Development of Functional Polymer Surfaces with Controlled

Invited Feature Article pubs.acs.org/Langmuir

Development of Functional Polymer Surfaces with Controlled Wettability Spiros H. Anastasiadis* Institute of Electronic Structure and Laser, Foundation for Research and Technology - Hellas, P.O. Box 1527, 711 10 Heraklion Crete, Greece, and Department of Chemistry, University of Crete, P.O. Box 2208, 710 03 Heraklion Crete, Greece ABSTRACT: There is a demand for surfaces with new functional properties in almost all industrial branches. During the next few years, research input will be required for the development of coatings exhibiting an easy-to-clean or selfcleaning ability, switchability so that they can act as sensors/ actuators, and defined tribological/mechanical properties and long-term stability. To achieve such behavior, the development of new advanced functional coatings that exhibit the proper chemistry and surface structure is necessary. In this Feature Article, we provide a review of the research activities in our laboratory on the development of functional and, especially, reversibly switchable polymer surfaces where the emphasis is on controlling their wettability. We will first discuss the fabrication of superhydrophobic surfaces by hierarchically micro- and nanostructuring a substrate surface with an ultrafast laser followed by appropriate hydrophobization. Then, we will summarize the development of surfaces that can alter their wetting behavior in response to changes in external stimuli such as humidity and light illumination. Finally, we will present our investigations on utilizing responsive (organic) coatings on hierarchically roughened substrates for the development of surfaces, which would be able to switch reversibly from superhydrophilic to superhydrophobic and water-repellent in response to an external stimulus (in this case, pH). appropriate response to a certain stimulus.2 The current trend in surface coatings is to control the coating composition on a molecular level and the morphology on the micro- and nanometer scales. The idea of controlling the assembly of macromolecular layers and the development of materials that can form defined structures with unique properties is being explored as well for pure scientific research and industrial applications. Examples of smart coatings include stimuliresponsive, antimicrobial, antifouling, conductive, self-healing, and superhydrophobic systems. A recent article reviewed the emerging applications of stimuli-responsive polymer materials where illustrative schematics demonstrate the different classes of polymer nanostructures that can be utilized for such purposes.3 In this Feature Article, we will summarize some of our research work over the years on the development of functional and reversibly switchable surfaces. The key property we will emphasize is the wettability of the surfaces. We will discuss the fabrication of superhydrophobic surfaces and surfaces that can alter their wetting behavior in response to changes in external stimuli such as humidity, pH, and light illumination. We will also present our most recent endeavors on developing surfaces that would be able to alter their wetting behavior all the way from superhydrophilic to superhydrophobic and water-repellent in response to an external stimulus such as pH; these have been

I. INTRODUCTION There is a continuously increasing demand for surfaces and surface coatings with new functional and multifunctional properties in almost all industrial branches. A relatively recent study on the research agenda in surface technology identified that during the 10 years subsequent to that study most research input would be needed for the following three coating functions:1 (a) easy-to-clean and/or self-cleaning (antistaining, antisoiling, and antifingerprint) properties; (b) sensor/actuator properties, activity, and switchability (measurement of and reaction to external stimuli); and (c) defined tribological and mechanical properties and long-term stability. Multifunctionality, however, has been identified as another main research task in the field of surface technology; multifunctional coatings are meant to implement several functions in one coating (e.g., easy-to-clean properties combined with scratch/mar resistance or hardness coupled with switchability). To achieve the desired functions, the development of new or advanced coating materials is necessary. Among them, the most important could be functional materials such as polymers, inorganics, metal alloys, and diamondlike carbon layers; micro- and nanostructured materials and surfaces; hybrid materials with complex morphology; nanocomposites and functional nanoparticles (e.g., quantum dots, core−shell particles at different lengths scales, (photo)catalytic particles, and new printing inks with tailored properties (especially for new fields of application such as printing electronics, nano- and microstructuring of surfaces, etc.)). The term “smart coatings” was coined to refer to coatings being able to sense their environment and produce an © XXXX American Chemical Society

Received: February 12, 2013 Revised: April 23, 2013

A

dx.doi.org/10.1021/la400533u | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Invited Feature Article

atmosphere,26 followed by a chloroalkylsilane monolayer deposition that introduces the appropriate hydrophobicity. Compared to the use of complex, multistep patterning procedures,8−15 the use of ultrafast laser irradiation constitutes a very simple scheme of micro/nanomanufacturing in a single step that enables the reproducible creation of complex hierarchical surface topologies exhibiting two characteristic lengths on the micrometer and the hundred-nanometer scale. In particular, quasi-ordered arrays of cone-shaped microtips (i.e., spikes) covering the irradiated area can be fabricated on silicon with the topology of the surface features being controlled by the laser beam fluence and its pulse duration (in the nanosecond or femtosecond range). The dependence of the characteristics of the surface topology on the laser fluence has been investigated before as has their relation to the wetting characteristics of the resulting surface.26 Scanning electron microscopy (SEM) images of the siliconbased artificial surfaces are shown in Figure 1d,e. The surface comprises protrusions with conical or pyramidal asperities with

developed by utilizing responsive (organic) coatings on hierarchically micro- and nanostructured surfaces.

II. SUPERHYDROPHOBIC SURFACES There have been great efforts by the scientific community to understand and control the wettability of solid surfaces and manufacture functional surfaces exhibiting superhydrophobic, water-repellent, and self-cleaning properties.4 This has been due to the importance of such surfaces for a vast range of potential applications in daily life, industry, and agriculture. Mimicking nature5,6 has been a central strategy in this area because biological species furnish numerous surface structures exhibiting extraordinary wetting properties. The wings of insects7 such as Cicada orni and Rhinotermitidae and leaves of plants such as Nelumbo nucifera5 (the sacred Lotus) and Colocasia esculenta exhibit remarkable wetting characteristics. The water repellency and self-cleaning properties of such surfaces have been attributed to both the chemical constituents of the hydrophobic cuticle covering their surface and, even more importantly, the particularly textured topography of the surface.5,6 It is generally understood that the roughened surface enhances the effect of hydrophobic surface chemistry into superhydrophobicity and water repellency. In the cases of the most famous water-repellent biosurfaces (e.g., the leaves of Nelumbo nucifera), a dual-scale roughness exists on their surfaces created by ten-micrometer-sized papillose epidermal cells decorated by an additional layer of hundred-nanometersized epicuticular waxes.5 This hierarchical roughness of the papillae leads to reduced contact between a liquid drop and the surface with droplets residing only on the tips of the hydrophobic wax crystals on top of the papillae. This leads to hydrophobic surfaces exhibiting contact angles greater than 150° and very small values of the contact angle hysteresis, defined as the difference between the advancing and receding contact angles, of less than 5°; both of these characteristics are required for a surface to be water-repellent. Moreover, contaminating particles on the surface can be picked up by liquid droplets and carried away as the drop rolls off the leaf, thus, leading to self-cleaning behavior; this has been coined as the Lotus effect by Barthlott and Neinhuis.5 The approach, thus, frequently employed consists of mimicking superhydrophobic biosurfaces5,6 by designing rough substrates out of hydrophobic materials.8,9 This has been implemented in a variety of bottom-up or top-down approaches10 (e.g., the growth of aligned carbon nanotube11 or ZnO nanorod films,12 the deposition of functionalized particles13 or micelles on surfaces, anisotropic plasma etching,14 and X-ray lithography15). Attempts to develop surfaces with dual-scale roughness were successful in creating surfaces with high contact angles (∼150−160°) and small contact angle hysteresis or sliding angles16,17 of ∼2.5−5°.15,18−23 It has been emphatically pointed out that very low values of the contact angle hysteresis are particularly important with respect to water repellency.24 At the same time, research efforts aim at understanding how superhydrophobicity may break down, thus leading to the wetting of an otherwise superhydrophobic surface.25 We have prepared artificial surfaces that possess hierarchical micro- and nanostructures and are able to mimic both the structure and the water repellency of the Lotus leaf quantitatively.20−23 These surfaces were prepared with a simple one-step production process utilizing ultrafast (femtosecond) laser irradiation of a silicon surface under a reactive gas

Figure 1. (a) Scanning electron microscopy (SEM) image of the surface of a Lotus leaf and (b) a higher-magnification SEM image where the existence of the hierarchical structures of micrometer-sized papillose epidermal cells decorated by an additional layer of 100 nm epicuticular waxes is clearly resolved. (c) A water droplet on the surface of the Lotus leaf attains a nearly spherical shape; the measured static contact angle is 153 ± 1°. (d) SEM image of the artificial laserstructured silicon surface and (e) a higher-magnification SEM image showing the dual-length-scale hierarchical structure resulting from micrometer-scale conical or pyramidal asperities and 100 nm nanostructures on the slopes of the protrusions. (f) A water droplet on the hierarchical Si surface after silanization; the measured static contact angle is 154 ± 1°. The surface was structured in the presence of 500 Torr of SF6 utilizing a regenerative amplified Ti/sapphire laser (λ = 800 nm) delivering 180 fs pulses at a repetition rate of 1 kHz at a laser fluence 2.47 J/cm2 with an average of 500 pulses. Adopted with permission from refs 20−22. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, and Copyright 2009 Elsevier BV. B


Langmuir


an average size of ∼10 μm with a surface density of ∼1.0 × 106 cm−2 decorated with nanoprotrusions of sizes of up to a few hundred nanometers. It is noted that the laser pulse fluence (in the range of 0.37−2.47 J cm−2) influences both the surface density of the protrusions on the textured surface26 and the resulting values of the water contact and sliding angles.26 The surface structure in Figure 1d,e mimics the morphology of the Lotus leaf (Figure 1a,b), which comprises almost semispherically topped papillae with sizes of 5−10 μm with a surface density of 4.2 × 105 cm−2 exhibiting branchlike protrusions with sizes of about 150 nm. The deposition of a hydrophobic silane coating on the artificial surface leads to almost spherical droplets (Figure 1f) with contact angle values (measured with the tensiometer described in ref 27) of 154 ± 1° and very small sliding angles16 of 5 ± 2° (for a 10 μL water droplet), which are both very similar to those obtained on the surface of a Lotus leaf (Figure 1c, contact angles 153 ± 1° and sliding angles 4 ± 2° for a 10 μL water droplet). The water repellency of the surfaces was quantified by investigating the bouncing of free-falling water droplets impacting onto them as a function of the droplet impact velocity. Movies of water droplets (0.84 mm radius) bouncing back numerous times following impact on a silanized laserstructured Si surface and on a surface of a water leaf have been shown as Supporting Information in ref 20. A measure of the elasticity of the collisions is the restitution coefficient, ε = V′/V, defined as the ratio of the center-of-mass velocity just after impact, V′, to that just before impact, V, and its dependence on the impact velocity, V, which is shown in Figure 2 for the surface of the Lotus leaf (Figure 2a) and for the silane-coated artificial surface (Figure 2b). One should point to the high values of elasticity observed at intermediate velocities for the artificial surface with the restitution coefficient exceeding 0.90, denoting that ∼90% of the drop’s initial kinetic energy is restored upon impact; this value matches that of the Lotus leaf and is among the highest ever reported.8 Moreover, the threshold velocity (i.e., the minimum velocity required for a water droplet to bounce off the surface) for the artificial surface compares very favorably with that of the Lotus leaf. Note that the smaller this threshold velocity, which depends on the size of the droplet, the more water-repellent the surface. The actual values of the threshold velocities in Figure 2 are close20 to those calculated within a simple model that states that the drop will rebound when its kinetic energy can overcome the energy dissipated by the pinning of the drop to the surface. The kinetic energy dissipation arises mainly from the presence of surface defects, where the contact line pins on such defects, resulting in a difference between the advancing and receding contact angles, θa and θr (i.e., in hysteresis). The pinning force per unit length is28 F = γLV(cos θr − cos θa) = γLVΔ(cos θ), with γLV being the liquid surface tension. Therefore, the dissipated energy scales as γLV R2Δ(cos θ) whereas the drop kinetic energy scales as ρR3V2. Equating the two energies leads to an estimate of the threshold velocity for water repellency, which, for the measured θa = 157° and θr = 152° for the silanized artificial surface and for a drop with radius R ≈ 0.84 mm, is estimated to be ∼0.06 m s−1, a value close to the experimental value. This kind of surface exhibits self-cleaning properties (i.e., a drop rolling down an inclined surface is able to pick up contaminating particles away from the surface). This was illustrated21 by covering a slightly tilted silanized micro/nanostructured surface with carbon black particles (Figure 3a) and

Figure 2. Restitution coefficient ε = V′/V, where V′ is the center-ofmass velocity right after impact, and V is that right before impact, as a function of the impact velocity V for a Lotus leaf surface (a) and an artificial silanized structured silicon surface (b) for falling water droplets with a radius R of 0.84 mm. The similarly high values of the restitution coefficients (exceeding 0.90) at intermediate velocities as well as the similarities in the threshold velocities necessary to avoid sticking of the drops between the two surfaces are evident; the dashed lines signify the threshold velocity of 0.11 m s−1 for the artificial surface. The artificial surface was structured as described in the caption of Figure 1. Adopted with permission from refs 20−22. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, and Copyright 2009 Elsevier BV.

Figure 3. Macroview of a silanized structured silicon surface covered with carbon particles (a). The size of the micro/nano-structured surface is 3 mm × 8 mm. Image of the path left behind by the rolling droplet (b, c) and image of the drop with the carbon particles collected on it (d). The artificial surface was structured as described in the caption of Figure 1. Reprinted with permission from ref 21. Copyright 2009 Elsevier BV.

allowing a water drop to roll over it. The droplet leaves a clean path behind it (Figure 3b,c) with nearly no particles as it rolls down the slightly tilted structured surface, whereas at the end of C


Langmuir


Above its LCST, the PNIPAM−water interactions become unfavorable and the mixture undergoes phase separation, resulting in the formation of a compact, globular polymer that is phase separated from the water-enriched phase. Under these conditions, PNIPAM can be considered to be incompatible with water and hence hydrophobic. Thus, a minute variation of temperature in the vicinity of the LCST can lead to reversible and dramatic changes in the polymer properties from hydrophilic to hydrophobic and back.3 The controlled reorganization of interfacial layers based on environmentally sensitive polymers has been widely utilized for the design and fabrication of smart/responsive material surfaces.23,42,47,52 III.1. Surfaces Responding to Changes in the Humidity of the Environment. Some years ago, we presented a methodology for the development of responsive polymer surfaces and, more specifically, surfaces that would reversibly modify their hydrophobicity/hydrophobicity in response to changes in the humidity of their environment.47 The employed methodology took advantage of two concepts: the partitioning of an AB diblock copolymer to the polymer/air interface via its low-energy A block and the surface reorganization of a functional end group X. The AB diblock copolymer acts as a surface delivery vehicle, a notion first introduced by Koberstein,53 to bring a functional group near the free surface. When the copolymer is mixed with a homopolymer B, the copolymer partitions to the free surface54 with the surface-active A block acting as an anchor to the surface and the B block (compatible with the homopolymer) forming a dangling chain55 that keeps the additive tethered to the matrix polymer. This surface partitioning brings the functional (hydrophilic) end group X, chemically attached at the end of the low-energy block, into the vicinity of the surface. The high-energy end group is hidden below the free surface in a dry environment whereas it appears at the surface when exposed to water vapor because of its polarity;53 this is schematically shown in Figure 4a,b. It is noted that such a methodology was utilized56 to enhance the adhesion selectively toward materials with the proper receptor for the functional moiety. The methodology was tested by utilizing polystyrene as the matrix and polyisoprene-block-polystyrene diblock copolymers of varying composition. Two different end groups of varying polarity, a dimethylamine and a sulfobetaine (zwitterion), were chemically attached to the end of the low-surface-energy polyisoprene block. The schematic behavior of Figure 4a was verified with respect to the segregation of the diblock copolymer to the polymer film surface by neutron reflectivity experiments and surface tension measurements. Moreover, because the zwitterionic end group possesses nitrogen and sulfur atoms, X-ray photoelectron spectroscopy (XPS) verified the surface reorganization of these end groups when exposed to the humid environment (Figure 4b). Figure 5 shows the photoelectron spectra in the binding-energy range of S 2p (Figure 5a) and N 1s (Figure 5b) for various exposure times to water vapor.47 It is evident that following exposures of a few days, both elements are clearly observed at the surface; the binding energies associated with both elements agreed with the chemical structure of the end group (sulfozwitterion). It is noted that the presence of nitrogen and sulfur is evident only when they are really at the surface (although the escape depth for X-ray photoelectrons for polymers is ∼7 nm) because each end group possesses only one nitrogen atom and one sulfur

its path, the droplet is covered by the carbon particles that were collected by it (Figure 3d). Thus, the artificial micro/nano-structured biomimetic surfaces exhibit structural features, water contact angle properties, and water repellency and self-cleaning capabilities very similar to those of Lotus leaves in nature. Moreover, the preparation of surfaces that exhibit anisotropic wetting and sliding properties has been accomplished by various authors by mimicking the anisotropic structure of different biosurfaces such as water-strider legs29 and rice leaves;30 therein, a threelevel macro/micro/nano-structured surface is prepared where the macroscale anisotropic patterning utilizes photolithography and soft lithography,30 interference lithography,31 or femtosecond laser structuring.32

III. RESPONSIVE SURFACES The understanding and fabrication of functional surfaces with wetting properties that can be controllably altered on demand have generated significant interest in the scientific community33 as a result of their wide variety of potential applications, including micro/nanofluidics,34 bioimmobilization and bioseparation,35 energy and green engineering,36 and antifouling and self-cleaning surfaces.4 These surfaces are able to switch their hydrophobicity/hydrophilicity reversibly in response to external stimuli such as the temperature,37,38 pH,23,38,39 photon energy,40−45 electric field,46 humidity,47 and electrochemical48 and chemical treatments.49 A great number of responsive materials and material surfaces are based on macromolecules that are often coined with names such as stimuli-responsive, smart, intelligent, and environmentally sensitive polymers.3 This is due to the fact that synthetic polymers offer a wealth of opportunities to design sophisticated materials by varying the length, chemical composition, architecture, and topology of the chains and adding functional end groups and/or side groups. Smart polymers can reversibly alter their physicochemical properties even upon slight variation in the conditions of the surrounding environment; they respond by modifying the conformation and/or location of the backbone, side chains, pendant groups, and/or end groups, which give rise to cooperative rearrangements and/or phase transitions occurring on the microscale and macroscale. In pH-responsive polymers, for example, protonation/ deprotonation events occur because of pH variations around their effective pKα values, which alter the degree of ionization of the monomer repeat units (weak acid or weak base behavior) and thus modify the polymer−solvent interactions leading to changes in the polymer chain conformations, the formation of smart polymer nanostructures,50,51 and changes in their hydrophilicity/hydrophobicity. Polymers that incorporate photon-driven reversibly isomerizable moieties such as photochromic spiropyran and azobenzene units respond to light illumination at the appropriate wavelength by undergoing changes in molecular conformation or bond-breaking/bondforming events that lead to the modification of their physical properties such as the dipole moment or polarity and thus the solubility in different solvents and hydrophilic/hydrophobic behavior. Thermally responsive polymers such as poly(Nisopropylacrylamide) (PNIPAM) take advantage of the existence of a lower critical solution temperature (LCST) in aqueous media. Below the LCST, PNIPAM is miscible with water, adopts a highly swollen coiled chain conformation, and can be considered to be water-soluble and hence hydrophilic. D


Langmuir


Figure 4. Schematic diagram of the methodology employed for the development of surfaces that respond to the humidity conditions: When a mixture of X−AB (X is the hydrophilic functional end group) in B is spin-coated onto a substrate and then annealed above the glasstransition temperature, the diblock copolymer partitions to the free surface where the low-energy A block (blue) acts as an anchor to the free surface and the B block (thick red) forms a dangling chain within the matrix homopolymer (thin red). The hydrophilic end group (orange) is brought into the vicinity of the free surface but is hidden below the surface when the specimen is exposed to dry air (a). When the specimen is exposed to water vapor, the hydrophilic end group responds to the new environment by presenting itself to the surface (b). This significantly reduces the water contact angle with representative drops shown in panels c and d. The measurements shown correspond to the case of a sulfozwitterionic end group at the end of a polyisoprene-block-polystyrene AB diblock copolymer (ZIS, Mw = 25 600, Mw/Mn = 1.06, 31 wt % polyisoprene) added to a polystyrene homopolymer (PS-1, Mw = 204 000, Mw/Mn = 1.02) at a concentration of 10 wt %. The contact angle values in panels c and d are 82 and 46°, respectively. Reprinted with permission from ref 47. Copyright 2003 American Chemical Society.

Figure 5. X-ray photoelectron spectra (in arbitrary units shifted for clarity) for a 10 wt % ZIS/PS-1 (caption of Figure 4) film for various exposure times to water vapor: 1 day (○), 4 days (●), 7 days (◊), 11 days (▲), and 14 days (∇). The spectra are shown in the bindingenergy range of S 2p (a) and N 1s (b). Between 4 and 7 days of exposure, the nitrogen and sulfur elements of the sulfozwitterion end group appear at the near surface. The specimen composition is the one described in the caption of Figure 4. Reprinted with permission from ref 47. Copyright 2003 American Chemical Society.

atom. Therefore, neutron reflectivity, surface tension, and XPS clearly verified the schematic presented in Figure 4a,b. As a result of the structure of Figure 4a,b, the equilibrium contact angle27 of water is significantly reduced when the film is exposed from a dry to a humid environment (Figure 4c,d), with the magnitude of the changes in the surface hydrophobicity/ hydrophilicity being a function of the end-group type, blocklength ratio, and additive concentration.47 The modification of the surface properties occurs in a reversible way. This is depicted in Figure 6, where representative drop images are shown for a zwitterion end group containing a film specimen before exposure (a) and following 5 days (b) and 14 days (c) of exposure to water vapor. When the specimen in panel c is held in vacuum only for 2 days, its contact angle increases back to 76° (d) whereas only 1 day of subsequent exposure to water vapor is sufficient to reduce its water contact angle to 50° (e). This switchable behavior, which did not even require the initial 14 days, could be continued for about five times (on average). This was attributed to the possibility that the sulfozwitterion participates at the beginning in some kind of cluster, which slows the reorganization the first time. Subsequently, the zwitterion does not have time to reform the clusters, and thus it switches back and forth from the surface faster. This behavior can be thought as a kind of “education” of the specimen during the first cycle and following its adaptiveness in the next cycles until it does not function any more, possibly because of surface contamination or end-group decomposition.

Figure 6. “Education” of the responsive surface. Photographs of representative water drops and the respective equilibrium contact angles on a film specimen with composition 10 wt % ZIS/PS-1 (caption of Figure 4) following (a) no exposure, (b) 5 days of exposure to water vapor, (c) 14 days of exposure to water vapor, (d) 2 days in vacuum after the conditions for panel c; and (e) 1 day of exposure to water vapor after the conditions for panel d. Reprinted with permission from ref 47. Copyright 2003 American Chemical Society.

We believe that this kind of methodology can have numerous extensions in the areas of sensors and biomaterials provided that the behavior is optimized with respect to the appropriate end groups or side groups, the number of functional groups per anchor chain, and the macromolecular architecture. E


Langmuir


Figure 7. (a) Characteristic images of water droplets residing on the initial, the UV-irradiated, and the green-irradiated surfaces for the first two irradiation cycles of a flat 5 wt % SP/PEMA-co-PMA substrate. (b) Average water contact angle values on the initial, the UV-irradiated, and the green -irradiated flat surfaces of 5% SP/PEMA-co-PMA substrates (●) and of PEMA-co-PMA substrates without photochromic molecules (■). The first two irradiation cycles are presented, with each irradiation cycle consisting of 50 UV laser pulses of 40 mJ cm−2 fluence and 200 green laser pulses of 45 mJ cm−2 fluence. Reprinted with permission from refs 42 and 45. Copyright 2006 American Chemical Society and 2008 Koninklijke Brill NV.

III.2. Surfaces Responding to Changes in the Wavelength of the Incident Light. The development of substrates and substrate surfaces that can reversibly alter their wetting characteristics when illuminated by photons of a certain energy and/or intensity has been attracting the interest of the scientific community. Photocatalytic oxides such as titania (TiO2) can be made hydrophilic when exposed to ultraviolet (UV) radiation, a trend that is reversed upon visible light illumination.57 Similarly, substrates decorated with aligned ZnO nanorods were observed to be superhydrophilic or superhydrophobic after exposure to UV light or the dark, respectively.40 Photoisomerizable units have been employed that can modify their conformation and/ or polarity when illuminated with light of the appropriate wavelength. Thymine-terminated self-assembled monolayers attached to gold surfaces undergo reversible photodimerization when exposed to UV radiation of the appropriate energy, which in turn leads to reversible changes in the water contact angle.41 Photosensitive azobenzene chromophores undergo a photoinduced cis−trans isomerization, which causes a different orientation of the dipole moment of the azobenzene, thus affecting the substrate surface energy; the azobenzene units can be employed as an end group of a monolayer58 or they can be attached as side groups to polymer chains.59 The photochromic molecules of the spiropyran family have been utilized in the form of a monolayer60 or as additives within a polymer matrix42−45 in order to affect the surface wettability. Photochromism is defined as a reversible transformation of chemical species, induced by electromagnetic radiation, between two states (isomer forms) having light absorption bands in distinctively different regimes. Spiropyran (SP) absorbs in the UV spectral region, and upon irradiation with UV light, it is converted to its colored merocyanine (MC) isomer by the photochemical cleavage of the C−O bond in the SP ring and the consequent ring opening (Figure 1 of ref 44). It is the enhanced dipole moment of the MC stereoisomers as

compared to that of the SP stereoisomers that potentially leads to the enhancement of hydrophilicity. The isomerization process is reversible, with the MC being converted back to the SP upon irradiation with visible light. This property is retained when the photochromic molecules are incorporated within macromolecular matrices.61 Figure 7a shows characteristic images of water droplets lying on a flat surface of a film of poly(ethyl methacrylate)-copoly(methyl acrylate) random copolymer, PEMA-co-PMA (Mw ≈ 100 000, 70 wt % in EMA), doped with 5 wt % SP before irradiation and after successive irradiation with 50 UV laser pulses and 200 green pulses (for the first and second cycles). When the SP-doped polymer film is irradiated with UV pulses, the SP molecules convert to their MC isomers and the surface of the specimen becomes more hydrophilic, and thus the water contact angle27 decreases. Subsequent irradiation of the sample with green laser pulses converts the molecules back to the SP form, and thus the surface becomes more hydrophobic, resulting again in an increase in the contact angle. The average values of the contact angles of drops lying on the initial, the UV-irradiated, and the green-irradiated flat photochromic− polymeric surfaces are presented in Figure 7b. The average values are taken for different droplets on a number of different specimens of the same composition. The contact angle values after the UV and green irradiation are shown for the first two UV−green irradiation cycles to ensure limitation of photochemical phenomena that leads to a deterioration of the behavior most probably due to the degradative photooxidation of the photochromic molecules.62 The maximum difference between the average contact angles measured on flat surfaces, shown in Figure 7b, is 7 ± 1°. The number and energy densities of the UV (and green) pulses used in the experiment presented in Figure 7 ensured complete conversion of the SP molecules to their MC isomers (and vice versa) according to spectroscopic studies whereas additional laser pulses did not F


Langmuir


Figure 8. (a) Characteristic images of water droplets on the initial and the UV- and green-irradiated microimprinted surfaces for the first two irradiation cycles of a 5 wt % SP/PEMA-co-PMA substrate. (b) Average contact angle values on the initial and the UV- and green-irradiated microimprinted surfaces of 5 wt % SP/PEMA-co-PMA substrates having an initial contact angle >105° (▲) and of PEMA-co-PMA substrates without photochromic molecules (■). The first two irradiation cycles are presented, with each cycle consisting of 50 UV laser pulses of 40 mJ cm−2 fluence and 200 green laser pulses of 45 mJ cm−2 fluence. Reprinted with permission from refs 42 and 45. Copyright 2006 American Chemical Society and 2008 Koninklijke Brill NV.

the particular 1.3 μm grating). Note that when films of PEMAco-PMA without SP molecules are microimprinted, their water contact angle (94°, Figure 8) is again higher than that of the flat films (76°, Figure 7); however, their water contact angle is not affected by successive exposure to the UV and green illumination sequence (Figure 8), which again illustrates that the switchability is due to only the presence of the photochromic molecules. In the present system, patterning enhances the hydrophobicity of the initial surfaces as compared to that of the flat ones. This behavior can be understood only in terms of the model of Cassie and Baxter63 that describes the wettability of rough surfaces; only partial wetting can be assumed to occur as a result of the trapping of air underneath the drop in recessed regions of the surfaces. The alternative Wenzel model64 proposes that roughness increases the liquid−solid interfacial area, and thus the hydrophilicity or hydrophobicity should be enhanced geometrically (i.e., hydrophobic surfaces (θ > 90°) should become more hydrophobic and hydrophilic surfaces (θ < 90°) should become more hydrophilic), which was not observed in these works. Thus, the pattern here can be considered to be an ordered roughness where the final contact angle, θrough, will be an average between the value on air (180°) and the value on the flat surface θ. Because the drop is situated partially on air, the rough (patterned) surface always exhibits an increased contact angle compared to that of the corresponding flat surface. Within the Cassie−Baxter model,

influence the wetting behavior of the surfaces any more. For reference, a flat PEMA-co-PMA polymer substrate without SP molecules was exposed to the same illumination sequence, and the average values of the contact angles of water droplets are plotted in Figure 7b to illustrate that the light-induced interconversions of the dopant photochromic molecules are exclusively responsible for the changes in the wetting properties because laser irradiation did not affect the contact angle values for the undoped system. As was extensively discussed in section II, the structuring of a surface greatly affects its wettability. In this context, the SPdoped polymeric substrates were microstructured with nanoimprint lithography, and the effects of this patterning were investigated with respect to both the extent of the wettability changes and the reversibility of the effect.42−45 Nanoimprint lithography was utilized to imprint a grating on the surface with a certain predetermined period and depth of the grooves. Figure 8a shows characteristic images of water droplets situated on the surface patterned with a grating (1.3 μm period, 430 nm depth) before irradiation and after successive UV and subsequent green irradiation (first and second cycles). Figure 8b shows the average water contact angle values27 on the patterned surfaces for the first two UV/green irradiation cycles. The values of the water contact angles on the patterned surfaces are always greater than on the flat surfaces with differences of a few degrees observed between surfaces patterned with the same replica as a result of structural imperfections of the imprinted gratings. One can observe that microstructuring quite substantially enhances the differences between the contact angles of the water droplets formed on the initial and the the UV- and green-irradiated surfaces compared to those on flat surfaces (Figure 7), with the maximum average contact angle difference measured in these being 19 ± 3°;42,45 the light-induced contact angle response on the smooth surfaces due to the photochromic transformations are further enhanced by a factor of almost 3 on the patterned surfaces (for

cos θrough = f (1 + cos θ) − 1

(1)

where f ≤ 1 is the solid fraction of the surface in contact with the liquid. The average initial contact angle on the flat surface is θ ≈ 77.5° (Figure 7) whereas the initial contact angle on the patterned surface θrough is ∼112° (i.e., the factor f is ∼0.51). If one assumes42,45 a fully symmetric pattern of period T with G


Langmuir


contact angles of water drops on a flat SP-doped polymer surface are shown as well. It is demonstrated that any patterning enhances the hydrophobicity of the surfaces compared to that of the flat ones whereas both the absolute values and the differences in the contact angle values between the UV- and the green-irradiated surfaces are a monotonic function of the periodicity of the grating (for the pattern depths of between 1.3 and 180 μm investigated) with the difference between the extreme cases increasing by decreasing the period of the grating44 from ∼6° for the 180 μm periodicity to ∼13.5° for the 1.3 μm periodicity. Alternatively, the doped polymer surfaces were micropatterned43,45 by interferometric photopolymerization lithography. The polymer used was an acrylate-based cross-linkable photopolymer, poly(pentaerythritol triacrylate) (PPETIA), with the photopolymerization taking place at 532 nm in the presence of a sensitizer dye (Eosin Y) and an amine photoinitiator (N-methyldiethanolamine). The patterning of the surface of the photopolymerized PPETIA films (with or without SP) was achieved using interferometric lithography at 532 nm, which is, in principle, ideal for creating periodic structures on SP-containing substrates because the SP molecules do not absorb in the spectral region from 450 to 550 nm where the photocurable system is sensitive. The flat PPETIA films without SP exhibit contact angle values27 of ∼45° whereas the contact angles are unaffected by the UV- and green-light illumination protocol as shown in Figure 10.43,45 The flat SP/PPETIA films exhibit higher contact

channel width x = T/2 and vertical channel walls with a smooth cutting surface and flat ends, then the factor f can be calculated as f = (T − x)/T = 0.50. It is noted that imprinting imperfections modified the value of θrough and thus the value of f in the range of 0.50−0.75, signifying that the stripes do not always have perfect top-hat profiles, and thus the water droplets may sink down into the channels, resulting in higher f values. Therefore, the variations of the initial contact angle on the patterned surfaces depend on the slope variations of the side walls of the channels because they can affect the area of the surface that is wetted by the water drop, which determines the factor f in eq 1. When the film is irradiated with UV, the same equation can be used where the initial contact angle is now 71° (as determined on the flat surfaces of Figure 7 under UV illumination) and that on the rough illuminated surface is 92° (Figure 7). In this case, f is determined to be 0.73. This means that the fraction of the solid in contact with the liquid apparently increases upon irradiation with UV pulses. This behavior is reversed upon irradiation with green pulses. This increase in the value of f is due to the light-induced structural changes on the microimprinted grating resulting from the macroscopic shrinkage of the film upon irradiation with UV, which was observed earlier.65 The average value of contraction of the microimprinted stripes can be translated to a ∼30 nm decrease in the full width at half-maximum for the 1.3 μm pattern.42,45 These structural changes that occur to the pattern following UV irradiation allow the water drops to penetrate more deeply in the channels of the UV-irradiated pattern, which contributes further to the reduction of the contact angles originally caused by the photochromic transformation of SP to MC. In a continuation of that work, the possibility of tuning the wettability and the amplitude of the wettability changes of the surface between extreme conditions was explored by modifying the topographical parameters of the introduced pattern. Figure 9 shows the average water contact angle values for the first five UV-/green-irradiation cycles for SP-doped substrates patterned with nanoimprint lithography where the characteristics of the patterning was varied. In all cases, mean values of the contact angles are plotted for 10 different samples for each patterning period and for at least 10 flat samples. The average values of the

Figure 10. Average contact angle values of water drops on the initial, the UV-irradiated, and the green-irradiated surfaces of an interferometrically microimprinted 0.3 wt % SP/PPETIA film (⧫), a flat 0.3 wt % SP/PPETIA film (●), and a flat film of a PPETIA sample without photochromic molecules (■). Three irradiation cycles are presented, with each cycle consisting of 50 UV laser pulses of 40 mJ cm−2 fluence and 200 green laser pulses of 45 mJ cm−2 fluence. Adopted with permission from refs 43 and 45. Copyright 2006 Springer Verlag and 2008 Koninklijke Brill NV.

angle values than PPETIA with initial contact angles of around 58° (still more hydrophilic than the SP/PEMA-co-PMA films). Figure 10 shows the values of the contact angle for water droplets lying on flat SP/PPETIA film surfaces before and after multiple UV and green irradiation. After irradiation with 50 UV laser pulses, the surface becomes more hydrophilic because of the conversion of SP to MC. However, subsequent irradiation with 200 green pulses does not bring the hydrophobicity of the system to its previous value but further enhances the hydrophobicity of the surface after every irradiation cycle.

Figure 9. Average contact angle values of water droplets situated on the initial, the UV-irradiated, and the green-irradiated surfaces of 10% SP/PEMA-co-PMA samples for five irradiation cycles for flat substrates (●) and those patterned with the following characteristics: period 1.3 μm, depth 520 nm (■), period 28.0 μm, depth 550 nm (▼), and period 180.0 μm, depth 6.6 μm (▲). Adopted with permission from ref 44. Copyright 2008 Springer Verlag. H


Langmuir


response to appropriate external stimuli,23,37,38,40 whereas multiresponsive surfaces have been explored as well.38 Endgrafting stimuli-sensitive polymers49,52 onto surfaces with hierarchical micro/nanoroughness has been frequently utilized for the development of such smart surfaces that can respond to changes in pH,23,38,39 temperature,37,38,66 light,66 and solvent quality.52 In such cases, it is important that the organic coating is conformal to the underlying surface, thus preserving its 3D micro/nanostructure; besides the approach utilizing anchored polymer chains, such conformity can be preserved by utilizing the chemical vapor deposition polymerization of appropriate monomers.67 In recent work, we have developed polymer-functionalized pH-responsive surfaces that exhibit hierarchical roughness on the micro- and nano-scale that can reversibly switch between superhydrophilicity at low pH and superhydrophobicity and water repellency at pH >8.23 That was the first time that such a surface was realized: until this time, polymers that became superhydrophobic at low pH and anionic and superhydrophilic at high pH had been reported;38 however, the latter may impose certain limitations for many applications (e.g., the anionic polymer surfaces are not able to interact with DNA, enzymes, and polyanionic drugs by attractive electrostatic interactions). In that respect, surfaces functionalized with pHresponsive polymers, which become cationic and superhydrophilic, can be especially desirable. A dual-length-scale roughened silicon surface was first prepared by utilizing ultrafast (femtosecond) laser irradiation of a silicon surface under a reactive gas (SF6) atmosphere,26 such as those described in section II of this Feature Article. In that case a chloroalkylsilane monolayer was deposited onto the roughened surface to introduce the appropriate hydrophobicity; the dual-scale roughness amplified this hydrophobicity into superhydrophobicity and water repellency.20−22 In the present case,23 the artificial micro/nanostructured surfaces (Figure 11a) w e r e f u n c t i o n a l i z e d b y “g r a f t i n g f r o m” p o l y ( 2 (diisopropylamino)ethyl methacrylate) (PDPA) chains at an anchoring density of σ = 0.78 chains/nm2 using surfaceinitiated atom-transfer radical polymerization (ATRP); this is schematically shown in Figure 12. Both the microscale conical features and the nanoscale protrusions are evident in the SEM image of Figure 11b following the functionalization process, verifying that the polymerization reaction did not perturb the hierarchical surface roughness. The PDPA homopolymer is a pH-responsive weak polybase that undergoes a reversible protonation/deprotonation process upon changing the solution pH and thus can alter its hydrophilicity/hydrophobicity. The switching behavior of the surface will thus be driven by the combined effects of the polymer chemical response and the hierarchical microscale and nanoscale roughness of the surface.23 Images of a water droplet lying on the PDPA-functionalized artificial surface are shown in Figure 13 following successive immersions in aqueous solutions at pH 8.5 and pH 2.5. The complete wetting (superhydrophilicity) following the immersion at pH 2.5 and the superhydrophobic behavior (contact angle27 of 154 ± 1°) following the immersion at pH 8.5 are due to the reversible protonation/deprotonation of DPA (the pKα of PDPA is 6.3) shown in the schemes of Figure 13. It was demonstrated23 that the superhydrophobic state of such surfaces requires sufficient hydrophobicity of the functionalizing polymer so that, combined with the dual-scale roughness, it can lead to superhydrophobicity.

This was attributed to the UV and green irradiation causing further polymerization of the material, apart from the photochromic transformations. It is noted that in the absence of SP interfering in the initial photopolymerization the initial laser intensity was sufficient to photopolymerize the film, as evidenced by the insensitivity of the contact angles of the SPfree films to the irradiation cycles (Figure 10). Interferometric patterning of the films formed surface relief gratings of period 2.5 μm. In this case, the initial values of the water contact angles on the patterned surfaces are always smaller than on the flat surfaces. Figure 10 presents the water contact angle values on a patterned surface before irradiation, after UV irradiation with 50 pulses, and after subsequent green irradiation with 200 pulses. The light-induced wettability variations were significantly enhanced upon alternating laser irradiation, compared to that of the flat surfaces, similar to the case of the soft molding lithographic patterns on PEMA-co-PMA. For this initially hydrophilic system, it appears than it is the Wenzel model64 that is more appropriate for explaining the behavior. Wenzel proposes that the roughness increases the liquid−solid interfacial area and thus the hydrophilicity or hydrophobicity is enhanced geometrically. In the present case, the hydrophilic flat surface becomes more hydrophilic after patterning, meaning that the water droplets penetrate the grooves of the grating simply because the initial polymer is very hydrophilic. In this model, the apparent contact angle θrough is related to that on flat surfaces θ by cos θrough = r cos θ

(2)

where r ≥ 1 is the surface roughness defined as the ratio of the actual wetted surface over the surface as measured on the plane of the interface. In the present case, the significant enhancement of the light-induced wettability variations of the patterned surfaces should then be due to the fact that the water−solid interfacial area is increased. Therefore, the water molecules are in contact with an increased number of photochromic molecules, which undergo transformations from the nonpolar SP to the polar MC and back and are responsible for the wettability changes. Note again the continuing increase in the hydrophobicity after each irradiation cycle, again most probably due to further polymerization of the patterned material as well. It was thus demonstrated that the photoinduced reversible wettability changes of a spiropyran-containing polymer film surface can be greatly enhanced and, in principle, controlled by microimprinting the surface with patterning methods such as soft molding lithography or interferometric photopolymerization techniques. The hydrophilicity is enhanced upon UV laser irradiation because the nonpolar spiropyran molecules convert to their polar merocyanine isomers and the process is reversed upon green laser irradiation whereas these light-induced alterations in wettability are greatly enhanced after microstructuring of the surfaces; the extent and the range of the contact angle changes are determined by the surface topographical characteristics as well as by the properties of the polymer matrices.

IV. SUPERHYDROPHILIC TO SUPERHYDROPHOBIC SURFACES The utilization of functional coatings based on responsive polymers on artificial hierarchically roughened surfaces can be expected to result in a surface that can, in principle, switch between superhydrophobicity and superhydrophilicity in I


Langmuir


PDPA-brush-functionalized flat silicon surface, which is responsive as well but with contact angle variations that are only within ∼30°; however, this illustrates a way of developing polymer surfaces responsive to pH variations as mentioned at the beginning of section III. The water repellency of the surfaces in their superhydrophobic state can be quantified by drop elasticity measurements (i.e., by investigating the bouncing of free-falling water droplets impacting onto them as a function of the droplet impact velocity). The inset of Figure 15 presents selected snapshots of a free-falling water droplet (radius 1.35 mm) impinging on the PDPA-functionalized hierarchical surface following its immersion in a pH 8.5 solution. The drop impacts the surface with a velocity that corresponds to a dimensionless Weber number of We = 3.5; the Weber number is defined as We = ρV2R/γLV, where ρ is the liquid density, γLV is its surface tension, R is the drop radius, and V is its impact velocity. The surface is so water-repellent that the drop bounces back numerous times, and selected maxima of its trajectory are shown as a function of time. The drop finally comes to rest on the surface after several rebounds. A movie of a water droplet (1.35 mm radius) bouncing back numerous times following its impact on the PDPA-functionalized artificially structured surface following immersion in a solution at pH 8.5 was shown as Electronic Supporting Information in ref 23. Note that a movie was also shown there of the complete wetting of the functionalized, artificially structured surface following immersion in a solution at pH 2.5. A direct measure of the elasticity of the PDPA-functionalized artificially structured surface is the restitution coefficient, ε= V′/ V, discussed in section II; this is shown in Figure 15 as a function of the impact velocity V in comparison to that of the natural Lotus leaf.20 The restitution coefficient exceeds 0.90 at intermediate velocities, from ∼0.18 to ∼0.30 m/s, indicating very high elasticity of the collisions, whereas for small velocities, ε decreases abruptly with decreasing V and reaches zero at a certain velocity (0.17 m/s). This is the threshold that quantifies the water repellency of the surface. The similarity of the restitution coefficient (to our knowledge, among the highest ever reported) as well as the threshold velocity value (necessary to keep the drops from sticking) between the functionalized artificial and the Lotus leaf surface is evident. The bouncing to nonbouncing transition has been discussed in section II above. Work is in progress on utilizing a photoresponsive (e.g., spiropyran-based polymer) coating (section III) applied to the hierarchically roughened surfaces in order to develop surfaces that can be reversibly switched from superhydrophilic (following UV irradiation) to superhydrophobic and waterrepellent (following green irradiation). It is also noted that, when the laser-structured surface is coated with ZnO nanograins grown by pulsed laser deposition (PLD), dynamic optical control of the wetting behavior is obtained by alternating UV illumination and dark storage or thermal heating;68 in that case, however, the mechanism of the photosensitivity of ZnO is the generation of electron−hole pairs in the ZnO lattice upon UV irradiation, which then react with lattice oxygen to form oxygen vacancies with water dissociatively adsorbing onto these vacancies, leading to hydrophilicity. Multiresponsiveness to external stimuli can be achieved by utilizing more than one type of monomer in the same randomcopolymer chains38,66 or the utilization of a ZnO coating of a few nanometers thickness on top of the hierarchical surfaces,68

Figure 11. (a) Scanning electron microscopy (SEM) image of an artificially micro/nano-structured silicon surface comprising protrusions with conical or pyramidal asperities with average sizes of ∼10 μm and a surface density of 5.0 × 106 cm−2 (scale bar 10 μm). (Inset) High-magnification SEM image of a single protrusion depicting nanostructures of sizes of up to few hundred nanometers on the slopes of the protrusions (scale bar 1 μm). The surface was structured in the presence of 500 Torr of SF6 at a laser fluence of 2.47 J cm−2 with an average of 500 pulses. (b) SEM image of the PDPA-functionalized artificially structured surface (σ = 0.78 chains/nm2) after ten pH cycles, where the protrusions remain unperturbed (scale bar 10 μm). (Inset) High-magnification SEM image of a single protrusion depicting that the few hundred nanometer nanostructures on the slopes of the protrusions remain unperturbed as well (scale bar 1 μm). Reprinted with permission from the ESI of ref 23. Copyright 2010 The Royal Society of Chemistry.

The average contact angles of water drops residing on the PDPA-functionalized hierarchically structured surface are shown in Figure 14a following successive immersions at pH 8.5 and pH 2.5. It is evident that the responsiveness of the functionalized surface holds for at least 10 cycles, with very stable values of the contact angles of both the superhydrophobic and the superhydrophilic states. This proves that the end-anchored polymer layer synthesized on the hierarchical surface is resilient to pH variations, which is especially important because the layer is immersed in basic and strongly acidic solutions. Figure 14b shows the contact angle values for a J


Langmuir


Figure 12. Schematic representation of the functionalization process of the hierarchically structured artificial surface. The initiator self-assembled monolayer is formed on top of the HF-treated hydrophilic silicon surface. A pH-responsive anchored polymer layer is grown using the surfaceinitiated atom-transfer radical polymerization of the PDPA monomer. Reprinted with permission from the ESI of ref 23. Copyright 2010 The Royal Society of Chemistry.

Figure 13. Contact angles and images of water droplets residing on the PDPA-functionalized hierarchically structured surface following immersion at pH 8.5, pH 2.5 (complete wetting), and again at pH 8.5 (▲). The schemes on the side show the protonation/deprotonation process of PDPA. Adopted with permission from ref 23. Copyright 2010 The Royal Society of Chemistry.

Figure 14. Average contact angle values of water droplets residing on the PDPA-functionalized hierarchically structured (a) and flat (b) surfaces following successive immersions at pH 8.5 and pH 2.5. Reprinted with permission from ref 23. Copyright 2010 Royal Society of Chemistry.

which would not destroy the micro/nano-roughness, followed by the synthesis of functional end-anchored polymer chains on top. This can lead to surfaces that can effectively respond to multiple external stimuli. In every case, however, the surfaces should be optimized with respect to their multifunctionality, anisotropy, sensitivity to the external stimulus, control of the range, and the kinetic evolution of the responses as well as stability upon repeated exposure to the changing stimuli. The respective design of superoleophobic surfaces, especially when submerged in aqueous media, has drawn research attention as well where the combination of hierarchical micro/nano-structures and the hydrophilic chemistry on the surfaces of fish scales is being mimicked.69 Surfaces with switchable underwater superoleophilicity and superoleophobicity have also recently been prepared by utilizing end-

functionalized diblock copolymers with a pH-responsive oleophilic/hydrophobic block;70 the copolymer was endanchored on nonwoven textiles, which proved effective in oil/ water separation.70

V. CONCLUDING REMARKS In this Feature Article, we made an attempt to present certain approaches to the development of novel functional surfaces, which, in this particular case, would be able to alter their wetting behavior in response to changes in external stimuli, such as humidity, pH, and light illumination. We first illustrated a convenient one-step methodology for creating hierarchically micro/nanoroughened surfaces; such surfaces can lead to K


Langmuir


product identity (e.g., protection of trademarks), heatable layers, UV/IR absorption properties, biological activity (e.g., antifouling, antimicrobial), and photovoltaic activity. However, biochemistry, environmental sciences, and biomedical sciences are certain fields of application that will benefit greatly from the further development of applications of stimulus-responsive polymeric materials and material surfaces. A great challenge in these areas will also be the development of artificial systems where a hierarchical material organization would allow the separation of the functions of receiving the signal and responding to it by modifying the material properties; this would prohibit possible interference of the changes induced by the stimuli with the desired changes in the material properties.3 It should be noted, however, that in all cases, besides the sensitivity of such surface coatings to external stimuli as well as the control of the range and the kinetic evolution of the responses, a very significant aspect of the engineering is related to the stability and durability of the surfaces upon long-time exposure to their environment (temperature, UV light, solvents, and abrasive agents) and the ability to retain their function and response to the changing stimuli. Finally, cost should not be a critical factor in the development of functional surfaces because, in most cases, the functionality is introduced into a very thin layer near the surface of the material, which may or may not require an extra processing step. The added functionality that such a thin layer may introduce to a specific product will most probably counterbalance any extra costs of its introduction.

Figure 15. Restitution coefficient as a function of the impact velocity for the PDPA-functionalized surface after immersion at pH 8.5 (■) and a natural Lotus leaf surface20 (○). The dashed line signifies the threshold velocity. (Inset) Selected snapshots of a water drop (radius R = 1.35 mm) impinging on the surface for dimensionless Weber number We = 3.5. Maxima of the drop trajectory are shown as a function of time. Reprinted with permission from ref 23. Copyright 2010 Royal Society of Chemistry.

superhydrophobicity and water repellency provided that an appropriate hydrophobic monolayer is deposited on them. Moreover, we demonstrate how the introduction of photochromic molecules in the spiropyran family can lead to surfaces that reversibly switch their wetting behavior in response to irradiation with laser pulses of the appropriate wavelength whereas we showed that the systematic patterning of these photoresponsive surfaces can significantly amplify the response. Finally, we showed how synthesizing end-anchored chains of a responsive polymer attached to the hierarchical micro/nanostructured surfaces leads to the development of surfaces where the hydrophilic−hydrophobic variations due to the pHresponsive polymer are amplified so that the surfaces can reversibly switch from superhydrophilic to superhydrophobic and water-repellent. The introduction of a third level of macrostructuring by patterning the substrate surfaces using photolithography and/or electron-beam lithography would lead to macroscopic anisotropy in the wetting behavior, which may be of interest in many applications. Moreover, multiresponsive surfaces could be designed by taking advantage of copolymers composed of unlike monomers able to respond to changes in different external stimuli or multilayered nanocoatings exhibiting different functions; this could lead to multifunctional coatings that can implement several functions in one coating. The development of functional surfaces is, in general, a subject that is much broader than what is discussed in this Feature Article. Herein, we focused on surfaces with controlled wettability being either permanent or switchable in response to an external stimulus. In general, functional coatings can be developed for very diverse applications, aiming either at the introduction of novel important functions to, for example, the paint industry or the development of switchable surfaces and adhesives, protective coatings that adapt to the environment, sensors, and drug-delivery systems. For the paint industry, it is anticipated1 that, beyond the classical functions of protection and decoration, some of the important desired new functions are going to be ease of cleaning and/or self-cleaning, scratch and mar resistance, adjustment of tribological properties, paints or coupling layers enabling the recycling of the coated substrate (e.g., delamination initiated by an external stimulus), switchable color, electrochromic behavior, electroluminescence, integrated

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biography

Spiros H. Anastasiadis is a Professor of polymer science and engineering in the Department of Chemistry at the University of Crete and an affiliated researcher at the Foundation for Research and Technology − Hellas. He holds a Diploma in Chemical Engineering (Aristotle University of Thessaloniki, 1983) and a Ph.D. in chemical engineering (Princeton University, 1988). He was a visiting scientist at the IBM Almaden Research Center from 1988 to 1989, a Professor of materials in the Department of Physics at the University of Crete (1993−2005) and a Professor of materials science and engineering at Aristotle University of Thessaloniki (2005−2008). He was awarded the John H. Dillon Medal of the American Physical Society in 1998 and was elected a Fellow of the American Physical Society in 2000. He received the Materials Research Society Graduate Student Award in L


Langmuir


(12) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. Reversible super-hydrophobicity to super hydrophilicity transition of aligned ZnO nanorod films. J. Am. Chem. Soc. 2004, 126, 62−63. (13) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Designing superoleophobic surfaces. Science 2007, 318, 1618−1622. (14) Dorrer, C.; Rühe, J. Wetting of silicon nanograss: from superhydrophilic to superhydrophobic surfaces. Adv. Mater. 2008, 20, 159−163. (15) Fürstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Wetting and self-cleaning properties of artificial superhydrophobic surfaces. Langmuir 2005, 21, 956−961. (16) The sliding angle, α, is defined as the critical angle above which a water droplet of a certain size begins to slide down an inclined plate. It is related to the contact angle hysteresis by the equation of Furmidge,17 mg sin(α)/w = γLV(cosθr − cosθa) = γLVΔ(cosθ), where m is the mass of the drop, g is the gravitational constant, θa and θr are the advancing and receding contact angles, respectively, γLV is the water surface tension, and w is the width of the drop (assumed to be 2R, with R being the drop radius). For a water drop of radius R = 1.35 mm (10 μL volume) and for θa = 154°, a sliding angle of 5° corresponds to a contact angle hysteresis, θa − θr, of ∼5°. In this Feature Article, the sliding angles are quoted for 10 μL drops. (17) Furmidge, C. G. L. Studies at phase interfaces. I. The sliding of liquid drops on solid surfaces and a theory for spray retention. J. Colloid Sci. 1962, 17, 309−324. (18) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Ö ner, D.; Youngblood, J.; McCarthy, T. J. Ultrahydrophobic and ultralyophobic surfaces: some comments and examples. Langmuir 1999, 15, 3395−3399. (19) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. Dual-scale roughness produces unusually water-repellent surfaces. Adv. Mater. 2004, 6, 1929−1932. (20) Zorba, V.; Stratakis, E.; Barberoglou, M.; Spanakis, E.; Tzanetakis, P.; Anastasiadis, S. H.; Fotakis, C. Biomimetic artificial surfaces quantitatively reproduce the water repellency of a lotus leaf. Adv. Mater. 2008, 20, 4049−4054. (21) Barberoglou, M.; Zorba, V.; Stratakis, E.; Spanakis, E.; Tzanetakis, P.; Anastasiadis, S. H.; Fotakis, C. Bio-inspired water repellent surfaces produced by ultrafast laser structuring of silicon. Appl. Surf. Sci. 2009, 255, 5425−5429. (22) Stratakis, E.; Zorba, V.; Barberoglou, M.; Spanakis, E.; Rizopoulou, S.; Tzanetakis, P.; Anastasiadis, S. H.; Fotakis, C. Laser structuring of water-repellent biomimetic surfaces. SPIE Newsroom 2009, DOI: 10.1117/2.1200901.1441. (23) Stratakis, E.; Mateescu, A.; Barberoglou, M.; Vamvakaki, M.; Fotakis, C.; Anastasiadis, S. H. From superhydrophobicity and water repellency to superhydrophilicity: smart polymer-functionalized surfaces. Chem. Commun. 2010, 46, 4136−4138. (24) Callies, M.; Quéré, D. On water repellency. Soft Matter 2005, 1, 55−61. (25) Papadopoulos, P.; Mammen, L.; Deng, X.; Vollmer, D.; Butt, H.-J. How superhydrophobicity breaks down. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 3254−3258. (26) Zorba, V.; Persano, L.; Pisignano, D.; Athanassiou, A.; Stratakis, E.; Cingolani, R.; Tzanetakis, P.; Fotakis, C. Making silicon hydrophobic: wettability control by two-lengthscale simultaneous patterning with femtosecond laser irradiation. Nanotechnology 2006, 17, 3234−3238. (27) Anastasiadis, S. H.; Hatzikiriakos, S. G. The work of adhesion of polymer/wall interfaces and its association with the onset of wall slip. J. Rheol. 1998, 42, 795−812. (28) Reyssat, M.; Pepin, A.; Marty, A.; Chen, Y.; Quéré, D. Bouncing transitions on microtextured materials. Europhys. Lett. 2006, 74, 306− 312. (29) Yao, Y.; Chen, Q.; Xu, L.; Li, Q.; Song, Y.; Gao, X.; Quéré, D.; Jiang, L. Bioinspired ribbed nanoneedles with robust superhydrophobicity. Adv. Funct. Mater. 2010, 20, 656−662. (30) Wu, D.; Wang, J.-N.; Wu, S.-Z.; Chen, Q.-D.; Zhao, S.; Zhang, H.; Sun, H. B.; Jiang, L. Three-level biomimetic rice-leaf surfaces with

1987 and the Society of Plastics Engineers - Plastics Analysis Division Best Paper Award during ANTEC 1985. He served as an editor of the Journal of Polymer Science, Part B: Polymer Physics (5/2006−7/ 2010). He has been appointed as a consulting editor for AIChE Journal (8/2012−present). His research interests are in the areas of polymer surfaces/interfaces and thin films, polymer blends and homopolymer/copolymer blends, organic/inorganic nanohybrid materials, nanoparticulate catalysts within polymer matrices, and responsive polymer systems.

■

ACKNOWLEDGMENTS I acknowledge the co-workers in the works reviewed in this Feature Article: E. Stratakis, V. Zorba, M. Barberoglou, E. Spanakis, P. Tzanetakis, C. Fotakis, S. Rhizopoulou, H. Retsos, S. Pispas, N. Hadjichristidis, S. Neophytides, M. I. Lygeraki, A. Athanassiou, M. Farsari, D. Pisignano, A. Mateescu and M. Vamvakaki. Moreover, A. Manousaki is acknowledged for her support with the scanning electron microscopy experiments. Special thanks to C. Tsoumplekas, L. Papoutsakis, and H. Papananou for the design of the composite image shown in the cover of this issue. The author acknowledges that this research has been partially supported by NATO’s Scientific Affairs Division (Science for Peace Programme), by the Greek General Secretariat of Research and Technology (ΠENEΔ Programme, projects 01EΔ587 and 03EΔ581), by the European Union (projects NMP3-CT-2005-506621 and CP-IP 246095-2), and by the Integrated European Laser Laboratories LASERLABEUROPE (contract no. RII3-CT-2003-506350).

■

REFERENCES

(1) Uhlmann, P.; Frenzel, R.; Voit, B.; Mock, U.; Szyszka, B.; Schmidt, B.; Ondratschek, D.; Gochermann, J.; Roths, K. Research agenda surface technology: future demands for research in the field of coatings materials. Prog. Org. Coat. 2007, 58, 122−126. (2) Baghdachi, J. Smart Coatings. In Smart Coatings II; Provder, T., Baghdachi, J., Eds.; ACS Symposium Series American Chemical Society: Washington, DC, 2009; Vol. 1002, p 3. (3) Cohen Stuart, M. A.; Huck, W. T. S.; Genzer, J.; Müller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Emerging applications of stimuli-responsive polymer materials. Nat. Mater. 2010, 9, 101−113. (4) Shirtcliffe, N. J.; McHale, G.; Newton, M. I. The superhydrophobicity of polymer surfaces: recent developments. J. Polym. Sci., Part B: Polym. Phys. 2011, 49, 1203−1217. (5) Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 1997, 202, 1−8. (6) Liu, K.; Yao, X.; Jiang, L. Recent developments in bio-inspired special wettability. Chem. Soc. Rev. 2010, 39, 3240−3255. (7) Watson, G. S.; Watson, J. A. Natural nano-structures on insectspossible functions of ordered arrays characterized by atomic force microscopy. Appl. Surf. Sci. 2004, 235, 139−144. (8) Quéré, D. Wetting and roughness. Ann. Rev. Mater. Res. 2008, 38, 71−99. (9) Li, X.-M.; Reinhoudt, D.; Crego-Calama, M. What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces. Chem. Soc. Rev. 2007, 36, 1350−1368. (10) Ma, M. L.; Hill, R. M. Superhydrophobic surfaces. Curr. Opin. Colloid Interface Sci. 2006, 11, 193−202. (11) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Superhydrophobic carbon nanotube forests. Nano Lett. 2003, 3, 1701− 1705. M


Langmuir


controllable anisotropic sliding. Adv. Funct. Mater. 2011, 21, 2927− 2932. (31) Wu, S. Z.; Wang, J.-N.; Niu, L.-G.; Yao, J.; Wu, D.; Li, A.-W. Reversible switching between isotropic and anisotropic wetting by one-direction curvature tuning on flexible superhydrophobic surfaces. Appl. Phys. Lett. 2011, 98, 081902. (32) Chen, F.; Zhang, D.; Yang, Q.; Wang, X.; Dai, B.; Li, X.; Hao, X.; Ding, Y.; Si, J.; Hou, X. Anisotropic wetting on microstrips surface fabricated by femtosecond laser. Langmuir 2011, 27, 359−365. (33) Russell, T. P. Surface-responsive materials. Science 2002, 297, 964−967. (34) Ionov, L.; Houbenov, N.; Sidorenko, A.; Stamm, M.; Minko, S. Smart microfluidic channels. Adv. Funct. Mater. 2006, 16, 1153−1160. (35) Chen, L.; Liu, M.; Bai, H.; Chen, P.; Xia, F.; Han, D.; Jiang, L. Antiplatelet and thermally responsive poly(N-isopropylacrylamide) surface with nanoscale topography. J. Am. Chem. Soc. 2009, 131, 10467−10472. (36) Nosonovsky, M.; Bhushan, M. Superhydrophobic surfaces and emerging applications: non-adhesion, energy, green engineering. Curr. Opin. Colloid Interface Sci. 2009, 14, 270−280. (37) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Reversible switching between superhydrophilicity and superhydrophobicity. Angew. Chem., Int. Ed. 2004, 43, 357−360. (38) Xia, F.; Feng, L.; Wang, S.; Sun, T.; Song, W.; Jiang, W.; Jiang, L. Dual-responsive surfaces that switch between superhydrophilicity and superhydrophobicity. Adv. Mater. 2006, 18, 432−436. (39) Zhang, Q.; Xia, F.; Sun, T.; Song, W.; Zhao, T.; Liu, M.; Jiang, L. Wettability switching between high hydrophilicity at low pH and high hydrophobicity at high pH on surface based on pH-responsive polymer. Chem. Commun. 2008, 44, 1199−1201. (40) Feng, X.; Feng, L.; eihua Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. Reversible super-hydrophobicity to super-hydrophilicity transition of aligned ZnO nanorod films. J. Am. Chem. Soc. 2003, 126, 62−63. (41) Lake, N.; Ralston, J.; Reynolds, G. Light-induced surface wettability of a tethered DNA base. Langmuir 2005, 21, 11922−11931. (42) Athanassiou, A.; Lygeraki, M. I.; Pisignano, D.; Lakiotaki, K.; Varda, M.; Fotakis, C.; Cingolani, R.; Anastasiadis, S. H. Photocontrolled variations in the wetting capability of photochromicpolymers enhanced by surface nanostructuring. Langmuir 2006, 22, 2329−2333. (43) Athanassiou, A.; Varda, M.; Mele, E.; Lygeraki, M. I.; Pisignano, D.; Farsari, M.; Fotakis, C.; Cingolani, R.; Anastasiadis, S. H. Combination of microstructuring and laser-light irradiation for the reversible wettability of photosensitised polymer surfaces. Appl. Phys. A: Mater. Sci. Process. 2006, 83, 351−356. (44) Lygeraki, M. I.; Tsiranidou, E.; Anastasiadis, S. H.; Pisignano, D.; Cingolani, R.; Athanassiou, A. Controlling the reversible wetting capability of smart photochromic-polymer surfaces by micro patterning. Appl. Phys. A: Mater. Sci. Process. 2008, 91, 397−401. (45) Anastasiadis, S. H.; Lygeraki, M. I.; Athanassiou, A.; Farsari, M.; Pisignano, D. Reversibly photo-responsive polymer surfaces. J. Adhes. Sci. Technol. 2008, 22, 1853−1868. (46) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. A reversibly switching surface. Science 2003, 299, 371−374. (47) Anastasiadis, S. H.; Retsos, H.; Pispas, S.; Hadjichristidis, N.; Neophytides, S. Smart polymer surfaces. Macromolecules 2003, 36, 1994−1999. (48) Isaksson, J.; Tengstedt, C.; Fahlman, M.; Robinson, N.; Berggren, M. A solid-state organic electronic wettability switch. Adv. Mater. 2004, 16, 316−320. (49) Minko, S.; Patil, S.; Datsyuk, V.; Simon, F.; Eichhorn, K.-J.; Motornov, M.; Usov, D.; Tokarev, I.; Stamm, M. Synthesis of adaptive polymer brushes via “grafting to” approach from melt. Langmuir 2002, 18, 289−296. (50) Rodríguez-Hernán dez, J.; Chéc ot, F.; Gnanou, Y.; Lecommandoux, S. Toward ‘smart’ nano-objects by self-assembly of block copolymers in solution. Prog. Polym. Sci. 2005, 30, 691−724.

(51) Vamvakaki, M.; Papoutsakis, L.; Katsamanis, V.; Afchoudia, T.; Anastasiadis, S. H.; Fragouli, P. G.; Iatrou, H.; Hadjichristidis, N.; Armes, S. P.; Sidorov, S.; Zhirov, D.; Zhirov, V.; Kostylev, M.; Bronstein, L. M. Micellization in pH-sensitive amphiphilic block copolymers in aqueous media and the formation of metal nanoparticles. Faraday Discuss. 2005, 128, 129−147. (52) Luzinov, I.; Minko, S.; Tsukruk, V. V. Adaptive and responsive surfaces through controlled reorganization of interfacial polymer layers. Prog. Polym. Sci. 2004, 29, 635−698. (53) Koberstein, J. T. Tailoring Polymer Interfacial Properties by End Group Modification. In Polymer Surfaces, Interfaces, and Thin Films; Karim, A., Kumar, S., Eds.; World Scientific: River Edge, NJ, 2000; pp 115−180. (54) Kunz, K.; Anastasiadis, S. H.; Stamm, M.; Schurrat, T.; Rauch, F. The segregation of poly(styrene-b-isoprene) diblock copolymers to the surface of a polystyrene melt: the effect of the ratio of block lengths. Eur. Phys. J. B 1999, 7, 411−419. (55) Retsos, H.; Terzis, A. F.; Anastasiadis, S. H.; Anastassopoulos, D. L.; Toprakcioglu, C.; Theodorou, D. N.; Smith, G. S.; Menelle, A.; Gill, R. E.; Hadziioannou, G.; Gallot, Y. Mushrooms and brushes in thin films of diblock copolymer/homopolymer mixtures. Macromolecules 2002, 35, 1116−1132. (56) Koberstein, J. T.; Duch, D. E. D.; Hu, W.; Lenk, T. J.; Bhatia, R.; Brown, H. R.; Lingelser, J.-P.; Gallot, Y. Creating smart polymer surfaces with selective adhesion properties. J. Adhes. 1998, 66, 229− 249. (57) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Studies of surface wettability conversion on TiO2 single-crystal surfaces. J. Phys. Chem. B 1999, 103, 2188−2194. (58) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Light-driven motion of liquids on a photoresponsive surface. Science 2000, 288, 1624−1626. (59) Bobrovsky, A.; Boiko, N.; Shibaev, V.; Stumpe, J. Comparative study of photoorientation phenomena in photosensitive azobenzenecontaining homopolymers and copolymers. J. Photochem. Photobiol., A: Chem. 2004, 163, 347−358. (60) Rosario, R.; Gust, D.; Hayes, M.; Jahnke, F.; Springer, J.; Garcia, A. Photon-modulated wettability changes on spiropyran-coated surfaces. Langmuir 2002, 18, 8062−8069. (61) Smets, G. Photochromic phenomena in the solid phase. Adv. Polym. Sci. 1983, 50, 17−44. (62) Athanassiou, A.; Sahinidou, D.; Arima, V.; Georgiou, S.; Cingolani, R.; Fotakis, C. Influence of laser wavelength and pulse duration on the degradation of polymeric films embedding photochromic molecules. J. Photochem. Photobiol., A: Chem. 2006, 183, 182− 189. (63) Cassie, A. B. D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (64) Wenzel, R. N. Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 1936, 28, 988−994. (65) Athanassiou, A.; Kalyva, M.; Lakiotaki, K.; Georgiou, S.; Fotakis, C. All-optical reversible actuation of photochromic-polymer microsystems. Adv. Mater. 2005, 17, 988−992. (66) Joseph, G.; Pichardo, J.; Chen, G. Reversible photo-/ thermoresponsive structured polymer surfaces modified with a spirobenzopyran-containing copolymer for tunable wettability. Analyst 2010, 135, 2303−2308. (67) Chen, H.-Y.; Lahann, J. Designable biointerfaces using vaporbased reactive polymers. Langmuir 2011, 27, 34−48. (68) Papadopoulou, E.; Barberoglou, M.; Zorba, V.; Manousaki, A.; Pagkozidis, A.; Stratakis, E.; Fotakis, C. Reversible photoinduced wettability transition of hierarchical ZnO structures. J. Phys. Chem. C 2009, 113, 2891−2895. (69) Liu, M. J.; Wang, S.; Wei, Z.; Song, Y.; Jiang, L. Bioinspired design of a superoleophobic and low adhesive water/solid interface. Adv. Mater. 2009, 21, 665−669. (70) Zhang, L.; Zhang, Z.; Wang, P. Smart surfaces with switchable superoleophilicity and superoleophobicity in aqueous media: toward controllable oil/water separation. NPG Asia Mater. 2012, 4, e8 DOI: 10.1038/am.2012.14. N


Development of Functional Polymer Surfaces with Controlled

Recommend Documents