Article pubs.acs.org/Macromolecules
Janus Anisotropic Hybrid Particles with Tunable Size from Patchy Composite Spheres Chen Tang, Chengliang Zhang,* Yijing Sun, Fuxin Liang, Qian Wang, Jiaoli Li, Xiaozhong Qu, 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 have developed a facile approach to synthesize anisotropic Janus hybrid particles by selective modification of bulgy patches on the composite spheres, which are in situ grown by phase separation during seeded emulsion polymerization. Janus particles are achieved by dissolution of the seed polymer particles. It is the key to control phase separation to form multiple bulges and tune their size by pH and monomer fraction. The size of the Janus particles is continuously tunable from submicrometer to nanometer scale. They can be massively synthesized since many bulges are derived from one seed sphere.
1. INTRODUCTION Janus particles have two different compositions and thus properties compartmentalized onto the corresponding two parts,1 which have gained growing interests due to their unique performances and emerging applications.2 They can sever as solid surfactants to stabilize emulsions or as building units to construct complex superstructures.3 Anisotropic Janus particles have superior performances over their isotropic counterparts.4 For example, anisotropic Janus particles can bring additional spatial confinement to form unique structures,5 exhibit new optical properties as photonic crystals with complete bands,6 or give enhanced interfacial stability.7 From the viewpoint of practical applications, it is important to develop methods to large scale synthesize anisotropic Janus particles with tunable size. Self-organization of block copolymers is efficient to derive Janus objects with tunable shape from disk to rod to sheet.8 Narrow molecular weight distribution of the copolymers is necessary in order to achieve the shape uniformity. The composition is also restricted. Alternatively, Pickering emulsion assisted synthesis has been extensively employed to massively prepare Janus particles by selective modification of exposed side of the particles at the emulsion oil/water interface.9 We have extended this approach to create nonspherical Janus particles by either wet etching silica particles at the interface or stretching the deformable PS particles by interfacial stress difference at the three phase contact line.10 However, the Pickering emulsion approach becomes ineffective to prepare nanosized Janus particles. Because of low adsorption energy of nanoparticles, their motion at Pickering emulsion interface becomes considerable. Besides, the capillary wave among the particles significantly broadens the particle− interface interaction beyond the particle radius. Multiple layers of nanoparticles may form at the interface, which will cause failure in synthesis of Janus nanoparticles.11 © 2012 American Chemical Society
Emulsion polymerization is promising to large scale synthesize anisotropic Janus particles due to their easier scalability and flexibility in controlling microstructures of particles. Heterogeneous snowman-like Janus particles have been extensively synthesized by polymerization-induced phase separation.12 In order to achieve only one bulge on the particle surface, the phase separation should be strictly controlled. In many cases, the number of the bulges on the surface is not uniform, and heterogeneous composite spheres with many isolated patchy bulges on the surface are usually obtained. They are regarded as byproducts in respect to the Janus ones. No report correlates the composite spheres with Janus particles. In fact, the isolated bulgy patches appear like particles embedded onto the sphere surface, similar to the particles at a Pickering emulsion interface. The patches are partially protected to allow a selective modification. We immediately realize that a new approach is open to synthesize Janus particles as illustrated in Figure 1a. Since the bulges are in situ grown from the seed sphere with their root part tightly embedded onto the sphere surface, rotation of the bulges is prohibited. During the selective chemical modification of their exposed surface, the root part remained buried and not influenced if the polymer matrix is not swelled during the modification. After removal of the polymer parent sphere, Janus particles are achieved by collection of the modified bulges. Spherical cap of the exposed part of the bulges is acquired due to interfacial stress, while the root part keeps flat. The size of the bulgy patches is tunable from 101 to 102 nm by alteration of growth parameters and thus the size of the resultant Janus particles. Received: October 4, 2012 Revised: November 13, 2012 Published: December 24, 2012 188
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Figure 1. (a) Schematic synthesis of the Janus anisotropic hybrid particles from the heterogeneous patchy composite sphere. (b) Two reactions of MPS: free radical polymerization and hydrolysis/condensation. spectroscopy on AVANCE III 400 MHz. Gel permeation chromatography (GPC) was performed on a Waters 515 HPLC pump and a Waters 2414 refractive index detector; DMF was used as an eluent at a flow rate of 1.0 mL/min at 35 °C. pH value was measured using a Mettler Seven Go Duo.
2. EXPERIMENTAL METHODS 2.1. Materials. 3-Methacryloxypropyltrimethoxysilane (MPS), aminopropyltriethoxysilane (APS), and octyltrichlorosilane (OTS) were purchased from Alfa Aesar. Potassium persulfate (KPS), sodium dodecylbenzenesulfonate (SDS), N,N-dimethylformamide (DMF), aqueous ammonia (NH3−H2O, 28 wt %), ethanol, and hexane were purchased from Sinopharm Chemical Reagent Beijing Co. Ltd. All reagents were used as received. 2.2. Synthesis of Heterogeneous Patchy Composite Spheres. 1 g of the freeze-dried powder of polystyrene (PS) hollow sphere12e was dispersed in 20 g of water under stirring at 70 °C to form a seed emulsion. A desired amount of aqueous ammonia (NH3− H2O, 28 wt %) was introduced to tune pH of the seed emulsions. 2 g of 3-methacryloxypropyltrimethoxysilane (MPS), 2 g of 1 wt % aqueous potassium persulfate (KPS), 0.02 g of sodium dodecylbenzenesulfonate (SDS), and 10 g of water were mixed under ultrasonication at room temperature for 10 min to form a monomer emulsion. After the monomer emulsion was dropped into the seed emulsion in 0.5 h, the mixture was held for polymerization at 70 °C for varied time. Heterogeneous patchy composite spheres were obtained after centrifugation and rinsing with water and ethanol. 2.3. Janus Particles Derived from the Heterogeneous Patchy Composite Spheres. 0.1 g of freeze-dried powder of the as-prepared heterogeneous patchy composite sphere was dispersed in 20 g of ethanol. 0.1 g of aqueous ammonia (NH3−H2O, 28 wt %) and 0.4 g of γ-aminopropyltriethoxysilane (APS) were added, and the mixture was refluxed for 24 h. After centrifugation and rinsing with ethanol, the modified heterogeneous composite spheres were obtained. The corresponding APS modified Janus particles were derived after the PS seed spheres were dissolved. In a parallel procedure, 0.1 g of the patchy composite sphere was dispersed in 20 g of hexane, and 0.4 g of octyltrichlorosilane (OTS) was added. The mixture was stirred at room temperature for 24 h. Similarly, the OTS modified Janus particles were achieved. 2.4. Characterization. Structure and morphology of the particles were characterized using transmission electron microscopy (JEOL 1011 at 100 kV) and scanning electron microscopy (Hitachi S-4300 at 15 kV). The samples for SEM characterization were prepared by vacuum sputtering with Pt on the ambient dried samples. TEM samples were prepared by spreading very dilute dispersions in ethanol on carbon-coated copper grid. The degree of hydrolysis/condensation of MPS and PMPS was characterized by 29Si solid-state NMR
3. RESULTS AND DISCUSSION A commercial PS hollow sphere with an outer diameter of 400 nm and a cavity diameter of 250 nm was selected as the
Figure 2. Microstructures of the heterogeneous patchy composite spheres controlled by MPS/PS weight ratio and pH value. In the SEM images all the scale bars are 400 nm; in the inset TEM images all the scale bars are 200 nm.
example seed sphere.12e The shell is thin (about 75 nm), providing a matrix to grow bulgy particles. While the seed emulsion was heated at 70 °C, another monomer emulsion containing MPS, initiator KPS, and surfactant SDS was slowly dropped in 0.5 h. After a further reaction, the bulgy patches grow from the seed sphere surface by polymerization-induced phase separation, forming heterogeneous composite spheres. Since the silane MPS has a vinyl group, two reactions of free 189
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Figure 5. SEM images of the submicrometer anisotropic Janus particles before (a) and after (b) being labeled with sulfonated PS nanoparticles.
Figure 3. Morphological evolution of the heterogeneous composite spheres with reaction time at a MPS/PS weight ratio 2:1 and pH = 8. Inset SEM images of the derived insoluble organosilica hybrid particles after dissolution of PS.
radical polymerization of CC and hydrolysis/condensation of Si−OR could occur to derive polyMPS (PMPS) and organosilica hybrid (Figure 1b). The two reactions can be controlled to be either simultaneous or separated by alteration of pH, which determines phase separation dynamics and thus microstructure of the heterogeneous patchy composite spheres. Besides pH, the MPS/PS weight ratio is also important to control phase separation. Effects of MPS/PS weight ration and pH value on microstructure of the patchy composite spheres were systematically investigated (Figure 2). At a fixed pH = 8, the microstructural evolution of the composite spheres with MPS/PS weight ratio was recorded. When the MPS/PS ratio is kept at a sufficiently high level, for example 2:1, only one bulge grows dominantly from the PS seed sphere surface (Figure 2a1). The bulge diameter is about 300 nm. Such resultant snowman-like composite spheres are composed of the parent seed particles and the newly grown bulges, which are usually regarded as Janus ones.12 The characteristic size of the snowman-like Janus particles is usually submicrometer or above. It is difficult to create less
Figure 6. (a) SEM image of the nanoparticles after the sample shown in Figure 2c4 was treated by DMF to remove linear polymers. (b) TEM image of the nanoparticles about 20−30 nm after fractionalization by centrifugation. TEM images of the nanosized Janus particles (c) before and (d) after being labeled with citric acid conjugated Au nanoparticles.
submicrometer-sized snowman-like Janus particles. When MPS/PS ratio is decreased to 1:1, several smaller bulges with a diameter about 150 nm appear on the surface (Figure 2b1).
Figure 4. (a) GPC diagrams and (b) 29Si solid-state NMR spectra of the heterogeneous composite spheres with reaction time at MPS/PS weight ratio 2:1 and pH = 8. T0: (CH3O)3SiR; T1: (CH3O)2Si(OSi)R; T2: (CH3O)Si(OSi)2R; T3: Si(OSi)3R. 190
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Figure 7. SEM images and inset photographs of the wax/water emulsions stabilized using different submicrometer particles (the particle concentration is 0.5 wt %): (a) the as-prepared fresh anisotropic particles just after removal of the seed polymer; (b) the anisotropic hydrophilic particles after further hydrolysis; (c) the Janus particles after modification of the spherical cap surface by OTS. Methyl orange was added to water as a chromogenic agent. Inset: the magnified SEM image revealing orientation of the particles at the interface. The symbols in the inset SEM images represent (+) the spherical cap side directing the aqueous phase, (−) the flat side directing the aqueous phase, and (⊥) the particles standing across the interface.
size becomes smaller and more polydispersed ranging within 50−100 nm (Figure 2a4). At a low MPS/PS ratio 1:1, the effect of pH value on the bulge size and number becomes more remarkable (Figures 2b1−b4). At a lower MPS/PS ratio 1:2, the effect is further enhanced (Figures 2c1−c4). A majority of nanosized bugles ranging within 20−80 nm is achieved at pH = 12 (Figure 2c4). In order to elucidate the forming mechanism of such morphology, the structural evolution and composition conversion of the example composite snowman-like spheres prepared at MPS/PS ratio 2:1 (Figure 2a1) were monitored with reaction time. At pH = 8, free radical polymerization dominates at early stage of the reaction, while the hydrolysis/ condensation proceeds slowly. From after completion of MPS feeding (0.5 h) to additional reaction for 4 h, the composite spheres are rather weak and collapsed after ambient drying since the PS seed sphere shell is highly swelled (Figure 3a,b), which can be completely dissoluble. GPC results show that the molecular weight of PMPS increases at early stage within 2 h, which changes less from 2 to 4 h (Figure 4a). This indicates that the free radical polymerization has finished after 2 h. Along with prolonging reaction time to 8 and 12 h, the composite spheres become less collapsed and dissoluble due to crosslinking of PMPS by a slow hydrolysis/condensation of Si−OR (Figure 3c,d). One organosilica hybrid bulge grows from the PS seed sphere matrix, implying that the phase separation develops progressively. After dissolution of PS, organosilica hybrid particles are derived (right Figure 3c,d). The organosilica hybrid bulge further grows up to be hemisphere on the surface with prolonging reaction time (Figure 3e,f). After removing linear polymer by DMF, the derived organosilica hybrid hemispheres are isolated without aggregation (right Figure
Figure 8. (a, b) SEM images of the frozen wax/water emulsions stabilized using the nanosized Janus particles after modification by OTS and (c, d) the nanoparticles without modification shown in Figure 6a. The particle concentration is 0.5 wt %.
At a lower MPS/PS ratio 1:2, the number of the bulges increases remarkably (Figure 2c1). The bulges are about 80 nm in diameter. At a fixed MPS/PS ratio for example 2:1, microstructure of the composite spheres is easily controlled by pH value. At a slightly high pH = 9, two bulges grow from both poles of the seed sphere (Figure 2a2). At pH = 10, the sphere surface becomes irregular, and more bulges grow (Figure 2a3). The size of the bulges becomes polydispersed. At pH = 12, the number of the bulges further increases, and the
Figure 9. SEM images of some representative nanosized particles at the wax/water interface: (a) homogeneous single layer of the Janus nanoparticles; (b) homogeneous triple layers of the Janus nanoparticles; (c) multiple layered irregular aggregates of the as-prepared nanoparticles without modification. 191
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The nanosized particles (Figure 6a) with a diameter of 20− 80 nm can be prepared from the patchy composite spheres (Figure 2c4). They are isolated rather than aggregated, implying they are separated within the polymer shell matrix, which ensures to synthesize individual nanosized particles. It is required to fractionate the samples to be more uniform in size distribution by ultracentrifugation. The size distribution of asprepared nanoparticles becomes shaper within 20−30 nm at the ∼30% of weight fraction (Figure 6b). Similar to the selective modification of the exposed side the submicrometer hybrid hemispheres to achieve Janus hemispherical particles (Figure 5), the nanosized Janus particles were achieved (Figure 6c). A citric acid conjugated Au nanoparticle about 2−5 nm was used to label such nanosized Janus particles. The TEM image shows that the Au nanoparticles are absorbed preferentially onto one side (Figure 6d). The particles were used to emulsify oil/water mixture to form emulsions. The wax with a melt temperature about 52−54 °C was used as an example oil to form emulsions at high temperature. Upon cooling to room temperature, orientation of the particles at the interface is frozen. When the as-prepared submicrometer anisotropic particles derived from the snowman-like spheres (Figure 2a1) are used to emulsify the mixture, the corresponding stable emulsions form. In the case of the asprepared fresh particles, the spherical cap side of all the particles faces to the aqueous phase (Figure 7a). This means that the particles are Janus but temporal. It is understandable that the exposed side of the bulgy particles is hydrophilic due to Si−OH groups after hydrolysis of PMPS, while the embedded flat side remains hydrophobic due to the embedded Si−OR groups. In fact, the particles are well dispersible in both polar and apolar solvents. After such particles were further hydrolyzed in aqueous ammonia, the flat side is also hydrolyzed and becomes hydrophilic. The particles are only dispersible in polar solvents, revealing that the Janus performance is lost. Although such hydrophilic particles can stabilize the wax/water emulsion due to Pickering effect, they lay at the interface randomly with spherical cap side and the flat side directing the aqueous phase. Some particles stand across the interface (Figure 7b). After the spherical cap side was hydrophobically modified with octyltrichlorosilane (OTS), Janus particles were generated. The particles have a preferential orientation at the emulsion interface with the flat side exclusively directing the aqueous phase (Figure 7c). When the nanosized Janus particles were used to stabilize the wax/water emulsion, the wax core is completely coated, and the coating is homogeneous (Figure 8a,b). In comparison, although an emulsion is formed in the presence of the nanoparticles without modification, the wax core is not completely coated with some wax naked (Figure 8c,d). Fracture surface images indicate that the single layer of the Janus nanoparticles dominates in the homogeneous coating (Figure 9a). Interestingly, in some areas triple layered coating of the Janus nanoparticles is observed (Figure 9b). This may be related with capillary wave far beyond one particle radius at the interface when the particle size is nanosized.11 If the Janus particles form multiple layers onto the interface, the layer number should be odd, for example 1, 3, 5, etc., to ensure the most interior surface and the most exterior surface of the coverage are different in wettability. In the case of the nanoparticles without modification, they are stacked at the interface forming irregular aggregates (Figure 9c). Interfacial behavior of the nanosized particles requires further investigation.
3e,f). Because the resulting submicrometer hemispheres can keep a relative uniformity in both size and shape, a simple centrifugation is sufficient to separate the samples. The process of hydrolysis/condensation was monitored with 29Si solid-state NMR spectra (Figure 4b). The signals at 41.5, 48.9, 56.9, and 66.1 ppm are assigned to T0 ((CH3O)3SiR), T1 ((CH3O)2Si(OSi)R), T2 ((CH3O)Si(OSi)2R), and T3 (Si(OSi)3R) species, respectively. With prolonging reaction time, the signals of T0 and T1 become weaker while the signals of T2 and T3 become stronger, revealing that the degree of hydrolysis/condensation gradually increases. The cross-linking of PMPS and further hydrolysis forming hydrophilic Si−OH group facilitate phase separation. At later stage, the dominant hydrolysis/condensation process can enhance phase separation. At a low MPS/PS weight ratio 1:1, the PS seed spheres cannot be swelled sufficiently and preserve their spherical shape without collapse. Diffusion restricted growth results formation of multiple bulges, which grow with reaction time (Figure S1). After 4−8 h from completion of monomer feeding, bulges start to form. When pH value increases to 12, the hydrolysis/ condensation of both MPS and PMPS is accelerated. A faster cross-linking and hydrophilicity conversion of MPS and PMPS can facilitate phase separation to form bulges. Many small bulges rather than one appear earlier within shorter reaction time 1 h (Figure S2). The composite spheres are slightly sticky, consistent with low conversion of the monomer. After 3 h, the spheres are separated without aggregation and the shape changes less. Higher pH can accelerate hydrolysis/condensation of MPS or PMPS and thus a faster phase separation. The approach forming patchy composite spheres by phase separation is versatile, which can be easily extended to larger seed hollow spheres or solid spheres (Figure S3). However, longer diffusion path causes irregular shape after phase separation. Similar to Pickering emulsion assisted synthesis of Janus particles, the exposed part of the bulges on the patchy composite sphere surface can be selectively modified. Although the patchy composite spheres achieved by electrostatic absorption of small particles onto large hard beads have been reported to derive Janus particles, it is questionable whether the contact part on the bead has been well protected and not influenced during modification.13 In our case, the bulges grow in situ from the polymer shell matrix and the flat root part is tightly buried within the matrix. The principle guarantees that the flat part is not influenced during the selective modification of exposed part of the bulges. It is a prerequisite that the polymer matrix is not swollen by the dispersants. After removal of the seed polymer, Janus particles are achieved. As an example, the heterogeneous patchy composite spheres shown in Figure 2a1 were treated with γ-aminopropyltriethoxysilane (APS) in ethanol to introduce amine group preferentially onto the exposed part of the bulges. The submicrometer hemispherical Janus particles were derived after dissolution of the seed polymer (Figure 5a). Zeta potential changes from −28.3 mV (Si−O−) of the as-prepared particles to 33.2 mV (−NH4+) of the Janus ones. The sulfonated PS (sPS) nanoparticles with a diameter of 20 nm were used to selectively label the amine group conjugated region of the particles by specific interaction. The sPS nanoparticles are exclusively present onto the spherical cap surface, while no nanoparticles are found on the flat part (Figure 5b). As comparison, no sPS nanoparticles are found on the as-prepared particle surface (Figure S4). 192
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4. CONCLUSION We have proposed a novel approach to synthesize Janus particles derived from patchy composite spheres by seeded emulsion polymerization. The bulgy patches are in situ grown from the seed spheres surface by phase separation, which are automatically protected to allow a further selective modification of the exposed part. Meanwhile, the anisotropic shape of the bulges is guaranteed. The size of the bulges can be tunable from submicrometer to nanometer scale by alternation of phase separation dynamics. The Janus particles have a preferential orientation at the emulsion interface. The method is facile and can be scaled up for large quantity of nanosized Janus particles. We have already scaled up the recipes in a 500 mL reactor with a high solid content of 10−20 wt %. Both structure and performance of the resultants are preserved as the same as descriptions in this report. The family of Janus particles will be greatly enriched by further selective modification and growth of functional materials onto desired sides of the particles.
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S4. 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] (C.Z.);
[email protected] (Z.Y.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the NSF of China (51173192, 51173191, 51233007) and MOST (2012CB933200). REFERENCES
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