Triblock Janus Particles by Seeded Emulsion Polymerization

Dec 18, 2018 - Triblock Janus particles are prepared by two-step seeded emulsion polymerization against eccentric polymer hollow particles. Two silica...
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Triblock Janus Particles by Seeded Emulsion Polymerization Xiaotian Yu,†,§ Yijing Sun,† Fuxin Liang,*,† Bingyin Jiang,† and Zhenzhong Yang*,†,‡ †

State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Polymer Institute, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China § University of Chinese Academy of Sciences, Beijing 100049, China

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S Supporting Information *

ABSTRACT: Triblock Janus particles are prepared by twostep seeded emulsion polymerization against eccentric polymer hollow particles. Two silica bulges are sequentially grown at the eccentric zone transiting from the thick shell to the thin shell. The angle between the two silica bulge axes is tunable within 0−180°, determined by the eccentricity degree of the PS/PDVB hollow seed particles. The two silica bulges and the PS/PDVB domain of the triblock Janus particles can be selectively modified to enrich composition and functionality. As an example, the paramagnetic PS/PEO modified triblock Janus composite particle can serve as a functional solid surfactant to manipulate emulsion droplets with a magnet and deliver desired materials toward the interface.

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extensively used to synthesize snowman-like particles.19−26 When polymer particle dimers with varied cross-linking degrees were used as seeds, the elastic forces are directional, which can induce formation of nonspherical particles. By growing another block along the axis which the cross-linking degree differentiates, triblock rod-like particles are generated. On the other hand, triangle polymer particles will be obtained after growing the third head perpendicular to the dimer axis when the cross-linking degrees are equal.22,23 The elastic stress driven by the entropy change of the swollen network during polymerization is responsible for the macroscopic deformation of the particles.24 We have previously reported snowman-like PS/PDVB−silica Janus particles by PS/PDVB hollow particle seeded emulsion polymerization using a gellable acrylate: 3-methacryloxypropyltrimethoxysilane (MPS).27 Different from the reported entropy-driven phase separation by polymerization,19−24 polymerizing oligomers of MPS can experience partial hydrolysis during the seeded emulsion polymerization. The enthalpy gain further facilitates a complete phase separation. As a consequence, the silica bulge and the PS/PDVB domain are distinctly compartmentalized at the particle surface. Because the PS/PDVB seeded hollow particles are highly eccentric, we recently revisited growth location of the silica bulge which was not noticed previously.27 It is curious if the snowman-like PS/PDVB−silica Janus particle can serve as a new seed to grow another silica bulge.

INTRODUCTION Janus particles with two different materials and thus properties distinctly compartmentalized onto one object have been attracting more interest in both academic and industrial considerations. They can serve as solid surfactants, self-driven motors, and building blocks toward complex superstructures.1−4 The shape of the Janus particles plays important roles in determining their performances. The snowman-like shape of diblock Janus particles is the representative model.5,6 Different from the spherical counterparts, nonspherical particles are more effective to control light scattering and fluid properties.7,8 Recently, ABC triblock Janus particles have gained growing concerns. As an example, ABC Janus nanoparticles can be easily prepared by disassembly of selfassembled bulk morphology of triblock copolymers.9−11 The narrow molecular weight distribution and cross-linking capability of one block are prerequisites to ensure uniformity of the Janus NP shape. Multiblock particles are fabricated by electrohydrodynamic cospinning and skiving.12−14 The characteristic size is large on the micrometer scale or above. Triblock Janus spheres are prepared by the sandwich printing method.15−18 The yield of Janus particles by the above methods is rather low. It is highly required to develop facile methods to produce triblock Janus particles with nonspherical contours on a large scale. Seeded emulsion polymerization is promising for the easy control of particle morphology, and it is also capable of largescale production. The interplay of dynamic internal stress retraction and thermodynamic factors during the swelling polymerization is key to determining the final morphologies.19,20 The method allows a precise control over shape and phase and surface composition of the particles, which has been © XXXX American Chemical Society

Received: October 1, 2018 Revised: November 30, 2018

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DOI: 10.1021/acs.macromol.8b02101 Macromolecules XXXX, XXX, XXX−XXX

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under stirring at 70 °C. 0.600 g of MPS, 0.600 g of aqueous KPS (1 wt %), and 0.012 g of SDS were mixed in 6.000 g of water under ultrasonication at room temperature for 10 min, forming the MPS emulsion. After the MPS emulsion was added into the modified PS/ PDVB−silica block Janus particle emulsion at 70 °C within 30 min, the mixture was polymerized at 70 °C for 24 h. Afterward, 0.5 mL of aqueous ammonia (28 wt %) was added under stirring at 70 °C. A further sol−gel process of PMPS was performed for 6 h. The ABC triblock Janus particle was obtained after centrifugation and washing with water and ethanol for three times. Synthesis of the ABC Triblock Janus Composite Particles. Grafting PS onto the First Silica Bulge by ATRP. 0.050 g of the ABC triblock Janus particle, 30 mg of Me6TREN, 0.600 g of styrene, and 6.0 mL of methanol were mixed. The mixture was degassed after three cycles of freeze−pump−thaw. In a frozen state, 15 mg of CuBr was added under nitrogen. The mixture was stirred at 30 °C for 24 h for the polymerization. After termination by exposure to air, the product was washed with ethanol for three times and vacuum-dried at 35 °C for 24 h. Hydrophilic Modification of the Second Silica Bulge. 200 mg of the PS-grafted ABC triblock Janus particle was dispersed in 20.00 g of ethanol. 50 μL of 2-methoxy(polyethyleneoxy) 6−9 propyltrimethoxysilane was added and refluxed at 70 °C for 24 h. The 2methoxy(polyethyleneoxy)6−9propyl group was introduced to the other silica bulge. After centrifugation and washing with ethanol and water for three times, the sample was purified. Maleic Anhydride Copolymerization at the PS/PDVB Domain. 100 mg of the modified ABC triblock Janus particle, 50 mg of maleic anhydride, and 85 mg of AIBN were mixed in 50 mL of toluene under stirring at 70 °C for 24 h. Maleic anhydride was copolymerized at the PS/PDVB domain. After centrifugation and washing with toluene, the ABC triblock Janus particle was obtained. Synthesis of Paramagnetic ABC Triblock Janus Particles. 10 mg of amine-group-capped Fe3O4 nanoparticle (synthesized according to ref 30) and 100 mg of the maleic anhydride modified ABC triblock Janus particle were mixed in 30 mg of water. After 100 mg of 1-(3(dimethylamino)propyl)-3-ethylcarbodiimide hydrochloride and 100 mg of N-hydroxysuccinimide were added, the mixture was stirred at 30 °C for 24 h. After centrifugation and washing with water for three times, a paramagnetic ABC triblock Janus composite particle was obtained. Characterization. The morphology of the Janus particles was characterized with a scanning electron microscope (Hitachi S-4800 at 15 kV) equipped with an energy-dispersive X-ray (EDX) analyzer and a transmission electron microscope (JEOL1011 at 100 kV). The samples for SEM observation were prepared by vacuum sputtering with Pt onto the ambient dried samples. The samples for TEM observation were prepared by spreading very dilute suspensions in ethanol onto carbon-coated copper grids. Fourier-transform infrared spectroscopy was performed on the sample/KBr pressed pellets using a BRUKER EQUINOX 55 spectrometer. The size distribution and zeta-potential measurement were performed on a NANO ZS (Malvern Instruments) at 25 °C. Thermogravimetric analysis was performed using the PerkinElmer Pyris 1 TGA in air at a scanning rate of 5 °C/min. The interfacial tension was measured with a Dataphysics OCA20 using n-hexane as the oil phase at 25 °C.

Herein we propose a facile method to synthesize ABC triblock Janus particles by two-step seeded emulsion polymerization. In the first step, a snowman-like PS/PDVB−silica Janus particle is synthesized by seeded emulsion polymerization against the cross-linked polymer hollow particle using a gellable acrylate MPS. Especially, when the seed particle is highly eccentric, the silica bulge grows at the eccentric transition zone. The silica bulge is modified with silanes to render the hydrophobic property. By the second step, the second silica bulge forms from the transition zone of the modified Janus particle again via seeded emulsion polymerization, achieving the ABC triblock Janus particle. Hydrophobic modification of the first silica bulge is determinative to grow the second silica bulge during the seeded emulsion polymerization. The angle between the two silica bulge axes is broadly tunable according to the eccentricity degree of the polymer hollow particle seeds.

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EXPERIMENT

Materials. The polystyrene (PS) hollow particle (HP-433) emulsion with a solid content of 37.5 wt % was purchased from the former Rohm & Hass. Divinylbenzene (DVB) and styrene (St) were purchased from Aldrich and further purified by passing through basic Al2O3 column. 3-Methacryloxypropyltrimethoxysilane (MPS) was purchased from ACROS. 2-Methoxy(polyethyleneoxy)6−9propyltrimethoxysilane, 4-chloromethylphenyltrimethoxysilane, azobis(isobutyronitrile) (AIBN), copper bromide (CuBr), and 1-(3(dimethylamino)propyl)-3-ethylcarbodiimide hydrochloride were purchased from J&K Chemicals. Tris2-(dimethylamino)ethylamine (Me6TREN) was purchased from TCI. N-Hydroxysuccinimide was purchased from Aladdin. Ethanol, potassium persulfate, maleic anhydride, and sodium dodecyl sulfate (SDS) were purchased from Sinopharm Chemical Reagent. Synthesis of Cross-Linked PS/PDVB Hollow Particles. 13.335 g of the PS hollow particle HP-433 emulsion (solid content of 37.5 wt %) was dispersed in 160.000 g of water as the seed emulsion. The mixture of DVB and AIBN was emulsified under ultrasonication in the presence of SDS for 1 min, forming a DVB/AIBN emulsion. After the DVB/AIBN emulsion was added into the seed emulsion under stirring at 300 rpm at 30 °C, the system was stirred for 8 h to swell the PS particle with the monomer/initiator mixture. Afterward, the swollen emulsion was heated for polymerization at 70 °C for 12 h. Cross-linked PS/PDVB hollow particles were obtained. The eccentricity of the hollow particles is tunable by alternation of the DVB feeding amount. Synthesis of PS/PDVB−Silica Janus Particles. Diblock Janus Particles. 24.600 g of one example cross-linked PS/PDVB hollow particle emulsion (containing 1.000 g of the PS/PDVB hollow particle) was added into a flask and stirred at 300 rpm at 70 °C. 0.750 g of MPS, 0.750 g of aqueous KPS (1.0 wt %), 0.020 g of SDS, and 10.000 g of water were mixed under ultrasonication at room temperature for 10 min, forming the MPS emulsion. The MPS emulsion was added into the PS/PDVB hollow particle emulsion within 30 min under stirring at 300 rpm at 70 °C. After the polymerization at 70 °C for 24 h, 2 mL of aqueous ammonia (28.0 wt %) was added. A further sol−gel process of poly(3-methacryloxypropyltrimethoxysilane) (PMPS) was performed under stirring at 70 °C for 6 h. An example PS/PDVB−silica diblock Janus particle was obtained after washing with water and ethanol for three times. Hydrophobic Modification of the Silica Bulge. 0.100 g of freezedried powder of the PS/PDVB−silica diblock Janus particle was dispersed in 20.000 g of ethanol. 25.0 μL of 4-chloromethylphenyltrimethoxysilane was added and refluxed for the hydrophobic modification of the silica bulge at 70 °C for 24 h. One example modified PS/PDVB−silica Janus particle was obtained after washing with ethanol for three times. Synthesis of ABC Triblock Janus Particles. 0.300 g of the modified PS/PDVB−silica Janus particle was dispersed in 25.0 mL of water

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RESULTS AND DISCUSSION The PS/PDVB hollow particles were synthesized by swelling emulsion polymerization of divinylbenzene (DVB) against a commercial polystyrene (PS) hollow particle. The PS/PDVB hollow particles become robust which can retain hollow structure in solvents, implying the PDVB forms a continuous network within the PS matrix. As previously reported,28,29 the PS/PDVB hollow particles become eccentric when the DVB feeding amount is high. The eccentric structure originates from an asymmetric phase separation in a highly viscoelastic matrix. The eccentricity degree is tunable by simple alteration of DVB B

DOI: 10.1021/acs.macromol.8b02101 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. TEM images of the representative PS/PDVB hollow particles synthesized at varied DVB/PS weight ratios: (a) 1:5, (b) 3:5, (c) 5:5, and (d) 7:5. (e) Size distribution of different PS/PDVB seed hollow particles. (f) Eccentricity degree of different PS/PDVB seed hollow particles.

Scheme 1. Illustrative Synthesis of the Triblock Janus Particle by Seeded Emulsion Polymerizationa

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A snowman-like PS/PDVB−silica diblock Janus particle is synthesized against the PS/PDVB hollow particle by seeded emulsion polymerization of MPS. The silica bulge (blue) surface of the Janus particle is selectively modified with a silane to introduce a hydrophobic as example 4chloromethylphenyl group (yellow). Against the modified PS/PDVB−silica diblock Janus particle, the ABC triblock Janus particle is obtained by growing a second silica bulge by seeded emulsion polymerization. The two silica bulges and the PS/PDVB domain can be selectively modified to further conjugate functional materials at the three parts.

ratio of 5:5 (Figure 1c). At a higher ratio from 7:5 to 10:5, the half-shell becomes much thicker than the other part (Figure 1d and Figure S1b). The size distribution of PS/PDVB seed particles is shown in Figure 1e and Figure S2. 300 particles per sample are taken to conduct a statistical analyze about their size distribution. They are 494 ± 14 nm (Figure 1a), 510 ± 15

feeding amount. As a comparison, the original PS hollow particle is centric with the same thick shell (Figure S1a). After polymerization of DVB at a low DVB/PS weight ratio, for example 1:5, the shell becomes thicker yet uniform (Figure 1a). At the ratio of 3:5, the shell appears eccentric (Figure 1b). The eccentricity becomes more remarkable at the DVB/PS C

DOI: 10.1021/acs.macromol.8b02101 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules nm (Figure 1b), 571 ± 9 nm (Figure 1c), and 611 ± 14 nm (Figure 1d) in diameter. We have previously reported on the synthesis of PS/PDVB− silica snowman-like Janus hollow particles by emulsion polymerization of MPS against the PS/PDVB hollow seed particles,27 where the silica bulge grown from the seed particle is unknown. The PS/PDVB hollow particles with tunable eccentricity degree provide a model to determine the growth location of the silica bulge. The eccentricity degree (de) of the PS/PDVB hollow seed particles is defined as following equation (the meaning of de, r1, r2, and d is shown in Figure S3):

transiting from the thick shell to the thin one. It is understandable that the internal stress is prone to accumulate at the transition zone, further facilitating release of the polymerization-induced internal stress. As a consequence, the silica bulge is favorably grown thereby. In the case of a highly eccentric hollow particle (DVB/PS ratio of 7:5), it is obvious that the silica bulge is located at the transition zone (Figure 2d). The silica bulge of the snowman-like Janus particle derived from the highly eccentric hollow particle (synthesized at a DVB/PS ratio of 10:5) is exactly present at the transition zone (Figure S4). It is noted that the silica bulge keeps the same size regardless of the eccentricity degree of the hollow particles. DLS analysis shows that the example PS/PDVB hollow particle (synthesized at DVB/PS weight ratio of 3:5) is 440 nm in diameter, while the derived diblock Janus particle becomes larger to 562 nm (Figure S5). TGA analysis gives a silica content of 40.5% (Figure S6). As previously reported, the silica bulge size is easily tunable by the MPS feeding amount.27 The snowman-like PS/PDVB−silica Janus particle can serve as a new seed; another silica bulge can grow by the second seeded polymerization. A triblock Janus particle is thus expected. For example, when the as-synthesized PS/PDVB− silica Janus particle is used (as shown in Figure 2c), the silica bulge surface covered with silanol groups (Si−OH) is hydrophilic. During the second seeded swelling polymerization, the growing oligomer species tend to grow onto the silica bulge surface. The silica bulge becomes larger (Figure 3a). No other silica bulges are found on the shell surface.

de = (d1 − d 2)/d1

Using the above equation, we can measure the eccentricity degree of the PS/PDVB seed particles in Figure 1, as shown in Figure 1f. 300 particles per sample are taken to conduct the measurement of eccentricity degree. The eccentricity degree is 0.113 ± 0.011 (Figure 1a), 0.523 ± 0.009 (Figure 1b), 0.674 ± 0.010 (Figure 1c), and 0.819 ± 0.009 (Figure 1d). Against the original PS hollow particle, a PS−silica Janus particle was derived (Figure 2a). First, monomer MPS and

Figure 2. (a) Snowman-like diblock Janus particle against the PS hollow particle; the Janus particles from those PS/PDVB hollow particles at varied DVB/PS weight ratios: (b) 1:5, (c) 3:5, and (d) 7:5. The MPS/particle weight ratio is fixed at 0.9:1.

water are emulsified to form the MPS-in-water emulsion with the assistance of SDS because the MPS is a hydrophobic monomer. The emulsion should be dropped into the seed emulsion within 30 min as described in the Experiment section. The hydrophobic seed particles can be well swelled by MPS. Otherwise, the silane group in MPS tends to hydrolyze, and the monomer becomes more hydrophilic. Then the swelling of seed particles is almost impossible. After the MPS emulsion is added into the system, the reaction is carried on for 24 h to finish the phase separation process. The PS shell retains uniform in thickness. When the PS/PDVB hollow particle (synthesized at the DVB/PS ratio of 1:5) is used, the PS/PDVB−silica Janus particle keeps the uniform PS shell but thicker (Figure 2b). An eccentric hollow particle (synthesized at the DVB/PS ratio of 3:5) was used to determine growth location of the silica bulge. It is interesting that the silica bulge is grown at neither the thicker half shell zone nor the thinner zone (Figure 2c). The bulge is located at the eccentricity zone

Figure 3. (a) SEM and inset TEM images of the Janus particle by seeded emulsion polymerization against the as-synthesized PS/PDVBsilica Janus particle. (b) The triblock Janus particle against the 4chloromethylphenyl modified PS/PDVB−silica Janus particle. Two triblock Janus particles synthesized at two MPS/Janus particle weight ratios during the second swelling polymerization: (c) 0.75:1 and (d) 4:1.

When the silica bulge surface was modified for example to introduce 4-chloromethylphenyl hydrophobic group, a second silica bulge formed while the first bulge remained the same size as before (Figure 3b). The chlorine element is detected exclusively from one bulge, while the other silica bulge contains no chlorine (Figure S7). The triblock Janus particle becomes larger to 677 nm in diameter (Figure S8). The silica content is further increased from 40.5% to 65.0% (Figure S9). To demonstrate the hydrophobicity requirement of the silica surface to form the second silica bulge, another example silane D

DOI: 10.1021/acs.macromol.8b02101 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules of n-octadecyltrichlorosilane was used to modify the first silica bulge surface. A second silica bulge is indeed grown thereby (Figure S10). Similarly, the size of the second silica bulge can be easily adjusted. At a MPS/Janus particle weight ratio of 0.75:1 during the second seeded emulsion polymerization, the second silica bulge is smaller than the first one (Figure 3c). At the weight ratio of 4:1, the second bulge is larger (Figure 3d). Similar to the first silica bulge, the second silica bulge is also grown at the eccentricity transition zone. It is anticipated that the angle between the two bulge axes is tunable according to the eccentricity degree of the hollow seed particles. Against the example concentric hollow particle (synthesized at DVB/PS ratio of 1:5), the two silica bulges are located at opposite poles of the hollow particle (Figure 4a). The angle between two

example oil of n-hexane (Figure S11). It is shown that the interfacial tension keeps the same before and after polymerization. The critical micellization concentration (CMC) of SDS is measured to be ∼1.09 mg/mL. At low SDS concentration regions, the interfacial tension drops dramatically. This means that SDS is in the starvation state to form micelles. Because the surfactant is insufficient to completely cover the particle during the second swelling seeded emulsion polymerization, the partially hydrolyzed silica species growing from the polymer shell can act as surfactants to further stabilize the particles. Meanwhile, the silica species are further coalesced forming the second silica bulge. At the concentration of 0.40 mg/mL, the triblock Janus particles are obtained (Figure S12). When SDS concentration is lower, for example 0.20 mg/mL, a number of small silica particles are present in the serum, while the second silica bulge forms onto the polymer shell (Figure S13). On the other hand, at a high SDS concentration above 0.60 mg/mL, the silica appears onto the whole surface of the diblock Janus particle. A core−shell structure thus forms (Figure S14a). After removal of polymers by calcination in air, a silica hollow particle was derived (Figure S14b). The SDS concentration window of 0.28−0.45 mg/mL is experimentally determined to guide the synthesis of triblock Janus particles. The two silica bulges are isolated at the PS/PDVB shell. After calcination in air, each triblock Janus particle (as shown in Figure 4c) can give two isolated silica caps (Figure S15). It is allowed to selectively modify the two silica bulges. The chloromethyl group on the first silica bulge surface can further initiate grafting polymers, for example PS by ATRP (Figure 5a−d). After staining with RuO4, elemental Ru is found at one

Figure 4. SEM and inset TEM images of the representative triblock Janus particles against the PS/PDVB−silica Janus particles synthesized at varied DVB/PS weight ratios: (a) 1:5, (b) 3:5, (c) 5:5, and (d) 10:5.

lobes is 175 ± 5° based on statistical analysis. When using a slightly eccentric hollow particle (synthesized at DVB/PS ratio of 3:5), the two silica bulges are arrayed in an obtuse angle (116 ± 12°) (Figure 4b). The angle becomes acute (84 ± 9°) when using a highly eccentric hollow particle (synthesized at DVB/PS ratio of 5:5) (Figure 4c). When the eccentricity degree is extremely high (in the case of the hollow particle synthesized at the ratio of 10:5), the angle is so small that the two silica bulges are coalesced, forming a larger one (Figure 4d). As previously reported, directional elastic stress induced phase separation is usually used to control position of a newly formed bulge either along the dimer axis or perpendicular to the axis.19,20,22 In our case, a new method is provided to simply control position of the new bulge according to eccentricity degree of the seed particles. The bulges tend to the position where the thickness gradient changes most in the eccentric hollow seed particles, which makes the control of triblock Janus particles’ morphology more easily. The size of the bulges along with other parameters of the triblock particles can be tuned conveniently. What is more, the angle between the two silica bulge axes can be continuously tunable ranging within 0− 180°. The formation mechanism of the PMPS bulge and the influence of the internal stress still need further investigation. The concentration of the surfactant SDS is another key parameter to synthesize the triblock Janus particles.24 The interfacial tension of the serum after centrifugation of particles was measured using the pendent drop method against an

Figure 5. (a) SEM and inset TEM images of the triblock Janus particle after the silica bulge surface is grafted with PS. (b) EDS analysis of the triblock Janus particle before (left) and after (right) grafting PS onto the first silica bulge; the PS side is stained with RuO4. (c) PS/PEO modified triblock Janus particle. (d) EDS analysis of the PS modified triblock Janus particle (left) and the PS/PEO modified triblock Janus particle (right).

silica bulge surface while no Ru at the other one (Figure 5a− d). The hydrophilic PEO moiety can be introduced to the second silica bulge surface by modification with 2-methoxy(polyethyleneoxy)6−9propyltrimethoxysilane (Figure 5a−d). The characteristic peaks at 1104 and 2925 cm−1 show the presence of PEO (Figure S16). After staining with phosphotungstic acid, elemental W can be detected at one silica bulge. This implies that the second silica bulge has been E

DOI: 10.1021/acs.macromol.8b02101 Macromolecules XXXX, XXX, XXX−XXX

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To observe the orientation of the Janus particles at the oil− water interface, paraffin with a high melting temperature (52− 54 °C) was used to solidify the interface upon cooling to room temperature. All the hydrophilic heads were found pointing toward the external aqueous phase (Figure 6c). It is reasonable that the hydrophobic heads should face toward the internal oil phase. The PS/PDVB domain should locate at the interface. In the case of the paramagnetic PS/PEO modified triblock Janus composite particle, the emulsion droplets can be driven with a magnet and eventually collected (Figure 6d). On the other hand, desired materials, for example the functional materials (Fe3O4 nanoparticles in the current case), can be delivered toward the emulsion interface after loaded with the triblock Janus particles.

selectively modified with PEO (Figure 5a−d). An amphiphilic PS/PEO modified triblock Janus particle is thus obtained. The PS/PDVB domain of the triblock Janus particles contains residual styrenic double bonds, which can start a further modification, for example grafting maleic anhydride, thereby. The characteristic peaks at 1720 and 1784 cm−1 confirm the presence of the carbonyl group from the maleic anhydride (Figure S17). The morphology of the triblock Janus particle is less influenced after the modification (Figure 6a).

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CONCLUSION We have proposed a new approach to synthesize the triblock Janus composite particles via two-step seeded swelling emulsion polymerization. The two silica bulges are grown from the eccentricity transition zone of the eccentric PS/ PDVB hollow seed particles. The angle between the two silica bulge axes is broadly tunable within 0−180° according to the eccentricity degree. The two silica bulges and the PS/PDVB domain can be selectively modified to further extend the composition and performance. As an example, PS and PEO are introduced to the two silica bulges separately, while carboxyl groups are grafted to the PS/PDVB domain surface which can further induce favorable growth of functional materials such as paramagnetic nanoparticles. The example magnetic responsive triblock Janus composite particle can be used to manipulate interfaces with a magnet. The triblock Janus particles are capable to functionalize interfaces and deliver payloads toward the interfaces.

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Figure 6. (a) SEM and inset TEM images of the PS/PEO modified triblock Janus particle after introduction of carboxylic acid group onto the PDVB/PS domain. (b) SEM and inset TEM images of the paramagnetic PS/PEO modified triblock Janus composite particle. (c) SEM image of the emulsion paraffin particle emulsified with paramagnetic PS/PEO modified triblock Janus composite particle. (d) SEM image of the emulsion paraffin particle surface with a preferential orientation of the Janus particles at the interface. (e1) The top dyed toluene phase and the bottom phase of paramagnetic PS/PEO modified triblock Janus composite particle aqueous dispersion. (e2) A toluene-in-water emulsion stabilized with the Janus particle upon shaking. (e3) The toluene droplets are collected with a magnet. (f) The conformal image of the toluene-in-water emulsion stabilized by the paramagnetic PS/PEO modified triblock Janus composite particles.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02101. FT-IR spectra, TGA traces, size distribution and energydispersive X-ray spectra of some representative particles (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Bingyin Jiang: 0000-0002-9526-5727 Zhenzhong Yang: 0000-0002-4810-7371

Amine-group-capped Fe3O4 nanoparticles were selectively adsorbed onto the carboxylic acid modified PS/PDVB surface. By a further amidation, the nanoparticles were covalently bound onto the surface. A paramagnetic PS/PEO modified triblock Janus composite particle was obtained (Figure 6b). The polymer domain becomes slightly coarsening. Crystallinity of the Fe3O4 nanoparticles is preserved after binding (Figure S18). Both the PS/PEO modified triblock Janus particles and the paramagnetic composite particles are amphiphilic. They can be well dispersible in both polar and nonpolar solvents, for example water and toluene (Figures S19a,b). As a solid surfactant, the paraffin/water emulsion forms (Figure S19c).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (51622308 and 51233007). REFERENCES

(1) Walther, A.; Müller, A. H. E. Janus Particles: Synthesis, SelfAssembly, Physical Properties, and Applications. Chem. Rev. 2013, 113, 5194−5261.

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DOI: 10.1021/acs.macromol.8b02101 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02101 Macromolecules XXXX, XXX, XXX−XXX