Robust Anisotropic Composite Particles with Tunable Janus Balance

Jun 4, 2012 - Jun-Bing FanHong LiuYongyang SongZhen LuoZhongyuan LuShutao Wang .... Ji Hoon Park , Nuri Han , Ji Eun Song , Eun Chul Cho...
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Robust Anisotropic Composite Particles with Tunable Janus Balance Bao Liu, Jiguang Liu, Fuxin Liang, Qian Wang, Chengliang Zhang, Xiaozhong Qu, Jiaoli Li, Dong Qiu, and Zhenzhong Yang* State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: We report a general emulsion approach to protrude a lobe by swelling the polymer core from a core−shell structure, achieving anisotropic Janus composite particles with tunable chemistry, shape, size, and size ratio of the two parts thus Janus balance. Oil-in-water emulsion is employed to swell a polymer core through the crack open hole within the shell, and the core protrusion is restricted in the particle/oil confined compartments enveloped with surfactant. When monomers are used as the oil solvents, cross-linking can strengthen the polymer lobe to tolerate against organic solvents. By tuning polymerization time and monomer/particle weight ratio, the size ratio of the polymer/inorganic parts thus Janus balance of the composite particles is continuously tunable across from more hydrophilic to more lipophilic. The robust anisotropic particles with tunable Janus balance can be further used as solid surfactants to tune microstructure of emulsions. hydrodynamic jetting.18 It is noticed that the methods are essentially based on phase separation. As a result, it is unavoidable that a major phase should contain another minor component due to incomplete phase separation in a highly viscoelastic system.19 In order to achieve some desired particle sizes, one of the methods can be chosen as an attempt. It is necessary to develop a general method to fabricate Janus particles with tunable size, shape, and composition thus Janus balance. Core−shell structure has gained increasing attention since two components are compartmentalized distinctively inside the cavity and within the shell. Many approaches have been well developed to tune characteristic size and compositions broadly. The structure has been extensively employed to create hollow spheres after removal of the core materials.20,21 The shell is usually perforated even fractured by osmotic pressure during removal of the core materials.22,23 One hole usually appears due to kinetic factors.24 Almost all the reports focus on final hollow spheres after a complete removal of the core; few are concerned with intermediate stages and structures. Wang et al. first demonstrated to synthesize anisotropic colloids with two compositions compartmentalized distinctively via protrusion of polystyrene (PS) from PS@polyelectrolyte multilayer core− shell capsules in a tetrahydrofuran/water solvent mixture.25 The

1. INTRODUCTION Janus particles with two different components compartmentalized onto the same surface have attracted increasing attention due to their diversified potential applications, for example solid surfactants to stabilize emulsions,1,2 building blocks toward complex structures,3 manipulation of liquid droplets,4,5 and detector for molecular interaction.6 Especially, anisotropic Janus particles possess additional spatial confinement besides directional interaction, forming unique structures for example helical and other new structures,7−9 which are not accessible from their spherical counterparts. The structures could exhibit new optical properties as photonic crystals with complete bands.10 Anisotropic Janus particles are also more kinetically stable at the interface.11,12 In analogy to hydrophile−lipophile balance (HLB) of small surfactant molecules as a guidance to tune microstructure of self-assembled suparmolecules, the concept of Janus balance is proposed to describe the hydrophile−lipophile balance of anisotropic particles.13 Janus balance is important to determine the stability and type of emulsions when they serve as solid surfactants. Chemical composition, size, and shape of the two corresponding sides essentially determine Janus balance. Snowman-like Janus particles are the most typical anisotropic particles with tunable chemical composition and shape. To date, snowman-like Janus polymer particles in shape have been extensively synthesized based on seeded emulsion polymerization that induces phase separation,14,15 precipitation polymerization,16 emulsion interfacial synthesis,17 and electro© 2012 American Chemical Society

Received: February 27, 2012 Revised: May 25, 2012 Published: June 4, 2012 5176

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Scheme 1. Schematic Synthesis of Anisotropic Janus Particle by Emulsion Swelling the Core−Shell Structure (a) with the Solvent Emulsion in Water, To Achieve Anisotropic Particle (b) with the Surfactant Enveloped around the Polymer Lobe, Following by Another Swelling Polymerization To Derive a New Anisotropic Particle (c) with Tunable Size Ratio of the Two Parts; Eventually, the Surfactant Is Washed with Ethanol from the Polymer Lobe Surface, Giving a “True” Janus Particle (d) in Both Chemistry and Shape

2.2. PS@Titania Core−Shell Composite Particles.22 A typical procedure to synthesize the core−shell composite particles is as follows. The freeze-dried submicrometer polystyrene particle (as shown in Figure S2a, Supporting Information) was immersed in a large quantity of concentrated sulfuric acid under stirring at 40 °C for varied sulfonation time to control the sulfonated PS shell thickness. After the product was separated by centrifugation at 4500 rpm and washed with ethanol, sulfonated PS core−shell gel particles were obtained. Then the freeze-dried gel particles were immersed in large amount of TBT/ ethanol (1:1 vol/vol) mixture for 8 h to allow a saturated absorption of TBT within the gel shell. After the TBT swollen gel particles were separated by centrifugation, 0.1 g of the particles was dispersed into 10 mL of ethanol/water (1:1 vol/vol) under stirring at ambient temperature for 2 h to allow the sol−gel process forming titania shell. 2.3. Janus PS@Titania Composite Particles. A typical procedure was described as following. 0.05 g of freeze-dried PS@ titania composite particle was dispersed in 5 mL of aqueous solution containing 10 mg mL−1 of SDS. 0.2 g of toluene and 5 mL of aqueous solution containing 10 mg mL−1 of SDS were emulsified at ambient temperature under ultrasonication for 5 min. The emulsion was added into the composite dispersion within 1 min under stirring at ambient temperature, and the system stood there for a varied time to control swelling extent. The swelling was terminated by adding excess ethanol. The samples were centrifugated and washed with ethanol three times to remove toluene. Meanwhile, the surfactant was removed from both the continuous phase and particle surface. As comparison to the above emulsion swelling, a controlled amount of toluene was added into ethanol dispersion of PS@titania composite particle within 1 min under stirring at ambient temperature.25 2.4. Janus Poly(St-DVB)@Titania Composite Particles. A typical procedure was described as follows. 0.05 g of freeze-dried PS@titania composite particle was dispersed in 5 mL of 10 mg mL−1 SDS aqueous solution. 0.1 g of St, 0.1 g of DVB, 0.002 g of AIBN, and 5 mL of 10 mg mL−1 SDS aqueous solution were emulsified at ambient temperature under ultrasonication for 5 min. The emulsion was added into the composite particle dispersion under stirring at ambient temperature. After swelling for 20 min, the emulsion was heated and held for polymerization at 70 °C for 10 h. Janus poly(StDVB)@titania composite particles were achieved by centrifugation. 2.5. Janus Poly(MPS)@Titania Composite Particles. A typical procedure was described as follows. 0.05 g of freeze-dried PS@titania composite particle was dispersed in 5 mL of 10 mg mL−1 SDS aqueous solution. 0.3 g of MPS, 0.05 g of DVB, 0.004 g of AIBN, and 5 mL of 10 mg mL−1 SDS aqueous solution were emulsified at ambient temperature under ultrasonication for 5 min. The emulsion was added into the composite particle dispersion under stirring at ambient temperature. After swelling for 20 min, the emulsion was heated and held for polymerization at 70 °C for 10 h. Janus poly(MPS)@titania composite particles were achieved by centrifugation. 2.6. Emulsification with the Janus Composite Particles. The Janus poly(St-DVB)@titania composite particles were washed with DMF to remove linear polymers. 0.02 g of freeze-dried powder of the

swell-assisted protrusion approach has a high potential to fabricate anisotropic Janus particles. But the core sulfated PS chains are easily precipitated forming secondary particles. It will lead to failure in the synthesis of anisotropic Janus particles at higher swell extent. Besides, the polyelectrolyte part is weak and would easily collapse. In order to achieve robust Janus particles, we selected previously reported polymer/inorganic PS@titania core−shell structure.22 Following the route by Wang, we attempted to replace water with ethanol or methanol in order to extend the method using other solvents, for example toluene. Several small PS lobes rather than one form onto the surface, and the particles are heavily aggregated when we attempted to tune the PS/titania ratio of the anisotropic particles after prolonged swelling time (Figure S1, Supporting Information). This resulted from coalescence among PS lobes in the presence of solvents. Herein, we propose a modified swell protrusion approach, e.g., emulsion approach to protrude the core polymer by swelling to form a lobe onto the shell of a core−shell structure, achieving anisotropic Janus composite particles as illustrated in Scheme 1. Although the process is based on protrusion of the core, it is essentially different that the process is undertaken with emulsions rather than a mixture solvent. The swelling induced protrusion occurs in confined compartments enveloped with the surfactant on the exterior surface of the TiO2 shells, which is dispersed in the aqueous phase. This guarantees that the polymer lobes do not coalesce with the protection of surfactant layer, and the ratio of PS/titania parts can be broadly controlled with increasing swell extent. Besides, yield of individual anisotropic Janus particles is ensured to be a high level at high solid content. When monomers are used as solvents, composition and size of the lobe are further tunable by a further polymerization. Eventually, the surfactant is washed from the particle surface, giving true Janus particles with two different parts in both chemistry and structure. The method is general with the prerequisite that the core should be swellable.

2. EXPERIMENTAL METHODS 2.1. Materials. Styrene (St), sodium dodecyl benzenesulfonate (SDS), azobis(isobutyronitrile) (AIBN), tetrabutyltitania (TBT), N,Ndimethylformamide (DMF), toluene, concentrated sulfuric acid (H2SO4, 98 wt %), polyvinylpyrrolidone (PVP-K30, Mw = 30 000 g mol−1), and ethanol were purchased from Sinopharm Chemical Reagent Beijing Co. Paraffin wax (Tm: 52−54 °C) was purchased from Nan Yang Wax Fine Chemical Plant. Divinylbenzene (DVB) was purchased from Fluka. Polyoxyethylene sorbitan monooleate (Tween80) was purchased from TCI. 3-Methacryloxypropyltrimethoxysilane (MPS) was purchased from Alfa Aesar. 5177

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Figure 1. Some representative anisotropic composite particles by emulsion swelling at varied toluene/particle weight ratio: (a) 0.5:1; (b) 1.5:1; (c) 8.0:1; (d) 16.0:1. (e) Morphological evolution of the Janus composite particles dependent on toluene/particle weight ratio: The PS lobe size (▲), the titania part size (◆), and the lobe/titania size ratio (■). The as-used PS/titania core/shell composite particles are synthesized from the sulfonated PS particles with sulfonation time of 1 h. Janus poly(St-DVB)@titania composite particles was dispersed in 2 g of water, followed by addition of 1 mL of decane. A trace of methylene orange was introduced into water to easily distinguish water phase. Emulsions formed after the mixture was vigorously shaken for 1 min. After the emulsions were added into liquid nitrogen, they were freezedried for SEM observation. Similarly, 0.02 g of freeze-dried powder of the Janus poly(St-DVB) @titania composite particles was dispersed in 2 g of water, followed by addition of 1 g of wax (Tm: 52−54 °C). The mixture was vigorously shaken at 70 °C for 1 min to form a melt wax-in-water emulsion. After the emulsion was naturally cooled down to room temperature, the melt wax was solidified. 2.7. Characterization. Very dilute suspensions of the samples in ethanol were dropped onto carbon-coated copper grids for transmission electron microscopy (TEM) characterization (JEOL 100CX operating at 100 kV). Scanning electron microscopy (SEM) measurements were performed with a HITACHI S-4800 apparatus operated at an accelerating voltage of 15 kV. The samples were ambient dried and vacuum sputtered with Pt. Optical microscopy images were performed with an Olympus optical microscope. Surface tension of the emulsions was measured with a KYOWA CBVP surface tensionmeter A3. Titania content of the composite spheres was determined by thermogravimetric analysis (Perkin-Elmer analyzer Pyris 1 TGA). Optical microscopy images were performed with an Olympus optical microscope. Particle size measurement was carried out with a Brookhaven Zetaplus analyzer (Brookhaven Instruments Corp.).

3. RESULTS AND DISCUSSION In the present research, we focus on those core−shell structures achieved by a postcoating onto the core template since distinct compartmentalization of the core and shell compositions is guaranteed in principle. As previously reported titania can grow favorably within the sulfonated polystyrene (SPS) layer forming composite shell,22 we selected polystyrene particles (about 230 nm in diameter) (Figure S2a, Supporting Information) as seed particles to prepare submicrometer PS@titania core−shell composite particle (Figure S2c, Supporting Information). The parent PS particles are relatively uniform (Figure S2b, Supporting Information), which can form colorful opal structure. The formed PS core and the titania shell are compartmentalized distinctively (Figures S2c and S3c,d, Supporting Information). The SPS shell becomes thicker with increase of the sulfonation time; thus, the titania shell is approximately proportional with SPS shell (Figure S2d, Supporting Information) and becomes more coarser (Figure S3a,b, Supporting Information). Besides, particle size and size distribution of the as-synthesized PS@titania particle are similar to the seed PS particle, which varies less with sulfonation time. This implies that titania forms within the SPS matrix. When the polymer core was dissolved with dimethylformamide (DMF), the interior surface of the formed titania hollow particles is smooth (Figure S3e, Supporting Information). This indicates that PS core and the titania/SPS composite shell are compartmentalized distinctly. 5178

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Figure 2. Morphological evolution of the anisotropic composite particles with different SDS concentration (mg mL−1): (a) 0.29; (b) 0.63; (c) 1.48; (d) 2.48; (e) 5.00; (f) 10.00. (g) Surface tension isotherm of the as-formed dispersion with different SDS concentration at 25 °C. Toluene/particle weight ratio is 4:1. The as-used PS/titania core/shell composite particles are synthesized from the sulfonated PS particles with sulfonation time of 1 h.

A typical procedure is given to synthesize anisotropic composite particles by swelling assisted protrusion. After the particle was dispersed in water containing the surfactant SDS, a toluene in water emulsion was fed under stirring within 1 min. When the particle meets the toluene droplets, toluene diffuses inwardly via the shell to swell the PS core. The arisen osmotic pressure increases sufficiently to crack the shell forming a crevice. As the pressure is released, part of the core PS is protruded through the crevice, forming a lobe anchored onto the exterior shell surface. Although we attempted to monitor the whole formation process of the composite particles especially at the beginning stage, a nascent PS lobe forms shortly after 2 min upon feeding the toluene emulsion (Figure S4a). The process is too fast to record. We monitored the lobe growth with time. The PS lobe gradually becomes larger with swelling time until 2 h (Figure S4b−d, Supporting

Information). After the swelling duration is further prolonged to 4 h, the lobe starts to deform until disappear (Figure S4e,f, Supporting Information). Free PS particles start to appear in the aqueous phase. In all the following experiments, the swelling duration is fixed for 20 min. The lobe size is mainly controlled by the feeding amount of the toluene-in-water emulsion. At a small toluene/particle weight ratio for example 0.5:1, no lobe forms but a small crevice in the shell under TEM (Figure 1a). At a high weight ratio of 1.5:1, the lobe becomes distinguished about 82 nm in diameter, forming an anisotropic snowman-like particle (Figure 1b). The lobe grows with increasing the amount of toluene (Figure 1e), while more PS is protruded from the core. At a weight ratio of 4:1, titania part becomes slightly collapsed (Figure S4b, Supporting Information). At a weight ratio of 8:1, the crack mouth becomes more open while the titania part is collapsed 5179

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Figure 3. Some other representative anisotropic composite particles by emulsion swelling: (a, b) derived from two PS@titania core−shell particles with varied core sizes, the particles are template synthesized from the corresponding core−shell gel particles by sulfonation of the PS particles for 0.5 and 2 h, respectively; (c) the anisotropic PS@silica composite particle; (d) the composite particle from the wax@phenolic resin core−shell composite.

Figure 4. Anisotropic composite particles synthesized after varied swelling polymerization time (h): (a) 0.5; (b) 1.0; (c) 1.3; (d) 1.6; (e) 2.0; (f) lobe diameter (▲), titania part diameter (◆), and lobe/titania size ratio (■) as a function of polymerization time. Monomer/particle weight ratio is 16:1. The as-used PS/titania core/shell composite particles are synthesized from the sulfonated PS particles with a sulfonation time of 2 h.

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Figure 5. Morphological evolution of the anisotropic composite particles at varied monomer/particle weight ratio: (a) 2:1, (b) 4:1, (c) 8:1, (d) 16:1. (e) Lobe diameter (▲), titania part diameter (◆), and lobe/titania size ratio (■) as a function of monomer/particle weight ratio. (f) Janus particle as shown in (b) after being treated with toluene. Polymerization is carried out at 70 °C for 10 h. The as-used PS/titania core/shell composite particles are synthesized from the sulfonated PS particles with sulfonation time of 1 h.

mg mL−1. We conjecture that stability of the dispersion below cmc of SDS may originate from the participation of the titania part to stabilize the dispersion together with SDS as shown in Scheme 1c. The polymer lobe forms in 2 min, and the size is less dependent on the surfactant concentration (Figure 2c−f). In all other experiments, SDS concentration is kept at a high level above cmc to ensure that the core protrusion is achieved within the particle/toluene confined compartments. Besides SDS, other representative surfactants for instance nonionic surfactant Tween-80 and polymeric PVP-K30 also work for the emulsion swelling to form anisotropic particles (Figure S7, Supporting Information). In order to demonstrate generality of the methodology, some other representative anisotropic composite particles with varied composition and size are synthesized. When the PS@titania particle with a larger core about 200 nm in diameter is swelled, the lobe is larger (Figure 3a). It becomes smaller when the core about 176 nm in diameter is smaller (Figure 3b). Similarly, anisotropic PS@silica composite particles are derived from the PS@silica core−shell particles synthesized following ref 26 (Figure 3c). Janus wax/phenolic resin composite particles of micrometers in size are synthesized from the core−shell capsules reported in ref 27 (Figure 3d). Similar to other anisotropic Janus particles, for example of PS@polyelectrolyte,25 the lobes are weak and easily dissolved from the Janus particles in the presence of good solvents. In

(Figure 1c). This implies that most PS core has been protruded to form the lobe. The lobe size changes less with more excess toluene for example at a weight ratio 16:1 (Figure 1d). Some free PS particles appear in the continuous aqueous phase. At a weight ratio above 100:1, all polymers including the lobe are dissolved, giving a hollow sphere with a single open hole (Figure S5, Supporting Information). Deformation of the PS lobe and eventual detachment from titania shell with excess amount of the solvent can be explained by the weakening of the swelled PS. In order to elucidate role of SDS in forming the anisotropic particles, we also carried out another experiment without SDS in both the toluene/water mixture and the particle dispersion. No lobe forms when the simple mixture is fed although the swelling time is prolonged to 10 h (Figure S6, Supporting Information). This implies that less solvent contacts the core− shell particles in the absence of SDS. Similarly, at low level of the surfactant no lobe forms (Figure 2a,b). The dispersion becomes partially de-emulsified to release some solvent forming a thin top layer of toluene. With increasing SDS concentration, the as-formed toluene/water emulsion becomes more stable while surface tension of the dispersion decreases remarkably (Figure 2g). Above a certain SDS value 1.48 mg mL−1, surface tension approaches a plateau. The dispersion becomes rather stable, and no free toluene is released. It is noted that the value is below cmc of SDS of 2.48 5181

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Figure 6. Janus characteristics of the anisotropic particles. (a) Left: a decane-in-water emulsion stabilized with the Janus particle as shown in Figure 5a; right: a water-in-decane emulsion stabilized with the Janus particle as shown in Figure 5d; water/decane volume ratio is 2:1, and the particle/ water weight ratio is 1:100. (b) Optical microscopy images of the decane-in-water emulsion and (c) water-in-decane emulsion. (d) Left: immiscible toluene/water mixture; right: a water-in-toluene emulsion stabilized with the Janus particle as shown in Figure 5d. Water/toluene volume ratio is 2:1, and the particle/water weight ratio is 2:1000.

becomes larger with the weight ratio (Figure 5b). At the weight ratio of 8:1, the polymer lobe is comparable with the titania part (Figure 5c). When the weight ratio is further increased for example to 16:1, the polymer lobe is larger than the titania part (Figure 5d). The lobe size and the lobe/titania size ratio are continuously tunable with the monomer/particle weight ratio (Figure 5e). When the Janus particle by swelling polymerization of DVB is dispersed in toluene for long time, the polymer part remains the same in size although linear PS is removed (Figure 5f). This means that the anisotropic composite particles are robust sufficiently to tolerate organic solvents. In comparison with the lobe protrusion from the core−shell structure simply by swelling using water-soluble solvents,25 our emulsion protrusion is more complicated with many steps synchronously involved. The solvent/monomer droplets should meet the core−shell structure and input the solvent/monomer to swell the PS core and protrude the core. When the droplets and the core−shell structure meet together, a collision must occur. The question is: is the collision elastic or inelastic? In the elastic case, if the droplet breaks into many smaller ones after collision with the core−shell structure. In the inelastic case, if the core−shell structure is anchored onto the droplet, or a new one is formed with the core−shell particle surrounded with the solvent/monomer shell. Nevertheless, we believe that both processes of collision and swelling are important. The former process provides substance to swell the core. The latter one causes emergence of lobe by protrusion and further growth. Since the collision is rather faster than the swelling although they are both fast, swelling is the control step. This can explain why all the particles become Janus ones after swelling. In order to elucidate the mechanism, further experiments are required to monitor how the lobe emerges at the early stage and morphological evolution of the solvent/monomer droplets with time.

order to strengthen the polymer lobe onto the inorganic part, a monomer St/DVB mixture containing oil soluble initiator AIBN was used to swell the core−shell particle instead of toluene. After swelling for 20 min, a free radical polymerization was carried out at 70 °C. A polymer lobe grows fast at early stage of polymerization (Figure 4a,b). The lobe is soluble and smaller than the titania part. With increasing polymerization time, the lobe becomes less dissoluble, while the titania part keeps unchanged. Different from the independence of lobe size with swelling time using toluene (Figure S4, Supporting Information), a further growth of the polymer lobe is related to elastic stress during polymerization.19 After 1.3 h, the lobe size becomes comparable with the titania part (Figure 4c). The polymer lobe becomes insoluble. With prolonging polymerization time after 1.6 h, the growth becomes slower (Figure 4d). After polymerization over 2 h, the polymer lobe is larger than the titania part (Figure 4e). The lobe size and lobe/titania size ratio are continuously tunable with polymerization time (Figure 4f). The polymer lobe becomes much coarse with the increase of DVB (Figure S8, Supporting Information). Similarly, larger Janus PS@titania particles of micrometers in size are synthesized by emulsion swelling polymerization against the corresponding larger core−shell particles (Figure S9a, Supporting Information). Many other monomers can be used to swell the core−shell particles,28,29 composition of the polymer lobe is broadly controlled. For example, using the MPS/DVB mixture instead of the St/DVB mixture, anisotropic Janus poly(MPS)@titania composite particles are achieved (Figure S9b, Supporting Information). Their derivative organic−silica hybrid@titania composite particles are synthesized after a further sol−gel process of poly(MPS). At a given St/DVB weight ratio, for example 1:1, the polymer lobe size is also controlled by the monomer/particle weight ratio. At a small weight ratio below 2:1, a small polymer lobe appears (Figure 5a) while the whole particle expands slightly from original 230 to 255 nm in diameter. The polymer lobe 5182

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Figure 7. Janus characteristics of the anisotropic particles: (a, b) SEM image of wax/water emulsion stabilized by the Janus particle as shown in Figure 5b; (c, d) SEM image of water/wax emulsion stabilized by the Janus particle as shown in Figure 5d; the bright circular areas are corresponded to the internal cavity after water droplets evaporate.

The Ti element is present exclusively onto the inorganic part (Figure S10a, Supporting Information). After being treated with HCl, the Ti element is absent (Figure S10b, Supporting Information). The inorganic part becomes smooth, consistent with the dissolution of titania and exposure of the sulfonated PS layer (Figure S10c, Supporting Information). PS and titania are distinctively compartmentalized onto the same surface. The asprepared composite particles are only dispersible in water, meaning the polymer lobe is covered with a layer of surfactant. After removal of the surfactant by washing, the particles can be dispersible in both water and oil, revealing they are amphiphilic. They act as solid surfactants to easily emulsify a representative immiscible water/decane mixture (Figure 6a). When the Janus particle (as shown in Figure 5a) with a smaller PS lobe is used, an O/W emulsion forms since the particles are more hydrophilic (Figures 6a (left) and 6b). The droplets are hundreds of micrometers. When the polymer lobe is larger than the titania part (as shown in Figure 5d), a W/O emulsion forms (Figures 6a (right) and 6c). This implies that the Janus particles are more lipophilic. At decreasing amount of the Janus particles, th esize of the emulsion droplets increases. For example, when the Janus particle content is 0.2 wt %, the emulsion droplets are millimeters in diameter (Figure 6d). It becomes easier to manipulate larger droplets for example simple filtration for separation. When the Janus particle with linear PS lobe (Figure 1c) is used to emulsify a representative good solvent for example toluene, an O/W emulsion forms and is stable in a short time. After a long time, PS is dissolved forming an interfacial layer after the emulsion is dried. Some particles peel off leaving pores (Figure S11a, Supporting Information). When the Janus particle prepared by swelling polymerization with DVB is used, the cross-linked polymer lobe is tolerant against toluene, and thus no coalescence occurs among the Janus particles (Figure S11b, Supporting Information). After being treated with toluene to remove linear polystyrene from the Janus particle (Figure 5a), a toluene in water emulsion forms. The Janus particles arrange regularly at the interface, and the coarse

titania part faces toward the exterior continuous aqueous phase (Figure S11c, Supporting Information). Similarly, a water in toluene emulsion forms with the as-prepared Janus particle (Figure 5d); the smooth polymer lobe mainly directs toward the exterior continuous oil phase (Figure S11d, Supporting Information). Although major individual particles have a preferential orientation, a minority of the particles orients oppositely after drying. In order to reveal their “real” orientation at the interface, the wax is employed instead of toluene as oil phase. Upon cooling, the melt wax becomes solidified. Orientation of the Janus particles at the interface is frozen. For the oil in water emulsion stabilized with the Janus particle (as shown in Figure 5b), the titania part directs exclusively toward the aqueous phase while the polymer lobe is partially embedded into wax phase (Figure 7a,b). When the Janus particle (as shown in Figure 5d) is used to form a water-in-oil emulsion, the titania part faces the interior aqueous phase and the polymer lobe is embedded into the wax continuous phase (Figure 7c,d). Besides, we also have demonstrated that the Janus particles have a well-defined orientation at the interface when they are used as solid surfactants to emulsify the oil/water mixture.19

4. CONCLUSION We have proposed a simple approach to synthesize anisotropic Janus composite particles by emulsion swelling core−shell composite particles. It is determinative that the polymer lobes do not coalesce among the anisotropic particles under protection of surfactants even when swelling extent is high at a high solid content. In combination of swelling polymerization, composition of the polymer lobe is further controlled while the lobe size is further tunable. Cross-linking the polymer can strengthen the lobe to tolerate organic solvents. Ratio of polymer/inorganic parts thus Janus balance is tunable across from more hydrophilic to more lipophilic. The method can be scaled up for high quantity of Janus particles with varied composition, shape, and characteristic size. For example, at a high solid content for example 5 wt %, the anisotropic Janus 5183

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(22) Yang, Z. Z.; Niu, Z. W.; Lu, Y. F.; Hu, Z. B.; Han, C. C. Angew. Chem., Int. Ed. 2003, 42, 1943−1945. (23) Yang, M.; Ma, J.; Zhang, C. L.; Yang, Z. Z.; Lu, Y. F. Angew. Chem., Int. Ed. 2005, 44, 6727−6730. (24) Giersig, M.; Ung, T.; Liz-Márzan, L. M.; Mulvaney, P. Adv. Mater. 1997, 9, 570−575. (25) Yu, H. K.; Mao, Z. W.; Wang, D. Y. J. Am. Chem. Soc. 2009, 131, 6366−6367. (26) Niu, Z. W.; Yang, Z. Z.; Hu, Z. B.; Lu, Y. F.; Han, C. C. Adv. Funct. Mater. 2003, 13, 949−954. (27) Jin, Z. G.; Wang, Y. D.; Liu, J. G.; Yang, Z. Z. Polymer 2008, 49, 2903−2910. (28) Wei, W.; Yang, Z. Z. Adv. Mater. 2008, 20, 2965−2969. (29) Ding, S. J.; Wei, W.; Yang, Z. Z. Polymer 2009, 50, 1609−1615.

composite particles are also produced accordingly (Figure S12, Supporting Information).

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details; SEM and TEM images of some representative Janus particles. 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 This work was supported by the NSF of China (50733004, 20720102041, 50973121, and 51173191), MOST (2011CB933700, 2012CB933200), and CAS (KJCX2-YWH20).

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dx.doi.org/10.1021/ma300409r | Macromolecules 2012, 45, 5176−5184