Recent Developments in Polymeric Superoleophobic Surfaces

Transcription

1 POLYMER SCIENCE REVIEW Recent Developments in Polymeric Superoleophobic Surfaces Zhongxin Xue, 1,2 Mingjie Liu, 1,2 Lei Jiang 1,3 1 Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing , People s Republic of China 2 Graduate School of Chinese Academy of Sciences, Beijing , People s Republic of China 3 School of Chemistry and Environment, Beihang University, Beijing , People s Republic of China Correspondence to: L. Jiang ( jianglei@iccas.ac.cn) Received 1 April 2012; accepted 4 June 2012; published online 23 June 2012 DOI: /polb ABSTRACT: The construction and application of superoleophobic surfaces have aroused worldwide interest during the past few years. These surfaces are of great significance not only for fundamental research but also for various practical applications in self-cleaning, oil-repellent coatings, and antibioadhesion. The unique properties of polymers have made them one of the most important materials for constructing superoleophobic materials. This article reviews recent developments in the design, fabrication, and application of polymeric superoleophobic surfaces. VC 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: , 2012 KEYWORDS: multifunctional superoleophobicity; oil adhesion; oil-repellent coating; oil/water separation; polymer; superoleophobicity; underwater superoleophobicity INTRODUCTION Wettability is a fundamental property of a solid surface that plays important roles in daily life, industry, and agriculture. 1 3 Superantiwetting surfaces such as superhydrophobic and superoleophobic surfaces (that display contact angles with water or oil droplets larger than 150 ) are one of the most extensively studied in this field In learning from nature two key surface parameters, surface energy, 12 and roughness, are studied in depth. A number of artificial superhydrophobic surfaces have been developed, 2,17 24 and these materials have broad applications in self-cleaning, anti-icing, antifogging, antifouling, and drag reduction However, superoleophobic surfaces are difficult to achieve due to the low surface tension of organic liquids (Table 1) Superoleophobic surfaces are of great significance not only for fundamental research but also for various economically important applications in oil-repellent coatings, marine antifouling, antioil treatment of oil pipelines, and antibioadhesion. These demands emphasize the need for more research on superoleophobic surfaces. Superoleophobic surfaces were first reported by Tsujii et al. in They fabricated a fluorinated monoalkylphosphate-modified fractal anodically oxidized aluminum surface, which showed a contact angle of 150 for rapeseed oil droplets. They concluded the basic strategy of design as superoleophobic surfaces is treating a surface of enough roughness with trifluoromethyl groups of low enough surface tension. From then on, the study of superoleophobicity aroused with broad interest. In 2001 and 2004, our group reported nanostructured aligned carbon nanotube (ACNT) films 30 and micro/nano hierarchically structured polymer surfaces 33 with excellent superoleophobicity, respectively. Those studies revealed the significance of nanostructures and micro/nano hierarchical structures for constructing superoleophobic surfaces. In 2007, Tuteja et al. showed that the introduction of re-entrant surface curvature was another effective way to design superoleophobic surfaces. 29 This concept considerably advanced the development of the field. Recently, our group reported a new concept of constructing underwater superoleophobic surfaces in oil/water/solid three-phase systems, which is inspired by the antiwetting behavior of oil droplets on fish scales in water. 34 By introducing the repulsive liquid phase into the micro/nano hierarchically structured surface, the superoleophobicity is easily achieved in water in a simple and fluoride-free way. Compared with superoleophobicity in air, the underwater superoleophobicity showed special applications on marine antifouling, antioil adhesion for oil pipelines, oil spill cleanup, and antibioadhesion in vivo. 1,35 Research on superoleophobicity has advanced significantly in recent years, and the number of articles published on the topic shows rapid growth (Fig. 1). Among them, polymers with their flexibility, diversiform molecular design, good processability, and low cost have become one of the most promising materials to construct superoleophobic surfaces. Fluorinated polymers are one of the most popular VC 2012 Wiley Periodicals, Inc. POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

2 REVIEW POLYMER SCIENCE Zhongxin Xue is currently a Ph.D. student at the Institute of Chemistry at the Chinese Academy of Sciences (ICCAS), China. She received her B.S. degree in chemistry from Northeast Normal University (2008). In 2008, she joined Prof. Lei Jiang s group and received her M.S. degree in Her current research interest is focused on using special wettability to design novel materials for oil/water separation. Mingjie Liu received his B.S. degree (2005) in chemistry from Beijing University of Chemical and Technology. In 2005, he joined Prof. Lei Jiang s group and received his M.S. degree (2007) and Ph.D. degree (2010) from National Center for Nanoscience and Technology, China (NCNST). He now works as a postdoctoral fellow in Prof. Takuzo Aida s group in Riken. His current research interest is focused on bioinspired design of functional soft materials. Lei Jiang received his B.S. degree (1987), M.S. degree (1990), and Ph.D. degree (1994) from the Jilin University of China (Jintie Li s group). He then worked as a postdoctoral fellow in Prof. Akira Fujishima s group at Tokyo University. In 1996, he worked as a senior researcher at the Kanagawa Academy of Sciences and Technology under Prof. Kazuhito Hashimoto. He joined the Institute of Chemistry at the Chinese Academy of Sciences (ICCAS) as part of the Hundred Talents Program in He is currently a professor at ICCAS and his scientific interests are focused on bioinspired surfaces and interfacial materials. materials to fabricate superoleophobic surfaces due to their extremely low surface energy. In addition, the low cost and easy processing properties of polymers are convenient for large-area fabrication. 36,37 Furthermore, multifunctional polymers can be easily fabricated through copolymerization, atom transfer radical polymerization (ATRP), and graft polymerization process, which provided a facile way to produce functionally integrated superoleophobic surfaces. More importantly, responsive polymers can change their chemical or physical properties reversibly with the external stimuli exerted on them. 26,38 Due to this feature, polymers have been used to prepare smart superoleophobic surfaces (that can switch between superoleophobicity and superoleophilicity reversibly using external stimuli like temperature, ph, light, electricity, magnetism, and so on), with attractive applications such as controllable oil/water separation and bioseparation. 39 In this review, we will cover the design, fabrication, and recent developments of polymer superoleophobic surfaces. First, we will present the basic rules of design for superoleophobic surfaces in air and in water, respectively. Then, we will summarize the fabrication methods of polymeric superoleophobic surfaces in air. In the next section, we will introduce superoleophobic surfaces in water. After that, we will conclude several applications of the polymer superoleophobic surfaces. Finally, the challenges and outlook on the future of this subject will be discussed. DESIGN OF SUPEROLEOPHOBIC SURFACES Superoleophobic Surfaces in Air Three key surface parameters play complementary roles on the design of superoleophobic surfaces in air: chemical composition, roughness, and re-entrant surface curvature. Chemical Composition The chemical composition of a material determines the surface free energy and thus has a great impact on wettability. To achieve a superoleophobic surface, the contact angle of the liquid drop on a flat surface is expected to be larger than 90. Then it is possible for the surface to repel the liquid completely by introducing enough roughness. Usually, The contact angle h on a flat surface is determined by Young s equation (eq 1), 40 cos h ¼ðc sv c sl Þ=c lv (1) where h is the contact angle in the Young s mode, c sv, c sl,and c lv are the different surface tensions (solid/vapor, solid/ liquid, and liquid/vapor). If h is 90, c sv is equal to c sl.we assume the interaction force between the two materials is of the same order, the c sl can be approximated by the following equation (eq 2), p c sl ¼ c sv þ c lv 2 ffiffiffiffiffiffiffiffiffiffiffi c sv c lv (2) Combining eqs 1 and 2, we can conclude that the surface free energy of the solid substrates (c sv ) should be low 1210 POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

3 POLYMER SCIENCE REVIEW TABLE 1 The Surface Tension of Various Liquids Liquid n-hexane 18.4 n-octane 21.6 n-decane 23.8 n-dodecane 25.4 Amyl alcohol 25.8 n-hexadecane 27.5 Octanol 27.5 Dodecanol 29.4 Methyl myristate 29.4 Diethyl glutarate 32.3 Cyclohexane 33.4 Diethyl adipate 36.0 Chlorobenzene 36.0 Ethyl benzoate 37.2 Dimethyl malonate 37.4 Benzyl chloride 40.0 Monoacetin 41.8 Diethylene glycol 45.2 Glycerol 63.4 Water 72.3 Surface Tension (mn/m) enough to be smaller than one quarter of that of the liquid (c lv ). Typical surface tensions of oils are much smaller than water (Table 1), such as 35 m Nm 1 for rapeseed oil and 27.5 m Nm 1 for hexadecane. Therefore, to be superoleophobic, the surface energy of the solid surface should only be a few m Nm 1. Such a low surface energy can only be realized with the -CF 3 group. This stringent requirement highlights the difficulty of fabricating superoleophobic surfaces and limits the choice of materials. Rough Structure The rough structure of the surface is also an important factor that influences the wettability. As described by Wenzel s equation (eq 3), 41 trapped air layer will contribute greatly to repel the liquid. Therefore, the rough structure is a key factor of constructing superoleophobic surfaces. Special Surface Curvature Structure In addition to chemical composition and rough structure, a third parameter, re-entrant surface curvature structure is essential to achieve superoleophobicity. It introduces the requirement for overhang structures, mushroom-like structures, inverse-trapezoidal microstructures, negative slopes, and fibers. 29,43 54 The concept of re-entrant surface curvature was put forward by Tuteja et al. 29 In their studies, they synthesized a class of polyhedral oligomeric silsesquioxane (POSS) molecules [the insert of Fig. 2(C)] and fabricated fiber mats with them [Fig. 2(C)]. Unexpectedly, all the intrinsic contact angles of the compositions were lower than 90 ; however, the fiber mats displayed superoleophobicity. They ascribed this phenomenon to the re-entrant surface curvature structures and discussed the possible mechanism. As shown in Figure 2(A,B) w is the local geometric angle of the texture; h is the contact angle of the liquid on the flat solid surface. When h > w [Fig. 2(A)], the net traction on the liquid vapor interface is directed upward. In this case, the liquid vapor interface recedes to the top of the pillars, creating a composite solid liquid air interface, which will stabilize the oleophobic Cassie state. On the contrary, if h < w [Fig. 2(B)], the net traction is downward, promoting the imbibition of the liquid into the solid textures. 29,44,47 They also fabricated a regular microhoodoo surface to study the influence of the re-entrant surface curvature factor [Fig. 2(D)] and backed up their study with theoretical calculations. 44 This work has inspired many efforts to construct superoleophobic surfaces using the re-entrant surface curvature factor, such as a poly(dimethylsiloxane) (PDMS) surface with inverse-trapezoidal microstructures [Fig. 2(E,F)] 45 and porous silicon films with overhang structures [Fig. 2(G,H)]. 43 Surfaces possessing re-entrant textures (W > 90 ) facilitate extremely high apparent contact angles, even if h < 90, and cos h 0 ¼ r cos h (3) where h 0 is the apparent contact angle on a rough surface, while h is the intrinsic contact angle on a flat surface, the roughness r can enhance both the wetting and antiwetting behavior of liquid on the solid. The modified Cassie s equation (eq 4), 42 cos h 0 ¼ f cos h ð1 f Þ (4) in which f is the fraction of the solid/liquid interface, while (1 f) is that of the air/liquid interface, indicates that, when a rough surface, which possesses nano structures 30 or micro/nano hierarchical structures, 33 comes into contact with liquid, air may be trapped in the rough area. The FIGURE 1 The number of articles indexed in the ISI web of science under the topic of superoleophobicity. POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

4 REVIEW POLYMER SCIENCE prepare superoleophobic artificial surfaces: (1) constructing rough structures on low surface energy substrates or (2) chemically modifying rough substrates with low surface energy materials. In the meanwhile, the introduction of reentrant surface curvature will enhance the stability of the superoleophobicity and be helpful to choose relative oleophilic materials to prepare superoleophobic surfaces. Superoleophobic Surfaces in Water Hydrophilic Chemical Composition The wettability of solid surface is commonly evaluated by the contact angle given by Young s equation. Although Young s equation was originally applied for a liquid droplet on a solid surface in air, it has also been applied to a liquid droplet on a solid surface in the presence of a second liquid. 55,56 Using Young s equation, we could infer the following equation 34,57 (eq 5) [Fig. 3(A)]: cos h 3 ¼ c l 1 g cos h 1 c l2 g cos h 2 c l1 l 2 (5) FIGURE 2 Critical role of re-entrant surface curvature on the design of superoleophobic surfaces in air. (A and B) Schematic diagrams illustrating possible liquid vapor interfaces on two different surfaces. (Reproduced from Ref. 44, with permission from National Academy of Sciences.) (C and D) SEM images of electrospun fluorodecyl POSS surface and microhoodoo surface. The insert shows the molecule structure of fluorodecyl POSS molecules. (Reproduced from Ref. 44, with permission from National Academy of Sciences.) (E and F) SEM images of PDMS surface with inverse-trapezoidal microstructures array. (Reproduced from Ref. 45, with permission from Royal Society of Chemistry.) (G and H) porous silicon films with overhang structures. (Reproduced from Ref. 43, with permission from American Chemical Society.) are therefore important for designing superoleophobic surfaces. Generally speaking, to design a superoleophobic surface in air, the above parameters should be considered comprehensively and complementarily. Two approaches can be used to where c l1 g, c l2 g, andc l1 l 2 are the liquid 1/gas interface tension, the liquid 2/gas interface tension, and the liquid 1/liquid 2 interface tension, respectively. h 1, h 2 and h 3 are the contact angle of liquid 1 in air, liquid 2 in air, and liquid 1 in liquid 2, respectively. When oil drops contact with the surface in water, liqud1 and 2 are oil and water, respectively [Fig. 3(A)]. To achieve an oleophobic surface, cos h 3 should be negative. Therefore, c l1 g cos h 1 should be lower than c l2 g cos h 2.Since the surface tension of oils and organic liquids (c l1 g) ismuch lower than that of water (c l2 g), cos h 1 and cos h 2 are inferred to be positive, which means the surfaces are hydrophilic. Through the above analysis, we can see that hydrophilic surfaces in air can become oleophobic in water. Micro/Nano Hierarchical Structure For a rough surface composed of solid and air, Cassie et al. proposed a model describing the contact angle in a water/air/ solid system. 42 In an oil/water/solid system, on the other hand, where the rough surface is composed of solid and water [Fig. 3(B,C)], the Cassie model is expressed as follows (eq 6): cos h 0 3 ¼ f cos h 3 þ f 1 (6) where f is the area fraction of solid, h 3 is the contact angle of an oil droplet on a smooth surface in water, and h 0 3 is the contact angle of an oil droplet on a rough surface in water. FIGURE 3 Diagrams illustrate the effect of surface structure on the wetting behaviors of solid substrates in oil/water/solid threephase systems. A liquid 1 droplet on (A) smooth surface, (B) microstructured surface, and (C) micro/nanostructured surface in a liquid 2 phase. (Reproduced from Ref. 34, with permission from Wiley-VCH.) 1212 POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

5 POLYMER SCIENCE REVIEW FIGURE 4 Structures of superoleophobic polymer surfaces from various fabrication approaches. (A) Electrospinning (Reproduced from Ref. 64, with permission from American Chemical Society.), (B) electrochemical polymerization (Reproduced from Ref. 65, with permission from American Chemical Society.), (C) organic/inorganic hybrid (Reproduced from Ref. 66, with permission from American Chemical Society.), (D) plasma treatment (Reproduced from Ref. 67, with permission from American Chemical Society.), (E) natural template (Reproduced from Ref. 68, with permission from American Chemical Society.), (F) artificial template (Reproduced from Ref. 69, with permission from The Royal Society of Chemistry.), (G) on-step casting (Reproduced from Ref. 33, with permission from Wiley-VCH.), (H) hydrolyze-self-assembly (Reproduced from Ref. 70, with permission from Wiley-VCH.), (I) layerby-layer approach (Reproduced from Ref. 71, with permission from Elsevier.) When surfaces with micro/nano hierarchical structure contact the oil drops, water molecules can be trapped in the micro/nanostructures, leading to a composite water solid interface [Fig. 3(C)]. This new composite interface shows superoleophobic properties. In other words, the water trapped in the hierarchically rough structures is a repulsive liquid phase for oil. Trapped water serves as a support to prevent the penetration of oil droplets, yielding superoleophobic and low-adhesive surfaces in water. Recently, the idea of introducing repulsive liquids into constructing superantiwetting surfaces has aroused increasing attention More interesting discoveries may appear in this area. Generally speaking, the hydrophilic chemical compositions and hierarchical rough structures are key parameters to design underwater superoleophobic surfaces. Research on underwater superoleophobicity has just started, and the theories are still being developed. FABRICATION OF POLYMER SUPEROLEOPHOBIC SURFACES The methods to fabricate superoleophobic surfaces usually follow bottom up or top down approaches. The bottom up approaches, which include electrospinning, self-assembly, layer-by-layer, and so on, are usually used to produce random surfaces, and are usually simple and low cost and therefore suitable for practical applications. The top down approaches, which involve template methods, lithographic approaches, plasma treatment, and so on, are generally used to prepare ordered surfaces, so are useful for fundamental research. Polymers play important roles in constructing POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

6 REVIEW POLYMER SCIENCE superoleophobic surfaces, because they have the unique combination of properties of flexibility, diverse molecular design, good processability, and low cost. Various methods to fabricate polymer superoleophobic surfaces have emerged. Electrospinning Electrospinning is a versatile technique to prepare fibers with diameters ranging from micrometers to nanometers In a typical electrospinning process, a high voltage is applied to a nozzle, through which the polymer solution is pumped. Because of the evaporation of the solvent, the solution jet solidifies and forms a nonwoven film on the collector. The morphology of the electrospinning products is strongly influenced by the concentration of the solution, providing ultrafine fibers from high concentration and microparticles from low concentration. By modifying the viscosity, porous nanofibers/microspheres composite three-dimensional network structures can be obtained [Fig. 2(C)]. In addition, a higher surface charge density on the solution-jet surface will result in smaller beads and thinner fiber diameters. Electrospinning is an effective method to construct polymeric superoleophobic surfaces. As mentioned above, Tuteja et al. synthesized a class of POSS molecules [the insert of Fig. 2(C)] with very low surface energy. They blended POSS with poly(methyl methacrylate) (PMMA), and then used electrospinning to fabricate fiber mats with beads on a string structures. 29,44 The re-entrant surface curvature of fiber mats makes the surfaces superoleophobic, even though all of the corresponding compositions are oleophilic. 29,44 Han and Steckl produced core-sheath-structured micro/ nanofibers with Teflon AF sheath and poly(e-caprolactone) (PCL) core by coaxial electrospinning technique [Fig. 4(A)], which showed superhydrophobic and oleophobic properties. 64 Teflon AF is a normally nonelectrospinnable material because of its low dielectric constant. The authors combined Teflon AF with an electrospinnable core material and successfully solved the problem. This method of coaxial electrospinning provides a new degree of freedom to choose materials for creating superoleophobic surfaces. Choi et al. reported a superoleophobic poly(2,2,2-trifluoroethyl methacrylate) fiber film with contact angle for hexadecane larger than This is a typical electrospinning method without any additional treatment, thus showing potential applications for industrial production. Electrochemical Polymerization and Electrochemical Deposition Electrochemical polymerization and electrochemical deposition are facile and inexpensive methods to construct rough surfaces regardless of the size and shape of the substrate. Nicolas et al. reported a series of work on preparation highly fluorinated conductive polymers with superoleophobicity by this method In 2006, they synthesized fluorinated thiophene monomers, and electrochemically prepared poly (fluorinated thiophene) films. 73 The electrochemical polymerization approach was quick, and led to a controlled rough surface. The poly(fluorinated thiophene) films showed excellent superoleophobic property. Then they synthesized a monomer containing a fluorinated chain grafted to 3,4-ethylenedioxypyrrole (EDOP) core. Through electrochemical polymerization, they successfully made another superoleophobic surface with this monomer. The oleophobicity can be tuned with the deposition charge, monomer concentration, nature of the doping anion, and nature of the substrate. 74,75 After that, they synthesized novel fluorinated monomers and their corresponding electrodeposited polymers with superoleophobic property. 65,76 78 The nanoporous structures [Fig. 4(B)] and the low surface energy of the monomer are the key reasons for the superoleophobicity. Other groups also used this method to develop superoleophobic surfaces. Yan et al. 79 and Chiba et al. 80 synthesized new monomers of fluorinated alkylpyrroles, and achieved highly oil-repellent films by electrochemical polymerization. Surface-Initiated Radical Polymerization and Vapor- Phase Polymerization Jung et al. reported on the preparation of oil-repellent surfaces using surface-attached monolayers of perfluorinated polymer molecules. 81 Covalent attachment of the polymer molecules to the substrate surfaces was achieved by generation of the polymer chains through surface-initiated radicalchain polymerization process. To start the polymerization reaction in situ, self-assembled monolayers of azo initiators were attached to SiO 2 substrates. The perfluorinated polymer films can be grown with controlled thicknesses on flat and even on porous silica surfaces, without changing the surface roughness. Wang et al. fabricated patternable, electrically conductive coatings with superoleophobicity by one-step vapor-phase polymerization of polypyrrole (PPy) in the presence of a fluorinated alkyl silane directly on fibrous substrates [Fig. 7(B)]. 82 The treated fabrics showed super repellence to both water and oil fluids; the fabrics were also conductive. Such multifunctional fabrics have potential applications in the development of intelligent clothing and electronics textiles. Organic/Inorganic Hybrid Surfaces Organic/inorganic hybrid materials are often used to constructing superoleophobic surfaces. Generally, they are composed of inorganic nanoparticles, which enhance the roughness, and organic fluoropolymers, which provide a low surface energy. 66,83 90 TiO 2, 83 ZnO, 85,91 66,84,86 90,92 and SiO 2 nanoparticles are the most common inorganic materials. There are three typical approaches: (1) the nanoparticles are first blended with the fluoropolymer, then the solution is coated onto the substrates; 83,85,91,93 (2) the nanoparticles are first modified with low surface energy compounds, then the modified particles are coated on the substrates; 84,88,92 (3) the nanoparticles are first stacked on the substrates, then coated with fluoropolymers. 66,86,87 Schutzius et al. followed the first approach, and made dispersions by combining fluoropolymer blends with clay nanoparticles or carbon nanowhiskers. 93 After spray processing onto substrates, the composite coatings showed water and water alcohol repellent properties. The coatings had strong adhesion with 1214 POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

7 POLYMER SCIENCE REVIEW substrates, which showed excellent durability. They also prepared fluoropolymers/zno nanoparticle dispersions, which were super repellent for water and isopropyl alcohol mixtures. 91 In the third approach, the binary nano- and submicrometer-scaled rough surfaces were proven to exhibit better superoleophobicity. Hsieh et al. stacked nanosized SiO 2 layers on submicrometer-sized SiO 2 layers, to make a surface with hierarchical roughness. After coating with fluoropolymer, the surface showed better superoleophobicity than the ones with a single size of particles [Fig. 4(C)]. 66,86,87 Plasma Treatment Plasma treatments used to create superoleophobic surfaces include plasma etching and plasma polymerization. They can be adopted as the only technique or coupled with other techniques as a combination to create superoleophobic surfaces. Plasma etching is one of the most commonly used methods to enhance the roughness of the substrates. In addition, through anisotropic etching, plasma treatment can cause a considerable change in the surface structures. Ellinas et al. coated PS beads on PMMA plates, after anisotropic and isotropic etching steps by O 2 plasma, a structure of PMMA pillar with rough PS sphere on the top was obtained [Fig. 4(D)]. 67 The sizes of the hierarchical micropillars can be tuned using the plasma etching process, which provides various rough structures for superoleophobicity. Furthermore, depending on the type of gas, fluoroelements can be easily introduced in the surface, 67,70,94 96 which is a simple way to modify a surface with low surface energy compounds. Template Methods Template methods are effective to construct regular surface patterns and even relatively complex structures. Generally speaking, the template process includes three steps: preparing a featured template master, molding the replica, and removing the templates. The templates can be natural surfaces, 34,57, nanoporous anodic aluminum oxide, 69,102,103 micropatterned Si surfaces, or other artificial structured surfaces. Jung and Bhushan used shark skins and micropatterned Si as templates, and then produced an epoxy resin replica through a two-step molding process. After that, the surface was modified with n-perfluoroeicosane, and a highly oleophobic surface was obtained. 57 Ghosh et al. chose Colocasia leaf as a template, and prepared a PDMS replica [Fig. 4(E)]. 68 The PDMS-replica was further modified with -CF 3 terminal silica nanoparticles, and became superhydrophobic and highly oil repellent. Liu et al. created Al and Al 2 O 3 molds with terraced micro/nanostructures as a template. 69 Using a molding process, the complex micro/nanostructures were imprinted into polymeric coatings, such as silicone elastomers, polyurethane, ultra high-molecular weight polyethylene and polytetrafluoroethylene. After coating with a sticky perfluoroalkyl, superoleophobic surfaces with low adhesion to a number of oils were obtained [Fig. 4(F)]. Im et al. reported a PDMS elastomer surface with perfectly ordered inversetrapezoidal microstructures using a template method [Fig. 2(E,F)]. 45 The introduction of overhang structures was thought to play positive role in superoleophobicity. Coating Processing Coating processing is a commonly used method to achieve superoleophobic surfaces, such as dip-coating, spin-coating, and spray-coating. The fluoropolymers with low surface energy are usually used. Choi et al. used blend solutions of fluoroposs, Teflon, or PMMA as coating materials and achieved superoleophobic fabrics and metal meshes. 46,51,104 Gao et al. 105 and Huang et al. 106 synthesized a series of POSS-based hybrid terpolymers and fluorinated polyacrylates; after a dip-coating process, various superoleophobic fabrics were obtained. Hsieh et al. investigated a one-step fabrication of fluoro-containing silica coatings on wooden substrates. Those coated woods showed super oil repellency, good durability, and low adsorption capacity of moisture properties. 107 Other Methods One-Step Casting Process Utilizing Difference in Solubility of Polymers Xie et al. reported a superoleophobic PMMA and fluorineend-capped polyurethane film. 33 This film was fabricated by utilizing difference in solubility of the two polymers in one solvent. The SEM image showed the polymer surfaces possess natural lotus-like micro/nano structures [Fig. 4(G)]. Hydrolyze-Self-Assembly Zhang et al. presented a novel grow-from approach for the fabrication of superoleophobic surfaces by the combination of versatile organosilanes. 70 Silicone nanofilaments with different microstructures were grown in toluene onto glass slides by simply regulating the water concentration during hydrolysis and condensation of trichloromethylsilane. Subsequently, the nanofilaments were activated using O 2 plasma and then modified with 1H,1H,2H,2H-perfluorodecyltrichlorosilane [Fig. 4(H)]. The surfaces had high CA and ultra low SA for various nonpolar liquids; they also possessed excellent transparency, and chemical and environmental stability. Layer-by-Layer Approach Yang et al. developed a simple and novel layer-by-layer (LBL) assembly technology and obtained superoleophobic surfaces. 71 The technology is based on the LBL deposition of polyelectrolytes on the textured aluminum surface. By exchanging the counterion [Fig. 4(I)], the oleophobicity can be manipulated. Interestingly, by consecutively exchanging ions between PFO and DYS anions, the wettability can be rapidly switched between superoleophobicity and superoleophilicity for multiple cycles. The combination of the LBL technique and ion-exchange chemistry shows potential applications in the design of surfaces with tunable oleophobicity on other substrates. UNDERWATER SUPEROLEOPHOBICITY Learning from nature, a number of superhydrophobic surfaces have been fabricated. After further improvement of the chemical composition design and special structure POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

8 REVIEW POLYMER SCIENCE FIGURE 5 Surface structures of fish scales and biomimic PAM hydrogel surface. (A) Optical image of the fish scales. (B and C) SEM images of fish scales at low and high magnifications. (D) PAM hydrogel film with fish scale structures (the inserts are oil droplet on the surface underwater and high magnification image). (Reproduced from Ref. 34, with permission from Wiley-VCH.) construction, superoleophobic surfaces are successfully achieved. However, the superoleophobicity, which we introduced above, only exists in air. When we put it in water, superoleophobicity is lost and the surface shows superoleophilic properties. 57,108 Underwater superoleophobicity has broad potential applications, such as oil-repellent coatings on ships and other marine equipments, antimarine biological adhesion, antioil adhesion of oil pipeline, oil spill cleanup, and antibioadhesion materials in vivo. It is therefore of great importance to develop surfaces with superoleophobicity in water. Underwater superoleophobicity in an oil/water/solid three-phase system is a new challenge for scientists. A phenomenon got our attention that seabirds rather than fish are endangered by pollution from oil during a shipwreck. To find out why fish could keep their body clean from oil pollution in water, we studied the chemical compositions and structures of fish scales, tried to search the reasons of the super-antiwetting in water, and then directed the construction of artificial underwater superoleophobic surfaces. 34 Fish scales are composed of calcium phosphate, protein, and a thin layer of mucus that leads to their hydrophilic behavior. For the SEM images, we can clearly observe the micro/nano hierarchical structures [Fig. 5(A C)]. In air, fish scales showed superoleophilicity. However, when put in water, fish scales showed superoleophobicity with oil contact angle larger than 150. We thought water trapped in the hydrophilic rough structures may play an important role in oil repellency. To prove our assumption and better understand the detailed wetting behavior in the oil/water/solid system, smooth, microstructured, and micro/nano hierarchically structured silicon surfaces were selected as our modeling samples. We observed that the smooth silicon surface was oleophobic under water with a contact angle of [Fig. 6(A)], while the microstructured [Fig. 6(B)] and micro/nanostructured silicon surfaces [Fig. 6(C)] showed underwater superoleophobicity with contact angles larger than 150. However, they were significantly different in adhesive behaviors. The smooth silicon surface exhibited remarkable adhesion to an oil droplet of 24.7 ln, the microstructured one showed a relatively lower adhesion of 10.2 ln, and interestingly, the micro/nanostructured one showed nearly no adhesion for oil droplets smaller than 1 ln [Fig. 6(A C)]. Based on the above experimental phenomenon, we undertook a series of theoretical analyses and concluded that hydrophilic chemical compositions and hierarchical rough structures are key parameters to design underwater superoleophobic surfaces. The details were discussed in Design of Superoleophobic Surfaces. According to the simple strategy proposed above, we used hydrogels to fabricate artificial superoleophobic surfaces by replicating fish scales [Fig. 5(D)]. 34 Hydrogels have excellent hydrophilic, water absorbing, and water retaining properties, which are a good choice for constructing underwater superoleophobic materials. The fish scale-structured PAM hydrogel film showed oil contact angle of , and extremely low oil adhesion. Then, we fabricated macromolecule-nanoclay composite hydrogel films, successfully enhancing the mechanical strength of the materials and the stability of the underwater superoleophobicity POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

9 POLYMER SCIENCE REVIEW FIGURE 6 (A C) SEM images of the substrates and shapes of oil droplets taken at different stages during normal adhesive force measurement process on different substrates: on the (A) smooth, (B) microstructured, and (C) micro/nanostructured silicon substrates. (Reproduced from Ref. 34, with permission from Wiley-VCH.) Except for fish scales, we also discovered underwater superoleophobicity on the lower side of a lotus leaf, and fabricated Janus interface materials with in-air superhydrophobicity on one side and underwater superoleophobicity on the other side inspired by the Janus feature of the lotus leaf. 110 The ingenious design on lotus leaf surfaces, superhydrophobicity on its upper side and underwater superoleophobicity on its lower side, not only helps us better understand the special surface wettability of the lotus leaf, but also gives a typical example of multifunctionality in biological systems. To develop an underwater superoleophobic system, we tried to construct multifunctional underwater superoleophobic surfaces, such as a wettability switchable surface, 111 a controllable oil adhesion surface, oil drop manipulation, 115 controllable oil motion, 113 and oil/water separation, 116 which show great potential for applications. We will describe the details in the next section. MULTIFUNCTIONAL SUPEROLEOPHOBIC POLYMERS AND THEIR APPLICATIONS In this section, we focus on the recent developments in the practical and potential applications of superoleophobic polymer surfaces both in air and in water. Transparency Optical transparency is a basic requirement for many optical materials and devices, such as building windows, optical widows for electronic devices, eye-glasses, optical mirrors and lens. For most of them, the oil repellent property or selfcleaning ability is also useful or even necessary. The key for fabricating transparent superoleophobic surfaces is to optimize the surface roughness, since surface roughness usually enhances light scattering and reduces transparency. Generally, for optically transparent surfaces, the roughness dimension should be lower than the wavelength of incident light, and is usually in the range of nanometers. Im et al. fabricated a transparent PDMS surface with perfectly ordered inverse-trapezoidal microstructure. 45 After coating with Teflon, the rough surface became superoleophobic. The transmittance of the structured PDMS was approximately 72% at the visible light wavelength in the range of 400 nm to 700 nm. After Teflon coating, the transmittance was increased to 77% due to antireflective effects arising from the Teflon layer. He et al. fabricated a transparent superamphiphobic coating with PDMS, hydrophobic SiO 2 nanoparticles, and perfluorosilane modification. 117 The transmittance of glass substrates with these coatings is higher than 80% in the wavelength region of 340 to 800 nm. The average height of the coatings is about 380 nm and the average roughness of all samples is about 80 nm, which is lower than the allowable maximum (100 nm) for good transparency. Cao and Gao reported a transparent superhydrophobic and highly oleophobic surface through LBL assembly of SiO 2 nanoparticles and sacrificial polystyrene (PS) nanoparticles. After removing the PS nanoparticles by calcination, a porous network of SiO 2 nanoparticles were achieved. After modification with a fluorocarbon molecule, the surface became highly oleophobic. The optical transmittance of the surface is higher than 90%. 118 Super Oil-Repellent Coatings The broadest application for superoleophobic surfaces is constructing super oil-repellent coatings for textiles, 46,82,89,90,105,106, woods, 107 metal, 53,104,123 and other substrates. 83,88 Oil-repellent coatings can keep substrates from fouling or corrosion by oils and other liquids, and thus show great significance. Fluoropolymers are the most commonly used materials because of their flexibility, diversiform molecular design, good processability, and low cost. The common methods to prepare such coatings are dipcoating, spin-coating, and spray-coating processes, which we POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

10 REVIEW POLYMER SCIENCE FIGURE 7 Conductive superoleophobic fabrics. (A and B) SEM images of the polyester fabric before and after the PPy FAS treatment. (D F) Colored water (green) and hexadecane (red) droplets on (C) PPy FAS-treated, (D) untreated, and (E) PPy-treated polyester fabrics. The PPy FAS-treated polyester fabrics show superoleophobicity. (Reproduced from Ref. 82, with permission from The Royal Society of Chemistry.) have introduced in Fabrication of Polymer Superoleophobic Surfaces. The super oil-repellent coatings are also significant for underwater applications, such as marine oil-repellent coatings for ships, antioil and biological fouling coatings for marine instruments, and antioil-adhesion pipes for crude oil transportation. We have successfully fabricated hydrogel films with underwater superoleophilicity, extremely low oil adhesion and high mechanical strength. 1,34,35,109 In the next step, we will try to use these materials in practical applications. This work is now in progress. A major challenge for superoleophobic coatings is the durability. Compared with many years of development on durability of superhydrophobic surfaces, 124 the research on superoleophobic surfaces has just begun. Wang et al. reported a superoleophobic rough alumina surface with selfhealing property, which showed good durability to external mechanical damage. 125 It possesses a large number of nanopores that act as nanoreservoirs. The low surface energy materials in the nanopores can consecutively release and heal the damaged surface. Electrical Conductivity Among various multifunctional superoleophobic surfaces, electrically conductive superoleophobic coatings are of particular interest owing to their potential applications as electromagnetic interference shielding materials, electrical devices, protective clothing, electronics textiles, and so forth. Conductive superoleophobic surfaces are mainly prepared by electrochemical polymerization using fluorinated monomers. Nicolas et al., 65,73 78 Yan et al., 79 and Chiba et al., 80 have reported a series of work on preparation of highly fluorinated conductive polymers with superoleophobicity by this method, which have been discussed in Fabrication of Polymer Superoleophobic Surfaces. Das et al. developed a simple method to prepare large-area electrically conducting films with super oil-repellent property. They prepared a CNF/polymer dispersion, which can be easily coated on substrates using a spray process. 126 Wang et al. further introduced electrically conductive superoleophobic coatings to fabrics (Fig. 7). 82 This work presented a new pathway to develop intelligent clothing and electronics textiles. Cargo Carriers and Oil Droplets Manipulation Using superoleophobicity to realize floating, load bearing and mechanical manipulation of oil drops is an interesting approach. Inspired by floatation of insects on water, Jin et al. fabricated superoleophobic nanocellulose aerogels, which could support considerable load on a water surface and also on oils. 127 The aerogel was capable of supporting a weight nearly three orders of magnitude larger than the weight of the aerogel itself. It may have applications in dirt-repellent coatings for miniature sensors and other devices floating on generic liquid surfaces. Su et al. successfully applied underwater superoleophobic materials to manipulate oil drops freely in water. 115 Due to the hydrophilic property and hierarchical structures of frosted glass plates, water was trapped in the rough structures of glass microstructures. Trapped water repelled oils from penetrating, yielding superoleophobic and low adhesive surfaces in water. Then, oil droplets, whether heavier or lighter than water, could be manipulated arbitrarily by a pair of tweezers with superoleophobic glass surfaces on the tips. This work proposed a novel, fluoride POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

11 POLYMER SCIENCE REVIEW free strategy for manipulation of oil droplets, which might provoke new ideas for controllable droplet motion and the design of miniature reactors. Smart Surfaces with Switchable Wettability and Controllable Adhesion for Oils Stimuli-responsive materials with their surface free energy or morphology sensitive to the external environment are being used to alter the surface wettability and adhesion properties. 18,20,128 However, the range of wettability transition is usually very limited, which restricts many important applications. Research results show that increasing the surface roughness is an effective way to amplify the surface wettability. Therefore, reversible transition between superoleophobicity and superoleophilicity can be realized by combining the responsive materials and the surface roughness. The study of this subject has just begun and needs more attention. Surfaces with Controllable Wettability Plasma treatment is an easy way to realize the wettability transition. Ellinas et al. prepared an ordered, triple-scale, PMMA polymer substrates through microparticle colloidal lithography. 67 When treated with oxygen plasma, the surface was amphiphilic. When treated with fluorocarbon plasma, the surface became amphiphobic. Photoresponsive switch of wettability is also studied. Zhang et al. fabricated superoleophobic perfluorosilane-modified TiO 2 /single-walled carbon nanotube composite coatings. 129 The wettability conversion of the composite coatings from superoleophobic to superhydrophilic was realized by the gradual decomposition of fluorosilane on the coating surface using UV irradiation. Interestingly, by controlling the UV irradiation dose, liquids with surface tension difference smaller than 5 mn/m can exist in completely converse wetting states on the same coating surface, which means the surface is superphobic for one liquid while superphilic for another with lower surface tension. Mixed organic liquids with different surface tension can be separated through a coated mesh. 129 Kim et al. also used UV irradiation to realize the transition from highly oil repellent to amphiphilic surfaces. 130 Yang et al. used a simple LBL assembly and counterion exchange technology to realize a rapidly and reversibly wettability switch between superoleophobicity and superoleophilicity. 71 The counterion exchange in this polyelectrolyte multilayer emerged easily to control the surface composition, which leads to tunable wettability. The underwater wettability switch behavior was reported by Liu et al. 111 They fabricated PPy aligned conducting polymer nanotube arrays and realized underwater reversible switching between superoleophobicity and superoleophilicity by tuning the electrical potential applied on the nanotube arrays. The underwater wettability switch is considered to result from the cooperative effect of chemical variation of the surface, electrical double layer, and aligned nanostructures of the films. FIGURE 8 Smart control of an oil droplet motion on the nanostrctured PPy surface by adjusting the electrochemical potentials. (A) An oil droplet was parking on the surface by adhesion. (B) The contact angle of oil droplet increased and adhesion decreased when the PPy surface was reduced. (C) The oil droplet began to run. (D) The oil droplet braked and adhered on the surface again. (E G) The oil droplet came back to the superoleophobic and low-adhesion state and rolled down. (Reproduced from Ref. 113, with permission from American Chemical Society.) Surfaces with Controllable Adhesion Control of oil adhesion is a fundamental issue in many applications for special wettable surfaces. The transition between low and high adhesion is hard to achieve, because for the same liquid drop, the wetting-state transition from Wenzel s to Cassie s state is usually irreversible. Yao et al. described a convenient approach to fabricate superoleophobic surfaces through perfluorothiolate reaction on Cu(OH) 2 nanostructure surfaces. 131 The prepared surfaces exhibit controllable oil adhesive force depending on surface nanostructures or external preloads on the oil droplet. Liu et al. developed an in situ underwater controllable oiladhesion surface using an electroresponsive system. 113 The wetting state of oil droplets in aqueous medium could be rapidly and reversibly switched between Wenzel s and POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

12 REVIEW POLYMER SCIENCE FIGURE 9 Underwater oil capture three-dimensional polymer network with superoleophobicity in air. (A) The SEM image of the FTS-derived rough surface. (B D) The surface shows superhydrophobic and superoleophobic in air, superoleophilic in water. (C) The oil capture and collection process with a FTS-derived glass tube. (Reproduced from Ref. 108, with permission from Wiley-VCH.) Cassie s state during a redox reaction of the conducting polymer films. Accordingly, the adhesion on the liquid/solid interface could be switched between an adhesive state and a nonadhesive state. By switching the electrochemical potential of the film, the oil droplet could selectively adhere onto or roll down the surface (Fig. 8). The in situ switched oil adhesion property of PPy films can be utilized to realize smart control of the liquid droplet s motion. In an oil/water/solid three-phase system, there are other excellent research results on controllable oil adhesion based on underwater superoleophobicity, such as thermal-responsive oil-adhesion controllable poly(n-isopropylacrylamide) (PNIPAM) hydrogels from Chen et al.; 112 and morphology-impacting oil-adhesion controllable colloidal crystals assembly from Huang et al. 114 Antibioadhesion When compared with superoleophobic surfaces in air, the underwater superoleophobic surfaces are more important for antibioadhesion applications, because of the liquid environment in vivo. Our group constructed a nanoscaled topography on a thermally responsive PNIPAAm surface by grafting the polymer from silicon nanowire arrays. 132 The asprepared surface showed superoleophobic behavior and low adhesion to oil droplets in water both below and above PNI- PAAm s lower critical solution temperature (LCST), which implied a relatively high ratio of water molecules trapped in the nanostructures. As a result, largely reduced platelet adhesion was achieved on the as-prepared surface, both below and above LCST. It is demonstrated that underwater superoleophobic surfaces are good candidates for preparing materials with antiadhesion for platelets. The works on antiadhesion for cells are also in progress. Oil Capture and Oil Water separation Oil water separation is a worldwide challenge because of the increasing industrial oily waste water as well as the frequent oil spill accidents Materials with both hydrophobic and oleophilic properties have aroused broad attention in recent years. They realized filtration or absorption of oils from water selectively and effectively However, this type of materials is easily fouled even blocked up by oils because of their intrinsic oleophilic property. The adhered oils seriously affect the separation efficiency, service life, and the recycle of oils and materials. Recently two works improve the problems by utilizing superoleophobic systems in air and in water, respectively. Jin et al. created a FTS-derived surface with nanofibers and microbumps intertwined in a three-dimensional network structure [Fig. 9(A)]. 108 The cooperation of low free-energy and micro/nanoscale roughness made the surface superamphiphobic in air [Fig. 9(B,C)]. Interestingly, this surface showed a contrary superoleophilic property in water [Fig. 9(D)]. The surfaces can be used to capture and collect oil droplets in water because of their superoleophilic property in water and can be easily cleaned when removed from water because of the superoleophobic surface in air [Fig. 9(E G)]. Xue et al. fabricated a novel superhydrophilic and underwater superoleophobic hydrogel coated mesh in oil/water/ solid three-phase systems, which consists of rough nanostructured hydrogel coatings and microscale porous substrates [Fig. 10(A,B)]. 116 It can selectively separate water from oil/water mixtures effectively, such as vegetable oil, gasoline, diesel, and even crude oil/water mixtures without any extra power [Fig. 10(D,E)]. During the separation process, the underwater superoleophobic interface with low affinity for oil drops [Fig. 10(C)] prevents the coated mesh getting fouled by oils, making the recycling of oils and materials easy. The underwater superoleophobic surfaces overcome the easy-fouling and hard-recycling limitations of the traditional materials. Those two works of introducing 1220 POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,

13 POLYMER SCIENCE REVIEW FIGURE 10 Underwater superoleophobic hydrogel-coated mesh for oil/water separation. (A and B) SEM images of the PAM hydrogel-coated mesh, consisting of microscale porous metal substrates and nanostructured hydrogel coatings. (C) The surface shows extremely low oil adhesion even under large preload. (D and E) The separation process of crude oil/water mixtures by utilizing the hydrogel-coated mesh. (Reproduced from Ref. 116, with permission from Wiley-VCH.) superoleophobicity in air and in water are new attempts to using special wettability to design next generation materials for oil/water separation, which suggest attractive potential applications in industrial oily waste water treatments and oil spill cleanup. CONCLUTIONS AND OUTLOOK This article reviews the design, fabrication, applications, and recent developments of polymeric superoleophobic surfaces. Polymers with their flexibility, diverse molecular design, good processability, and low cost have become one of the most important materials to create superoleophobic surfaces. Through years of efforts, great achievements have been made in this field. However, there are still many challenges. First and foremost, fundamental theories of superoleophobic systems need to be further studied, especially the underwater superoleophobic system. What role the impulsive phase plays in enhancing the antiwetting property needs more research. Second, the practical applicability of superoleophobic surfaces needs to be improved. To fulfill the needs of large-scale production, low-cost materials and simple approaches are necessary. In the meantime, the enhancement of mechanical stability, robustness, and self-healing ability of superoleophobic materials is also important. Furthermore, using environmentally friendly materials is also significant for applications. Third, the development of multifunctional and stimuli-responsive superoleophobic surfaces remains great challenges in front of us, and more research needs to be done in this subject. Functional integrated and intelligent superoleophobic surfaces will connect interfacial science with other fields, such as biology, physics, material and engineering. We believe this collaboration is helpful to push this field forward, so as to bring great benefit to society. ACKNOWLEDGMENTS The authors are grateful for financial support from the National Research Fund for Fundamental Key Projects (2011CB935700, 2010CB934700, and 2009CB930404) and the National Natural Science Foundation ( and ). The authors gratefully acknowledge the Chinese Academy of Sciences. REFERENCES AND NOTES 1 Liu, M. J.; Zheng, Y. M.; Zhai, J.; Jiang, L. Acc. Chem. Res. 2010, 43, Sun, T.; Feng, L.; Xuefeng, G.; Jiang, L. Acc. Chem. Res. 2005, 38, Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces; Wiley-Interscience, New York, Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50,