Superhydrophobic Films Fabricated by Electrospraying Poly(methyl

Article pubs.acs.org/JPCC

Superhydrophobic Films Fabricated by Electrospraying Poly(methyl methacrylate)‑b‑poly(dodecafluoroheptyl methacrylate) Diblock Copolymers Yudi Guo, Dongyan Tang,* and Zailin Gong Department of Chemistry, School of Science, Harbin Institute of Technology, Harbin 150001, China S Supporting Information *

ABSTRACT: We present the fabrication of superhydrophobic surfaces by electrospraying structure controlled poly(methyl methacrylate-bromine) (PMMA-Br) homopolymer and degree of fluorination controlled poly(methyl methacrylate)-b-poly(dodecafluoroheptyl methacrylate) (PMMA-b-PDFMA) copolymers synthesized via atom transfer radical polymerization (ATRP). The flow rate has great effects on the surface hydrophobicity of PMMA50.6, and superhydrophobic surface was obtained at a higher flow rate of 5 μL/min. For fluorinated diblock copolymers, superhydrophobic surfaces can be obtained by electrospraying PMMA50.6-b-PDFMA0.8 and PMMA147.9-b-PDFMA17.5 at the selected flow rates. X-ray photoelectron spectroscopy (XPS) detections reveal that, during the film formation, more hydrophobic surfaces would be fabricated due to the surface segregations of C−C and C−H groups with THF as solvent instead of C−O−CO group with DMF as solvent. This investigation may be of great value in fabricating superhydrophobic polymer materials.

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low molecular weight polymers. Furthermore, the fiber diameter or particle size can be controlled effectively by adjusting the solution characters or the processing conditions.22 In order to fabricate superhydrophobic surfaces, fluorinated polymers are often used as the low surface energy materials to chemically modify a micro/nanostructural surface. The materials that have the advantages of perfluorinated materials yet low fluorine content are very attractive.23−25 Incorporation of fluorinated monomers into block copolymers with controllable polymeric degree is a viable and usual route for obtaining partially fluorinated polymers.26,27 Up to now, a number of copolymer architectures with functional chain ends, random or block distribution of the fluorinated monomers, or with different topologies have been constructed successfully.25 In recent years, living or controlled radical polymerization (CRP) methods have been widely used to synthesize well-defined (co)polymers with various architectures (i.e., telechelic, block, graft, or star copolymers).28−30 Among these methods, atom transfer radical polymerization (ATRP) is one of the most effective techniques for the structural design of polymers and for the synthesis of various fluorinated (meth) acrylic copolymers with low polydispersities and well-defined architectures.30−32 Recently, electrospinning/electrospraying of fluorinated polymers has emerged as a powerful route to achieve the

INTRODUCTION Superhydrophobic surfaces have attracted much attention because of their potential applications in self-cleaning interfaces,1,2 electrowetting-based applications,3−5 and anticorrosion layers.6,7 Previous studies have indicated that the surface wettability is governed by both the chemical compositions and the surface geometrical structures of the solid surfaces. Therefore, the combination of suitable surface roughness and low-surface-energy materials is responsible for the superhydrophobicity.8,9 Physical and chemical methods have been developed to create hierarchical micro/nanostructures on a hydrophobic substrate, such as plasma etching,10 electrospinning,11,12 sol− gel,13 chemical vapor deposition,14−16 chemical etching,17,18 electrochemical deposition,19 and solution-immersion. 20 Among these methods, surfaces with different morphology and roughness can be obtained via electrospinning and/or electrospraying. Electrospinning or electrospraying is a process based on the application of an electric field to an injected polymer solution, which can deposit either submicrometer polymer fibers or polymeric particles on a surface.21 The film morphology formed during the electrospinning/electrospraying process is strongly depended on the polymer solution properties (viscosity, conductivity, and surface tension) and the process conditions (applied voltage, flow of the polymer solution, and nozzle-to-collector distance). For instance, pure fibers can be achieved by electrospinning high molecular weight polymers with appropriate viscosity, while particles or mixture of fibers and particles would be produced by electrospraying © 2012 American Chemical Society

Received: June 6, 2012 Revised: November 22, 2012 Published: November 29, 2012 26284

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columns (300 × 7.5 mm2). THF was used as eluent with a flow rate of 1.0 mL/min. Polystyrene was used as internal standard. Surface morphology was examined by a QUANTA200F field emission scanning electron microscope (FEI Co. USA). Contact angle measurements were carried out by the sessile drop method at room temperature using a JC2000 C3 contact angle goniometer (Shanghai Zhongchen Powereach Co., CN). Typically, three drops of the liquid were deposited on the film surface, and three readings of contact angles were taken for each drop. The average of nine readings was used as the final contact angle of each sample. Deionized water (3−5 μL) was used as the wetting liquid for contact angle measurements. Advancing and receding contact angles were measured upon adding and withdrawing liquid through a flat-tipped probe. Surface elemental compositions of the electrosprayed films were detected on a PHI 5700 ESCA system (Physical Electronics Co., USA) employing an Al Kα X-ray resource (1486.6 eV) and a concentric hemispherical energy electron analyzer operating at 12.5 kV and 250 W with a chamber pressure of ∼5 × 10−9 Torr. The survey spectrum was recorded at constant pass energy of 187.9 eV, and the high-resolution analysis was conducted at constant pass energy of 29.4 eV. The instrumental error in terms of the binding energy was within ±0.1 eV. Data were recorded at different incident angles of 90° and 30° in order to probe the surface and bulk features of the electrosprayed films. All spectra were calibrated by the C1s peak of the C−C bond at 285.0 eV. ATRP Procedure for the Synthesis of PMMAm-Br Homopolymers. The typical bromine terminated macroinitiators of PMMA-Br were synthesized as follows. Twenty milliliters of MMA (0.1886 mol) and 20 mL of THF were added into a Schlenk flask equipped with a magnetic stirrer and sealed with a rubber septum. Then, the Schlenc flask was immersed in an ice−water bath, thoroughly purged by vacuum, and flushed with nitrogen (three cycles) to deoxygenate the reactants. Afterward, 0.2706 g (1.886 mmol) of CuBr was added as catalyst, and the system was purged by vacuum again. Then, 0.785 mL (3.772 mmol) of PMDETA was added as ligand before the system was flushed with nitrogen again. The above mixture was heated to 70 °C gradually, and 0.246 mL (1.886 mmol) of EbiB was injected into the mixture as initiator under stirring with monomer, initiator, catalyst, and ligand at the molar ratio of 100:1:1:2. After 12 h reaction time, the reaction mixture was cooled down to ambient temperature, diluted with THF, and filtered through a neutral Al2O3 column. After filtering, the polymers were concentrated and precipitated into methanol. The crude product was purified by repeatedly dissolving in THF and precipitating into methanol three times. Finally, the product was dried under vacuum at 50 °C for 3 days. The polymerization degrees of the MMA monomer in a series of macroinitiators were presented as m in the form of a suffix. ATRP Procedure for the Synthesis of PMMAm-bPDFMAn Diblock Copolymers. PMMAm-b-PDFMAn diblock copolymers were prepared by using PMMAm-Br as macroinitiator and using monomer, initiator, catalyst, and ligand at the molar ratio of 20:1:1:2. In a typical procedure, PMMA100-Br (the suffix value was estimated based on the theoretical value) (1 g, ∼1 × 10−4 mol) and THF (10 mL) were added into a 100 mL Schlenk flask equipped with a magnetic stirrer and sealed with a rubber septum, and mixed completely by ultrasonic. Then, the Schlenc flask was immersed in an ice−water bath, thoroughly purged by vacuum, and flushed with nitrogen (three

combination of surface roughness and low surface free energy. However, because of the insolubility and collapsing of fluorine moieties in common solvents, fluorinated copolymers usually form unimers, micelles, and other aggregates in solution with fluorinated segments packed in soluble segments; thus, the surface segregation of fluorinated segments becomes difficult.26 It is well-known that the orientation and packing density of fluorocarbon segments in the surface layer are very important in fabricating a superhydrophobic surface.33 However, several studies demonstrate that, by adjusting the polymer chain architectures and the solvent compositions, superhydrophobic surfaces with well packed perfluoroalkyl chains can be accomplished.34−36 In the case of electrospinning/electrospraying, the polymer chain architectures and the solvent properties have great effects on the surface morphology and wettability. Therefore, in terms of the practical application of such materials, it is very important to study the fluorinated polymer chain structures and the film-forming conditions with the aim of controlling the surface roughness and surface chemical compositions. However, few researches focus on the effects of film-forming conditions/processes on the selfassembly behaviors of the fluorinated block copolymers during the electrospraying process. In this work, a series of PMMA-Br macroinitiators and PMMA-b-PDFMA diblock copolymers with various MMA block and DFMA block lengths were synthesized via ATRP technique. The superhydrophobic surfaces were fabricated by electrospraying PMMA-Br homopolymer and PMMA-bPDFMA diblock copolymer solutions. The aims of the present work were to investigate the effects of flow rates, solvent compositions, film-formation conditions, and lengths of block on the wettability of the synthesized polymers and on the surface compositions of the electrosprayed films.

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EXPERIMENTAL SECTION Materials. Dodecafluoroheptyl methacrylate (DFMA) was supplied by Xeogia Fluorine−Silicone Chemical Co. (Harbin, China). Methyl methacrylate (MMA) was purchased from Kemiou Chemical Co. (Tianjin, China). The above monomers were washed with 5 wt % aqueous solution of sodium hydroxide to remove the inhibitor, and then washed with deionized water for several times until neutralization. The organic layer was collected and dried over anhydrous magnesium sulfate before polymerization. Copper(I) bromide (CuBr), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), and ethyl-2-bromoisobutyrate (EbiB) were all from Aladdin Reagent Co. Ltd. (Shanghai, China) and employed without further purification. Tetrahydrofuran (THF), N,N-dimethylformamide (DMF), and methanol were purchased from Xilong Chemical Industry Incorporated Co., Ltd. (Santou, China) and used as received. Characterization. The FT-IR spectra of PMMA-Br homopolymer and PMMA-b-PDFMA diblock copolymer were accomplished on an AVATER-360B FT-IR spectrometer (Nicolet Co., USA) in the range from 400 to 4000 cm−1. 1H NMR detections were performed on an AVANCE III 400 MHz spectrometer (Bruker Co. CH) with CDCl3 as solvent and tetramethylsilane (TMS) as internal standard. Molecular weights and molecular weight distributions of homopolymers and diblock copolymers were determined by size exclusion chromatography (SEC) using an Agilent 1100 instrument (Agilent Co. USA) equipped with a PL gel 5 μm guard column (50 × 7.5 mm2) and two PL gel 5 μm mixed D26285

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Scheme 1. Synthetic Procedures of PMMAm-Br Macroinitiators and PMMAm-b-PDFMAn Diblock Copolymers

macroinitiator was then used to initiate the polymerization of DFMA to obtain a series of diblock copolymers of PMMAm-bPDFMAn via the following ATRP process. The general synthetic procedures are shown in Scheme 1. The surface structures and chemical compositions of PMMAm-Br homopolymers and PMMAm-b-PDFMAn diblock copolymers were confirmed by FT-IR and 1H NMR spectra. Figure 1 shows the FT-IR spectra of PMMA50.6-Br homopol-

cycles) to deoxygenate the reactants. Subsequently, CuBr (0.0143 g, 1 × 10−4 mol) was introduced into the flask, and the system was purged by vacuum again. After that, PMDETA (0.042 mL, 2 × 10−4 mol) was added as ligand using a syringe before the system was backfilled with nitrogen again. The above mixture was heated to 70 °C gradually, and DFMA (0.503 mL, 2 × 10−3 mol) was introduced into the mixture. After 12 h reaction time, the flask was removed from the bath and cooled to room temperature. Then, the mixture was diluted with THF and passed through a neutral Al2O3 column to remove the copper catalyst. The resulting solution was concentrated and precipitated into anhydrous methanol. The product was separated by filtration and further purified twice by redissolving in THF and reprecipitation into methanol before drying under vacuum at 50 °C for 3 days. The polymerization degrees of the PMMA and PDFMA segments in a series of diblock copolymers were presented as m and n in the form of a suffix. Self-Assembled Superhydrophobic Films Fabricated by Electrospraying. The electrospraying apparatus used in this work consisted of a high voltage power, a syringe infusion pump, and an aluminum plate covered with aluminum foil mounted on an insulating stand as the collector. The macroinitiators of PMMAm-Br and fluorinated diblock copolymers of PMMAm-b-PDFMAn were dissolved in a mixture of DMF and THF (DMF/THF = 1:1, w/w) or DMF (or THF) with the polymer concentration of 2% (w/w), respectively. Electrospraying was performed on the apparatus with a 14 cm gap between the substrate electrode and the tip of the needle at 14 kV. The flow rates of the polymer solution were 1 μL/min, 5 μL/min, and 2 mL/h, respectively.

Figure 1. FT-IR spectra of PMMA50.6-Br homopolymer (a) and PMMA50.6-b-PDFMA0.8 diblock copolymer (b).

ymer (Figure 1a) and PMMA50.6-b-PDFMA0.8 diblock copolymer (Figure 1b). As observed in Figure 1a,b, the characteristic peaks at 1450 and 1387, 1734, and 2830−3000 cm−1 could be attributed to the distortion vibration of −CH2−, the stretching vibration of ester carbonyl group, and stretching vibration of −CH3 and −CH2−, respectively. For PMMA50.6-b-PDFMA0.8, a new peak is observed at 2930 cm−1, which could be ascribed to the asymmetric stretching vibration of the methylene groups [CH2(s)] that are connected to the perfluoroalkyl group.37 The

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RESULTS AND DISCUSSION Synthesis and Characterization of PMMAm-Br and PMMAm-b-PDFMAn. After the MMA monomer was transformed into PMMAm-Br macroinitiator via ATRP process, the 26286

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Figure 2. 1H NMR spectra of PMMA147.9-b-PDFMA17.5 diblock copolymer (a) and PMMA147.9-Br homopolymer (b).

broadened absorption peaks at 1270−1068 cm−1 may be due to the overlap between the stretching vibration bands of −CF2 and −CF3 groups at about 1289−1068 cm−1 and the asymmetrical stretching vibration band of C−O−C at 1242 cm−1. The peaks at around 660 and 713 cm−1 result from the combination of the rocking and wagging vibrations of CF2 groups. Figure 2 exhibits the 1H NMR spectra of PMMA147.9-bPDFMA17.5 diblock copolymer (Figure 2a) and PMMA147.9-Br homopolymer (Figure 2b). The peaks at 3.60, 1.81, and 1.02 ppm are assigned to the typical bands of −OCH3, −CH2−, and −C−CH3 in MMA segments existed in both PMMA147.9-Br and PMMA147.9-b-PDFMA17.5. As shown in Figure 2a, the weak characteristic δH signals of O−CH2 and −CFH in DFMA at 3.93 and 5.58 ppm are due to the intrinsic association of the DFMA in CDCl3.38 Both the FT-IR and 1H NMR spectra confirm that PMMAm-Br and PMMAm-b-PDFMAn have been successfully synthesized. The reaction recipes, molecular weights (M n ), and polydispersity indexes (PDI) are summarized and listed in Table 1. The structures of the polymers are given by a numeric code, which refers to the number of repeating units per polymeric block (e.g., PMMA50.6-b-PDFMA0.8), and the molecular weight distributions of the synthesized PMMAm-Br and PMMAm-b-PDFMAn are shown in Figure 3. In the case of ATRP polymerization, a specific set of conditions would be required because of the intrinsic radical propagation rate possessed by each type of monomer.39,40 Therefore, as shown in Table 1, the diblock copolymers have relatively low degrees of polymerization of the fluorinated block with only a few fluorinated monomer units, which is most probably due to the conditions chosen for the polymerization including the bulkiness of the monomer. However, because of the poor solubility of fluorinated segments in the solvent, the

Table 1. Molecular weights (Mn), Polydispersity Indexes (PDI) of PMMAm-Br Homopolymers and PMMAm-bPDFMAn Diblock Copolymers Ia/MMA/DFMA (mol/mol/mol)

Mn, in theory (g·mol−1)

Mn,from SEC (g·mol−1)

PDI (mol)

1:50 1:100 1:100 1:150 1:50:20

5000 10000 10000 15000 13000

5071 13554 14822 18192 5377

1.13 1.34 1.40 1.23 1.12

1:100:20

18000

14996

1.25

1:100:60

18000

21837

1.20

1:150:20

23000

19841

1.17

a

PMMAm-Br and PMMAm-bPDFMAn PMMA50.6 PMMA135.4 PMMA147.9 PMMA181.7 PMMA50.6-bPDFMA0.8 PMMA135.4-bPDFMA3.6 PMMA147.9-bPDFMA17.5 PMMA181.7-bPDFMA4.1

I: initiator, ethyl-2-bromoisobutyrate (EbiB).

micelles with the fluorinated segments as core would be formed during the polymerization process, which would inhibit further chain propagation. Consequently, the PDI of diblock copolymer is lower than that of the corresponding homopolymer, and the narrow PDI (Mw/Mn, 1.1−1.4) obtained for all the polymers indicates that the polymerizations are typically living and controllable. Since no high molecular weight tails and low molecular weight shoulders are observed in Figure 3, the monomodal SEC curves for all polymers reveal no obvious evidence of permanent termination from recombination in the initial and final stages of the polymerization. Compared with the SEC curve of the corresponding homopolymers, the SEC curves of the diblock copolymers with low degree of fluorination show an unobvious shifting toward lower elution 26287

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Figure 3. SEC curves of PMMAm-Br homopolymers and PMMAm-b-PDFMAn diblock copolymers.

μL/min, while sunken spheres or continuous branches-like morphology were formed at the higher flow rate of 5 μL/min. Furthermore, nanopore shaped spheres are formed for the diblock copolymer of PMMA147.9-b-PMMA17.5 at the flow rate of 2 mL/h. The fast evaporation of solvents during the electrospraying process at the lower flow rate might probably lead to the collapsing of solvent-containing spherical polymer aggregates, which further induce the formation of the shrunken spherical morphology. By contrast, the aggregates have more time for chain rearrangement at the higher flow rate and thus prefer to form sunken spherical particles. As for PMMA50.6-Br, the high chain mobility favors the formation of continuous branches-like morphology at the higher flow rate during the film forming process. For homopolymers, as shown in Figure 4, the chain length of PMMA shows great effects on the polymer surface morphology generated at the higher flow rate. By increasing the PMMA chain length, the individual chains would be fixed more strongly in their original positions with subtle deformations because of the low chain mobility, which results from the enhanced interactions between the polymer chains; thus, independent sunken spherical particles are formed. Moreover, the solvent evaporation rate would decrease, which means a longer time for chain rearrangement, thus the sunken spherical particles become flat. In comparison with PMMA homopolymers, the length of PMMA block has little effects on the surface morphology of diblock copolymers at the higher flow rate; only particles are formed because the PMMA blocks can only rearrange around the core composed of fluorinated segments. By further increasing the flow rate and length of fluorinated block, the

times, which might be attributed to the low degree of polymerization of fluorinated monomers. Effects of Flow Rates and Lengths of Block on the Surface Morphology and Wettability of the SelfAssembled Electrosprayed Films. Electrospinning of semifluorinated copolymers as a possible way for imparting roughness and low surface free energy to the polymer surfaces concurrently has attracted much academic attention,41,42 whereas the self-assembly of the incompatible fluorinated segments in diblock copolymer would affect the surface compositions and the phase morphology and, consequently, affect the properties of such fluoro-modified polymers.43,44 In the following section, SEM and CA measurements are used to investigate the relationship between the self-assembled behaviors of PMMA-b-PDFMA during the electrospraying process and the surface morphology and wettability, respectively. Figures 4, 5, and 6 show the SEM images of the electrosprayed films of polymers prepared at different flow rates with the mixture of DMF and THF (DMF/THF = 1:1, w/w) as solvent, and the optical photographs of water droplets on these films are embedded in each SEM image. The surface morphology of the electrosprayed polymeric film is strongly correlated to the solution property, solution flow rate, and the polymer intrinsic properties.45,46 As shown in Figures 4−6, only polymer particles are formed in all cases except continuous branches like morphology for PMMA50.6 formed at the flow rate of 5 μL/min. The formation of particles might be due to the insufficient molecular weight and polymer concentration, as observed for other semifluorinated block copolymers.41,47 For both homopolymers and diblock copolymers, shrunken spheres were formed at the lower flow rate of 1 26288

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Figure 4. SEM images of PMMAm-Br films electrosprayed from the mixture of DMF and THF (DMF/THF = 1:1, w/w) at the flow rate of 1 μL/ min (a−c) and 5 μL/min (a′−c′) ((a,a′) PMMA50.6-Br; (b,b′) PMMA135.4-Br; (c,c′) PMMA181.7-Br).

long fluorinated blocks tend to migrate toward the surface because of the dynamic driving force to minimize the interfacial free energy with sufficient rearrangement time. Meanwhile, the strong incompatibility between fluorinated segments and hydrocarbon chains leads to the phase separation and results in the formation of nanopore shaped spheres. The static contact angle measurements listed in Table 2 indicate that hydrophobic surfaces (CA > 130°) and even superhydrophobic surfaces (CA > 150°) could be fabricated by electrospraying the homopolymers and diblock fluorinated copolymers. For both homopolymers and diblock copolymers, the CA values increase with increasing the flow rate because the high flow rate would provide more time for hydrophobic

groups to segregate to the polymer−air interface. However, the CA values decrease with increasing the chain length of PMMA because the high degree of chain entanglement in polymers would limit the migration of hydrophobic groups toward the polymer−air interface. Compared with the corresponding homopolymers, fluorinated diblock copolymers having long PMMA blocks and short PDFMA blocks do not show extremely high CA values, as shown in Table 2. This result can be attributed to the restricted surface segregation of fluorinated blocks, which is due to the formation of polymer micelles consisting of a core of short fluorinated blocks and a shell of long PMMA blocks during the film formation. However, the surface hydrophobicity could be 26289

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Figure 5. SEM images of PMMAm-b-PDFMAn films electrosprayed from the mixture of DMF and THF (DMF/THF = 1:1, w/w) at the flow rate of 1 μL/min (a−c) and 5 μL/min (a′−c′) ((a,a′) PMMA50.6-b-PDFMA0.8; (b,b′) PMMA135.4- b-PDFMA3.6; (c,c′) PMMA181.7-b-PDFMA4.1).

surfaces during the compression−relaxation process show that the water droplet cannot be deposited on the superhydrophobic surfaces even after the compression. These results reveal that superhydrophobic surfaces are easily fabricated with short-chain polymers. For short-chain polymers, the strong chain mobility would favor the reorientation of molecules at the polymer−air interface, which results in the high surface concentration of hydrophobic groups. In terms of PMMA50.6, more opportunities are available for the hydrophobic groups to migrate toward the air interface during the film formation at a higher flow rate, which is responsible for the fabrication of superhydrophobic surfaces, whereas for PMMA50.6-b-PDFMA0.8, the formation of micelles with fluorinated blocks as core and the short MMA blocks as shell

enhanced greatly by increasing the weight fraction of fluorinated monomers in diblock copolymers. For a superhydrophobic surface with lotus effect, a large contact angle (θ > 150°) as well as a low contact angle hysteresis (θh < 10°) is needed.48,49 In this work, superhydrophobic surfaces were obtained by electrospraying PMMA50.6 (θ = 156°, θh = 3°, 5 μL/min), PMMA50.6-bPDFMA0.8 (θ = 154°, θh = 8°, 1 μL/min; θ = 155°, θh =6°, 5 μL/min), and PMMA147.9-b-PDFMA17.5 (θ = 160°, θh = 3°, 1 μL/min; θ = 160°, θh = 3°, 5 μL/min; θ = 160°, θh = 2°, 2 mL/ min). The small hysterisis angles indicate that the superhydrophobic surfaces have nonwetting property (i.e., selfcleaning ability). The optical photographs (see Supporting Information) of water droplets on the superhydrophobic 26290

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Figure 6. SEM images (a,b) and optical photograph (c) of PMMA147.9-b-PDFMA17.5 films electrosprayed at the flow rate of 1 μL/min (a) and 2 mL/ h (b).

Table 2. Water Contact Angles of Surfaces Fabricated by Electrospraying Homopolymers and Copolymers with Different Block Lengths at Different Flow Rates contact angle (deg) under different flow rates sample PMMA50.6 PMMA135.4 PMMA181.7 PMMA50.6-b-PDFMA0.8 PMMA135.4-b-PDFMA3.6 PMMA181.7-b-DFMA4.1 PMMA147.9-b-PDFMA17.5

fluorine content (wt %)

1 μL/min

5 μL/min

3.2 5.5 4.7 18.3

138 135 134 154 140 132 160

156 140 139 155 144 136 160

2 mL/h

5 μL/min (in THF)

5 μL/min (in DMF)

138

83

135

98

160

the polymer concentration of 2% (w/w). The polymer solutions were electrosprayed at the flow rate of 5 μL/min or cast on aluminum foil followed by drying at 40 °C for 24 h. The SEM images of the electrosprayed or casting films of the diblock copolymers are given in Figure 7, and the optical photographs of water droplets on the films were embedded in each image. As shown in Figure 7b, rather flat sunken spheres are obtained by electrospraying the polymer solution from DMF, while shrunken spheres are formed from THF. Since the boiling point of DMF is higher than that of THF, the longer evaporation process could be expected for polymer chains to rearrange, which can be used to interpret the above phenomena. The solvent components play important roles in the migration of hydrophobic groups toward the surface.50 Contact angle measurements confirm that films generated from THF (CA ≈ 135° for electrosprayed film; CA ≈ 99° for casting film) have stronger hydrophobicity than that generated from DMF

would make the surface segregation of hydrophobic groups much easier; thus, a superhydrophobic surface could be obtained even at a lower flow rate of 1 μL/min. By further increasing the fluorinated block length, the concentration of the fluorinated blocks at the outer shell of the micelle would increase because of the incomplete shielding of the fluorinated blocks by PMMA chains and the tendency to decrease the surface free energy driven by thermodynamic force, which would lead to the formation of superhydrophobic surfaces (e.g., the superhydrophobic surfaces could be obtained by electrospraying PDFMA147.9-b-PDFMA17.5 at any selected flow rate). Surface Composition Analysis of PMMA 181.7 -bPDFMA4.1 Copolymer Films Fabricated from Different Solvents. To investigate the important roles of solvents and film forming techniques in the determination of the surface morphology and wettability, PMMA181.7-b-PDFMA4.1 was selected and dissolved in DMF and THF, respectively, with 26291

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Figure 7. SEM images of the electrosprayed films (a,b) and casting films (c,d) of PMMA181.7-b-PDFMA4.1 films from different solvents ((a,c) THF; (b,d) DMF).

reveals that hydrophilic groups enrich at the film−air interface, which results in a lower CA value, whereas, as shown in the high-resolution spectra and Table 3, it is surprising that the −CF2− group is detected at both probe depths of the polymer film, which is attributed to the surface segregation of C−O− CO during the electrospraying process with DMF as solvent. For films electrosprayed from THF, as shown in Figure 8c and Table 4, −CF2− group could hardly be detected in the outermost layer, but a large quantity of −C−C− and C−H groups (68%) are detected. Therefore, a higher contact angle (135°) is observed. From Table 4 and Figure 8d, −CF2− group can be detected by increasing the probing depth, which verifies the formation of aggregates composed of PDFMA blocks as core and PMMA blocks as shell. For the polymer film electrosprayed from a mixed solvent, the hydrophobic groups would preferentially segregate to the air interface. Also, the longer time for chain rearrangement accompanied by the presence of DMF contributes to the higher CA value (136°). From the above discussion, we propose that THF favors the surface segregation of −C−C− and C−H groups in PMMA blocks; while DMF favors the surface segregation of C−O− CO.

(CA ≈ 96° for electrosprayed film; CA ≈ 86° for casting film) regardless of the film forming techniques. In terms of film forming techniques, with THF as solvent, the electrosprayed film (CA ≈ 135°) is more hydrophobic than the casting film (CA ≈ 99°), which can be attributed to the increased surface roughness, but with DMF as solvent, little difference in the surface wettability can be observed between the two films. Apparently, the surface compositions have dominant effects on the surface wettability, which are closely related to the solvent properties. Surface compositions of a few atomic layers and the arrangements of the hydrophobic groups at the surfaces are of great importance to the surface wettability. To understand the variations in the surface free energies of PMMA181.7-bPDFMA4.1 films fabricated from different solvents, the surface compositions of the films are examined by XPS. Figure 8 shows the C1s XPS high-resolution spectra of the PMMA181.7-bPDFMA4.1 films electrosprayed from DMF (Figure 8a,b) and THF (Figure 8c,d) under different incident angles of 30° (Figure 8a,c) and 90° (Figure 8b,d). The spectra of C1s are separated into sub-Gaussian curve fitted peaks corresponding to the C bonding states. The compositions of the functional groups at different probing depths of the films are summarized in Tables 3 and 4. From the spectra of films electrosprayed from DMF, four types of functional groups are observed in Figure 8a,b. The peaks with binding energies of 285.0, 286.5, 288.5, and 291.5 eV can be assigned to hydrocarbon (C−C, C− H), methoxy group carbon (C−O−CO), carbon in the ester group (CO) or (−C−F), and −CF2−, respectively. Table 3

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CONCLUSIONS In summary, we have synthesized a series of PMMAm-Br homopolymers and PMMAm-b-PDFMAn diblock copolymers with low polydispersity indexes via ATRP method, and superhydrophobic surfaces with lotus effect have been fabricated by electrospraying the PMMA-Br macroinitiators 26292

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Figure 8. C1s XPS high-resolution spectra of the PMMA181.7-b-PDFMA4.1 films electrosprayed from DMF (a,b) and THF (c,d) under different incident angles of 30° (a,c) and 90° (b,d).

closely related to the solvent properties. During the electrospraying process, DMF favors the surface segregation of C−O− CO, while the C−C and C−H groups tend to enrich at the film−air interface with THF as solvent. Therefore, the film electrosprayed from THF is more hydrophobic than that from DMF. It can be concluded that, by adjusting the polymeric degree of the two segments in diblock copolymer, superhydrophobic surfaces having nonwetting property can be obtained using the one-step electrospraying method.

Table 3. Surface Chemical Functionalities and the Concentrations Calculated from C1s XPS Fitting Spectra (DMF) incident angle (deg)

C−C and C− H (%)

−C−O−CO (%)

−CO and C−F (%)

−CF2 (%)

30 90

36 49

17 16

39 30

8 5

Table 4. Surface Chemical Functionalities and the Concentrations Calculated from C1s XPS Fitting Spectra (THF) incident angle (deg)

C−C and C− H (%)

−C−O−CO (%)

−CO and C−F (%)

−CF2 (%)

30 90

68 27

15 27

17 31

0 15

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ASSOCIATED CONTENT

S Supporting Information *

Optical photographs of water droplets on the surfaces during the process of compression and relaxation. This material is available free of charge via the Internet at http://pubs.acs.org.

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and PMMAm-b-PDFMAn copolymers. Results indicate that the surface hydrophobicity can be enhanced by increasing the flow rates, and superhydrophobic surfaces were achieved by electrospraying PMMA50.6 at the flow rate of 5 μL/min. PMMA-b-PDFMA copolymers having long PMMA blocks and short PDFMA blocks exhibit similar surface wettability to the corresponding PMMA homopolymers; however, superhydrophobic surfaces can be obtained by electrospraying diblock copolymers having short PMMA blocks or long fluorinated blocks. XPS detections indicate that the surface composition is the major determinant of the surface wettability, which is

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the Program for New Century Excellent Talents in University (NCET-08-0165). 26293

dx.doi.org/10.1021/jp305562s | J. Phys. Chem. C 2012, 116, 26284−26294

The Journal of Physical Chemistry C

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Article

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dx.doi.org/10.1021/jp305562s | J. Phys. Chem. C 2012, 116, 26284−26294