Article pubs.acs.org/Langmuir
Thermal Dewetting Behavior of Polystyrene Composite Thin Films with Organic-Modified Inorganic Nanoparticles Masaki Kubo,*,† Yosuke Takahashi,† Takeshi Fujii,† Yang Liu,† Ken-ichi Sugioka,† Takao Tsukada,† Kimitaka Minami,‡ and Tadafumi Adschiri§ †
Department of Chemical Engineering, Tohoku University, 6-6-07, Aramaki, Aoba-ku, Sendai 980-8579, Japan Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8565, Japan § WPI Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡
ABSTRACT: The thermal dewetting of polystyrene composite thin films with oleic acid-modified CeO2 nanoparticles prepared by the supercritical hydrothermal synthesis method was investigated, varying the nanoparticle concentration (0−30 wt %), film thickness (approximately 50 and 100 nm), and surface energy of silanized silicon substrates on which the composite films were coated. The dewetting behavior of the composite thin films during thermal annealing was observed by an optical microscope. The presence of nanoparticles in the films affected the morphology of dewetting holes, and moreover suppressed the dewetting itself when the concentration was relatively high. It was revealed that there was a critical value of the surface energy of the substrate at which the dewetting occurred. In addition, the spatial distributions of nanoparticles in the composite thin films before thermal annealing were investigated using AFM and TEM. As a result, we found that most of nanoparticles segregated to the surface of the film, and that such distributions of nanoparticles contribute to the stabilization of the films, by calculating the interfacial potential of the films with nanoparticles.
1. INTRODUCTION Polymer thin films on nonwettable substrates are either metastable or unstable, and may dewet upon thermal annealing or solvent exposure. The dewetting of polymer films, the rupture and breakup of films on the substrate, is a crucial technological problem for the industrial applications of thin films, and is influenced by several factors such as wettability of substrate, viscoelasticity of polymer, and interfacial potential between polymer and air. The introduction of additives into polymers is one of the potential strategies for stabilizing thin polymer films and retarding dewetting. Barnes et al.1 demonstrated that the addition of a small amount of C60 fullerene nanoparticles inhibits the dewetting of polystyrene thin films. According to their study, such dewetting inhibition might be attributed to the following: fullerene nanoparticles, which are segregated to the interface between the polystyrene film and the silicon substrate, pin the contact line of polystyrene films on the substrate surface, or change the wettability of the substrate surface. Over the past decade, by using various additives, such as dendrimers,2 polystyrene nanoparticles,3,4 cadmium selenide quantum dots,4 carbon nanotubes,5,6 perovskite compounds,7 silsesquioxane nanofillers,8 star comb-like polymers,9 gold nanoparticles,10−12 and palladium nanoparticles,12 the effect of additives on the retardation or inhibition of dewetting in polymer thin films has been investigated. © 2014 American Chemical Society
To understand the mechanism of the dewetting inhibition of polymer thin films with nanoparticles or nanofillers, it is necessary to elucidate the spatial distribution of additives in polymer films. Concerning the dewetting behavior of polystyrene thin films with polystyrene nanoparticles on a silicon substrate, Krishnan et al.4 revealed that polystyrene nanoparticles segregate near the substrate surface, by neutron reflectivity measurement. McGarrity et al.13 numerically demonstrated the segregation of nanoparticles near solid surfaces observed by Krishnan et al.,4 using the fluid density functional theory (F-DFT). Luo and Gersappe14 proved the relation between dewetting inhibition and nanoparticle segregation on a solid surface using molecular dynamics simulations. On the other hand, Krishnan et al.4 observed, by TEM, that hydrocarbon-coated cadmium selenide nanoparticles mostly segregated to the air interface of polystyrene thin films, although nanoparticles were partially pushed to the silicon substrate. In this case, also, the dewetting of the polymer thin films was inhibited. Mukherjee et al.11 found that the stability of PS thin films changed from complete dewetting to partial dewetting followed by complete stability with increasing the concentration of uncapped gold nanoparticles, where the Received: May 23, 2014 Revised: June 30, 2014 Published: July 3, 2014 8956
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elucidate the dewetting mechanism of the films, the surface morphologies of the films were observed by atomic force microscopy (AFM), and the spatial distributions of nanoparticles in the films were measured by transmission electron microscopy (TEM). In addition, the interfacial potential of polymer thin films with nanoparticles was evaluated to discuss the contribution of nanoparticles to the stability of polymer films.
nanoparticles did not segregate to the substrate surface but aggregated in the central part of the rim surrounding the dewetted hole. All of the above-mentioned previous studies suggest that the dewetting of polymer thin films can be inhibited by adding a small amount of nanoparticles that segregate to the substrate or the air interface of the polymer film. However, the mechanism of the dewetting inhibition of nanoparticles in polymer thin films remains to be elucidated. In addition, the primary aim of most previous studies was simply to retard and inhibit the dewetting of polymer thin films by adding a small amount of nanoparticles. Materials composed of polymers into which nanoscale inorganic particles are incorporated, polymer nanocomposites, have attracted much attention because of their significantly different properties from pure host polymers. The properties of polymer nanocomposites can be controlled not only by combining host polymer and nanoparticle functionalities, but also by controlling the size and morphology of the nanoparticles and their spatial structure in the host polymer. Therefore, numerous applications utilizing the functional and structural properties of polymer nanocomposites are expected for chemical sensors,15 self-healing surfaces,16 conjugated polymer photovoltaic cells,17,18 optical and optoelectronic devices,19 and electrochemical devices.20 However, to date, there have been few studies in which the dewetting behavior of polymer nanocomposite thin films was discussed, particularly at relatively high concentration of inorganic nanoparticles. When fabricating a polymer nanocomposite film, controlling the affinity between the polymer and the inorganic nanoparticles by organic modification is a critical issue.21 Recently, Adschiri et al.21−28 have developed a new synthesis method for organic-modified inorganic nanoparticles of single-nanometer size, which gives rise to better dispersion of nanoparticles in organic solvents.21,23,25−28 In this method, organic species (e.g., amino acids, carboxylic acids, amines, alcohols, and aldehydes) are introduced in supercritical hydrothermal synthesis,28−30 and metal oxide nanoparticles with surfaces modified by the organic materials are consequently obtained. Such nanoparticles have the potential to adjust their physical properties by changing the combination of their ligand organic compound and core metal oxide materials. In addition, such surface modification allows dispersion of inorganic nanoparticles in organic solvents, and the control of spatial assemblies of nanoparticles, that is, the two- or threedimensional nanostructures, in host polymers. Therefore, it is expected that polymer nanocomposite materials with novel characteristics can be created using organic−inorganic hybrid nanoparticles synthesized by the supercritical hydrothermal synthesis method. However, to the author’s knowledge, there has been no study of the dewetting behavior of polymer nanocomposites with organic-modified metal oxide nanoparticles. In this study, the dewetting behavior of polystyrene (PS) composite thin films with oleic acid-modified CeO2 nanoparticles prepared by the supercritical hydrothermal synthesis method was investigated. Here, toluene solutions of PS with CeO2 nanoparticles were spin-coated on silicon substrates, and then the resulting composite films on the substrate were thermally annealed. For various concentrations of nanoparticles added to PS thin films, film thicknesses, and substrate surface energies, the dewetting behavior of the PS composite thin films during annealing was observed by an optical microscope. To
2. EXPERIMENTAL METHOD 2.1. Materials. A polystyrene (PS) standard with a weight-average molecular weight of 2000 (GL Sciences Inc., Japan) was used as the host polymer matrix; the usage of such PS with a relatively low molecular weight was to promote faster dewetting of the polymer film. Toluene, the solvent for dissolving the polymer, was purchased from Wako Pure Chemical Industries, Ltd. The starting materials for synthesizing oleic acid-modified CeO2 nanoparticles, that is, cerium hydroxide and oleic acid, were purchased from Sigma-Aldrich Co. and Wako Pure Chemical Industries, Ltd., respectively. A 23 × 23 mm2 silicon wafer (Nilaco Co., Japan) was used as a substrate whose surface was silanized using octadecyltriethoxysilane (Shin-etsu Chemical Co., Ltd., Japan) to investigate the effect of the surface energy of the substrate on the dewetting behavior of a PS composite thin film. First, the wafer was cleaned in a piranha bath (70% H2SO4 and 30% H2O2) for 12 h or longer to remove any organic compounds and make its surface hydrophilic. The cleaned wafer was rinsed with pure water by ultrasound irradiation for 10 min, and then dried in an oven at 348 K. The dried silicon wafers were then dipped in cyclohexane solution containing 0.01 M octadecyltriethoxysilane (ODS) at 333 K to silanize its surfaces. The silanized substrate was rinsed with acetone and water, and then dried at 393 K for 2 h. The surface energy of the substrate was adjusted by changing the time of immersion in the ODS/cyclohexane solution. Because the surface energy of the substrate is closely linked with the surface hydrophobicity of the substrate, first, the relationship between the two was measured in this study. Here, the surface energy of the substrate was measured by the following Owens−Wendt method:31
γL(1 + cos θ) = 2 γLpγSp + 2 γLdγSd
(1)
γL = γLp + γLd
(2)
In eqs 1 and 2, γ denotes surface energy, subscripts S and L denote solid and liquid, superscripts p and d denote the polar and nonpolar components of γ, and θ is the contact angle. The two components of surface energy of the substrate, γpS and γdS, were determined by eqs 1 and 2 using the measured contact angles of two different liquids with known surface energies, γpL and γdL, that is, water and diiodomethane. On the other hand, the surface hydrophobicity of the substrate was evaluated using the contact angle of water on the substrate. The surface energy of the substrate in each dewetting experiment shown below then was evaluated along the following procedure: A sessile water droplet of approximately 7 μL was placed on each of the 3 × 3 regions of the substrate surface used in the experiment, and then the contact angle of water was determined from the profile of the droplet near the contact line, taken using a digital camera (EOS Kiss Digital X, Canon Inc., Japan). The surface energies corresponding to both nine contact angles and their mean value were evaluated using the above relationship between the surface energy and surface hydrophobicity of the substrate. 2.2. Preparation of Ceria Nanoparticles. Ceria (CeO2) nanoparticles, whose surfaces were modified with oleic acid, were prepared by the supercritical hydrothermal synthesis method.22 First, 2.5 mL of 0.1 M Ce(OH)4 aqueous solution was poured into a pressure-resistant SUS316 vessel (inner volume, 5 mL). To modify the surface of the nanoparticles, 0.48 mL of oleic acid was also loaded into the reactor vessel. The hydrothermal reaction was performed at 673 K for 10 min and terminated by submerging the reactor in a water bath at room temperature. The oleic acid-modified nanoparticles were 8957
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Figure 1. Optical microscope images of the surfaces of PS thin films annealed at 393 K, where the thicknesses of the films are approximately 50 nm. (a) Nanoparticle concentration, 0 wt %; surface energy, 27 mJ/m2. (b) Nanoparticle concentration, 10 wt %; surface energy, 26 mJ/m2. (c) Nanoparticle concentration, 20 wt %; surface energy, 31 mJ/m2. (d) Nanoparticle concentration, 30 wt %; surface energy, 29 mJ/m2. extracted from the product mixture with 10 mL of cyclohexane. The final products were precipitated from the resulting cyclohexane phase by adding ethanol as the antisolvent reagent, and then separated by centrifugation. The nanoparticles obtained were redissolved in toluene. The average diameter of the nanoparticles evaluated using TEM (HD2700, Hitachi High-Technologies Co., Japan) was approximately 7 nm. 2.3. Annealing of Polymer Composite Thin Film. A toluene solution containing PS and oleic acid-modified ceria nanoparticles was prepared. The PS concentration in the solution was adjusted to either 2.6 or 5.0 wt %, depending on the final thickness of polymer films, that is, approximately 50 or 100 nm, respectively. The amount of nanoparticles added was determined so that the nanoparticle concentration of the polymer composite thin film could range up to 30 wt %. 140 μL of PS/toluene solution with nanoparticles was spincast (1H-D3, Mikasa Co., Ltd., Japan) at 4500 rpm for 30 s onto a silicon substrate, and then dried at room temperature for 24 h in a clean bench (PCV-750APG, Hitachi Co., Ltd., Japan) to form PS composite films on the substrates. Film thickness was measured by ellipsometry (DHA-XA/S3-T, Mizojiri Optical Co., Ltd., Japan). The PS composite films with nanoparticles on a silicon substrate were annealed in air at 393 K for up to 1 h using a handmade PIDcontrolled hot plate. The thermal annealing of PS composite films for 24 h has been performed as preliminary experiments, and then the following was confirmed: the dewetting behaviors of PS films did not change more than 1 h later, and the surface morphologies of PS films annealed for 24 h were the same as those for 1 h. Therefore, the thermal annealing for 1 h was considered to be sufficient for investigating the dewetting behavior of PS composite thin films. The dewetting behavior during thermal annealing was observed by reflected-light optical microscope (Fabulous, Olympus Co., Japan), and images were captured using a CCD camera. In addition, photo images of the entire area of the thin film surface after annealing for 1 h were taken using a digital camera. This photo image was converted to binary (black and white) image to evaluate the degree of dewetting,
which was defined as the ratio of the dewetting area to the entire surface area of the substrate. The surface morphologies of the films were observed by AFM (Nanocute, SII Nano Technology Inc., Japan). The spatial distributions of nanoparticles in the cross section of the films were observed by TEM (HF-2100, Hitachi High-Technologies Co., Japan).
3. RESULTS AND DISCUSSION Figure 1 shows optical microscope images of the surfaces of PS thin films annealed at 393 K when the concentrations of nanoparticles in PS thin films are (a) 0, (b) 10, (c) 20, and (d) 30 wt %, respectively. Here, the film thickness in all cases was approximately 50 nm. The surface energies of the substrates were (a) 27, (b) 26, (c) 31, and (d) 29 mJ/m2, respectively. The number at the upper left of each image indicates the annealing time. From Figure 1a without nanoparticles, it can be seen that, in the early stage of dewetting, isolated circular holes formed and grew in the film. The rims of adjacent holes then merged with each other and formed interconnected rim structures. Finally, the rims decayed into small droplets due to the Rayleigh instability of a cylindrical fluid. Figure 1a shows the typical dewetting behavior of polymer thin films. For PS thin films with 10 wt % nanoparticles shown in Figure 1b, also, isolated holes formed in the film, the rims of the growing holes merged with each other, and interconnected rim structures formed. However, as compared to those in Figure 1a, most of the rims remained with undulating cylindrical shape and did not decay into small droplets, but some broke into the droplets with irregular contact lines. Xu et al. observed the similar morphologies during thermal annealing of PS thin films with star comb-like polymer nanoparticles, and suggested that this was due to the increase in viscosity of the rims in which the 8958
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Figure 2. Optical microscope images of the surface of PS thin films annealed at 393 K, where the thicknesses of the films are with 50 nm thickness. (a) Nanoparticle concentration, 0 wt %; surface energy, 50 mJ/m2. (b) Nanoparticle concentration, 10 wt %; surface energy, 50 mJ/m2. (c) Nanoparticle concentration, 20 wt %; surface energy, 51 mJ/m2. (d) Nanoparticle concentration, 30 wt %; surface energy, 47 mJ/m2.
Figure 3. Optical microscope images of the surfaces of PS thin films annealed at 393 K, where the thicknesses of the films are approximately 100 nm. (a) Nanoparticle concentration, 0 wt %; surface energy, 32 mJ/m2. (b) Nanoparticle concentration, 5 wt %; surface energy, 28 mJ/m2. (c) Nanoparticle concentration, 10 wt %; surface energy, 23 mJ/m2. (d) Nanoparticle concentration, 30 wt %; surface energy, 30 mJ/m2.
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Figure 4. Effect of surface energy on degree of dewetting for (a) film thickness of 50 nm, and (b) film thickness of 100 nm. The open plots denote the degree of dewetting in each of the 3 × 3 regions of the substrate surface, and the solid plots denote the mean value of them.
nanoparticles accumulated as the dewetting holes grew.9 However, the definite mechanism of dewetting behavior shown in Figure 1b remains unknown. The dewetting behavior of PS thin films with 20 wt % nanoparticles is shown in Figure 1c. It was clear that the dewetting morphology markedly changed by increasing the concentration of nanoparticles. The structure of the growing holes in Figure 1c was very similar to fingering structures observed in dewetting thin films of evaporating colloidal nanoparticle suspensions, that is, nanofluids,32−34 where the fingering structures were suggested to be influenced strongly by the mobility of nanoparticles in the nanofluid on the substrate. Figure 1d shows the surfaces of PS thin films with 30 wt % nanoparticles. In this case, the dewetting did not occur even after 1 h annealing. Thus, the dewetting was completely suppressed in this case. Figure 2 shows the dewetting morphologies of PS composite thin films on the silicon substrates, which were higher surface energies than those used in Figure 1. The surface energies of the substrate were approximately 50 mJ/m2. The thicknesses of PS films were approximately 50 nm. Comparing the dewetting morphologies in Figure 2 to those in Figure 1, it was found that the dewetting behaviors in both cases were almost the same, although the dewetting rate decreased and the number density of initial holes increased with the surface energy of the substrate. In the case of PS thin film with 30 wt % nanoparticles, also, the dewetting was completely suppressed similar to Figure 1d.
Figure 3 shows optical microscope images of the surfaces of the PS thin films when the film thicknesses are approximately 100 nm. The surface energies of the substrate were around 30 mJ/m2. Here, to investigate the effect of thickness of PS composite thin films on the dewetting morphologies of the films, the figure was compared to Figure 1. In the case of the film with 5 wt % nanoparticles, the dewetting holes with irregularly shaped contact lines grew. In the case with 10 wt % nanoparticles, the channels, not the holes, were generated and progressed in the film. Thus, it is clear that film thickness affects the dewetting behaviors of the films containing nanoparticles. In contrast, for 30 wt % nanoparticles, the dewetting was suppressed similarly to the films with 50 nm thickness shown in Figure 3d. To clarify the effect of the surface energy of silanized silicon substrate on the dewetting behavior of the spin-coated PS thin films, the degrees of dewetting were plotted against the substrate surface energies in Figure 4 for the film thicknesses of approximately (a) 50 and (b) 100 nm. In the figures, the contact angles of water on the substrate are also shown to comprehend the relationship between the surface energy and surface hydrophobicity of the silanized substrate used for determination of surface energy. The surface energies of SiO2 and octadecane, which are the surface compositions of the unsilanized substrate with contact angle of approximately 30° and fully silanized substrate with contact angle of 100°, are 62.135 and 28.336 mJ/m2, respectively, and thus the estimated values of surface energy in this study are appropriate. The open 8960
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Figure 5. AFM images of film surface when film thicknesses are 50 nm and surface energies are between 55 and 65 mJ/m2. Nanoparticle concentrations are (a) 0 wt %, (b) 10 wt %, (c) 20 wt %, and (d) 30 wt %.
Figure 6. AFM images of film surface when film thicknesses are 50 nm and surface energies are around 30 mJ/m2. Nanoparticle concentrations are (a) 0 wt %, (b) 10 wt %, (c) 20 wt %, and (d) 30 wt %.
concentrations in the figure, the dewetting did not occur at surface energy higher than 70 mJ/m2. In addition, it is obvious that there is a critical value of surface energy beyond which the degree of dewetting suddenly decreases. At lower surface energy, the dewetting occurred in the entire area of the substrate. The critical value of surface energy decreased with increasing nanoparticle concentration. In the case of pure polystyrene film, that is, 0 wt % nanoparticles, the critical value of the surface energy was 70−75 mJ/m2. When the nanoparticle concentrations were 10 and 20 wt %, the critical values
plots shown in Figure 4 denote the relationship between the surface energy and the degree of dewetting in each of the 3 × 3 regions of the substrate surface, and the solid plots denote the mean value of them. The degree of dewetting is defined as the ratio of the dewetting area to the entire substrate surface area. If the degree of dewetting is 0, dewetting is completely inhibited, and if the degree of dewetting is 1, dewetting occurs in the entire area of the substrate. Figure 4a shows the results for the degree of dewetting of PS thin films with thickness of 50 nm. For all nanoparticle 8961
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were 40−60 and 25−55 mJ/m2, respectively. When the nanoparticle concentration was 30 wt %, dewetting did not occur within the present experiment range. Thus, the addition of nanoparticles is effective for inhibiting dewetting. Figure 4b shows the results when the film thickness is approximately 100 nm. The higher is the nanoparticle concentration, the lower is the critical surface energy. The critical values were 50−60 and 25−35 mJ/m2 for 0 and 10 wt % nanoparticle concentrations, respectively. The dewetting did not occur for 30 wt % nanoparticle concentration at all. In addition, by comparing the results in Figure 4b with those in Figure 4a, it was clear that the critical value of the surface energy decreased as film thickness increases, although the dewetting behaviors of the films containing nanoparticles depended on the film thickness as shown in Figures 1 and 3. To study the mechanism of the suppression or inhibition of dewetting by nanoparticle addition, the surface morphologies of PS composite thin films were observed by AFM, and the spatial distributions of nanoparticles in the films were observed by TEM. Figure 5 shows the AFM images of the film surface before thermal annealing for three nanoparticle concentrations. The thicknesses of all films were approximately 50 nm. The surface energies of the substrates were between 55 and 65 mJ/ m2. In all cases with nanoparticles, the dewetting did not occur as shown in Figure 4a. The surfaces of the films with 0, 10, and 20 wt % nanoparticles were smooth, and their root-meansquare (RMS) roughness values were 0.470, 0.756, and 0.647 nm, respectively. In contrast, for the film with 30 wt % nanoparticles, the RMS roughness was 1.89 nm, which may be responsible for the segregation of nanoparticles to the film surface. Figure 6 shows the surface morphologies of PS composite thin films before thermal annealing when the surface energies of the substrates are approximately 30 mJ/m2, which are lower than those in Figure 5. In this case, the dewetting occurred for nanoparticle concentrations of 0, 10, and 20 wt % except for 30 wt %, as shown in Figure 4. The surfaces of films with 0 and 10 wt % nanoparticles were smooth; the values of RMS roughness were 0.857 and 0.911 nm, respectively. On the other hand, the values of RMS for 20 and 30 wt % were 1.80 and 1.61, respectively. The change in the surface roughness of films with lower surface energy also may reveal the segregation of nanoparticles to the film surface. Figure 7 shows a cross-sectional TEM image of the spatial distributions of nanoparticles in the PS composite thin films before annealing. Here, the film thickness was 50 nm, the nanoparticle concentration was 30 wt %, and the surface energy of the substrates was 61 mJ/m2. Most of the nanoparticles segregated on the film surface (film/air interface), although a
small number of nanoparticles also existed on the substrate surface. The spatial structures of oleic acid-modified CeO2 nanoparticles in PS thin films on silicon substrates are determined by the balance of entropic and enthalpic effects concerned with polymer and nanoparticles. According to Krishnan et al.4,18 and Stamm and Sommer,37 the nanoparticles might segregate to the substrate surface, because the conformational entropic gain, which is provided by moving the linear PS chains away from the substrate and replacing them with nanoparticles, is superior to the losses of translational entropy of nanoparticles and enthalpic contact energy between nanoparticles and PS chains. However, if surface energy of the oleic acid modifying the surface of CeO2 nanoparticles is lower than that of PS, the nanoparticles should migrate to the PS film/air interface so as to minimize the total energy of the system. Here, note that the surface tension of hydrocarbons, approximately 20−25 mN/ m,36 is lower than that of PS, 40.7 mN/m,38 considering that the surface property of oleic acid on the nanoparticles is similar to that of hydrocarbons. In the system of this work, such an enthalpic effect is superior to the conformational entropic effect of polymers and nanoparticles, and, consequently, the spatial structure of nanoparticles shown in Figure 7 might be generated. Moreover, the wetting stability of a PS thin film was evaluated on the basis of the spatial structure of nanoparticles in the film shown in Figure 7 and the Hamaker constant of the film.4 In the case of the film with nanoparticles, a three-layer system consisting of nanoparticles, PS, and ODS was considered. In the case of the film without nanoparticles, on the other hand, the system consisting of PS and ODS was considered in which the surface of PS film was in contact with air. The Hamaker constants of the polymer films in the case with or without nanoparticles were estimated using the following Lifshitz theory:39 A132 =
⎛ ε − ε3 ⎞⎛ ε2 − ε3 ⎞ 3hνe 3 kBT ⎜ 1 ⎟⎜ ⎟+ 4 8 2 ⎝ ε1 + ε3 ⎠⎝ ε2 + ε3 ⎠ (n12 − n32)(n22 − n32)
(n12 + n32)1/2 (n22 + n32)1/2 {(n12 + n32)1/2 + (n22 + n32)1/2 } (3)
where εi and ni denote the dielectric constant and refractive index, respectively. kB, h, and νe are the Boltzmann constant, Planck constant, and frequency for dominant relaxation in the ultraviolet. The refractive indices of CeO2, oleic acid, PS, and ODS are 2.2,40 1.46,41 1.59,39 and 1.44,4 respectively. The dielectric constants of CeO2, oleic acid, PS, and ODS are 6.0,42 2.3,41 2.5,39 and 2.0443 (it is assumed to be similar to that of octadecane), respectively. Assuming that the length of oleic acid on CeO2 is 2.5 nm,44 the refractive index and dielectric constant of nanoparticles are 1.69 and 3.47, respectively, with a simple volumetric average of those two properties. Using these values, the Hamaker constant of the film without nanoparticles was found to be 23.0 zJ, and that with nanoparticles was −3.7 zJ. A negative Hamaker constant denotes that the film with nanoparticles is stable.4
4. CONCLUSIONS The thermal dewetting of polystyrene composite thin films with oleic acid-modified CeO2 nanoparticles prepared by the supercritical hydrothermal synthesis method was investigated,
Figure 7. TEM image of cross section of film when film thickness is 50 nm, surface energy is 61 mJ/m2, and nanoparticle concentration is 30 wt %. 8962
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varying the nanoparticle concentration (0−30 wt %), film thickness (approximately 50 and 100 nm), and surface energy of the silicon substrate on which the composite films were coated. The dewetting behavior of the composite thin films during thermal annealing was observed by an optical microscope. The presence of nanoparticles on the composite thin films affected the morphology of dewetting holes. When the nanoparticle concentration was 30 wt %, dewetting was suppressed or inhibited. The surface energy of the substrate had a critical value at which the dewetting occurred. The observation of the cross-section of the thin film by TEM elucidated that the nanoparticles segregated to the air surface of the film. The surface roughness by AFM also revealed that the nanoparticles segregated to the air surface. The interfacial potential of polymer thin films with nanoparticles suggested that the presence of nanoparticles, which segregated to the film surface, primarily contributed to the stabilization of the films.
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(10) Kim, J.; Green, P. F. Phase Behavior of Thin Film Brush-Coated Nanoparticles/Homopolymer Mixtures. Macromolecules 2010, 43, 1524−1529. (11) Mukherjee, R.; Das, S.; Das, A.; Sharma, S. K.; Raychaudhuri, A. K.; Sharma, A. Stability and Dewetting of Metal Nanoparticle Filled Thin Polymer Films: Control of Instability Length Scale and Dynamics. ACS Nano 2010, 4, 3709−3724. (12) Xavier, J. H.; Sharma, S.; Seo, Y. S.; Isseroff, R.; Koga, T.; White, H.; Ulman, A.; Shin, K.; Satija, S. K.; Sokolov, J.; Rafailovich, M. H. Effect of Nanoscopic Fillers on Dewetting Dynamics. Macromolecules 2006, 39, 2972−2980. (13) McGarrity, E. S.; Frischknecht, A. L.; Mackay, M. E. Phase Behavior of Polymer/Nanoparticle Blends Near a Substrate. J. Chem. Phys. 2008, 128, 154904. (14) Luo, H.; Gersappe, D. Dewetting Dynamics of Nanofilled Polymer Thin Films. Macromolecules 2004, 37, 5792−5799. (15) Holmes, M.; Mackay, M.; Giunta, R. Nanoparticles for Dewetting Suppression of Thin Polymer Films used in Chemical Sensors. J. Nanopart. Res. 2007, 9, 753−763. (16) Gupta, S.; Zhang, Q.; Emrick, T.; Balazs, A. C.; Russell, T. P. Entropy-driven Segregation of Nanoparticles to Cracks in Multilayered Composite Polymer Structures. Nat. Mater. 2006, 5, 229−233. (17) Coakley, K. M.; McGehee, M. D. Conjugated Polymer Photovoltaic Cells. Chem. Mater. 2004, 16, 4533−4542. (18) Krishnan, R. S.; Mackay, M. E.; Duxbury, P. M.; Pastor, A.; Hawker, C. J.; Van Horn, B.; Asokan, S.; Wong, M. S. Self-assembled Multilayers of Nanocomponents. Nano Lett. 2007, 7, 484−489. (19) Jeeju, P. P.; Sajimol, A. M.; Sreevalsa, V. G.; Varma, S. J.; Jayalekshmi, S. Size-dependent Optical Properties of Transparent, SPIN-coated Polystyrene/ZnO Nanocomposite Films. Polym. Int. 2011, 60, 1263−1268. (20) Bhatt, A. S.; Bhat, D. K. Influence of Nanoscale NiO on Magnetic and Electrochemical Behavior of PVDF-based Polymer Nanocomposites. Polym. Bull. 2011, 68, 253−261. (21) Adschiri, T.; Byrappa, K. Nanohybirdization of OrganicInorganic Materials,11 Supercritical Hydrothermal Synthesis of Organic-Inorganic Hybird Nanoparticles. Adv. Mater. Res. 2009, 13, 247−280. (22) Zhang, J.; Ohara, S.; Umetsu, M.; Naka, T.; Hatakeyama, Y.; Adschiri, T. Colloidal Ceria Nanocrystals: A Tailor Made Crystal Morphology in Supercritical Water. Adv. Mater. 2007, 19, 203−206. (23) Adschiri, T. Supercritical Hydrothermal Synthesis of OrganicInorganic Hybrid Nanoparticles. Chem. Lett. 2007, 36, 1188−1193. (24) Takami, S.; Sato, T.; Mousavand, T.; Ohara, S.; Umetsu, M.; Adschiri, T. Hydrothermal Synthesis of Surface-Modified Iron Oxide Nanoparticles. Mater. Lett. 2007, 61, 4769−4772. (25) Rangappa, D.; Ohara, S.; Naka, T.; Kondo, A.; Ishii, M.; Adschiri, T. Synthesis and Organic Modification of CoAl 2O 4 Nanocrystals under Supercritical Water Conditions. J. Mater. Chem. 2007, 17, 4426−4429. (26) Rangappa, D.; Naka, T.; Kondo, A.; Ishii, M.; Kobayashi, T.; Adschiri, T. Transparent CoAl2O4 Hybrid Nano Pigment by Organic Ligand-Assisted Supercritical Water. J. Am. Chem. Soc. 2007, 129, 11061−11066. (27) Arita, T.; Yoo, J.; Ueda, Y.; Adschiri, T. Highly Concentrated Colloidal Dispersion of Decanoic Acid Self-assembled Monolayerprotected CeO2 Nanoparticles Dispersed to a Concentration of up to 77 wt % in an Organic Solvent. Chem. Lett. 2012, 41, 1235−1237. (28) Byrappa, K.; Adschiri, T. Hydrothermal Technology for Nanotechnology. Prog. Cryst. Growth Charact. Mater. 2007, 53, 117− 166. (29) Adschiri, T.; Kanazawa, K.; Arai, K. Rapid and Continuous Hydrothermal Crystallization of Metal-Oxide Particles in Supercritical Water. J. Am. Ceram. Soc. 1992, 75, 1019−1022. (30) Adschiri, T.; Hakuta, Y.; Arai, K. Hydrothermal Synthesis of Metal Oxide Fine Particles at Supercritical Conditions. Ind. Eng. Chem. Res. 2000, 39, 4901−4907. (31) Owens, D. K.; Wendt, R. C. Estimation of the Surface Free Energy of Polymers. J. Appl. Polym. Sci. 1969, 13, 1741−1747.
AUTHOR INFORMATION
Corresponding Author
*Phone: +81-22-795-7261. Fax: +81-22-795-7261. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported in part by Grant-in-Aid for Scientific Research (B) no. 21360375 from the Japan Society for the Promotion of Science (JSPS). We thank Mr. Takamichi Miyazaki of Tohoku University for providing technical support by taking TEM images.
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REFERENCES
(1) Barnes, K. A.; Karim, A.; Douglas, J. F.; Nakatani, A. I.; Gruell, H.; Amis, E. J. Suppression of Dewetting in Nanoparticle-Filled Polymer Films. Macromolecules 2000, 33, 4177−4185. (2) Mackay, M. E.; Hong, Y.; Jeong, M.; Hong, S.; Russell, T. P.; Hawker, C. J.; Vestberg, R.; Douglas, J. F. Influence of Dendrimer Additives on the Dewetting of Thin Polystyrene Films. Langmuir 2002, 18, 1877−1882. (3) Krishnan, R. S.; Mackay, M. E.; Hawker, C. J.; Van Horn, B. Influence of Molecular Architecture on the Dewetting of Thin Polystyrene Films. Langmuir 2005, 21, 5770−5776. (4) Krishnan, R. S.; Mackay, M. E.; Duxbury, P. M.; Hawker, C. J.; Asokan, S.; Wong, M. S.; Goyette, R.; Thiyagarajan, P. Improved Polymer Thin-Film Wetting Behavior through Nanoparticle Segregation to Interfaces. J. Phys.: Condens. Matter 2007, 19, 356003. (5) Besancon, B. M.; Green, P. F. Polystyrene-based Single-walled Carbon Nanotube Nanocomposite Thin Films: Dynamics of Structural Instabilities. Macromolecules 2005, 38, 110−115. (6) Koo, J.; Shin, K.; Seo, Y.; Koga, T.; Park, S.; Satija, S.; Chen, X.; Yoon, K.; Hsiao, B. S.; Sokolov, J. C.; Rafailovich, M. H. Stabilizing Thin Film Polymer Bilayers against Dewetting using Multiwalled Carbon Nanotubes. Macromolecules 2007, 40, 9510−9516. (7) Xue, L. J.; Cheng, Z. Y.; Fu, J.; Han, Y. C. Dewetting Behavior of Polystyrene Film Filled with (C6H5C2H4NH3)2PbI4. J. Chem. Phys. 2008, 129, 054905. (8) Hosaka, N.; Otsuka, H.; Hino, M.; Takahara, A. Control of Dispersion State of Silsesquioxane Nanofillers for Stabilization of Polystyrene Thin Films. Langmuir 2008, 24, 5766−5772. (9) Xu, L.; Yu, X.; Shi, T.; An, L. Investigation of the Dewetting Inhibition Mechanism of Thin Polymer Films. Soft Matter 2009, 5, 2109−2116. 8963
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(32) Thiele, U.; Vancea, I.; Archer, A. J.; Robbins, M. J.; Frastia, L.; Stannard, A.; Pauliac-Vaujour, E.; Martin, C. P.; Blunt, M. O.; Moriarty, P. J. Modelling Approaches to the Dewetting of Evaporating Thin Films of Nanoparticle Suspensions. J. Phys.: Condens. Matter 2009, 21, 264016. (33) Pauliac-Vaujour, E.; Stannard, A.; Martin, C. P.; Blunt, M. O.; Notingher, I.; Moriarty, P. J.; Vancea, I.; Thiele, U. Fingering Instabilities in Dewetting Nanofluids. Phys. Rev. Lett. 2008, 100, 176102. (34) Stannard, A. Dewetting-Mediated Pattern Formation in Nanoparticle Assemblies. J. Phys.: Condens. Matter 2011, 23, 083001. (35) Sangjan, S.; Traiphol, N.; Traiphol, R. Influences of Poly[(styrene)x-stat-(chloromethylstyrene)y]s Additives on Dewetting Behaviors of Polystyrene Thin Films: Effects of Polar Group Ratio and Film Thickness. Thin Solid Films 2012, 520, 4921−4928. (36) Jasper, J. J. The Surface Tension of Pure Liquid Compounds. J. Phys. Chem. Ref. Data 1972, 1, 841−1110. (37) Stamm, M.; Sommer, J.-U. Polymer-Nanoparticle Films: Entropy and Enthalpy at Play. Nat. Mater. 2007, 6, 260−261. (38) Wu, S. Surface and Interfacial Tensions of Polymer Melts. II. Poly(methyl methacrylate), Poly(n-butyl methacrylate), and Polystyrene. J. Phys. Chem. 1970, 74, 632−638. (39) Islaelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (40) Parlak, O.; Demir, M. M. Toward Transparent Nanocomposites Based on Polystyrene Matrix and PMMA-grafted CeO2 Nanoparticles. ACS Appl. Mater. Interfaces 2011, 3, 4306−4314. (41) de Sousa, F. F.; Moreira, S. G. C.; dos Santos da Silva, S. J.; Nero, J. D.; Alcantara, P. Dielectric Properties of Oleic Acid in Liquid Phase. J. Bionanosci. 2009, 3, 139−142. (42) Yamamoto, T.; Momida, H.; Hamada, T.; Uda, T.; Ohno, T. First-principles Study of Dielectric Properties of Cerium Oxide. Thin Solid Films 2005, 486, 136−140. (43) Dasgupta, S.; Smyth, C. P. Influence of Intramolecularly Compensated Dipoles upon Collision Absorption in Liquids. J. Chem. Phys. 1974, 60, 1746−1750. (44) Motte, L.; Billoudet, F.; Pileni, M. P. Self-Assembled Monolayer of Nanosized Particles Differing by Their Sizes. J. Phys. Chem. 1995, 99, 16425−16429.
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Thermal Dewetting Behavior of Polystyrene Composite Thin Films with Organic-Modified Inorganic Nanoparticles Masaki Kubo,*,† Yosuke Takahashi,† Takeshi Fujii,† Yang Liu,† Ken-ichi Sugioka,† Takao Tsukada,† Kimitaka Minami,‡ and Tadafumi Adschiri§ †
Department of Chemical Engineering, Tohoku University, 6-6-07, Aramaki, Aoba-ku, Sendai 980-8579, Japan Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8565, Japan § WPI Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡
ABSTRACT: The thermal dewetting of polystyrene composite thin films with oleic acid-modified CeO2 nanoparticles prepared by the supercritical hydrothermal synthesis method was investigated, varying the nanoparticle concentration (0−30 wt %), film thickness (approximately 50 and 100 nm), and surface energy of silanized silicon substrates on which the composite films were coated. The dewetting behavior of the composite thin films during thermal annealing was observed by an optical microscope. The presence of nanoparticles in the films affected the morphology of dewetting holes, and moreover suppressed the dewetting itself when the concentration was relatively high. It was revealed that there was a critical value of the surface energy of the substrate at which the dewetting occurred. In addition, the spatial distributions of nanoparticles in the composite thin films before thermal annealing were investigated using AFM and TEM. As a result, we found that most of nanoparticles segregated to the surface of the film, and that such distributions of nanoparticles contribute to the stabilization of the films, by calculating the interfacial potential of the films with nanoparticles.
1. INTRODUCTION Polymer thin films on nonwettable substrates are either metastable or unstable, and may dewet upon thermal annealing or solvent exposure. The dewetting of polymer films, the rupture and breakup of films on the substrate, is a crucial technological problem for the industrial applications of thin films, and is influenced by several factors such as wettability of substrate, viscoelasticity of polymer, and interfacial potential between polymer and air. The introduction of additives into polymers is one of the potential strategies for stabilizing thin polymer films and retarding dewetting. Barnes et al.1 demonstrated that the addition of a small amount of C60 fullerene nanoparticles inhibits the dewetting of polystyrene thin films. According to their study, such dewetting inhibition might be attributed to the following: fullerene nanoparticles, which are segregated to the interface between the polystyrene film and the silicon substrate, pin the contact line of polystyrene films on the substrate surface, or change the wettability of the substrate surface. Over the past decade, by using various additives, such as dendrimers,2 polystyrene nanoparticles,3,4 cadmium selenide quantum dots,4 carbon nanotubes,5,6 perovskite compounds,7 silsesquioxane nanofillers,8 star comb-like polymers,9 gold nanoparticles,10−12 and palladium nanoparticles,12 the effect of additives on the retardation or inhibition of dewetting in polymer thin films has been investigated. © 2014 American Chemical Society
To understand the mechanism of the dewetting inhibition of polymer thin films with nanoparticles or nanofillers, it is necessary to elucidate the spatial distribution of additives in polymer films. Concerning the dewetting behavior of polystyrene thin films with polystyrene nanoparticles on a silicon substrate, Krishnan et al.4 revealed that polystyrene nanoparticles segregate near the substrate surface, by neutron reflectivity measurement. McGarrity et al.13 numerically demonstrated the segregation of nanoparticles near solid surfaces observed by Krishnan et al.,4 using the fluid density functional theory (F-DFT). Luo and Gersappe14 proved the relation between dewetting inhibition and nanoparticle segregation on a solid surface using molecular dynamics simulations. On the other hand, Krishnan et al.4 observed, by TEM, that hydrocarbon-coated cadmium selenide nanoparticles mostly segregated to the air interface of polystyrene thin films, although nanoparticles were partially pushed to the silicon substrate. In this case, also, the dewetting of the polymer thin films was inhibited. Mukherjee et al.11 found that the stability of PS thin films changed from complete dewetting to partial dewetting followed by complete stability with increasing the concentration of uncapped gold nanoparticles, where the Received: May 23, 2014 Revised: June 30, 2014 Published: July 3, 2014 8956
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elucidate the dewetting mechanism of the films, the surface morphologies of the films were observed by atomic force microscopy (AFM), and the spatial distributions of nanoparticles in the films were measured by transmission electron microscopy (TEM). In addition, the interfacial potential of polymer thin films with nanoparticles was evaluated to discuss the contribution of nanoparticles to the stability of polymer films.
nanoparticles did not segregate to the substrate surface but aggregated in the central part of the rim surrounding the dewetted hole. All of the above-mentioned previous studies suggest that the dewetting of polymer thin films can be inhibited by adding a small amount of nanoparticles that segregate to the substrate or the air interface of the polymer film. However, the mechanism of the dewetting inhibition of nanoparticles in polymer thin films remains to be elucidated. In addition, the primary aim of most previous studies was simply to retard and inhibit the dewetting of polymer thin films by adding a small amount of nanoparticles. Materials composed of polymers into which nanoscale inorganic particles are incorporated, polymer nanocomposites, have attracted much attention because of their significantly different properties from pure host polymers. The properties of polymer nanocomposites can be controlled not only by combining host polymer and nanoparticle functionalities, but also by controlling the size and morphology of the nanoparticles and their spatial structure in the host polymer. Therefore, numerous applications utilizing the functional and structural properties of polymer nanocomposites are expected for chemical sensors,15 self-healing surfaces,16 conjugated polymer photovoltaic cells,17,18 optical and optoelectronic devices,19 and electrochemical devices.20 However, to date, there have been few studies in which the dewetting behavior of polymer nanocomposite thin films was discussed, particularly at relatively high concentration of inorganic nanoparticles. When fabricating a polymer nanocomposite film, controlling the affinity between the polymer and the inorganic nanoparticles by organic modification is a critical issue.21 Recently, Adschiri et al.21−28 have developed a new synthesis method for organic-modified inorganic nanoparticles of single-nanometer size, which gives rise to better dispersion of nanoparticles in organic solvents.21,23,25−28 In this method, organic species (e.g., amino acids, carboxylic acids, amines, alcohols, and aldehydes) are introduced in supercritical hydrothermal synthesis,28−30 and metal oxide nanoparticles with surfaces modified by the organic materials are consequently obtained. Such nanoparticles have the potential to adjust their physical properties by changing the combination of their ligand organic compound and core metal oxide materials. In addition, such surface modification allows dispersion of inorganic nanoparticles in organic solvents, and the control of spatial assemblies of nanoparticles, that is, the two- or threedimensional nanostructures, in host polymers. Therefore, it is expected that polymer nanocomposite materials with novel characteristics can be created using organic−inorganic hybrid nanoparticles synthesized by the supercritical hydrothermal synthesis method. However, to the author’s knowledge, there has been no study of the dewetting behavior of polymer nanocomposites with organic-modified metal oxide nanoparticles. In this study, the dewetting behavior of polystyrene (PS) composite thin films with oleic acid-modified CeO2 nanoparticles prepared by the supercritical hydrothermal synthesis method was investigated. Here, toluene solutions of PS with CeO2 nanoparticles were spin-coated on silicon substrates, and then the resulting composite films on the substrate were thermally annealed. For various concentrations of nanoparticles added to PS thin films, film thicknesses, and substrate surface energies, the dewetting behavior of the PS composite thin films during annealing was observed by an optical microscope. To
2. EXPERIMENTAL METHOD 2.1. Materials. A polystyrene (PS) standard with a weight-average molecular weight of 2000 (GL Sciences Inc., Japan) was used as the host polymer matrix; the usage of such PS with a relatively low molecular weight was to promote faster dewetting of the polymer film. Toluene, the solvent for dissolving the polymer, was purchased from Wako Pure Chemical Industries, Ltd. The starting materials for synthesizing oleic acid-modified CeO2 nanoparticles, that is, cerium hydroxide and oleic acid, were purchased from Sigma-Aldrich Co. and Wako Pure Chemical Industries, Ltd., respectively. A 23 × 23 mm2 silicon wafer (Nilaco Co., Japan) was used as a substrate whose surface was silanized using octadecyltriethoxysilane (Shin-etsu Chemical Co., Ltd., Japan) to investigate the effect of the surface energy of the substrate on the dewetting behavior of a PS composite thin film. First, the wafer was cleaned in a piranha bath (70% H2SO4 and 30% H2O2) for 12 h or longer to remove any organic compounds and make its surface hydrophilic. The cleaned wafer was rinsed with pure water by ultrasound irradiation for 10 min, and then dried in an oven at 348 K. The dried silicon wafers were then dipped in cyclohexane solution containing 0.01 M octadecyltriethoxysilane (ODS) at 333 K to silanize its surfaces. The silanized substrate was rinsed with acetone and water, and then dried at 393 K for 2 h. The surface energy of the substrate was adjusted by changing the time of immersion in the ODS/cyclohexane solution. Because the surface energy of the substrate is closely linked with the surface hydrophobicity of the substrate, first, the relationship between the two was measured in this study. Here, the surface energy of the substrate was measured by the following Owens−Wendt method:31
γL(1 + cos θ) = 2 γLpγSp + 2 γLdγSd
(1)
γL = γLp + γLd
(2)
In eqs 1 and 2, γ denotes surface energy, subscripts S and L denote solid and liquid, superscripts p and d denote the polar and nonpolar components of γ, and θ is the contact angle. The two components of surface energy of the substrate, γpS and γdS, were determined by eqs 1 and 2 using the measured contact angles of two different liquids with known surface energies, γpL and γdL, that is, water and diiodomethane. On the other hand, the surface hydrophobicity of the substrate was evaluated using the contact angle of water on the substrate. The surface energy of the substrate in each dewetting experiment shown below then was evaluated along the following procedure: A sessile water droplet of approximately 7 μL was placed on each of the 3 × 3 regions of the substrate surface used in the experiment, and then the contact angle of water was determined from the profile of the droplet near the contact line, taken using a digital camera (EOS Kiss Digital X, Canon Inc., Japan). The surface energies corresponding to both nine contact angles and their mean value were evaluated using the above relationship between the surface energy and surface hydrophobicity of the substrate. 2.2. Preparation of Ceria Nanoparticles. Ceria (CeO2) nanoparticles, whose surfaces were modified with oleic acid, were prepared by the supercritical hydrothermal synthesis method.22 First, 2.5 mL of 0.1 M Ce(OH)4 aqueous solution was poured into a pressure-resistant SUS316 vessel (inner volume, 5 mL). To modify the surface of the nanoparticles, 0.48 mL of oleic acid was also loaded into the reactor vessel. The hydrothermal reaction was performed at 673 K for 10 min and terminated by submerging the reactor in a water bath at room temperature. The oleic acid-modified nanoparticles were 8957
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Figure 1. Optical microscope images of the surfaces of PS thin films annealed at 393 K, where the thicknesses of the films are approximately 50 nm. (a) Nanoparticle concentration, 0 wt %; surface energy, 27 mJ/m2. (b) Nanoparticle concentration, 10 wt %; surface energy, 26 mJ/m2. (c) Nanoparticle concentration, 20 wt %; surface energy, 31 mJ/m2. (d) Nanoparticle concentration, 30 wt %; surface energy, 29 mJ/m2. extracted from the product mixture with 10 mL of cyclohexane. The final products were precipitated from the resulting cyclohexane phase by adding ethanol as the antisolvent reagent, and then separated by centrifugation. The nanoparticles obtained were redissolved in toluene. The average diameter of the nanoparticles evaluated using TEM (HD2700, Hitachi High-Technologies Co., Japan) was approximately 7 nm. 2.3. Annealing of Polymer Composite Thin Film. A toluene solution containing PS and oleic acid-modified ceria nanoparticles was prepared. The PS concentration in the solution was adjusted to either 2.6 or 5.0 wt %, depending on the final thickness of polymer films, that is, approximately 50 or 100 nm, respectively. The amount of nanoparticles added was determined so that the nanoparticle concentration of the polymer composite thin film could range up to 30 wt %. 140 μL of PS/toluene solution with nanoparticles was spincast (1H-D3, Mikasa Co., Ltd., Japan) at 4500 rpm for 30 s onto a silicon substrate, and then dried at room temperature for 24 h in a clean bench (PCV-750APG, Hitachi Co., Ltd., Japan) to form PS composite films on the substrates. Film thickness was measured by ellipsometry (DHA-XA/S3-T, Mizojiri Optical Co., Ltd., Japan). The PS composite films with nanoparticles on a silicon substrate were annealed in air at 393 K for up to 1 h using a handmade PIDcontrolled hot plate. The thermal annealing of PS composite films for 24 h has been performed as preliminary experiments, and then the following was confirmed: the dewetting behaviors of PS films did not change more than 1 h later, and the surface morphologies of PS films annealed for 24 h were the same as those for 1 h. Therefore, the thermal annealing for 1 h was considered to be sufficient for investigating the dewetting behavior of PS composite thin films. The dewetting behavior during thermal annealing was observed by reflected-light optical microscope (Fabulous, Olympus Co., Japan), and images were captured using a CCD camera. In addition, photo images of the entire area of the thin film surface after annealing for 1 h were taken using a digital camera. This photo image was converted to binary (black and white) image to evaluate the degree of dewetting,
which was defined as the ratio of the dewetting area to the entire surface area of the substrate. The surface morphologies of the films were observed by AFM (Nanocute, SII Nano Technology Inc., Japan). The spatial distributions of nanoparticles in the cross section of the films were observed by TEM (HF-2100, Hitachi High-Technologies Co., Japan).
3. RESULTS AND DISCUSSION Figure 1 shows optical microscope images of the surfaces of PS thin films annealed at 393 K when the concentrations of nanoparticles in PS thin films are (a) 0, (b) 10, (c) 20, and (d) 30 wt %, respectively. Here, the film thickness in all cases was approximately 50 nm. The surface energies of the substrates were (a) 27, (b) 26, (c) 31, and (d) 29 mJ/m2, respectively. The number at the upper left of each image indicates the annealing time. From Figure 1a without nanoparticles, it can be seen that, in the early stage of dewetting, isolated circular holes formed and grew in the film. The rims of adjacent holes then merged with each other and formed interconnected rim structures. Finally, the rims decayed into small droplets due to the Rayleigh instability of a cylindrical fluid. Figure 1a shows the typical dewetting behavior of polymer thin films. For PS thin films with 10 wt % nanoparticles shown in Figure 1b, also, isolated holes formed in the film, the rims of the growing holes merged with each other, and interconnected rim structures formed. However, as compared to those in Figure 1a, most of the rims remained with undulating cylindrical shape and did not decay into small droplets, but some broke into the droplets with irregular contact lines. Xu et al. observed the similar morphologies during thermal annealing of PS thin films with star comb-like polymer nanoparticles, and suggested that this was due to the increase in viscosity of the rims in which the 8958
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Figure 2. Optical microscope images of the surface of PS thin films annealed at 393 K, where the thicknesses of the films are with 50 nm thickness. (a) Nanoparticle concentration, 0 wt %; surface energy, 50 mJ/m2. (b) Nanoparticle concentration, 10 wt %; surface energy, 50 mJ/m2. (c) Nanoparticle concentration, 20 wt %; surface energy, 51 mJ/m2. (d) Nanoparticle concentration, 30 wt %; surface energy, 47 mJ/m2.
Figure 3. Optical microscope images of the surfaces of PS thin films annealed at 393 K, where the thicknesses of the films are approximately 100 nm. (a) Nanoparticle concentration, 0 wt %; surface energy, 32 mJ/m2. (b) Nanoparticle concentration, 5 wt %; surface energy, 28 mJ/m2. (c) Nanoparticle concentration, 10 wt %; surface energy, 23 mJ/m2. (d) Nanoparticle concentration, 30 wt %; surface energy, 30 mJ/m2.
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Figure 4. Effect of surface energy on degree of dewetting for (a) film thickness of 50 nm, and (b) film thickness of 100 nm. The open plots denote the degree of dewetting in each of the 3 × 3 regions of the substrate surface, and the solid plots denote the mean value of them.
nanoparticles accumulated as the dewetting holes grew.9 However, the definite mechanism of dewetting behavior shown in Figure 1b remains unknown. The dewetting behavior of PS thin films with 20 wt % nanoparticles is shown in Figure 1c. It was clear that the dewetting morphology markedly changed by increasing the concentration of nanoparticles. The structure of the growing holes in Figure 1c was very similar to fingering structures observed in dewetting thin films of evaporating colloidal nanoparticle suspensions, that is, nanofluids,32−34 where the fingering structures were suggested to be influenced strongly by the mobility of nanoparticles in the nanofluid on the substrate. Figure 1d shows the surfaces of PS thin films with 30 wt % nanoparticles. In this case, the dewetting did not occur even after 1 h annealing. Thus, the dewetting was completely suppressed in this case. Figure 2 shows the dewetting morphologies of PS composite thin films on the silicon substrates, which were higher surface energies than those used in Figure 1. The surface energies of the substrate were approximately 50 mJ/m2. The thicknesses of PS films were approximately 50 nm. Comparing the dewetting morphologies in Figure 2 to those in Figure 1, it was found that the dewetting behaviors in both cases were almost the same, although the dewetting rate decreased and the number density of initial holes increased with the surface energy of the substrate. In the case of PS thin film with 30 wt % nanoparticles, also, the dewetting was completely suppressed similar to Figure 1d.
Figure 3 shows optical microscope images of the surfaces of the PS thin films when the film thicknesses are approximately 100 nm. The surface energies of the substrate were around 30 mJ/m2. Here, to investigate the effect of thickness of PS composite thin films on the dewetting morphologies of the films, the figure was compared to Figure 1. In the case of the film with 5 wt % nanoparticles, the dewetting holes with irregularly shaped contact lines grew. In the case with 10 wt % nanoparticles, the channels, not the holes, were generated and progressed in the film. Thus, it is clear that film thickness affects the dewetting behaviors of the films containing nanoparticles. In contrast, for 30 wt % nanoparticles, the dewetting was suppressed similarly to the films with 50 nm thickness shown in Figure 3d. To clarify the effect of the surface energy of silanized silicon substrate on the dewetting behavior of the spin-coated PS thin films, the degrees of dewetting were plotted against the substrate surface energies in Figure 4 for the film thicknesses of approximately (a) 50 and (b) 100 nm. In the figures, the contact angles of water on the substrate are also shown to comprehend the relationship between the surface energy and surface hydrophobicity of the silanized substrate used for determination of surface energy. The surface energies of SiO2 and octadecane, which are the surface compositions of the unsilanized substrate with contact angle of approximately 30° and fully silanized substrate with contact angle of 100°, are 62.135 and 28.336 mJ/m2, respectively, and thus the estimated values of surface energy in this study are appropriate. The open 8960
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Figure 5. AFM images of film surface when film thicknesses are 50 nm and surface energies are between 55 and 65 mJ/m2. Nanoparticle concentrations are (a) 0 wt %, (b) 10 wt %, (c) 20 wt %, and (d) 30 wt %.
Figure 6. AFM images of film surface when film thicknesses are 50 nm and surface energies are around 30 mJ/m2. Nanoparticle concentrations are (a) 0 wt %, (b) 10 wt %, (c) 20 wt %, and (d) 30 wt %.
concentrations in the figure, the dewetting did not occur at surface energy higher than 70 mJ/m2. In addition, it is obvious that there is a critical value of surface energy beyond which the degree of dewetting suddenly decreases. At lower surface energy, the dewetting occurred in the entire area of the substrate. The critical value of surface energy decreased with increasing nanoparticle concentration. In the case of pure polystyrene film, that is, 0 wt % nanoparticles, the critical value of the surface energy was 70−75 mJ/m2. When the nanoparticle concentrations were 10 and 20 wt %, the critical values
plots shown in Figure 4 denote the relationship between the surface energy and the degree of dewetting in each of the 3 × 3 regions of the substrate surface, and the solid plots denote the mean value of them. The degree of dewetting is defined as the ratio of the dewetting area to the entire substrate surface area. If the degree of dewetting is 0, dewetting is completely inhibited, and if the degree of dewetting is 1, dewetting occurs in the entire area of the substrate. Figure 4a shows the results for the degree of dewetting of PS thin films with thickness of 50 nm. For all nanoparticle 8961
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were 40−60 and 25−55 mJ/m2, respectively. When the nanoparticle concentration was 30 wt %, dewetting did not occur within the present experiment range. Thus, the addition of nanoparticles is effective for inhibiting dewetting. Figure 4b shows the results when the film thickness is approximately 100 nm. The higher is the nanoparticle concentration, the lower is the critical surface energy. The critical values were 50−60 and 25−35 mJ/m2 for 0 and 10 wt % nanoparticle concentrations, respectively. The dewetting did not occur for 30 wt % nanoparticle concentration at all. In addition, by comparing the results in Figure 4b with those in Figure 4a, it was clear that the critical value of the surface energy decreased as film thickness increases, although the dewetting behaviors of the films containing nanoparticles depended on the film thickness as shown in Figures 1 and 3. To study the mechanism of the suppression or inhibition of dewetting by nanoparticle addition, the surface morphologies of PS composite thin films were observed by AFM, and the spatial distributions of nanoparticles in the films were observed by TEM. Figure 5 shows the AFM images of the film surface before thermal annealing for three nanoparticle concentrations. The thicknesses of all films were approximately 50 nm. The surface energies of the substrates were between 55 and 65 mJ/ m2. In all cases with nanoparticles, the dewetting did not occur as shown in Figure 4a. The surfaces of the films with 0, 10, and 20 wt % nanoparticles were smooth, and their root-meansquare (RMS) roughness values were 0.470, 0.756, and 0.647 nm, respectively. In contrast, for the film with 30 wt % nanoparticles, the RMS roughness was 1.89 nm, which may be responsible for the segregation of nanoparticles to the film surface. Figure 6 shows the surface morphologies of PS composite thin films before thermal annealing when the surface energies of the substrates are approximately 30 mJ/m2, which are lower than those in Figure 5. In this case, the dewetting occurred for nanoparticle concentrations of 0, 10, and 20 wt % except for 30 wt %, as shown in Figure 4. The surfaces of films with 0 and 10 wt % nanoparticles were smooth; the values of RMS roughness were 0.857 and 0.911 nm, respectively. On the other hand, the values of RMS for 20 and 30 wt % were 1.80 and 1.61, respectively. The change in the surface roughness of films with lower surface energy also may reveal the segregation of nanoparticles to the film surface. Figure 7 shows a cross-sectional TEM image of the spatial distributions of nanoparticles in the PS composite thin films before annealing. Here, the film thickness was 50 nm, the nanoparticle concentration was 30 wt %, and the surface energy of the substrates was 61 mJ/m2. Most of the nanoparticles segregated on the film surface (film/air interface), although a
small number of nanoparticles also existed on the substrate surface. The spatial structures of oleic acid-modified CeO2 nanoparticles in PS thin films on silicon substrates are determined by the balance of entropic and enthalpic effects concerned with polymer and nanoparticles. According to Krishnan et al.4,18 and Stamm and Sommer,37 the nanoparticles might segregate to the substrate surface, because the conformational entropic gain, which is provided by moving the linear PS chains away from the substrate and replacing them with nanoparticles, is superior to the losses of translational entropy of nanoparticles and enthalpic contact energy between nanoparticles and PS chains. However, if surface energy of the oleic acid modifying the surface of CeO2 nanoparticles is lower than that of PS, the nanoparticles should migrate to the PS film/air interface so as to minimize the total energy of the system. Here, note that the surface tension of hydrocarbons, approximately 20−25 mN/ m,36 is lower than that of PS, 40.7 mN/m,38 considering that the surface property of oleic acid on the nanoparticles is similar to that of hydrocarbons. In the system of this work, such an enthalpic effect is superior to the conformational entropic effect of polymers and nanoparticles, and, consequently, the spatial structure of nanoparticles shown in Figure 7 might be generated. Moreover, the wetting stability of a PS thin film was evaluated on the basis of the spatial structure of nanoparticles in the film shown in Figure 7 and the Hamaker constant of the film.4 In the case of the film with nanoparticles, a three-layer system consisting of nanoparticles, PS, and ODS was considered. In the case of the film without nanoparticles, on the other hand, the system consisting of PS and ODS was considered in which the surface of PS film was in contact with air. The Hamaker constants of the polymer films in the case with or without nanoparticles were estimated using the following Lifshitz theory:39 A132 =
⎛ ε − ε3 ⎞⎛ ε2 − ε3 ⎞ 3hνe 3 kBT ⎜ 1 ⎟⎜ ⎟+ 4 8 2 ⎝ ε1 + ε3 ⎠⎝ ε2 + ε3 ⎠ (n12 − n32)(n22 − n32)
(n12 + n32)1/2 (n22 + n32)1/2 {(n12 + n32)1/2 + (n22 + n32)1/2 } (3)
where εi and ni denote the dielectric constant and refractive index, respectively. kB, h, and νe are the Boltzmann constant, Planck constant, and frequency for dominant relaxation in the ultraviolet. The refractive indices of CeO2, oleic acid, PS, and ODS are 2.2,40 1.46,41 1.59,39 and 1.44,4 respectively. The dielectric constants of CeO2, oleic acid, PS, and ODS are 6.0,42 2.3,41 2.5,39 and 2.0443 (it is assumed to be similar to that of octadecane), respectively. Assuming that the length of oleic acid on CeO2 is 2.5 nm,44 the refractive index and dielectric constant of nanoparticles are 1.69 and 3.47, respectively, with a simple volumetric average of those two properties. Using these values, the Hamaker constant of the film without nanoparticles was found to be 23.0 zJ, and that with nanoparticles was −3.7 zJ. A negative Hamaker constant denotes that the film with nanoparticles is stable.4
4. CONCLUSIONS The thermal dewetting of polystyrene composite thin films with oleic acid-modified CeO2 nanoparticles prepared by the supercritical hydrothermal synthesis method was investigated,
Figure 7. TEM image of cross section of film when film thickness is 50 nm, surface energy is 61 mJ/m2, and nanoparticle concentration is 30 wt %. 8962
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varying the nanoparticle concentration (0−30 wt %), film thickness (approximately 50 and 100 nm), and surface energy of the silicon substrate on which the composite films were coated. The dewetting behavior of the composite thin films during thermal annealing was observed by an optical microscope. The presence of nanoparticles on the composite thin films affected the morphology of dewetting holes. When the nanoparticle concentration was 30 wt %, dewetting was suppressed or inhibited. The surface energy of the substrate had a critical value at which the dewetting occurred. The observation of the cross-section of the thin film by TEM elucidated that the nanoparticles segregated to the air surface of the film. The surface roughness by AFM also revealed that the nanoparticles segregated to the air surface. The interfacial potential of polymer thin films with nanoparticles suggested that the presence of nanoparticles, which segregated to the film surface, primarily contributed to the stabilization of the films.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +81-22-795-7261. Fax: +81-22-795-7261. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported in part by Grant-in-Aid for Scientific Research (B) no. 21360375 from the Japan Society for the Promotion of Science (JSPS). We thank Mr. Takamichi Miyazaki of Tohoku University for providing technical support by taking TEM images.
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