Imprinting Dimples on Narrowly Dispersed Polymeric Spheres by

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Imprinting dimples on narrowly dispersed polymeric spheres by heterocoagulation between hard polymer particles and soft oil droplets Kanji Kadowaki, Haruyuki Ishii, Daisuke Nagao, and Mikio Konno Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02688 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 14, 2016

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Imprinting dimples on narrowly dispersed polymeric spheres by heterocoagulation between hard polymer particles and soft oil droplets Kanji Kadowaki, Haruyuki Ishii, Daisuke Nagao,* Mikio Konno

Department of Chemical Engineering, Tohoku University 6-6-07 Aoba, Aramaki-aza Aoba-ku, Sendai, 980-8579 (Japan) E-mail: [email protected]

KEYWORDS. Dimple, Heterocoagulation, narrowly dispersed, Oil droplet.

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ABSTRACT. Golf ball-like particles having a number of dimples on their spherical surfaces were prepared by a combined method of a heterocoagulation between hard polymer particles and soft silicone oil droplets, a polymerization of the oil droplets and a dissolution of the polymer particles with tetrahydrofuran. In the heterocoagulation polystyrene (PSt) particles with three different sizes were employed as hard particles. Distribution of dimples formed with small-sized PSt particles (SPS) was less homogeneous than that with middle-sized particles (MPS). A high number ratio of MPS to oil droplets successfully prepared narrowly dispersed golf ball-like particles with dimples homogeneously distributed. The employment of large-sized particles (LPS) in the heterocoagulation decreased the number of PSt particles required for stabilization of the oil droplets, which created polyhedron-like particles having dimples on their surface. Additional experiments in which polymer particles with different surface affinities to the oil droplets were heterocoagulated with the droplets revealed that a high affinity surface of particles to the droplets could deeply embed the polymer particles into the droplets and form dimples with a low contact angle.

Introduction. Surface functionalization of materials has gained growing interests in industry and also in various scientific fields since key components of functional materials are downsized to have a large surface-to-volume ratio in a meso- or micro-scale these days. Control over surface roughness of materials is an effective functionalization in hydrophobic coatings1,2, friction reducing coatings 3 and optical measurements.4 The functionalization is also applicable to particulate products. Golf ball-like particles that have a number of dimples on their spherical


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surfaces are a structural architectures to control surface features of materials for hydrophobic, tribological and optical properties.5-8 Building up of such monodisperse particles with surface morphologies well-defined is expected to be a promising material processing to develop new functional materials.9 An approach commonly used for synthesis of the golf ball-like particles is phase separation of polymers in seeded polymerization.8, 10, 11 In the polymerization, polymer domains that can be a template for the dimples of golf ball-like particles are formed at the surface of seed particles. Then, the polymer domains are selectively removed by dissolution with solvent good for the domain polymer. It was reported that the relative hydrophilicities of the seed polymer and the polymer formed in the seeded polymerization could have a high impact on the morphologies of particles finally obtained.8 Because of phase separation used in the methods, it is difficult to control the surface composition of golf ball-like particles independently of the morphologies of dimples. Another approach for golf ball-like particles is a heterocoagulation in which large-sized droplets are stabilized by a number of small-sized particles similarly to pickering emulsion.12, 13 Takahara et al. reported that toluene oil droplets containing hydrophobic polymers were heterocoagulated with silica particles to stabilize the oil droplets in water.6 The stabilizer particles used in the heterocoagulation were prepared in advance by partial surface-modification of hydrophilic silica particles with n-octadecyltrimethoxysilane on the interface between oil droplets and aqueous phase. Golf ball-like particles were created by a combination of a toluene evaporation and a dissolution of the silica particles partially surface-modified. However, golf ball-like particles with low polydispersity have not been prepared because a mechanical downsizing process of sonication was employed to disperse the polymer-containing droplets in water.


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To overcome this problem, the present work proposes a novel technique that employs a buildup process for preparing narrowly dispersed oil droplets applied to heterocoagulation with the stabilizer particles. Figure 1 shows our synthetic procedure for narrowly dispersed golf ball-like particles. Firstly, narrowly dispersed silicone oil droplets are synthesized by hydrolysis and condensation of 3-methacryloxypropylmethoxy silane (MPTMS) that has methacryl groups polymerizable in the droplets.14 On the other hand, polystyrene (PSt) particles used as stabilizer particles are prepared in soap-free emulsion polymerization with an amphoteric initiator of 2,2'azobis[N-(2-carboxyethyl)-2-methylpropionamidine] (VA- 057) in the absence of surfactants.15 The PSt particles were designed to be smaller than the silicone droplets and to be cationized in an acidic condition where the silicone droplets are anionic or almost zero-charged. Then, the narrowly dispersed droplets were electrostatically heterocoagulated with PSt particles and polymerized with potassium persulfate. Finally, the PSt particles on the polymerized droplets are dissolved with tetrahydrofuran solvent good for PSt. The particles obtained by the combined process of heterocoagulation and polymer dissolution were observed with a scanning electron microscope for examination of size distributions and surface morphologies of the polymerized droplets. Different stabilizer particles of PSt particles and polymethylmethacrylate (PMMA) particles were also prepared in soap-free emulsion polymerization in the presence and the absence of surfactant, respectively, to examine the surface affinity of stabilizer particles to silicone oil droplets on dimple formation in the combined process. For understating the mechanism on dimple formation, size and depth of the dimples formed with the different stabilizer particles were compared together with contact angles calculated from the dimple dimensions and the size of polymer particles.


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Experimental: Materials: 3-Methacryloxypropyltrimethoxysilane (MPTMS, 95%) was purchased from JNC Corporation (Tokyo, Japan). Styrene (St, 99%), methylmethacrylate (MMA, 98%), aqueous ammonia solution (25 wt%, NH3), sodium dodecylbenzenesulfonate (SDBS, 95 %), sodium hydroxide solution (0.1 mol/L analytical grade), hydrochloric acid solution (0.1 mol/L analytical grade) and potassium persulfate (KPS, 95%) were obtained from Wako Pure Chemical Industries (Osaka, Japan). The inhibitors of St and MMA were removed by inhibitor removal columns, and water was deionized in advance (>18.2 MΩ cm). The other chemicals were used as received. Preparation of MPTMS droplets: MPTMS oil-in-water emulsions were prepared according to the method by Obey et al.14, 16, 17 The hydrolysis of MPTMS was initiated by adding MPTMS to an aqueous ammonia solution under vigorous stirring. Deionized water bubbled with nitrogen for 30 min was used in preparation of the ammonia solution. The mixed aqueous solution of MPTMS and ammonia was stirred for 4 min and kept for 24 h without stirring. The reaction of hydrolysis and condensation was conducted at 35°C with a reaction volume of 50 mL. The concentrations of MPTMS and ammonia in the solution were adjusted to 86 mM and 200 mM, respectively. Preparation of polymer particles: Narrowly dispersed PSt or PMMA particles with low impurity were prepared in soap-free emulsion polymerization according to our method previously reported14.

Briefly, the polymerization of St or MMA was initiated by adding an

aqueous solution of VA-057 initiator to an aqueous solution of monomer and sodium hydroxide. The polymerization was conducted at 65°C under stirring at 300 rpm. The concentration of St monomer was varied 0.044―0.354 M at the fixed NaOH concentration of 0.45 mM to obtain PSt particles with different sizes.


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Preparation of golf ball-like particles: The polymer particles prepared were electrostatically heterocoagulated with the silicone oil droplets by lowering the suspension pH with the addition of HCl solution at 35°C under stirring. The electrostatic heterocoagulation was conducted in an acidic suspension with a volume of 30 mL. After stirring the suspension for 15 min and heating it to 70°C, an aqueous solution of KPS was added to polymerize methacryl groups incorporated into the oil droplets. The addition of KPS was performed 30 min after the addition of HCl to induce the heterocoagulation. The polymerization initiated by the KPS was conducted under stirring for 6 h. The suspension obtained by the polymerization was centrifuged and redispersed in tetrahydrofuran to remove the polymer particles.

Results and discussion: Silicone oil droplets and PSt particles used in the heterocoagulation Narrowly dispersed silicone oil droplets were prepared by hydrolysis and condensation of 3methacryloxypropyltrimethoxysilane (MPTMS) in a basic solution of ammonia.14,

16, 17

The

initial concentrations of MPTMS and ammonia were 86 mM and 200 mM, respectively, to prepare approximately 1.0 µm oil droplets with low polydispersity. As shown in the optical microscope image and the size distribution in Figure 2, the oil droplets prepared had a narrow size distribution and an average size of dDLS = 1.1 µm in water. Zeta potentials of the oil droplets were approximately -50 mV at pH=11 and ±5 mV or less at pH 2. In heterocoagulations between two different particles electrostatically stabilized in aqueous solution, size ratio of the two particles is an important factor to selectively coagulate the small particles onto the large particles. For preparation of narrowly dispersed polymer particles with different particle sizes as designed, soap-free emulsion polymerization with styrene monomer


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was employed. Figure 3 presents SEM images of the PSt particles with average sizes of 213, 307 and 602 nm. Hereafter, the PSt particles with different sizes are indicated with SPS, MPS and LPS, respectively. Styrene concentration in the polymerizations was varied in the range of 0.044―0.354 M at the fixed NaOH concentration of 0.45 mM to adjust the size of PSt particles. Narrowly dispersed PSt particles with coefficients of variations (CV) lower than 4.0 % were obtained in the polymerizations. Another important factor for the selective heterocoagulation is the solution pH to change the electrostatic interaction between the small and large particles electrostatically destabilize the small particles in the presence of the large ones maintaining colloidal stability. Zeta potentials of the PSt particles measured in the pH range of 2―13 are plotted in Figure 4. The potential profiles exhibited isoelectric points (iep) in the pH range of 4―6, which was caused by VA-057 used as an amphoteric initiator in the polymerization to prepare PSt particles. According to the potential profiles, the PSt particles can be positively charged at pH values lower than 4 whereas they are anionizd at pH values higher than 6. The measurement of potentials suggested that a decrease in solution pH to acidic one could induce heterocoagulation between PSt particles and oil droplets dispersed in water because silicone oil droplets have silanol groups with isoelecrtric point around 2 on their surfaces are negatively charged in a wide pH range.

Formation of dimples on polymerized MPTMS for golf ball-like particles The effect of solution pH on the heterocoagulation was examined by the addition of HCl to MPS suspensions with the particle number ratio of 100 to oil droplets. According to the number ratio of Ncover = 79 that was estimated with the equation (1),18 the experimental number ratio of 100 was high enough to form a mono-layered coverage of MPS on the droplets.


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N cov er

2π  d Large + d small  = d small 3 

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  

2

(1)

where Ncover is the number of monolayered small particles having a diameter of dSmall and being hexagonally close-packed on the surface of large particles with a diameter of dLarge.18 An addition of HCl solution to the mixed suspensions (pH~11) of oil droplets and PSt particles in the process 1 of Fig. 1 was conducted 30 min before initiating the polymerization of oil droplets. The suspension pHs for the HCl additions at 30, 35, 40 and 45 mM were measured to 7.7, 2.6, 2.1 and 2.0, respectively. The raspberry-like particles formed by the heterocoagulation are presented in Fig. S1 of Supporting Information. Because of a large number of PSt particles unadsorbed, it was difficult to recognize the PSt particles adsorbed onto the oil droplets polymerized. To examine the formation of dimples on the polymerized particles, the raspberrylike particles in Fig. S1 were treated with THF to dissolve PSt particles adsorbed and unadsorbed. Figure 5 shows SEM images of golf ball-like particles obtained by the combined process of heterocoagulation and the dissolution with THF. The THF dissolution of raspberrylike particles in Fig. 1 selectively removed PSt component to form hemispherical dimples on the surfaces of products as presented in Fig. 5. Aggregated particles with a dimple feature were formed at [HCl] = 35 mM or lower, since the oil droplets could not maintain their colloidal stability during the heterocoagulation insufficiently lowering the pH. On the other hand, the HCl additions at 40 mM or higher could form golf ball-like particles having dimples homogeneously located on their surfaces. Although the difference in solution pH between Fig. 5(b) and (c) was as small as 0.5 after the HCl addition, the difference in pH at the high number ratio of 100 probably caused the difference in adsorption rate of polymer particles to form the different products shown in Fig. 5(b) and (c).


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The present heterocoagulation includes a dynamic process in which the suspension pH of oil droplets and PSt particles was decreased by the addition of HCl solution. Strongly acidic pHs around 2 are, therefore, not the starting point to initiate the heterocoagulation. The pH values in the range of 3―4 after the HCl addition cause the electrostatic heterocoagulation, meaning that the oil droplets and the PSt particles are being heterocoagulated with each other during the decrease in suspension pH. Another important aspect of the heterocoagulation is described in the previous reports.19, 20 It was shown in the reports that in the case of dissimilar double layer interaction at constant surface potential an electrostatic attraction is produced between the particles when they approach a certain distance even though both surface potentials are of the same sign, which suggests that the electrostatic heterocoagulation is not only governed by the sign of charged particles but also affected by difference in the magnitude of surface potentials, the number ratio of the different particles and their size ratio. It is of great interest that another treatment of heating at a high temperature of 500°C for 3 h could also remove the PSt component of heterocoagulates accompanying particle shrinkage to almost half of the droplet sizes as shown in Figure S2 of supporting information. The particle shrinkage was caused by pyrolyzing the organic structure of polymerized MPTMS molecules leaving the rest of inorganic structure in the golf ball-like particles. The number ratio of PSt particles to oil droplets was varied from 25 to 200 (Ncover = 0.32― 2.53) to examine dominant factors for stabilizing oil droplets dispersed in solution at a fixed HCl concentration of 40 mM. SEM images of dimple products obtained in the number ratio range of 25―200 are shown in Figure 6.

Dissolution of PSt component with THF was applied to the

raspberry-like particles to obtain the golf ball-like products. Particle aggregation or formation of


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elongated particles covered with many dimples were observed at the number ratios of 50 or lower as shown in in Figs (a) and (b). On the other hand, the high number ratios of 100 and 200 led to golf ball-like particles having a plenty of dimples on their surfaces, resulting in formation of golf ball-like particles with low polydispersity. The heterocoagulations at different number ratios indicated that the raspberry-like particles in Fig. 1 were stabilized by the PSt particles that were cationized in the acidic solution. The mechanism on dimple formation in the heterocoagulation was examined in additional experiments where the HCl addition was conducted in the coexistence of oil droplets and PSt particles 30 min or 2 h after initiating the droplet polymerization. Figures 7(b) and (c) indicate SEM images of the products obtained by the HCl additions together with the case of HCl addition 30 min before the polymerization (Figure 7(a)).

The concentration of HCl added was

fixed at 40 mM in the heterocoagulations of Fig. 7. As shown in Fig. 7(c) no dimples were observed on the micron-sized spheres of polymerized-MPTMS for the HCl addition 2 h after the initiation. Droplet surfaces hardened by the polymerization of MPTMS probably prevented the adsorption of PSt particles onto the surface of spheres. The HCl addition 30 min after the polymerization could adsorb the PSt particles onto soft surfaces, although the low softness of surfaces led to dimple number smaller than that of golf ball-like particles in Fig. 7(a). SPS and LPS shown in Fig. 3 were also employed as stabilizer particles to form dimples with different sizes. Figure 8 shows the golf ball-like particles prepared using the different PSt particles at the different number ratios in a range of 0.5Ncover―2Ncover where the numbers of Ncover calculated for SPS and LPS were 138 and 29, respectively. The concentration of HCl added was fixed at 40 mM for the heterocoagulations in Fig. 8. As presented in the top raw of Fig. 8 dimples formed by the SPS were located less homogeneously than those by MPS (again


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see Figs. 6 (c) and (d)).

Heterocoagulation with SPS more than twice of Ncover could form

inhomogeneous distribution of dimples on the polymerized particles. Such an inhomogeneous distribution of dimples was probably due to weak electrostatic interactions between SPSs, which was experimentally supported by their zeta potential lower than that of MPS (see Table S1 in Supporting Information). The weak stabilization by the SPS might cause coalescence of droplets in the heterocoagulation and lead to the polymerized particles larger than the droplets. On the other hand, according to the number of dimples observed in Fig. 8(d), it is deduced that the LPS could electrostatically stabilize oil droplets with a smaller number of LPS than the cases of SPS and MPS, which was probably caused by the high zeta potentials of LPS (again see Table S1).

The number of dimples on the polymerized particles in Fig. 8(d) was lower than Ncover =

29 for LPS. The stabilization of oil droplets by a small number of particles is very interesting because the products obtained by the stabilization can be a model architecture to experimentally elucidate stable packing structures of variously shaped hard particles in a spherical confinement.21

The particles shown in Fig. 8(d) also provide an important implication for

heterocoagulation process since it was reported that size ratios (dSmall/dLarge) lower than 0.33 were required for completely covering large particles with small particles in heterocoagulations.22, 23 The present golf ball-particles experimentally revealed that even a high size ratio of 0.6 could selectively adsorb small particles onto large oil droplets. In this heterocoagulation, aggregation of the oil droplets was well suppressed so that the sizes of golf-ball particle became narrowly dispersed. On the other hand, the oil-droplets in the cases of Figs. 8(e) and (f) aggregated in the heterocoagulation, possibly because rapid neutralization between the cationic polymer particles and the anionic oil droplets. It is of importance to control the particle number for suppression of the oil droplet aggregation.


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Diameters of dimples fabricated with the different PSt particles were compared in Table 1 where the depths estimated from the dimple diameter and the PSt particle size are presented. The diameter and the depth of dimples were increased by an increase in PSt particle sizes. The contact angles, which was calculated according to the previous reports,24,

25

exhibited no

significant differences between the dimples fabricated using the PSt particles in the size range of 213―602 nm. PMMA particles with an average size (dV) similar to that of MPS were also prepared for heterocoagulation to create different surface features of the golf ball-like particles. The PMMA particles prepared were shown in a SEM image of Figure S3(a).

The heterocoagulation using

the PMMA particles resulted in a lower contact angle of 95° than that for MPS (see Table S2 and Figure S4). The high affinity between the PMMA surface and the oil droplets having methacryl groups could embed the particles more deeply than PSt particles. Another comparison experiment was that PSt particles prepared in the presence of anionic surfactant (SDBS) were employed as stabilizer particles. The PSt particles prepared at the SDBS concentration of 0.2 mM (PStSDBS) were shown in Figure S3(b). The contact angle of dimples fabricated with the PStSDBS was 141° that was higher than that for MPS prepared without SDBS (again see Table S2). The comparison experiment suggested that the hydrophilic groups of surfactants left on the polymer particles prevented them from being deeply embedded into the oil droplets, which led to the high contact angle of dimples. The present method proposed is in principal applicable to other hard particles that are cationized in an acidic conditions and can be removed with maintaining the morphology of polymerized droplets. For instance, amino-functionalized silica particles that are dissolved in an alkaline solution can also be a candidate for hard particles for the heterocoagulation.


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Conclusion Narrowly dispersed golf ball-like particles were successfully prepared by a combination of the electrostatic heterocoagulation between silicone oil droplets and polymer particles, the polymerization of heterocoagulates and the dissolution of polymer component. A high number ratio of middle-sized PSt particles to oil droplets was required for homogeneous distribution of dimples formed onto the polymerized particles. The number of PSt particles required for the droplet stabilization was decreased with an increase in the size of PSt particles. In the case of large-sized PSt particles with a size ratio of 0.6 to the droplet size, polyhedron-like particles having dimples on their surface could be successfully created with a low number ratio to oil droplets.

Comparative experiments using polymer particles with different affinities to oil

droplets revealed that a high affinity to oil droplets could deeply embed the polymer particles into droplets and form dimples with a low contact angle.

Figure 1. Scheme on synthesis of narrowly dispersed golf ball-like particles. Figure 2. Optical microscope image of silicone oil droplets (a) and their size distribution measured with dynamic light scattering (b). Figure 3. SEM images of PSt particles prepared in soap-free emulsion polymerization with VA057 at different styrene concentrations of 0.044 (a), 0.089 (b) and 0.354 M (c), respectively. The concentrations of VA-057 and NaOH were fixed at 4.5 mM and 0.45 mM, respectively. Figure 4. Zeta potentials of PSt particles measured in the pH range of 2―13.

See Fig. 3 for the

synthetic conditions for PSt particles.


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Figure 5. SEM images of golf ball-like particles obtained by HCl addition at different concentrations of 30(a), 35(b), 40(c) and 45 mM(d). Figure 6. SEM images of dimple products obtained at the different number ratio of PSt particles to oil droplets. The images of (a)―(d) correspond to the ratios of 25(a), 50(b), 100(c) and 200(d), respectively. Figure 7. SEM images of products obtained by HCl additions 30 min before the initiation of droplet polymerization (a), 30 min after the initiation (b) and 2 h after the initiation (c). The number ratio of PSt particles to oil droplets was 100 in the heterocoagulations. Figure 8. SEM images of golf ball-like particles obtained by heterocoagulations with SPS (a-c) or LPS (d-f). The number ratios of PSt particles to oil droplets were 75 (a), 150 (b) and 300 (c) for SPS and 15 (d), 30 (e) and 60 (f) for LPS. Table 1. Comparison on dimensions of golf ball–like particles prepared with PSt particles with different sizes. ASSOCIATED CONTENT Supporting Information. SEM image of golf ball-like particles prepared by calcination at 500°C for 3h. Zeta potentials of PSt particles with different sizes. EM images of particles used in the heterocoagulation. Dimensions of golf ball–like particles prepared with different particles. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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Corresponding Author *Tel: +81-22-795-7239, Fax: +81-22-795-7241, E-mail: [email protected]. ACKNOWLEDGMENT This research was mainly supported by the Ministry of Education, Culture, Sports, Science and Technology (JSPS KAKENHI Grant Number 26286019). ABBREVIATIONS CV, coefficient of variation; iep, isoelectric point; MPS, middle-sized polystyrene particles; MPTMS, 3-methacryloxypropylmethoxy silane; LPS, Large-sized polystyrene particles; PMMA, polymethylmethacrylate; PSt, polystyrene; SPS, small-sized polystyrene particles; VA-057, 2,2'azobis[N-(2-carboxyethyl)-2-methylpropionamidine].

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(4) Dashtdar, M.; Mohammadzade, A.; Hosseini-Saber, S. M.-A Measurement of Roughness Based on the Talbot Effect in Reflection from Rough Surfaces. Applied Optics 2009, 54, 5210– 5216 (5) Zhou, Q.; Ziang, H.; Fan, H.; Yang, X. Zhao, N.; Xu J. Facile Fabrication of Golf Ball-like Hollow Microspheres of Organic-Inorganic Silica. J. Mater. Chem. 2011, 21, 13056–13061. (6) Takahara, Y. K.; Ikeda, S.; Ishino, S.; Tachi, K.; Ikeue, K.; Sakata, T.; Hasegawa, T.; Mori, H.; Matsumura, M.; Ohtani, B. Asymmetrically Modified Silica Particles: A Simple Particulate Surfactant for Stabilization of Oil Droplets in Water. J. Am. Chem. Soc. 2005, 127, 6271–6275. (7) Konishi, N.; Fujibayashi, T. Tanaka, T.; Minami, H. Okubo, M. Effects of Properties of the Surface Layer of Seed Particles on the Formation of Golf Ball-like Polymer Particles by Seeded Dispersion Polymerization. Polymer J. 2010, 42, 66-71. (8) Okubo, M.; Fujiwara, T.; Yamaguchi, A. Morphology of Anomalous Polystyrene/Polybutyl Acrylate Composite Particles Produced by Seeded Emulsion Polymerization. Colloid Polymer Sci. 1998, 276, 186–189. (9) Wang, Y.; Wang, Y.; Zheng, X.; Yi, G.-R.; Sacanna S.; Pine D.; Weck, M. ThreeDimensional Lock and Key Colloids. J. Am. Chem. Soc. 2014, 136, 6866−6869. (10) Okubo, M.; Fujiwara, T.; Yamaguchi, A. Formation Mechanism of Anomalous "Golf Ball-like" Composite Polymer Particles by Seeded Emulsion Polymerization. Colloid Polymer Sci. 1996, 274, 520–524.


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(11) Fujibayashi, T.; Komatsu, Y.; Konishi, N.; Yamori, H.; Okubo, M. Effect of Polymer Polarity on the Shape of “Golf Ball-like” Particles Prepared by Seeded Dispersion Polymerization. Ind. Eng. Chem. Res. 2008, 47, 6445–6449. (12) Narongthong, J.; Nuasaen, S.; Suteewong, T.; Tangboriboonrat, P. One-Pot Synthesis of Organic-Inorganic Hybrid Hollow Latex Particles via Pickering and Seeded Emulsion Polymerizations. Colloid Polymer Sci. 2015, 293, 1269–1274. (13) Dai, M.; Song, L.; Nie, W.; Zhou, Y. Golf Ball-like Particles Fabricated by Nonsolvent/Solvent-Induced Phase Separation Method. J. Colloid Interface. Sci. 2013, 391, 168171. (14) Obey, T. M.; Vincent, B. Novel Monodisperse "Silicone Oil"/Water Emulsions. J. Colloid Interface Sci. 1994, 163, 454–463. (15) Shibuya, K.; Nagao, D.; Ishii, H. Konno, M. Advanced Soap-Free Emulsion Polymerization for Highly Pure, Micronsized, Monodisperse Polymer Particles. Polymer 2014, 55, 535–539. (16) Sacanna, S.; Irvine, W. T. M.; Rossi, L. Pine, D. J. Lock and key colloids through polymerization-induced buckling of monodisperse silicon oil droplets. Soft Matter 2011, 7, 16311634. (17) Ohta, T.; Nagao, D.; Ishii, H.; Konno, M. Preparation of oil-containing, polymeric particles having a single depression with various shapes. Soft Matter 2012, 8, 4652−4658. (18) Taniguchi, T.; Obi, S.; Kamata, Y.; Kashiwakura, T., Kasuya, M.; Ogawa, T.; Kohri, M.; Nakahira, T. Preparation of Organic/Inorganic Hybrid and Hollow Particles by Catalytic


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Deposition of Silica onto Core/Shell Heterocoagulates Modified with Poly[2-(N,Ndimethylamino)ethyl methacrylate]. J. Colloid Interface Sci. 2012, 368, 107-114. (19) Derjaguin, B. V. A Theory of the Heterocoagulation, Interaction and Adhesion of Dissimilar Particles in Solutions of Electrolytes. Discussions Faraday Soc., 1954, 18, 85-98. (20) Usui, S., Interaction of Electrical Double Layers at Constant Surface Charge. J. Colloid Interface Sci. 1973, 44, 107-113. (21) Teich, E. G.; Anders, G. V, Klotsa, D.; Dshemuchadse, J.; Glotzer, S. C. Clusters of Polyhedra in Spherical Confinement. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 669-678. (22) Furusawa, K; Anzai, C. Preparation of Composite Fine Particles by Heterocoagulation. Colloid Polymer Sci. 1987, 265, 882–888. (23) Furusawa, K; Anzai, C. Heterocoagulation Behavior of Polymer Lattices with Spherical Silica. Colloid Surf. 1992, 63, 103–111. (24) Fan, X.; Niu, L.; Wu, Y.; Chen, J.; Yang, Z. Assembly Route toward Raspberry-like Composite Particles and Their Controlled Surface Wettability through Varied Dual-size Binary Roughness. Appl. Surf. Sci. 2015, 332, 393–402. (25) Ikeda, S.; Takahara, Y.; Ishino, S.; Matsumura, M. Ohtani, B. Direct Observation of Amphiphilic Silica Particles Assembled at an Oil–Water Interface. Chem. Lett. 2005, 34, 13861387.


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Figure 1 Scheme on synthesis of narrowly dispersed golf ball-like particles. The method is composed of a heterocoagulation (1), a polymerization of the oil droplet (2) and a dissolution of polymer particles (3).


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Figure 2

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Optical microscope image of silicone oil droplets (a) and their size distribution

measured with dynamic light scattering (b).


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Figure 3 SEM images of PSt particles prepared in soap-free emulsion polymerization at different styrene concentrations of 0.044 (a), 0.089 (b) and 0.354 M (c), respectively. The concentrations of VA057 and NaOH were fixed at 4.5 mM and 0.45 mM, respectively.


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Figure 4 Zeta potentials of PSt particles measured in the pH range of 2―13. See Fig. 3 for the synthetic conditions for PSt particles


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Figure 5

SEM images of golf ball-like particles obtained by HCl addition at different

concentrations of 30(a), 35(b), 40(c) and 45 mM(d).


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Figure 6 SEM images of dimple products obtained at the different number ratio of PSt particles to oil droplets. The images of (a)―(d) correspond to the ratios of 25(a), 50(b), 100(c) and 200(d), respectively.


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Figure 7 SEM images of products obtained by HCl additions 30 min before the initiation of droplet polymerization (a), 30 min after the initiation (b) and 2 h after the initiation (c). The number ratio of PSt particles to oil droplets was 100 in the heterocoagulations.


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Figure 8 SEM images of golf ball-like particles obtained by heterocoagulations with SPS (a-c) or LPS (d-f). The number ratios of PSt particles to oil droplets were 75 (a), 150 (b) and 300 (c) for SPS and 15 (d), 30 (e) and 60 (f) for LPS.


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Table 1 Comparison on dimensions of golf ball–like particles prepared with PSt particles with different sizes.

The number ratios of PSt particles to oil droplets were 300, 100 and 15 for SPS, MPS and LPS, respectively


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Insert Table of Contents Graphic and Synopsis Here


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Imprinting Dimples on Narrowly Dispersed Polymeric Spheres by

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