Rationally Designed, Multifunctional Self-Assembled Nanoparticles for

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Rationally Designed, Multifunctional Self-Assembled Nanoparticles for Covalently Networked, Flexible and Self-Healable Superhydrophobic Composite Films Yujin Lee, Eun-Ah You, and Young-Geun Ha ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19045 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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ACS Applied Materials & Interfaces

Rationally Designed, Multifunctional Self-Assembled Nanoparticles for Covalently Networked, Flexible and SelfHealable Superhydrophobic Composite Films Yujin Lee,a Eun-Ah You,*b and Young-Geun Ha*a a. Department of Chemistry, Kyonggi University, Suwon, Gyeonggi-Do, 16227, Republic of Korea. b. Center for Nano-Bio Measurement, Korea Research Institute of Standards and Science, Daejeon, 34113, Republic of Korea. KEYWORDS : multifunctional nanoparticle, covalent network, superhydrophobicity, hybrid composite, self-healing

ABSTRACT. For constructing bio-inspired functional films with various superhydrophobic functions including selfcleaning, anti-corrosion, anti-bioadhesion, and oil-water separation, hydrophobic nanomaterials have been widely used as crucial structural components. In general, hydrophobic nanomaterials, however, cannot form strong chemical bond networks in organic-inorganic hybrid composite films because of the absence of chemically compatible binding components. Herein, we report the rationally designed, multifunctional self-assembled nanoparticles with tunable functionalities of covalent cross-linking and hydrophobicity for constructing three-dimensionally interconnected superhydrophobic composite films via a facile solution-based fabrication at room temperature. The multifunctional self-assembled nanoparticles allow the systematic control of functionalities of composite films, as well as the stable formation of covalently linked superhydrophobic composite films with excellent flexibility (a bending radius of 6.5 and 3.0 mm, 1000 cycles) and self-healing ability (water contact angle > 150°, ≥ 10

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cycles). The presented strategy can be a versatile and effective route to generating other advanced functional films with covalently interconnected composite networks.

1. INTRODUCTION Bio-inspired functional nanostructures have attracted great attention for their potential applications in constructing films with various superhydrophobic functions such as self-cleaning,1–3 anti-icing, 4–6 anticorrosion,7–9 anti-bioadhesion,10–12 drag reduction,13–15 and oil-water separation.16–18 For the functional films, inorganic nanomaterials have been extensively used as primary building blocks and constituents because they can provide robust mechanical properties as well as functionalizable surface platforms.19– 26

In general, the surfaces of the inorganic nanomaterials used for superhydrophobic films have been

modified with hydrophobic molecules such as hydro- and fluoro-carbon based self-assembled monolayers (SAMs).27–31 These assembled low surface energy materials allow avoiding the aggregation of nanomaterials and possessing the hydrophobic characteristics of organic-inorganic hybrid composite films.32,33 However, the hydrophobic SAMs are not able to form strong chemical bond networks between nanomaterials and their surrounding components, which does not ensure the stable and robust film formation of the hybrid composites. Therefore, it is highly desirable to design and create multifunctionalized nanomaterials with moieties of both chemical binding and hydrophobicity for the formation of stably interconnected, advanced superhydrophobic composite films via strong bonding of chemically compatible binding components. Herein, we present the rational design and systematic synthesis of multifunctional self-assembled nanomaterials with tunable molecular functionalities of covalent cross-linking and hydrophobicity to produce covalently interconnected, highly flexible, durable and self-healable superhydrophobic composite films via a facile room-temperature process of spraying and UV irradiation. As a model system of multifunctionalized nanomaterials, we employed Al2O3 nanoparticles (NPs) as surface-


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functionalizable nanomaterials as well as the mixed SAMs of phosphoric acid 2-hydroxyethyl methacrylate ester (PHME) for covalent cross-linking34 and octadecylphosphonic acid (ODPA) for hydrophobicity.35 The multifunctional NPs allow the precise control of the surface coverage ratio of the heterogeneous molecular assemblies, which provides a crucial platform to not only tailor the compositions and functionalities of the materials but also achieve the optimization of film properties. For the complete formation of the flexible hybrid composite films, we included 1,6-hexanediol diacrylate (HDDA) as a bifunctional cross-linking agent as well as a flexible organic component of hybrid films. On the basis of the chemically compatible binding compositions, the multifunctional NPs with tunable ratios of hydrophobic ODPA to UV cross-linkable PHME SAMs can form covalently linked networks through surrounding methacrylate-terminated molecules including HDDA and other PHME SAMs on Al2O3 under UV irradiation. Therefore, the rational design and synthesis of the multifunctional NPs can play a pivotal role in the construction of robust superhydrophobic composite films with covalently interconnected 3D networks and optimized film properties. We demonstrate that the composition- and functionality-controlled synthesis of multifunctional self-assembled NPs allows the formation of advanced superhydrophobic films through covalently linked composite networks with the excellent multiple properties of flexibility, durability, and self-healing ability as well as roomtemperature processability, which can be further applied to plastic- and bio-electronics.

2. EXPERIMENTAL SECTION 2.1 Materials All chemical reagents were used as received without further purification. All organic solvents were purchased from Sigma-Aldrich and Dae-Jung Chemicals. Deionized (DI) water was obtained from Milli-Q water purification system (resistivity ~18 MΩ·cm, Millipore). 20 nm Al2O3 NPs (diameter < 20 nm, nanopowder) were purchased from Sky Spring Nanomaterials, USA. Octadecylphosphonic acid (ODPA, > 97 %), phosphoric acid 2-hydroxyethylmethacrylate ester (PHME, 90 %) and Irgacure 651


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were obtained from Sigma-Aldrich. 1,6-hexanediol diacrylate (HDDA, 99 %) was received from Alfa Aesar. Silicon wafers (150 mm diameter, N type doped with antimony, (100)-oriented, 525 µm thick) were supplied by LG Siltron, South Korea. Plastic substrates including polypropylene and polystyrene were purchased from Office Depot. 2.2 Synthesis and Characterization of Multifunctional Self-Assembled NPs Prior to the self-assembly of ODPA and PHME, Al2O3 NPs were dispersed in 2-propanol at a concentration of 0.04 g/mL by ultrasonication for 30 min. For the heterogeneous SAMs formed on Al2O3 NPs, the varied molar ratios of ODPA to PHME of 10:0, 7:3, 5:5, 3:7, and 0:10 were dissolved in the pre-dispersed NP solution so that the total concentration of ODPA and PHME was 18 mM for each ratio. All mixtures were ultrasonicated for 10 min and then stirred for 1 h at room temperature. For each suspension, multifunctional self-assembled NPs were collected by centrifugation at 5000 rpm for 30 min and then rinsed three times by repeating resuspension in 2-propanol and centrifugation (5000 rpm, 30 min). After being washed, the multifunctional NPs were dried in a vacuum oven at 100 °C overnight. Subsequently, the synthesized multifunctional NPs were characterized by thermogravimetric analysis (TGA) and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. The thermal degradation behaviors of the multifunctional self-assembled NPs and the ODPA and PHME reagents were examined by TGA performed under a nitrogen atmosphere at a heating rate of 10 °C/min up to 700 °C (TGA-DSC1, Mettler Toledo). The precisely controlled surface coverage ratios of ODPA to PHME on Al2O3 NPs were further proved by ATR-FTIR spectra measured with the FT-IR spectrometer (TENSOR 27, Bruker) using the ATR option and ZnSe crystal plate. Transmission spectra were collected at a resolution of 4 cm−1 by clamping 3 mg of dry powder of the multifunctional NPs to the ATR crystal plate. 2.3 Fabrication of Multifunctional NP-Based Composite Films


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To prepare the homogeneous suspensions of multifunctional NPs with different ratios of ODPA to PHME SAMs before spray coating, the synthesized multifunctional NPs (0.2 wt %) were dispersed in 5 mL of 2-propanol, and then the each suspension was ultrasonicated for 1 h. Subsequently, 0.003 M of HDDA as a cross-linking agent and 0.1 wt % of Irgacure 651 as a photo-initiator were added to make the complete suspensions of multifunctional self-assembled NPs for the formation of covalently interconnected superhydrophobic composite films. Before the film coating, various substrates, including silicon, glass, polypropylene, and polystyrene, were cleaned by ultrasonication in ethanol for 2 min and dried under a stream of nitrogen. They were then subjected to UV/ozone (UVO) cleaning (UVO cleaner, Yuil UV) for 5 min to remove organic contaminants and improve the wettability of substrates. Then, the multifunctional NP solution was sprayed over the substrates for 50 s via a spray coater (Airbrush, Anest Iwata) while the substrates were heated up to 80 °C to evaporate the solvent. Finally, these spray-coated films were cured under 365 nm UV irradiation (UV lamp output 4 W, Spectroline) for 1 h to accelerate the cross-linking reaction. 2.4 Film Characterization The morphologies of the films composed of multifunctional NPs were observed by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). The surface wetting properties were examined by using a contact angle measurement system (SEO Phoenix-I) under ambient conditions. The water contact angle (WCA) of each composite film sample was measured by the sessile drop method using a 5 µL droplet of DI water. All WCA values were averaged over 20 measurements on different areas of each sample. The sliding angle (SA) of each film was determined by using a tilting stage and the sample stage was tilted until the droplet (5 µL) started to move. For the evaluation of flexibility and self-healing ability of the films, we used the optimized composite films, made of the multifunctional NPs with the SAM ratio of ODPA to PHME of 7:3. For the bending test, the optimized composite film was fabricated on a polypropylene substrate (substrate thickness ≈ 200 µm) and the flexible composite film was wrapped around a glass tube with a radius of 6.5 and 3.0 mm. The WCA of


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the flexible composite film was measured as a function of the number of bending cycles, which was performed up to 1000 cycles. To examine self-healing ability of the multifunctional NP-based composite films, the optimized composite films (7:3) were exposed to various damage-inducing processes, including UVO, immersion in strong acid, and continuous impact with acidic raindrops. In essence, the pristine composite films were treated with UVO for 2 min, immersed in a strong acid solution (6 M HCl) for 10 min, and impacted by continuous acidic water droplets (80 µL, pH 5.0) of 3 L at a rate of 20 drops per min from a dropping height of 20 cm onto the sample stage with a slope of 45°. Then, the damaged films were heated at 100 °C for 30 min in a vacuum oven to recover. The damage/ healing processes were repeated from 10 to 20 cycles. To further verify the self-healing ability, the surface compositions of the differently treated composite films, such as initial, damaged, and healed films, were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi spectrometer, Thermo-Scientific) equipped with a monochromated Al Kα X-ray radiation source. The binding energy values were calibrated using the reference peak of C1s at 284.6 eV.

3. RESULTS AND DISCUSSION 3.1 Design and Synthesis of Multifunctional Self-Assembled NPs with Tunable Functionalities Figure 1a depicts the design and synthesis of multifunctional self-assembled NPs with tunable molecular functionalities for producing covalently networked, highly flexible, durable, and selfhealable superhydrophobic composite films. The multifunctional NPs were created by co-assembling phosphonic acid-based molecules with two different functionalities of PHME and ODPA onto Al2O3 NPs. Basically, phosphonic acid-based SAMs can be easily formed on metal oxide surfaces through metal-ligand coordination (M–O–P) between oxide and phosphonate.36–38 The phosphonic acid-based SAMs were thus composed of methacrylate-terminated PHME for covalent cross-linking and long alkyl-chained ODPA for hydrophobicity. The molecular functionalities were also tailored by mixing the varied molar ratios of ODPA to PHME such as 10:0, 7:3, 5:5, 3:7, and 0:10 (Figure 1b). The surface


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coverage ratios of the heterogeneous SAMs were confirmed by conventional analytical techniques including TGA and ATR-FTIR spectroscopy. Figure 2a shows the thermal degradation profiles of multifunctional self-assembled NPs with representative mixing ratios of ODPA to PHME (10:0, 5:5, and 0:10). For the ODPA-assembled NPs (10:0), the abrupt weight loss at 500 °C is attributed to the decomposition of the alkyl chains via C-C bond cleavage.39–41 With a gradual weight loss above 200 °C, the PHME-coated NPs (0:10) exhibited a total weight loss of 10 % at 700 °C, which was lower than that of ODPA-assembled NPs (total weight loss of 16.5 %) because of the relatively lower molecular weight of PHME. The thermal degradation behavior of ODPA- and PHME-coated NPs was corroborated by the similar TGA profiles of the corresponding ODPA and PHME molecules (Figure S1). For the multifunctional self-assembled NPs with a mixing ratio of 5:5, a total weight loss of 13 % corresponded to the average weight loss of ODPA- and PHME-coated NPs, revealing the equal surface coverage of the heterogeneous SAMs. The systematically controlled surface coverage ratios of ODPA to PHME were further verified by ATR-FTIR spectra. As the ODPA portion of the heterogeneous SAMs was lowered, the intensities of –CH2– (∼2850 and 2920 cm-1) and –CH3 (∼2970 cm-1) stretching modes, corresponding to the vibration of methylene and methyl groups of ODPA alkyl chain, gradually decreased (Figure 2b).41 Therefore, these results demonstrate that the multifunctionality of NPs can be precisely controlled by changing the mixing ratios of ODPA to PHME. 3.2 Covalently Linked, Multifunctional NP-Based Composite Films with Tunable Wettability Based on the multifunctional self-assembled NPs with tunable functionalities, we constructed covalently networked superhydrophobic composite films via a facile room-temperature process of spraying and UV irradiation (Figure 3). The long alkyl-chained ODPA SAMs of the multifunctional NPs can inhibit the aggregation of NPs and provide the hydrophobicity of the NP-based composite films. The methacrylate-terminated PHME SAMs on Al2O3 NPs enable covalent cross-linking with other methacrylate-terminated molecules through radical reaction upon exposure to UV light.42 In addition to the multifunctional self-assembled NPs, we thus included HDDA as a dimethacrylate cross-


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linker and a flexible organic component of hybrid composite films. More specifically, HDDA, containing the bifunctional cross-linking groups, can improve the cross-linking efficiency between NPs for covalently linked composite networks. In addition, HDDA can increase the organic portion of the hybrid composite films and thus it can work as a spacer between NPs to enhance the flexibility. Then, covalently interconnected hybrid composite films could be stably formed by spray-coating and subsequent UV-curing of a homogeneous suspension of multifunctional NPs and methacrylateterminated cross-linking agents. The multifunctional NPs with tunable surface coverage ratios of ODPA to PHME provide a pivotal platform to tailor the functionalities of the materials and therefore optimize the composite film properties. To generate robust and flexible superhydrophobic composite films, we investigated the effect of an ultrasonication-induced force on the wettability of the hybrid films composed of multifunctional NPs with various ratios of ODPA to PHME SAMs, as well as the influence of a repeated bending stress on the hydrophobicity of the optimized composite film. Figure 4 shows the variation in WCAs and SAs of the composite films of the multifunctional NPs with the different ratios of ODPA to PHME before and after ultrasonication. In general, the hydrophobicity of a surface depends on both surface roughness and surface chemistry.43–45 In particular, superhydrophobicity can be achieved by the combination of hierarchical micro/nano dual-scale surface roughness and chemical composition with low surface energy. The hierarchical roughness structures with low surface energy reduce the liquid-solid contact area by forming many trapped air pockets on the surface, and thus lead to the substantial increase of WCAs and the decrease of SAs.46–49 For the surface roughness shown in Figure 5 and Figure S2, all the composite films of multifunctional NPs with various SAM ratios of ODPA to PHME (10:0, 7:3, 5:5. 3:7, and 0:10) exhibited hierarchical micro/nano dual-scale surface roughness, which is a prerequisite for superhydrophobic surfaces. Therefore, the different wetting properties of the hybrid composite films, shown in Figure 4, are attributed to the varied proportion of hydrophobic ODPA SAMs on Al2O3 NPs, contributing to lower surface energy. The pristine composite films composed of the NPs with a larger


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proportion of ODPA such as 10:0 and 7:3 exhibited high WCAs (> 150°) and low SAs (< 5°), affording effective non-wetting superhydrophobic surfaces. For example, the WCA of the pristine film composed of the NPs with a SAM ratio of 7:3 was 155°, similar to that of the film of NPs with a SAM ratio of 10:0 (156°). As the ratio of ODPA to PHME was further lowered, the WCA of composite films decreased constantly (Figure 4 and Figure S3). In contrast with the pristine composite film, after ultrasonication (ethanol, 5 min.), the WCA of the film of the NPs with ODPA only (10:0) was reduced from 156° to 138°; however, the other films of the multifunctional NPs with PHME (7:3, 5:5, 3:7, and 0:10) showed no significant decrease (< 2°) in WCAs. The SAs of the films of the multifunctional NPs with PHME (7:3 and 5:5) also exhibited insignificant changes (< 2°) from 5° to 6° for 7:3 and from 81° to 83° for 5:5 after ultrasonication while the SA of the film of the NPs with ODPA only (10:0) markedly increased from 3° to 70°. Note that the SA was not measurable for the composite films of the NPs with the SAM ratios of 3:7 and 0:10 (Figure S3). These results reveal the crucial roles of multifunctional NPs in not only tailoring the surface wettability of films because of tunable ODPA SAMs, but also constructing robust covalent networks of the hybrid composite films because of covalent cross-linking PHME SAMs, which thus allows preserving the initial wettability of films from external forces. 3.3 Highly Flexible and Self-Healable Multifunctional NP-Based Superhydrophobic Composite films On the basis of the robust network and the superhydrophobicity with a high WCA (> 150°) and a low SA (5°), we chose the optimized SAM ratio of 7:3 for further investigation of hybrid composite films (film thickness ≈ 4µm). Besides Si substrates, the multifunctional NP-based superhydrophobic composite films can be versatilely fabricated on various substrates including glass, plastic, paper, and cotton fabric regardless of the substrate material while exhibiting excellent water repellency (Figure 6). To further evaluate the flexibility and durability for potential wearable applications, we performed the bending test of the multifunctional NP-based superhydrophobic composite film (4 µm) on the plastic


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(polypropylene) substrate (200 µm) up to 1000 times. Figure 7 displays the bending test configurations of the flexible composite films at different tensile strains. Under a repeated bending stress at the bending radius (R) of 6.5 mm to induce the substantial tensile strain for 1000 cycles,50–52 the multifunctional NP-based hybrid composite film showed the consistent superhydrophobicity with a high WCA of 158°. The flexible superhydrophobic film can retain the micro/nano dual-scale roughness structures after repeated bending process (Figure S4). The durable superhydrophobicity was also shown in the reference sample (10:0) for repeated bending cycles (Figure S5). To further examine the flexibility of the multifunctional NP-based composite film, we carried out the bending test under the much larger bending stress at the bending radius of 3.0 mm, which can be useful for the demanding flexible electronics.53–56 Under the high bending stress (R = 3.0 mm), the multifunctional NP-based hybrid composite film also exhibited the consistent superhydrophobicity with a WCA larger than 150° (Figure 7). These results reveal the excellent flexibility and durability of the superhydrophobic composite films. Therefore, these highly flexible and durable superhydrophobic composite films can be used for plastic- and bio-electronic applications.57–59 To assess the self-healing ability of the superhydrophobic composite films following chemical damage, the multifunctional NP-based composite films were treated with UVO (2 min.) to induce chemical damage and then treated with heat (100 °C, 30 min.) to recover (Figure 8a). The pristine superhydrophobic composite film changed to a hydrophilic surface with a very low WCA ( 150°) mostly in 20 cycles of damage and healing (Figure 8e). The self-healing performance of the multifunctional NP-based superhydrophobic composite film is comparable to those of other reported self-healable superhydrophobic composite films (Table S1).64–70 Consequently, because of the superior stability and self-healing abilities, the multifunctional NP-based superhydrophobic hybrid composite films can retain their superhydrophobicity even under harsh environmental conditions.


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4. CONCLUSION In summary, we developed the rationally designed, multifunctional self-assembled NPs with tunable molecular functionalities of covalent cross-linking and hydrophobicity for generating a new class of highly flexible, durable and self-healable superhydrophobic films through covalently linked composite networks. Because of tunable functional SAMs, the multifunctional self-assembled NPs allowed for the systematic control and optimization of superhydrophobic film properties while forming a threedimensionally interconnected composite network. The covalently linked superhydrophobic composite films exhibited the excellent multiple properties of flexibility, durability, and self-healing ability as well as room-temperature processability, which are desirable integrated properties for plastic- and bioelectronics. Furthermore, based on the versatile applicability, the presented approach can be adapted to construct other advanced functional films and devices through the multifunctional NPs with desired multiple molecular functionalities including a chemically compatible binding function, forming stably interconnected composite networks.

ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge on the ACS Publications website. TGA data of ODPA and PHME molecules; tilted SEM images of the superhydrophobic composite film; contact angles and sliding angles of water droplets on the composite films; SEM images of the composite films after repeated bending test; bending test of reference sample; XPS spectra of the multifunctional NP-based superhydrophobic composite films; self-healing test of reference sample; comparison of the self-healing performance.


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AUTHOR INFORMATION *Eun-Ah You, E-mail: [email protected] *Young-Geun Ha, E-mail: [email protected]

ACKNOWLEDGMENT This research was supported by the Basic Science Research Program (2016R1D1A1B03933571) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education and the Development of Platform Technology for Innovative Medical Measurements funded by Korea Research Institute of Standards and Science (KRISS-2017-GP2017-0020).


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Figure 1. Schematic representation of the synthesis of multifunctional self-assembled NPs with tunable functionalities of hydrophobicity and covalent binding. (a) Multifunctional NP created by co-assembly of hydrophobic ODPA and covalently cross-linkable PHME on Al2O3 NP. (b) Cross-sectional view of multifunctional NPs with various surface coverage ratios of ODPA to PHME, which are generated by assembling the different molar ratios of ODPA to PHME on Al2O3 NPs.


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Figure 2. Characterization of the multifunctional self-assembled NPs with different surface coverage ratios of ODPA to PHME. (a) TGA data of the multifunctional NPs with representative surface coverage ratios of ODPA to PHME of 10:0, 5:5, and 0:10. (b) ATR-FTIR spectra of the multifunctional NPs with systematically varied surface coverage ratios of the heterogeneous SAMs.


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Figure 3. Schematic illustration of a two-step procedure for constructing covalently networked, highly flexible, durable and self-healable superhydrophobic composite films by spray-coating of a multifunctional NP suspension including bifunctional cross-linkers (HDDA) and subsequent UV-curing. The multifunctional NPs with methacrylate-terminated PHME SAMs can form covalent linkages with other methacrylate-terminated NPs and dimethacrylate cross-linkers to form covalently interconnected functional composite films, as illustrated in the magnified cross-sectional view of the UV cured film.


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Figure 4. Variation in WCAs and SAs of the hybrid composite films composed of multifunctional selfassembled NPs with various ratios of ODPA to PHME SAMs before and after ultrasonication. (The SAs of the composite films with the SAM ratios of 3:7 and 0:10 were not measurable.)


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Figure 5. Top-down SEM images of the composite films of multifunctional NPs with various SAM ratios of ODPA to PHME. The different SAM ratios are (a-c) 10:0, (d-f) 7:3, (g-i) 5:5, (j-l) 3:7, and (mo) 0:10 in the SEM images taken at different magnifications. All composite films show hierarchical micro/nano dual-scale surface roughness.


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Figure 6. Optical images of water-repellent multifunctional NP-based composite films on various substrates. (a) Versatile fabrication of the superhydrophobic composite film of multifunctional NPs with an optimized SAM ratio of 7:3 on diverse substrates of glass, plastic (polypropylene), paper, and cotton fabric. (b) Immersion of the superhydrophobic composite film (7:3) coated on cotton fabric in water, showing an obviously bright layer due to light reflection on the air pocket-formed waterrepellent surface.


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Figure 7. WCA of the flexible hybrid composite film of multifunctional NPs with an optimized SAM ratio of 7:3 as a function of the number of bending tests for 1000 cycles under the different tensile strains at the bending radius (R) of 6.5 and 3.0 mm. For the bending test, the 4 µm-thin hybrid composite films were prepared on the 200 µm-thick plastic (polypropylene) substrates.


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Figure 8. Self-healing ability of the superhydrophobic composite film of multifunctional NPs with an optimized SAM ratio of 7:3, followed by the different damage-inducing processes, including UVO exposure, immersion in a strong acid solution, and continuous dropping of acidic raindrops. The damaged composite films were healed by heating. (a) Schematic presentation of the self-healing behavior of the multifunctional NP-based superhydrophobic composite film. (b) Change in WCAs of the multifunctional NP-based superhydrophobic composite film for 10 cycles of UVO damage and healing. (c) XPS spectra of the multifunctional NP-based superhydrophobic composite film after UVO damage and subsequent healing. The inset shows the elemental compositions of the surfaces damaged


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by UVO and then healed by heating, which were determined by XPS analysis. Change in WCAs of the multifunctional NP-based superhydrophobic composite films damaged by (d) the immersion in a strong acid solution and (e) the impact of persistent dropping of artificial acidic raindrops for repeated cycles of damage and healing.

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Rationally Designed, Multifunctional Self-Assembled Nanoparticles for

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