Robust Reactive Janus Composite Particles of Snowman Shape

Apr 16, 2015 - We present a facile approach toward snowman-like silica@PDVB/PS Janus particles by seed emulsion polymerization using a gelable monomer...
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Robust Reactive Janus Composite Particles of Snowman Shape Yijing Sun, Fuxin Liang,* Xiaozhong Qu, Qian Wang, 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 present a facile approach toward snowman-like silica@PDVB/PS Janus particles by seed emulsion polymerization using a gelable monomer MPS against a PDVB/PS hollow particle. Individual silica bulge is protruded from the seed particle surface, whose size is tunable. The silica@PDVB Janus particles are derived after dissolution of PS, which are robust to tolerate against organic solvents. Both sides are reactive for selective modifications to grow desired materials with tunable wettability and functionality. As solid emulsifiers, the Janus balance of the particles is tunable from more hydrophobic to more hydrophilic by changing either aspect size ratio or composition of the two sides.

1. INTRODUCTION Janus particles possess two different compositions and properties compartmentalized onto the opposite sides. They have broad potential applications in solid surfactants, selfpropelled motors, display and building blocks toward more complex superstructures.1−3 It is significant to tune their shape along with anisotropic composition.4 Among the possible shapes, the two-headed snowman-like shape is especially promising. The asymmetric shape imposes a steric constraint to induce formation of chiral strings under external fields. At high aspect size ratio, a single helical string is achieved.5 The asymmetric shape of a snowman-like Janus particle can lead a jamming structure at the emulsions interface. As a result, nonspherical emulsion droplets are obtained.6 Synchronous variations of the aspect size ratio and wettability determine equilibrium configuration of the particle at emulsions interface. Some kinetically metastable configurations are permitted with an intermediate interfacial orientation at high aspect ratio.7 It is important to develop methods to large scale synthesize snowman-like Janus particles with tunable aspect size ratio and distinct compartmentalization of two compositions onto both sides. Snowman-like PS/silica composite particles are synthesized by further coalescence of the heterodoublets consisting of two particles, which are initially connected by electrostatic absorption onto a glass substrate.8 Although the two compositions are distinctly compartmentalized, yield of the particles is rather low. Other aggregates composed of more particles are coexistent. Tedious sorting process is required to achieve pure Janus particles. Alternatively, seed emulsion polymerization is more promising due to easy controlling particle morphology, which has been extensively used to large scale synthesize snowman-like particles by phase separation. Both internal stress-retraction dynamic and thermodynamic variables during monomer swelling and polymerization are responsible for the unique morphology.9 During emulsion polymerization, polymer particles are usually used as seeds to synthesize snowman-like particles. Microsized polystyrene © XXXX American Chemical Society

snowman-like particles are synthesized by dynamic swelling polymerization against a cross-linked polystyrene seed particle.10 Although two sides are different in cross-linking density, they are the same in composition of PS therefore the similarity in wettability. The particles are not Janus in chemistry although they are anisotropic in shape. It is difficult to selectively modify one side to further differentiate composition thus wettability of the two sides. In order to create anisotropic chemistry, some functional monomers for example glycidyl methacrylate (GMA) are previously polymerized in the PS seed particle. Exterior surface of the seed particle side becomes hydrophilic after further conversion of the functional polymer, while the other bulge side is preserved hydrophobic.12 Besides introduction of functional monomers in the seed particle, different monomers are used for the swelling polymerization against the seed particle to achieve polymer/polymer snowmanlike Janus particles.11−14 As an example, a kind of PnBA− PMMA composite particle is prepared by emulsion polymerization against the PnBA seed particle.11 The particle can selforientate when coating onto a substrate due to wettability difference between the two sides. Similarly, PS/PnBA snowman-like Janus particle is achieved against a PS seed particle.15 Both size and number of the PnBA lobes are tunable. Similarly, snowman-like Janus particles such as PS/PMMA and PS/ PBMA are fabricated.12,13 It is noted that the particle shape is mainly determined by cross-linking degree and phase separation kinetics. PMMA/PS snowman-like Janus particle is obtained via the swelling polymerization of styrene against a PMMA seed.14 The PMMA seed particle is not cross-linked, and the PS bulge should contain a minority of PMMA after the phase separation (vice versa). PS and PMMA are not distinctly compartmentalized onto the two sides. We have previously reported submicrometer snowman-like Janus particles.16 PAN/ Received: January 30, 2015 Revised: March 20, 2015

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DOI: 10.1021/acs.macromol.5b00207 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules PS snowman-like particles are synthesized by a seed emulsion polymerization against the cross-linked PAN particle. The PS bulge is pure without PAN. It is a key step to further hydrolyze the PAN side to introduce poly(acrylic acid)which can induce a favorable growth of inorganic materials. It is clear that seed emulsion polymerization is so powerful to synthesize snowmanlike particles with tunable shape and composition. However, almost all the reported snowman-like particles possess linear polymeric side, which are easily swollen and dissoluble in solvents. Their diversified potential applications are greatly restricted. For example, it is incapable to emulsify good solvents when using such Janus particles. How to synthesize snowman-like Janus particles which are robust to tolerate solvents and reactive for easy selective modification using seed emulsion polymerization becomes crucial. Compared with those polymer/polymer snowman-like particles, cross-linked polymer/inorganic composite snowman-like particles are advantageous in strength and easiness for further selective modification. For example, PS/silica particles have been synthesized by polymerization induced phase separation onto a seed silica particle surface.17 Careful controlling surface density of methacrylate group on the silica surface at an intermediate level is determinative to achieve the snowman shape. Otherwise, PS will be separated from the silica particle or fully encapsulate the particle forming a core/shell structure. Besides the thick PS bulge, a thin layer of PS should be present onto the opposite side of the bulge. It is confirmed that a cavity is left after etching the seed silica particle from the snowmanlike composite particle.18 The particles are only dispersible in oil and not truly Janus in chemistry. In order to achieve a truly Janus particle, the PS thin layer should be removed to expose a fresh hydrophilic silica surface.19 Recently, a swelling protrusion against a core−shell structure has been proposed to prepare snowman-like Janus composite particles with varied shell composition including polymer and inorganic. However, the protruded polymer cores remains linear which are easily dissoluble in good solvents.20−22 Until now, there is no report on synthesis of solvent tolerant and reactive snowman-like Janus composite particles by emulsion polymerization against a polymer seed particle. Once it is successful, it will become much easier to large scale produce the particles at high solid content. This is rather important to further explore practical applications of the Janus particles. Herein, we report the synthesis of snowman-like silica@ PDVB/PS Janus particles by seed emulsion polymerization after extending our previous finding.19 After dissolution of PS, silica@PDVB Janus particles (a) are derived. Silica and PDVB are distinctly compartmentalized onto the opposite sides. The silica/PDVB Janus particles are robust and stable in organic solvents. Both sides possess reactive groups to allow selective growth of desired materials. Their composition, functionality and microstructure are tunable. As an example, after the silica side is selectively modified with a silane the side becomes hydrophobic (b). The PDVB side can be grafted with polymers for example polyamide and derivative poly(acrylic acid)(PAA) to render hydrophilic and pH responsive performance (c). A huge family of composite Janus particles is derived.

Scheme 1. Illustrative Synthesis of the Robust Reactive Janus Composite Particles: (a) Silica@PDVB Composite Snowman-Like Janus Particle; (b) OTES−Silica@PDVB Composite Particle; (c) OTES−Silica@PDVB−PAA Composite Janus Particle

triethoxysilane (OTES) were purchased from Alfa Aesar. Aqueous ammonia (28 wt %), decane, ethanol, potassium peroxydisulfate (KPS), sodium dodecyl benzenesulfonate (SDS), N,N-dimethylformamide (DMF), and azobisisbutyronitrile (AIBN) were purchased from Sinopharm Chemical Reagent. Synthesis of PDVB/PS Hollow Particles. A 5.0 g sample of freeze-dried powder of polystyrene (PS) hollow particle HP-433 (Rohm and Haas product) was dispersed in 160.0 g of water containing 0.048 g of SDS. 3.0 g of DVB, and 0.03 g of AIBN was mixed and emulsified in the presence of SDS under ultrasonication for 5 min forming a monomer emulsion. The emulsion and the dispersion were mixed under stirring at ambient temperature for 8 h to swell the PS shell with the monomer/initiator mixture. The emulsion was heated to 70 °C to initiate the polymerization for 12 h. The PDVB/PS hollow particles were obtained after centrifugation and vacuum-dried at 40 °C. Synthesis of Silica@PDVB/PS Janus Particles. A 1.0 g sample of freeze-dried powder of the PDVB/PS hollow particle was dispersed in 20.0 g of water under stirring at 70 °C. 0.6 g of MPS, 0.6 g of 1 wt % aqueous KPS, 0.02 g of SDS, and 10.0 g of water were mixed under ultrasonication at room temperature for 2 min, forming a monomer emulsion. After the monomer emulsion was dropped into the dispersion at 70 °C within 30 min, the mixture stood for the polymerization for varied time. After the polymerization, a desired amount of aqueous ammonia (28 wt %) was added under stirring at 70 °C for 1 h to induce a further sol−gel process of PMPS. The silica@ PDVB/PS Janus particles were obtained after centrifugation and washing with water and ethanol. A 1.0 g sample of freeze-dried powder of the silica@PDVB/PS Janus particle was dispersed in 20.0 g of DMF for 4 h to remove PS. After washing with DMF, the Janus silica@PDVB particles were obtained after centrifugation and washing with water and ethanol. Synthesis of the OTES−Silica@PDVB Particles. A 0.1 g sample of freeze-dried powder of the silica@PDVB Janus particle was dispersed in 20.0 g of ethanol. Then, 50.0 μL of aqueous ammonia (28%) and 200.0 μL of N-octyltriethoxysilane (OTES) were added and refluxed for 24 h. After centrifugation and washing with ethanol and water, the OTES modified silica@PDVB (OTES−silica@PDVB) particles were obtained. Synthesis of the OTES−Silica@PDVB−PAA Janus Particles. A 1.0 g sample of acrylamide (AM) was dispersed in 20.0 g of ethanol. Then 0.02 g of AIBN was added at 70 °C within 5 min, and 2.0 g of freeze-dried powder of the OTES−silica@PDVB particle was dispersed in 10.0 g of ethanol. After the particle dispersion was mixed in the monomer solution, the mixture was held under stirring for polymerization at 70 °C for 8 h. After centrifugation and washing with ethanol, the OTES−silica@PDVB−PAM Janus particles were obtained. A 1.0 g sample of freeze-dried powder of the OTES−silica@ PDVB−PAM Janus particle was dispersed in 20.0 g of 1 M aqueous HCl. The mixture was refluxed under stirring at 80 °C for 72 h to hydrolyze the PAM shell. After washing until the continuous phase became neutral, the OTES−silica@PDVB−PAA Janus particles were obtained after centrifugation.

2. EXPERIMENTAL SECTION Materials. Styrene (St), divinylbenzene (DVB), and acrylamide (AM) were purchased from Aldrich and destabilized over an Al2O3 column. 3-Methacryloxypropyltrimethoxysilane (MPS) and N-octylB

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Figure 1. SEM and inset TEM images of the PDVB/PS composite hollow particles synthesized at varied DVB/seed particle weight ratio and the corresponding derivative PDVB hollow particles after dissolution of PS: (a, b) 1/5; (c, d) 3/5; (e, f) 5/5. Emulsification with the Janus Particles. A 0.05 g sample of freeze-dried powder of the Janus particle was dispersed in 10.0 g of water, followed by adding 1.0 g of wax (Tm = 52−54 °C) at 70 °C under vigorously stirring for 10 min. The emulsion was naturally cooled to room temperature. Then, 0.05 g of freeze-dried powder of the Janus particle was dispersed in 10.0 g of water, followed by adding 1.0 g of decane at 25 °C under vigorously stirring for 10 min. Characterization. Morphology of the Janus particles was characterized with scanning electron microscopy (Hitachi S-4800 at 15KV) equipped with an energy dispersive X-ray (EDX) analyzer and transmission electron microscopy (JEOL1011 at 100 kV). The samples for SEM observation were prepared by vacuum sputtering with Pt on the ambient dried samples. The samples for TEM observation were prepared by spreading very dilute emulsions in ethanol onto carbon-coated copper grids. FT-IR spectroscopy was performed on the sample/KBr pressed pellets using a BRUKER EQUINOX 55 spectrometer. Florescence microscopy images were observed using a confocal laser scanning microscope (Leica TCS-sp2). Zeta potential measurement was carried out on a NANO ZS (Malvern Instruments) at 25 °C.

ization forming polymer. Wettability will be dramatically varied during the sol−gel process, which can drive a phase separation. Some snowman-like silica/PS composite particles were found after carefully adjusting pH and surfactant concentration. However, the composite particles contain linear PS side which remains dissoluble in PS good solvents. In order to achieve robust polymer side, a cross-linked polymer hollow particle seed should be used instead. The cross-linked polymer hollow particles are synthesized by swelling emulsion polymerization against the particle (HP-433) using divinylbenzene (DVB). The seed PS particle possesses an average outer diameter of 440 nm and a cavity diameter of 240 nm. At a DVB/seed particle weight ratio of 1/5, the PDVB/PS composite hollow particles are concentric (Figure 1a). The shell becomes thicker from 100 to 120 nm, while the cavity becomes smaller from 240 to 200 nm. The outer diameter remains the same 440 nm as the seed particle. After dissolution of PS with DMF, the shell preserves integrity deriving the crosslinked PDVB hollow particles (Figure 1b). This implies that PDVB forms a penetration network within the continuous PS matrix. The shell thickness is preserved the same irrelevant with removal of PS. The PDVB hollow particles possess transverse channels across the shell and a coarsening exterior surface. The PDVB particles become slightly shrunk due to internal stress release from the PDVB network. At DVB/seed particle ratio of 3/5, the hollow particles become eccentric (Figure 1c). The eccentric structure is arisen by a phase separation driven by

3. RESULTS AND DISCUSSION We previously reported on synthesis of PS/silica patchy composite particles via swelling polymerization using gelable monomer MPS against the commercial polystyrene (PS) hollow seed particle (HP-433) as a seed.19 MPS can serve as a cross-linked by sol−gel process besides free radical polymerC

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Figure 2. Morphological evolution of the silica@PDVB/PS composite Janus particles with reaction time after completion of feeding monomer (h): (a) 0; (b) 4; (c) 6; (d) 12; (e) 16; (f) 24. The monomer MPS/seed particle weight ratio is fixed at 6/10. The PDVB/PS hollow seed particle is synthesized at DVB/PS seed particle ratio of 3/5.

high cross-linking degree resultant elastic stress.23 After treatment with DMF, the corresponding eccentric PDVB hollow particles are derived (Figure 1d). No obvious pores are observed within the shell. At higher DVB/seed particle ratio of 5/5, the eccentricity becomes more remarkable (Figure 1e). The shell becomes thicker while the cavity becomes smaller. Shell of the PDVB hollow particles is completely enclosed (Figure 1f). Some unsaturated vinyl- groups should be residual at the PDVB hollow spheres, which can facilitate a further modification and growth of other materials. In the FT-IR curve (Figure S1), a new peak at 3040 cm−1 assigned to CH2CHgroup reveals the presence of pendant styrene. The peak at 2940 cm−1 is assigned to −CH−CH2 group after polymerization of the styrene. Using the PDVB/PS composite hollow particle as a new seed, the silica@PDVB/PS composite snowman-like Janus hollow particles are synthesized by seed emulsion polymerization. The PDVB/PS seed hollow particle synthesized at DVB/PS seed particle weight ratio of 3/5 is used as an example (as shown in Figure 1c). Different from the previous report using the linear PS seed particle,18 the PDVB/PS hollow seed particle is highly cross-linked. This can ensure that polymer side of the Janus particles is robust to tolerate against solvents. Otherwise, the Janus particles will lose the polymer side after dissolution and become incapable to emulsify good solvents. The PDVB/PS composite hollow particle is dispersed in water

in the presence surfactant of SDS. Monomer MPS is emulsified in water containing initiator KPS, forming a monomer-in-water emulsion at ambient temperature. After dropping the monomer emulsion into the seed dispersion under stirring at 70 °C within 30 min, the seed emulsion polymerization was carried out for varied time. At early stage of polymerization (0−4 h), no bulges form, and the particle morphology is less affected with time (Figures 2a, b). The particles are individual without coalescence. During this period, polymerization is nearly completed to form a polyMPS (PMPS) network within the shell matrix. With increasing reaction time (6 h), a small bulge appears (Figure 2c). Besides coalescence between deformable bulges, some bulges are adhered onto the polymer side. This implies that the both the bulge and exterior surface of the polymer side are covered with Si−OH group. Sol−gel process of PMPS is gradually progress. After 12 h, the bulges keep the same size, and have developed a regular semispherical shape (Figure 2d). This is understandable that the bulges become stronger with further sol−gel process. After 16 h, the bulges are individual and no adhesion onto the polymer surface is found (Figure 2e). The bulges keep the same size even after 24 h (Figure 2f). Size of the silica bulge is tunable with MPS feeding content. When MPS/seed particle ratio (R) is at a low level for instance 3.0/10.0, a small silica bulge ∼90 nm in diameter is protruded from the seed particle surface (Figure 3a). Silica/polymer D

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Figure 3. SEM and inset TEM images of the silica@PDVB/PS composite Janus particles synthesized at varied weight ratio of MPS to PDVB/PS seed particle (R): (a) 3/10; (b) 6/10; (c) 15/10; (d) 18/10. The PDVB/PS seed hollow particle is synthesized at DVB/PS seed particle weight ratio of 3/5.

Figure 4. (a) Dispersions of the silica@PDVB/PS composite Janus particle (as shown in Figure 3b) in water (left) and oil (right); (b) SEM and inset TEM images of the silica particle after removal of PDVB/PS by calcination in air at 450 °C; (c) SEM and inset TEM images of the PDVB/PS hollow particle after removal of silica by etching with aqueous NH4F; (d) SEM and inset TEM images of the silica@PDVB composite Janus particle after removal of PS with DMF.

weight ratio is 2.27/10.0 measured by TGA (Figure S2). TEM image indicates that the bulge is solid. A cavity ∼180 nm is preserved at the PDVB/PS side. No individual silica particles form in the continuous aqueous phase. At R = 9.0/10.0, the silica bulge becomes larger ∼180 nm (Figure 3b). Silica/ polymer weight ratio is 5.72/10.0. At R = 15/10, the silica bulge becomes further larger which are comparable with the PDVB/ PS side (Figure 3c). Silica/polymer weight ratio is increased to 8.72/10.0. At R = 18/10, the silica bulge becomes larger than the PDVB/PS side (Figure 3d). The cavity becomes smaller ∼100 nm. Silica/polymer weight ratio is 9.96/10.0. It is

important to tune the aspect size ratio across 1.0/1.0 to further adjust wettability mismatch and steric constrain content of the Janus particles. A further analysis indicates that the silica/ polymer weight ratio increases proportionally with MPS feeding content (Figure S3). Sol−gel process of PMPS is further confirmed by FT-IR spectrum. The peak at 1160 cm−1 is assigned to the asymmetrical stretching vibration of Si−O−Si bond (Figure S4), implying the presence of silica after sol−gel process of PMPS. PMPS is the major phase which is confirmed by the strong peak at 1726 cm−1 assigned to vibration of CO. E

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Figure 5. (a) SEM and inset TEM images of the OTES−silica@PDVB composite particle; (b) EDX results of the silica@PDVB particle before (1) and after (2) modification with OTES; (c) SEM and inset TEM images of the OTES−silica@PDVB−PAM composite Janus particle; (d) SEM and inset TEM images of the OTES−silica@PDVB−PAA composite Janus particle; (e) dispersions of the OTES−silica@PDVB−PAA composite Janus particle in water at high pH = 8 (left) and oil (right). (f) pH response of the OTES−silica@PDVB−PAA composite particle in water: (a) at pH = 2; (b) at pH = 4.5.

PDVB side becomes nanoscale coarsening, while the silica bulge preserves smooth. Two sides of the silica@PDVB composite Janus particles are reactive which can be selectively modified to grow other materials. Composition and performance of the Janus particles will be broadly extended. As proof of the concept, desired organic species are selectively introduced onto the silica bulge surface using silanes for example N-octyltriethoxysilane (OTES) to introduce hydrophobic octyl group. The modified OTES−silica@PDVB composite particles are achieved (Figure 5a). The ζ-potential is greatly lowered from −39.0 (Si−OH) to −10.6 mV (Si−OC8H17) (Figure S5). This implies that a majority of the Si−OH group onto the silica bulge surface has been terminated with the octyl- group. EDX result reveals a remarkable increase of C element content onto the silica bulge (Figure 5b). Presence of octyl group is also confirmed by FT-IR spectrum (Figure S6). The silica bulge becomes hydrophobic from hydrophilic. The particles are dispersible only in oil but not in water. Onto the PDVB side, there exist residual vinyl groups to allow a further grafting of other polymers for example poly(acrylamide) (PAM) by free radical polymerization. The OTES−silica@PDVB−PAM composite Janus particles are achieved (Figure 5c). A fibril structure onto the polymer side is found. The characteristic peaks at 1690−1650 cm−1 are

The as-synthesized silica@PS/PDVB composite particles contain surfactant SDS onto the exterior surface. They are dispersible only in water but not in oil. After washing SDS from the surface, the hydrophobic PS/PDVB side is exposed. The composite particles become well dispersible both in water and oil (Figure 4a). This implies that the particles are Janus in wettability. In order to clarify that silica and PDVB/PS are distinctly compartmentalized, the PDVB/PS and the silica sides are selectively removed from the composite particles. After the PDVB/PS side is removed by calcination in air at 450 °C, the silica caps are obtained (Figure 4b). The smooth concave side is originally connected onto the polymer surface. No other small silica particles are found. This implies that nearly all the monomer MPS has been converted into the bulge. Only ∼40% weight of the silica cap is left during TGA measurement, implying a majority of organic species is present in the bulge. After the silica part is removed by etching with NH4F, the polymer hollow particles are derived (Figure 4c). No holes or dents are left on the polymer surface, indicating that no other small silica particles form during formation of the bulge. In order to obtain good solvent tolerant robust Janus particles, PS is dissolved from the PDVB/PS side with DMF. The silica@ PDVB composite Janus particles are achieved (Figure 4d). The F

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Figure 6. (a) SEM image of the frozen wax-in-water (1/9 w/w) emulsion stabilized with the silica@PDVB Janus particle (as shown in Figure 3b); (b) SEM image of the frozen wax-in-water (1/9 w/w) emulsion stabilized with the OTES−silica@PDVB−PAA particle (as shown Figure 5d); (c, d) SEM images of the frozen water-in-wax (1/9 w/w) emulsions stabilized with the silica@PDVB Janus particle and the OTES−silica@PDVB−PAA composite Janus particle, respectively. pH is fixed at 8.

Figure 7. (a) Photographs of the immiscible decane/water mixtures at varied weight ratio: (1) 7/3, (2) 6.2/3.8, (3) 6/4, and (4) 5/5, which are stabilized with the silica@PDVB Janus particle. (b) Immiscible mixtures at varied ratio: (1) 7/3, (2) 6/4, (3) 5.4/4.6, and (4) 5/5, which are stabilized with the OTES−silica@PDVB−PAA composite Janus particle. pH is fixed at 8.

phase (Figure 6a). This indicates that the Janus particles adopt a well-defined standing orientation at the interface. Janus performance of the particles can be easily reverted after selective modifications of the two sides. In contrast, the silica side of the OTES−silica@PDVB−PAA composite Janus particles becomes hydrophobic, while the PDVB side becomes hydrophilic. In order to ensure hydrophilic performance of PAA, pH is fixed at high level such as 8. Although a wax-inwater emulsion forms, the coarsening hydrophilic PDVB−PAA sides of all the particles direct toward the aqueous phase (Figure 6b). On the other hand, if wax is a major phase, a water-in-wax (1/9) emulsion forms. After the sample is fractured and freeze-dried, many voids are left. At the void interface, the smooth silica sides of the silica@PDVB Janus particles direct toward the internal aqueous phase (Figure 6c). In the case of OTES−silica@PDVB−PAA composite Janus particles, the coarsening PDVB−PAA sides direct inwardly (Figure 6d). The Janus particles are robust, and other good solvents including toluene can be easily emulsified in water forming a stable emulsion. Janus balance of the particles with varied emulsification capability can be precisely tunable with composition besides

assigned to amide group (Figure S7). The particles become dispersible in water besides in oil. The dispersion keeps stable irrelevant with pH. After hydrolyzing PAM into poly(acrylic acid)(PAA), the OTES−silica@PDVB−PAA composite Janus particles are derived (Figure 5d). The peak at 1280 cm−1 is assigned to acrylic acid group (Figure S8). The OTES−silica@ PDVB−PAA composite particles preserve dispersible in oil and water at high pH for example 8. It is interesting that Janus performance of the particles is dependent on pH. At low pH level for example ∼2, the particles float at the top layer, some large aggregates are visible and present in the aqueous phase (Figure 5f1). When pH is increased above a critical value of 4.5 (corresponding to pKa of COOH group), the particles start to become homogeneously dispersible in water (Figure 5f2). Silica@PDVB composite Janus particles can serve as a solid emulsifier to stabilize emulsions. In order to reveal orientation of the Janus particles at the emulsion interface, wax (Tm = 52− 54 °C) is used as an example oil. A wax-in-water (1/9) emulsion is formed at high temperature. Upon cooling to room temperature, orientation of the Janus particles at the wax particle surface is frozen. The smooth hydrophilic silica bulges of all the Janus particles direct toward the external aqueous G

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(6) Kim, J. W.; Lee, D.; Shum, H. C.; Weitz, D. A. Adv. Mater. 2008, 20, 3239−3243. (7) Lee, D.; Park, B. J. Soft Matter 2012, 8, 7690−7698. (8) Chaturvedi, N.; Juluri, B. K.; Hao, Q. Z.; Huang, T. J.; Velegol, D. J. Colloid Interface Sci. 2012, 371, 28−33. (9) Sheu, H.; Ei-Aasser, M.; Vanderhoff, J. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 629−653. (10) Okubo, M.; Wang, Z.; Ise, E.; Minami, H. Colloid Polym. Sci. 2001, 279, 976−982. (11) Pfau, A.; Sander, R.; Kirsch, S. Langmuir 2002, 18, 2880−2887. (12) Kim, J. W.; Larsen, R. J.; Weitz, D. A. J. Am. Chem. Soc. 2006, 128, 14374−14377. (13) Kim, J.; Larsen, R. J.; Weitz, D. A. Adv. Mater. 2007, 19, 2005− 2009. (14) Hoffmann, M.; Lu, Y.; Schrinner, M.; Ballauff, M.; Harnau, L. J. Phys. Chem. B 2008, 112, 14843−14850. (15) Skelhon, T. S.; Chen, Y.; Bon, S. A. F. Langmuir 2014, 30, 13525−13532. (16) Tang, C.; Zhang, C. L.; Liu, J. G.; Qu, X. Z.; Li, J. L.; Yang, Z. Z. Macromolecules 2010, 43, 5114−5120. (17) Nagao, D.; Hashimoto, M.; Hayasaka, K.; Konno, M. Macromol. Rapid Commun. 2008, 29, 1484−1488. (18) Zhang, C. L.; Liu, B.; Tang, C.; Liu, J. G.; Qu, X. Z.; Li, J. L.; Yang, Z. Z. Chem. Commun. 2010, 46, 4610−4612. (19) Tang, C.; Zhang, C. L.; Sun, Y. J.; Liang, F. X.; Wang, Q.; Li, J. L.; Qu, X. Z.; Yang, Z. Z. Macromolecules 2013, 46, 188−193. (20) Yu, H. K.; Mao, Z. W.; Wang, D. Y. J. Am. Chem. Soc. 2009, 131, 6366−6367. (21) Park, J. G.; Forster, J. D.; Dufresne, E. R. J. Am. Chem. Soc. 2010, 132, 5960−5961. (22) Monteiro, M. J.; Cunningham, M. F. Macromolecules 2012, 45, 4939−4957. (23) Li, J. J.; Ding, S. J.; Zhang, C. L.; Yang, Z. Z. Polymer 2009, 50, 3943−3949.

aspect size ratio. In the presence of the silica@PDVB Janus particles, oil (decane)-in-water emulsions always form at the top phase when W/O ratio is above 7/3 (Figure 7a). At low W/O ratio below 6/4, water-in-oil emulsions form at the bottom phase. Therefore, the silica@PDVB particles are more hydrophobic. This is consistent with larger PDVB side than the silica bulge. Decreasing the W/O ratio close to ∼6.2/3.8 leads the emulsion to inverse from O/W to W/O. When the OTES− silica@PDVB−PAA composite Janus particles are used, the phase inversion point is at 5.4/4.6 (Figure 7b). At a W/O ratio of 6/4, the emulsion is the O/W type. In comparison, the emulsion is the W/O type using the silica@PDVB Janus particles. This implies that the particles are more hydrophilic since the PDVB−PAA side is larger than the OTES−silica side. More detailed characterization of the emulsion phase inversion is in progress.

4. CONCLUSION We have reported on large scale synthesis of the snowman-like silica@PDVB/PS composite Janus particles by seed emulsion polymerization. Dissolution of PS from the PDVB/PS side leads nanoscale roughness. The silica@PDVB Janus particles are robust to tolerate organic solvents. Size aspect ratio of two sides is tunable across 1:1, implying that Janus balance is tunable from more hydrophobic to hydrophilic. Both sides of the silica@PDVB Janus particles are reactive to allow selectively growth of desired materials to derive a huge family of Janus particles such as OTES−silica@PDVB−PAA with tunable Janus balance and functionality such as pH responsive. The current research will open an avenue to systematically characterize performance of snowman-like Janus particles and further explore their practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

FT-IR spectra, TGA curves, ζ-potential distribution, and the weight ratio curves 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 Authors

* E-mail: [email protected] (F.L.). *E-mail: [email protected] (Z.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.Y. and F.L. acknowledge support by MOST of China (2012CB933200) and NSF of China (51233007 and 51173191).

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REFERENCES

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DOI: 10.1021/acs.macromol.5b00207 Macromolecules XXXX, XXX, XXX−XXX