Ultrasonics Sonochemistry xxx (2014) xxx–xxx
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Size-controlled spherical polymer nanoparticles: Synthesis with tandem acoustic emulsification followed by soap-free emulsion polymerization and one-step fabrication of colloidal crystal films of various colors Yuki Hirai a, Koji Nakabayashi a, Maya Kojima b, Mahito Atobe a,b,⇑ a b
Department of Environment and System Sciences, Yokohama National University, Yokohama 240-8501, Japan Department of Electronic Chemistry, Tokyo Institute of Technology, Yokohama 226-8502, Japan
a r t i c l e
i n f o
Article history: Received 13 November 2013 Received in revised form 6 December 2013 Accepted 8 December 2013 Available online xxxx Keywords: Tandem acoustic emulsification Size-controlled polymer particle Colloidal crystal film Structural color Polymethylmethacrylate (PMMA)
a b s t r a c t We have developed a novel synthesis method for size-controlled polymer nanoparticles using soap-free emulsion polymerization. This new synthetic method involves sequential ultrasonic irradiation (20 kHz ? 500 kHz ? 1.6 MHz ? 2.4 MHz) for acoustic emulsification of a water-insoluble monomer such as methylmethacrylate (MMA) in an aqueous medium, followed by emulsion polymerization in the obtained solution without using any surfactants. The sequential ultrasonication (tandem acoustic emulsification) could provide a clear and stable emulsified solution containing monomer droplets with relatively narrow size distribution in the nanometer range. The subsequent polymerization in this solution yielded size-controlled polymethylmethacrylate (PMMA) nanoparticles and monodisperse PMMA nanoparticles of different sizes. Furthermore, colloidal crystal films could be easily prepared from the as-polymerized nanoparticle solution using the fluidic-cell method. Moreover, we succeeded to modify the structural color of colloidal crystal films by the addition of a small amount of organic solvent to the as-polymerized nanoparticle solution for the fluidic-cell method. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, structural colors have attracted much attention in a wide variety of research fields. Structural colors are quite widespread in nature, such as in opals and some animals. On the other hand, they are deeply connected with recent rapidly growing fields as photonics and have been extensively studied to clarify their peculiar optical properties. In principle, their mechanisms are of a purely physical origin, which differs completely from the ordinary coloration mechanisms such as in pigments and dyes, where the colors are produced by virtue of the interaction with light. It is recognized that structural colors are mainly based on several elementary optical processes, including thin-layer interference, diffraction, light scattering. Therefore, in general, the structural color is maintained for a long period of time, unlike the color of pigments or dyes [1–4]. A colloidal crystal is an ordered array of colloid particles and one of the most promising structural color materials. Colloidal crystal can be prepared as a film structure by simple methods such ⇑ Corresponding author at: Department of Environment and System Sciences, Yokohama National University, Yokohama 240-8501, Japan. Tel./fax: +81 045 339 4214. E-mail address: [email protected] (M. Atobe).
as gravity sedimentation [5,6], dipping [7,8], solvent evaporation [9], spin coating [10] and microfluidic cell method [11]. On the other hand, the use of monodisperse fine particles is strongly required for the brilliant coloration of colloidal crystals [12–18]. Until now, the best-established and most commonly used method for the production of monodisperse fine particles is emulsion polymerization. However, this method requires the use of a relatively large amount of surfactant. The presence of the surfactants increases cost and difficulty of purification of the obtained particles. Therefore, the development of a new method for surfactant-free (soap-free) emulsion polymerization has been strongly desired not only for the colloidal crystal synthesis but also for the industrial synthesis of monodisperse polymer particles. On the other hand, ultrasonic irradiation provides stable emulsions without using surfactants by means of mechanical forces generated from acoustic cavitation at the liquid/liquid interphase boundaries [19–21]. This has been termed ‘acoustic emulsification’ and is regarded as one of the powerful tools for rapid and environmental-friendly emulsion production. Recently, our group reported a new technique for the preparation of a highly clear and transparent emulsified aqueous solution containing water immiscible organic droplets with diameters of a few tens of nanometers under surfactant-free conditions using several ultrasonic devices having different frequencies [22]. We observed that the droplet size was clearly reduced by using sequential ultrasonic processing. This
1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.12.006
Please cite this article in press as: Y. Hirai et al., Size-controlled spherical polymer nanoparticles: Synthesis with tandem acoustic emulsification followed by soap-free emulsion polymerization and one-step fabrication of colloidal crystal films of various colors, Ultrason. Sonochem. (2014), http://dx.doi.org/ 10.1016/j.ultsonch.2013.12.006
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Table 1 Details of each ultrasonication step in the sequential ultrasonications. Frequency Power/ W cm2
Sonication time/ min
20 kHz
44
8
500 kHz 1.6 MHz 2.4 MHz
4 16 7
10 10 10
Devise used Ultrasonic stepped horn (13 mm diameter, titanium alloy) connected with a 20 kHz oscillator (SONIFIER-250D, Branson Ultrasonics Co.) Ultrasonic transducer (Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm) Ultrasonic transducer (Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm) Ultrasonic transducer (Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm)
novel technique, tandem acoustic emulsification, was found to be adequate for producing emulsion nanodroplets with desired size in the absence of any surfactant. Hence, we envisioned that size-controlled polymer nanoparticles could be synthesized by soap-free polymerization with a tandem acoustically emulsified monomer solution. In fact, we have already reported that this approach works in the preparation of size-controlled polymer particles. Furthermore, colloidal crystal films were found to be easily prepared from such size-controlled polymer particles. The successful results promoted us to perform a systematic study on the fabrication of colloidal crystal films with a variety of structural colors using size-controlled polymer particles.
The PMMA particles were synthesized from the emulsified solution using radical polymerization method [23]. Polymerization was started by the addition of 0.025 g of APS as an initiator at 78 °C for 15 min and then the reaction vessel was cooled to 25 °C to stop the polymerization. By this procedure, polymerization of MMA droplets was completed (the conversion of MMA monomer was 100%).
2. Experimental
2.7. Fabrication of fluidic cells
2.1. Abbreviation In the present paper, MMA, PMMA, APS, DLS, and SEM are used to indicate methylmethacrylate, polymethylmethacrylate, ammonium peroxodisulfate, dynamic light scattering, and scanning electron microscopy, respectively.
The fluidic cell used in this study is composed of two flat glass substrates (Matsunami, S-1226, 76 26 1 mm) and two spacers sandwiched between them. Two stick tapes (Nitto Denko Corporation, No. 5601, 10 lm in thickness) were used as spacer. The cell has two openings with the same thickness as the spacers. A fluid reservoir is connected with one side of the opening (Fig. 1).
2.2. Chemicals and materials
2.8. Preparation of colloidal crystal films
MMA (analytical grade) was purchased from Tokyo Chemical Industry Co., Ltd. and passed through a basic alumina column for the removal of polymerization inhibitor (6-tert-butyl-2,4-xylenol). Aluminum oxide 90 active basic (0.063–0.200 mm) (activity stage I) for column chromatography was purchased from MERCK. APS (analytical grade) was purchased from Wako Pure Chemical Industries, Ltd. and used as received. Methanol, 2-propanol, and ethylene glycol were purchased from Kanto Chemical Co., Inc. Water for emulsification and polymerization was deionized to a resistivity of 18.2 MX cm using a Barnstead DI water system.
As-polymerized nanoparticle solutions were simply used for preparation of colloidal crystal films. The dispersion of PMMA spheres penetrated into the space of the fluidic cell caused under capillary forces (Fig. 2). The fluidic cell was held to supply the dispersion from the reservoir, and then the colloidal crystal film constructed across the inside of the fluidic cell. The colloidal crystal grew gradually from the edge of another opening and reached over several square centimeters after several days (Fig. 3).
2.3. Acoustic emulsification
After the polymerization, the PMMA colloidal crystal films were prepared by fluidic cell method and then dried under reduced pressure. After this process, the surface morphology of the films was observed by SEM (VE-7800, Keyence Co.). The accelerating voltage of SEM was 15 kV.
2.134 g of MMA was added to 22.7 ml of deionized water in glass beaker cell (1:10 volume ratio of MMA to water). Then, the sequential ultrasonications (20 kHz ? 500 kHz ? 1.6 MHz ? 2.4 MHz) were conducted to the water/MMA mixture. Details of each ultrasonication step are summarized in Table 1. The ultrasound intensity (power level) in indicated by the electrical input power to the oscillator in this paper. 2.4. Photographic recording of MMA emulsion The appearance of the MMA emulsion was monitored with photographic recording. The photographs were taken with a digital camera (IXY 300, Cannon Co.). 2.5. Measurement of droplet size and distribution Droplet size distribution was determined by DLS at 25 °C with light scattering photometer (nano-ZS ZEN 3600, Sysmex Co.) with-
out diluting the mixture. The minimal measurement time of 10 min was required for setting and stabilizing the sample before the first data point was taken. 2.6. Preparation of PMMA nanoparticles
2.9. Scanning electron microscopy (SEM)
2.10. Modification of structural color 1.0 ml of organic solvent (methanol, 2-propanol, or ethylene glycol) was added to 10 ml of an as-polymerized nanoparticle solution in order to expand the size of PMMA nanoparticle. Then, the solution of nanoparticles swelled for 30 min was used for the fabrication of colloidal crystal films by the fluidic cell method. 2.11. Measurement of transmittance spectra Optical properties of PMMA colloidal crystal films are evaluated by measuring their transmittance spectra, using a UV–Vis spectrometer (UV-1800, Shimadzu Co.). The spectra were measured over a range from 400 to 700 nm.
Please cite this article in press as: Y. Hirai et al., Size-controlled spherical polymer nanoparticles: Synthesis with tandem acoustic emulsification followed by soap-free emulsion polymerization and one-step fabrication of colloidal crystal films of various colors, Ultrason. Sonochem. (2014), http://dx.doi.org/ 10.1016/j.ultsonch.2013.12.006
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3. Results and discussion 3.1. Preparation of MMA nanoemulsion by tandem acoustic emulsification
Fig. 1. Schematic drawing of the fluidic cell used to fabricate colloidal crystal films.
Sequential ultrasonic irradiation (20 kHz ? 500 kHz ? 1.6 MHz ? 2.4 MHz) for acoustic emulsification of a water-insoluble monomer such as MMA was carried out in an aqueous medium (Fig. 4). A milky white solution was obtained immediately after 20 kHz ultrasonic treatment, and this appearance was not changed by elongation of the sonication time (Fig. 4b). Sequential ultrasonication with 500 kHz treatment after the 20 kHz treatment gave a clear emulsified solution (Fig. 4c). Subsequent ultrasonication with 1.6 MHz gave a further clearer solution (Fig. 4d), and finally a perfectly clear and transparent solution was obtained by further sequential ultrasonication with 2.4 MHz treatment after the 20 kHz, 500 kHz, and 1.6 MHz treatments (Fig. 4e). However, Tyndall scattering of the laser light could be observed in this emulsified solution. It was noted that direct ultrasonication with 500 kHz, 1.6 MHz or 2.4 MHz could not emulsify the original MMA/water mixture, due to its low mechanical effect (less cavitation intensity), and therefore a tandem treatment is required for preparation of a clear and transparent emulsified solution. In addition, tandem emulsified solution was so stable that the clear and transparent appearance was maintained for at least 6 months, even under surfactant-free conditions. The actual reason why the tandem acoustic emulsified solution was so stable even without the addition of surfactants has not been clarified at present. However, it has been proposed by Fogler et al. that the acoustic emulsions are stabilized as a result of the preferential adsorption of OH ions that are always present in the aqueous media [19–21]. Therefore, we can speculate this phenomenon on the basis of preferential adsorption of OH ions at the MMA-water interface. On the other hand, it is well known that ultrasound can generate radicals to initiate polymerization reaction [24]. However, the aims of this work are preparation of the MMA nanodroplets of different sizes by the use of tandem acoustic emulsification and their application to synthesis of the PMMA nanoparticles of different sizes using a chemical initiator. Therefore, ultrasound should not play a role of generator for the polymerization initiator but a role of emulsifier. Fortunately, polymerization was hardly occurred under acoustic emulsification conditions employed in this work.
3.2. Droplets size and their size distribution Fig. 2. Schematic illustration of growth of colloidal crystal film.
Fig. 3. Photograph of the fluidic cell used to fabricate colloidal crystal films.
To estimate the droplets size quantitatively, we measured the size distribution of the MMA droplets after the acoustic emulsification treatments by DLS (Fig. 5). After 20 kHz ultrasonication, a single peak in the number-mode was observed at 220 nm. In addition, the emulsion contained a large amount of droplets larger than 380 nm, which is the upper limit of the visible region, and hence the solution appeared cloudy. Sequential ultrasonication at a higher frequency could break up the larger droplets to form smaller droplets. Ultrasonication with 500 kHz treatment after the 20 kHz shifted a peak to 112 nm and further sequential ultrasonication with 1.6 MHz resulted in a peak shift to 51 nm. Finally, ultrasonication with 2.4 MHz after 20 kHz, 500 kHz and 1.6 MHz led to a peak shift to 23 nm. Since this emulsified solution did not contain any droplets larger than 380 nm, it appeared a clear and transparent as shown in Fig. 4e. From these measurements, it can be stated that the size of the MMA nanodroplets was controlled intentionally by selecting the number of ultrasonication steps in the tandem operation.
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Fig. 4. Photographic observations of tandem acoustic emulsification of MMA in aqueous solution. Photograph (a) represents the original MMA-in aqueous solution mixture. Emulsification conditions were (b) 20 kHz, 8 min, (c) 20 kHz, 8 min ? 500 kHz, 10 min, (d) 20 kHz, 8 min ? 500 kHz, 10 min ? 1.6 MHz, 10 min, and (e) 20 kHz, 8 min ? 500 kHz, 10 min ? 1.6 MHz, 10 min ? 2.4 MHz, 10 min.
Fig. 5. Size distributions of MMA droplets in acoustically emulsified aqueous solutions. Emulsification conditions used were (a) 20 kHz, 8 min (average droplet size: 220 nm), (b) 20 kHz, 8 min ? 500 kHz, 10 min (average droplet size: 112 nm), (c) 20 kHz, 8 min ? 500 kHz, 10 min ? 1.6 MHz, 10 min (average droplet size: 51 nm), and (d) 20 kHz, 8 min ? 500 kHz, 10 min ? 1.6 MHz, 10 min ? 2.4 MHz, 10 min (average droplet size: 23 nm).
Fig. 6. Size distributions of PMMA particles prepared from acoustically emulsified aqueous solutions. PMMA samples were synthesized from (a) emulsion A (average particle size: 241 nm), (b) B (average particle size: 191 nm), (c) C (average particle size: 164 nm), and (d) D (average particle size: 129 nm) of Fig. 4.
size distribution of all PMMA samples obtained by this synthesis method was found to be relatively narrow. From these facts, it can be stated that non-sequential ultrasonication in the megahertz range is not suitable to emulsify the original oil-in-water mixture, due to the low mechanical effect [25], however, it does have potential for further size reduction of droplets larger than a few hundred nanometers because the size of a cavitation bubble decreases as the acoustic frequency is increased [26].
3.3. Preparation of PMMA nanoparticles and measurement of their size and size distribution In order to obtain size-controlled PMMA nanoparticles, we carried out soap-free emulsion polymerization using these emulsified solutions (emulsions A–D in Fig. 4). Polymerization was started by the addition of APS as an initiator at 78 °C for 15 min, and the reaction vessel was then cooled to 25 °C to stop the polymerization. Fig. 6 shows the size distribution of the PMMA nanoparticles polymerized from emulsions A–D. Although the average size of the PMMA prepared from the 20 kHz sonicated solution (emulsion A) was almost equal to the corresponding MMA droplet size, it was not in accord with the corresponding MMA droplet size when using the tandem sonicated solutions (emulsions B–D). This fact would indicate that the elevated temperature for the polymerization activates Brownian motion of MMA droplets, and thus the coalescence of smaller MMA droplets occurred during the polymerization. However, the size order between PMMA samples reflected that of the corresponding MMA droplets. Moreover, the
3.4. Preparation of PMMA colloidal crystal films and their structural color Colloidal crystals consist of monodispersed submicrometer spheres, and they exhibit structural colors due to Bragg’s law of diffraction of visible light. Therefore, colloidal crystals have attracted much interest owing to their potential in applications such as optoelectronics and photonics [27]. Surfactant-free polymer latexes are suitable for the preparation of colloidal crystals because pretreatments for removing impurities such as surfactants are not needed. Using the fluidic-cell method [11], we fabricated colloidal crystal films composed of size-controlled PMMA nanoparticles prepared by the present synthesis method. In this demonstration, an as-polymerized nanoparticle solution was simply used for the fabrication. Upon cell injection into the reservoir of an as-polymerized nanoparticle solution, the dispersion of PMMA spheres penetrated into the space of the fluidic cell caused by capillary forces. Then colloidal crystal film with structural color grew gradually from the vicinity of another opening. Fig. 7 shows photographs of PMMA colloidal crystal films formed within a fluidic cell. As shown in Fig. 7, different optical properties were observed for the film samples. Film A composed of PMMA particles prepared from emulsion A of Fig. 4 showed a vivid green1 color. On the other hand, film B composed of the parti1 For interpretation of color in Fig. 4, the reader is referred to the web version of this article.
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Fig. 7. Photographic images of colloidal crystal films composed of size-controlled PMMA particles prepared from (a) emulsions A, (b) B, (c) C, and (d) D of Fig. 4.
cles synthesized from emulsion B exhibited an aqua blue color, and film C composed of PMMA particles prepared from emulsion C displayed a deep blue color. However, no color was exhibited in film D composed of size-controlled PMMA particles prepared from emulsion D. Fig. 8 shows their transmission spectra. In general, the transmission peak wavelength (kpeak) is strongly related to the sphere diameter, that is, kpeak of the colloidal crystal film composed of bigger spheres is higher because of the longer interparticle distance. In fact, as shown in Fig. 8, kpeak of film A composed of bigger PMMA spheres was longer than that of other film samples at 551 nm. On the other hand, kpeak of films B and C was found to be 509 and 417 nm, respectively. kpeak of film D was not detected in the visible-light region. This may be ascribed to the fact that visible light was not diffracted in film D that was composed of smaller PMMA particles. 3.5. Theoretical calculation of the PMMA particle size from the observed transmission peak wavelength (kpeak) The structural color of the colloidal crystal depends on the periodic particle array and refractive index. This is expressed by the Bragg’s equation with Snell’s law as follows [9]:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 2 neff sin h ; k ¼ 2d
ð1Þ
where d indicates interspace of plane between particle array, neff indicates average refractive index, and h indicates incident angle. By using the transmission peak wavelength (kpeak), Eq. (1) is modified to Eq. (2), and hence the interspace of plane between particle array (d) is calculated from neff and transmission peak wavelength (kpeak) observed for the film sample:
kpeak ¼ 2dneff :
ð2Þ
On the other hand, the neff is calculated from the following Eq. (3) [28,29]:
neff ¼ ð1 /Þns þ /nsphere ;
ð3Þ
where / is the volume fraction of particles in colloidal crystal, ns is the refraction index of solvent around the particle, and nsphere is the particle refraction index. In Eq. (3), / is 0.74 when the structure of the colloidal crystal is densely packed. In addition, when the particle material and solvent around the particle are PMMA and water, respectively, the nsphere and ns are 1.49 and 1.33, respectively. Hence, the actual neff is calculated by Eq. (3) using these values. Since it is well known that the (1 1 1) plane of face-centered cubic (FCC) lattice coated on the whole area of the substrate for colloidal crystal [27], d is regarded as d1 1 1. Therefore, the d1 1 1 can be calculated from Eq. (2), and then by the substitution of the d1 1 1 value for the following Eq. (4), the particle size (D) of PMMA is theoretically estimated:
D¼
12 3 d1 1 1 : 2
ð4Þ
Table 2 shows the calculated and DLS measured particle sizes. From these results, the measured particle sizes seem to be in fair agreement with those calculated from Eqs. (1)–(4). 3.6. Crystallinity of the film surface
Fig. 8. Transmission spectra of colloidal crystal films composed of size-controlled PMMA nanoparticles prepared from (a) emulsion A, (b) B, (c) C, and (d) D of Fig. 4.
In order to evaluate the crystallinity (i.e., the ordering of the PMMA spheres), the surface of the films was observed by SEM (Fig. 9). The SEM images clearly revealed that both sample films
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Table 2 Calculated and DLS measured PMMA particle size.
a
Emulsification condition
Calculated particle size/nm
Measured particle sizea/nm
Size distribution curve in Fig. 6
20 kHz 20 kHz ? 500 kHz 20 kHz ? 500 kHz ? 1.6 MHz 20 kHz ? 500 kHz ? 1.6 MHz ? 2.4 MHz
233 215 176 –
241 191 164 129
a b c d
Determined by size distribution curves in Fig. 6.
Fig. 9. SEM micrographs of colloidal crystal films. (a) Film A, (b) film D. Fig. 11. Transmission spectra of colloidal crystal films composed of PMMA particles swelled by the addition of (a) methanol and (b) 2-propanol.
(films A and D of Fig. 7) have a densely packed structure with a periodic array of size-controlled PMMA polymer particles. Therefore, it can be stated that the vivid coloration observed in Fig. 7 greatly depends on these highly ordered structure of the films.
Table 3 Size of PMMA nanoparticles swelled by the addition of alcohols.
3.7. Modification of structural color The biggest size of PMMA particles obtained in the present work is about 240 nm, and hence the film composed of them exhibited a vivid green color. If the structural colors with red tones are required, further bigger PMMA particles should be prepared for the fabrication of colloidal crystal films. To meet this request, we added a small amount of alcohol (methanol, 2-propanol, or ethylene glycol) to the as-polymerized nanoparticle aqueous solution in order to expand of the size of PMMA nanoparticles by their swelling. Then, the solution of nanoparticles swelled was used for the fabrication of colloidal crystal films by the fluidic cell method. Fig. 10 shows photographs of colloidal crystal films composed of PMMA nanoparticles swelled by the addition of methanol and 2-propanol. The colloidal crystal with vivid orange and red colors grew gradually from the edge of the fluidic cell by the addition of methanol and 2-propanol, respectively. Fig. 11 shows their transmission spectra. The transmission peak wavelengths (kpeak) for the films composed of the nanoparticles swelled by the addition of methanol and 2-propanol are 598 and 601 nm, respectively, and corresponded exactly to their observed structural colors. These facts suggest that the size of PMMA nanoparticles was expanded further by the addition of methanol and 2-propanol. To confirm
a
Solvent added
Measured particle sizea/nm
– Methanol 2-Propanol
241 272 307
Determined by DLS.
this, we then measured the size of the swelled PMMA nanoparticles by DLS (Table 3). Although the average size of PMMA nanoparticles was 241 nm before the addition of alcohols, the size was actually expanded to 272 and 307 nm by the addition of methanol and 2-propanol, respectively. Thus the addition of alcohols enabled the expansion of particle size by swelling, so tuning the dimension of the particles, and, as consequence, the color of the colloidal crystal film. On the other hand, the crystal did not construct across the inside of the fluidic cell by the addition of ethylene glycol since the vaporization from the top edge opening of the fluidic cell did not occur. 4. Conclusion We successfully prepared size-controlled polymer nanoparticles under a surfactant-free condition using tandem acoustic emul-
Fig. 10. Photographic images of colloidal crystal films composed of PMMA particles swelled by the addition of (a) methanol and (b) 2-propanol.
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sification. Thus, tandem acoustic emulsification offers a ‘‘green’’ method and is clearly a powerful tool for the synthesis of size-controlled polymer nanoparticles in the absence of any surfactants. In addition, structural color materials were successfully made from such size-controlled polymer nanoparticles prepared by tandem acoustic emulsification. The structural color of colloidal crystal films could be modified by the addition of a small amount of alcohols to the as-polymerized nanoparticle solution for the fluidic-cell method, and consequently colloidal crystal films with various colors were obtained in this work.
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Acknowledgements This work was financially supported by The Grant-in-Aid for Scientific Research on Innovative Areas ‘‘Organic Synthesis based on Reaction Integration. Development of New Methods and Creation of New Substances’’ (No. 2105). References
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Size-controlled spherical polymer nanoparticles: Synthesis with tandem acoustic emulsification followed by soap-free emulsion polymerization and one-step fabrication of colloidal crystal films of various colors Yuki Hirai a, Koji Nakabayashi a, Maya Kojima b, Mahito Atobe a,b,⇑ a b
Department of Environment and System Sciences, Yokohama National University, Yokohama 240-8501, Japan Department of Electronic Chemistry, Tokyo Institute of Technology, Yokohama 226-8502, Japan
a r t i c l e
i n f o
Article history: Received 13 November 2013 Received in revised form 6 December 2013 Accepted 8 December 2013 Available online xxxx Keywords: Tandem acoustic emulsification Size-controlled polymer particle Colloidal crystal film Structural color Polymethylmethacrylate (PMMA)
a b s t r a c t We have developed a novel synthesis method for size-controlled polymer nanoparticles using soap-free emulsion polymerization. This new synthetic method involves sequential ultrasonic irradiation (20 kHz ? 500 kHz ? 1.6 MHz ? 2.4 MHz) for acoustic emulsification of a water-insoluble monomer such as methylmethacrylate (MMA) in an aqueous medium, followed by emulsion polymerization in the obtained solution without using any surfactants. The sequential ultrasonication (tandem acoustic emulsification) could provide a clear and stable emulsified solution containing monomer droplets with relatively narrow size distribution in the nanometer range. The subsequent polymerization in this solution yielded size-controlled polymethylmethacrylate (PMMA) nanoparticles and monodisperse PMMA nanoparticles of different sizes. Furthermore, colloidal crystal films could be easily prepared from the as-polymerized nanoparticle solution using the fluidic-cell method. Moreover, we succeeded to modify the structural color of colloidal crystal films by the addition of a small amount of organic solvent to the as-polymerized nanoparticle solution for the fluidic-cell method. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, structural colors have attracted much attention in a wide variety of research fields. Structural colors are quite widespread in nature, such as in opals and some animals. On the other hand, they are deeply connected with recent rapidly growing fields as photonics and have been extensively studied to clarify their peculiar optical properties. In principle, their mechanisms are of a purely physical origin, which differs completely from the ordinary coloration mechanisms such as in pigments and dyes, where the colors are produced by virtue of the interaction with light. It is recognized that structural colors are mainly based on several elementary optical processes, including thin-layer interference, diffraction, light scattering. Therefore, in general, the structural color is maintained for a long period of time, unlike the color of pigments or dyes [1–4]. A colloidal crystal is an ordered array of colloid particles and one of the most promising structural color materials. Colloidal crystal can be prepared as a film structure by simple methods such ⇑ Corresponding author at: Department of Environment and System Sciences, Yokohama National University, Yokohama 240-8501, Japan. Tel./fax: +81 045 339 4214. E-mail address: [email protected] (M. Atobe).
as gravity sedimentation [5,6], dipping [7,8], solvent evaporation [9], spin coating [10] and microfluidic cell method [11]. On the other hand, the use of monodisperse fine particles is strongly required for the brilliant coloration of colloidal crystals [12–18]. Until now, the best-established and most commonly used method for the production of monodisperse fine particles is emulsion polymerization. However, this method requires the use of a relatively large amount of surfactant. The presence of the surfactants increases cost and difficulty of purification of the obtained particles. Therefore, the development of a new method for surfactant-free (soap-free) emulsion polymerization has been strongly desired not only for the colloidal crystal synthesis but also for the industrial synthesis of monodisperse polymer particles. On the other hand, ultrasonic irradiation provides stable emulsions without using surfactants by means of mechanical forces generated from acoustic cavitation at the liquid/liquid interphase boundaries [19–21]. This has been termed ‘acoustic emulsification’ and is regarded as one of the powerful tools for rapid and environmental-friendly emulsion production. Recently, our group reported a new technique for the preparation of a highly clear and transparent emulsified aqueous solution containing water immiscible organic droplets with diameters of a few tens of nanometers under surfactant-free conditions using several ultrasonic devices having different frequencies [22]. We observed that the droplet size was clearly reduced by using sequential ultrasonic processing. This
1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.12.006
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Table 1 Details of each ultrasonication step in the sequential ultrasonications. Frequency Power/ W cm2
Sonication time/ min
20 kHz
44
8
500 kHz 1.6 MHz 2.4 MHz
4 16 7
10 10 10
Devise used Ultrasonic stepped horn (13 mm diameter, titanium alloy) connected with a 20 kHz oscillator (SONIFIER-250D, Branson Ultrasonics Co.) Ultrasonic transducer (Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm) Ultrasonic transducer (Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm) Ultrasonic transducer (Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm)
novel technique, tandem acoustic emulsification, was found to be adequate for producing emulsion nanodroplets with desired size in the absence of any surfactant. Hence, we envisioned that size-controlled polymer nanoparticles could be synthesized by soap-free polymerization with a tandem acoustically emulsified monomer solution. In fact, we have already reported that this approach works in the preparation of size-controlled polymer particles. Furthermore, colloidal crystal films were found to be easily prepared from such size-controlled polymer particles. The successful results promoted us to perform a systematic study on the fabrication of colloidal crystal films with a variety of structural colors using size-controlled polymer particles.
The PMMA particles were synthesized from the emulsified solution using radical polymerization method [23]. Polymerization was started by the addition of 0.025 g of APS as an initiator at 78 °C for 15 min and then the reaction vessel was cooled to 25 °C to stop the polymerization. By this procedure, polymerization of MMA droplets was completed (the conversion of MMA monomer was 100%).
2. Experimental
2.7. Fabrication of fluidic cells
2.1. Abbreviation In the present paper, MMA, PMMA, APS, DLS, and SEM are used to indicate methylmethacrylate, polymethylmethacrylate, ammonium peroxodisulfate, dynamic light scattering, and scanning electron microscopy, respectively.
The fluidic cell used in this study is composed of two flat glass substrates (Matsunami, S-1226, 76 26 1 mm) and two spacers sandwiched between them. Two stick tapes (Nitto Denko Corporation, No. 5601, 10 lm in thickness) were used as spacer. The cell has two openings with the same thickness as the spacers. A fluid reservoir is connected with one side of the opening (Fig. 1).
2.2. Chemicals and materials
2.8. Preparation of colloidal crystal films
MMA (analytical grade) was purchased from Tokyo Chemical Industry Co., Ltd. and passed through a basic alumina column for the removal of polymerization inhibitor (6-tert-butyl-2,4-xylenol). Aluminum oxide 90 active basic (0.063–0.200 mm) (activity stage I) for column chromatography was purchased from MERCK. APS (analytical grade) was purchased from Wako Pure Chemical Industries, Ltd. and used as received. Methanol, 2-propanol, and ethylene glycol were purchased from Kanto Chemical Co., Inc. Water for emulsification and polymerization was deionized to a resistivity of 18.2 MX cm using a Barnstead DI water system.
As-polymerized nanoparticle solutions were simply used for preparation of colloidal crystal films. The dispersion of PMMA spheres penetrated into the space of the fluidic cell caused under capillary forces (Fig. 2). The fluidic cell was held to supply the dispersion from the reservoir, and then the colloidal crystal film constructed across the inside of the fluidic cell. The colloidal crystal grew gradually from the edge of another opening and reached over several square centimeters after several days (Fig. 3).
2.3. Acoustic emulsification
After the polymerization, the PMMA colloidal crystal films were prepared by fluidic cell method and then dried under reduced pressure. After this process, the surface morphology of the films was observed by SEM (VE-7800, Keyence Co.). The accelerating voltage of SEM was 15 kV.
2.134 g of MMA was added to 22.7 ml of deionized water in glass beaker cell (1:10 volume ratio of MMA to water). Then, the sequential ultrasonications (20 kHz ? 500 kHz ? 1.6 MHz ? 2.4 MHz) were conducted to the water/MMA mixture. Details of each ultrasonication step are summarized in Table 1. The ultrasound intensity (power level) in indicated by the electrical input power to the oscillator in this paper. 2.4. Photographic recording of MMA emulsion The appearance of the MMA emulsion was monitored with photographic recording. The photographs were taken with a digital camera (IXY 300, Cannon Co.). 2.5. Measurement of droplet size and distribution Droplet size distribution was determined by DLS at 25 °C with light scattering photometer (nano-ZS ZEN 3600, Sysmex Co.) with-
out diluting the mixture. The minimal measurement time of 10 min was required for setting and stabilizing the sample before the first data point was taken. 2.6. Preparation of PMMA nanoparticles
2.9. Scanning electron microscopy (SEM)
2.10. Modification of structural color 1.0 ml of organic solvent (methanol, 2-propanol, or ethylene glycol) was added to 10 ml of an as-polymerized nanoparticle solution in order to expand the size of PMMA nanoparticle. Then, the solution of nanoparticles swelled for 30 min was used for the fabrication of colloidal crystal films by the fluidic cell method. 2.11. Measurement of transmittance spectra Optical properties of PMMA colloidal crystal films are evaluated by measuring their transmittance spectra, using a UV–Vis spectrometer (UV-1800, Shimadzu Co.). The spectra were measured over a range from 400 to 700 nm.
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3. Results and discussion 3.1. Preparation of MMA nanoemulsion by tandem acoustic emulsification
Fig. 1. Schematic drawing of the fluidic cell used to fabricate colloidal crystal films.
Sequential ultrasonic irradiation (20 kHz ? 500 kHz ? 1.6 MHz ? 2.4 MHz) for acoustic emulsification of a water-insoluble monomer such as MMA was carried out in an aqueous medium (Fig. 4). A milky white solution was obtained immediately after 20 kHz ultrasonic treatment, and this appearance was not changed by elongation of the sonication time (Fig. 4b). Sequential ultrasonication with 500 kHz treatment after the 20 kHz treatment gave a clear emulsified solution (Fig. 4c). Subsequent ultrasonication with 1.6 MHz gave a further clearer solution (Fig. 4d), and finally a perfectly clear and transparent solution was obtained by further sequential ultrasonication with 2.4 MHz treatment after the 20 kHz, 500 kHz, and 1.6 MHz treatments (Fig. 4e). However, Tyndall scattering of the laser light could be observed in this emulsified solution. It was noted that direct ultrasonication with 500 kHz, 1.6 MHz or 2.4 MHz could not emulsify the original MMA/water mixture, due to its low mechanical effect (less cavitation intensity), and therefore a tandem treatment is required for preparation of a clear and transparent emulsified solution. In addition, tandem emulsified solution was so stable that the clear and transparent appearance was maintained for at least 6 months, even under surfactant-free conditions. The actual reason why the tandem acoustic emulsified solution was so stable even without the addition of surfactants has not been clarified at present. However, it has been proposed by Fogler et al. that the acoustic emulsions are stabilized as a result of the preferential adsorption of OH ions that are always present in the aqueous media [19–21]. Therefore, we can speculate this phenomenon on the basis of preferential adsorption of OH ions at the MMA-water interface. On the other hand, it is well known that ultrasound can generate radicals to initiate polymerization reaction [24]. However, the aims of this work are preparation of the MMA nanodroplets of different sizes by the use of tandem acoustic emulsification and their application to synthesis of the PMMA nanoparticles of different sizes using a chemical initiator. Therefore, ultrasound should not play a role of generator for the polymerization initiator but a role of emulsifier. Fortunately, polymerization was hardly occurred under acoustic emulsification conditions employed in this work.
3.2. Droplets size and their size distribution Fig. 2. Schematic illustration of growth of colloidal crystal film.
Fig. 3. Photograph of the fluidic cell used to fabricate colloidal crystal films.
To estimate the droplets size quantitatively, we measured the size distribution of the MMA droplets after the acoustic emulsification treatments by DLS (Fig. 5). After 20 kHz ultrasonication, a single peak in the number-mode was observed at 220 nm. In addition, the emulsion contained a large amount of droplets larger than 380 nm, which is the upper limit of the visible region, and hence the solution appeared cloudy. Sequential ultrasonication at a higher frequency could break up the larger droplets to form smaller droplets. Ultrasonication with 500 kHz treatment after the 20 kHz shifted a peak to 112 nm and further sequential ultrasonication with 1.6 MHz resulted in a peak shift to 51 nm. Finally, ultrasonication with 2.4 MHz after 20 kHz, 500 kHz and 1.6 MHz led to a peak shift to 23 nm. Since this emulsified solution did not contain any droplets larger than 380 nm, it appeared a clear and transparent as shown in Fig. 4e. From these measurements, it can be stated that the size of the MMA nanodroplets was controlled intentionally by selecting the number of ultrasonication steps in the tandem operation.
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Fig. 4. Photographic observations of tandem acoustic emulsification of MMA in aqueous solution. Photograph (a) represents the original MMA-in aqueous solution mixture. Emulsification conditions were (b) 20 kHz, 8 min, (c) 20 kHz, 8 min ? 500 kHz, 10 min, (d) 20 kHz, 8 min ? 500 kHz, 10 min ? 1.6 MHz, 10 min, and (e) 20 kHz, 8 min ? 500 kHz, 10 min ? 1.6 MHz, 10 min ? 2.4 MHz, 10 min.
Fig. 5. Size distributions of MMA droplets in acoustically emulsified aqueous solutions. Emulsification conditions used were (a) 20 kHz, 8 min (average droplet size: 220 nm), (b) 20 kHz, 8 min ? 500 kHz, 10 min (average droplet size: 112 nm), (c) 20 kHz, 8 min ? 500 kHz, 10 min ? 1.6 MHz, 10 min (average droplet size: 51 nm), and (d) 20 kHz, 8 min ? 500 kHz, 10 min ? 1.6 MHz, 10 min ? 2.4 MHz, 10 min (average droplet size: 23 nm).
Fig. 6. Size distributions of PMMA particles prepared from acoustically emulsified aqueous solutions. PMMA samples were synthesized from (a) emulsion A (average particle size: 241 nm), (b) B (average particle size: 191 nm), (c) C (average particle size: 164 nm), and (d) D (average particle size: 129 nm) of Fig. 4.
size distribution of all PMMA samples obtained by this synthesis method was found to be relatively narrow. From these facts, it can be stated that non-sequential ultrasonication in the megahertz range is not suitable to emulsify the original oil-in-water mixture, due to the low mechanical effect [25], however, it does have potential for further size reduction of droplets larger than a few hundred nanometers because the size of a cavitation bubble decreases as the acoustic frequency is increased [26].
3.3. Preparation of PMMA nanoparticles and measurement of their size and size distribution In order to obtain size-controlled PMMA nanoparticles, we carried out soap-free emulsion polymerization using these emulsified solutions (emulsions A–D in Fig. 4). Polymerization was started by the addition of APS as an initiator at 78 °C for 15 min, and the reaction vessel was then cooled to 25 °C to stop the polymerization. Fig. 6 shows the size distribution of the PMMA nanoparticles polymerized from emulsions A–D. Although the average size of the PMMA prepared from the 20 kHz sonicated solution (emulsion A) was almost equal to the corresponding MMA droplet size, it was not in accord with the corresponding MMA droplet size when using the tandem sonicated solutions (emulsions B–D). This fact would indicate that the elevated temperature for the polymerization activates Brownian motion of MMA droplets, and thus the coalescence of smaller MMA droplets occurred during the polymerization. However, the size order between PMMA samples reflected that of the corresponding MMA droplets. Moreover, the
3.4. Preparation of PMMA colloidal crystal films and their structural color Colloidal crystals consist of monodispersed submicrometer spheres, and they exhibit structural colors due to Bragg’s law of diffraction of visible light. Therefore, colloidal crystals have attracted much interest owing to their potential in applications such as optoelectronics and photonics [27]. Surfactant-free polymer latexes are suitable for the preparation of colloidal crystals because pretreatments for removing impurities such as surfactants are not needed. Using the fluidic-cell method [11], we fabricated colloidal crystal films composed of size-controlled PMMA nanoparticles prepared by the present synthesis method. In this demonstration, an as-polymerized nanoparticle solution was simply used for the fabrication. Upon cell injection into the reservoir of an as-polymerized nanoparticle solution, the dispersion of PMMA spheres penetrated into the space of the fluidic cell caused by capillary forces. Then colloidal crystal film with structural color grew gradually from the vicinity of another opening. Fig. 7 shows photographs of PMMA colloidal crystal films formed within a fluidic cell. As shown in Fig. 7, different optical properties were observed for the film samples. Film A composed of PMMA particles prepared from emulsion A of Fig. 4 showed a vivid green1 color. On the other hand, film B composed of the parti1 For interpretation of color in Fig. 4, the reader is referred to the web version of this article.
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Fig. 7. Photographic images of colloidal crystal films composed of size-controlled PMMA particles prepared from (a) emulsions A, (b) B, (c) C, and (d) D of Fig. 4.
cles synthesized from emulsion B exhibited an aqua blue color, and film C composed of PMMA particles prepared from emulsion C displayed a deep blue color. However, no color was exhibited in film D composed of size-controlled PMMA particles prepared from emulsion D. Fig. 8 shows their transmission spectra. In general, the transmission peak wavelength (kpeak) is strongly related to the sphere diameter, that is, kpeak of the colloidal crystal film composed of bigger spheres is higher because of the longer interparticle distance. In fact, as shown in Fig. 8, kpeak of film A composed of bigger PMMA spheres was longer than that of other film samples at 551 nm. On the other hand, kpeak of films B and C was found to be 509 and 417 nm, respectively. kpeak of film D was not detected in the visible-light region. This may be ascribed to the fact that visible light was not diffracted in film D that was composed of smaller PMMA particles. 3.5. Theoretical calculation of the PMMA particle size from the observed transmission peak wavelength (kpeak) The structural color of the colloidal crystal depends on the periodic particle array and refractive index. This is expressed by the Bragg’s equation with Snell’s law as follows [9]:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 2 neff sin h ; k ¼ 2d
ð1Þ
where d indicates interspace of plane between particle array, neff indicates average refractive index, and h indicates incident angle. By using the transmission peak wavelength (kpeak), Eq. (1) is modified to Eq. (2), and hence the interspace of plane between particle array (d) is calculated from neff and transmission peak wavelength (kpeak) observed for the film sample:
kpeak ¼ 2dneff :
ð2Þ
On the other hand, the neff is calculated from the following Eq. (3) [28,29]:
neff ¼ ð1 /Þns þ /nsphere ;
ð3Þ
where / is the volume fraction of particles in colloidal crystal, ns is the refraction index of solvent around the particle, and nsphere is the particle refraction index. In Eq. (3), / is 0.74 when the structure of the colloidal crystal is densely packed. In addition, when the particle material and solvent around the particle are PMMA and water, respectively, the nsphere and ns are 1.49 and 1.33, respectively. Hence, the actual neff is calculated by Eq. (3) using these values. Since it is well known that the (1 1 1) plane of face-centered cubic (FCC) lattice coated on the whole area of the substrate for colloidal crystal [27], d is regarded as d1 1 1. Therefore, the d1 1 1 can be calculated from Eq. (2), and then by the substitution of the d1 1 1 value for the following Eq. (4), the particle size (D) of PMMA is theoretically estimated:
D¼
12 3 d1 1 1 : 2
ð4Þ
Table 2 shows the calculated and DLS measured particle sizes. From these results, the measured particle sizes seem to be in fair agreement with those calculated from Eqs. (1)–(4). 3.6. Crystallinity of the film surface
Fig. 8. Transmission spectra of colloidal crystal films composed of size-controlled PMMA nanoparticles prepared from (a) emulsion A, (b) B, (c) C, and (d) D of Fig. 4.
In order to evaluate the crystallinity (i.e., the ordering of the PMMA spheres), the surface of the films was observed by SEM (Fig. 9). The SEM images clearly revealed that both sample films
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Table 2 Calculated and DLS measured PMMA particle size.
a
Emulsification condition
Calculated particle size/nm
Measured particle sizea/nm
Size distribution curve in Fig. 6
20 kHz 20 kHz ? 500 kHz 20 kHz ? 500 kHz ? 1.6 MHz 20 kHz ? 500 kHz ? 1.6 MHz ? 2.4 MHz
233 215 176 –
241 191 164 129
a b c d
Determined by size distribution curves in Fig. 6.
Fig. 9. SEM micrographs of colloidal crystal films. (a) Film A, (b) film D. Fig. 11. Transmission spectra of colloidal crystal films composed of PMMA particles swelled by the addition of (a) methanol and (b) 2-propanol.
(films A and D of Fig. 7) have a densely packed structure with a periodic array of size-controlled PMMA polymer particles. Therefore, it can be stated that the vivid coloration observed in Fig. 7 greatly depends on these highly ordered structure of the films.
Table 3 Size of PMMA nanoparticles swelled by the addition of alcohols.
3.7. Modification of structural color The biggest size of PMMA particles obtained in the present work is about 240 nm, and hence the film composed of them exhibited a vivid green color. If the structural colors with red tones are required, further bigger PMMA particles should be prepared for the fabrication of colloidal crystal films. To meet this request, we added a small amount of alcohol (methanol, 2-propanol, or ethylene glycol) to the as-polymerized nanoparticle aqueous solution in order to expand of the size of PMMA nanoparticles by their swelling. Then, the solution of nanoparticles swelled was used for the fabrication of colloidal crystal films by the fluidic cell method. Fig. 10 shows photographs of colloidal crystal films composed of PMMA nanoparticles swelled by the addition of methanol and 2-propanol. The colloidal crystal with vivid orange and red colors grew gradually from the edge of the fluidic cell by the addition of methanol and 2-propanol, respectively. Fig. 11 shows their transmission spectra. The transmission peak wavelengths (kpeak) for the films composed of the nanoparticles swelled by the addition of methanol and 2-propanol are 598 and 601 nm, respectively, and corresponded exactly to their observed structural colors. These facts suggest that the size of PMMA nanoparticles was expanded further by the addition of methanol and 2-propanol. To confirm
a
Solvent added
Measured particle sizea/nm
– Methanol 2-Propanol
241 272 307
Determined by DLS.
this, we then measured the size of the swelled PMMA nanoparticles by DLS (Table 3). Although the average size of PMMA nanoparticles was 241 nm before the addition of alcohols, the size was actually expanded to 272 and 307 nm by the addition of methanol and 2-propanol, respectively. Thus the addition of alcohols enabled the expansion of particle size by swelling, so tuning the dimension of the particles, and, as consequence, the color of the colloidal crystal film. On the other hand, the crystal did not construct across the inside of the fluidic cell by the addition of ethylene glycol since the vaporization from the top edge opening of the fluidic cell did not occur. 4. Conclusion We successfully prepared size-controlled polymer nanoparticles under a surfactant-free condition using tandem acoustic emul-
Fig. 10. Photographic images of colloidal crystal films composed of PMMA particles swelled by the addition of (a) methanol and (b) 2-propanol.
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sification. Thus, tandem acoustic emulsification offers a ‘‘green’’ method and is clearly a powerful tool for the synthesis of size-controlled polymer nanoparticles in the absence of any surfactants. In addition, structural color materials were successfully made from such size-controlled polymer nanoparticles prepared by tandem acoustic emulsification. The structural color of colloidal crystal films could be modified by the addition of a small amount of alcohols to the as-polymerized nanoparticle solution for the fluidic-cell method, and consequently colloidal crystal films with various colors were obtained in this work.
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Acknowledgements This work was financially supported by The Grant-in-Aid for Scientific Research on Innovative Areas ‘‘Organic Synthesis based on Reaction Integration. Development of New Methods and Creation of New Substances’’ (No. 2105). References
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Please cite this article in press as: Y. Hirai et al., Size-controlled spherical polymer nanoparticles: Synthesis with tandem acoustic emulsification followed by soap-free emulsion polymerization and one-step fabrication of colloidal crystal films of various colors, Ultrason. Sonochem. (2014), http://dx.doi.org/ 10.1016/j.ultsonch.2013.12.006
