Time Evolution of Precursor Thin Film of Water on Polyelectrolyte

Aug 13, 2018 - Phone/Fax: +81-42-628-4551. ... In front of the contact line, a water film with a nanometer-scale thickness, referred to as a precursor...
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Article Cite This: Langmuir 2018, 34, 10276−10286

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Time Evolution of Precursor Thin Film of Water on Polyelectrolyte Brush Shohei Shiomoto,† Kazuo Yamaguchi,‡ and Motoyasu Kobayashi*,‡ †

Graduate School of Engineering and ‡School of Advanced Engineering, Kogakuin University, 2665-1 Nakanomachi, Hachioji, Tokyo 192-0015, Japan

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S Supporting Information *

ABSTRACT: The microscopic wetting behavior of a water film on the line-patterned surface of a polyelectrolyte brush was directly visualized using an optical microscope by dyeing procedures. Surface line patterns of 5 and 5 μm width or 10 and 5 μm width for the polyelectrolyte brush and hydrophobic monolayer, respectively, were prepared by a photolithography process, chemical vapor adsorption method, and surface-initiated polymerization. A droplet of water containing dye was placed on the line-patterned surface. In front of the contact line, a water film with a nanometer-scale thickness, referred to as a precursor film, elongated along the polymer brush line with time. The elongation velocity at the first stage increased as the brush line width increased. On the other hand, at the second stage after the macroscopic contact line stopped moving, the precursor film continued to elongate in proportion to the 0.6 power of time, independent of the brush thickness, line width, and droplet volume.

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INTRODUCTION Super-hydrophilic polymer brushes have attracted considerable attention because of their application for self-cleaning, antifouling,1−7 antifogging,8 switchable,9 and lubrication10−13 systems. Even when a water droplet is placed on the flat surface of a water-soluble polymer brush, complete wetting does not occur.1 Possible reasons for this have been discussed from various viewpoints. Cohen Stuart et al. theoretically investigated this issue by employing the self-consistent field theory for poly(ethylene oxide) brush systems.14 They proposed that the partial wetting is the result of surface pressure due to the adsorption of polymer chains onto the liquid−vapor interface. On the basis of the Young−Dupré equation, Muller et al. discussed the contact angles of water droplets on polystyreneblock-poly(acrylic acid) (PS-b-PAA) diblock copolymer layers on polystyrene substrates.15 The infinitesimal work dW performed by the displacement of the contact line was described by considering the change of the free enthalpy ΔGPAAhyd for the hydration of polyelectrolyte monomers as follows ij dW = jjjj(γPS/H O − γPAA/PS)(1 − σ ) + γair/H O cos θ 2 2 k hyd yz ΔG PAA deAA + + Π − γPAA/air zzzzdA MAA {

phase, d is the density of PAA, eAA is the thickness of the PAA layer, MAA is the molecular weight of the AA monomer, and dA is the infinitesimal unit area. The model equation includes four kinds of interfacial energies: at PS/H2O γPS/H2O, PAA/PS γPAA/PS, air/H2O γair/H2O, and PAA/air γPAA/air. Murakami et al. characterized water on the poly[2(methacryloyloxy)ethyltrimethylammonium chloride] poly(MATC) brush by high-spatial-resolution IR spectroscopy16,17 to observe a characteristic O−H stretching absorption attributable to highly structured water by hydrogen bonds18 even outside a macroscopic droplet on the brush surface. This result indicated that water permeated the thin brush layer from the macroscopic contact line, to produce a water-swollen brush layer19 outside a water droplet. Structured water in a swollen poly(MATC) brush layer was also observed by synchrotronbased soft X-ray absorption spectroscopy and X-ray emission spectroscopy.20 It is supposed that the hydrated swollen brush thin film outside the water droplet changes the force balance at the macroscopic contact line to prevent the complete spreading of water on the polyelectrolyte brushes. In general, the liquid thin film extending ahead of the macroscopic contact line of the droplet placed on a flat and smooth surface is called the “precursor film”, which usually has a thickness from 10 nm to a few micrometers.21−24 In the strict sense, the water thin film formed on the polyelectrolyte brush outside the

(1) Received: June 18, 2018 Revised: August 5, 2018 Published: August 13, 2018

where Π is the interfacial pressure of the brush, (1 − σ) is the reduction of the contact area between PS and the external © 2018 American Chemical Society

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macroscopic droplet is different from the precursor film observed by Hardy in 1919.25 However, the water thin film on the polymer brush also extends beyond the macroscopic contact line of the water droplet and behaves like a general precursor film, as described latter. Therefore, this article used the term precursor film to express the liquid thin film spreading outside the macroscopic droplet on the polymer brush. Joanny and de Gennes showed that the precursor film development area consists of two regions.26 Immediately after the droplet contacts the surface, an “adiabatic” precursor film region develops ahead of the macroscopic contact line and expands several micrometers along with the contact line. The region is dominated by the balance between the viscous force and a disjoining pressure caused by van der Waals forces. At the later stage of spreading droplet, the “diffusive” precursor film develops in the region ahead of the adiabatic precursor film. After the droplet stops spreading, the length of the diffusive precursor film continues to develop in proportion to the square root of time and expand much further than the adiabatic precursor film. Various visualization methods have been developed for observing the behavior of the precursor film. At the early stage of research, polarized reflection microscopy was used by Ausserré et al. and Léger et al. for directly visualizing the existing region of the precursor film of silicone oil on Si surfaces.27,28 More recently, an interference phenomenon was used to observe the dynamics of the precursor film. Kavehpour et al. observed the time dependency of the precursor film profile by using a novel interference microscope comprising an electro-optically phase-shifting laser feedback interferometer combined with a reflecting light microscope.29 The time-series of the existing length was also evaluated by Hirose et al. using an elliptic polarization interferometer.30 On the other hand, Hoang et al. observed the film of silicone oil containing the photochromic dye using an epifluorescence inverted microscope in view of its suitable range, resolution, and high signalto-noise ratio.31 Furthermore, Franken et al. developed an observation method based on total internal reflection fluorescence microscopy.32 In other examples of optical methods, Hashimoto et al. used a Brewster angle microscope and a confocal laser displacement sensor and observed both the adiabatic and diffusive regions.33 Tani et al. simply used an optical camera to visualize the film evolution on a structural vertical surface, which has open-capillary channels.34 Different from optical methods, atomic force microscopy (AFM) was used by Xu et al.35,36 A droplet of the melt of bottle-brush polymers was placed on a flat surface and the diffusion of individual macromolecules in the film was monitored directly. In the present study, we propose a new approach using an optical microscope for the direct visualization of the precursor water film on the polyelectrolyte brush. We prepared micrometer-scale line-patterned polyelectrolyte brush/hydrophobic monolayer surfaces by combining the chemical vapor adsorption (CVA) method,10,37−39 vacuum ultraviolet (VUV) photolithography technique,39,40 and surface-initiated activators generated by electron transfer for atom transfer radical polymerization41 (SI-AGET ATRP) of the ionic monomer. Using colored water and dyed line-patterned brushes, we successfully visualized the precursor film extending from the water droplet. The dynamics of microscopic wetting along the polyelectrolyte brush and the dependency on the brush thickness, line width, and bulk liquid volume were investigated.

Article

EXPERIMENTAL SECTION

Materials. Trimethoxysilane (Tokyo Chemical Industry Co., Ltd. (TCI), 99.0%), 5-hexen-1-ol (TCI, 95.0%), 2-bromo-2-methylpropionyl bromide (Sigma-Aldrich Co., 98%), platinum(0)-1,3-divinyl1,1,3,3-tetramethyldisiloxane complex solution (Karstedt catalyst, Sigma-Aldrich, in xylene, Pt content = 2%), calcium hydride (CaH2, Nacalai Tesque, Inc., 90.0%), phosphorus oxide (P2O5, Yoneyama Yakuhin Kogyo Co., Ltd., 98%), [2-(perfluorohexyl)ethyl]trimethoxysilane (FAS, Wako Pure Chemical Industries, Ltd., 97.0%), dehydrated toluene (Kanto Chemical Co., Inc., 99.5%), ethanol (Kanto, 99.5%), 3-sulfopropyl methacrylate potassium salt (SPMK, Sigma-Aldrich, 98%), copper(II) bromide (CuBr2, Wako, 99%), 2,2′-bipyridyl (bpy, Wako, 99.5%), methanol (Kanto, 99.5%), L(+)-ascorbic acid (Wako, 99.6%), ethylene glycol (Kanto, 99.0%), methylene blue hydrate (TCI, 70.0%), Acid Red 1 (TCI), hydrogen peroxide (H2O2, TCI, 35.0% in water), and sulfuric acid (H2SO4, Kanto, 95.0%) were used as received. Triethylamine (Kanto, 98.0%) was purified by distillation with CaH2. Dichloromethane (Kanto, 99.5%) was refluxed over P2O5 for 6 h and distilled under a N2 atmosphere. 5′-Hexenyl 2-bromoisobutylate was synthesized from 5hexen-1-ol and 2-bromo-2-methylpropionyl bromide in the presence of triethylamine, and then purified by distillation with CaH2 under reduced pressure. The surface initiator, (2-bromo-2-methyl)propionyloxyhexyltrimethoxysilane (BHM), was synthesized by hydrosilylation of 5′-hexenyl 2-bromoisobutylate treated with trimethoxysilane in the presence of a Karstedt catalyst.42 Deionized water was obtained from a water purification system (Direct-Q UV 3, Merck Millipore Co.). Preparation of Poly(SPMK) Brush/FAS Monolayer LinePatterned Surface. The substrates of glass slides (Matsunami Glass Ind., borosilicate glass, 0.8−1.0 mm) were cut into 26 × 38 mm2 pieces. The substrates were immersed in a H2SO4/35% H2O2 aq = 70:30 (v/v) mixture at 373 K for 1 h to obtain the Si−OHterminated surface. The substrates were exposed to VUV-ray (λ = 172 nm, Xe excimer lamp, UER20-172, Ushio Electric, Ltd.) under 60−70 Pa for 6 min. Surface initiator, BHM, was immobilized on glass substrates by the CVA method,11,38−40 as follows. The substrates and a glass vessel filled with a 5 vol % toluene solution of BHM were packed in a custom-made separable flask (inside diameter: 75 mm × height 105 mm) purged with N2 gas. The flask was left to stand in an oven at 418 K for 7 h (Figure 1), after which the substrates were rinsed with ethanol. The BHM-immobilized substrate was placed in an evacuated vacuum chamber for photolithography.39,40 The substrate was covered with a photomask (LS5 or L10S5, Mitani Micronics Co., Ltd.) which had 5 μm Cr lines and 5 μm slits (5:5 μm), or 10 μm Cr lines and 5 μm slits (10:5 μm), in an area of 30 × 30 mm2 on a quartz glass plate with a thickness of 2.3 mm. On the photomask, two stainless-steel rings (outside diameter: 60 mm, inside diameter: 30 mm, height: 9.5 mm and weight: 160 g per piece) were loaded. The surface was irradiated for 6 min with VUV light under 20−40 Pa. To remove the decomposed residue of BHM monolayers, the patterned surface was rinsed with ethanol. The second organosilane monolayer of FAS was then introduced between the lines of the first patterned monolayer of BHM using a similar CVA method.39,40 The substrates and a glass vessel filled with FAS were packed in the separable flask purged with N2 gas, and then heated at 373 K for 3 h, after which the substrate was rinsed with ethanol. By the above method, line-patterned BHM monolayer/FAS monolayer surfaces with 5:5 or 10:5 μm line widths, respectively, were prepared. The poly(SPMK) brushes were prepared using the SI-AGET ATRP of SPMK from the residual BHM monolayer on the linepatterned surface (Table S1). The SPMK (3.74−8.12 mmol), CuBr2 (6.7 μmol), bpy (15.4 μmol), deionized water (3.0 mL), and methanol (7.0 mL) were mixed and added to the substrate in a glass tube in a typical case. After the mixture was stirred under a stream of N2 gas for 30 min, a 0.20 M ascorbic acid aqueous solution (6.7 10277

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average of more than five measurements was used as the data in both the parallel and the orthogonal directions to the line pattern. Observations of Wetting Behavior on the Line-Patterned Surface. The spreading behavior of the macroscopic droplet was observed from directly above using a video camera (HDR-CX370, Sony Co.). The elongation behavior of the precursor film was observed by an upright transmitted light microscope (BX51, Olympus Co.) equipped with an objective lens (UPlanFI, 40×/0.75 Ph2, Olympus Co.) and a digital camera (C-4040ZOOM, Olympus Co.). For both the observations of wetting behavior, two types of dyes were used: methylene blue and Acid Red 1. On the line-patterned surface, 2.2 wt % methylene blue aqueous solution was placed. The substrates were rinsed with deionized water and dried completely by blowing air. In the case of observing the droplet spreading behavior, the substrate was placed horizontally on the bottom of a 500 mL glass beaker. For observing the elongation behavior of the precursor film, the substrate was placed inside a plastic dish. To maintain a highly humid air atmosphere, wetting paper was placed on the wall of the glass beaker or the plastic dish. A droplet of 3.6 wt % Acid Red 1 aqueous solution was placed on the surface and the timing was set as time t = 0 s. The gap between the objective lens and the dish was closed with plastic wrap. The schematic diagram of the observation method is shown in Figure S5 in the Supporting Information. Image analysis was performed with the ImageJ program (National Institutes of Health).

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RESULTS AND DISCUSSION Characterization of Poly(SPMK) Brush/FAS Monolayer Line-Patterned Surfaces. Poly(SPMK) brush/FAS monolayer line-patterned surfaces with line widths of 5:5 or 10:5 μm, respectively, were prepared by photolithography and surface-initiated polymerization. Figure 2a shows a typical Figure 1. Preparation of the poly(SPMK) brush/FAS monolayer linepatterned surface by the CVA method, VUV photolithography technique, and SI-AGET ATRP. μmol) was added as a reducing agent to start polymerization. The mixture was stirred under a N2 atmosphere at 303 K. The polymerization time was changed from 0.5 to 12 h for preparing a polymer brush with a different thickness. The polymerization was terminated by opening the vessel to air. The substrate was rinsed with deionized water, ethylene glycol, and deionized water again. The substrate was dried by air blowing. Characterization of the Line-Patterned Surface. AFM (NanoWizard 3 Ultra system, JPK Instruments AG) was used in the dynamic force mode at room temperature. Imaging was performed under an ambient atmosphere for the measurement of the brush thickness in the dry state, or in water for the swollen brush thickness. For imaging of the precursor film, a highly humid air atmosphere was maintained by the same method of the observation by optical microscopy. Rectangular silicon cantilevers with a backside gold coating (HyperDrive PPP-NCHAuD, Nanosensors, NanoWorld AG, tip radius 7 nm, spring constant 42 N m−1 and resonance frequency 330 kHz) were used for imaging. The scanning for imaging was conducted in the traverse direction of the line pattern in an area of 30 × 30 μm2. X-ray photoelectron spectroscopy (XPS) (Quantum 2000 Scanning ESCA Microprobe, Physical Electronics, Inc.) was operated with a monochromatized Al Kα X-ray source at 1.48 keV at 24.7 W under 1 × 10−6 Pa. The emission angle of photoelectrons was set to 45° and the beam diameter was 100.0 μm. The neutralizer was set at 1.0 V and 20.0 μA. Wide-scan spectra (0−800 eV) were acquired at an energy step of 1.0 and high-resolution spectra (narrow scan) of F1s, K2p, C1s, and S2p were at 0.125 eV. Static contact angles of the water droplets (2 μL) were measured with a contact angle meter (Model Standard 100, Excimer Inc.) equipped with a zoom camera with a USB interface. The half-angle method was applied to the images to determine the angles. The

Figure 2. (a) AFM topography image and (b) the cross section of the poly(SPMK) brush and FAS monolayer line-patterned surface with line widths of 5 and 5 μm, respectively, in the dry state. The brush thickness was measured to be 83 nm.

AFM topographic image of the line-patterned surface in the dry state. Line-patterned concavo-convex morphology of the brush showed line widths identical to that of the photomask line and space, indicating that the line/space period on the photomask was successfully transcribed onto the surface. Figure 2b shows a cross-sectional profile of the line-patterned surface. Higher and lower areas correspond to the poly(SPMK) brush growth area and FAS monolayer area, respectively. The distance from the top to the valley on the 10278

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One of the reasons for the contact angle’s anisotropy can be explained by the wetting energy barrier. A theoretical explanation was proposed by Youngblood et al. to explain the wettability on a poly(tetrafluoroethylene)/air complex surface.43 In the case of a line-patterned surface, Morita et al. applied this explanation to understand the contact angles on liquidphobic-fluoroalkylsilane monolayer/silanol line-patterned surfaces.39 According to this theory, when a contact line advances in the orthogonal direction to the pattern line, in other words, spreads across the pattern line, the hydrophobic FAS monolayer functions as the wetting energy barrier. On the other hand, in the parallel direction, there is no energy barrier. Therefore, the orthogonal contact angles are higher than the parallel contact angles. The thickness dependency of the contact angle on a homogeneous polymer brush surface has already been reported by several researchers.15,44−48 In particular, the contact angles on the brush with thin thickness are largely affected by the surface free energy of the substrates covered with the monolayer of the surface initiator. When the brushes have sufficiently large thicknesses and high grafting densities, the contact angles become the values represented by the surface free energy of the grafting polymers. In this study, linepatterned poly(SPMK) brush with thin thickness (h = 8 nm) with the line widths of 5:5 μm showed a relatively higher water contact angle (θpara = 62.7°, θortho = 75.4°), as shown in Figure S4, probably due to the high hydrophobicity of FAS and BHM monolayer immobilized on the glass substrate. The water contact angles of the homogeneous FAS monolayer and the BHM monolayer on the glass were 116.5 and 80.0°, respectively. By increasing the brush thickness from 8 to 83 nm, the water contact angle on the brush decreased to θpara = 6.1° and θortho = 7.8°, which was the similar value of the nonpatterned poly(SPMK) brush on the glass substrate (θ = 7.5°). Line-patterned surface consisting of the poly(SPMK) brush (h = 28 nm) and the FAS monolayer with the line widths of 10:5 μm showed relatively lower water contact angles of θpara = 31.9° and θortho = 41.5°, compared with the surface of the 5:5 μm line-patterned thin brush, owing to the larger area ratio of the hydrophilic poly(SPMK) brush than that of the hydrophobic FAS. However, the significant reduction in water

line pattern structure shows the thickness of the poly(SPMK) brush, h. In the case of Figure 2, h was measured as 83 nm. Polymer brushes with h = 8, 35, 58, and 83 nm were prepared on the line-patterned surfaces with 5:5 μm line widths, and h = 28, 39, 42, and 65 nm were with 10:5 μm line widths. The case of 10:5 μm line widths is shown in Figure S1. Formation of the poly(SPMK) brush and the FAS monolayer was also confirmed by XPS analysis based on the observation of characteristic F1s, K2p, and S2p peaks corresponding to the poly(SPMK) brush and the FAS monolayer, as shown in Figure S2. The water contact angles on the poly(SPMK) brush/FAS monolayer line-patterned surfaces were measured by placing 2 μL of deionized water at room temperature (298 K). The droplet on the substrate was observed from both the parallel and orthogonal directions to the line pattern. Contact angles in the orthogonal direction were larger than those in the parallel direction. Figure 3 shows optical images of the contact angles

Figure 3. Optical images of the static wetting for a water droplet on the poly(SPMK) brush/FAS monolayer line-patterned surfaces. The contact angles θpara and θortho in (a) the parallel and (b) the orthogonal direction to the line pattern, respectively, are shown. The line widths of the poly(SPMK) brush and FAS monolayer were 5 and 5 μm, respectively. The brush thickness in the dry state was 35 nm.

on the line-patterned surface with 5:5 μm line widths and h = 35 nm. The 10:5 μm line widths show the same tendency as the 5:5 μm line widths, as shown in Figure S3. Increasing the poly(SPMK) brush thickness tended to lower the contact angle in both directions in the case of 5:5 μm line widths (Figure S4). For example, the contact angles in the orthogonal direction were θortho = 75.4° at h = 8 nm and 7.8° at h = 83 nm. The contact angles of the 10:5 μm line widths were not heavily dependent on the brush thickness.

Figure 4. (a) Optical images of the spreading behavior of the droplet of Acid Red 1 aqueous solution on the poly(SPMK) brush and FAS monolayer line-patterned surface at time t = 0, 77, 153, and 699 s after the placing of the droplet (2 μL). The line widths of the brush and monolayer were 10 and 5 μm, respectively. The brush thickness in the dry state was 42 nm. The brush area was dyed with methylene blue. The droplet/surface interface area A on the line-patterned surface was measured. (b) Optical micrographs of the precursor film on the line-patterned surface at t = 132, 320, 618, and 922 s. The precursor film was observed as decolored lines along the line-patterned poly(SPMK) brush area by the mixing Acid Red 1 in a red-colored water droplet and methylene blue which had dyed the brush. The elongation length X of the precursor film was determined by the distance from the macroscopic contact line to the border of the decolored region on the line-patterned brush area. Both the observations were carried out at room temperature under a humid air atmosphere. 10279

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Langmuir contact angle was not observed on the 10 μm line-patterned surface, even though a thicker brush was fabricated on the glass substrate. Water contact angle of the brush with 65 nm thickness was θortho = 37.4°, which was much higher than that of 5 μm line-patterned brush with 58 nm thickness (θortho = 23.9°). In this work, we do not still have an appropriate reason why the patterned poly(SPMK) brush with the wider line width showed the higher water contact angle than that with the narrower line width. Further experiments are in progress using a polymer brush with various line widths in a range from 1 to 100 μm to understand the effect of the line width on the water contact angle dependency on the brush thickness. Observation of Wetting Dynamics on the LinePatterned Surfaces. The droplet contact area A (mm2) of Acid Red 1 began to expand just after the droplet contacted the surface at t = 0 s, and then spread until t = 150 s. Figure 4a shows typical optical images of a droplet of Acid Red 1 aqueous solution (2 μL) on the line-patterned surface with 10:5 μm line widths, brush thickness h = 42 nm, and dyed with methylene blue under a humid air atmosphere. The droplet was observed as a red circle, which had rectilinear sides because the contact line was situated along the line pattern. After t = 150 s, A remained constant and the spreading converged. The evolution of the precursor film was clearly visualized as decolored lines on the line-patterned surface by using two types of dyes. Figure 4b shows time-series optical micrographs of the area in front of a droplet of Acid Red 1 aqueous solution on the line-patterned surface with 10:5 μm line widths and h = 42 nm. Blue lines represent the brush area in a dry state, which was originally colorless, but the poly(SPMK) brush can be dyed with methylene blue. The bulk droplet was visible as a red area. At the area near the macroscopic contact line, the blue lines turned colorless and the decolored region elongated along the poly(SPMK) line with time. The decolored lines were considered to be the precursor water film. The elongation length X of the precursor film was measured as the distance from the macroscopic contact line to the border of the decolored region on the line-patterned brush area. It is hard to observe the wetting propagation of the precursor water film on the uniformly-grafted (non-patterned) polymer brush surface in a few hundred-micrometer regions from the contact line, even if a colored liquid and a dyed brush were used. By using the hydrophilic−hydrophobic linepatterned surface, in fact, we serendipitously found that the precursor film can be successfully visualized. Due to the large gap in the surface free energy between the hydrophilic poly(SPMK) brush and the hydrophobic FAS monolayer area, the direction of water extension from the macroscopic contact line was regulated to the direction parallel to the linepatterned brush layer, which also improved the visibility of the contact line. We supposed that the selective wetting on the patterned surface enhanced the contrast of wet and dry area to enable the clear observation of the elongation behavior of the precursor film. The observation of the precursor film on the line-patterned surface was conducted in humid air by surrounding the sample with a fence of wetting paper towel. A high humidity was maintained during the observation to prevent the shrinkage of the droplet and the precursor film due to the non-equilibrium evaporation. The poly(SPMK) brush is expected to be hydrated under a humid atmosphere before the precursor film reaches the brush. Swollen structures of hydrophilic

polymer brushes under humid air have also been reported by several researchers.49−51 We supposed that the hydration of the brush would also promote the development of the precursor film to some extent, however, additional experiment for the precursor film on the brushes with various hydrophilicities, such as ionic, non-ionic, and zwitterionic polymers, must be necessary to discuss quantitatively the effect of the vapor hydrated state of the brush on the precursor film development. Decoloration means that the brush dyed with methylene blue became wet from the Acid Red 1 aqueous solution. A chemical mechanism for decoloration is suggested as follows. Methylene blue displays a blue color when the molecule is in the oxidized form. The cationic methylene blue molecules and anionic sulfonic acid groups including the poly(SPMK) brush are held together by an ionic bond. Therefore, the poly(SPMK) brush takes on a blue color. Methylene blue is predisposed to be easily reduced to a colorless hydrogenated molecule by various agents.52−54 Acid Red 1 is an H-acid-based dye that has a phenyl-azo group connected to the naphthol ring.55,56 The hydrogen including the naphthol group can be eliminated and the molecular structure changes from a naphthol form to a naphthoquinone form. Thus, Acid Red 1 donates its hydrogen to methylene blue, which becomes decolored. As a result, visually distinguishable three areas consisting of the blue-colored brush in the dry state, the colorless precursor film, and the red-colored macroscopic droplet appeared on the line-patterned surface. Owing to the color variation, the elongation length of the precursor film was clearly observed by conventional optical microscopy. Molecular interaction between the dye and the poly(SPMK) brush is unclear. Considering that water contact angles of the nondyed and dyed brush surfaces were the same, the effect of dyeing on the wetting behavior of the precursor film would be negligible. The topography of the precursor film spreading in front of the water droplet was measured by AFM. The substrate of the poly(SPMK) brush/FAS monolayer line-patterned surface was placed horizontally in a plastic dish. Wetting paper was placed on the wall of the dish to maintain a highly humid air atmosphere. The dish was set on the AFM stage and a water droplet (2 μL) was placed on the line-patterned surface. After 15 min, AFM imaging was conducted, at room temperature, in front of the macroscopic contact line, where decolored lines had been observed by optical microscopy. As shown in Figure 5a, the L-axis was set in the parallel direction to the line pattern. The origin of the L-axis was positioned at the macroscopic contact line, which was visible by using an inverted optical microscope equipped on the AFM unit. The AFM cantilever was moved back from the contact line by operating the translational control knob. A cross-sectional AFM profile was obtained at 100 μm points from L = 10 to 510 μm. The measurement time for all the points was 5 min, which is a smaller time scale compared to the transport of the precursor film. As shown in Figure 5b, at the point located at L = 10 μm, the brush thickness was 125 nm. At L = 510 μm, the thickness was 65 nm, which is equal to that in the dry state. The brush thickness around the droplet was larger than that in the dry state and decreased with the increasing distance from the contact line. Since the polymer brushes are known to swell with good solvents and increase in thicknesses,19 the brush in front of the bulk water can be considered to form a swollen brush layer by absorbing water. It was also confirmed that the 10280

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AFM, as shown in the topographic cross-sectional profiles of Figure S6. The poly(SPMK) brush on the line-patterned surface had 83 nm thickness in the dry state under an ambient atmosphere, whereas the thickness increased to 163 nm by swelling in water. Time Evolution of the Precursor Film and Droplet. The development of the precursor film and the spreading of the droplet on the line-patterned surfaces was described by the time-power law. Both X and A are plotted in double logarithmic charts with time t as a function, as shown in Figures 6 and S7. Because X and A increased linearly during particular time periods, both were proportional to the power of time. In the case of 10:5 μm line widths and h = 42 nm (Figure 6b), A increased with a specific slope (an exponent of time) aA = 0.2 from the droplet contacting the surface at t = 0 s to tA = 153 s. After tA, A remained virtually constant. On the other hand, X also increased with time, but the slope aX of log(X) to log(t) changed at the time tX = 370 s. Therefore, the development behavior of X can be divided into two stages. At the first stage (t < tX), the slope aX1 was 1.3. At the second stage (tX < t), the development proceeded continuously with time, having a smaller slope aX2 = 0.6 as described by X ∼ t0.6. The expansion of the droplet in our observation can be explained as a regime dominated by viscous dissipation from the point of the spreading velocity. The viscous-dominant regime follows the inertia-dominant regime and starts at around 10 ms after contacting the liquid for aqueous formulations.57−60 This type of spreading kinetics is described using the following power law based on Tanner’s law61

Figure 5. (a) Optical micrograph of the area in front of the macroscopic contact line of the water droplet (2 μL) on the poly(SPMK) brush and FAS monolayer line-patterned surface at room temperature under a humid air atmosphere. This image was taken by an inverted optical microscope equipped on the AFM unit. The line widths of the brush and monolayer were 5 and 5 μm, respectively. The brush thickness in the dry state was 65 nm. (b) The height profile of the precursor film along the line-patterned poly(SPMK) brush at a distance L from the contact line of water toward the dry-state brush area. The height of the precursor film was determined by the cross-sectional AFM profiles scanning in the orthogonal direction of the line pattern at each position.

i γt y r(t ) ∼ R jjjj zzzz k ηR {

1/10

poly(SPMK) brush area decolored during the observation by optical microscopy correspond to the precursor film composed of the swollen brush layer. AFM observation of the precursor film was conducted by using a cantilever with a spring constant of 42 N m−1, which might be too high to observe a swelling brush. However, large attractive capillary force between the tip and the wet surface cannot be ignored in this case because the observation of the precursor film was carried out under a humid atmosphere, not in water. A cantilever with a low spring constant was easily drawn into the precursor film due to the capillary force during the tip approach process. Therefore, the cantilever with the hard spring constant was used here. On the other hand, the swollen poly(SPMK) brush was also observed in water by

(2)

where r is the instantaneous radius of the droplet at time t, R is the initial radius, γ is the surface tension of the liquid, and η is the viscosity. The contact line is usually in the shape of a circle; therefore, the interface area is given by i 2j j

γt y A(t ) ∼ πR jj zzzz k ηR {

1/5

(3)

The exponent “1/5” in eq 3 is expected to change within the range of 0.2−0.6, influenced by the viscosity of the liquid or the surface chemistry or texture as reported in experimental research by Grewal et al.62 The exponent aA of our observation took a range from 0.11 to 0.27 near or within the predictable

Figure 6. Time evolution of (filled circle) the precursor film length X and (open circle) the droplet interface area A on the poly(SPMK) brush and FAS monolayer line-patterned surfaces with the line widths of 10 and 5 μm, respectively. The brush thickness h in the dry state was (a) 28 nm, (b) 42 nm, or (c) 65 nm. The slopes of log(X) to the logarithm of the time log(t) at the first stage and the second stage are represented by aX1 and aX2, respectively. The slope of log(A) to log(t) while the droplet spreading is represented by aA. 10281

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Figure 7. Brush thickness dependency of the precursor film elongation velocity and the droplet spreading velocity on the poly(SPMK) brush and FAS monolayer line-patterned surfaces. (a) The slope of the logarithm of the droplet area log(A) to the logarithm of the time log(t) is represented by aA. The slopes of the logarithm of the precursor film length log(X) to log(t) are represented by aX1 at (b) the first stage and aX2 at (c) the second stage. The line widths of the brush and the monolayer were (filled circle) 5 and 5 μm, respectively, and (open circle) 10 and 5 μm.

The aX2, in the second stage, approached almost 0.6 with the increase in the brush thickness, although the values had small variations, as shown in Figure 7c. In addition, the aX2 on both the patterned surface with the 5:5 and 10:5 μm line widths also showed similar values. The reason for the constant value of aX2 is described in the coming paragraph. Interestingly, independent of the droplet volume, the slope aX2 at the second stage (filled circles) remained at 0.6, as shown in Figure 8. On the other hand, as the droplet volume

range of the viscous-dominant spreading regime. Thus, the macroscopic wetting behavior on the line-patterned surface can be described by a conventional general system. The relationship between the brush thickness h and the spreading velocity of the water droplet or the precursor film is summarized in Figure 7. The slope aA of the droplet on the brush with thin thickness h = 8 nm was 0.28, while the aA leveled off to 0.12 with the increase in the brush thickness, as shown in Figure 7a. This wetting process includes swelling of the hydrophilic brush and the increase in the thickness. Swollen brush thickness is determined by the energy balance between an enthalpy gain by the hydration of the brush and an entropy loss due to the chain stretching of the brush. The brush with a thickness thinner than 28 nm induced a smaller thickness change, however, a relatively large thickness change occurred in the brush with a thicker thickness than 28 nm. The swelling process of the brush retarded the macroscopic spreading of the droplet leading to the reduction in the aA value. The aA became constant in the thickness ranging from h = 28 to 83 nm probably due to the constant swelling rate. Cheesman et al. reported that the poly(2-(diethylamino)ethyl methacrylate) brushes with the thickness of 24−74 nm showed the same swelling velocity in water.63 No significant difference was observed between the aA on the surface with the line widths of the brush/FAS = 5:5 μm pattern and that of the 10:5 μm pattern. If the line width ratio of the hydrophilic and hydrophobic area was much larger, such as 1:100 and 100:1 μm, the effect of the line width on the aA might be observed. On the other hand, Figure 7b shows that aX1 depended on the line width. The aX1 of the 10:5 μm line widths (open circle) showed various values from 2 to 3 depending on the brush thickness, whereas aX1 of the 5:5 μm line widths (filled circle) showed the relatively constant values around 0.6. In fact, the observation of the aX1 of the precursor film was technically difficult because the first stage started just after placing the droplet and transferred to the second stage within a short period, in particular, on the patterned surface of the 5:5 μm line widths. For example, in the case of 5:5 μm line widths and h = 83 nm, the aX1 could not be obtained. In addition, the contact line was still advancing during the first stage which made the trace observation very hard. Therefore, the estimated aX1 value from Figures 6 and S7 might lack the precision to discuss the dependency of aX on the brush thickness and the line widths. The precise analysis of the elongation behavior of the precursor film at the first stage is in progress using a viscous liquid.

Figure 8. Droplet volume dependency of the precursor film elongation velocity on the poly(SPMK) brush and FAS monolayer line-patterned surfaces. The line widths of the brush and monolayer were 5 and 5 μm, respectively. The brush thickness in the dry state was 83 nm. The slopes of the logarithm of the precursor film length log(X) to the logarithm of the time log(t) are represented by (open circle) aX1 at the first stage and (filled circle) aX2 at the second stage. The droplet volumes of Acid Red 1 aqueous solution were 0.20, 2.0, 10.0, 20.0, or 30.0 μL.

increased, aX1 increased exponentially (open circles). To understand the penetration behavior of water, we considered the potential energy of the water droplet. The droplet with the larger volume forms a sphere-like shape with a larger diameter before attaching to the substrate, and has a larger potential energy attributed to the height between the top and the bottom of the droplet. When the water droplet contacts with the substrate, the droplet begins to spread immediately on the surface, and its potential energy is converted to kinetic energy while the height of the droplet reduced.64 We supposed that a larger kinetic energy promoted the elongation velocity at the initial development stage of the precursor film, to result in a higher aX1 by the larger droplet volume. At the second stage, the effect of the potential energy of the droplet disappeared on the precursor film, and then water proceeded by the other driving factors mainly due to the stabilization of hydrophilic 10282

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Figure 9. (a) Illustration of the two stages of precursor film elongation. The values of the time t and the exponents are the typical case of the poly(SPMK) brush and the FAS monolayer line-patterned surface with 10 and 5 μm line widths, and 42 nm thickness of the brush. (b) Illustration of the sectional side view of the precursor film, which takes account of the interfacial energies γLV and γSV at the border of the precursor film.

Now, the behavior of the precursor film is discussed in more detail from the viewpoint of energy dissipation of the droplet caused by swelling of the brush. Figure 9b shows a sectional side view of the precursor film on the line-patterned surface of the polyelectrolyte brush. The precursor film consisted of a brush layer swollen by water that permeated the brush layer. In the area outside the precursor film, the brushes are in a dry state and are shrinking. At the border line of the precursor film, two kinds of surface energies exist side by side: the surface energy γLV at the swollen layer as the liquid (L)/vapor (V) surface on the precursor film side opposes the surface energy γSV at the dry-state brush as the solid (S)/vapor surface in the dry area. When the precursor film advances per unit area, the energy dissipation ΔU arises from the change in energy due to replacing the dry-state brush with the swollen one. At that time, a change in the free energy per brush chain ΔGbrush is also caused by the brush swelling. Therefore, ΔU can be expressed as follows

brush by hydration. Accordingly, aX2 was independent of the droplet volume. The two stages of the precursor film elongation are schematically illustrated in Figure 9a, and are associated with macroscopic droplet spreading. At the first stage of 0 s < t < tA, when the macroscopic droplet was spread at a velocity of aA = 0.2, the precursor film elongated at a larger velocity of aX1 = 3. At the second stage tX < t after the droplet spreading converged, the elongation velocity decreased and took the value aX2 = 0.6. It is suggested that the precursor film on the poly(SPMK) brush/FAS monolayer line-patterned surfaces at the first stage corresponds to the adiabatic precursor film, and at the second stage corresponds to the diffusive precursor film. It is noteworthy that the precursor film at the second stage developed faster than that in the theoretical model predicted by Joanny and de Genes. According to the model, the diffusive precursor film length Xdif on a general solid surface is given by26 Xdif =

H × t 0.5 3πηhliq

ΔU = γLV − γSV − ΔG brushσ

(5)

where σ is the grafting density of the polymer brush. ΔGbrush is determined by balancing the enthalpy change ΔH and the entropy change ΔS at temperature T, given as follows

(4)

where H is the Hamaker constant and hliq is the thickness of the liquid film. In this equation, Xdif increases in proportion to the 0.5 power of time, but on our line-patterned surfaces, X developed with a larger exponent of aX2 = 0.6. The suggested reason is that the polymer brush spontaneously swells with water and stabilizes through hydration of the brush chains. At the second stage, the precursor film proceeded by the energy balances between the enthalpy gain due to the stabilization of the hydrophilic brush by the hydration and the entropic energy loss due to the stretched chain conformation of the swollen brush. Therefore, aX2 was independent of the line width and the brush thickness. The elongation mechanism of water in this brush film is different from the classical precursor film development. We supposed that the attractive hydration strongly promoted the elongation velocity of the precursor film to lead to the faster elongation compared with that of the surface without the brush. Another possible reason for the faster velocity on the line-patterned surface is the capillary force due to the narrow line structure. The capillary force largely depends on the fluid channel width, whereas the aX2 in this study was independent of the line widths. Therefore, the precursor film on the line-patterned polymer brush might be regarded as a unique case in the conventional precursor liquid film on the flat solid surface. However, both had similar features, such as the same-scale thicknesses and the two-stage development process. We supposed that a new theory would be necessarily based on the further improvement of the conventional theory of the precursor film.

ΔG brush = ΔH − T ΔS

(6)

In this particular polymer brush, ΔH expresses the energy gain due to stabilization through hydration of the brush chains. On the other hand, the swelling of the brush causes stretching of the polymer brush chains. This confinement of the conformation of the brush results in the entropy loss as expressed by ΔS in eq 6. The energy dissipation decelerated the elongation velocity of the precursor film, measured as X ∼ t0.6. For the polymer brush, the entropy loss was considered to act as a “brake” in the development of the precursor film. It has been suggested that the energy change plays an important role in the wetting of polymer brushes. For example, de Gennes et al. assumed that the energy for swelling F was the sum of the mixing energy Fmix per brush chain and the elastic energy Fel that resists the brush stretching as follows65,66 F = Fmix + Fel

(7)

Fmix is given by the following Fmix = kT ×

hD2 × ϕp ln(ϕp) b3

(8)

where k is the Boltzmann constant, h is the brush thickness, D is the distance between the attachment sites for the brush (D = bσ−1/2), b is the monomer size, and ϕP is the volume fraction of the solvent in the brush. Fel is given by 10283

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Langmuir Fel = kT ×

R 2 zy 3 jij h2 × jj 2 + 20 zzz 2 jk R 0 h z{

Article

(9)

where, R0 = N1/2b (here, N is the degree of polymerization). Muller et al. considered the free enthalpy of hydration by wetting and succeeded in explaining the contact angle on the polyelectrolyte brush surface by eq 1.15 For discussing the kinetics of the spreading behavior of a droplet on fractal agar gel surfaces, Nonomura et al. derived a model for the energy dissipation rate arising from viscous phenomena.67 To better understand the dynamics of a precursor film on the surface of a polyelectrolyte brush, it is necessary to clarify the energy dissipation caused by the swelling of the brush chains. We are presently conducting experiments to construct the theoretical model based on physical chemistry. Molecular weight distribution and grafting density of the polymer brush will be taken into account in future work.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +81-42628-4551. ORCID

Motoyasu Kobayashi: 0000-0001-8349-4365 Notes

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The authors declare no competing financial interest.

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CONCLUSIONS The development of the precursor film consisting of the swollen brush layer was successfully visualized on the poly(SPMK) brush/FAS monolayer line-patterned surfaces by optical microscopy. The line-patterned surfaces with hydrophilic/hydrophobic contrast of micrometer-scale width were prepared through the combination of the CVA method, VUV photolithography technique, and SI-AGET ATRP. Poly(SPMK) brush lines were dyed with methylene blue, and then droplets of Acid Red 1 were placed on the surface. The precursor film elongated outward the macroscopic contact line along the polymer brush lines following the time-power law. The evolution of the precursor film can be divided into two stages. The elongation velocity at the first stage increased as the brush line width or the droplet volume increased, and was not dependent on the brush thickness. At the second stage even after the macroscopic contact line stopped moving, the velocity became slower than that in the first stage, but the precursor film continued to elongate as described by X ∼ t0.6. The velocity at the second stage was not dependent on the brush thickness, droplet volume, or line width. As predicted by Joanny and de Genes in 1986, the precursor film on a general solid surface expands with Xdif ∼ t0.5. In the case of the linepatterned polyelectrolyte brush surface, the development was faster than the general case. The suggested reason for the difference is that the polymer brush spontaneously swells in the water film and stabilizes through hydration of the brush chains. On the other hand, the brush’s swelling causes stretching of the polymer brush chains, resulting in an entropy loss. This entropy loss leads to energy dissipation of the liquid and gradual deceleration of the development of the precursor film. This study is the first experimental investigation of the evolution of the precursor film on the polymer brushes. A more detailed theoretical model based on physical chemistry is a future goal.

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10:5 μm line widths; XPS spectra; optical images of the static wetting for a water droplet; brush thickness dependency of the water contact angle; observation method for the precursor film; AFM topography cross sections of the line-patterned surface under atmosphere and in water; time evolution of the precursor film length and the droplet interface area on the line-patterned surfaces with 5:5 μm line widths (PDF)

ACKNOWLEDGMENTS This research was supported by a Grant-in-Aid under the Japan Society for the Promotion of Science (JSPS) KAKENHI Scientific Research C (No. 17K05887) from the Ministry of Education, Culture, Science, Sports and Technology of Japan (MEXT), and by a Functional Microstructure Surfaces Research (FMS) program, and the Biomolecules System Research Center (BMSC) program for Strategic Research at Private Universities (Kogakuin University) from MEXT, Research Center for Biomedical Engineering (No. 4005), and performed under the Cooperative Research Program (Nos. 20161265 and 20171310) of the Network Joint Research Center for Materials and Devices. M.K. appreciates Dr. Hiroyuki Mayama in Asahikawa Medical University for his helpful suggestions on this work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02070. Preparation of the poly(SPMK) brush/FAS monolayer line-patterned surface; AFM topography images and the cross sections of the line-patterned surface with 5:5 and 10284

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DOI: 10.1021/acs.langmuir.8b02070 Langmuir 2018, 34, 10276−10286