Versatile Antireflection Coating for Plastics: Partial Embedding of

Research Article www.acsami.org

Versatile Antireflection Coating for Plastics: Partial Embedding of Mesoporous Silica Nanoparticles onto Substrate Surface Norihiro Mizoshita* and Hiromitsu Tanaka Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan

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S Supporting Information *

ABSTRACT: Antireflection (AR) coating for transparent plastic substrates is constructed by partially embedding mesoporous silica nanoparticles (MSNs) onto the surface of the substrates. Simulation of optical properties of polymer substrates coated with a single-particle MSN layer indicates that the surface has a low and graded refractive index in the direction of the thickness and effectively decreases the reflectance of visible light. The MSN-coated surfaces can be prepared by exposure of the MSN-painted substrates to a solvent vapor, irrespective of the shape of the polymer substrates. The plastic substrates with a single-particle layer of MSNs with diameters of 145−165 nm exhibit high transparency and good AR behavior as simulated. The mesoporous structures of MSNs play important roles not only in decreasing the refractive index but also in strengthening the adhesion between MSNs and substrate surfaces. Moreover, fixation of MSNs onto a thermosetting epoxy resin is successfully achieved by transfer of a single layer of MSNs from flexible films for which MSNs are weakly bonded. The present simple AR coating is applicable to a wide range of substrates with various materials and shapes, and useful for various applications such as optical devices, displays, and solar cells. KEYWORDS: antireflection coating, mesoporous silica nanoparticles, transfer, composite materials, plastic substrates

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framework components and air (n = 1.00).11−17 Mesoporous silica films have been energetically examined as low-n layers because silica framework has a relatively low refractive index (n = 1.46) among chemically stable transparent materials. Porous polymers have also been developed as AR coatings which can be prepared using block copolymers under moderate conditions. 18−21 Formation of porous nano-objects on substrate surfaces is another effective approach to low-n AR layers.22−28 Surface structures with AR properties have been developed by forming nanoscale projections, fibers, rods, or reliefs at the air−substrate interface. For example, broadband AR coatings exhibiting reflectance of ca. 0.1% have already been realized for silicon substrates with silica nanorod arrays.27,28 However, preparation of thin coatings with significant AR properties requires some complicated and/or expensive processes such as vacuum deposition, nanoimprinting, nanoetching, and multistep surface treatments.25,26,29−31 Most of the conventional AR coatings are also applicable only to flat surfaces owing to the requirement of nanometer-level thickness control of the AR layers. AR coatings for plastic substrates are more restricted than those for inorganic substrates because organic polymers exhibit low heat resistance, easy deformation, and low adhesion for inorganic low-n materials such as silica-

INTRODUCTION Antireflection (AR) coatings for visible light wavelength region have attracted much attention because they can improve performance of optical devices, visibility of windows, and efficiency of photovoltaic systems.1−3 For example, low visibility of displays of portable phones under sun light or outdoor show windows can be solved by decreasing the reflection of light. Luminescence of light emitting devices can also be enhanced by AR coatings because optical losses due to reflection at the surfaces of transparent covering are reduced. In addition to transparent materials, AR coatings on silicon substrates have recently been developed to enhance the light absorption of the substrates, which can improve performance of solar cells.4−8 Formation of low-refractive-index (low-n) layers with optimized thickness on the substrate surface is the key to induce significant AR properties. The low-n coatings are designed to suppress the reflection from the air−coating and coating−substrate interfaces, on the basis of thin film interference.9,10 For precise control of the distribution of refractive index, a variety of surface structures have been proposed. Conventional AR coatings are single or multilayer thin films consisting of inorganic low-n materials such as MgF2 (n = 1.38) and CaF2 (n = 1.43). However, there is a limitation for reducing refractive indices by the choice of materials. A large number of studies on AR coatings have focused attention on nanoporous materials exhibiting middle refractive indices of © 2016 American Chemical Society

Received: August 24, 2016 Accepted: October 21, 2016 Published: October 21, 2016 31330

DOI: 10.1021/acsami.6b10624 ACS Appl. Mater. Interfaces 2016, 8, 31330−31338

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ACS Applied Materials & Interfaces

approach reported by Zhao et al.39 The surfaces of the MSNs were protected with trimethylsilyl groups by the post-treatment reported by Okubo et al.37 The detailed synthetic procedures of the MSNs and their structural properties are described in the Supporting Information. Table 1 summarizes structural properties of the MSNs used in the present study. As a reference nonporous silica nanosphere, KE-P15 (diameter: 150−200 nm; Nippon Shokubai) was used. Polymer substrates used in the present study were made of poly(methyl methacrylate) (PMMA; Mn = 3.3 × 105, Mw/Mn = 3.6), polycarbonate (PC; Mn = 2.5 × 104, Mw/Mn = 3.4), polystyrene (Mn = 8.1 × 105, Mw/Mn = 3.1), or two-liquid-type epoxy resin Crystal Resin II SP-C (Nisshin Resin Co., Ltd.). Methods. Atomic force microscope (AFM) observation was performed using a NanoNavi E-sweep scanning probe microscope system (Hitachi) with a cantilever SI-DF20. For preparing samples after removal of silica coatings, the polymer substrates coated with nanoparticle layers were immersed in a 6 M aqueous NaOH solution at room temperature for 12 h to dissolve the silica species, and then washed with ethanol. Scanning electron microscopy (SEM) was conducted on a Hitachi S-4300 with an acceleration voltage of 2.0 kV or on a Hitachi SU-3500 with an acceleration voltage of 15 kV. Transmission electron microscopy (TEM) was conducted on a Jeol JEM-2010FEF with an acceleration voltage of 200 kV. Transmission spectra in a visible light wavelength region were recorded on a Jasco V670 spectrometer. The reflectance was measured using a Soma S-2650 system. For the optical measurements, light sources, samples, and photodetectors were fixed in a straight line (normal incidence). Measurement of haze was carried out on a haze meter HGM-3DP (Suga Test Instruments). Nitrogen adsorption−desorption isotherms were measured using a Quantachrome Autosorb-1 sorptometer at −196 °C. All samples were outgassed at 110 °C for 2 h before measurements. Pore size distributions were determined using the density functional theory (DFT) method (the DFT kernel used: N2 at 77 K on silica, cylindrical pore, NLDFT equilibrium model). Pore volumes were estimated by the t-plot method. Preparation of MSN Coatings by Exposure to a Solvent Vapor. For polymer substrates, dispersions of MSNs in ethanol (3.0− 7.0 wt %) were deposited onto both sides of the substrates by painting with brushes, spray-coating, or dip-coating (lift-off rate: 50 mm· min−1). After moderate evaporation of ethanol, the polymer substrates with multilayers of MSNs were exposed to a chloroform vapor in a sealed box at room temperature for 8 and 10 h for PMMA and PC, respectively. The excessive MSNs were washed out by sonication in ethanol, giving the polymer substrates equipped with single-particle MSN layers. For the preparation of PC films (thickness: 0.1 mm) on which MSN145 was fixed in a detachable state, the MSN145-coated films (one side) were exposed to a chloroform vapor in a nonsealed bell jar for 24 h in order to adjust the depth of embedding to be 15−25 nm. Preparation of MSN Coatings by Direct Transfer. Two-liquidtype epoxy resin (Crystal Resin II SP-C, Nisshin Resin Co., Ltd.) was coated on one side of a glass substrate and aged at room temperature for 9 h. For the unhardened epoxy resin, the MSN145-coated PC film (depth of embedding: 15−25 nm) was pressed under a pressure of ca. 1.5 kg·cm−2. Then, the PC film was peeled off. The MSN145transferred epoxy resin was aged at room temperature for 1 day to complete the hardening reaction.

based coatings. If AR layers can be formed on various plastic substrates by simple processes, then such AR materials will be useful in a wide variety of optical devices and applications. Here we report the facile preparation of AR coatings for plastic substrates by fixing a single-particle layer of mesoporous silica nanoparticles (MSNs). Syntheses of MSNs have energetically been developed in various approaches because MSNs are useful as nanocarriers in drug delivery systems, solid supports for catalysts, and low-n components for AR materials.32−39 The synthetic method recently reported by Zhao et al.,39 which has been described as a biphase stratification approach, is highly significant because MSNs with dendritic mesoporous structures and narrow particle size distribution can be obtained reproducibly. We reasoned that if MSNs with uniform size are arranged on the surface of plastic substrates as a singleparticle layer, the MSN layer functions as a versatile AR coating in visible light wavelength region. Our surface design is shown in Figure 1. In the present design, we intended to embed MSNs

Figure 1. Design of MSN-based AR coating for plastic substrates.

partially onto the substrate surfaces, utilizing plasticization or softening behavior of polymer substrates. The AR coating based on MSN arrays has advantages over conventional film-shaped AR layers. The thickness of the AR layer can be regulated by the sizes of MSNs, requiring no precise thickness control in the coating step. The refractive index of the MSN layer should be graded in the direction of the thickness, which is highly effective for decreasing reflectance. Moreover, the partial embedding of MSNs should secure the robustness of the coatings, such as solid fixation of MSNs owing to infiltration of polymers into mesopores and durability for deformation of substrates. In the present study, we demonstrate that partial embedding of a single-particle layer of MSNs is achieved by surface-selective plasticization of the substrates with a solvent vapor. The MSN arrays are stably fixed on the polymer substrates with different shapes and function as satisfactory AR coating layers. We also show that the AR coating can be prepared on a thermosetting epoxy resin by transfer of an MSN layer from a thermoplastic film.

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EXPERIMENTAL SECTION

Materials. All reagents and solvents were purchased from Aldrich, Wako Pure Chemical Industries, and Tokyo Chemical Industry and used without further purification. MSNs with different sizes and void volume fractions were synthesized using biphase stratification

Table 1. Structural Properties of MSNs Used in the Present Study MSN100 MSN145 MSN165 MSN190 a

particle diameter (nm)

pore size (nm)

pore volume (cm3·g−1)

void volume fraction (%)a

90−110 130−160 150−180 180−200

4.9 4.7 4.8 4.5

0.60 0.37 0.47 0.18

54 42 48 26

Calculated by assuming the density of silica framework as 2 g·cm−3. 31331


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Figure 2. (a) Surface models of plastic substrates with partially embedded MSNs; (b) simulated refractive index profiles in the direction of thickness for models A−C; and (c) simulation of reflectance for PMMA plates with MSN layers on both sides. The distribution of refractive indices was approximated by multilayers of 10 nm-thick (models A and B) or 15 nm-thick (model C) thin films with step-by-step refractive indices (in-plane distribution of refractive indices was ignored).

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RESULTS AND DISCUSSION Simulation of Optical Properties. AR properties expected for our surface design were simulated by simplifying and idealizing the MSN-coated surface structures. Refractive index of silica framework was set to be 1.46.17,40 As a polymer substrate, PMMA with n = 1.49 was selected. MSNs were supposed to be silica spheres with void volume fraction of 0.5 and the diameter of 100, 150, or 200 nm. It was hypothesized that MSNs were ideally arranged to a close-packed 2D hexagonal structure as a single-particle layer (surface coverage: 90.7%) and the depth of embedding was 50 nm in all cases (Figure 2a). For models A−C, the distribution of n in the direction of the thickness was calculated using the Lorentz− Lorenz relationship, (nf2 − 1)/(nf2 + 2) = (1 − Vp)(ns2 − 1)/ (ns2 + 2), where nf is the refractive index of the coating, Vp is the void volume fraction, and ns is the refractive index of the solid silica framework.17,40 The optical properties are principally determined by Vp because the dimensions of mesopores are sufficiently smaller than visible light wavelength. For the porous surfaces, refractive indices of thin layers sliced by the planes parallel to the substrate surface were calculated by considering the ratio of framework silica and air (mesopores and interparticle voids) at the cross section. For the embedded portions, the gradation of refractive index was ignored because the mesopores of MSNs were thought to be partially filled with the plasticized polymers. Figure 2b shows the refractive index profiles calculated for models A−C in the direction of thickness. The refractive indices are found to alter between 1.0 and 1.2 in the direction of thickness. For the calculation of light reflectance, the surface with graded refractive index was approximated by multilayers of thin films (10−15 nm-thick films) with step-by-step refractive indices. Reflectance at the MSN-coated surface was simulated using multilayer interference calculation.9,10 Figure 2c shows the calculated reflectance

of PMMA plates with MSN layers on both sides. The simulation results indicate that the single-particle layer of MSNs is effective for inducing significant AR properties. The optical properties are strongly dependent on the particle diameters. According to the present results, MSNs with diameters of 150−200 nm are suitable for AR treatment in the visible light wavelength region. Preparation and Structures of MSN-Coated Thermoplastics. MSNs with different particle diameters were synthesized using a biphase stratification approach reported by Zhao’s group.39 The surfaces of the MSNs were protected with hydrophobic trimethylsilyl groups to increase the affinity of the MSNs for hydrophobic polymer substrates. Structural properties of the MSNs are summarized in Table 1. MSNs with average diameters of 100−190 nm (denotes as MSNx; x: average diameter [nm]) were used for the AR coatings in the present study. We found that a single-particle layer of MSNs could be fixed on PMMA substrates by exposure to a solvent vapor. The procedure of the fixation of MSNs is schematically shown in Figure 3. MSNs are first placed on the surface of polymer substrates (Figure 3A → B). We can choose various coating methods, such as spray coating, painting, and dip-coating without taking care of the thickness of the deposited MSNs. The polymer substrates covered with MSNs are exposed to a solvent vapor to plasticize selectively the surface of the substrates, which induces sinking of the MSNs into the substrates (Figure 3B → C). Washing of the substrates with ethanol under sonication leads to removal of the excessive MSNs, giving the polymer substrates equipped with the MSN coating as a single-particle layer (Figure 3C → D). It should be noticed that it is needless to coat MSNs as a single-particle layer in the first step (Figure 3A → B), because only the lowest layer of MSNs contacting with the substrate surface is selectively 31332


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sporadically. The coverage was estimated to be ca. 70% from the AFM image. The height of the roughness was 90−110 nm, which suggests that the depth of embedding is ca. 50 nm. We confirmed that the use of 5.0−6.8 wt % dispersions of MSNs in ethanol led to the reproducible production of single-particle MSN coatings with coverages of 65−72%. When dilute dispersions of MSN are used, a large number of defects are formed on the surface. However, the use of 7.0 wt % or higher concentrations of MSN dispersions resulted in the detachment of MSN aggregates from the substrate surfaces in drying processes. Surface structure of the MSN145-coated PMMA plate was further examined by electron microscopy observations. Figure 4b shows an SEM image of the MSN145-coated surface of the PMMA plate. We can see that MSNs are partially embedded into the PMMA substrate as schematically shown in Figure 1. TEM observation of the MSN145 fixed to the PMMA surface supports the 50 nm embedding of the MSNs into the substrate (Figure 4c). However, the infiltration of polymers into the mesopores of MSNs was not confirmed from the TEM images. In order to examine the infiltration of polymers into the mesopores, MSN145 fixed to the PMMA plate was dissolved with a 6 M aqueous NaOH solution and the surface of the PMMA substrate was observed with an AFM. As a reference, a PMMA plate embedding nonporous silica nanospheres (diameter: 150−200 nm) instead of MSN145 was also prepared and the silica nanospheres were removed by the similar procedure. Figure 4d,e shows AFM images and height profiles of the PMMA surfaces after removing MSN145 and nonporous silica nanospheres, respectively. For the PMMA substrate after removal of the nonporous silica nanospheres, hemispherical hollow shapes along the nanoparticles were formed as expected (Figure 4e). In contrast, polymer components with a nanoscale roughness were observed in the hollows after removal of MSN145 (Figure 4d). This indicates

Figure 3. Fixation of single-particle layer of MSNs onto thermoplastic substrates. (A) Plastic substrate before loading MSNs and (B) after loading MSNs. (C) After exposure to solvent vapors. (D) Plastic substrate coated with a single-particle MSN layer.

fixed to the substrate by the partial embedding. The present method shows a striking contrast to conventional AR films requiring precise control of thickness in the first coating step. The most important step in the present method is the partial embedding of the MSNs (Figure 3B → C). The depth of the embedding of the MSNs can be controlled by the conditions of the exposure to solvent vapors. We examined the relationships between the exposure time and the depth of embedding for several conditions (Figure S3). MSNs were embedded ca. 50 nm into the PMMA substrates by 8 h exposure to a chloroform vapor in a sealed box. For PC substrates, it took 10 h for similar embedding. Figure 4a shows an AFM image of the surface of the MSN145-coated PMMA plate, which was prepared by painting a 5.5 wt % dispersion of MSN145 in ethanol, followed by exposure to a chloroform vapor for 8 h in a sealed box. After sonication in ethanol, the whole surface was covered with a single-particle layer of MSN145 although defects were observed

Figure 4. (a) AFM image of an MSN145-coated PMMA substrate (fixed by CHCl3 vapor treatment). (b) SEM image of partially embedded MSN145 onto a PMMA substrate. (c) TEM image of MSN145 fixed on a PMMA substrate. (d) AFM image and height profile of a PMMA surface after dissolving MSN145. (e) AFM image and height profile of a PMMA surface after dissolving nonporous silica nanospheres. 31333


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MSN-coated PMMA plates is lowered in the order MSN100 > MSN145 > MSN165 > MSN190. For example, the reflectance at λ = 600 nm was 5.5, 3.0, 2.2, and 1.2% for MSN100, MSN145, MSN165, and MSN190, respectively. Although the profile for the MSN190-coated PMMA plate contained the light scattering component, the other MSN-coated plates also exhibited much lower reflectance than bare PMMA and the reflection spectra were in good agreement with those simulated in Figure 2. For the PMMA plates with MSN100, MSN145, and MSN165 layers, the transmittance curves are approximately coincident with the reflectance curves (Figure 5), which indicates high transparency of the PMMA plates with singleparticle MSN layers. Figure 6a shows a photograph of a PMMA

that a part of the polymer substrate was infiltrated into the mesopores of MSN145. In order to exclude the possibility of deterioration of PMMA with the alkaline solutions, we also confirmed the infiltration of substrate polymer into MSN145 for the alkali-stable polystyrene plate by the same method, which gave more remarkable results (Figure S4). The surface of the polymer substrates should be highly fluidic in order to induce sinking of MSNs into the substrates and infiltration of the polymers into the mesopores. The exposure to a solvent vapor is a suitable method for selective and effective plasticization of the surface polymers. Optical Properties of MSN-Coated Thermoplastics. Figure 5 shows transmittance and reflectance of PMMA plates

Figure 6. (a) Photograph of an MSN165-coated PMMA substrate. (b) AFM image of the surface of the MSN165-coated PMMA substrate.

Figure 5. Transmittance (a) and reflectance (b) of MSN-coated PMMA plates.

plate coated with the MSN165 layer on both sides of the right half. The photograph indicates that the letters printed on the paper are reflected on the surface of the bare PMMA plate, whereas the reflection is strongly suppressed on the MSN165coated region. It is also shown that the single-layer coating of MSN165 causes no noticeable light scattering. The haze of the MSN165-coated PMMA was only 0.6%. Figure 6b shows an AFM image of the surface of the MSN165-coated PMMA substrate. MSN165 is densely fixed on the surface as a singleparticle layer and the 100 nm-scale rough surface is formed by the spherical shape of the MSNs. The highly porous surface structures contribute to the induction of good optical properties as simulated in Figure 2. Slight differences between the simulated curves and the experimental results are probably caused by the imperfect surface coverages (ca. 70%) with the MSNs. Similar improvement of optical properties by fixing a single-particle layer of MSNs was confirmed for other thermoplastics such as PC and polystyrene plates. For example, transmittance of PC plate was improved from 88.0 to 93.5% at

equipped with single-particle layers of MSNs on both sides. Compared to a bare PMMA plate, MSN-coated samples exhibited higher transmittance and lower reflectance, which indicates that the MSN layers function as an AR coating as simulated in Figure 2. The optical properties were strongly dependent on the size of MSNs used for the coating. The transmittances of the MSN-coated PMMA plates became higher in the order MSN100 < MSN145 < MSN165. The transmittance of the MSN165-coated PMMA plate reached 98.0% at λ = 500 nm. These results suggest that fixation of MSNs with larger diameters are more effective for improving transmittance. However, the MSN190-coated PMMA plate showed lower light transmittance in the short wavelength region. The array of MSN190 with diameters of 180−200 nm probably induces light scattering by the in-plane defects in visible light wavelength region more remarkably than the other smaller MSNs. Relatively low void volume fraction (Table 1), that is, high refractive index of MSN190 is another cause of detectable light scattering. On the other hand, reflectance of the 31334


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ACS Applied Materials & Interfaces λ = 600 nm by coating with a single-particle layer of MSN145 on both sides (Figure S5). We examined relationships between optical properties and coverage of the PMMA substrates with MSNs. As mentioned above, the use of MSN dispersions containing less than 5.0 wt % of MSNs for painting onto substrates leads to the lowering of surface coverages. Thus, we prepared a 3.5 wt % dispersion of MSN145 in ethanol and used it for coatings of MSNs with different methods. Figure 7a−c shows AFM images of the

Figure 8. (a) Photograph of an MSN165-coated PMMA substrate with a curved surface. (b) Photograph of an MSN165-coated PMMA dome and a bare PMMA dome.

coated PMMA curved plate and a PMMA dome. The fixation of MSNs was performed by the same procedure as that shown in Figure 3. The MSN coatings on the curved and spherical surfaces exhibited AR properties similar to those formed on the flat substrates. It should be noted that the MSN-based AR treatment is easily achievable for both convex and concave surfaces of the substrates. In addition, we confirmed that the MSN-based AR layers can be formed on flexible polymer thin films. The present AR technique based on MSNs is also advantageous for large-scale treatment. Deposition of MSNs onto large polymer substrates is achievable by simply spreading MSN dispersions with paint brushes. Therefore, the amount of MSN dispersions necessary for coating can be minimized according to the size of the substrates. For example, deposition of MSNs onto 1 m2 substrate requires only several grams of 5.0 wt % MSN dispersions in ethanol. The MSN-coated surfaces were found to be stable in the air for a long period and durable for deformation of the substrates. No noticeable degradation of optical properties has been observed for the MSN-coated PMMA plates for more than two years at least. This is because the surface of the MSNs is protected with hydrophobic trimethylsilyl groups, minimizing the influence of moisture absorption. The MSN-based AR coatings were also highly durable for the bending or thermal expansion of polymer substrates. The MSNs partially embedded on the substrate surface are separated one by one; therefore, neither bending nor thermal expansion of the polymer substrates results in the destruction or peeling of the MSN arrays. We confirmed that optical properties of an MSN145-coated PMMA plate were unchanged after ten quick cycles between hot (80 °C, heated with hot air) and cool (20 °C, immersed in a cool water) conditions (Figure S7). The mechanical strength of the MSN-coated surface is strongly dependent on the depth of the embedding of MSNs. The MSN145-coated PMMAs with 50 nm embedding were durable for wiping with clothes under surface pressures less than 1 kg·cm−2. The MSN145-coated surfaces also resisted peeling with an adhesive tape (3 M Scotch tape). However, when the surface was strongly rubbed with a cotton wool under a pressure of more than 2 kg·cm−2, the PMMA plate became

Figure 7. AFM images of MSN145-coated PMMA plates prepared by (a) spray-coating, (b) painting, and (c) dip-coating (50 mm/min) of a 3.5 wt % dispersion of MSN145 in ethanol. (d) Transmittance of the MSN145-coated PMMA plates in a visible light wavelength region. Surface coverages calculated from the AFM images are in parentheses.

surfaces of MSN145-coated PMMA substrates prepared by spray coating, painting, and dip-coating of MSN145. For spray coating, MSNs were found to be densely packed and the coverage was calculated to be 71% (Figure S6). In contrast, painting of the 3.5 wt % dispersion resulted in a loose MSN array with the coverage of 56%. For the dip-coated sample, we can see inhomogeneity of the fixation of MSNs. The coverage was 54%. Light transmittances of these MSN145-coated PMMA substrates are shown in Figure 7d. The MSN145coated PMMA substrates with higher surface coverages exhibited higher transmittances. For the substrate prepared by dip-coating, depression of the transmittance in the shorter wavelength region was more conspicuous than the other MSN145-coated PMMA substrates although the coverage of the dip-coated sample was similar to that of the substrate obtained by painting of MSN145. This is attributable to the light scattering loss due to the micrometer-scale domains formed by the in-plane distribution of MSN arrays (Figure 7c). These results indicate that the single-layer MSN coating with high surface coverages is essential for enhancing optical properties and suppressing optical losses due to light scattering. Applicability and Durability of MSN-Coated Plastics. The most remarkable advantage of the present AR coating is the applicability to curved and spherical surfaces. This is owing to no requirement of thickness control for the deposited MSNs (Figure 3A → B). Figure 8 shows photographs of a MSN16531335


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Figure 9. (a) Preparation of an MSN coating layer for an epoxy resin by transfer method. (b) AFM image and height profile of the surface of the MSN145-coated PC film (part A in a). (c) AFM image and height profile of the surface of the PC film after transfer (part B in a). (d) AFM image and height profile of the surface of the epoxy resin after transfer of MSN145 layer (part C in a).

shown in Figure 3. However, this method was not applicable to thermosetting polymers which were viscous fluids in the initial states and insoluble polymer networks after hardening. We developed a transfer method of an MSN layer from the surface of thermoplastic films onto a thermosetting epoxy resin in an unhardened state (Figure 9a). As discussed in the previous section, MSNs can be fixed on thermoplastic films in a detachable state by tuning the depth of embedding to be less than 25 nm. Thus, we prepared MSN145-coated flexible PC films (thickness: 0.1 mm) with a depth of embedding of ca. 20 nm by tuning the condition of exposure to a chloroform vapor (Figure S3). We succeeded in the transfer of the single-particle MSN145 layer from the PC film to a commercially available two-liquidtype epoxy resin (Crystal Resin II SP-C, Nisshin Resin Co., Ltd.). Figure 9 shows the schematic of the transfer process and the AFM images of the surface of the substrates. The AFM image of the MSN145-coated PC film before transfer shows that the height of the roughness is ca. 120−130 nm, indicating that the depth of embedding is 15−25 nm (Figure 9b). The weakly fixed MSN145 layer was transferred by contacting the PC film with the epoxy resin coated on the glass substrate. The timing of the transfer was important to obtain the desired surface structure with partially embedded MSNs. For the present epoxy resin, it was appropriate to age the resin for 9 h at room temperature after preparation of the epoxy/hardener mixture. The epoxy resin aged for less than 7 h was still fluidic and highly sticky. When the aging time was over 11 h, the MSN layers could not be transferred to the surface of the epoxy resin due to the excessive hardening reaction. For the unhardened epoxy resin aged for 9 h, the MSN145 layer on the PC film was pressed under a pressure of ca. 1.5 kg·cm−2. After removal of the PC film, the MSN145 layer was transferred to the surface of the epoxy resin. Then, the epoxy resin was aged for 1 day to complete the hardening reaction. Figure 9c,d shows the AFM

cloudy. In this case, the MSNs remained on the PMMA surface without detachment. The surface rubbing caused deformation of the MSN layer together with the PMMA substrate (Figure S8a). Strong adhesion of the MSNs onto the substrate surface was induced by the infiltration of substrate polymers into the mesopores of the MSNs (Figure 4d). The pencil hardness of the surface was HB for both bare PMMA and MSN145-coated PMMA plates, indicating that the surface strength was mainly determined by the hardness of the polymer substrate. However, when the depth of embedding of MSNs was less than 25 nm, the MSNs were easily eliminated from the surface by wiping with a cloth or peeling with a Scotch tape. When the depth is less than 5 nm, most of the MSNs were detached by washing with sonication in ethanol. Embedding of MSNs with more than 35 nm depth is necessary for stable fixation of MSNs onto the surface of the polymer substrates. As a control, we prepared PMMA plates onto which nonporous silica nanospheres with diameters of 150−200 nm were fixed with 50−70 nm embedding. The silica nanospheres were easily detached by wiping with a cloth. AFM observations of the surface after wiping showed that there were holes and tracks formed by the detachment of the nanospheres (Figure S8b). These results also verify that mesoporous structures of MSNs with large surface areas play an important role in the stable fixation of MSNs onto the polymer substrates. It was reported that AR coatings based on a single nanoparticle layer could be obtained by physical adsorption of nonporous colloidal particles onto substrate surfaces using electrostatic attraction.41 In this case, however, neither durability for deformation of substrates nor mechanical strength of the surface has been examined. Preparation of MSN-Coated Thermosetting Resin by Transfer. Coating with single-particle MSN layers for thermosetting resins was achieved using a transfer method. Partial embedding of the MSN layers onto thermoplastics could be performed utilizing plasticization behavior of the substrate as 31336


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Figure 10. Transmittance (a) and reflectance (b) of an epoxy-coated (on one side) glass substrate before and after transfer of a single-particle layer of MSN145.

images of the surfaces of the PC film and the epoxy resin after the transfer process, respectively. On the surface of the PC film, no remaining MSNs were observed. There was an array of shallow hollows (ca. 10 nm in depth) corresponding to the positions of the embedded MSNs (Figure 9c). However, an array of MSN145 was fixed on the surface of the epoxy resin (Figure 9d). The height of the roughness was 90−110 nm, corresponding to ca. 45 nm embedding of the MSN145. The transmittance of visible light was enhanced by 2.0−2.5% for the MSN145-coated epoxy resin, accompanied by the corresponding decrease in the reflectance (Figure 10). These results indicate that the MSN layer transferred onto the thermosetting resin functions as a satisfactory AR coating. The preparation of the AR coating by the transfer method is applicable to nonflat thermosetting resins owing to the flexibility of the film substrates with the detachable MSN layer. The MSNs layers fixed on the epoxy resin also exhibited enhanced mechanical durability. When the surface was rubbed with a cotton wool under a pressure of ca. 2 kg·cm−2, the transparency and AR behavior of the substrate were unchanged in contrast to the MSN-coated PMMA plate, suggesting that the strengthening of the polymer substrate is effective for the induction of high wear resistance of the MSN-based AR layers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mr. Juntaro Seki for performing SEM observation.

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REFERENCES

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CONCLUSIONS We proposed the MSN-based versatile AR coatings applicable to various polymer substrates. The surface layer consisting of a partially embedded MSN array is easy to prepare and can be coated onto various substrates with different shapes. The fixation of the MSN arrays for thermoplastics was performed by exposure to a chloroform vapor. Moreover, the MSN layer could be formed on thermosetting resin by the transfer method. The chemical stability of the MSN layers and their durability for the deformation of substrates are useful in practical applications. In addition, we have preliminarily confirmed that the fixation of MSNs onto thermoplastics is achievable by only hot air blast (Figure S9),42 which leads to more facile preparation of MSN-coated AR surfaces. The induction of AR properties by fixing single-particle MSN layer can open a new way to the production of practical transparent materials with improved optical properties.

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Synthesis of MSNs, additional AFM images, and supplementary experimental results (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10624. 31337


Research Article

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Versatile Antireflection Coating for Plastics: Partial Embedding of

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