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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25436−25444

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Smart Copolymer-Functionalized Flexible Surfaces with Photoswitchable Wettability: From Superhydrophobicity with “Rose Petal” Effect to Superhydrophilicity Chuanyong Zong,† Mei Hu,† Umair Azhar,† Xu Chen,† Yabin Zhang,*,† Shuxiang Zhang,*,† and Conghua Lu*,‡

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†

Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, P. R. China ‡ School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P. R. China S Supporting Information *

ABSTRACT: Realizing smart surfaces with switchable wettability inspired by nature continues to be fascinating as well as challenging. Herein, we present a versatile dip-coating approach to fabricate smart polymer-functionalized flexible surfaces with photoswitchable superwettability. Decorated with novel acrylate copolymers bearing a trifluoromethyl side chain and fluorine-containing azobenzene derivative moieties, the modified cotton fabric possesses a rose petal-like superhydrophobicity with contact angles larger than 150° and high water adhesion. This smart surface exhibits rapid phototriggered wettability transformation between superhydrophobicity and superhydrophilicity via alternate irradiation with ultraviolet and visible light, respectively. Meanwhile, the as-prepared flexible smart surfaces have excellent chemical and physical stabilities, which could tolerate harsh environmental conditions and repetitive mechanical deformation (e.g., stretching, curling, folding, and twisting) as well as multiple washing. More importantly, based on the excellent photocontrollability, various erasable and rewritable patterns with distinct wetting properties upon selective photoirradiation can be obtained. This simple strategy and the developed smart surface may find more advanced potential applications in controllable liquid transport, patterning droplet microarrays, and microfluidic devices. KEYWORDS: azobenzene, superhydrophobic, photo-responsive, super-wettability, fluorine-containing

1. INTRODUCTION Smart surfaces with switchable wettability offer tremendous potential applications in controllable liquid transport,1 microfluidic devices,2 printing techniques,3 intelligent valves,4 sensors,5,6 and biomedicine7,8 among others. Nowadays, various materials capable of response to external stimuli, such as temperature,8−10 electric potential,6,11,12 magnetic field,1,13 light irradiation,1,14−16 pH value,17−19 counterions,14 and mechanical forces,20−22 provide on-demand choices for the design and engineering of smart surfaces. Among them, photoresponsive materials have attracted extensive attention in triggering surface wettability change, owing to rapid contactfree stimulation provided by light, locally as well as with high resolution.1,14,23 The optical triggering-yielded electron−hole pairs on inorganic transition-metal oxide surfaces, for example, ZnO,16,24 SnO2,25 Fe2O3,26 TiO2,15,27 V2O528 and WO3,25,29 as well as the photoisomerization-induced reversible conformational conversion and/or the dipole moment change of organic molecules, such as spiropyran30−32 and azobenzene,1,33,34 have been widely used to regulate the wettability of the functional surfaces. Compared with the former, the photoresponsive © 2019 American Chemical Society

organic molecules, especially azobenzene and its derivatives, exhibit faster response to the photoswitchable wetting/ dewetting transition, which puts them in the list of more suitable candidates for realizing photoresponsive wettability on the target surface.15,33,35 So far, many researchers have extensively studied these photoresponsive surfaces constructed by “grafting from,” “grafting to,” and/or self-assembly of azobenzene-containing polymer layers as well as the azobenzene derivative modification.1,23 Generally, the surface wettability switching behavior is governed by the combined effects of the surface chemical composition and the surface morphology.14 Hence, a morphology-enhanced photoswitchable wettability on the azobenzene-modified surface can be realized via tailor-made surfaces with hierarchical micro/ nanostructures.34,36 For example, Jiang et al.36 have realized a large photoreversible water contact angle (WCA) change (∼66°) on the azobenzene monolayer, which is electrostatiReceived: May 3, 2019 Accepted: June 24, 2019 Published: June 24, 2019 25436

DOI: 10.1021/acsami.9b07767 ACS Appl. Mater. Interfaces 2019, 11, 25436−25444

Research Article

ACS Applied Materials & Interfaces

2. EXPERIMENTAL SECTION

cally self-assembled on a rough silicon substrate with patterned square pillars. Cho et al.34 have constructed a nanoporous, fluorinated azobenzene-containing organic−inorganic hybrid multilayer with a photoswitchable superhydrophobicity/superhydrophilicity transition via the layer-by-layer technique. Chen and Besenbacher37 have presented a facile electrospinning method for introducing complex topographic roughness (containing randomly aligned micro/nanoscale fibers, beads, and pores) on the azobenzene-modified polycaprolactone surfaces, which resulted in a larger surface wettability change between hydrophobicity and hydrophilicity. Recently, Lu et al.38 have also used the electrospinning technique to construct azobenzene polymer membranes with a controllable surface morphology and roughness, which exhibited remarkable amplified surface photo-responsive wettability changes and superior efficiency for the separation of oils from contaminated water. However, despite these outstanding achievements, simple and convenient preparation of large-area photoresponsive smart surfaces with rapid remarkable wettability changes is still a challenging issue. Natural cellulosic fabrics, such as cotton fabrics, with high porosities and containing abundant reactive groups on the surface, have been widely used as separation materials, coatings, and sensors as well as flexible wearable electronics based on various programmable surface functional modifications.39,40 Recently, various cotton fabric-based intelligent textiles with excellent temperaturesensitive surface wettability modified by the thermo-responsive polymers, such as poly(N-isopropylacrylamide), poly(n-isopropylacrylamide-co-ethylene glycol methacrylate), and poly(triethylene glycol methyl ether methacrylate-co-ethylene glycol methacrylate-co-acrylamide azobenzene), have been reported.41−44 Note that, a few studies have reported the fabrication of cellulose-based smart surfaces with photoreversible wettability via surface modification with azobenzene derivatives.45,46 However, slow responsiveness or a relatively small change in the surface wettability of the photoresponsive surfaces limits the practical applications. It is noteworthy that the inclusion of fluorine-containing groups with a low surface energy (e.g., trifluoromethyl group) into the azobenzene derivatives or the azobenzene-containing polymers can effectively enhance the hydrophobicity as well as the wettability changes of the photoresponsive coatings.34,38 In this study, we report a facile dip-coating approach to obtain a cotton fabric-based smart surface with a rapid and large photoswitchable wettability transformation based on the combined effects of the chemical variation of the fluorinecontaining azobenzene component and the inherent porous micro/nanostructure of the fabric. Owing to the surface decorated with novel, designed acrylate copolymers bearing a trifluoromethyl side chain and fluorine-containing azobenzene derivative moieties, the modified fabric possessed a rose petallike superhydrophobicity with a large contact angle (WCA ≈ 150°) as well as a high water adhesion. This smart fabric owned rapid photoswitchable wettability change between superhydrophobicity and superhydrophilicity via simple alternate ultraviolet (UV)/visible light irradiation. Especially, upon selective exposure, various rewritable patterns with different wetting properties could be constructed conveniently. The as-prepared fabric surfaces exhibited excellent flexibility, chemical and physical stabilities, surface wettability, and photocontrollability, which may find promising applications in microfluidics, patterning droplet microarrays, and cell capture/release.

2.1. Materials. 2,2,2-Trifluoroethyl methacrylate (FMA, 98%) was purchased from Weihai Newera Chemical Co. Ltd. and purified by an Al 2O 3 column. 6-[4-[2-[4-(trifluoromethoxy)phenyl]diazenyl]phenoxy]hexyl acrylate (FAzo) and 6-[4-[2-(phenyl)diazenyl]phenoxy]hexyl acrylate (Azo) were prepared in our laboratory (for detailed information see Supporting Information Scheme S1). 2,2Azobisisobutyronitrile (AIBN, 98%) was purchased from SigmaAldrich and recrystallized twice from ethanol. All other reagents were analytically pure and used directly. Different photomasks were purchased from ZKSZ Technology Development Ltd. (Beijing, China). 2.2. Synthesis of Poly(FAzo-co-FMA). Random copolymers of poly(FAzo-co-FMA) with different molar ratios (Supporting Information Table S1) were synthesized by free-radical solution polymerization. First, the FAzo/FMA monomers with different molar ratios (10 g) and AIBN (0.1 g) were dissolved in N,Ndimethylformamide (25 mL) under a nitrogen atmosphere. Then, the polymerization was carried on at 80 °C for 24 h. After polymerization, the mixed solution was added dropwise to deionized water (200 mL). The obtained precipitate was dissolved in tetrahydrofuran (THF) and reprecipitated from deionized water three times. The resulting product was collected as an orange-colored solid and dried in a vacuum oven for 48 h. Azobenzene-containing homopolymer (polyAzo) was also synthesized by following the same procedure as described above. 2.3. Fabrication of Cotton Fabric-Based Smart Surfaces. The pure cotton fabrics used here belong to the plain weaving fabrics and were purchased from Weifang qianjia Textile Co., Ltd. (Weifang, China). The light-responsive cotton fabric-based flexible surface was fabricated via a simple dip-coating approach. Briefly, the cotton fabric was ultrasonically washed (15 min) with ethanol, acetone, and deionized water three times. Then, the cotton fabric was cut into a rectangle (2 cm × 3 cm) and immersed into the copolymer poly(FAzo-co-FMA)/THF solution or homopolymer (poly-Azo)/ THF solution for 0.5 h. As a contrast, the polymer solution was also spin-coated (2000 rpm) on a glass slide substrate. All samples were dried at 100 °C in a vacuum oven for 48 h. The FAzo/FMA molar ratio of the copolymer and the concentration of the azo-polymer solution that were used to modify the cotton fabric were 60:40 and 10 wt %, respectively, unless otherwise stated. 2.4. Characterizations. 1H nuclear magnetic resonance (NMR) and 19F NMR spectra were characterized via an AVANCE III 400 MHz NMR spectrometer (Bruker BioSpin, Karlsruhe) in CDCl3 and acetone-d6 for monomers and polymers, respectively. The Fourier transform infrared (FT-IR) spectra were obtained on a Nicolet Is10 FT-IR spectrophotometer (Thermo Fisher, USA). The molecular weight and dispersity (Đ) were analyzed using gel permeation chromatography (Waters 1500) with a column of THF solvent, a high performance liquid chromatography pump, and a refractive index detector. The UV−visible (UV−vis) absorption spectra were recorded on a TU-1901 spectrophotometer. The morphology and elemental distribution of the fabric surface were investigated by scanning electron microscopy (SEM) (S-2500, Hitachi Seiki). Surface wettability of the fabric was investigated by WCA measurement (OCA40, Dataphysics, Germany) at room temperature. Water droplets of 3 μL were dripped onto the fabric surface, and digital images of the water droplets were taken after ensuring that the droplet profiles do not change any more with time. During the chemical stability measurements, the residual acid/base solution was removed from the cotton fabric by ultrasonication with deionized water (15 min, 3 times) prior to measuring the contact angles. Contact angles for each sample were taken for five measurements at different spots. A halogen lamp (CEL-HXF300, zhongjiao Aulight, China) equipped with different filters was used to irradiate the above samples. The intensity of the used UV and visible light were 95 and 300 mW/cm2, respectively. The light intensity was measured with an optical power meter (zhongjiao Aulight, China). 25437

DOI: 10.1021/acsami.9b07767 ACS Appl. Mater. Interfaces 2019, 11, 25436−25444

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the procedure to fabricate a cotton fabric-based smart surface via dip-coating (a); 1H NMR (b) and 19F NMR (c) analysis of poly(FAzo-co-FMA) copolymers; UV−vis absorption spectrum evolution of the copolymer solution with UV light irradiation (d) and visible light irradiation (e).

Figure 2. (a) Photographs of water droplets on the azo-copolymer-modified fabric surfaces before and after exposure to UV and/or visible light; (b) dependence of WCAs on the modified fabric surfaces and the weight gain ratios of the modified cotton fabrics with dip-coating solution concentration; (c) reversible wettability transition on the fabric surfaces modified with different copolymers under alternate UV/visible light irradiation; frame (d) shows the variation in the WCA on the fabric surfaces against the irradiation time with visible light (red line), UV (black line), and visible-light irradiation after UV exposure (green line).

3. RESULTS AND DISCUSSION

copolymers of poly(FAzo-co-FMA) were synthesized by freeradical polymerization. All obtained copolymers had a relatively similar molecular weight (Mn) ranging from 1.03 × 104 to 1.59 × 104 and a moderate dispersity (D̵ ) (i.e., Mw/Mn =

The light-responsive cotton fabric-based flexible surface with controllable wettability was fabricated via a simple dip-coating approach (Figure 1a). The azobenzene-containing random 25438

DOI: 10.1021/acsami.9b07767 ACS Appl. Mater. Interfaces 2019, 11, 25436−25444

Research Article

ACS Applied Materials & Interfaces

Figure 3. SEM images of the cotton fabrics before (a,b) and after (c,d) modification with the copolymer; SEM−EDS elemental mapping images acquired from the modified fabric (e−i) and the corresponding atomic percentages (j).

solution (Figure 1d,e) and/or in the film state (Supporting Information Figure S2). Upon UV irradiation, an evident decrease of π−π* absorption at 350 nm and an increase of n−π* absorption at 440 nm were observed, indicating photoisomerization from the trans to cis isomer of the azobenzene moiety (Figure 1d and Supporting Information Figure S2a). Further irradiation with visible light caused an increase of π−π* absorption and a decrease of n−π* absorption, accompanied by the cis to trans conversion of the azobenzene groups (Figure 1e and Supporting Information Figure S2b). This reversible trans/cis/trans isomerization cycle of the azobenzene units resulted in the photoswitchable surface wettability transformation on the poly(FAzo-co-FMA)-based coatings (for detailed discussion, see the following section).33,34,37 Decorated with the fluoro-containing azo-copolymers, the wettability of the commercially available cotton fabric surface was changed from the pristine superhydrophilic (WCA = 0°) to superhydrophobic property with the maximum WCAs about 150° (Figure 2a,b). As compared to the azo-homopolymer without trifluoromethyl group-modified fabrics, the fluorinecontaining azo-copolymer-modified fabrics exhibited higher hydrophobicity (Supporting Information Figure S3). The enhanced hydrophobicity of the azo-copolymer-modified fabric surface resulted from the existence of the trifluoromethyl-

1.2−1.4) (Supporting Information Table S1). The chemical structures of copolymers with different comonomer molar ratios were characterized by 1H NMR and 19F NMR (Figure 1b,c). As demonstrated in Figure 1b, the feature peaks at 7.00, 7.49, and 7.93 ppm were assigned to −CH− in the phenyl group of azobenzene (4H, Ar-H).34 The signals at 3.08−4.63 and 2.65 ppm belonged to −CH2− groups in the side chain (2H, −CH2O−)34 and −CH− groups in the main chain,47 respectively. Furthermore, the characteristics peaks of −CH3− −CH2− groups in the main chain and −CH2− groups in the side chain were observed at 0.89−2.01 ppm.47 Meanwhile, the characteristic absorption peaks of −CF3 in the copolymers evidently emerged at −58.12 ppm (3F, Ph−OCF3) and −73.69 ppm (3F, −CH2−CF3) (Figure 1c).48 In addition, FTIR analysis also confirmed the chemical constitutions of the copolymers (Supporting Information Figure S1). These results indicated the successful synthesis of azobenzene-containing copolymers. As an azobenzene-containing copolymer, there existed two peaks in the UV−vis absorption spectrum at approximately 350 and 440 nm, corresponding to π−π* and n−π* transitions of the fluorine-containing azobenzene derivative moieties, respectively (Figure 1d). 49,50 The azobenzene units in the poly(FAzo-co-FMA) copolymers showed rapid photoisomerization between the trans and cis state upon UV light or visible light irradiation in dilute THF 25439

DOI: 10.1021/acsami.9b07767 ACS Appl. Mater. Interfaces 2019, 11, 25436−25444

Research Article

ACS Applied Materials & Interfaces

the pristine hydrophobicity of the fabric surface against visiblelight exposure indicated that the wettability change was essentially related to the light-induced photoisomerization of the azobenzene moieties in the copolymers (Figure 2d). To further understand the inherent mechanism of the initial petal effect superhydrophobicity and the rapid photoswitchable wettability transformation on the modified fabric surface, the surface topography and the surface elemental compositions were characterized by SEM and energy-dispersive spectroscopy (EDS), respectively (Figure 3). As shown in Figure 3a, the pristine cotton fabric was composed of a large amount of microscale fibers with diameters of 7−20 μm, providing a rough porous microstructure. The high magnification of the SEM image indicated that the surface of the individual fiber was smooth (Figure 3b). After dip-coating with the copolymer solution, the modified fabric maintained the porous structure without any obvious change, but the fiber surface became rougher compared to the pristine (Figure 3c,d). As will be discussed later, this porous microstructure played a key role in the formation of the optically switchable bionic superhydrophobic/superhydrophilic surface. The EDS spectra revealed that the modified fabric surface consisted of C, O, N, and F elements with atomic percentages of 51.00, 37.77, 6.33, and 4.90%, respectively (Figure 3e−j). Among these, the F element stemmed from trifluoromethyl tethered to the copolymer side chain, which was an ideal low-surface-energy group for enhancing the surface hydrophobicity.34,56,57 It was evident that the combined effect of the modified chemical composition and the surface hierarchical micro/nanostructures endowed the fabric with superhydrophobicity.58,59 Note that the ratio of atomic percentages between O and F in the fabric surface according to the EDS spectra (∼7:1) was much larger than that in the copolymer composition (∼1:1), implying that some hydroxyl groups are exposed on the surface. These hydrophilic groups would act as the hydrophobic defects and result in a high adhesive force to water. Meanwhile, the air trapped in the porous microstructures formed a barrier layer for water permeation, leading to the Cassie impregnating wetting state on the fabric surface.53 At the pristine state, the rodlike trans state of azobenzene derivative moieties usually presented as vertically extended chains with the −CF3 tail at the outermost, yielding a hydrophobic surface. UV (365 nm) light irradiation triggered a rapid isomerization of azobenzene from a rodlike trans state to a curving cis state (Figure 1d and Supporting Information Figure S2). It is well known that azobenzene chromophores in the cis state had a shorter length and a larger dipole moment (0.55 nm/3.1 D) than those in the trans state (0.9 nm/0.5 D).60,61 Therefore, after UV exposure, the bent cis isomers with high polarity-exposed nitrogen atoms at the outermost replaced the pristine −CF3 groups, offering a high surface energy on the fabric surface with an enhanced water affinity.33,34,36 Subsequently, the superhydrophilic surface, with WCA decreased to about 0°, was observed and can be attributed to the capillary effect from the porous structures of the fabric, which was consistent with the Wenzel’s equation.62,63 Conversely, the large change in the surface dipole moment could be recovered upon visible light irradiation with the resulting reversible cis to trans isomerization of azobenzene chromophores (Figure 1e). Consequently, the surface wettability invertible change from superhydrophilicity to superhydrophobicity occurred (Figure 2d).

containing side chain in the copolymers, which decreased the surface free energy. Noticeably, the modified fabric surface demonstrated super-high water adhesion with water droplets that could hang on the surface, just like the “rose-petal effect” (Figure 2a and Supporting Information Figure S4).51,52 Most probably, the Cassie impregnating wetting state of the fabric surface with a high adhesive force was attributed to the porous structures of the fabric as well as the hydrophobic defects, owing to the existence of hydrophilic components on the surface (e.g., the ester group of the copolymers).53,54 This “sticky” superhydrophobicity of the modified fabric surfaces could be maintained under laboratory conditions for several months. The pristine hydrophobicity of the fabric surface and the weight gain ratios of the modified cotton fabrics depended on the concentration of the dip-coating copolymer solution, which may be influenced the surface chemical property as well as the surface roughness (Figure 2b). The WCA on the fabric surface was at a maximum value of 150 ± 4.28° with the copolymer solution concentration increased to 10 wt % (Figure 2b). Surprisingly, exposure to UV light led to an obvious wettability transition from superhydrophobicity to hydrophilicity or superhydrophilicity on the fabric surface and reversibly to its initial superhydrophobic state upon visiblelight irradiation (Figure 2a,c). These photoswitchable wettability transformations with excellent reversibility exhibited larger WCA changes compared to the corresponding copolymer film surface when deposited on a smooth substrate (Figure 2c and Supporting Information Figure S5). It was evident that the proper composition of the copolymers (e.g., the molar ratio of FAzo/FMA = 60:40) also played a key role for the intensified wettability transformation of the fabric surface, especially for the wettability transformation from superhydrophobicity to superhydrophilicity (Figure 2c). It is worth noting that the molar ratios of the comonomers had no obvious effect on the pristine hydrophobicity of the modified fabric owing to the presence of the low-surface-energy trifluoromethyl terminal group in both comonomers; but it could significantly lower the WCA changes on the modified fabric surface, when the azobenzene in the copolymers was reduced to a certain amount (e.g., the mole content of FAzo less than 60%). It was due to the smaller polarity decrease on the fabric surface induced by the photoisomerization of azobenzene molecules in the copolymers (for detailed discussion, see the following section). These results indicated that the light-controlled wettability switching on the fabric surface was related to the combined effects of the surface chemistry and the inherent porous structure of the fabric. The former provided the photoresponsive surface wettability change between hydrophobicity and hydrophilicity (Supporting Information Figure S5), and the latter dramatically magnified this property (Figure 2c).55,56 Moreover, as shown in Figure 2d, the smart fabric surface exhibited rapid photoswitchable wettability transformation between superhydrophobicity and superhydrophilicity by the alternate irradiation with UV and visible light, respectively. The WCAs on the modified fabric surface gradually reduced from 150° to 0° under UV irradiation within 90 s and rapidly recovered to about 150° within 50 s after exposure to higher intensity visible light (Figure 2d). Meanwhile, this photocontrolled wettability transformation can be repeated for more than 50 cycles without obvious loss of responsiveness, demonstrating good stability of the as-prepared smart fabric surface (Supporting Information Figure S6). The inertness of 25440

DOI: 10.1021/acsami.9b07767 ACS Appl. Mater. Interfaces 2019, 11, 25436−25444

Research Article

ACS Applied Materials & Interfaces

Figure 4. Photographs of the coated fabrics immersed in different pH aqueous solutions for 24 h (a) and the corresponding solution after removal of the fabric (b); (c) effect of acidic/basic solution corrosion for 24 h on the wettability photoresponse.

superhydrophobic robustness of the coated fabric was evaluated by repetitive mechanical deformation tests including folding, curling, twisting, and stretching (Figure 5a). As shown

The superhydrophobicity of the azo-copolymer-coated cotton fabric could be maintained in the air for a long period (e.g., 2 years) with a constant value of WCA (Supporting Information Figure S7), indicating that the photoresponsive superhydrophobic surfaces possess long-lasting stability. In order to evaluate the durability to resist various corrosive environments, the fabric samples were further immersed in different pH aqueous solutions for 24 h (Figure 4a,b). Evidently, there was no obvious deterioration in the superhydrophobicity and the wettability photoresponse of the fabric surface over the aqueous solution pH range from 1 to 14 (Figure 4c), showing excellent chemical corrosion resistance of the as-prepared surfaces. Interestingly, the fluorine-containing azo-copolymer-modified fabrics exhibited an enhanced acid/ base resistance compared to the azo-homopolymer-modified fabrics (Figure 4b and Supporting Information Figure S8). Note that, although a slight shedding of the fabric coatings in the strong alkaline solution was observed (Figure 4b), which may be due to the hydrolysis of ester groups in the copolymer, the WCA values as well as the color of the surface coatings were almost unchanged after 24 h of immersion (Figure 4c and Supporting Information Figure S9) and even after immersion for 10 days (Supporting Information Figure S10). This excellent chemical robustness against acid/base environments should be stemmed from the hydrophobicity and the chemical resistive nature of the trifluoromethyl side chain and the fluorine-containing azobenzene derivative moieties of the designed acrylate copolymer layers that were deposited on the cotton fabric. The trifluoromethyl terminal groups with a low-surface-energy close-packing on the coating surface made them highly stable and withstand strong acid and alkali attacks, which is very important for expanding the application under harsh conditions.64−66 Generally, apart from the chemical stability, the mechanical durability of the coated fabric, which was a combined result of the intrinsic strength of the fibers and the adhesion strength of the surface coating, was also a significant factor that influenced their reusability for practical applications. Hereby, the

Figure 5. Photographs of the mechanical durability tests (folding, curling, twisting, and stretching) on the coated fabrics (a); WCAs and the wettability change on the coated fabrics upon UV or visible light irradiation after different treatments (b); SEM images of the coated fabric after repeated washings (c,d).

in Figure 5b, the superhydrophobic coatings had no obvious decay under multiple deformation tests (e.g., more than 20 cycles) (with respect to the initial WCA of 150 ± 5°), and the coated fabrics surface also maintained the rapid photoswitchable wettability transformation during the alternate UV/vis irradiation (Figure 5b). Furthermore, the fabric surface-wetting properties were retained without noticeable degradation even after passing through severe washing test for 10 cycles. This demonstrated that the azo-copolymer coating 25441

DOI: 10.1021/acsami.9b07767 ACS Appl. Mater. Interfaces 2019, 11, 25436−25444

Research Article

ACS Applied Materials & Interfaces provided excellent mechanical flexibility and strong adhesion to the underlying cotton fabric, with the latter attributed to the existence of numerous hydrogen bonds between the ester groups in the copolymer and the hydroxyl groups on the fabric surface.67,68 SEM observation also identified that the morphology of the azo-copolymer coating kept the good integrality on the fabric surface without any observable damage after the washing test (Figure 5c,d). Furthermore, encouraged by the convenient spatiotemporal control of light, the selective exposure and/or the blanket exposure of the coated fabric could be easily realized through different photomasks, which resulted in the coated fabrics with distinct extreme wetting properties (Figure 6 and Supporting Information Figure S11). Upon selective exposure to UV light, the coated fabric wettability in the exposed region was transformed from the initial superhydrophobicity to superhydrophilicity (Figure 6a→ 6b). Subsequently, as expected, the

intelligently triggered by the alternation of UV exposure and visible-light illumination with the time shortened to tens of seconds. More importantly, the as-prepared photoresponsive cotton fabric exhibited excellent chemical robustness and mechanical durability, which could tolerate strong corrosive (acid/base attack) and repetitive large mechanical deformation as well as multiple washings. The smart fabric surface also offered a rewritable platform for creating different extreme wetting patterns upon selective light irradiation, which had widely potential applications in microfluidics, microcontact printing, cell biology, and other surface sciences.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07767.

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Figure 6. Photographs of the coated fabric with photoswitchable wetting properties: (a) the as-prepared superhydrophobic fabric; (b) patterned superhydrophilic fabric surface after selective UV irradiation; (c) globally superhydrophilic fabric resulting from blanket exposure to UV; and (d) patterned superhydrophobic fabric restored as a result of selective visible light irradiation.

Synthesis of FAzo; molecular weight and dispersity of the copolymers; FT-IR analysis of poly(FAzo-co-FMA) copolymers; UV−vis absorption spectra of the copolymer film on a glass slide substrate with different light irradiations; photographs of water droplets on the azocopolymer-modified fabric surfaces; SEM image of the smooth poly(FAzo-co-FMA) film; reversible wettability transition on the smooth copolymer surfaces upon different light irradiations; superhydrophobicity of the modified fabric surface after 24 months of storage; photographs of the coated fabrics after immersion in various aqueous solutions; and photographs of the coated fabrics with different patterned wetting properties (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Z.). *E-mail: [email protected] (S.Z.). *E-mail: [email protected] (C.L.). ORCID

fabric wettability in the unexposed region could be thoroughly changed by blanket exposure to UV light, yielding a globally superhydrophilic fabric surface (Figure 6b → 6c). The resulting superhydrophilic fabric surface could be further tailored via selective exposure to visible light, which patterned the fabric surface with restored superhydrophobicity (Figure 6c → 6d). Moreover, after blanket exposure with visible light, the as-prepared “peony” pattern vanished, that is, the fabric wettability recovered to its initial state (Figure 6d → 6a). Notably, as presented in Figure 2c, the coated fabrics had good durability in cycling phototriggered surface wettability transformation in the alternate UV and/or visible light irradiations. Thus, taking advantage of the photosensitive fabric combined with selective exposure durably, rewritable patterned surfaces with varied superwettability can be easily realized. This highefficiency wettability-patterning technique may find significant potential applications in micro/nanopatterning functional materials, (bio)sensors, microfluidic devices, and microreactors as well as in cell culture.65,69,70

Chuanyong Zong: 0000-0002-1415-1497 Conghua Lu: 0000-0003-3995-8067 Author Contributions

C. Z. and M. H. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 21704033, 21574099, 21875160), the Natural Science Foundation of Shandong Province of China (ZR2017ZC0529), the Key Research Program of Shandong Province of China (2018GGX102002), and the Natural Science Foundation of Tianjin City (15JCZDJC31400).

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4. CONCLUSIONS In conclusion, we have presented a facile dip-coating approach for the fabrication of an azopolymer-functionalized cotton fabric surface with switchable wettability. The reversible cycle of superhydrophobicity to superhydrophilicity transition was

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DOI: 10.1021/acsami.9b07767 ACS Appl. Mater. Interfaces 2019, 11, 25436−25444

Research Article

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