WO2009110667A1

WO2009110667A1 - A novel random copolymer for water-repellent coating and substrate coated with the same

Info

Publication number: WO2009110667A1
Application number: PCT/KR2008/005196
Authority: WO
Inventors: Sang Yong Jon; Sangjin Park; In Sungs Choi; Woo Kyung Cho
Original assignee: Gwangju Institute Of Science And Technology
Priority date: 2008-03-04
Filing date: 2008-09-03
Publication date: 2009-09-11
Also published as: KR20090095123A; KR100941464B1

Abstract

The present invention relates to a random copolymer for use as a water- repellent coating material and a substrate coated with a random copolymer of the present invention. The coating of a random copolymer of the present invention onto the surface of a substrate can provide a substrate having a superhydrophobic surface. Furthermore, compared to the conventional methods for fabricating fluorinated polymer surfaces, a process of the present invention may be more facile and applicable to various oxide-based surfaces.

Description

[DESCRIPTION] [Invention Title]

A NOVEL RANDOM COPOLYMER FOR WATER-REPELLENT COATING AND SUBSTRATE COATED WITH THE SAME [Technical Field]

<i> The present invention relates to a random copolymer for water-repellent coating and a substrate coated with the random copolymer. [Background Art]

<2> When a liquid droplet contacts a solid surface, it will either remain as a droplet or spread out on the surface to form a thin liquid film. This physicochemical property is represented as surface wettability. The wetting behavior of solid surfaces against a liquid is of universal importance in many different areas as diverse as biology, microsystems engineering, and the painting of cars, ships, and buildings. In particular, water repellency is very useful for practical applications, such as dust-free coating, prevention of snow sticking, and others [I]. Water repellency of solid surfaces is governed by the surface free energy and geometrical structures of the surface [2]. Since the surface free energy is determined by the chemical nature of the topmost molecular layer of the surface, materials with a low surface free energy are required to make surfaces water-repellent. Among the materials, fluorinated compounds have been used widely for this purpose [3-5]. Fluorine is the most electro-negative element, with a Pauling electro-negativity value of 3.98, which is much larger than the value of carbon (2.55) [6]. Despite the polar nature of the C-F bond, perfluorinated linear alkanes and polymers, such as poly(tetraf luoroethylene) , possess no permanent dipole moment because of the symmetric distribution of charges [7]. The non-polar nature of fluorinated polymers confers a range of properties, including water repellency, solubility in supercritical carbon dioxide, and a low dielectric constant.

<3> The conventional methods of coating a fluorinated polymer on the surface of a substrate to provide hydrophobicity to the surface have the following drawbacks: (i) it is difficult to achieve a desired level of water- repellency on the surface of a substrate, and (ii) it is difficult to apply the conventional methods to the formation of self-assembled monolayer of monomer or the modification of a substrate with various degrees of roughness.

<4> The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. [Disclosure] [Technical Problem]

<5> The present inventors have exerted extensive researches to develop a water-repellent coating material that can be applied to various type of substrate. As a result, they have synthesized a novel random copolymer comprising (i) a surface-reactive part that can react with and anchor onto the surface of a substrate and (ii) a water-repellent functional part, and ascertained that water-repellency of the surface of a substrate is remarkably improved by coating this random copolymer on the substrate, thereby finally completing the present invention.

<6> Therefore, the present invention aims to provide a novel random copolymer for use as a water-repellent coating.

<7> The present invention also aims to provide a substrate coated with a random copolymer of the present invention.

<8> The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents. [Technical Solution]

<9> As described above, a random copolymer of the present invention is prepared by a radical polymerization of a monomer having a surface reactive part that can react with a substrate and a monomer having a water-repellent functional part. The coating of a random copolymer of the present invention onto the surface of a substrate can provide a substrate having a superhydrophobic surface. [Advantageous Effects]

<io> Therefore, a copolymer of the present invention is useful for practical applications such as dust-free coating, the painting of an automobile, a ship or a building and surface treatment for developing biochips. Furthermore, compared to the conventional methods for fabricating fluorinated polymer surfaces, a process of the present invention may be more facile and applicable to various oxide-based surfaces. [Description of Drawings]

<π> The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein:

<i2> Figure 1 shows the chemical structure of poly(TMSMA-r-fluoroMA) (m:n =

1:1) <13> Figure 2 shows FE-SEM images of the textured aluminum sheet: (a) top view and (b) tilted view, and the scale bar is lμm. <i4> Figure 3 shows FE-SEM images of the nanoporous AAO substrates fabricated by anodizing for 30 min at 155 V and wet-chemical etching at 45°C for 30 min: (a) top view, (b) slightly tilted view, and (c) cross-sectional view, and the scale bar is 1 μm. <i5> Figure 4 shows static water contact angles on substrates having diverse roughnesses without/with the pSAMs of poly(TMSMA-r-fluoroMA) <i6> Figure 5 shows XPS spectrum of the nanoporous AAO substrate coated with poIy(TMSMA-r-f luoroMA) <π> Figure 6 shows sliding behavior of a 5 μl water droplet on the poly(TMSMA-r-fluoroMA)-coated nanoporous AAO substrate, and the substrate was

-1 tilted at a rate of 3 s . <18> It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment . [Best Mode]

<19> In an aspect, the present invention discloses a random copolymer of Formula 1:

<20> [Formula 1]

<21> <22>

<23> wherein each of R₁, R2 and R3 is independently a hydrogen or a C1-C5 alkyl group; R₄ is a fluorocarbon compound; X is an oxygen, sulfur or nitrogen atom; each of m and n is independently an integer of 1-10,000; and each of p and q is independently an integer of 1-20.

<24>

<25> The present inventors have designed a new random copolymer presenting fluorocarbon groups for use as a water-repellent coating material.

<26> A random copolymer of the present invention comprises a 'surface- reactive part' and a 'functional part'. As used herein, the term of a 'surface-reactive part' refers to a part that can react with a substrate and anchor onto the surface. The term of a 'functional part' is a part that provides a water-repellent property to a surface of a substrate. <27> In a preferred embodiment, each of Ri, R₂ and R3 is independently a hydrogen or a C1-C₃ alkyl group, more preferably a hydrogen or a methyl group. <28> In another embodiment, R4 is -(CF2)_r ^~CF₃, and r is an integer of 1-40.

<29> In still another embodiment, a random copolymer of the present invention has a structure of Formula 2: <30> [Formula 2]

<31>

<32>

<33> wherein each of m and n is independently an integer of 1-10,000.

<34>

<35> Various monomers can be used for the formation of a random copolymer of the present invention. 3-(Trimethoxysilyl) propyl methacrylate (TMSMA) and a f luoromonomer bearing methacrylate moiety (fluoroMA) are preferred as a comonomer in the present invention. The TMSMA and the fluoroMA serve as a 'surface-reactive part' and a 'functional part', respectively. Poly(TMSMA-r- fluoroMA), one of random copolymers of the present invention, can be prepared by a radical polymerization using a mixture of the two monomer and an initiator such as azobisisobutyronitrile (AIBN).

<36>

<37> In another aspect, the present invention provides a substrate coated with a random copolymer of the present invention. <38> In a preferred embodiment, the surface of a substrate coated with a random copolymer of the present invention has a water-repellent characteristic. <39> A 'surface reactive part' herein react with a substrate to form multiple covalent bonds. A 'functional part' herein is a part of low surface free energy that provides a water-repellent property. In poIy(TMSMA-/^*- fluoroMA) random copolymer, for example, a trimethoxysilyl group (a surface reactive part) forms multiple covalent bonds onto the surface of a oxide- based substrate [8], and a perfluoro group (a functional part) provides a water-repellent property to a substrate coated with a random copolymer. <40> In another preferred embodiment, a random copolymer of the present invention is coated on the surface of a substrate in the form of polymeric self-assembled monolayers (pSAMs). Polymeric self-assembled monolayers (pSAMs) used herein have several advantages over SAMs of monomeric compounds: rapid formation, increased stability, and smooth surface roughness of the SAMs.

<41>

<42> In still another preferred embodiment, a random copolymer of the present invention is applied to the coating of a variety of substrates, more preferably hydroxy1-present ing substrates. Examples of such substrates include without limitation glass, silicon wafers, polymers, semi-conductors and metal oxides. Moreover, a random copolymer of the present invention is applicable to various oxide-based surfaces having diverse surface roughness.

<43> In yet another preferred embodiment, a substrate coated with a random copolymer of the present invention comprises a plurality of hydroxyl groups on the surface of the substrate. A random copolymer of the present invention can be applied to a hydrophilic substrate comprising a plurality of hydroxyl groups, thus providing a substrate with a superhydrophobic surface.

<44>

<45> The above features and advantages of the present invention will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description, which together serve to explain by way of example the principles of the present invention. [Mode for Invention] <46> Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the drawings attached hereinafter, wherein like reference numerals refer to like elements throughout. The embodiments are described below so as to explain the present invention by referring to the figures.

<47> The following examples illustrate the invention and are not intended to limit the same.

<48>

<49> Experimental details <50> 1. Materials

<5i> Pure aluminum sheets (99.999%, 0.25 mm thickness) were purchased from Goodfellow (Cambridge, UK). Perchloric acid (HCIO4, 70%, Junsei), absolute ethanol (99.8%, Merck), acetone (extrapure, Dae Jung Chemical & Metal Co., Ltd, Siheung, South Korea), phosphoric acid (H3PO4, 85%, Junsei), chromium(VI) oxide (CrO₃, 99.9%, Sigma-Aldrich) , chloroform (HPLC grade, Merck), 3-

(trimethoxysilyl)propyl methacrylate (TMSMA, Aldrich), Zonyl TM fluoromonomer (fluoroMA average Mn of about 534, Aldrich), azobisisobutyronitrile (AIBN, 98%, Aldrich), and tetrahydrofuran (THF, 99.9%, anhydrous, inhibitor-free,

Aldrich) were used as received. Ultrapure water (18.3 MΩcm ) from the Human Ultra Pure System (Human Corp., Seoul, South Korea) was used.

<52>

<53> 2. Synthesis of polv(TMSMA-r-fluoroMA)

<54> PoIy(TMSMA-r-fluoroMA) was synthesized as an example of a water- repellent coating material. TMSMA (1.25 g, 5 mmol), f luoromonomer (fluoroMA, 2.67 g, 5 mmol), and AIBN (9 mg, 0.1 mmol) were placed in a vial and dissolved in 10 ml of THF. The mixture was degassed for 20 min by passing a continuous stream of dry argon, after which the vial was sealed with a Teflon-lined screw cap. The polymerization reaction was carried out at 70 °C for 24 h. O

<55> ¹H nuclear magnet i c resonance (NMR) (CDCl₃ , 300 MHz) : δ (ppm) 4.20

(br , 2H, CO₂-CH₂ at f luoroMA) , 3.92 (br , 2H, CO₂-CH₂ at TMSMA) , 3.54 (s , 9H) , 2.42 (br , 2H) , 1.99-1.68 (m, 6H) , 0.99 (br , 2H) , 0.83 (s , 4H) , 0.60 (br , 2H) .

<56>

<57> 3. Formation of nanostructured model surfaces

<58> The practical application of poly(TMSMA-r-fluoroMA) as a water- repellent coating material was investigated using a model system, anodic aluminum oxide (AAO) membranes with diverse surface roughnesses. The parameters of AAO membranes such as membrane thickness, pore diameter and spacing can easily be tuned by changing the anodization voltage, time and post-treatment [10]. Moreover, we fabricated several substrates with different surface roughnesses by changing the parameters.

<59> Anodic aluminum oxide (AAO) membranes were fabricated by following the well-known, two-step anodization process [10].

<60> In the first step, the aluminum sheet was degreased in acetone for 5 min by sonication, rinsed several times with acetone, water and ethanol, and dried under a stream of argon. The cleaned aluminum sheet was electropolished in a mixture of perchloric acid and ethanol (HCIO4 : C₂HsOH =

1 : 4 in volume ratio) to remove surface irregularities: the aluminum sheet was used as an anode, while the graphite was used as a cathode. The distance between the anode and the cathode was adjusted to be about 5 cm, and a constant cell voltage of 20 V was applied between two electrodes for 2 min. The solution temperature was kept at 7 ^°C during electropolishing. After electropolishing, the plate was rinsed sufficiently with ethanol and water, and then dried under a stream of argon.

<6i> In the second step, the surface-finished aluminum sheet was anodized in 0.9 M H₃PO4 for 6 h under a constant cell voltage of 155 V, using the graphite cathode. The temperature of the anodizing electrolyte solution was kept at 3 °C . Subsequently, the porous aluminum oxide layer was completely removed by immersing the resulting substrate into an acidic mixture of 2 wt% chromic acid (H₂CrO₄) and 8.4 wt% H3PO4 at 65 °C for 3 h to obtain a textured surface on the aluminum sheet. The resulting textured aluminum was washed thoroughly with water, and then dried under a stream of argon. The textured aluminum, which was also called a concave plate, was anodized again under the same conditions used in the first anodization except for the anodizing time. The second anodizing time was 30 min. After the second anodization, the pores of the aluminum substrate were widened by wet-chemical etching with 5 wt% H₃PO₄ at 45 °C for 30 min.

<62>

<63> 4. Formation of polymeric self-assembled monolayers (pSAMs) <64> The prepared substrates including nanoporous AAO membranes were chemically modified by forming polymeric self-assembled monolayers (pSAMs) of poly(TMSMA-/-fluoroMA), and the wetting properties of the substrates were investigated.

<65> Prior to the formation of the pSAMs of poly(TMSMA-r-fluoroMA), the aluminum oxide-based substrate was oxidized by an oxygen plasma cleaner (Harrick PDC-002, medium setting) for 15 min to maximize the surface density of (-0H) groups. The oxidized substrate was immersed in 1 wt% chloroform solution of poIy(TMSMA-r-fluoroMA). After 1 h, the substrate was taken out, washed by sonication in chloroform for 1 min, rinsed with chloroform, and dried under a stream of argon. Finally, the substrate was cured at 100 °C for 1 min.

<66>

<67> 5. Instruments and characterization

<68> The ¹H NMR (300 MHz) spectrum was recorded on a JEOL JNM-LA300WB FT-NMR

(Tokyo, Japan). The morphologies of the fabricated concave-like aluminum plate and nanoporous AAO membrane were investigated using a field emission scanning electron microscope (FE-SEM; Philips XL30SFEG) . <69> The formation of the pSAMs presenting f luoropolymer was confirmed by x- ray photoelectron spectroscopy (XPS) study, which was performed with a VG- Scientific ESCALAB 250 spectrometer (UK) with a monochromatized Al Ka xray source (1486.6 eV). Emitted photoelectrons were detected by a multichannel detector at a take-off angle of 90 relative to the surface. During the

-9 -10 measurements, the base pressure was 10 -10 Torr. The survey spectrum was obtained at a resolution of 1 eV from one scan.

<70> Contact angle measurements were performed using a Phoenix 300 goniometer (Surface Electro Optics Co., Ltd, Korea) and a DSA-IO goniometer (Kruss, Germany). Dynamic advancing (θadv) and receding (θrec) water contact angles were determined by tilting experiments [11, 12]. Contact angles were measured at five different locations on each sample and the average values are reported in this paper.

<71>

<72> Results and discussion

<73> I_^ Design and synthesis of _a random copolymer for water-repellent coating

<74> When designing a water-repellent coating material, we considered two main factors: the material should be attached easily onto targeted surfaces and possess a pertinent functional group for water repel lency.

<75> Among the surface modification methods, self-assembly-based coating on solid surfaces is a well-studied process, due to both its fundamental scientific interest and its far-reaching technological applications [12, 13]. The formation of self-assembled monolayers (SAMs) using monomeric compounds that bear a surface-reactive group has been known to be a simple and practical technique for controlling wettability [14], corrosion [15], and adhesion [16] of solid surfaces.

<76> In an embodiment of the present invention, we selected a trimethoxysilyl group as a surface-reactive moiety for oxide surfaces and utilized a self-assembly process for the formation of polymeric SAMs (pSAMs) [8].

<77> It has been demonstrated by us [8] that pSAMs have several advantages over SAMs of monomeric compounds: rapid formation, increased stability, and smooth surface roughness of the SAMs. As a functional group for water- repellent applications, we chose a fluorine-based moiety. The low intermolecular forces of highly fluorinated organic compounds result in many unique characteristics, such as repel lency against both polar and non-polar liquids (e.g., water and oils), chemical resistance, low coefficient of friction, and small dielectric constant values [17].

<78> Related to the water-repellent property, it was reported that the surface free energy decreased in the order -CH₂ > -CH₃ > -CF₂ > -CF₂H > -CF₃, indicating that the closest hexagonal packing of -CF₃ groups gives the lowest surface free energy [18]. For example, flat platinum surfaces showed a low surface energy (yc = 5.6 mNm ) when they were modified with the SAMs of perf luorolauric acid [18]. <79> A random copolymer, poly(TMSMA-r-fluoroMA), was synthesized by a radical polymerization of 3-(trimethoxysilyl)propylmethacrylate (TMSMA) and a

® f luoromonomer having a methacrylate moiety (fluoroMA) in THF at 70 ^°C for 24 h. The feed ratio of two monomers was 1:1, and 0.01 equivalent of AIBN was used as an initiator. The chemical structure of poIy(TMSMA-r-fluoroMA) is expressed as Formula 2.

<80> The copolymer was characterized by H NMR spectroscopy. When the integration value of the peak at δ = 4.20 (m, 2H, CO₂-CH₂ at fluoroMA) was compared with that of the peak at 3.92 (m, 2H, CO₂-CH₂ at TMSMA), the molar ratio of the two monomers turned out to be the same as the corresponding feed ratio.

<81>

<82> 2. Preparation of model substrates

<83> To estimate the feasibility of poly(TMSMA-r-fluoroMA) as a water- repellent coating material, we fabricated model solid surfaces by the controlled anodic oxidation of aluminum plates. <84> Figure 2(a) shows the typical FE-SEM image of the textured aluminum sheet that was formed by anodizing an electropolished aluminum sheet in 0.9 M H3PO4 solution and subsequently removing the AI2O3 layer.

<85> The surface structure of textured aluminum was characterized as hexagonal-like arrangements of hemispherical concaves. The concave-like shape could be confirmed by the tilted FE-SEM image of the plate (figure 2(b)). Further anodization of the textured aluminum produced nanoporous AAO with cylindrical pore channels at the precise center of concave areas. The pore length could be controlled by varying the anodization time, and the pore diameter could be enlarged by the controlled wetchemical etching of the as- prepared AAO.

<u> The anodization of the textured aluminum plate was performed for 30 min. The generated pores were further widened by immersing the anodized substrate in 5 wt% H₃PO₄ at 45 ^°C for 30 min. As shown in figure 3(a), the average pore diameter was about 280 nm. The prepared nanoporous AAO had a needle-like shape, showing a very rough surface (figure 3(b)). The pore channels were straight and parallel along the pore, with a length of about 2 μm (figure 3(c)). The fabricated AAO substrates in this study were not highly ordered nanoporous structures, which were different from previously reported studies related to AAO [19].

<87>

<88> 3. Wettability of aluminum oxide-based surfaces

<89> The wettability of the fabricated model surfaces was characterized by contact angle measurements with ~5 μ 1 of water droplets as an indicator. Figure 4 shows the static water contact angles of three different substrates (flat, concavetextured, and nanoporous substrates) without/with the pSAMs of the f luoropolymer, ρoly(TMSMA-r-fluoroMA) .

<90> Before the contact angle measurements, the bare substrates without pSAMs were dried in vacuum oven at 50 °C overnight to remove moisture from the substrates completely. The flat aluminum sheet prepared by the electropolishing process showed a static contact angle of 72.0^° + 0.3° whereas the concave plate showed a decreased angle of 57.9° + 0.6° . This phenomenon could be explained by equation (1), derived by Wenzel to describe the contact angle for a liquid droplet onto a rough solid surface [20]:

<9i> cos θr = r cos θs (1)

<92> where θs and θr are the contact angles of a smooth surface and a rough surface made of the same material, respectively. r is the roughness factor, which is defined as the ratio of the true surface area A to the apparent surface area A' (r = A/A', where r is always greater than 1).

<93> This equation assumes that a liquid completely fills up the space between protrusions of a rough surface, which is classified as homogeneous wetting. From the equation, we realized that an increase in surface roughness resulted in the decrease of the actual contact angle for hydrophilic substrates (θ < 90^° and an increase for hydrophobic substrates (θ > 90° ). Therefore, the water contact angle on the hydrophilic aluminum plate decreased when the surface was roughened to the concave-like shape.

<94> We reasoned that we could achieve further increased contact angles if the surface was fabricated to possess much rougher structures than the concave shape. The concave plate was, therefore, anodized a second time for 30 min to make nanoporous AAO membranes, whose pores were further widened by a wet-chemical etching method. The resulting nanoporous structure dramatically increased the surface roughness and led to a situation [21] in which air became trapped within the grooves beneath the liquid (water). The large fraction of air trapped within the interstices of the nanostructured surface could effectively prevent the penetration of water into the grooves, resulting in an increase in the water contact angle from 72.0^° + 0.3^° to 138.1° ± 0.3° (the bottom left picture of figure 4).

<95> The above-mentioned relationships between surface roughness and surface wettability could be explained using the equation proposed by Cassie and Baxter [22]:

<96> cos θr = fi cos θs - f₂ (2) <97> where fi and (2 are the fractions of a solid surface and air in contact with a liquid droplet, respectively (fi + f2 = 1). Provided that fi is less than 1, the equation (2) always predicts an increase in θr, and is independent of θs. Equation (2) assumes that a liquid does not completely wet a rough surface. Once air is trapped in the interstices of a rough surface, a liquid droplet interacts with the composite surface, which consists of a solid substrate and air pockets: this is classified as heterogeneous wetting. The larger the value of f2, the larger the contact area between the trapped air and the liquid and the larger the increase in the water contact angle.

<98> From the water contact angles on the flat aluminum sheet (72.0° + 0.3 ) and the nanoporous substrate (138.1° + 0.3° ), ii was calculated to be

0.8046, indicating that ~80% of the surface was occupied by air. Therefore, the significant increase in the water contact angle on the nanoporous surface was caused by the trapped air. The water contact angle, however, did not increase any more, even when the nanopore length of the substrate was elongated from 2 to 4.2 μm, showing an almost identical value of 138.4° + 0.4° ). The water contact angle of as much as 138° indicated that the fabricated surface was relatively hydrophobic. However, the hydrophobicity was not kept stable: the surface was easily wetted when a stream of water was applied to the surface. Coating of poly(TMSMA-r-fluoroMA) was, therefore, attempted to achieve long-term and enhanced hydrophobicity compared with that of the bare, uncoated surfaces.

<99>

<ioo> 4. Water-repellent coating with DOIv(TMSMA-/-f luoroMA) <ioi> The surface modification of the aluminum oxide-based substrates was performed by the formation of pSAMs of poly(TMSMA-/-fluoroMA), which led to significantly increased water contact angles for all three substrates employed herein (figure 4, right panel). For example, the water contact angle of the flat aluminum sheet dramatically increased from 72.0° + 0.3° to 117.2° ± 1.6° after pSAM formation. The value of 117.2^° ± 1.6^° was near to the maximum for flat surfaces. It was reported that a surface coated with densely packed -CF3 groups, which are known to possess the lowest surface free energy, showed a contact angle of 119° [3]. Therefore, this result implies that any flat oxide surfaces may be converted to be water-repellent if the surfaces are coated with poIy(TMSMA-r-fluoroMA).

<iO2> In addition to static water contact angle measurements, x-ray photoelectron spectroscopy (XPS) was also used to characterize the pSAMs of poIy(TMSMA-r-f luoroMA) . Figure 5 shows the XPS spectrum of poly(TMSMA-r -fluoroMA)-coated AAO substrate. In the XPS spectrum, we observed the fluorine peaks at 688.0 (F Is) and 33.0 (F 2s) eV, indicating that the pSAMs on the nanoporous AAO substrate was successfully formed. Besides the fluorine peaks, there were additional peaks at 531.4 (0 Is), 119.0 (Al 2s), and 74.0 (Al 2p) eV, which correspond to aluminum oxide peaks. The peak of carbon (C Is) was also observed and was mainly split into 290.8 and 264.5 eV. Based on the previously reported studies [23], we thought that the peaks at 290.8 and 264.5 eV came from the perfluoro group (-CF2-) and the alkyl chain

(-CH2-), respectively.

<i03> It should be noticed that the water contact angle of the poly(TMSMA-r- fluoroMA)-coated nanoporous substrate was 163.4° ± 0.1° (figure 4), which is indicative of a superhydrophobic surface.

<i04> A superhydrophobic surface, defined as a surface with a water contact angle of 150° and above and a sliding angle of less than 10° [24], has been reported to be extremely water-repellent [4, 24, 25]. The observed water contact angle of 163.4° was comparable to the previously reported values of superhydrophobic surfaces. For example, the layer-by-layer films of poly(diallyldimethylammonium chloride) and sodium silicate showed a water contact angle of 157.1° after coating with fluoroalkylsilane [26]. The treatment of gold-coated silver nanostructures with n-dodecanethiol also led to superhydrophobic surfaces, giving a water contact angle as high as 162° [27]. To verify further the superhydrophobicity of the poly(TMSMA-/- f luoroMA)-coated nanoporous AAO surface, we evaluated the rolling-off or sliding behavior of a water droplet on the surface.

<iO5> The driving force (F) needed to make a water droplet move on a solid surface is given by equation (3) [28]:

<i06> F = Y LV(cos θrec - cosθadv) (3)

<iO7> where Y LV is the interfacial tension of water at the water-air interface, and θrec and θadv are the receding and advancing contact angles, respectively. The difference between the advancing and receding angles, ΘH = (θadv - θrec), is defined as contact angle hysteresis. As indicated in equation (3), a water droplet can easily slide down on a solid surface when the contact angle hysteresis value is small.

<1O8> We measured the advancing (θadv) and receding (Θrec) water contact angles by using tilting experiments [11, 12]. The water contact angle hysteresis of the poly(TMSMA-r-fluoroMA)-coated nanoporous substrate was calculated to be only 1.4° (the advancing water contact angle is 164.0° ± 0.1^° the receding water contact angle is 162.6° + 0.1° ). Therefore, water drops easily slide down on the tilted surface, which could be directly proved by measuring the sliding angle at which a water droplet with a certain weight begins to slide down on the tilted substrate.

<i09> The poly(TMSMA-r-f luoroMA)-coated nanoporous substrate was tilted at a rate of 3° s . The sliding behavior of a water droplet (5 μl) on the surface was photographed with a speed of 0.2 s/frame (figure 6). During the first 1 s, there was no motion of the water droplet. As the substrate was tilted more, however, the water droplet began to move and finally slid down along the substrate at an angle of 5.4° . clearly showing the extreme water- repellent characteristic of the superhydrophobic surface. In addition, even when a stream of water was continuously applied onto the surface, water never wetted the surface but was bounced from the surface.

<πo>

<iπ> The aforementioned results ascertains that a method of coating the surface of a substrate with a random copolymer of the present invention is a facile, fast, and reliable methods for the preparation of superhydrophobic surfaces with water-repel lency.

<112>

<ii3> The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

<114>

<ii5> References

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Claims

[CLAIMS] [Claim 1]

A random copolymer of Formula 1: Formula 1

wherein each of R₁, R₂ and R₃ is independently a hydrogen or a C₁-C₅ alkyl group! R4 is a fluorocarbon compound; X is an oxygen, sulfur or nitrogen atom! each of m and n is independently an integer of 1-10,000; and each of p and q is independently an integer of 1-20. [Claim 2]

The random copolymer of claim 1, wherein R₄ is -(CF₂)_r-CF₃, and r is an integer of 1-40. [Claim 3]

The random copolymer of claim 1, which has a structure of Formula 2: Formula 2

wherein each of m and n is independently an integer of 1-10,000. [Claim 4]

A substrate comprising a coating layer of the random copolymer of any of claims 1-3. [Claim 5]

The substrate of claim 1, wherein a surface of the substrate has a water-repellent property. [Claim 6]

The substrate of claim 1, wherein the coating is has a form of polymeric self-assembled monolayers (pSAMs). [Claim 7]

The substrate of claim 1, which is selected from the group consisting of glass, silicon wafers, polymers, semi-conductors and metal oxides. [Claim 8]

The substrate of claim 1, which comprises a plurality of hydroxyl groups (-0H).