Facile Surface Immobilization of ATRP Initiators on Colloidal Polymers

pubs.acs.org/Langmuir © 2011 American Chemical Society

Facile Surface Immobilization of ATRP Initiators on Colloidal Polymers for Grafting Brushes and Application to Colloidal Crystals Yi-Yu Liu,† Hui Chen,*,† and Koji Ishizu‡ †

Department of Chemical and Materials Engineering, National Central University, 300 Jhongda Road, Jhongli 32001, Taiwan, and ‡Department of Organic Materials and Macromolecules, International Research Center of Macromolecular Science, Tokyo Institute of Technology, 2-12-1-H133, Ookayama, Meguro-ku, Tokyo 152-8552, Japan Received September 6, 2010. Revised Manuscript Received December 11, 2010 Bromo-initiators for atom transfer radical polymerization (ATRP) were successfully immobilized on the surfaces of cross-linked poly(methyl methacrylate) (PMMA) spheres by soap-free emulsion polymerization using CBr4 as the chain transfer agent. Subsequent surface-initiated ATRP (SI-ATRP) afforded a layer of PMMA brushes covalently attached to the sphere surfaces. Colloidal crystal films of these monodisperse spheres were then studied to identify the relationship between variation in particle diameter and the optical properties. The particle diameters were controlled by varying the feed monomer proportions in soap-free emulsion polymerization and the thickness of the grafted brush layer. It was found that the particle diameter could successfully be controlled to obtain crystal films that produce a variety of brilliant colors in the visible region. The results of this study can provide useful information for facile preparation of surfaceimmobilized ATRP initiators on colloidal polymers and can be employed for grafting polymer brushes.

Introduction Monodisperse colloids have long been used as advanced materials in various fields such as colloidal crystals, pigments, and bioassays.1-3 In the past few years, there has been much interest in colloidal crystal materials with three-dimensional periodic structures because of their ability to selectively control the propagation of electromagnetic waves in a particular frequency range.4 The crystal structure of the materials results in various brilliant colors in the visible region, as determined by the photonic band gap of the crystal lattice and period.5 In addition, these materials have also found application in lasers, optical fibers, and quantum optical devices.6-8 Throughout history, colloids have been successfully prepared by different synthesis methods: the St€ ober method,9,10 emulsion 11 polymerization, dispersion polymerization,12,13 soap-free emulsion polymerization,14-16 and so forth. Among these methods, soap-free emulsion polymerization is the simplest procedure *To whom correspondence should be addressed. Telephone: þ886 3 4227151 ext. 34216. Fax: þ886 3 4273643. E-mail: [email protected]. (1) Muscatello, M. M. W.; Stunja, L. E.; Thareja, P.; Wang, L.; Bohn, J. J.; Velankar, S. S.; Asher, S. A. Macromolecules 2009, 42, 4403–4406. (2) Lin, C.; Li, Y.; Yu, M.; Yang, P.; Lin, Jun. Adv. Funct. Mater. 2007, 17, 1459–1465. (3) Park, S. Y.; Handa, H.; Sandhu, A. Nano Lett. 2010, 10, 446–451. (4) Wong, S.; Kitaev, V.; Ozin, G. A. J. Am. Chem. Soc. 2003, 125, 15589–15598. (5) Lin, Y. S.; Hung, Y.; Lin, H. Y.; Tseng, Y. H.; Chen, Y. F.; Mou, C. Y. Adv. Mater. 2007, 19, 577–580. (6) Zhang, J.; Sun, Z.; Yang, B. Curr. Opin. Colloid Interface Sci. 2009, 14, 103–114. (7) Song, J. H.; Kretzschmar, I. Langmuir 2008, 24, 10616–10620. (8) Paquet, C.; Yoshino, F.; Levina, L.; Gourevich, I.; Sargent, E. H.; Kumacheva, E. Adv. Funct. Mater. 2006, 16, 1892–1896. (9) Ketelson, H. A.; Pelton, R.; Brook, M. A. Langmuir 1996, 12, 1134–1140. (10) Deng, T. S.; Zhang, J. Y.; Zhu, K. T.; Zhang, Q. F.; Wu, J. L Colloids Surf., A 2010, 356, 104–111. (11) Cui, L.; Zhang, Y.; Wang, J.; Ren, Y.; Song, Y.; Jiang, L. Macromol. Rapid Commun. 2009, 30, 598–603. (12) Schmid, A.; Fujii, S.; Armes, S. P. Langmuir 2005, 21, 8103–8105. (13) Kim, O. H.; Lee, K.; Kim, K.; Lee, B. H.; Choe, S. Polymer 2006, 47, 1953–1959. (14) Wang, W.; Gu, B.; Liang, L. J. Colloid Interface Sci. 2007, 313, 169–173. (15) Ou, J. L.; Yang, J. K.; Chen, H. Eur. Polym. J. 2001, 37, 789–799. (16) Zhang, S.; Zhao, X. W.; Xu, H.; Zhu, R.; Gu, Z. Z. J. Colloid Interface Sci. 2007, 316, 168–174.

1168 DOI: 10.1021/la103560j

for preparing monodisperse colloidal polymers, and this method can be used in the construction of colloidal crystals. In typical soap-free emulsion polymerization, synthesis parameters such as the kind of monomer, agitation rate, and temperature strongly determine the obtained particle diameter, coefficient of variation in particle size distribution (Cv), and polymerization rate. Zhang et al. proposed a rapid soap-free emulsion polymerization process, in which the monomers were polymerized at the boiling status of the solvent, for synthesizing monodisperse polymer spheres with various particle diameters to be used in colloidal crystals.16 Cui et al. have also successfully used monodisperse particles with various diameters to control the optical properties of the colloidal crystals.11 Precise control over particle diameter and low Cv are thus critical to achieving accurate control over the optical properties of the obtained colloidal crystal. Therefore, soap-free emulsion polymerization is one method that promises a high degree of control. Surface functionalization of spherical particles is a widely used technique in both academia and industry.17-20 The synthesis of a polymer brush grafted sphere can be accomplished by the grafting-from method.21 This method is carried out in conjunction with surface-initiated atom transfer radical polymerization (SIATRP): linear chains are grown from the surfaces of the spherical particles to yield a covalently attached brush layer. SI-ATRP is an attractive polymer grafting method because it affords a narrow shell layer of well-defined polymer brushes with high grafting density. Most previous studies employed several synthetic steps to anchor the initiator groups necessary for SI-ATRP.22-25 Mizutani (17) Zhou, Z.; Zhu, S.; Zhang, D. J. Mater. Chem. 2007, 17, 2428–2433. (18) Tsyalkovsky, V.; Klep, V.; Ramaratnam, K.; Lupitskyy, R.; Minko, S.; Luzinov, I. Chem. Mater. 2008, 20, 317–325. (19) Lee, D. H.; Tokuno, Y.; Uchida, S.; Ozawa, M.; Ishizu, K. J. Colloid Interface Sci. 2009, 340, 27–34. (20) Roohi, F.; Fatoglu, Y.; Titirici, M. M. Anal. Methods 2009, 1, 52–58. (21) Zou, Y.; Kizhakkedathu, J. N.; Brooks, D. E. Macromolecules 2009, 42, 3258–3268. (22) Nagase, K.; Kobayashi, J.; Kikuchi, A.; Akiyama, Y.; Kanazawa, H.; Okano, T. Langmuir 2007, 23, 9409–9415. (23) Nagase, K.; Kobayashi, J.; Kikuchi, A.; Akiyama, Y.; Annaka, M.; Kanazawa, H.; Okano, T. Langmuir 2008, 24, 10981–10987.

Published on Web 01/07/2011

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Figure 1. Schematic representation of preparation of grafting of polymer brushes on monodisperse spheres.

et al. reported the synthesis of poly(N-isopropylacrylamide) brush grafted polystyrene beads by SI-ATRP.25 The ATRP initiatorimmobilized polystyrene beads were prepared by two-step synthesis. The modification process can be simplified by the use of chain transfer agent during the particle growth period. A chain transfer agent is also employed in free radical polymerization to terminate chain growth.26 Harrisson et al. reported evidence for halogen-capping of chain ends by CCl4 and CBr4 as chain transfer agents.27 Therefore, the latter chain transfer agent was employed to attach halogen-capped chain end ATRP initiators to polymer particle surfaces. Subsequently, brushes can be grafted on by SI-ATRP. This paper describes the attachment of SI-ATRP initiators on colloidal polymers for the purpose of grafting brushes, as shown in Figure 1. The grafting process was carried out in organic solvent by SI-ATRP. Therefore, in order to ensure the colloidal polymers used had sufficient organic-solvent tolerance, crosslinked poly(methyl methacrylate) (PMMA) spheres (CPS) with different degrees of cross-linking, determined by the weight fraction of divinylbenzene (DVB), were subject to soap-free emulsion polymerization at the boiling temperature. The morphologies, particle diameters, and Cv of the obtained CPS were characterized. CBr4 was added to the boiling polymerization reaction mixture when conversion reached 70% to immobilize bromoinitiators on CPS surfaces. Subsequently, the polymer brushes were grafted by SI-ATRP (grafting-from method). Colloidal crystal films were produced form the colloidal polymers by evaporation. The optical properties of the films were then investigated. Next, the distinct relationship between particle diameter and variation in optical properties with the incident angle, which is important for the proper design of the photonic band gap, was recorded. The results of this study are expected to provide useful information for the facile attachment of ATRP initiators on colloidal polymers and can be employed for grafting brushes.

Experimental Section Materials. Methyl methacrylate monomer (MMA, 99.0% SHOWA, Japan) and divinylbenzene monomer (DVB, 50% (meta þ para) isomers and the remnant is mainly ethylvinylbenzene (Tokyo Chemical Industry, Japan) were purified by vacuum distillation and stored at 4 °C until use. Potassium peroxodisulfate (24) Wei, J.; He, P.; Liu, A.; Chen, X.; Wang, X.; Jing, X. Macromol. Biosci. 2009, 9, 1237–1246. (25) Mizutani, A.; Nagase, K.; Kikuchi, A.; Kanazawa, H.; Akiyama, Y.; Kobayashi, J.; Annaka, M.; Okano, T. J. Chromatogr., A 2010, 1217, 522–529. (26) Kuil, L. A.; Grove, D. M.; Gossage, R. A.; Zwikker, J. W.; Jenneskens, L. W.; Drenth, W.; Koten, G. Organometallics 1997, 16, 4985–4994. (27) Harrisson, S.; Kapfenstein-Doak, H.; Davis, T. P. Macromolecules 2001, 34, 6214–6223.

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Table 1. Polymerization Conditions and Characterization of Cross-Linked PMMA Spheres (CPS) Code

MMA (g)

DVB (g)

CPS0 10 0 CPS5 9.5 0.5 CPS15 8.5 1.5 CPS25 7.5 2.5 CPS35 6.5 3.5 a Measured by DLS in H2O.

dn (nm)

dh (nm)a

Cv (%)

277 237 234 223 212

280 241 237 228 217

2.04 2.10 2.18 2.55 3.34

(KPS, 98.0% SHOWA, Japan), tetrabromomethane (CBr4, 99.0% Sigma-Aldrich), copper(I) bromide (CuBr, 98.0% SigmaAldrich), 2,20 -bipyridyl (bpy, 99.0% Sigma-Aldrich), 2-bromo-2methylpropionic acid (98.0% Sigma-Aldrich), Dowex Marathon MSC ion-exchange resin (Sigma-Aldrich), methanol (MeOH, 99.9% ECHO, Taiwan), and tetrahydrofuran (THF, anhydrous 99.9% Acros Organics, Belgium) were used as received without purification. Water was processed through reverse osmosis and deionized (DI) to have an electric resistance higher than 18 MΩ cm. Preparation of Monodisperse CPS. The cross-linked PMMA spheres were prepared via soap-free emulsion polymerization at the boiling temperature. Here, 85 mL of DI water and 10 g of monomer (various proportions of DVB to MMA in the feed) were added to a two-necked flask equipped with a reflux condenser and a standard Teflon-coated stirring bar. Subsequently, the mixture was brought to boiling with stirring at ca. 600 rpm. After 5 min, a KPS solution (0.087 g KPS in 5 mL DI water) was added and boiling was continued for 2 h under stirring. The polymerization recipes are shown in Table 1. The colloids were purified with MeOH and DI water to remove impurities by the centrifugal technique and then resuspended in DI water. The monomer conversion was in the range of 82%-90%, as determined by the gravimetric method. SI-ATRP Initiators on Colloidal Polymer. CBr4 was used in the soap-free emulsion polymerization system to afford CPS with surface-immobilized bromo-initiators (CPI). First, a mixture of monomer (6.5 g of MMA and 3.5 g of DVB) in DI water (85 mL) and KPS solution (0.087 g KPS) was brought to a boil under constant stirring. After 10 min of boiling, when conversion was nearly 70%, a CBr4 solution (0.01 g of CBr4 in 1 g of monomer) was added. The polymerization was allowed to proceed for 2 h. Finally, the obtained CPI was rinsed with THF repeatedly by the centrifugal technique and then resuspended in THF. Grafting of Polymer Brushes. A suspension of the obtained CPI (0.8 g) in 40 mL of THF was mixed in a 100 mL dry flask with a stirring bar. After ultrasonication for 30 min and degassing with argon for 30 min under stirring, MMA (8 g, 80 mmol), CuBr (0.0573 g, 0.4 mmol), and bpy (0.1248 g, 0.8 mmol) were added to the flask. Subsequently, the synthetic solution was frozen, and three freeze-pump-thaw cycles were performed to remove oxygen. Polymerization was then carried out at 60 °C under stirring 500 rpm) DOI: 10.1021/la103560j

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for 24 h. After polymerization, ion-exchange resin was added to the synthetic solution (to remove the copper ions) until the solution color turned from green to white. The obtained CPS with attached brushes (CPB) were rinsed with THF by repeated sequential centrifugation and then resuspended in THF. The monomer conversion was 7.26%, as determined by the gravimetric method. In addition, in order to obtain the molecular weight (Mn) and polydispersity index (Mw/Mn) of the polymer brushes, the additional (sacrificial) ATRP initiator was added into the SI-ATRP system. Construction of Colloidal Crystals. Colloidal crystal films were made from CPS, CPI, and CPB. For this, 0.5 mL of latex of polymer spheres was dropped on glass substrates and then dried at 60 °C for 6 h. After evaporating, these polymer spheres were fabricated to ordered colloidal crystal films on glass substrate. Measurements. Each colloid was observed by field emission scanning electron microscopy (FE-SEM, HITACHI, model S80, Japan) to determine the morphology. More than 200 particle diameters were picked from the FE-SEM photographs to determine the number-averaged diameter (dn), the standard deviation (σ), and the coefficient of variation in particle size distribution (Cv), defined as follows:28 X X  ð1Þ dn ¼ ni d i = ni X  2 X 1=2 σ ¼ di - dn = ni Cv ¼

σ  100 dn

ð2Þ ð3Þ

Here, ni is the number of particles with diameter (di). In addition, the hydrodynamic diameter (dh) was measured by dynamic light scattering (DLS, MALVERN, Zetasizer nano ZS, U.K.) for each colloid at 25 °C (633 nm laser, 173° scattering angle). The surface chemical compositions of the colloids were characterized by electron spectroscopy for chemical analysis (ESCA, Thermo VG Scientific, Sigma Probe, U.K.) with an Al KR source (1486.6 eV). Elemental surface stoichiometries were obtained from the sensitivity factor-corrected spectral area ratios. The molecular weight and polydispersity index were determined by gel permeation chromatography (GPC, Viscotek, TDA 300). The instrument consisted of a pump, three 300 mm mixed columns (Shodex, KF-805 L, Japan), and a differential refractiveindex detector in series. Data were collected and analyzed with Viscotek’s TriSEC gel permeation chromatography software package. The GPC system was operated at 40 °C with tetrahydrofuran as an eluent at 1 mL/min. A molecular weight calibration curve was obtained via polystyrene standards. The grafting density (Ds, chains/nm2) was calculated according to the equation:29 WNA ð4Þ Ds ¼ M nS Here, W is the weight of polymer brushes, NA is Avogadro’s number, Mn is the number-average molecular weight, and S is the surface area. The surface area was calculated from dn to obtain the number of spheres and surface area of a sphere. Optical properties of the colloidal crystal films were evaluated by measuring reflection spectra to determine wavelength of maximum reflected intensity (λmax), using an ultraviolet-visible spectrophotometer (UV-vis, JASCO, V-670) with an ARSN-733 absolute reflectance measurement accessory. The band gap position with a face-centered cubic (FCC) lattice was according to the modified Bragg’s law:6,10,14 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð5Þ λcal ¼ 2d 111 neff 2 - sin2 θ (28) Nagao, D.; Sakamoto, T.; Konno, H.; Gu, S.; Konno, M. Langmuir 2006, 22, 10958–10962.

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where λcal is the wavelength, d111 is the grating constant which is related to the particle diameter (D) by (2/3)1/2D for FCC, θ is the angle between the incident light and the normal to the diffraction planes, neff is an effective refractive index of the photonic crystals determined as neff = npolymer φ þ nair (1 - φ). Here, n is the refractive index and φ is the volumetric proportion. Assuming that the photonic crystals are close-packed with an ideal particle volume fraction (φ= 0.74) in the colloidal crystal films, the refractive index of PDVB and PMMA is 1.62 and 1.49, respectively.

Results and Discussion Characterization of CPS. In order to obtain organic-solvent tolerance of polymer spheres, the CPS with different degrees of cross-linking were investigated. The morphologies, number-averaged diameter (dn), hydrodynamic diameter (dh), and Cv were determined by field emission scanning electron microscopy (FESEM; see Figure 2) and dynamic light scattering (DLS). In all of the images, it was apparent that the spheres were extremely uniform. Moreover, as seen in Figure 2a, a necking phenomenon occurred between the polymer spheres due to the fact that homoPMMA (CPS0) is slightly hydrophilic. The addition of more DVB (an extremely hydrophobic monomer) led to a decrease in necking; furthermore, the calculated dn decreased from 277 to 212 nm. The reactivity of DVB is significantly greater than that of MMA (rm-DVB = 0.62, rMMA 0.43; rp-DVB = 0.85, rMMA = 0.11),30 which should lead to rapid depletion of DVB during copolymerization. Therefore, we conclude that the increase in the amount of DVB led to a proliferation of nucleation sites, consequent rapid nucleation of initial copolymerization, and thus the decreased particle diameters.31 Figure 3 shows particle size distribution and dh values obtained by DLS in DI water against the proportion of DVB used in CPS synthesis. From Figure 3, it is obvious that the dh values decreased from 280 to 217 nm as the proportion of DVB increased. The precise values are shown in Table 1. The dh values were slightly higher than the particle diameters in the dry state (dn), since the particles develop an extended double layer of ions in aqueous solution. In addition, the Cv values for all CPS produced in this study were less than 4.0%, which confirms the practicality of soap-free emulsion polymerization. Dried CPS with different cross-linking degrees were resuspended in THF (10 wt % solid content) and then again dried at 60 °C to observe the organic-solvent tolerance (Figure 4). It could be clearly observed that CPS with low cross-linking (CPS5 and CPS15) suffered the largest deformation (flattening), whereas CPS with high cross-linking (CPS25 and CPS35) maintained their spherical shape. Overall, sufficient tolerance was obtained at > 25 wt % DVB. Note that subsequent surface immobilization of ATRP initiators and brush grafting was exclusively performed with CPS35. Polymer Brush Grafted Spheres. The conversion-time curve of CPS35 is shown in Figure 5. Obviously, the maximum value of conversion (∼80%) was achieved after 30 min. In order to immobilize bromo-initiators on CPS surfaces, CBr4 was added at conversion nearly 70% (at 10 min). Electron spectroscopy for chemical analysis (ESCA; Figure 6) showed composition of the CPS and CPI, respectively. The C 1s core-level spectra clearly showed that the immobilization of bromo-initiators on the CPI surfaces was verified by the appearance of the C-Br peak (286.4 eV). After surface atom calculations using survey spectral (29) Li, G. L.; Wan, D.; Neoh, K. G.; Kang, E. T. Macromolecules 2010, 43, 10275-10282. (30) Shim, S. E.; Yang, S.; Jung, H.; Choe, S. Macromol. Res. 2004, 12, 233–239. (31) Gong, T.; Wang, C. C. J. Mater. Sci. 2008, 43, 1926–1932.

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Figure 2. FE-SEM photographs of CPS with various DVB proportions: (a) 0 wt %, (b) 5 wt %, (c) 15 wt %, (d) 25 wt %, and (e) 35 wt %.

Figure 3. Size distributions of CPS with various DVB proportions suspended in H2O: (a) 0 wt %, (b) 5 wt %, (c) 15 wt %, (d) 25 wt %, and (e) 35 wt %.

data, the attached amount of bromine atoms was 0.05% (Table 2). The ESCA results give evidence of the presence of bromo-initiators on surfaces and that the reaction proceeded with grafting of polymer brushes via surface-initiated atom transfer radical polymerization (SI-ATRP). The grafting process of PMMA brushes was carried out from the CPI surfaces by SI-ATRP. The ESCA results of the elemental surface stoichiometries were evaluated from the sensitivity factor-corrected spectral area ratios. As listed in Table 2, the O Langmuir 2011, 27(3), 1168–1174

concentration of CPB was higher than that of CPI due to the addition of the homo-PMMA brushes grafted on CPI surfaces. After the grafting process, the number-average molecular weight (Mn) and polydispersity index of the PMMA brushes were 17 200 and 1.38, respectively. The grafting density of the CPB could be calculated based on eq 4 which was 0.89 chains/nm2. In addition, the particle diameter characteristics of CPI and CPB are shown in Figure 7. From the FE-SEM photographs, the slight necking between the polymer spheres of CPB appeared due to the DOI: 10.1021/la103560j

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Figure 6. C 1s ESCA spectra of CPS and CPI. Table 2. Characterization of CPI and CPB electron levela code

C 1s (%)

O 1s (%)

Br 3d (%)

dn (nm)

dh (nm)b

Cv (%)

CPI 86.8 13.1 0.05 202 206 3.04 CPB 83.1 16.8 0.06 216 322 2.89 a Surface percentages are mole percents of each type of surface atom calculated using survey spectral data. b Measured by DLS in THF.

Figure 4. FE-SEM photographs of CPS with various DVB proportions for THF solvent resistance test: (a) 5 wt %, (b) 15 wt %, (c) 25 wt %, and (d) 35 wt %.

Figure 5. Conversion versus polymerization time for CPS with DVB proportion of 35 wt %.

homo-PMMA brushes on surface. The average dn (n = 200) for CPI and CPB is shown in Table 2. This value of dn suggests that the thickness of the grafted brush layer was 7 nm. Furthermore, dh increased from 206 to 322 nm in THF, which shows that the grafted PMMA brushes can greatly expand in THF. These results indicate that the monodisperse CPI could be successfully prepared and employed for grafting polymer brushes. Optical Properties of Colloidal Crystals. The monodisperse colloidal polymers were fabricated by the evaporation 1172 DOI: 10.1021/la103560j

method to form colloidal crystal films. The brilliant color of colloidal crystal films covering the visible regions could be controlled through the various particle diameters. Figure 8a shows color photographs of colloidal crystal films with various particle diameters. The CPS0, CPS15, and CPS35 films exhibited brilliant colors of red, green, and blue-green, respectively. Colloidal crystal films could exhibit brilliant color due to prevention of the propagation of electromagnetic waves in a particular frequency range through the crystal structure in the visible light region. Figure 8b shows the reflection spectra of the CPS0, CPS15, and CPS35 films at and incident angle of 0°. As clearly seen in Figure 8b, the reflection wavelengths in the visible region were controlled by varying the particle diameter. Furthermore, the maximum reflected intensity decreased moving from CPS0 to CPS35 due to the influence of defects such as cracks, point defects, and dislocations (Figure 2). The measured wavelengths of maximum reflected intensity (λmax) are listed in Table 3. In addition, the theoretical wavelength (λcal) of the colloidal crystal films could be estimated from the modified Bragg’s law, which describes the band gap position of a face-centered cubic (FCC) lattice. As shown in Table 3, the peak reflected intensity wavelength (λmax) was greater than the value predicted form the modified Bragg’s law (λcal) because of an increase in the distance between colloidal crystals for FCC lattice with close-packed structure (Figures 2 and 7). Nevertheless, agreement is good, which suggests that Bragg’s law is practical for the colloidal crystals. From the modified Bragg’s law for the FCC structure, two parameters (the particle diameter and incidence angle) determine λmax. Therefore, the varying incident angles of CPS0 colloidal crystal film were evaluated as shown in Figure 9. As shown in Figure 9, λmax for incidence at 20°, 40°, 50°, and 60° on the CPS0 film was 635 nm (red region), 584 nm (yellow region), 555 nm (green region), and 519 nm (green region). Clearly, λmax exhibited significant blue-shift with an increase in the incidence angle. For PMMA brush grafted CPI, Figure 10 shows color photographs and the reflection spectra of colloidal crystal films for CPI Langmuir 2011, 27(3), 1168–1174

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Figure 7. FE-SEM photographs and size distributions of CPI (a, a0 ) and CPB (b, b0 ) suspended in THF. Table 3. Optical Properties of the Colloidal Crystal Films code

dn (nm)

λcal (nm)a

λmax (nm)b

color

CPS0 277 619 642 red CPS15 234 528 549 green CPS35 212 484 491 blue-green CPI 202 462 482 blue CPB 216 480 501 green a λcal was determined from the diameters calculated through Bragg’s law. b λmax was determined from the wavelength of reflection.

Figure 8. Color photographs (a) and reflection spectra (b) of colloidal crystal films in various particle diameters.

and CPB, respectively. The CPI and CPB films exhibited brilliant blue and green colors, as shown in Figure 10a, with λmax at an incident angle of 0° being 482 nm (blue region) and 501 nm (green region), respectively, as shown in Figure 10b. The additional 7 nm thick brush layer on CPB should lead to a longer λmax compared Langmuir 2011, 27(3), 1168–1174

Figure 9. Reflection spectra of CPS0 colloidal crystal film with various incident angles.

with CPI. From these results, the PMMA brushes could be successfully grafted on polymer spheres and applied in colloidal crystals. DOI: 10.1021/la103560j

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Conclusions Monodisperse colloidal spheres with organic-solvent tolerance were successfully prepared by soap-free emulsion polymerization when the DVB proportion was higher than 25 wt %. Furthermore, various particle diameters were obtained ranging from 212 to 277 nm, by varying the proportions of MMA and DVB in the feed. Surface-immobilized bromo-initiators were obtained using CBr4 as a chain transfer agent. These modified spheres could be employed for grafting polymer brushes through surface-initiated atom transfer radical polymerization (SI-ATRP). The average thickness (n = 200) of the grafted PMMA brush layer was 7 nm, which compared with the grafted PMMA brushes could greatly expand in THF. Colloidal crystal films of these spheres exhibited brilliant colors in the visible region, which could be controlled by varying the particle diameter. Additionally, significant blue-shift was seen with increasing incidence angle for colloid crystals. The results of this study provide useful information for facile preparation of surface-immobilized ATRP initiators on colloidal polymers. Furthermore, these spheres can be employed for grafting various functionalities of polymer brushes and will therefore find wide application in various fields. Figure 10. Color photographs (a) and reflection spectra (b) of colloidal crystal films for CPI and CPB.

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Acknowledgment. This work was financially supported by the National Science Council of Taiwan.

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