PDMS Photochromic

Dec 26, 2012 - Emulsifier-free emulsion polymerization was employed to synthesize polystyrene (PS) microspheres, which were then self-assembled into a...
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Preparation and Evaluation of the Bioinspired PS/PDMS Photochromic Films by the Self-Assembly Dip−Drawing Method Jen-Yu Shieh,† Jen-Yu Kuo,† Hsueh-Ping Weng,† and Hsin Her Yu*,†,‡ †

Graduate Institute of Electro-Optical and Materials Science and ‡Department of Biotechnology, National Formosa University, 64 Wenhua Road, Huwei, Yunlin 63208, Taiwan ABSTRACT: Emulsifier-free emulsion polymerization was employed to synthesize polystyrene (PS) microspheres, which were then self-assembled into an ordered periodic structure. A photochromic film was formed by adding polydimethylsiloxane (PDMS) around the self-assembly of PS microspheres on a PDMS substrate. During polymerization, the PS microspheres shrunk depending on the amount of the hydrophilic comonomer, sodium 4styrenesulfonate (NaSS). Variation in structural color was strongly affected by the size of the PS microspheres. Scanning electron microscopy was used to observe the surface and cross sections of the self-assembled microspheres. Results showed that the order and stacking thickness of microspheres were dependent on the drawing rate of the substrate and suspension concentration. High-transmittance photochromic films could be prepared when the drawing rate was lower than 1 μm/s and the suspension concentration was 20 wt %. PDMS surrounding the vacant space between regularly arranged PS microspheres could not only protect them but also increase the degree of matching between the refractive indices of PS and PDMS. The stability of the reflected structural color increased, and the optical transmittance of the photochromic film approached 95% after PDMS was poured between the PS microspheres. A special pattern could be designed and embedded inside the photochromic film. The PS/PDMS photochromic films can also be applied in decorative painting as well as in security devices, color-changing false nails, and privacy filters.

1. INTRODUCTION Nature endows organisms with different structural colors, which the animals can change for defense against natural enemies,1 camouflage and mimicry,2 thermoregulation,3 and courtship and mating.4 The mysteries of structural colors are better understood since the discovery of nanostructures. Therefore, nanotechnology has been employed to simulate the photonic crystal structure of biological organisms and extend its applications in varied research fields. Various techniques for self-assembly of latex particles have been developed with the aim of fabricating colloidal crystals and complicated assembly structures. Recent research in this direction has been immensely successful.5−7 However, several problems are associated with the current self-assembly approaches and practical applications of colloidal crystals,8 the most important challenge being the large-scale fabrication of colloidal crystals without cracks for high-quality optical devices. Zhang et al.9 suggested that point defects, crack defects, and dislocation line defects in colloidal crystals are the major reasons for light-forming chromatic dispersion in air, and as a result, the structural color tends to be white. In their study, nigrosine dye was mixed with poly(MMA-co-DVB-co-MAA) nanoparticles in solution; since nigrosine shows absorption over the entire visible-light range and inhibits light-forming dispersion and rainbow effects inside the colloidal crystal, the structural color could be effectively enhanced. Wang et al.10 used a suspension of polystyrene microspheres as the core material and elastic PMMA/poly(acrylic acid) as the shell © 2012 American Chemical Society

material to construct a perfectly dense colloidal crystal with a very small amount of air between the microspheres because of the strong intramolecular hydrogen bonding with the carboxyl group. The colloidal crystal film was dense with separate air spaces, and hence, its mechanical strength was greatly improved; however, the crystal continued to collapse under external force. Therefore, enhancing the mechanical stability of colloidal crystal films remains an important challenge. Nagayama11,12 prepared ordered multilayers of polystyrene (PS) spherical particle arrays by the evaporation method. Such an ordered multilayer exhibits a stepwise change in color each time the top particle layer is removed. This property could be utilized for recording micrometer-scale colored images. In addition, Nagayama’s group reported a method to veriify thin, freely suspended liquid films containing a single layer of nanometer-sized particles. Large particles (lipid or protein vesicles and latex spheres) can be used to produce well-ordered particle arrays to appropriately control the thickness of the liquid films.13 Nagayama’s group also published some patents on formation of a two-dimensional thin film of particles14 and a method for rapid fabrication of high-quality two-dimensional aggregates.15 The seemingly transparent Hymenoptera (wasp) and Diptera (fly) insect wings show an astonishing display of structural Received: August 29, 2012 Revised: December 24, 2012 Published: December 26, 2012 667

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colors.16 About 80% of the incident light can penetrate the wings, and the remaining 20% is reflected from the extremely thin single layer of chitin. Interference patterns are formed as light interacts with the convex ridges and spherical cellular structure of the wings. Such interference patterns result from different chitin thicknesses, hair placement, pigment depositions, and convex ridges. The convex ridges effectively enhance optical refraction and cause stable reflection of colors. In recent years, a highly efficient dip−drawing method to assemble tightly stacked polystyrene microsphere arrays at room temperature has been reported.17−20 The method is advantageous because it affords colloidal crystal arrays within a very short time, irrespective of the preparation environment. PS microsphere arrays, however, are easily peeled off from the substrate, similar to the arthropod scales from the wing of a butterfly; hence, polydimethylsiloxane (PDMS) has been used to overcome this disadvantage. Once the spaces surrounding the PS microspheres are filled with PDMS, the adhesion between the microsphere arrays and the substrate can be enhanced and the flexibility of the overall structure can be increased. A photochromic film was formed when PDMS filled the voids between the PS microspheres, and it was then cured. Fudouzi21 reported formation of PS-PDMS hybrid opal films by the evaporation and crystallization method for dynamic tuning of the structural colors. The distance between the crystal lattice planes could be varied by swelling the PDMS with silicone oil or applying mechanical stretching. The hybrid opal films have potential applications in the preparation of smart sensing materials, pigment-free color imaging, and strain mapping of plastic deformation. For comparison, in this study, PS microsphere arrays were prepared by a dip−drawing method at different drawing rates and PS suspension concentrations at room temperature. The influence of the PS suspension concentration on the optical properties of photochromic film was investigated. Highly transparent photochromic films have potential applications in the anticounterfeit security design of banknotes.

plate. Because of the hydrophobicity of the substrate surface, oxygen plasma treatment was unavoidable. Oxygen plasma (PCD-150, All Real Technology Co., Ltd.) treatment was carried out at a pressure of 250 mTorr, with 15 sccm oxygen flow under 100 W electric power. The PDMS surface became hydrophilic, which contributed to formation of a meniscus between the PS suspension and the PDMS substrate, as shown schematically in Figure 1. Thus, the PS microspheres could be stacked and arranged by self-assembly under capillary force on the PDMS surface.

Figure 1. Schematic of self-assembly by the dip−drawing method. 2.4. Preparation of Transparent Photochromic Films. First, the suspension of PS spheres was diluted to a definite concentration using deionized water. Then, the plasma-treated hydrophilic PDMS substrate was vertically immersed into the suspension for several minutes, withdrawn at different drawing rates, and dried at 50 °C for 20 min. Subsequently, the PDMS prepolymer was poured onto the PS/PDMS structure and cured at 50 °C for 8 h. 2.5. Characterization. The average diameters of the PS spheres in water were determined using Zetasizer (Malvern 3000HS, U.K.). The transmittance and reflectance of the PS colloidal crystal films were measured using a variable-angle UV−near-IR spectrophotometer (Hitachi U-4100, Japan). Field-emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F, Japan) was used to determine the photochromic film surface and the cross-sectional features. A thin layer of Pt was sputtered onto the samples prior to SEM imaging. Atomic force microscopy (AFM, NT-MDT SOLVER P47H-PRO, RUS) imaging of the spacing between the PS spheres in the film was carried out in tapping mode using a 10 nm OMEGA silicon cantilever tip (NSG01).

2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (St) and ethanol, potassium persulfate (KPS), sodium 4-styrenesulfonate (NaSS), and polydimethylsiloxane (PDMS) SYLGARD-184 were purchased from Sigma-Aldrich, Inc., J.T. Baker, Inc., Alfa Aesar, Inc., and Dow Corning, Inc., respectively. Supplies were presented in a kit containing two separate components: the base material and the curing agent. All chemicals were used as received without further purification. 2.2. Synthesis of Monodisperse Polystyrene Spheres. PS spheres were synthesized by emulsifier-free emulsion polymerization using St and NaSS as comonomers and KPS as the initiator. The size of the PS microspheres was controlled by adjusting the NaSS dosage.22 The polymerization procedure was as follows. A mixture of deionized water (135 mL), St 15 (mL), and different amounts of NaSS was added to a four-necked flask equipped with a reflux condenser and a mechanical stirrer. After being homogeneously mixed, the reaction mixture was maintained at the boiling point, and a deoxygenated aqueous solution of 0.1 g of KPS was added. Emulsion polymerization was carried out in a 250 mL glass reactor under nitrogen atmosphere, and the reaction was terminated after 24 h. 2.3. Preparation and Surface Treatment of the Substrate. PDMS was formed by uniformly mixing the base material and the curing agent in a mass ratio of 10:1 with colloidal gas bubbles removed by vacuum. The PDMS prepolymer colloid was poured onto a cleaned glass plate surface and cured at 50 °C for 8 h. The resulting PDMS substrate obtained as a hardened template was removed from the glass

3. RESULTS AND DISCUSSION 3.1. Factors That Influence the Structural Colors. 3.1.1. Particle Size. The structural colors varied with the particle size of the synthesized PS spheres. At the initial stage of polymerization, micelles were formed with increasing NaSS content but the particle size was small. The particle size of PS synthesized without NaSS and with 40 mg of NaSS was about 1155 and 258 nm, respectively. 3.1.2. Iridescence. In this study, we consider that iridescence is mainly characterized by the change in hue when an object is viewed from different angles of vision.23 As light scattering occurred, when the self-assembled PS microsphere array was irradiated with the incident light, the color of the array was white. However, the change in the color of the PS microsphere array with the viewing angle, shown in Figure 2, is referred to as iridescence. For the polymerized PS microspheres with a 668

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Figure 2. Reflected spectra of colloidal crystals measured at the angle between the normal line of the film surface and the detecting light for various detection angles: (a) 10°, (b) 20°, (c) 30°, (d) 40°, (e) 50°, and (f) 60°.

diameter of 258 nm, different angles of reflectance were observed using a variable-angle UV−near-IR spectrophotometer. The reflection peak showed a blue shift from 487 to 430 nm when the detection angles changed from 10° to 60°. 3.2. Self-Assembly of PS Spheres at Different Dip− Drawing Rates. To allow for self-assembly of the PS microspheres on the PDMS substrate, the surface of the substrate was made hydrophilic by oxygen plasma treatment. In addition, to observe the surface topography of the selfassembled PS microspheres on the PDMS substrate at different dip−drawing rates, microspheres with particle sizes of 258 nm were selected. The PS suspension was diluted with deionized water to 10 wt %, and dip−drawing was conducted at rates of 0.1−1000 μm/s. SEM images showed that at high drawing rates, i.e., 100−1000 μm/s (Figure 3 a and 3 b), the PS microspheres were arranged in a random array. This arrangement was due to van der Waals attraction and electrostatic repulsion between the PS microspheres, which were unbalanced at the air/suspension interface, when the PDMS substrate was quickly drawn out of the PS suspension surface. At moderate drawing rates of 1−10 μm/s, there was sufficient time for the aforesaid interactions to be balanced at the interface. As shown in Figure 3 c and 3 d, the PS microspheres were arranged densely, but local line defects and vacancies still existed. Figure 3 e shows that at a very low drawing rate of 0.1 μm/s, the PS microspheres are arranged in a compact ordered periodic structure. It was difficult to observe the stack thickness and layers of PS microspheres at different drawing rates from the cross-sectional view of the suspension at a low concentration (10 wt %). Therefore, the PS suspension concentration was increased to 50 wt %. When the drawing rate was decreased from 1000 to 0.1 μm/s, the stacking thickness of the PS microspheres (see Figure 3 f−j) increased from 0.487 to 330.9 mm. From the cross-sectional images in Figure 3 f−j and the top-view images in Figure 3 a−e it was found that if the drawing rate was too high (above 100 μm/s) the microspheres were in random arrangement, as the time for self-assembly was insufficient. As a result, the stacking thickness was low. On the other hand, when the drawing rate was low (below 10 mm/s), the microspheres were arranged in an ordered periodic structure, as the time for self-assembly was sufficiently long, and the stacking thickness was high. The particle sizes of the PS microspheres were found to be 258 and 237 nm, as determined using Zetasizer and AFM measurements, respectively. This difference resulted from the

Figure 3. Self-assembly of PS spheres by the dip−drawing method at different drawing rates: (a−e) SEM top-view images of 10 wt % PS suspension, (f−j) SEM cross-sectional images of 50 wt % PS suspension.

analysis method: in Zetasizer analysis, the particle size is based on the amount of light scattered from the PS microspheres and is likely to be erroneous, whereas in AFM observation the particle size is estimated from the minimum van der Waals force between the probe and the analyte.24 Thus, the PS particle sizes selected for discussion in this study were based on the AFM measurements. In general, the reflection peak, particle size of the photonic crystal, and refractive index of the medium are related to the detection angle. According to Bragg’s law and Snell’s law, the relationship between these parameters can be expressed by eqs 1 and 225,26 λ=2 n= 669

2 (D) n2 − sin 2 θ 3 2 2 nsphere × 0.74 + nair × 0.26

(1) (2)

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where λ is the position of the photonic band gap, D is the diameter of the photonic crystal microspheres, and n is the effective refractive index of the medium. According to the AFM images, the particle size of the PS microspheres is 237 nm. The photonic crystal microspheres occupy a total volume of about 74%, and the air gap occupies the remaining volume, since the photonic crystal film obtained by the dip−drawing method has a face-centered cubic (FCC) structure.25,26 In eq 2, the refractive index of the photonic crystal (nsphere) is 1.6 and the refractive index of air (nair) is 1. Table 1 summarizes the

monodisperse, which may be the other possible reason for the aforesaid difference. The blue-shift phenomenon was also evident in both methods. 3.3. Optical Properties of Photochromic Films. 3.3.1. Reflection Spectra. To observe the structural color changes of the PS microspheres in the visible region under different environmental conditions, the change in the reflectance (R%) of self-assembled PS microspheres with diameters of 258 nm, prepared on the PDMS substrate at different suspension concentrations and at a drawing rate of 1 μm/s, was recorded (Figure 4A (top)). As the PS suspension concentration was increased from 20 to 100 wt % (see Figure 4A (top) (a−e)), the reflectance (R%) decreased from 33.8% to 10.1% and the reflection peak shifted from 522 to 497 nm, indicating a blue shift. The stack thickness of the PS microspheres on the PDMS template was the primary factor influencing R%. The PS microspheres arranged on the PDMS substrate tended to be white since the PS suspension was a white emulsion. However, when the suspension concentration was increased to 100 wt %, the stack thickness of PS microspheres on the PDMS substrate increased, decreasing the amount of light scattered over the microspheres and thus R %.28 The blue shift resulted from the significantly large difference in refractivity between the PS microspheres (n = 1.6) and air (n = 1). Thus, light was strongly scattered, and the reflection peak was shifted.29 An ideal multilayer interference model is qualitatively understood in terms of a successive pair of thin layers. However, such a model is too simple to explain the mechanism of multilayer interference. In fact, this model is applicable only when the difference between the refractive indices of the two layers is sufficiently small. Otherwise,

Table 1. Reflection Peak Positions of the Polymerized PS Microsphere Arrays As Determined by Theoretical Calculation and Experimental Observation detection angle (θ)

theoretical value (nm)

experimental value (nm)

10° 20° 30° 40° 50° 60°

561 549 531 507 481 455

487 477 462 445 432 430

calculated theoretical values (according to eqs 1 and 2) and the experimentally observed values (from Figure 2) of the reflection peak positions for the PS microspheres when the detection angle was changed from 10° to 60°. However, there was a difference between the theoretical and experimental values, which resulted from the packing defects in the PS microspheres and the imperfect roundness of the particle surfaces.27 In addition, the particle size distribution was not

Figure 4. (A) Reflectance spectra and (B) transmittance spectra of PS microspheres (top) and photochromic film (bottom). PS suspension concentrations: (a) 20, (b) 40, (c) 60, (d) 80, and (e) 100 wt %. 670

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multiple reflections would significantly modify the interference conditions. Thus, quantitative evaluation of wavelengthdependent reflectivity is rather complex but important for understanding the principle underlying the origin of structural colors in nature because most of the colors originate from multilayer interference or its analog.30−33 The reflectivity of the photochromic films is shown in Figure 4A (bottom). When the PS suspension concentration was increased from 20 to 100 wt %, R% decreased from 8.8% (Figure 4A (bottom) (a)) to 2.7% (Figure 4A (bottom) (e)) and the reflection peak was located at 531 nm. R% was 33.8% before the voids between the PS microspheres were filled with PDMS, while it decreased to 8.8% after filling, as light was totally reflected by the photochromic film. According to Snell’s law, light enters the optically thinner medium (air, n = 1) from an optically denser medium (PDMS, n = 1.43) and when the incident angle is greater than the critical angle (θc = 44.4°) total reflection is observed. However, the reflection peak of the photochromic film was independent of the change in the PS suspension concentration. As the refractive indexes of the PS microspheres and the surrounding PDMS match in the photochromic film, the reflection peaks are close to 531 nm. 3.3.2. Transmission Spectra. Light was reflected and refracted when it irradiated the PS microspheres on the PDMS template. The transmittance in Figure 4B (top) shows a photonic band gap. When the suspension concentration increased from 20 to 100 wt %, light could not propagate and was reflected, and as a result, the average transmittance decreased from 77.5% to 8.3%, as shown in Figure 4B (top) (a−e). The stack thickness of the self-assembled PS microspheres increased with the suspension concentration, and the optical transmittance was reduced as light was scattered over the PS microspheres on the PDMS substrate. According to Figure 4B (bottom) (a−e), the voids between the PS microspheres in the photochromic film were filled with PDMS and the average transmittance decreased from 95.2% to 31.2% when suspension concentration was increased from 20 to 100 wt %. Here, we compared the average maximum transmittance before and after PDMS filling (for the PS suspension concentration of 20 wt %). The average transmittance of the PS microspheres on the PDMS substrate without PDMS filling was 77.5% (from Figure 4B (top) (a)), and the transmittance increased to 95.2% after filling (Figure 4B (bottom) (a)). Such a high transmittance resulted from the optical characteristics of PDMS. Since most of the light cannot penetrate the PS colloidal crystal, the incident light was refracted in different directions and reflected many times from the PS microspheres, and therefore, the PS microspheres tended to appear white. After the PDMS filling, light was scattered as it came in contact with the PS microspheres in the photochromic film. The scattered light propagated forward along the microspheres and penetrated the photochromic film because of the high transmittance of PDMS. Thus, the transparency of the photochromic film increased. 3.3.3. Applications. In this study, paper currency was used as the background, with a line of laser anticounterfeit pattern on its right-hand side. A photochromic film prepared in this study was placed on the left-hand side of a 500 dollar Taiwanese banknote, as shown in Figure 5a. High transparency was observed. The embedded structural color pattern could be reflected from the photochromic film when the note was tilted at a particular angle of view. A green letter “U” and a blue letter “N” were clearly displayed on the photochromic film, as shown

Figure 5. Photograph of a photochromic film changing the structural color: (a) transparent features and (b) reflected structural color observed upon tilting the banknote.

in Figure 5b. The laser anticounterfeit bar on the right-hand side of the banknote was pasted over the text, and thus, the text in the overlap zone of the banknote was covered with the anticounterfeit bar. In contrast, the transmittance of the photochromic film was as high as 95%, and when it covered the note, the pattern and text on the note were still legible. The colors and patterns in the photonic transparent film could be designed and changed, and the efficiency of counterfeit currency detection was better than that with the traditional ultraviolet-irradiated design. In addition to the above application, our present invention can also be used as a color-changing artificial nail when the nail is tilted at some special viewing angles. Photochromic nails are different from some commercial false nails that change color upon UV irradiation or with temperature. In principle, this property of the photochromic film can also be utilized (1) for privacy filters that ensure privacy and safety inside houses and help prevent onlookers from viewing one’s computer screen or hand-held-device screen and (2) for protecting the fragile surface of device screens from scuffs and scratches.

4. CONCLUSIONS PS microspheres were synthesized and self-assembled in an ordered periodic structure at a low drawing rate (