Preparation of Janus Particles with Different Stabilizers and Formation

Janus particles with two hemispheres having different stabilizers, a polystyrene (PS) phase stabilized by poly(acrylic acid) (PAA) (PSPAA) and a poly(...
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Preparation of Janus Particles with Different Stabilizers and Formation of One-Dimensional Particle Arrays Shohei Onishi, Masayoshi Tokuda, Toyoko Suzuki, and Hideto Minami* Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan ABSTRACT: Janus particles with two hemispheres having different stabilizers, a polystyrene (PS) phase stabilized by poly(acrylic acid) (PAA) (PSPAA) and a poly(methyl methacrylate) (PMMA) phase stabilized by poly(vinylpyrrolidone) (PVP) (PMMAPVP), were synthesized by the solvent-absorbing/releasing method of PSPAA/PMMAPVP composite particles with a core−shell structure. The PSPAA/PMMAPVP composite particles were prepared by seeded dispersion polymerization of MMA using PVP as stabilizer in the presence of PS seed particles stabilized by PAA. We also demonstrated the facile formation of the colloidal chains via hydrogen bonding interaction between different stabilizers.

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INTRODUCTION Janus particles comprising two surfaces with different chemical compositions have attracted considerable attention from an academic perspective owing to their anisotropic wettability and optical, electronic, and magnetic properties as well as their potential applications as particulate surfactants1−3 and in electronic paper display materials4−6 and chemical and biochemical sensors.7−10 Since the pioneering research by Casagrande et al.,1 Janus particles have been prepared by various techniques such as macrophase separation induced by seeded polymerization,11,12 microfluidic methods,13−16 toposelective surface modification based on the particle monolayer,1,17−22 and Pickering emulsions.23−26 Using these techniques, Janus particles responding to external stimuli such as electric27,28 and magnetic fields,29 pH,25,30 and temperature25 have been prepared. Moreover, one-dimensional (1D) particle arrays composed of Janus particles that have been produced via anisotropic interactions formed because of their anisotropic morphology.25,27 1D particle arrays having electronic, photonic, and energy transfer can became innovative products.31 Hydrogen bonding interactions between hydrogen bond donors and water-soluble hydrogen bond acceptor polymers often induce formation of precipitates in aqueous solutions.32 Recently, we demonstrated a facile preparation and development of raspberry-like particles by mixing large and small polystyrene (PS) particles stabilized by the hydrogen bond donor polymer (poly(acrylic acid) (PAA)) and hydrogen bond acceptor polymer (poly(vinylpyrrolidone) (PVP)), respectively. The particles formed because of the hydrogen bonding interactions between the PAA and PVP, and the morphology (coverage ratio) of the particles was controlled by changing the molecular weight of the stabilizers.33 Moreover, poly(methyl methacrylate) (PMMA)-core/PS-corona and PS/SiO434 rasp© 2014 American Chemical Society

berry-like particles were also obtained because of the hydrogen bonding interactions following a simple mixing of the corresponding dispersions. Janus-like particles containing both hydrogen donor- and acceptor-type stabilizers are expected to be able to form 1D particle arrays. However, to the best of our knowledge, the preparation of Janus particles with different stabilizers has not yet been reported. Herein, we describe the preparation of Janus particles with two hemispheres (PS stabilized by PAA (PSPAA) and PMMA stabilized by PVP (PMMAPVP)). The formation of a colloidal structure (1D particle array) via hydrogen bonding interactions between the PAA and PVP stabilizers in the Janus particles is also discussed.

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EXPERIMENTAL SECTION

Materials. Styrene and MMA (Nacalai Tesque, Inc., Kyoto, Japan) were purified via distillation under reduced pressure in a N2 atmosphere. Reagent-grade 2,2′-azobis(isobutyronitrile) (AIBN, Wako Pure Chemical Industries, Osaka, Japan) and 2,2′-azobis(2,4dimethyl valeronitrile) (V-65, Wako Pure Chemical Industries, Osaka, Japan) were purified by recrystallization in methanol. Reagent grade methanol, ethanol, and toluene, a standard hydrogen phthalate buffer solution (pH 4.0), PVP (weight-average molecular weight = 4.0 × 104, Nacalai Tesque, Inc., Kyoto, Japan), PAA (weight-average molecular weight = 2.5 × 105, Sigma-Aldrich Chemical Co.), and sodium dodecyl sulfate (SDS, Wako Pure Chemical Industries, Ltd., Japan) were used as received. The deionized water used in all experiments was obtained from an Elix UV (Millipore Co., Ltd., Japan) purification system and had a resistivity of 6 μS. Received: November 21, 2014 Revised: December 20, 2014 Published: December 25, 2014 674

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Figure 1. SEM images of the (a) PSPAA seed particles and (b) particles prepared via seeded dispersion polymerization of MMA in the presence of PSPAA seed particles. (c) TEM image of ultrathin cross sections of the initially prepared composite particles after drying and staining with RuO4. Preparation of PSPAA/PMMAPVP Composite Particles. First, PS seed particles stabilized by PAA (PSPAA) were prepared via dispersion polymerization as follows. Styrene (1.0 g), AIBN (0.01 g), and PAA (0.2 g) were dissolved in an ethanol/water mixture (7.0 g/2.0 g). The solution was placed in a glass tube, and degassed using several vacuum/N2 cycles, and the tube was sealed. The sealed glass tube was then placed in a water bath at 65 °C for 24 h with shaking at 80 cycle/ min (3 cm strokes). The obtained particles were washed with ethanol via centrifugation to remove excess PAA, and then the medium was replaced with methanol. Next, the PSPAA/PMMAPVP composite particles were prepared via seeded dispersion polymerization of MMA using PVP as the stabilizer and the synthesized PSPAA particles as seed particles. MMA (0.2 g), V-65 (5 mg), and PVP (0.04 g) were added to a PSPAA dispersion (10 wt % solids in methanol/water (2.8 g/ 1.2 g)). The polymerization was performed at 50 °C for 24 h with shaking at 50 cycle/min (3 cm strokes). After polymerization, the obtained particles were washed with methanol/water via centrifugation to remove excess PVP, and then the medium was replaced with water. Preparation of PSPAA/PMMAPVP Janus Particles. The PSPAA/ PMMAPVP Janus particles were prepared using the solvent-absorbing/ releasing method (SARM), which is proposed by Okubo et al.35 for adjusting the morphology of the composite particles to obtain a thermodynamically stable structure. A typical procedure was performed as follows. Toluene (1.2 g), which is a good solvent for PS and PMMA, and SDS (0.047 g) were added to the composite particle dispersion (0.7 wt % solids) in a 50 mL glass vial, and the mixture was stirred vigorously using a NISSEI ABM-2 homogenizer at 2000 rpm for 2 min (solvent-absorbing process). The toluene was evaporated from the dispersion by stirring with a magnetic stirrer at room temperature for 24 h in an uncovered glass cylindrical vessel (solvent-releasing process). The mobility of the polymer chains should be sufficiently high to phase separate, because the viscosity of the inner toluene-swollen particles was less than approximately 10 mPas due to addition of enough toluene (polymer/toluene, 1/12(w/w)).36 After the releasing process, the SDS residing on the surface of the obtained particles was removed by centrifuging five times with water. The amount of residual toluene in the dispersion after the release process for 24 h was confirmed by gas chromatography (GC-18A, SHIMADZU, Japan) (less than 1% of its original value (1.2 g)). We also examined SDS removal by measuring the supernatant of the dispersion after centrifugal washing using the electrical conductivity meter (CM-40S, TOA, Japan). The electric conductivity of the supernatant was changed from 1110 μS/cm to 5 μS/cm (deionized water), indicating that most SDS could be removed. Characterization of PSPAA/PMMAPVP Composite and Janus Particles. The obtained PSPAA/PMMAPVP composite and Janus particles were observed with an optical microscope (ECLIPSE 80i, Nikon) and, after being coated with platinum, with a scanning electron microscope (SEM, JSM-6510, JEOL, Tokyo, Japan) at 20 kV. The number-average diameter (Dn) and coefficient of variation (Cv) were determined by counting over 200 particles in SEM images using an image analysis software (WinROOF, Mitani Co., Ltd., Japan). To observe the interior morphology of the particles before and after the SARM process, the dried PSPAA/PMMAPVP composite particles were stained with ruthenium tetraoxide (RuO4) vapor at room temperature for 30 min in the presence of a 1.0 wt % aqueous RuO4 solution,

embedded in an epoxy matrix, cured at room temperature overnight, and subsequently microtomed. Ultrathin (100 nm-thick) cross sections were observed using a transmission electron microscope (TEM, JEOL JEM-1230) at 100 kV.

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RESULTS AND DISCUSSION Figure 1a,b shows SEM images of the PSPAA seed particles and the polymer particles prepared via seeded dispersion polymerization of MMA in the presence of PSPAA seed particles and using PVP as a stabilizer, respectively. The PSPAA seed particles exhibited a high monodispersity (Dn, 1.29 μm; Cv, 5.2%), and PAA should have been grafted on the PS particles because typically during dispersion polymerization some of the steric stabilizer is grafted onto the polymerized polymer particles via hydrogen abstraction.37 After the subsequent seeded dispersion polymerization was complete, the diameter of the obtained particles (Dn, 1.44 μm; Cv, 5.9%) was larger than that of the PSPAA seed particles and consistent with the theoretical diameter increase expected for the composite particles. In addition, monodispersity was maintained. These results indicated that the seeded dispersion polymerization proceeded successfully, and we inferred that PVP was grafted onto the PMMA phase (PMMAPVP). TEM observation of ultrathin cross sections of the composite particles stained with RuO4 clearly indicated that the obtained particles had a core−shell structure consisting of a PS core and a PMMA shell, with the PS phase preferentially stained with RuO4 and appearing darker than the PMMA phase. Okubo et al. reported that the seeded dispersion polymerization technique tends to yield core−shell particles, with the shell layers accumulating in the order of their formation when the seed particles have a high glass transition temperature.38 Moreover, they demonstrated the preparation of particles with various morphologies such as ex-centered, snowman-like, and Janus particles by subjecting the composite particles to the SARM process using various emulsifies and solvents.35 Notably, in the presence of SDS, Janus particles were obtained because the approach of the interfacial tensions of the PS/SDS aq and PMMA/SDS aq resulted from a decrease in interfacial tensions of the PS/SDS aq and PMMA/SDS aq. Therefore, in this study, changing the morphology of the composite particles from a core−shell structure to a Janus structure with grafted stabilizers was initially attempted using the SARM without SDS in the expectation that the PAA and PVP stabilizers would have the same effect as SDS (Scheme 1). Figure 2a,b shows an SEM image of the PSPAA/PMMAPVP composite particles after being subjected to the SARM process in water and a TEM image of the RuO4-stained ultrathin cross sections of the particles, respectively. In the TEM images, the ultrathin cross-sections appeared to be ellipsoidally deformed particles owing to the shear stress of the cutting process. Each 675

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(Janus particles), the SARM process was performed using the composite particles in an aqueous solution of SDS (3.3 g/L). Figure 3a,b shows an SEM image of the composite particles

Scheme 1. Preparation of PSPAA/PMMAPVP Janus Particles

Figure 3. (a) SEM image and (b) TEM image of RuO4-stained ultrathin cross sections of PSPAA/PMMAPVP composite particles after the SARM process in an aqueous solution of SDS.

after the SARM process in the presence of SDS and a TEM image of RuO4-stained ultrathin cross sections of the obtained particles, respectively. Notably, after the SARM process, the composite particles remained monodisperse because of an increase in their colloidal stability resulting from the addition of SDS. In addition, the morphology of the composite particles was changed from a core−shell structure to a structure with completely different hemispheres, indicating that PSPAA/ PMMAPVP Janus particles were successfully prepared via the SARM in the presence of SDS. Most importantly, as we intended, the two different surfaces of the obtained Janus particles separately contained the PAA and PVP stabilizers. To confirm that each surface of the PSPAA/PMMAPVP Janus particles contained different (PAA and PVP) stabilizers, the Janus particles were washed many times with water to remove the SDS and then mixed with submicron-sized silica particles. At pH 6.0, the pyrrolidone groups of PVP form hydrogen bonds with the silanol groups of silica particles, resulting in heterocoagulation,34 whereas the carboxy groups of PAA do not interact with silica particles because they are ionized. Therefore, if the obtained Janus particles had different stabilizers on each half of their surfaces, the submicron-sized silica particles would be expected to heterocoagulate on one side of the surface of each Janus particle because of the hydrogen bonding interactions. Figure 4 shows an SEM image of a mixture of aqueous, pH 6.0 dispersions of the PSPAA/PMMAPVP Janus particles and silica particles. The submicron-sized silica particles can be seen

Figure 2. (a, c) SEM images of the PSPAA/PMMAPVP composite particles after the SARM process in water and methanol/water, respectively, and (b, d) TEM images of RuO4-stained ultrathin cross sections of the obtained particles shown in parts a and c, respectively.

particle had a single dimple on the surface (Figure 2a), and the particle surfaces primarily comprised PMMA (Figure 2b). Moreover, the size distribution of the particles became polydisperse because of coalescence of the toluene-swollen particles (larger particles) and splitting toluene-swollen particles (smaller particles) by a stirring bar during the SARM process. The formation of the dimples at the particle surfaces has been reported by Okubo et al.35 During toluene evaporation, phase separation occurs in the PS/PMMA composite particles. In a moderate ex-centered PS-core/ PMMA-shell structure, the PMMA shell hardens before the PS core because the PMMA shell contains comparatively less toluene.39 After the PMMA shell hardens, the PS core contracts and is accompanied by the release of the residual toluene, resulting in a cave-in at the PS core surface. Thus, to increase the PSPAA phase on the particle surfaces, the SARM process of the PSPAA/PMMAPVP composite particles was performed in methanol/water (7.5 g/2.5 g), which is a relatively hydrophobic medium. However, as we can see in Figure 2c,d, the composite particles obtained after the SARM process in methanol/water still had an ex-centered PSPAA-core/PMMAPVP-shell structure, although the PS percent on the surface appeared to have increased slightly, and the particles had a spherical shape. This approach, therefore, was insufficient for decreasing the interfacial tension between the polymers and medium. To decrease the interfacial tension and obtain composite particles with a clear, hemispherically phase-separated structure

Figure 4. SEM image of a mixture of SiO2 particles and PSPAA/ PMMAPVP Janus particles. 676

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Figure 5. Optical micrographs of PSPAA/PMMAPVP (a−c) Janus and (d−f) core−shell particles in water at pH values of (a, d) 6.0, (b, e) 4.0, and (c, f) 13.

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on only one-half of each Janus particle surface. This result strongly suggests that the PVP stabilizer existed on one-half of the particle surface because of the phase separation of the PVP (grafted with the PMMA phase) during reconstruction of the morphology using the SARM. Moreover, the other half of the surface of each Janus particle was thought to be stabilized with PAA grafted onto the PS phase. At pH 6.0, both the core−shell and Janus particles were welldispersed (Figure 5a,d), because the carboxyl groups of the PAA (pKa 4.8) were nearly all ionized; thus, hydrogen bonding was not possible between the two stabilizers. However, the formation of a 1D particle array via hydrogen bonding between the PAA and PVP stabilizers on different particles was expected at a lower pH because the carboxyl groups would be protonated (Scheme 2). Notably, the core−shell particles were aggregated

CONCLUSION Janus particles composed of PSPAA and PMMAPVP were successfully prepared using the SARM, and we confirmed that each Janus particle had a different steric stabilizer on the surface of each hemisphere, reflecting the morphology of the particle. The facile formation of colloidal chains via hydrogen bonding interactions between the different stabilizers was also demonstrated. Recently, Darrell et al. reported that particles forming 1D particle arrays may behave similarly to monomers undergoing polymerization.40 We speculated that the behavior of the present Janus particles during structuralization of the particle array does indeed look similar to that of the monomers during polymerization. Controlling the number of arrayed particles and testing this assumption will be discussed in a future study.

Scheme 2. 1D Particle Array Composed of PSPAA/PMMAPVP Janus Particles and Formed via Hydrogen Bonding Interactions

Corresponding Author

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AUTHOR INFORMATION

*E-mail: [email protected]. Phone and fax: (+81) 78 803 6197. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partially supported by a Grant-in-Aid for Scientific Research (Grant 26288103) from the Japan Society for the Promotion of Science (JSPS) and by a Research Fellowship of JSPS for Young Scientists (given to M.T.)

in a disorderly fashion in a buffer solution at pH 4.0 (Figure 5b), indicating that the PAA and PVP coexisted on the surface of the composite particles. The Janus particles, on the other hand, formed a 1D particle array at pH 4.0 (Figure 5e). Moreover, when the pH of both dispersions was changed from 4.0 to 13, the aggregated Janus and core−shell particles were redispersed (Figure 5c,f, respectively). These results indicated that the driving force for aggregation was hydrogen bonding between the stabilizers and further confirmed that PSPAA/PMMAPVP Janus particles with different stabilizers (PAA and PVP) on different parts of their surfaces were successfully prepared.

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REFERENCES

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