Multilayered Composite Microgels Synthesized by Surfactant-Free

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Multilayered Composite Microgels Synthesized by Surfactant-Free Seeded Polymerization Daisuke Suzuki,* Tomoyo Yamagata, and Masaki Murai Graduate School of Textile Science & Technology, Shinshu University, 3-15-1, Tokida Ueda 386-8567 Japan S Supporting Information *

ABSTRACT: We report on a simple and rapid method to produce multilayered composite microgels. Thermosensitive microgels were synthesized by aqueous free radical precipitation polymerization using N-isopropylacrylamide (NIPAm) as a monomer. Using the microgels as cores, surfactant-free seeded polymerization of an oil-soluble monomer, glycidyl methacrylate (GMA), was carried out at 70 °C, where the microgels were highly deswollen in water. All of the oil-soluble monomers were polymerized, and the resultant polymers were attached on the pre-existing microgel cores, resulting in hard shell formation. It is worth mentioning that secondary particles of oil-soluble monomers have never been formed during the polymerization. The composite microgels were characterized by electron microscopy and dynamic light scattering. In particular, X-ray photoelectron spectroscopy (XPS) measurements revealed that the surface of the composite microgels was composed of a hydrogel layer, although microgel cores were covered by polyGMA shell. The mechanism of the trilayered composite microgel formation will be discussed.

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INTRODUCTION Design and synthesis of functional polymer particles has been an important research field to realize the next generation of advanced technologies In particular, recent research has focused on hydrogel particles (microgels), which are cross-linked, highly water-swellable particles.1−5 One of the most studied microgels is composed of poly(N-isopropylacrylamide) (pNIPAm), which is a representative thermo-responsive polymer exhibiting a lower critical solution temperature (LCST) at ∼31 °C.6,7 Microgels composed of pNIPAm show a volume phase transition temperature (VPTT) around the LCST.1 Microgels have been widely studied materials since they are considered to be the potential future of applications such as target drug/gene delivery,8−10 photonic crystals,11−13 and self-beating micropumps.14 To achieve these goals, construction of well-defined structured particles such as core/shell and Janus microgels have been proposed.15,16 To make microgels more attractive for future applications, complexation with other chemical compounds is useful. Composite microgels with inorganic nanoparticles or organic materials have been widely studied.17−26 For instance, Kumacheva et al. reported on a comprehensive study of synthesizing inorganic nanoparticles (CdS, fluorescent and nonfluoresent Ag, and Fe3O4) in poly(NIPAm-acrylic acid-2hydroxyethyl acrylate) microgels.17,18 Kawaguchi et al., have reported on hairy particles: pNIPAm linear chains were grafted from iniferter-immobilized polystyrene cores.24 More recently, the hydrogel core/hydrogel shell particles (i.e., core/shell microgels) have received great attention since Lyon’s group reported pNIPAm core/p(NIPAm-co-acrylic acid) shell microgels and the inverse microgels.16,25,26 Among them, multi© XXXX American Chemical Society

layered composite particles are of particular interest due to their potential applications as photonic and electronic devices.27,28 For instance, theoretical calculations indicate that a colloidal crystal of hybrid particles composed of refractive index periodicity can show a full photonic band gap.29 To achieve this, we tried to synthesize hybrid microgels with inorganic nanoparticles formed in situ using poly(glycidyl methacrylate) core/pNIPAm shell composite microgels as templates, and revealed that the template enabled the fabrication of a hybrid microgel in which a stable Au nanoshell was formed in situ at the surface of a rigid core. 22 However, creation of monodispersed composite particles with multi core/shell structures is a still challenge with regard to developing next generation advanced materials.28 In this work, we demonstrate that multilayered composite particles with soft core/hard shell/soft outer shell can be synthesized by seeded polymerization of an oil-soluble monomer, in the presence of deswollen microgels (Scheme 1). Although there are numerous reports on seeded precipitation polymerization of a water-soluble monomer using deswollen microgels to obtain soft core/soft shell particles,3,15,23,25,26,30,31 hardly any works on the seeded polymerization using an oil-soluble monomer on microgels have been conducted in detail. First, thermosensitive pNIPAm microgels were synthesized by aqueous free radical precipitation polymerization in water.32,33 Using the pNIPAm microgels as cores, surfactant-free seeded polymerization of Received: July 7, 2013 Revised: July 22, 2013

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Scheme 1. Synthetic Approach Used for the Multilayered Composite Microgels

filter (Sartorius Ltd., φ5 μm) to remove any aggregates. Finally, the dispersions were cleaned by means of dialysis for a day. Synthesis of Poly-GMA Particles by Surfactant-Free Emulsion Polymerization. For a control experiment, poly-GMA particles were synthesized by surfactant-free emulsion polymerization in the absence of the microgels. 95 mL of water was poured into a 300 mL three-neck, round-bottom flask equipped with a mechanical stirrer, a condenser, and nitrogen gas inlet. Water was bubbled for 30 min with nitrogen gas to purge oxygen at 70 °C. Under a stream of nitrogen and with constant stirring at 250 rpm, the initiator APS (0.091 g) dissolved in 5 mL of water was injected to the flask. After 5 min, a mixture of water (100 mL) which had been bubbled for 30 min with nitrogen gas and GMA (5.99 g, 200 mM) was poured into the flask to start the polymerization. Then the polymerization continued for 4 h. After the polymerization, the microgel dispersion was cooled to room temperature. The microgels were purified by centrifugation/redispersion with water twice using a relative centrifugal force (RCF) of 20 000g at 25 °C and by means of dialysis for a day. Mixing of Presynthesized Poly-GMA Particles and Deswollen pNIPAm Microgels. Small poly-GMA particles (∼70 nm, 0.1 wt %) and the deswollen pNIPAm microgels (0.85 wt %) were mixed in water at 70 °C for 30 min. After that, 1.5 μL of the mixture was dried on the polystyrene substrate to observe by SEM. Here, the small polyGMA particles were synthesized by adding hydroquinone to stop the polymerization after 1 min from the initiation. The polymerization condition for poly-GMA particles is the same as shown above. Characterization. The hydrodynamic diameter of microgels was characterized by dynamic light scattering (DLS, Malvern Instruments Ltd., ZetasizerNanoS). Data were an average of 15 measurements with 30 s acquisition times. Microgels were analyzed at a concentration of ∼0.005 wt %. NaCl was then used to adjust each dispersion to 1 mM total salt concentration unless noted otherwise. The samples were allowed to equilibrate thermally at the desired temperature for 10 min before the measurements. Scattered light collected at 173°. The hydrodynamic radii of microgels were determined from the measured diffusion coefficients by using the Stokes−Einstein equation (Zetasizer software v6.12). The microgels in the dried state were observed by field emission scanning electron microscopy (FE-SEM, Hitachi Ltd., S5000). Diluted microgel dispersions were dried on a polystyrene substrate. The samples were sputtered with Pt/Pd before observation. The ultrathin cross sections of the composite microgels were observed by transmission electron microscopy (TEM, JEOL Ltd., JEM2100). The composite microgels were dispersed in epoxy matrix. Then, the matrixes were cured at 35 °C for 20 h, and then at 60 °C for 24 h, and were microtomed for the observation. Microgels in an aqueous solution were observed with an optical microscope (BX51, Olympus) equipped with a digital camera (ImageX Earth type S-2.0 M ver.3.0.5, Kikuchi-Optical Co., Ltd.) and a temperature controller (NCB-1200, Eyela). Microgels were transferred into Vitrotube borosilicate rectangular capillaries (0.1 × 2.0 mm) by capillary action. The electrophoretic mobility (EPM) of microgels was measured with a

glycidyl methacrylate (GMA) was carried out at 70 °C where the microgels were highly deswollen state but still contained water. The obtained composite microgels were characterized by electron microscopy and dynamic light scattering. In particular, to clarify the surface structure of the composite microgels, X-ray photoelectron spectroscopy (XPS) was used for freeze-dried samples. Additionally, electrophoretic mobility was measured to estimate the surface softness of the composite microgels. Through the experiments, the mechanism of the seeded polymerization will be discussed.

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EXPERIMENTAL SECTION

Materials. All reagents were purchased from Wako Pure Chemical Industries and were used as received. Water for all reactions, solution preparation, and polymer purification was first distilled then ionexchanged (EYELA, SA-2100E1). Synthesis of Core Microgels. Chemically cross-linked core microgels were prepared by temperature-programmed precipitation polymerization as previously reported.33 A mixture of N-isopropylacrylamide (NIPAm, 8.402 g, 99 mol %), N,N′-methylenebis(acrylamide) (BIS, 0.116 g, 1 mol %), and water (497 mL) was poured into a 1000 mL three-neck, round-bottom flask equipped with a mechanical stirrer, a condenser, and nitrogen gas inlet. The total monomer concentration was fixed at 150 mM. The monomer solution was bubbled for 30 min with nitrogen gas to purge oxygen at 40 °C. Under a stream of nitrogen and with constant stirring at 250 rpm, the initiator ammonium peroxodisulfate (APS: 0.228 g) dissolved in 3 mL of water was injected to the flask to start the polymerization. After 15 min, the temperature was increased from 40 to 70 °C at a rate of ∼2 °C/min. Then the polymerization continued for 4 h. After the polymerization, the microgel dispersion was cooled to room temperature. The microgels were purified by centrifugation/redispersion with water twice using a relative centrifugal force (RCF) of 50 000g at 25 °C and by means of dialysis for a day. Surfactant-Free Seeded Polymerization of Glycidyl Methacrylate (GMA) in the Presence of Deswollen Core Microgels. Twenty mL of water was poured into a 200 mL three-neck, roundbottom flask equipped with a mechanical stirrer, a condenser, and nitrogen gas inlet. Water was bubbled for 30 min with nitrogen gas to purge oxygen at 70 °C. Under a stream of nitrogen and with constant stirring at 250 rpm, the core microgels (0.85 g in 25 mL of water) was poured into the flask, then 0.046 g of APS initiator dissolved in 5 mL of water was injected to the flask. After that, a mixture of glycidyl methacrylate (GMA) and 50 mL of water which had been bubbled for 30 min with nitrogen gas was poured into the flask to start the polymerization. Then the polymerization continued for 4 h. After the polymerization, the dispersion was cooled to room temperature. The composite microgels were purified with water twice using a RCF of 20 000g at 25 °C. Then the dispersions were filtered through a Minisart B

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ZetasizerNanoZS (Malvern, Zetasizer software ver.4.20). The samples were allowed to equilibrate thermally at the desired temperature for 10 min before the measurements. X-ray photoelectron spectroscopy (XPS) spectra were obtained on a spectrometer with a Mg Kα X-ray source (Shimazu Co., KRATOS AXIS Ultra DLD).

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RESULTS AND DISCUSSION First, core microgels of pNIPAm cross-linked chemically by N,N′-methylenebis(acrylamide) (BIS) were synthesized by aqueous free radical precipitation polymerization.1,32,33 Optical microscope observation revealed that they were monodispersed, and the diameter was ∼1.0 μm in their swollen state (Figure 1). In addition, the hydrodynamic diameter of the

Figure 1. Optical microscope image of colloidal crystal of core pNIPAm microgels assembled in deionized water at 25 °C. Microgel concentration is 4.0 wt %. Inset is photograph of the microgel assembly in the rectangular capillary tube.

Figure 2. Electron microscope observation: SEM images of (a) N, (b) NG20, (c) NG60, and (d) NG120 microgels dried on polystyrene substrate at 25 °C. Insets are close-up views of each microgel. TEM images of ultrathin cross sections of (e) NG60 and (f) NG120 microgels.

microgels was changed as a function of temperature, and showed abrupt change at the volume phase transition temperature (VPTT) of ∼32 °C (1005 nm at 25 °C and 422 nm at 70 °C, respectively.). As shown in Figure 1, these microgels were very monodispersed, which can be confirmed by colloidal crystal formation in the rectangular glass capillary shown in inset. Using the pNIPAm microgels as cores, surfactant-free seeded polymerization of glycidyl methacrylate (GMA) was carried out at 70 °C where microgels were highly contracted (water content ∼30%).1,34,35 GMA was chosen as a model oil-soluble monomer because poly-GMA particles with uniform size distribution can be synthesized by surfactant-free emulsion polymerization (shown below). In addition, GMA can be modified by functional materials (e.g., DNA, protein), which could lead to future applications such as chemical separation. First, influence of GMA concentration on the polymerization was checked. Figure 2 shows representative SEM images of core (N) and composite microgels (NG-X). In the sample code, N and G stand for NIPAm and GMA, respectively, while the number, X, following each letter represents the GMA concentration in the polymerization. Because of the evaporation of water during sample preparation, core microgels were largely deformed (Figure 2(a)). NG20 microgels were composite ones with small GMA particles as shown in Figure 2(b) Herein, GMA concentration is 20 mM during the seeded polymerization. With increasing GMA concentration, small GMA particles could not be observed, and the composite microgels were slightly deformed (NG60 (i.e., GMA concentration is 60 mM), Figure 2(c)). Lastly, spherical shaped particles were formed (NG120 (i.e., GMA concentration is 120 mM), Figure 2(d)). In this case, macroscopic aggregates were formed when the polymerization was carried out at higher GMA concentrations (more than 150 mM). Surprisingly,

secondary particles of GMA have never been observed after the seeded polymerization of GMA irrespective of the GMA concentrations (10−120 mM, Figure 2(b)−(d)). Taking into account that all GMA was polymerized within the polymerization time (4 h) in all cases, all poly-GMA must be composited with the core microgels. Therefore, a hard shell of poly-GMA must be formed on the pre-existing core microgels when the GMA concentrations were 60 and 120 mM. Note that all samples were not purified before the sample preparation for SEM observation in Figure 2 in order to check the absence of secondary particles, but the appearance of the samples was almost the same before and after the purification (see SI Figure 1). Figure 2(e,f) shows ultrathin cross sections of the composite microgels, (e) NG60 and (f) NG120, observed by TEM. For both samples, there were strong contrasts between core and shell, indicating that poly-GMA shells were indeed formed on the core microgels. In particular, shell thickness of NG120 microgels can be estimated to be ∼90 nm. To clarify the mechanism of the seeded polymerization, time dependence of NG120 microgel formation was checked by SEM (Figure 3). Within 1 min, small poly-GMA particles (∼50 nm) were formed, and all of them were fixed on the deformed core microgels (Figure 3(a)). After 10 min, the composite microgels became spherical due to the growth of poly-GMA particles attached on the microgels (Figure 3 (b)). Then, the size of the composite microgels was not changed largely until the end of the procedure (4 h, Figure 3 (d)), indicating that these composite microgels can be synthesized within a short period of time. Control experiment of poly-GMA particle synthesis without the core microgels showed that small polyC

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Figure 5. SEM image of a mixture of core microgels N (0.85 wt %), and small poly-GMA particles (0.1 wt %). These dispersions were mixed at 70 °C for 30 min. 1.5 μL of the mixture was dried on the polystyrene substrate to observe by SEM.

carried out further experiments to check the partitioning effect of GMA into the deswollen microgels. The water solubility of GMA is ∼120 mM at 25 °C,36 and theoretically, all of the GMA can be dissolved in water for all samples. Nonetheless, GMA droplets can be seen in water at 70 °C when GMA concentration is 120 mM (see SI Figure 2). Thus, a portion of GMA dissolved in water can partition into the deswollen microgels. First, as a control experiment, GMA and the deswollen pNIPAm microgels were mixed before the initiation of the seeded polymerization. (In this study, all of the seeded polymerizations were started by injecting GMA/water mixtures to the reactor, thus, GMA did not partition into the deswollen pNIPAm microgels before the seeded polymerization. See the Experimental Section.) As a result, the morphology of the composite microgel was largely changed from that shown in Figure 2 (d), and secondary particles were formed (see SI Figure 3(a)). Next, to remove the excess GMA which dissolved in water but did not partition into the deswollen microgels, the deswollen microgels were centrifuged and redispersed in hot water. Then, the seeded polymerization was carried out. The composite microgels could be obtained, but the morphology of them is not spherical (see SI Figure 3(b)). Additionally, secondary particles were not formed after the removal of the excess GMA dissolved in water. These data indicate that the partition of GMA monomers is important factor for the seeded polymerization, but the mechanism is not simple and may contain another process such as adsorption and insertion of poly-GMA small particles synthesized in aqueous phase into the exterior of the deswollen microgels during the polymerization. Further experiments, such as the effect of the swelling degree of the core microgels and the kinds of monomers are now being investigated to clarify the whole mechanism of the seeded polymerization, and will be reported elsewhere. Next, hydrodynamic diameters of these microgels were measured as a function of temperature by means of dynamic light scattering (DLS) (Figure 6). As is commonly known, core microgels of pNIPAm showed volume phase transition at 32 °C.1 All NG composite microgels also showed the thermoresponsive deswelling behavior. But the swelling ratio (i.e., V25 °C/ V50 °C) of these composite microgels was smaller than that of the parent microgels (13.7 for N, 5.3 for NG20, 3.8 for NG60, 2.5 for NG120). Jones and Lyon have reported on shellrestricted swelling of pNIPAm core microgels using soft core/ soft shell microgels.37 Similar to the soft core/soft shell microgels, hydrodynamic diameters at the swollen states (i.e., 25 °C) are complex for the NG composite microgels: after the seeded polymerization, swelling of core pNIPAm microgels

Figure 3. SEM images of NG120 microgels at different polymerization times. (a) 1 min, (b) 10 min (c) 60 min, and (d) 240 min from the initiation of the polymerization. Insets are close-up views of each microgel.

GMA particles (∼70 nm) were formed within 1 min, and the growth of poly-GMA particles was completed after 20 min (Figure 4). Considering the results of the control experiment,

Figure 4. SEM images of poly-GMA particles synthesized by surfactant-free emulsion polymerization. A small drop of the dispersion was obtained from the reactor at different polymerization times: (a) 1 min, (b) 5 min, (c) 20 min, and (d) 240 min. The mean diameters of these particles are (a) 76 nm, (b) 115 nm, (c) 334 nm, and (d) 338 nm (n = 50).

the mechanism of the composite microgel formation might be due to adsorption of small poly-GMA particles on the deswollen microgels. However, it is questionable that all of poly-GMA particles formed in an aqueous phase are adsorbed within 1 min. Therefore, we carried out an additional control experiment: the deswollen microgels and presynthesized polyGMA particles (∼70 nm) were mixed for 30 min, and we found that many poly-GMA particles were not adsorbed on the microgels (Figure 5). This implies that poly-GMA particles were formed by the assistance of the deswollen microgels. We D

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Table 1. Electrophoretic Mobilities (EPM) of Core (N), Composite Microgels (NG), and Poly-GMA Particles (G) in a 1 mM NaCl Solution EMP (10−8m2V−1s−1) code N NG20 NG60 NG120 G

25 °C −0.17 −0.54 −0.72 −0.74 −3.74

(±0.005) (±0.02) (±0.03 (±0.05) (±0.02)

40 °C −3.71 −4.29 −4.19 −4.42 −4.45

(±0.05) (±0.05) (±0.01) (±0.05) (±0.03)

coated with a hydrogel layer show a low value of EPM.38 Slight increases in EPM with GMA concentration increases during the polymerization indicate that the outer hydrogel shells became thinner. The formation of the outer shell of pNIPAm may be explained thermodynamically: during a seeded polymerization, core/shell particles with a more hydrophilic polymer forming the cores and a more hydrophobic polymer forming the shells is thermodynamically unstable in water, resulting in phase inversion.39 Therefore, the more hydrophobic shell of polyGMA migrated into deswollen microgels to some extent during the polymerization. Another possible formation mechanism is as follows: pNIPAm adsorbs a lot of water and pushes the polyGMA domains to the outer regions of the composite microgels. Thus, some pNIPAm chains are on the surface of the polyGMA shell. In order to clarify it, the dispersion of NG120 microgels was deposited on the polystyrene substrate at 70 °C before cooling. We could confirm that the morphology of NG120 microgels was not changed before and after cooling (see SI Figure 4). Therefore, the latter mechanism seems not to be the dominant mechanism for the seeded polymerization. The role of the outer shell of pNIPAm is versatile: high colloidal stability of the composite particles in their swollen state can be tuned simply by increasing the temperature above VPTT (Figure 8), for example.

Figure 6. Deswelling curves of hydrodynamic diameter for N (gray), NG20 (red), NG60 (blue), and NG120 microgels (yellow).

should be restricted by the presence of poly-GMA shell. Therefore, the hydrodynamic diameter at 25 °C is not necessarily increased with increasing the GMA monomer concentration. Most importantly, if the core microgels were covered with poly-GMA completely, thermo-sensitive volume change should be highly restricted, and the microgels should not show thermo-sensitive volume changes. Therefore, there are two possibilities for the structure of the composite microgels: (1) Core microgels are not covered with polyGMA completely. (2) A pNIPAm shell exists on the poly-GMA complete shell. Taking into account the electron microscope observation as shown in Figure 2, the reason for the thermosensitivity of NG20 must be the former (1). However, the reason for the thermo-sensitive volume changes of NG60 and NG120 microgels may be the latter (2), because the poly-GMA shell can be confirmed by the ultrathin cross sections as shown in Figure 2(e,f). In order to reveal the surface structures of the composite microgels, X-ray photoelectron spectroscopy (XPS) measurements were conducted using freeze-dried samples (Figure 7). As expected, NG60 and NG120 microgels showed

Figure 8. Observation of NG120 microgel dispersion (NaCl 150 mM) by changing the temperature from 25 °C to 70 °C to 25 °C. Note that the colloidal stability of the dispersion could be tuned by changing the temperature due to the presence of a thermosensitive, pNIPAm outer layer on the composite microgels.

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Figure 7. X-ray photoelectron spectra recorded for N (gray), NG20 (red), NG60 (blue), NG120 (yellow), and poly-GMA (G) particles.

CONCLUSIONS We discovered a simple and rapid method to produce composite microgels with trilayered structures. The composite microgels could be obtained by surfactant-free seeded polymerization of an oil-soluble monomer, GMA, in the presence of the deswollen pNIPAm microgels. Electron microscope observation showed that the composite microgels possess a distinct hard shell of oil-soluble chemical species. In addition, surface analysis revealed that the surface of the composite microgels is

that the nitrogen signal originated from pNIPAm as well as N, N20 microgels. This means that the structure of NG60 and NG120 microgels is as follows: hydrogel core/hard shell/ hydrogel outer shell, as shown in Scheme 1. Further evidence of the complex structures are the low values of the electrophoretic mobility (EPM) of the NG composite microgels in their swollen state, as shown in Table 1: particles whose surfaces are E

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(12) Hellweg, T.; Dewhurst, C. D.; Brúckner, E.; Kratz, K.; Eimer, W. Colloidal Crystals Made of Poly(N-isopropylacrylamide) Microgel Particles. Colloid Polym. Sci. 2000, 278, 972−978. (13) Lyon, L. A.; Debord, J. D.; Debord, S. B.; Jones, C. D.; McGrath, J. G.; Serpe, M. J. Microgel Colloidal Crystals. J. Phys. Chem. B 2004, 108, 19099−19108. (14) Suzuki, D.; Kobayashi, T.; Yoshida, R.; Hirai, T. Soft Actuators of Organized Self-Oscillating Microgels. Soft Matter 2012, 8, 11447− 11449. (15) Hu, X.; Tong, Z.; Lyon, L. A. Multicompartment Core/Shell Microgels. J. Am. Chem. Soc. 2010, 132, 11470−11472. (16) Suzuki, D.; Tsuji, S.; Kawaguchi, H. Janus Microgels Prepared by Surfactant-Free Pickering Emulsion-Based Modification and Their Self-Assembly. J. Am. Chem. Soc. 2007, 129, 8088−8089. (17) Zhang, J.; Xu, S.; Kumacheva, E. Polymer Microgels: Reactors for Semiconductor, Metal, and Magnetic Nanoparticles. J. Am. Chem. Soc. 2004, 126, 7908−7914. (18) Zhang, J.; Xu, S.; Kumacheva, E. Photogeneration of Fluorescent Silver Nanoclusters in Polymer Microgels. Adv. Mater. 2005, 17, 2336−2340. (19) Lu, Y.; Ballauff, M. Thermosensitive Core−Shell Microgels: From Colloidal Model Systems to Nanoreactors. Prog. Polym. Sci. 2011, 36, 767−792. (20) Wu, S.; Dzubiella, J.; Kaiser, J.; Drechsler, M.; Guo, X.; Ballauff, M.; Lu, Y. Thermosensitive Au-PNIPA Yolk-Shell Nanoparticles with Tunable Selectivity for Catalysis. Angew. Chem., Int. Ed. 2012, 51, 2229−2233. (21) Suzuki, D.; Kawaguchi, H. Modification of Gold Nanoparticle Composite Nanostructures Using Thermosensitive Core−Shell Particles as a Template. Langmuir 2005, 21, 8175−8179. (22) Suzuki, D.; Kawaguchi, H. Gold Nanoparticle Localization at the Core Surface by Using Thermosensitive Core−Shell Particles as a Template. Langmuir 2005, 21, 12016−12024. (23) Suzuki, D.; McGrath, J. G.; Kawaguchi, H.; Lyon, L. A. Colloidal Crystals of Thermosensitive, Core/Shell Hybrid Microgels. J. Phys. Chem. C 2007, 111, 5667−5672. (24) Tsuji, S.; Kawaguchi, H. Temperature-Sensitive Hairy Particles Prepared by Living Radical Graft Polymerization. Langmuir 2004, 20, 2449−2455. (25) Jones, C. D.; Lyon, L. A. Synthesis and Characterization of Multiresponsive Core-Shell Microgels. Macromolecules 2000, 33, 8301−8306. (26) Hendrickson, G. R.; Smith, M. H.; South, A. B.; Lyon, L. A. Design of Multiresponsive Hydrogel Particles and Assemblies. Adv. Funct. Mater. 2010, 20, 1697−1712. (27) Graf, C.; van Blaaderen, A. Metallodielectric Colloidal CoreShell Particles for Photonic Applications. Langmuir 2002, 18, 524− 534. (28) Soukoulis, C. M.; Wegener, M. Past Achievements and Future Challenges in the Development of Three-Dimensional Photonic Metamaterials. Nat. Photonics 2011, 5, 523−530. (29) Moroz, A. Three-Dimensional Complete Photonic-Band-Gap Structures in the Visible. Phys. Rev. Lett. 1999, 83, 5274−5277. (30) Berndt, I.; Pedersen, J. S.; Richtering, W. Structure of Multiresponsive Intelligent Core-Shell Microgels. J. Am. Chem. Soc. 2005, 127, 9372−9373. (31) Richtering, W.; Pich, A. The Special Behaviours of Responsive Core-Shell Nanogels. Soft Matter 2012, 8, 11423−11430. (32) Pelton, R. H.; Chibante, P. Preparation of Aqueous Lattices with N-isopropylacrylamide. Colloids Surf. 1986, 20, 247−256. (33) Meng, Z.; Smith, M. H.; Lyon, L. A. Temperature-Programmed Synthesis of Micron-Sized Multi-Responsive Microgels. Colloid Polym. Sci. 2009, 287, 277−285. (34) Pelton, R. Poly(N-isopropylacrylamide)(PNIPAM) is Never Hydrophobic. J. Colloid Interface Sci. 2010, 348, 673−674. (35) Suzuki, D.; Yamagata, T.; Horigome, K.; Shibata, K.; Tsuchida, A.; Okubo, T. Colloidal Crystallization of Thermo-Sensitive Gel Spheres of Poly(N-isopropyl acrylamide). Influence of Gel Size. Colloid Polym. Sci. 2012, 290, 107−117.

composed of a hydrogel layer despite the fact that core microgels are indeed covered with a hard shell. Although further investigations to clarify the formation mechanism and the structure of the composite microgels are necessary, this study revealed that GMA can be polymerized and coated onto the deswollen pNIPAm hydrogel particles without secondary particle formation. We believe that the seeded polymerization using microgels will lead to the creation of unique-shaped composite microgels.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of purified NG composite microgels (SI Figure 1), photographs of the mixture of GMA and water (SI Figure 2), SEM images of the composite microgels synthesized after mixing GMA and the deswollen pNIPAm microgels or removing excess GMA by centrifugation (SI Figure 3), SEM images of NG120 microgels before cooling, and after a cooling/ heating cycle (SI Figure 4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.S. acknowledges a Grant-in-Aid for Young Scientists (A) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (22685024). Yasuhisa Nagase is acknowledged for his help in TEM observation.

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REFERENCES

(1) Pelton, R. Temperature-Sensitive Aqueous Microgels. Adv. Colloid Interface Sci. 2000, 85, 1−33. (2) Kawaguchi, H. Functional Polymer Microspheres. Prog. Polym. Sci. 2000, 25, 1171−1210. (3) Nayak, S.; Lyon, L. A. Soft Nanotechnology with Soft Nanoparticles. Angew. Chem., Int. Ed. 2005, 44, 7686−7708. (4) Hoare, T.; Pelton, R. Characterizing Charge and Crosslinker Distributions in Polyelectrolyte Microgels. Curr. Opin. Colloid Interface Sci. 2008, 13, 413−428. (5) Richtering, W. Responsive Emulsions Stabilized by StimuliSensitive Microgels: Emulsions with Special Non-Pickering Properties. Langmuir 2012, 28, 17218−17229. (6) Heskins, M.; Guillet, J. E. Solution Properties of Poly(Nisopropylacrylamide). J. Macromol. Sci., Chem. 1968, A2, 1441−1455. (7) Schild, H. G. Poly(N-isopropylacrylamide)Experiment, Theory and Application. Prog. Polym. Sci. 1992, 17, 163−249. (8) Nayak, S.; Lee, H.; Chmielewski, J.; Lyon, L. A. Folate-Mediated Cell Targeting and Cytotoxicity Using Thermoresponsive Microgels. J. Am. Chem. Soc. 2004, 126, 10258−10259. (9) Bae, Y.; Jang, W. D.; Nishiyama, N.; Fukushima, S.; Kataoka, K. Multifunctional Polymeric Micelles with Folate-Mediated Cancer Cell Targeting and pH-Triggered Drug Releasing Properties for Active Intracellular Drug Delivery. Mol. Biosyst. 2005, 1, 242−250. (10) Guan, Y.; Zhang, Y. PNIPAM Microgels for Biomedical Applications: From Dispersed Particles to 3D Assemblies. Soft Matter 2011, 7, 6375−6384. (11) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Thermally Switchable Periodicities and Diffraction from Mesoscopically Ordered Materials. Science 1996, 274, 959−960. F

dx.doi.org/10.1021/la4025537 | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

(36) SciFinder® https://scifinder.cas.org/scifinder/view/scifinder/ scifinderExplore.jsf. (37) Jones, C. D.; Lyon, L. A. Shell-restricted Swelling and Core Compression in Poly(N-isopropylacrylamide) Core−Shell Microgels. Macromolecules 2003, 36, 1988−1993. (38) Makino, K.; Yamamoto, S.; Fujimoto, K.; Kawaguchi, H.; Ohshima, H. Surface-Structure of Latex-Particles Covered with Temperature-Sensitive Hydrogel Layers. J. Colloid Interface Sci. 1994, 166, 251−258. (39) Kirsch, S.; Doerk, A.; Bartsch, E.; Sillescu, H.; Landfester, K.; Spiess, H. W.; Maechtle, W. Synthesis and Characterization of Highly Cross-Linked, Monodisperse Core−Shell and Inverted Core−Shell Colloidal Particles. Polystyrene/Poly(tert-butyl Acrylate) Core−Shell and Inverse Core−Shell Particles. Macromolecules 1999, 32, 4508− 4518.

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dx.doi.org/10.1021/la4025537 | Langmuir XXXX, XXX, XXX−XXX