS: Antismudge coatings, Self-cleaning coatings, Water-based precursors, Various substrates, Smooth coatings

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INTRODUCTION

Second, without the roughness required for extremely high contact angles, we recently demonstrated that smooth surfaces were able to stay clean after they had been exposed to various liquids when they had been enriched with low-surface-tension liquid moieties.2 This phenomenon has been observed on slippery liquid-infused porous surfaces (SLIPS),20−22 on monolayers of grafted poly(dimethylsiloxane)23,24 and perfluorinated polyether,25,26 as well as on TEFLON,27 while our previous coatings distinguished themselves by their optical clarity, the tunable thickness and their facile and economical preparation.2,27,28 It is noteworthy that these smooth antismudge coatings were not hindered by the limitations that typically plague rough amphiphobic coatings. Both rough and smooth antismudge coatings are typically prepared from solvent-based precursors (with rare exceptions29,30), and this strategy utilizes costly and environmentally harmful solvents such as tetrahydrofuran, acetonitrile, and dimethyl carbonate.2,10−14,27,28 Ideally, these coatings should be prepared from water-based precursors to ensure that they are cost-effective and environmentally friendly. However, the lowsurface-tension components required for these coatings are inherently insoluble in water. In addition, the incorporation of hydrophilic functional groups needed for a water-based

Coatings play an important role in endowing materials with desirable surface properties. Antismudge coatings are known to repel both water- and oil-borne contaminants, thus extending the lifetimes of their substrates and minimizing the need for cleaning.1,2 Two fundamental principles have been proposed to help with the successful design of these coatings. First, hierarchical rough surfaces (particularly those having roughness on both the micro- and nanoscale as well as re-entrant surface structures) with low surface free energies can exhibit superamphiphobic behavior, having exceptionally high contact angles toward both water and oil (exceeding 150°) with sliding angles that are typically smaller than 10°.3−7 Numerous methods have been employed to prepare rough antismudge surfaces. For example, the substrates have been pretreated by plasma etching,8 the precursors have been melted and molded into fibers via electrospinning,9 and particles such as SiO2,10 TiO2,11 ZnO,12 multiwalled carbon nanotubes (MWCNTs),13 or raspberry-like polymer particles14 have been incorporated as hybrid coating components. However, rough coatings have certain limitations. For example, they often exhibit poor durability (as their intricate surfaces are easily damaged,15,16 while particle-based coatings typically do not adhere strongly to their substrates17). In addition, rough surfaces scatter light, which renders them unsuitable for applications requiring transparent materials.18,19 © 2017 American Chemical Society

Received: December 6, 2016 Revised: January 19, 2017 Published: January 30, 2017 2605

DOI: 10.1021/acssuschemeng.6b02957 ACS Sustainable Chem. Eng. 2017, 5, 2605−2613

Research Article

ACS Sustainable Chemistry & Engineering

resultant coatings.33,34 Finally, PFPG was used to provide the low-surface-tension and it is widely believed to be an environmentally friendly fluorinated material.35−37

component can weaken the resultant coating’s antismudge properties, and the use of water as a solvent can weaken the binding between a coating and its substrate due to dewetting effects encountered during drying.30,31 Our group recently reported a smooth antismudge coating that can be fabricated using a water-soluble precursor, while the strategy involved electrophoretic deposition and was applicable only for conductive substrates.29 Herein, we have extended our previous work and designed a versatile strategy using waterbased components to prepare smooth antismudge coatings that can not only be applied onto conductive materials, but also onto a wide range of substrates. Furthermore, it was discovered that the novel components allowed these coatings to retain their antismudge properties, even when the fluorinated content and the thickness were significantly decreased. The water-based formulation contains a water-dispersible polyacrylate-based polymers (WPAs), a random copolymer of perfluorinated poly(propylene oxide and ethylene oxide) glycol (PFPG), and a hexamethylene diisocyanate trimer (HDIT). The structures of the WPAs, PFPG, and HDIT along with the coating strategy are shown in Figure 1. Our strategy involves

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RESULTS AND DISCUSSION In order to optimize the coating strategy, a series of WPAs bearing different hydroxyl groups were investigated. The formulations used to prepare the WPAs and the hybrid coating solutions are listed in Tables S1 and S2, which are provided in the Supporting Information (SI). The water and diiodomethane contact angles of the resultant coatings are shown in Figures S1 and S2 in the SI. Based on the amphiphobicity of the resultant coatings, the hybrid solution containing WPA-3, PFPG (1.000 wt % with respect to the WPA), and HDIT (at an isocyanate-to-hydroxyl molar ratio of 1.500:1.000) was found to provide a promising candidate for the preparation of antismudge coatings. It was noteworthy that the applied PFPG content accounted for only 0.1449 wt % of the hybrid solution and represented only 0.7744 wt % of the resultant coating. The details regarding the characterization of WPA-3 and HDIT by 1H NMR and size-exclusion chromatography (SEC) analysis as well as the characterized structure of WPA-3 are provided in the SI (Figures S3, S4, S5, and S6, respectively). The subscripts x, y, z, and i in Figure 1 for each unit of WPA-3 were determined to be ∼0.29, ∼ 0.34, ∼ 0.10, and ∼0.15, respectively, and the overall repeat unit number n was determined to be ∼126. Figure 2a shows an image of the WPA-3 solution (14.49 wt %) and of the hybrid solution containing WPA-3 (14.49 wt %), PFPG (0.1449 wt %), and HDIT (5.397 wt %). The WPA-3 solution was blue and semitransparent, while the hybrid component containing WPA-3, PFPG, and HDIT yielded a milky white solution. The aggregates formed by WPA-3 and by the hybrid component were characterized via dynamic light scattering (DLS) measurements (Figure 2b). Those aggregates that were observed in the WPA-3 dispersion had diameters that were on the scale of tens of nanometers. Apparently, WPA-3 self-assembled into aggregates that bore anionic acid units on their external surfaces.38 These smaller aggregates were less numerous in the mixed dispersion, while larger aggregates with diameters on the scale of hundreds of nanometers were visible. It is noteworthy that WPA-3, PFPG, and HDIT were thoroughly mixed together prior to their dispersal into water, and that PFPG and HDIT have poor solubility in water. With this in mind, we believe that PFPG and HDIT were localized within the core of the larger mixed aggregates while the anionic acrylic acid groups of WPA-3 occupied the surfaces.2,29 In addition, it is likely that the thoroughly mixed aggregates could readily undergo cross-linking during the curing treatment, and the uniform distribution of PFPG would yield a coating matrix exhibiting antismudge properties not only on its surface but also throughout its bulk matrix. The formation of aggregates with diameters ranging from 5 to 20 nm may be attributed to intramolecular interactions between the WPA-3 units.39 The aggregates formed by WPA-3 and the hybrid components were further investigated via cryogenic transmission electron microscopy (cryo-TEM, Figures 2c−f). Both the WPA aggregates and the mixed aggregates exhibited spherical morphologies, which we had targeted via the selfassembly strategy. The diameters and the size distributions of the aggregates observed in the TEM images were consistent with those determined via DLS measurements.

Figure 1. Chemical structures of the WPA, PFPG, and HDIT (left). Schematic depiction of the coating procedure (right).

the self-assembly of the precursors first in water, along with subsequent coating and curing processes. For the self-assembly process, the WPAs were used as an amphiphilic carrier to disperse the PFPG and HDIT into the aqueous solution. After the hybrid aggregates were drop-cast onto the substrates, the resultant coating was subjected to a heating treatment to facilitate the cross-linking chemistry and the removal of the charges required in order to disperse the precursor into water. The WPAs, PFPG, and HDIT were all multifunctional and readily underwent condensation reactions that took place between the isocyanate groups of HDIT and the hydroxyl groups of the WPAs as well as PFPG, thus facilitating the preparation of the targeted cross-linked coating matrix and enhancing the coating’s long-term amphiphobicity and robustness.2,28,32 In addition, the charges were eliminated due to the evaporation of triethylamine and the disappearance of ion pairs formed between the ammonium and carboxylate moieties of the WPAs, thus limiting the undesired hydrophilicity of the 2606


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Figure 2. Images of the WPA-3 solution (left, 14.49 wt %) and the corresponding hybrid coating solution (right) containing 14.49 wt % WPA-3, 0.1449 wt % PFPG, and 5.397 wt % HDIT (a). DLS trace of the WPA-3 solution (14.49 wt %) and that of the hybrid solution containing 14.49 wt % WPA-3, 0.1449 wt % PFPG, and 5.397 wt % HDIT (b). Cryo-TEM images of the aggregates observed in the WPA-3 solution (14.49 wt %, c and d). Cryo-TEM images of the mixed aggregates in the hybrid coating solution containing 14.49 wt % WPA-3, 0.1449 wt % PFPG, and 5.397 wt % HDIT (e and f). These solutions were freshly prepared prior to the DLS and TEM characterization.

Figure 3. Three-dimensional (3D) topography image of the coating (a). Images of droplets of water, diiodomethane, and hexadecane recorded at various times after their application onto the coating (b). Plots of the contact angles of the droplets at various times after their application (c). Snapshots displaying the movement of water, diiodomethane, and hexadecane droplets on the coating at various times (d). These coatings were applied onto a tin substrate.

As the hybrid solution was cast onto the substrate, the subsequent thermal curing would evaporate the solvent, facilitate the condensation reaction, and yield a coating with a thickness of 5.0 ± 0.5 μm. FT-IR characterization indicated that the isocyanate groups were completely consumed during the curing process (Figure S7 in the SI). Compared with our previous antismudge coatings prepared via electrophoretic deposition, the thickness of these novel antismudge coatings was decreased from 70 ± 2 to 5.0 ± 0.5 μm, the content of the fluorinated components in the resultant coatings was decreased from 7.479 to 0.7744 wt %, and these coatings can not only be applied onto conductive materials but also onto a wide range of substrates.29 In addition, a previous study suggested that the ionic species existing in the form of ammonium and carboxylate

groups could be eliminated due to the evaporation of TEA when a polymer containing this salt was heated at 120 °C for 20 min.33 In our case, the coating was cured at 140 °C for 1 h and the ionized moieties of WPA-3 should have been removed. This should have helped to enhance the hydrophobicity of the resultant coatings. SEM characterization revealed that the surface and the crosssection of the coating were very smooth and that they did not exhibit evidence of particles (Figure S8). The surface of the coating was also characterized via AFM, which provided further evidence that no particles were present and revealed that the root-mean-square roughness was only 4.79 nm (Figure 3a). We believe that the smoothness of the surface may be attributed to the aggregates becoming fused together during the curing 2607


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ACS Sustainable Chemistry & Engineering treatment, ultimately yielding a highly cross-linked matrix. The hardness of the resultant coating was found to be 3H according to ASTM D 3363-00 standards, which is above the median score on the hardness scale and is a suitable hardness for various coating applications.40 Contact angle and sliding angle measurements were performed in order to evaluate the wettabilities of our surfaces with respect to water, diiodomethane, and hexadecane droplets, which had surface tensions of ∼72, 50, and 27 mN m−1, respectively.41 The contact angles of these liquid droplets observed immediately after they were applied onto the coating surface were 107 ± 3°, 89 ± 2°, and 65 ± 2°, respectively (Figure 3b). A higher degree of roughness can cause contact angles to become smaller when the intrinsic contact angle is 65°.33,42 Thus, even with only 0.7744 wt % of the PFPG, the incorporation of micro- or nanoparticles with the hybrid component could cause the coating’s water and oil contact angles to increase. In our previous work, we found that PFPG had promising potential to replace long-chain perfluorinated compounds as components of superhydrophobic and highly oleophobic self-cleaning coatings.32 After the test liquids were initially applied onto the coating, the contact angles tended to decrease over time (Figures 3b and c). It is possible that the liquid PFPG moieties adopted stretched conformations and covered more of the coating surface in air but contracted to cover less of the coating surface when they came into contact with a test liquid.43,44 The strong tendency of the PFPG moieties to contract resulted from their oily consistency and their low adhesion properties.45 Smaller changes in the contact angles were observed for hexadecane, which had the lowest surface tension. The PFPG moieties could not shrink extensively upon exposure to hexadecane, as this liquid was compatible with neither the PFPG moieties nor the other coating components. Figure 3d provides a comparison of the sliding behaviors of the different liquid droplets that were placed on our coatings. The sliding angles of water, diiodomethane, and hexadecane droplets (20 μL) were 57.5 ± 0.9°, 9.1 ± 0.3°, and 13.5 ± 0.4°, respectively. At the above sliding angles, it took the droplets ∼9, 5, and 6 s, respectively, to slide a distance of 3 cm along the coating surfaces. All of the liquids slid cleanly down the coatings without leaving residue along their paths. After the droplets had rested on the original positions where they had been applied onto the coatings for 20 min, the sliding angle of water was above 90°, while those of diiodomethane and hexadecane grew only slightly larger to 10.3 ± 0.3° and 13.8 ± 0.4°, respectively. The time dependent sliding properties could also be attributed to the surface conformations of the PFPG moieties. The coating also exhibited excellent repellency toward pump oil, cooking oil, and alcohol. These liquids left enduring liquid films on uncoated surfaces that did not contract or slide (Figure S9 in the SI). In the case of the coating surface, the pump oil films had contracted and the oil gradually flowed away from the coatings without leaving any noticeable traces (Figures 4a, b, c, and Movie S1 in the SI). Photographs demonstrating the shrinking and gliding behavior of cooking oil and alcohol are also shown in Figure S9 as well as Movies S2 and S3 in the SI. In addition, the ink-repellency of the antismudge coatings was also examined. A permanent marker readily left a uniform and prominent trace on an uncoated section of a tin plate, but this

Figure 4. Images of pump oil films on coated tin plates (a, b, and c). Traces of ink left by a permanent marker on uncoated and coated tin plates before (d) and after (e) the surface was wiped with a tissue. Ink traces left on the surface of a wood plate that was uncoated on the lefthand side but coated on the right-hand side (f). Ink traces left on the surface of a piece of uncoated and coated paper (g). Droplets of various common liquids, along with 50% KOH and 98% H2SO4 sitting on the coated paper (h). Ink traces left on an uncoated and a coated glass plate (i). Traces of artificial fingerprint liquid left on uncoated (j) and coated (k) glass plates, which were magnified via a microscope.

ink contracted when it was applied onto a coated tin plate (Figure 4d). Ink that had been applied onto the uncoated region and subsequently dried left behind a persistent marking that could not be wiped away. Meanwhile, ink that had been applied onto the surface contracted into a faint uneven mark and could be easily wiped away with a tissue (Figure 4e). These observations suggested that the coating could help prevent unwanted ink deposition onto a surface and also facilitate the subsequent removal of any ink markings. With this in mind, this coating could potentially be used to protect surfaces against graffiti. In order to illustrate the versatility of our coating, wood, paper, and glass were also used instead of tin plates as other kinds of commonly employed substrates. The coated wood, paper, and glass also exhibited similar outstanding antismudge properties comparable with that of the coated tin plate (Figures 4f, g, and i, and images of the ink traces after wiping are shown in Figure S10 in the SI). In addition, the uncoated region of the wood substrate only had a gloss value of 10 ± 1 GU, while the coated region exhibited a gloss value of 74 ± 1 GU, so that it would thus be classified as a high-gloss surface.46 Due to its high degree of glossiness and desirable visual appearance, this coating could have potential use in enhancing the aesthetic appearance of commercial wood products such as furniture.47 Figure 4h shows droplets of cola, milk, coffee, red wine, 2608


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Figure 5. Ink traces left on a section of a coated tin plate that had been subjected to bending tests (a). Opposite (left-hand side) and front (righthand side) sides of a coated tin plate after it had been subjected to an impact, and the traces of ink left on the damaged coating surface (b). Coating bearing gridlike knife scratches and traces left on the scratched surface (c). Ink traces left on a coating surface that had been subjected to 25 cycles of the sandpaper abrasion experiment (d). Ink traces left on a coating surface that had been subjected to 1500 cycles of the cotton fabric abrasion test (e). XPS analysis of the coating surface before and after the cotton fabric abrasion test (f). Tin plate bearing coated and uncoated sections after immersion in a 10 wt % NaCl solution for 48 h, and ink traces left on the coated region (g).

soybean oil, as well as 50% KOH and 98% H2SO4 sitting on the surface of the coated paper. The coating endowed the paper with enhanced antiwetting properties toward these common liquids as well as corrosives. Meanwhile, the uncoated paper became stained and damaged by the above liquids within seconds (Figure S10c in the SI). This behavior suggests that the coating could be used to protect vulnerable surfaces, such as valuable paintings and other fragile materials. When glass was used as the substrate, it was noteworthy that the coating was transparent in the visible region, possessing a transmittance of more than 97%. Thus, the coating could potentially be used on the touchscreen displays of smartphones or tablets. In a second antismudge test, the repellency of the coating toward an artificial fingerprint liquid (consisting of lactic acid, acetic acid, sodium chloride, sodium hydrogen phosphate, 1methoxy-2-propanol, hydroxyl-group-terminated polydimethylsiloxane, and deionized water) was investigated.48 An aqueous ink solution (1 wt %) was added to the artificial fingerprint liquid in order to present a clearer enlarged image of the trace that was impressed onto the uncoated and coated glass plates (Figures 4j and k). This liquid readily wetted the uncoated glass plate, while the liquid contracted into distinct droplets when it was applied onto the coated surface. Thus, the coating rejected the “fingerprint” liquid and exhibited outstanding antifingerprint performance. Bending, impact, knife scratching, sandpaper abrasion, fabric abrasion, salt solution corrosion, fluorescent UV, and thermal shock tests were performed to evaluate the robustness of the coating as well as the durability of its antismudge properties. As demonstrated by the bending test (Figure 5a), the coating remained securely bound to the tin substrate even after it was subjected to severe bending. Moreover, subjecting a coated tin plate to an impact (Figure 5b) did not weaken the adhesion between the coating and the tin or cause the coating to rupture. The resilience of this coating was observed both on the impacted side and the side opposite to the impact. In addition, the bending and impact damage had no significant effect on the

self-cleaning properties of the coating. The adhesion of the coating was also investigated via an ASTM D3359 standard test.30 None of the small cross-cut squares in the grid-like pattern had detached from the tin plate after a piece of tape was applied to the damaged coating and subsequently peeled away, thus demonstrating that our coating qualified for the adhesion scale’s top ranking of 5B.30 In addition, the knife scratches did not stop the ink traces from contracting, indicating that this damage did not influence the coating’s antismudge properties. In the case of the wooden and glass substrates, none of the squares were lost from the cross-cut patterns left on the coatings after the adhesion test (Figure S11 in the SI), indicating that the coatings applied onto these surfaces also had the highest ranking of 5B on the adhesion scale. After the coating was subjected to 25 cycles of the sandpaper abrasion experiment, 1.0 ± 0.3 μm of the coating was consumed (Figure 5d). It was noteworthy that the ink had contracted despite the removal of the top layer of the coating, although the extent of the contraction was reduced. In order to avoid the influence of the abrasion caused by roughness, a cotton fabric was used instead of the sandpaper for the second abrasion test. After 1500 cycles of the fabric abrasion, 1.0 ± 0.2 μm of the coating was consumed (Figure 5e), and the remaining layer of the smooth coating exhibited outstanding antismudge performance. These findings confirmed that the high degree of cross-linking enabled the PFPG moieties to become spread throughout the coating matrix in addition to enriching the surface. Therefore, the PFPG on the outer surface would be replenished by deeply buried PFPG moieties as the outer surface wore down, thus ensuring that the newly exposed surface still had a low surface energy. In order to further confirm the internal functional structure of our coating matrix, the pristine coating surface as well as the newly exposed surface obtained after the abrasion (∼1 μm of the coating was consumed) were investigated via XPS (Figure 5f). The atomic abundance of the coating surfaces are shown in Table S3 in the SI, and the analysis indicated that the F content 2609



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decreased to 8.41% on the newly exposed surface from 11.6% on the pristine surface. The reduction in the F content that was observed when the initial surface was worn down and the inner matrix became freshly exposed was reasonable upon consideration that the assemblies were anticipated to fuse during the coating curing process, and the PFPG moieties would thus migrate upward and become enriched on the surface due to their low surface energy.45 In addition, the properties of the freshly exposed surfaces were further investigated after they had been subjected to numerous abrasion circles. The contact angles and sliding angles toward water, diiodomethane, and hexadecane droplets, as well as the anti-ink and transmittance properties of the coatings after the abrasion tests are shown in Table S4 in the SI. Although the decreased contact angles and the increased sliding angles were observed along with the abrasion, the ink traces on the freshly exposed surfaces could still be wiped away. The transmittance spectra are shown in Figure S12, indicating that the transmittance increased slightly when the coating became thinner. Therefore, our coating had outstanding wear resistance and the antismudge properties were retained even when the surface of the coating was consumed by abrasion. As a further test of the coating’s durability, a tin plate that bore coated and uncoated regions was placed in a 10 wt % NaCl solution for 48 h (Figure 5g). Although rust had developed on the uncoated region, no rust was visible on the coated section. In addition, this immersion into the NaCl solution did not appear to damage the coating, thus demonstrating that the coating was highly robust. The high degree of corrosion resistance may have been due to the repellency and impermeability of the coating with respect to the NaCl solution. Strikingly, even after exposure to the NaCl solution for this prolonged period, the excellent antismudge performance of the coating became apparent as soon as the residual NaCl liquid was removed. Finally, the ability of the coatings to withstand weathering was evaluated by exposing them to fluorescent UV and thermal shock tests for various times. Tables S5 and S6 in the SI provide summaries of the contact angles and sliding angles toward various liquids (water, diiodomethane, and hexadecane), as well as the ink-resistance, hardness, and adhesion properties. We found that the duration of the weathering tests did not significantly influence these properties. Considering the robustness of this coating and its durable antismudge properties, we believe that materials bearing this antismudge coating will have prolonged service lifetimes and may be useful for applications where they may be exposed to physical wearing and chemical corrosion, as well as temperature changes.

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EXPERIMENTAL SECTION

Materials. Various reagents, including methyl methacrylate (MMA), butyl acrylate (BA), acrylic acid (AA), 2-hydroxypropyl methacrylate (HPMA), 2,2′-azobis(2-methylpropionitrile) (AIBN), and triethylamine (TEA) were purchased from Hersbit Chemical Co. Ltd. These precursors were of analytical reagent grade. A random copolymer of perfluorinated poly(propylene oxide and ethylene oxide) glycol (PFPG, with an average equivalent molecular weight of 750 g/ mol, fluorine content of 57 wt %, surface tension at 20 °C of 23 mN/ m, kinematic viscosity at 20 °C of 115 cSt, glass transition temperature of −100 °C, and a specific gravity at 20 °C of 1.73 g/cm3) was purchased from Solvay and distilled under reduced pressure prior to use.32 Hexamethylene diisocyanate trimer (HDIT, which had a NCO content of 21.8 ± 0.3 wt %) was purchased from Wanhua Chemicals Co., Ltd., and was used without further purification. n-Butyl acetate, diiodomethane, and hexadecane were all of analytical reagent grade and were purchased from Tianjin Damao Chemical Reagent Factory. Sandpaper (Grit No. 2000) was purchased from a local store. Synthesis of the WPAs. The WPAs were synthesized via free radical polymerization (Figure 1a). Typically, n-butyl acetate (90.00 g, 0.7748 mol) and AIBN (0.4000 g, 0.002436 mol) were added into a 500 mL four-necked round-bottom flask. This flask was equipped with feeding inlets, a mechanical stirrer, a thermometer, and a reflux condenser. The reaction mixture was purged with nitrogen for 30 min at room temperature before the flask was subsequently heated to 90 °C. Another mixture of MMA (38.72 g, 0.3867 mol), BA (44.00 g, 0.3433 mol), AA (7.000 g, 0.097 14 mol), HPMA (10.28 g, 0.071 32 mol), AIBN (0.4000 g, 0.002 436 mol), and n-butyl acetate (5.000 g, 0.043 04 mol) was added dropwise into the flask over a period of ∼2 h. AIBN (0.4000 g, 0.002436 mol) and n-butyl acetate (5.000 g, 0.04304 mol) were subsequently added into the above mixture, and the reaction was then allowed to proceed for another 12 h at 90 °C. This reaction mixture was subsequently cooled to room temperature, and the AA units were neutralized via the addition of TEA to obtain WPA1. The other WPAs were prepared using similar polymerization procedures except for variations in the monomer mass ratios, in order to obtain polymers bearing different amounts of cross-linkable units but with the same glass transition temperature (∼283 K). The monomer contents of the WPAs are summarized in Table S1 in the SI. Preparation of the Coatings. The compositions of the hybrid components are listed in Table S2 in the SI. The WPA, PFPG, and HDIT were thoroughly mixed together, and distilled water was subsequently added into the above mixture under vigorous stirring to achieve a 20 wt % solution. The obtained solution was shaken on a vortex machine for ∼10 s and subsequently drop-cast onto the substrates. The standard curing procedure that was performed at 140 °C for 1 h was subsequently implemented after the resultant coating was dried in a desiccator under a gentle nitrogen flow for 20 min. Characterization. The WPA was characterized via 1H NMR spectroscopy using a Bruker Avance-500 spectrometer. These characterizations were performed using acetone-d6 as the solvent. Samples of the WPA were characterized via SEC for molecular weight determination. Tetrahydrofuran (THF) was employed as the mobile phase (flow rate = 1.0 mL min−1). This characterization was performed at 40 °C using a Waters 515 system that was equipped with a Waters 2410 refractive index (RI) detector. Calibration was performed in THF using narrowly dispersed polystyrene standards. Dynamic light scattering (DLS) characterization was performed in order to determine the aggregate diameters encountered in the WPA solution and those in the mixed solution containing WPA, PFPG, and HDIT. These experiments were performed using a laser light scattering (LLS) spectrometer (ALV/SP-125) that was equipped with a multi-τ digital time correlater (ALV-5000). A solid-state He− Ne laser (22 mW) was used to generate the incident beam (λ = 632.8 nm). In addition, a scattering angle of 90° was used for these measurements, which were performed at 25.0 ± 0.1 °C. The correlation function was evaluated via CONTIN analysis.49 The aggregate morphologies observed in the WPA solution and those in the mixed solution containing WPA, PFPG, and HDIT were

CONCLUSIONS

A novel versatile and smooth antismudge coating was prepared from water-based hybrid precursors. The designed strategy included the assembly of the water-based precursors, the subsequent elimination of the ionic groups, and achieving a high degree of cross-linking in the coating matrix. This coating has numerous merits such as compatibility with a wide range of substrates, an extremely low fluorinated content, a facile coating procedure, use of environmentally friendly and cost-effective components, and outstanding antismudge properties, as well as excellent transparency and robustness. 2610


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(hardest).43 Each sharpened pencil was fixed to a VF2378 pencil hardness tester (Thermimport Quality Control, Netherlands) and pressed against the coated surface. The process began with the hardest pencil and proceeded toward softer pencils until the tested pencil was unable to scratch the surface. The glossiness (in gloss units of “GU”) of the coatings was measured using an IG-330 gloss meter (Horiba, Japan) according to the standard ISO 2813 protocol.51 The coated substrate was illuminated by a red diode laser (650 nm) at an angle of 60.0°. Five measurements were performed to yield an average value, which corresponded to each reported glossiness value. For the first abrasion test, the coated surface of a tin plate was placed facing downward toward the sandpaper. The strength of the abrasion cycles was enhanced with a 100 g weight that was placed above the plate to increase the force of the abrasion cycles. A single abrasion cycle corresponded to 10 cm of movement in each longitudinal and transverse direction.7 An A20-339 abrasion test system (Chuangheng, China) was used for the second abrasion test. A piece of cotton fabric was used as the abrasion material, and a weight of 500 g was placed above the fabric to enhance the force of the abrasion. One abrasion cycle corresponded to a total of 10 cm of the fabric’s movement against the coating. The X-ray photoelectron spectroscopy (XPS) measurements were performed using an electron takeoff angle of 45° and an XPS sampling depth of ∼6.6 nm.27 Samples of the coatings were drop-cast onto a mica plate and cured according to the previously described procedure performed for the other investigated substrates. The coated tin plates were irradiated with UV (λ = 340 nm) light at 30 ± 5 °C that was generated by a ZN-TX fluorescent UV weathering device (Beijing Zhongke Ring test Apparatus Co., Ltd.).29 In addition, the thermal shock tests were conducted on the coated tin plates according to the IEC 60068-2-1:2007 protocol using a HTS Thermal Shock Chamber (Dongguan Huatian Equipment Co., Ltd.).52 During these tests, the samples were subjected to multiple 12 h thermal shock cycles, which each included 2 h of cooling at −5 °C and 10 h of heating at 50 °C. Samples were collected after they had been subjected to 24, 47, 72, 96, 120, 144, and 168 h of exposure to the UV and thermal shock tests (two samples were collected at each time for each of the two experiments). The contact angles, sliding angles, anti-ink, hardness, and adhesion behavior of these collected coatings were subsequently evaluated.

investigated via cryo-TEM. The samples were prepared in an environmentally controlled environment vitrification system (CEVS).50 The droplets of freshly prepared sample solutions were placed onto copper TEM grids that were coated with a support film, and they were then soaked by the filter papers to form a thin liquid film. The copper grids were quickly immersed into a reservoir of liquid ethane, which was cooled to −165 °C with liquid nitrogen. The vitreous specimens were kept under liquid nitrogen until they were loaded into a cryogenic sample holder (Gatan 626) and examined using a JEOL JEM-1400 transmission electron microscope that was operated at 120 kV and at approximately −174 °C. Fourier-transform infrared (FT-IR) spectroscopy was employed to monitor the consumption of the isocyanate groups from the coatings. These characterizations were performed using a Tensor-27 spectrometer (Bruker Optics, Germany). In addition, KBr was employed as the sample matrix. These spectra were recorded in the range of 4000− 400 cm−1 at 25.0 ± 0.1 °C. A CM-8825FN digital coating thickness gauge with an accuracy of ±1−3% (Landtex, China) was employed to measure the coating thickness. Meanwhile, the transmittance of the coated glass was measured using a Varian CARY 300 Bio UV−visible spectrometer. The reported value represented the average of three measurements performed at a wavelength of 500 nm. The morphology of the coatings was observed using a Hitachi S-530 scanning electron microscope (SEM) and a Multimode atomic force microscopy (AFM) instrument that was operated in the ScanAsyst TM mode. The coating was drop-cast onto a polytetrafluoroethene plate and carefully removed and placed on the sample stage after the curing procedure for the SEM test. The coating was drop-cast onto a mica plate, and the AFM experiment was subsequently implemented after the curing procedure. These curing procedures were similar to that performed to treat the coatings on the other investigated substrates. Properties. The axisymmetric drop shape analysis (ADSA) method was employed for the contact angle (CA) measurements, which were conducted at 25.0 ± 0.1 °C using a JC2000A contact angle measuring instrument (Shanghai Zhongchen Powereach, China). The wettability tests were performed using ∼2 μL droplets of water, diiodomethane, and hexadecane as test liquids. Each reported contact angle corresponded to the average of five measurements. A homemade device that has been described in our previous work was employed for the sliding angle measurements.32 A level was used to adjust the tilting angle of the coated tin plates with respect to the horizontal plane. Droplets (20 μL) were allowed to fall onto the surface of the plate when it was in a horizontal alignment. The inclination of the plate was incrementally increased by adjusting the height of the shaft of the vertical micrometer. The angle between the plate and the horizontal plane at which the droplets began to roll corresponded to the sliding angle. Each reported value corresponded to the average of ten measurements that were recorded at various positions on the coating. The ASTM D3359 standard protocol was employed to evaluate the adhesion between the coatings and the substrates.30 The surfaces of the coatings were cut with a blade in a cross-cut pattern (4 × 4 lines) to form a grid. The distance between each line was ∼3 mm. A piece of 3 M 250 masking tape was subsequently placed at an angle of 45° with respect to the lines and pressure was applied continuously onto the surface to prevent the formation of air pockets beneath the tape. The tape was kept in contact with the coating for 45 s to provide ample opportunity for it to adhere to the coating. This tape was subsequently peeled away and the sample was examined for evidence of damage. A QCJ-50 membrane impact tester (Shanghai Modern Environmental Engineering Technology, China) was employed to evaluate the effects of an impact on the adherence and self-cleaning properties of the coating. After the coated tin plates were placed on the stage, an impact hammer (1000 ± 1 g) was allowed to fall from a height of 50 cm (thus inducing an impact) onto the coatings. The pencil hardness test was performed using a set of Faber Castell 9000 pencils with hardness values ranging from 6B (softest) to 6H

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02957. Figures S1−S12; water and diiodomethane contact angles of the coatings containing various HPMA and PFPG contents; 1H NMR spectra of WPA-3 and HDIT; SEC trace and structure of WPA-3; FT-IR spectra of the coating; SEM images of the coating surface and the cross-section; image of the behavior of pump oil, cooking oil, and alcohol on the uncoated tin plates; ink traces left behind by a permanent ink marker on a wood plate, a paper surface, and a glass plate that bearing uncoated and coated regions after wiping; cross-cut patterns remaining on the coated wooden and glass substrates after the adhesion test; droplets of common liquids and corrosive liquids siting on uncoated paper; transmittance curves for the coating with various abrasion cycles; Tables S1−S6; formulations employed to prepare the WPAs and the hybrid coating solutions; atomic percentage of the coating surfaces before and after the abrasion test; properties of the coatings with various abrasion cycles; properties of the coating after the UV irradiation and thermal shock tests for various durations (PDF) 2611


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Video of pump oil gradually flowing away from the coating without leaving any noticeable traces (AVI) Video of cooking oil gradually flowing away from the coating without leaving any noticeable traces (AVI) Video of alcohol gradually flowing away from the coating without leaving any noticeable traces (AVI)

(12) Steele, A.; Bayer, I.; Loth, E. Inherently superoleophobic nanocomposite coatings by spray atomization. Nano Lett. 2009, 9, 501−505. (13) Zhu, X. T.; Zhang, Z. Z.; Ren, G. N.; Men, X. H.; Ge, B.; Zhou, X. Y. Designing transparent superamphiphobic coatings directed by carbon nanotubes. J. Colloid Interface Sci. 2014, 421, 141−145. (14) Jiang, W. J.; Grozea, C. M.; Shi, Z. Q.; Liu, G. J. Fluorinated raspberry-like polymer particles for superamphiphobic coatings. ACS Appl. Mater. Interfaces 2014, 6, 2629−2638. (15) Kota, A. K.; Choi, W.; Tuteja, A. Superomniphobic surfaces: design and durability. MRS Bull. 2013, 38, 383−390. (16) Zhao, H.; Park, K. C.; Law, K. Y. Effect of surface texturing on superoleophobicity, contact angle hysteresis, and “robustness. Langmuir 2012, 28, 14925−14934. (17) Xue, C. H.; Ma, J. Z. Long-lived superhydrophobic surfaces. J. Mater. Chem. A 2013, 1, 4146−4161. (18) Budunoglu, H.; Yildirim, A.; Guler, M. O.; Bayindir, M. Highly transparent, flexible, and thermally stable superhydrophobic ORMOSIL aerogel thin films. ACS Appl. Mater. Interfaces 2011, 3, 539−545. (19) Deng, X.; Mammen, L.; Zhao, Y. F.; Lellig, P.; Müllen, K.; Li, C.; Butt, H. J.; Vollmer, D. Transparent, thermally stable and mechanically robust superhydrophobic surfaces made from porous silica capsules. Adv. Mater. 2011, 23, 2962−2965. (20) Wong, T. S.; Kang, S. H.; Tang, S. K. Y.; Smythe, E. J.; Hatton, B. D.; Grinthal, A.; Aizenberg, J. Bioinspired self-reparing slippery surfaces with pressure-stable omniphobicity. Nature 2011, 477, 443− 447. (21) Solomon, B. R.; Khalil, K. S.; Varanasi, K. K. Drag reduction using lubricant-impregnated surfaces in viscous laminar. Langmuir 2014, 30, 10970−10976. (22) Tesler, A. B.; Kim, P.; Kolle, S.; Howell, C.; Ahanotu, O.; Aizenberg, J. Extremely durable biofouling-resistant metallic surfaces based on electrodeposited nanoporous tungstite films on steel. Nat. Commun. 2015, 6, 8649. (23) Cheng, D. F.; Urata, C.; Yagihashi, M.; Hozumi, A. Hozumi, A statically oleophilic but dynamically oleophobic smooth nonperfluorinated surface. A. Angew. Chem., Int. Ed. 2012, 51, 2956−2959. (24) Cheng, D. F.; Urata, C.; Masheder, B.; Hozumi, A. physical approach to specifically improve the mobility of alkane liquid drops. A. J. Am. Chem. Soc. 2012, 134, 10191−10199. (25) Cheng, D. F.; Masheder, B.; Urata, C.; Hozumi, A. Smooth perfluorinated surfaces with different chemical and physical natures: Their unusual dynamic dewetting behavior toward polar and nonpolar liquids. Langmuir 2013, 29, 11322−11329. (26) Block, S.; Kleyer, D.; Hupfield, P.; Kitaura, E.; Itami, Y.; Masutani, T.; Nakai, Y. New anti-fingerprint coatings. Paint Coat. Ind. 2008, 24, 88−92. (27) Hu, H.; Liu, G. J.; Wang, J. Clear and durable epoxy coatings that exhibit dynamic omniphobicity. Adv. Mater. Interfaces 2016, 3, 1. (28) Rabnawaz, M.; Liu, G. J.; Hu, H. Fluorine-free anti-smudge polyurethane coatings. Angew. Chem. 2015, 127, 12913−12918. (29) Zhong, X. M.; Wyman, I.; Yang, H.; Wang, J. B.; Wu, X. Preparation of robust anti-smudge coatings via electrophoretic deposition. Chem. Eng. J. 2016, 302, 744−751. (30) Milionis, A.; Dang, K.; Prato, M.; Loth, E.; Bayer, I. S. Liquid repellent nanocomposites obtained from one-step water-based spray. J. Mater. Chem. A 2015, 3, 12880−12889. (31) Wang, J. Z.; Zheng, Z. H.; Li, H. W.; Huck, W. T. S.; Sirringhaus, H. Dewetting of conducting polymer inkjet droplets on patterned surfaces. Nat. Mater. 2004, 3, 171−176. (32) Chen, J. Y.; Zhong, X. M.; Lin, J.; Wyman, I.; Zhang, G. W.; Yang, H.; Wang, J. B.; Wu, J. Z.; Wu, X. The facile preparation of selfcleaning fabrics. Compos. Sci. Technol. 2016, 122, 1−9. (33) Xu, Z. G.; Zhao, Y.; Wang, H. X.; Wang, X. G.; Lin, T. A superamphiphobic coating with an ammonia-triggered transition to superhydrophilic and superoleophobic for oil-water separation. Angew. Chem. 2015, 127, 4610−4613. (34) Liu, S. H.; Han, G.; Shu, M. H.; Han, L.; Che, S. N. Monodispersed inorganic/organic hybrid spherical colloids: Versatile

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.W.). *E-mail: [email protected] (G.L.). *E-mail: [email protected] (Z.W.). ORCID

Xu Wu: 0000-0002-8907-6073 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank the Natural Science Foundations of China (21406040), the Important National Science and Technology Specific Project of China (2016ZX05013003-004 and 2016ZX05025003-007), the Science and Technology Project of Guangdong Province (2015A050502052), as well as the Science and Technology Project of Guangzhou (201610010018 and 201607010285) for sponsoring this research.

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REFERENCES

(1) Brown, P. S.; Bhushan, B. Mechanically durable, superomniphobic coatings prepared by layer-by-layer technique for selfcleaning and anti-smudge. J. Colloid Interface Sci. 2015, 456, 210−218. (2) Rabnawaz, M. G.; Liu, J. Graft-copolymer-based approach to clear, durable, and anti-smudge polyurethane coatings. Angew. Chem. 2015, 127, 6616−6620. (3) Tuteja, A.; Choi, W.; Ma, M. L.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Designing superoleophobic surfaces. Science 2007, 318, 1618−1622. (4) Chu, Z. L.; Seeger, S. Superamphiphobic surfaces. Chem. Soc. Rev. 2014, 43, 2784−2798. (5) Deng, X.; Mammen, L.; Butt, H. J.; Vollmer, D. Candle soot as a template for a transparent robust superamphiphobic coating. Science 2012, 335, 67−70. (6) Vogel, N.; Belisle, R. A.; Hatton, B.; Wong, T. S.; Aizenberg, J. Transparency and damage tolerance of patternable omniphobic lubricated surfaces based on inverse colloidal monolayers. Nat. Commun. 2013, 4, 2167−2176. (7) Lu, Y.; Sathasivam, S.; Song, J. L.; Crick, C. R.; Carmalt, C. J.; Parkin, I. P. Robust self-cleaning surfaces that function when exposed to either air or oil. Science 2015, 347, 1132−1135. (8) Ellinas, K.; Pujari, S. P.; Dragatogiannis, D. A.; Charitidis, C. A.; Tserepi, A.; Zuilhof, H.; Gogolides, E. Plasma micro-nanotextured, scratch, water and hexadecane resistant, superhydrophobic, and superamphiphobic polymeric surfaces with perfluorinated monolayers. ACS Appl. Mater. Interfaces 2014, 6, 6510−6524. (9) Choi, G. R.; Park, J.; Ha, J. W.; Kim, W. D.; Lim, H. Superamphiphobic web of PTFEMA fibers via simple electrospinning without functionalization. Macromol. Mater. Eng. 2010, 295, 995− 1002. (10) Zhang, G. W.; Lin, S. D.; Wyman, I.; Zou, H. L.; Hu, J. W.; Liu, G. J.; Wang, J. D.; Li, F.; Liu, F.; Hu, M. L. Robust superamphiphobic coatings based on silica particles bearing bifunctional random copolymers. ACS Appl. Mater. Interfaces 2013, 5, 13466−13477. (11) Ganesh, V. A.; Dinachali, S. S.; Nair, A. S.; Ramakrishna, S. Robust superamphiphobic film from electrospun TiO2 nanostructures. ACS Appl. Mater. Interfaces 2013, 5, 1527−1532. 2612


Research Article

ACS Sustainable Chemistry & Engineering synthesis and their gas-triggered reversibly switchable wettability. J. Mater. Chem. 2010, 20, 10001−10009. (35) Malinverno, G.; Pantini, G.; Bootman, J. Safety evaluation of perfluoropolyethers, liquid polymers used in barrier creams and other skin-care products. Food Chem. Toxicol. 1996, 34, 639−650. (36) Brady, R. F.; Aronson, C. L. Elastomeric fluorinated polyurethane coatings for nontoxic fouling control. Biofouling 2003, 19, 59−62. (37) Ding, N.; Jose, S. High-density lipoprotein coated medical devices. Patent US 7959659B2, 2011. (38) He, F.; Gädt, T.; Manners, I.; Winnik, M. A. Fluorescent “barcode” multiblock co-micelles via the living self-assembly of di- and triblock copolymers with a crystalline core-forming metalloblock. J. Am. Chem. Soc. 2011, 133, 9095−9103. (39) Wu, X.; Qiao, Y. J.; Yang, H.; Wang, J. B. Self-assembly of a series of random copolymers bearing amphiphilic side chains. J. Colloid Interface Sci. 2010, 349, 560−564. (40) Pedraza, E. P.; Soucek, M. D. Effect of functional monomer on the stability and film properties of thermosetting core-shell latexes. Polymer 2005, 46, 11174−11185. (41) Campos, R.; Guenthner, A. J.; Haddad, T. S.; Mabry, J. M. Fluoroalkyl-functionalized silica particles: synthesis, characterization, and wetting characteristics. Langmuir 2011, 27, 10206−10215. (42) Tian, Y.; Jiang, L. Wetting: Intrinsically robust hydrophobicity. Nat. Mater. 2013, 12, 291−292. (43) Vaidya, A.; Chaudhury, M. K. Surface properties of methacrylic copolymers containing a perfluoropolyether structure. J. Colloid Interface Sci. 2002, 249, 235−245. (44) Bongiovanni, R.; Malucelli, G.; Lombardi, V.; Priola, A.; Siracusa, V.; Tonelli, C.; Di Meo, A. Surface properties of methacrylic copolymers containing a perfluoropolyether structure. Polymer 2001, 42, 2299−2305. (45) Casazza, E.; Mariani, A.; Ricco, L.; Russo, S. Synthesis, characterization, and properties of a novel acrylic terpolymer with pendant perfluoropolyether segments. Polymer 2002, 43, 1207−1214. (46) Jančovičová, V.; Kindernay, J.; Jakubíková, Z.; Mrlláková, I. Influence of photoinitiator and curing conditions on polymerization kinetics and gloss of UV-cured coatings. Chem. Pap. 2007, 61, 383− 390. (47) Jančovičová, V.; Mikula, M.; Havlínová, B.; Jakubíková, Z. Influence of UV-curing conditions on polymerization kinetics and gloss of urethane acrylate coatings. Prog. Org. Coat. 2013, 76, 432− 438. (48) Wu, L. Y. L.; Ngian, S. K.; Chen, Z.; Xuan, D. T. T. Quantitative test method for evaluation of anti-fingerprint property of coated surfaces. Appl. Surf. Sci. 2011, 257, 2965−2969. (49) Wu, X.; Cai, X. X.; Hao, A. H.; Wang, J. B. Molecular design of brush-like amphiphilic statistical tripolymers and their self-assembly behaviors. J. Chem. Eng. Data 2013, 58, 927−931. (50) Zhao, Y. R.; Wang, J. Q.; Deng, L.; Zhou, P.; Wang, S. J.; Wang, Y. T.; Xu, H.; Lu, J. R. Tuning the self-assembly of short peptides via sequence variations. Langmuir 2013, 29, 13457−13464. (51) ENISO 2813: Paints and varnishes-determination of specular gloss of non-metallic paint films at 20°, 60° and 85°. CEN: Brussels, 1999. (52) Roitman, D.; Park, M.; Goyal, D.; Jose, S. Curable resins and articles made therefrom. Patent US 8197723, 2012.

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Smooth Water-Based Antismudge Coatings for Various Substrates

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