A Facile One-Step Approach toward Polymer@SiO2 Core–Shell

Article pubs.acs.org/Macromolecules

A Facile One-Step Approach toward Polymer@SiO2 Core−Shell Nanoparticles via a Surfactant-Free Miniemulsion Polymerization Technique Yongliang Zhao, Zhi Chen, Xiaomin Zhu,* and Martin Möller DWI − Leibniz-Institute for Interactive Materials e.V. and Institute for Technical and Macromolecular Chemistry, RWTH Aachen University, Forckenbeckstraße 50, Aachen 52056, Germany

ABSTRACT: We present a surfactant-free miniemulsion process for the preparation of monodisperse polystyrene@SiO2 nanoparticles with a well-defined core−shell structure. The strategy utilizes a silica precursor polymer, hyperbranched polyethoxysiloxane (PEOS), as a sole miniemulsion stabilizer due to its water insolubility and at the same time pronounced amphiphilicity induced by hydrolysis at the oil/water interface. The core−shell particles are obtained by emulsifying a PEOS/ styrene mixture in water and subsequent heating to initiate polymerization. As the polymerization proceeds, driven by osmotic pressure and incompatibility with polystyrene, PEOS macromolecules migrate continuously toward the oil/water interface where sol−gel reaction takes place. As soon as the polymerization is completed, PEOS is fully expelled from the polymer phase and is converted to silica on the polystyrene surface. This method allows an easy control of silica shell thickness by varying the PEOS concentration. The particle size, on the other hand, can be regulated not only by the shearing force but also by the pH of the aqueous medium. This process offers a new type of miniemulsion polymerization technique for the preparation of composite polymer particles that is facile, low cost, highly scalable, and environmentally friendly.

■

INTRODUCTION Composite technology, which combines multiple materials differing significantly in properties to produce a composite material with characteristics different from the constituents, has progressed rapidly since the past decades.1,2 By proper design the properties of the composites can often go far beyond that achievable with single materials.3 In this fast-growing field considerable effort is devoted to the development of colloidal inorganic−organic nanocomposites due to their promising applications in catalysis, chromatography, coating, biotechnology, etc.4,5 Polymer/silica nanocomposite particles with various morphologies are by far the most common and most studied system, and the combination of organic polymers with silica in an appropriate way can remarkably alter the practical performance like chemical, mechanical, optical, electrical, rheological, and surface properties.6 Coating a polymer core with a silica layer to form a core−shell polymer@SiO2 particle is an important technique to impart hydrophilicity, biocompatibility, and modifiability which are intrinsic properties of silica to the polymer.7−9 Furthermore, the chemical as well as © 2016 American Chemical Society

thermal stability of the core materials can be significantly improved thanks to the protective function of the silica coating.10 The polymer@SiO2 particles are often transformed into hollow silica spheres by removing the organic polymer using thermal or organic solvent treatments.11−32 In principle, there are two different means to synthesize core−shell polymer@SiO2 particles known in the literature, namely silica coating of preformed polymer particles either via a sol−gel process15−23,25−35 or via deposition of preformed silica particles12−14 and Pickering (mini)emulsion polymerization using silica/silicate nanoparticles as stabilizer.24,33 Sol−gel coating is by far the most common way to deposit a thin silica layer. For hydrophobic polymers, the polymer surface should, however, be modified to improve its affinity toward the hydrophilic silica layer. The methods for the surface modification of polymer particles include, for instance, the Received: January 7, 2016 Revised: February 15, 2016 Published: February 19, 2016 1552

DOI: 10.1021/acs.macromol.6b00038 Macromolecules 2016, 49, 1552−1562

Article

Macromolecules use of poly(vinylpyrrolidone),19,21,25,30,34 cetyltrimethylammonium bromide,11,31 or poly(L-lysine)29 as coupling agents, plasma treatment,20 and introduction of surface functional groups such as silanol27,28,36,37 or pyridine32 by copolymerization with γ-methacryloxypropyltrimethoxysilane or 4-vinylpyridine, respectively. An organic solvent often ethanol has to be used in the sol−gel coating step because of the low water solubility of the silica precursor tetraethoxysilane (TEOS); therefore, most of the preparation strategies involve the transfer of the polymer core from the aqueous phase to the alcohol medium with the help of surfactants or amphiphilic polymers. The surface decoration of polymer particles with silica nanoparticles is usually combined with the layer-by-layer deposition; i.e., oppositely charged silica particles and polyelectrolytes are adsorbed alternatively onto the polymer particle surface.12−14 In the case of Pickering emulsion polymerization, the polymerization takes place in the presence of silica nanoparticles, and afterward the particles become adsorbed on the surface of resulting latex particles.24 During Pickering miniemulsion polymerization, the emulsion droplets of a monomer in water stabilized with silica/silicate particles33 are polymerized to result in polymer particles armored with silica/silicate particles on the surface. The low-molecular-weight silica precursor TEOS is a hydrophobic liquid, so it can be used as a solvent for both polymers and monomers. A solution of a hydrophobic polymer like polystyrene (PS), polyethylene, and polydimethylsiloxane in TEOS with or without a cosolvent was emulsified in alkaline ethanol and then converted to polymer/silica hybrid particles.35−37 In the resulting composite particles, polymer and silica formed usually interpenetrating domains, and no core−shell structure was observed. In another study, a miniemulsion of an oil phase comprising styrene, γ-methacryloxypropyltrimethoxysilane, TEOS, and a radical initiator in water of pH 9 was formed upon ultrasonication. After polymerization at 70 °C, PS/silica asymmetric dimeric particles were obtained.38 In all these cases, surfactants were added in order to stabilize oil-in-water (o/w) (mini)emulsions. Hyperbranched polyethoxysiloxane (PEOS), which is prepared by the condensation of TEOS,39 is a silica precursor polymer whose hydrophobicity is substantially higher than that of TEOS.40 PEOS is miscible with most organic solvents; however, it can barely serve as a solvent for polymers due to its polymer nature. PEOS was used as glue to prepare all silica colloidosomes by linking silica nanoparticles at the oil/water interface in Pickering emulsions.41,42 In our recent paper the pronounced hydrolysis-induced interfacial activity of PEOS in oil/water systems was clearly demonstrated.43 It was shown that an o/w miniemulsion was obtained by dispersing the toluene solution of PEOS in water under ultrasonication. In this emulsion system methyl-functionalized silica nanoparticles catalyzed the conversion of PEOS to mechanically strong nanocapsules, and in the absence of the silica particles the resulting silica capsules were collapsed and stuck to each other after drying, indicating the high softness of the shell. Similar to Pickering miniemulsion polymerization,24,33 the mechanical stability of the capsules could in principle also be improved if the conversion of PEOS were accompanied by the solidification of the encapsulated organic phase. Hence, in this work we aim at the preparation of PS@SiO2 composite particles by combining the conversion of PEOS with the polymerization of styrene monomer. In this case, a miniemulsion of PEOS/styrene in water can be stabilized by hydrolyzed PEOS, so no surfactant should be

added. During the polymerization of styrene and PEOS conversion, the compatibility of inorganic and organic phases will be significantly decreased, leading to strong phase separation. As all these processes will take place within the monomer/PEOS droplets, the question arises then as to whether a core−shell structure can form in such a system. The influence of the reaction conditions such as PEOS content, shearing force, temperature profiling, pH of the aqueous medium, etc., on the size and morphology of the resulting particles will be the main topic of this study.

■

EXPERIMENTAL SECTION

Materials. Tetraethoxysilane (TEOS, GPR RECTAPUR, VWR), acetic anhydride (ACS reagent, ≥98.0%, Sigma-Aldrich) and titanium trimethylsiloxide (ABCR) were used as received. Styrene (ReagentPlus, ≥99.0%, Sigma-Aldrich) and 2,2′-azobis(2-methylpropionitrile) (AIBN, 98%, Sigma-Aldrich) were purified prior to use by distillation under vacuum and recrystallization with ethanol, respectively. Deionized water was used for all experiments. PEOS was synthesized according to the method published elsewhere.39 The resulting PEOS had the following characteristics: degree of branching 0.54, SiO2 content 49.2%, Mn 1740, and Mw/Mn 1.9 (measured by gel permeation chromatography in chloroform with evaporative light scattering detector calibrated using polystyrene standards). Preparation of PS@silica Composite Particles. The recipes are summarized in Table 1. The procedure was carried out as follows. An

Table 1. Recipes for the Preparation of PS@silica Composite Particles in 30 g of Water run

PEOS (g)

styrene (g)

pH

1 2 3 4 5 6b 7c 8 9 10 11 12 13 14

0.15 0.30 0.60 1.20 1.80 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20

1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20

7.0 7.0 7.0 7.0 7.0 7.0 7.0 1.5 2.5 5.0 9.5 10.5 11.0 7.0

15

1.20

1.20

7.0

16

1.20

1.20

7.0

17

1.20

1.20

7.0

emulsification method ultrasonication ultrasonication ultrasonication ultrasonication ultrasonication ultrasonication ultrasonication ultrasonication ultrasonication ultrasonication ultrasonication ultrasonication ultrasonication Ultra-Turrax, 5000 rpm Ultra-Turrax, 7500 rpm Ultra-Turrax, 13000 rpm Ultra-Turrax, 18000 rpm

particle sizea (nm) 167 157 151 152 154 232 152

± ± ± ± ± ± ±

15 9 6 3 14 18 24

159 ± 11 123 ± 8 109 ± 7 576 ± 40 423 ± 47 305 ± 6 178 ± 10

a

Particle size was estimated by averaging diameter of 500 particles in electron micrographs. bThe emulsion was stirred at room temperature for 48 h. cHeating started after stirring the emulsion at room temperature for 24 h and continued for 24 h. oil phase consisting of styrene, PEOS, and AIBN was added to water. The mixture was treated by ultrasonic irradiation for 15 min (Branson Sonifier 450 cell disrupter, 3 mm microtip, 0.9 time circle, 247 W output) or a high-performance homogenizer (T 18 digital ULTRATURRAX, IKA) to yield a milky o/w emulsion. The resulting emulsion was transferred into a three-necked flask equipped with a reflux condenser and a nitrogen inlet. After the flask was flushed with nitrogen for 10 min, the temperature was raised to 70 °C, and the reaction was allowed to proceed for another 24 h under mild stirring 1553

DOI: 10.1021/acs.macromol.6b00038 Macromolecules 2016, 49, 1552−1562

Article

Macromolecules and nitrogen protection. Afterward, the obtained polymer particles were isolated by centrifugation, rinsed three times with water, and then redispersed in water or freeze-dried for further measurements. The silica hollow spheres were obtained by heating the dried sample in a muffle oven at 500 °C for 4 h. In order to remove silica the resulting particles were dispersed in a 5 M sodium hydroxide aqueous solution. The dispersion was stirred at 70 °C for 12 h. Afterward, the particles were isolated by centrifugation and rinsed with water until a neutral reaction. This procedure was repeated twice. Interfacial Tension (IFT) Measurements. The interfacial tension between water and oil phase was measured at room temperature with a Krüss DSA100 tensiometer. The pendant drop method was used to determine the interfacial tension. During the measurements a droplet of water, initially contained in a syringe (1 mL) equipped with a needle (diameter ∼ 1.8 mm), was pumped into the oil phase placed in an optical glass cuvette (10 × 10 × 45 mm). Field-Emission Scanning Electron Microscopy (FE-SEM). FESEM measurements were performed on a Hitachi S4800 highresolution field emission scanning electron microscope with an accelerating voltage of 1.5 kV. A droplet of a sample dispersion was placed on the silicon wafer and air-dried under ambient conditions. Transmission Electron Microscopy (TEM). TEM measurements were carried out on a Zeiss Libra 120 transmission electron microscope. The accelerating voltage was set at 120 kV. The samples were prepared by placing a droplet of a diluted sample dispersion on a Formvar-carbon-coated copper grid with 200 meshes. Energy-Dispersive X-ray Spectroscopy−Scanning Transmission Electron Microscopy (EDX-STEM). EDX-STEM mapping was performed for particles on a Hitachi SU9000 field emission scanning electron microscope equipped with Oxford Xmax 80 EDXdetector and operated at 30 kV. The Si−K and C−K edges were used to collect chemical information on individual elements. Fourier Transform Infrared (FT-IR) Spectroscopy. FT-IR spectra were recorded on a Nicolet 60 SXR FT-IR spectrometer using the KBr pellet technique. The samples were dried at 45 °C overnight before measurements. Thermogravimetric Analysis (TGA). TGA measurements were conducted on a PerkinElmer STA 6000 unit operating under a nitrogen atmosphere with a flow rate of 20 mL/min. The particles were isolated from the emulsions by centrifugation and dried at 70 °C overnight. 5−10 mg of the dried sample was then placed in a standard PerkinElmer alumina 85 μL crucible for the measurements. Dynamic Light Scattering (DLS) Measurements. Hydrodynamic diameter of polymer particles in water was measured with a Malvern Zetasizer Nano Series at a scattering angle of 147° at 25 °C. Before the measurements, the stock dispersions were diluted to a particle concentration of 1.5 wt ‰.

Figure 1. IFT of a PEOS/styrene mixture of different weight ratios with water of pH 7 versus time.

Monodisperse nanospheres with an average diameter of 150 nm are observed. These particles have a smooth rather than particle-decorated rough surface. Calcination at 500 °C results in hollow particles of the same size (Figure 2d), indicating clearly that the obtained original particles have a PS@SiO2 core−shell structure, where a PS core is surrounded by a continuous silica shell. The thickness of the silica shell is approximately 15 nm. To verify further the core−shell structure, EDX-STEM mapping was used to investigate the distribution of silicon and carbon atoms in the hybrid particles. As shown in Figure 2c, the silicon atoms are concentrated at the periphery of the particles, and the carbon atoms are mostly observed in the particle core. The particle size decreases to 137 nm (Figure 3a) after silica is removed by an aqueous solution of NaOH. In the resulting particles, which do not possess a porous structure, silicon atoms are absent, confirming a “pure” core− shell structure of the original hybrid particles. The chemical composition of the product was further identified with FT-IR spectroscopy. Figure 4 demonstrates the FT-IR spectra of PEOS, pure PS, dried core−shell nanoparticles, and silica hollow spheres obtained after calcination. The spectrum of the core−shell particles appears to be a superposition of the spectra of PS and the hollow spheres consisting entirely of pure silica, indicating the full conversion of PEOS to silica and the successful polymerization of styrene. As discussed above, PEOS plays a dual role as a silica precursor as well as an emulsion stabilizer in the preparation of PS@SiO2 nanoparticles. In order to clarify the mechanism of the formation of well-defined core−shell particles, the polymerization process was monitored by DLS measurements (Figure 5). After the ultrasonic emulsification of the PEOS/styrene mixture in water, submicron droplets are formed with a relatively broad size distribution (mean diameter 310 nm, PDI 0.27). Only after heating for 1 h, the particle size and size distribution (mean diameter 210 nm, PDI 0.21) decrease significantly. The size and size distribution reach the plateau (mean diameter 160 nm, PDI 0.11) after 2 h heating at 70 °C, and further heating does not cause any significant change. This reaction can be considered as miniemulsion polymerization,44,45 since the size of the emulsion droplets is in the submicron range. The miniemulsion is stabilized by partially hydrolyzed PEOS macromolecules; at the same time, the presence of water-insoluble PEOS provides additional stability

■

RESULTS AND DISCUSSION Figure 1 illustrates that the IFT of a styrene solution containing different amounts of PEOS with water of pH 7 reaches with time the final value as low as 10 mN/m that is barely dependent on the PEOS concentration. The time needed to reach this value, however, increases with the dilution of the PEOS solution in styrene. It implies that the interfacial activity of PEOS is indeed induced by the hydrolysis whose rate depends obviously on the PEOS concentration. Under ultrasonic irradiation, the oil phase consisting of PEOS, styrene, and AIBN is easily emulsified in water of pH 7 to yield a stable o/w emulsion; meanwhile, the conversion of PEOS starts. Immediately afterward, the polymerization of styrene is initiated by raising the temperature to 70 °C. After stirring for 24 h at this temperature, a milky dispersion, which is stable for at least 6 months, is obtained. The product can be isolated by centrifugation and redispersed in water. Figure 2 shows the FE-SEM and TEM images of the particles prepared by this method using the weight ratio of styrene to PEOS 1:1. 1554

DOI: 10.1021/acs.macromol.6b00038 Macromolecules 2016, 49, 1552−1562

Article

Macromolecules

Figure 2. FE-SEM (a), TEM (b), and EDX-STEM mapping micrographs (c) of PS@SiO2 core−shell particles prepared by emulsifying a mixture of styrene and PEOS (1:1) in water of pH 7 via ultrasonication and subsequent heating at 70 °C (Table 1, run 4). (d) TEM image of silica hollow particles obtained by calcination of the core−shell particles. Blue color indicates the presence of silicon atoms, and red color corresponds to carbon atoms. The scale bars represent 100 nm.

Figure 3. TEM micrograph (a) and EDX-STEM mapping image (b) of particles obtained after NaOH etching of PS@SiO2 particles. Red color corresponds to carbon atoms and blue to silicon atoms. The scale bars represent 100 nm.

against Ostwald ripening by building up an osmotic pressure in the droplets.46−49 The polymerization seems to be confined in the emulsion droplets, since an oil-soluble initiator is used and the mass transport of the monomer through the aqueous phase and the creation of monomer swollen micelles as in the case of classical emulsion polymerization are not observed. The significant shrinkage of the particles upon heating at 70 °C is

due to the polymerization of styrene and for the most part due to the simultaneous conversion of PEOS to silica, which is accompanied by at least 50% weight loss. So far the polymerization was initiated immediately after the formation of the miniemulsion. The questions arise as to how the PEOS/styrene-in-water miniemulsion behaves without the polymerization and what happens if the polymerization starts 1555

DOI: 10.1021/acs.macromol.6b00038 Macromolecules 2016, 49, 1552−1562

Article

Macromolecules

Figure 6. Particle size distribution of initial PEOS/styrene(1:1)-inwater emulsion, emulsion stirred at room temperature for 48 h (Table 1, run 6), and emulsion where heating is started immediately after the emulsification (Table 1, run 4) or after stirring the emulsion at room temperature for 24 h (Table 1, run 7).

Figure 4. FT-IR spectra of PEOS, dried PS@silica nanoparticles, pure PS, and silica hollow spheres after calcination (Table 1, run 4).

and TEM images show the formation of a mixture of polydisperse core−shell nanoparticles, pure silica nanoparticles, and particle agglomerates (Figure 7b), indicating an uncontrolled reaction manner. Based on the experimental data presented above, it can be concluded that there is a synergistic effect between the polymerization of styrene and PEOS conversion in the formation of well-defined PS@SiO2 core−shell particles. The proposed mechanism of their formation is depicted in Scheme 1. PEOS and styrene are miscible; meanwhile, PS can no more be dissolved in PEOS. Furthermore, the hydrolysis of PEOS makes it hydrophilic and reduces its compatibility with both styrene and PS. When the polymerization is initiated as soon as the miniemulsion is formed, strong phase separation takes place in the emulsion droplets. As the polymerization proceeds, driven by the osmotic pressure and incompatibility with PS, PEOS molecules continuously migrate toward the oil/water interface where they are consumed. As the polymerization is completed, the PEOS molecules are fully expelled from the PS phase and are converted to silica on the PS surface. However, if the polymerization of styrene is initiated after stirring the PEOS/styrene-in-water miniemulsion at room temperature for 24 h, the thin soft silica layer already present at the oil/water interface wrinkles or is partially broken into small silica particles due to the shrinkage of the inner organic phase upon the polymerization and further PEOS conversion. In the deposition of silica via a sol−gel process onto the surface of preformed polymer particles, the silica thickness can be adjusted by the amount of silica precursor. The process may last very long to achieve a high thickness because the precursor cannot exceed a certain concentration in order to avoid homogeneous nucleation of silica. In our case, the thickness of the silica layer can in principle be tuned by the amount of PEOS in the oil phase. The weight ratio of PEOS to styrene in the oil phase was systematically varied in this work. It is shown that PS@SiO2 core−shell particles are obtained even with a very low concentration of PEOS in styrene (11 wt %) (run 1 in Table 1, cf. Figure 8a). In comparison to the PEOS/styrene ratio 1:1, the particles obtained here have a thinner silica shell, slightly bigger size, and broader size distribution. The PEOS amount is certainly enough to stabilize the o/w emulsion; the

Figure 5. Evolution of particle size distribution during the miniemulsion polymerization of PEOS/styrene (1:1) in water of pH 7 (Table 1, run 4).

after stirring the resulting emulsion for a longer time. The emulsion remains stable after stirring for 48 h. As shown by the DLS measurements, the mean particle size is increased to 520 nm (Figure 6). In the FE-SEM image of the dried sample, collapsed capsules can be seen (Figure 7a), showing the high softness of the silica shell most probably because of a low condensation degree of PEOS. The increase of the particle size is possibly due to the consumption of nonhydrolyzed highly hydrophobic PEOS macromolecules in the oil droplets, leading to Ostwald ripening. In the PEOS/styrene-in-water emulsion, PEOS molecules migrate continuously to the oil/water interface where the hydrolysis and condensation take place. However, due to the presence of a hydrophobic solvent at the interface, where ethoxysilane groups can be dissolved, the access of these groups toward water is restricted, thus resulting in a low conversion degree. On the other hand, the formation of a thin layer of hydrolyzed and partially condensed PEOS at the interface also increases the emulsion stability by preventing the coalescence of the droplets. In another experiment, the system was heated to 70 °C after stirring the PEOS/styrene-inwater miniemulsion for 24 h. In this case, the size distribution of the particles measured by DLS is very broad. The FE-SEM 1556

DOI: 10.1021/acs.macromol.6b00038 Macromolecules 2016, 49, 1552−1562

Article

Macromolecules

Figure 7. FE-SEM images of particles prepared (a) by stirring the PEOS/styrene(1:1)-in-water emulsion at room temperature for 48 h (Table 1, run 6) and (b) by heating started after stirring the emulsion at room temperature for 24 h and continued for 24 h (Table 1, run 7). The insets show the TEM micrographs of the corresponding samples. The scale bars represent 200 nm.

Scheme 1. Schematic Illustration of the Formation Process of PS@SiO2 Core−Shell Particles and Silica Hollow Nanospheres and Chemical Structure of Hyperbranched Polyethoxysiloxane (PEOS)

where mPEOS, mstyrene, and RPEOS/styrene are mass of PEOS and styrene and their ratio, respectively; SC is the silica content in PEOS and equals 49.2%. X is calculated to be 5.8, 11.0, 19.7, 33.0, and 42.5%, in good agreement with the values measured by TGA (Figure 9). In addition, the decomposition temperature of PS increases with the increase of silica content; thus, the protecting function of the silica coating is clearly demonstrated. It is well-known that pH affects significantly the hydrolysis and condensation of silica precursors like PEOS as well as the resulting silica structure.43,50,51 So far the synthesis was carried out under the neutral pH, and well-defined PS@SiO2 core− shell nanoparticles were obtained. Here different pH values ranging from very acidic to highly basic were tested for the preparation of PEOS/styrene-in-water emulsion as well as PS@ SiO2 core−shell particles by subsequent polymerization. When the aqueous pH is very acidic and close to the isoelectric point of silica (pH ≈ 2.0), polydisperse microsized composite spheres with ill-defined core−shell structures are formed (Figure 10a,b), although the emulsions can be obtained and they remain stable during the polymerization reaction. In the pH range of 5−10, monodisperse nanoparticles with a well-defined

increase of the particle size and size distribution can be accounted for by the decrease of the concentration of water insoluble PEOS in the oil phase. In the PEOS/styrene ratio range of 1:4 to 1:1 (runs 2−4 in Table 1) the size and size distribution of the PS@SiO2 core−shell particles remain almost unchanged; meanwhile, the thickness of the silica shell increases almost linearly with the increase of the PEOS content (Figures 2 and 8b,c). With increasing the PEOS/styrene ratio to 3:2, although the silica shell thickness increases further, quite polydisperse particles are obtained (Figure 8d). The broader size distribution of the particles can possibly be explained by the relatively high viscosity of the solution with a high PEOS concentration. The thermal stability of PS as well as the silica content in the PS@SiO2 core−shell particles was determined by means of TGA. The TGA curves of the particles obtained with different PEOS/styrene weight ratios are shown in Figure 9. The theoretical silica content X in the core−shell particles assuming the full conversion of PEOS can be calculated by eq 1. X=

RPEOS/styrene × SC mPEOS × SC = mPEOS × SC + mstyrene RPEOS/styrene × SC + 1 (1) 1557

DOI: 10.1021/acs.macromol.6b00038 Macromolecules 2016, 49, 1552−1562

Article

Macromolecules

Figure 8. FE-SEM images of PS@SiO2 core−shell particles prepared with different weight ratios of PEOS to styrene: (a) 1:8, (b) 2:8, (c) 4:8, and (d) 12:8 (Table 1, runs 1−3 and 5). The insets show the TEM images of the calcined samples. The scale bars represent 200 nm.

of hours. A sticky mass most probably composed of styrene, PS, and PEOS is precipitated, and meanwhile a mixture of polydisperse solid and hollow particles is formed in the aqueous dispersion (Figure 10f). In order to understand the pH dependence of the product morphology, the IFT of the PEOS/styrene 1:1 mixture with water of different pH was measured. As shown in Figure 11, at low pH the IFT decreases continuously with time, indicating an instable oil/water interface. In the pH range from 5 to 10, where monodisperse well-defined PS@SiO2 core−shell particles are obtained, the IFT reaches a plateau after a certain time period. At high pH the interface between PEOS/styrene and water becomes again not stable, and the IFT even increases with time at pH 11. In our previous study it was demonstrated that the interfacial activity of PEOS depended strongly on the pH value of the aqueous medium.43 The interfacial activity of the partially hydrolyzed PEOS increases with pH due to the increase of the degree of ionic dissociation of the silanol groups. This is most probably the reason for the decrease of the size of the resulting core−shell particles. At low pH close to the isoelectric point of silica, the silanol groups are mostly protonated. The big size of the particles obtained in this pH range can be accounted for by the nonionic character of the hydrolyzed PEOS molecules. In general, hydrolysis is the ratedetermining step of the whole sol−gel process.52 At very acidic pH, hydrolysis is very fast, and dominant over condensation reaction, so a thin layer of almost completely hydrolyzed yet still liquid PEOS is formed quickly at the oil/water interface.41 This layer is probably too hydrophilic to stabilize miniemulsions, thus causing instability of the system. In the intermediate pH range between 5 and 10, the hydrolysis is pretty slow and the condensation reaction becomes faster; therefore, the amphiphilicity of the hydrolyzed PEOS may reach a quasi-stationary state, resulting in emulsions that are

Figure 9. TGA curves obtained upon heating dried PS@silica core− shell nanoparticles prepared with different weight ratios of PEOS to styrene (Table 1, runs 1−5). The measured silica contents are 7.6, 10.6, 19.8, 32.0, and 41.9% for PEOS/styrene ratios 1:8, 2:8, 4:8, 8:8, and 12:8, respectively.

core−shell structure are prepared (Figures 10c,d and 2). The size of particles decreases with the increase of pH. At high pH, however, the particle surface turns partially wrinkled (e.g., at pH 9.5, Figure 10d). The surface wrinkling becomes more pronounced with the further increase of pH. At pH 10.5, the resulting PS@SiO2 core−shell particles have a mean diameter of 109 nm, but a totally irregular wrinkled surface (Figure 10e). In this case, the polymer cores are also not spherical. It seems that the polymer growth is somehow confined in a preformed silica framework. When the pH value is increased to 11, the emulsion is partially broken after heating at 70 °C for a couple 1558

DOI: 10.1021/acs.macromol.6b00038 Macromolecules 2016, 49, 1552−1562

Article

Macromolecules

Figure 10. FE-SEM images of PS@SiO2 core−shell particles prepared with PEOS/styrene 1:1 in water of different pH: (a) 1.5, (b) 2.5, (c) 5.0, (d) 9.5, (e) 10.5, and (f) 11.0 (Table 1, runs 8−13). The insets show the TEM micrographs of the corresponding samples. The scale bars in (c−f) represent 200 nm.

stable at least during the polymerization reaction. However, with the further increase of pH, both hydrolysis and condensation rates increase again; a highly cross-linked layer from PEOS is thus formed quickly after the emulsion formation. This causes the significant surface wrinkling at pH 10.5. At pH 11 and 70 °C the rate of the sol−gel reaction of PEOS is so fast that the emulsion is broken quickly, since the amphiphilic hydrolyzed PEOS in this case has a pretty short lifetime. The high silica solubility in water at this pH and the tendency of the system to reduce the interfacial energy might be the reason for the formation of solid silica particles. It is known that a high-shearing process is needed for the formation of miniemulsions, and the droplet size is dependent on the used devices.44 Up to now the emulsification of the PEOS/styrene mixture in water was carried out by means of ultrasonication. Rotor−stator systems such as Ultra-Turrax and Omni mixer relying on turbulence can be employed to prepare miniemulsions, and the minimum droplet size that can be achieved is controlled by the rotation speed. In this work the Ultra-Turrax homogenizer was also used for the emulsification

Figure 11. IFT of a PEOS/styrene (1:1) mixture with water of different pH versus time.

1559

DOI: 10.1021/acs.macromol.6b00038 Macromolecules 2016, 49, 1552−1562

Article

Macromolecules

Figure 12. FE-SEM images of PS@SiO2 core−shell particles prepared by emulsifying a mixture of styrene and PEOS (1:1) in water of pH 7 using Ultra-Turrax homogenizer under different circumferential speeds: (a) 5000, (b) 7500, (c) 13 000, and (d) 18 000 rpm and subsequent heating at 70 °C for 24 h (Table 1, runs 14−17). The scale bars represent 200 nm.

Furthermore, it has been shown that the thickness of the silica shell increases with the increase of PEOS content in the oil phase.

of the PEOS/styrene mixture in water. The resulting emulsions are stable and well-defined submicron PS@SiO2 core−shell particles are obtained (Figure 12). The particles are generally bigger than that obtained by using ultrasonication. The particle size and size distribution decrease with the increase of the rotation speed. At high circumferential speeds monodisperse core−shell particles can be synthesized; at the same time quite polydisperse particles are produced at low speeds.

■

AUTHOR INFORMATION

Corresponding Author

*(X.Z.) Tel +49-241-8023341; fax +49-241-8023301; e-mail [email protected].

■

CONCLUSIONS In summary, a surfactant-free miniemulsion polymerization technique has been developed for the preparation of monodisperse polystyrene@SiO2 core−shell nanoparticles using PEOS not only as a silica precursor but also as a sole miniemulsion stabilizer because of its water insolubility and meanwhile pronounced hydrolysis-induced interfacial activity. The core−shell particles are synthesized simply by emulsifying a PEOS/styrene mixture in water and subsequent heating to initiate polymerization. After removing the polymer core by calcination or dissolution, mechanically stable silica hollow spheres can be prepared. The influence of the reaction conditions such as PEOS content, shearing force, temperature profiling, pH of the aqueous medium on the emulsion stability, and the size and morphology of the resulting particles has been systematically investigated. A mechanism has been proposed to account for the formation of core−shell particles that is believed to be the result of a delicate interplay between the PEOS hydrolysis and condensation rate, polymerization kinetics, emulsion stability, and phase separation in the emulsion droplets. Well-defined core−shell particles are formed in the pH range of 5−10, and the particle size decreases with the increase of pH. The particle size can also be regulated by varying the shearing force in the emulsification process.

Notes

The authors declare the following competing financial interest(s): The results presented in this paper form a part of a patent submitted by Y.Z., X.Z., and M.M..

■

ACKNOWLEDGMENTS

■

ABBREVIATIONS

This work was performed in part at the Center for Chemical Polymer Technology CPT, which is supported by the EU and the federal state of North Rhine-Westphalia (Grant EFRE 30 00 883 02). The authors thank Sabrina Mallmann for the EDXSTEM measurements and Dr. Walter Tillmann for the assistance in the FT-IR measurements.

PEOS, hyperbranched polyethoxysiloxane; TEOS, tetraethoxysilane; AIBN, 2,2′-azobis(2-methylpropionitrile); IFT, interfacial tension; DLS, dynamic light scattering; FE-SEM, field emission−scanning electron microscopy; TEM, transmission electron microscopy; EDX-STEM, energy-dispersive X-ray spectroscopy−scanning transmission electron microscopy; TGA, thermogravimetric analysis; FT-IR, Fourier-transform infrared. 1560

DOI: 10.1021/acs.macromol.6b00038 Macromolecules 2016, 49, 1552−1562

Article

Macromolecules

■

nanocomposite particles prepared by alcoholic dispersion polymerization. Chem. Mater. 2007, 19, 2435−2445. (25) Song, X. F.; Gao, L. Synthesis, characterization, and optical properties of well-defined N-doped, hollow silica/titania hybrid microspheres. Langmuir 2007, 23, 11850−11856. (26) Sun, B.; Mutch, S. A.; Lorenz, R. M.; Chiu, D. T. Layered polyelectrolyte-silica coating for nanocapsules. Langmuir 2005, 21, 10763−10769. (27) Tissot, I.; Novat, C.; Lefebvre, F.; Bourgeat-Lami, E. Hybrid latex particles coated with silica. Macromolecules 2001, 34, 5737−5739. (28) Tissot, I.; Reymond, J. P.; Lefebvre, F.; Bourgeat-Lami, E. SiOH-functionalized polystyrene latexes. A step toward the synthesis of hollow silica nanoparticles. Chem. Mater. 2002, 14, 1325−1331. (29) Yang, J.; Lind, J. U.; Trogler, W. C. Synthesis of hollow silica and titania nanospheres. Chem. Mater. 2008, 20, 2875−2877. (30) Zhang, L.; D’Acunzi, M.; Kappl, M.; Auernhammer, G. K.; Vollmer, D.; van Kats, C. M.; van Blaaderen, A. Hollow Silica Spheres: Synthesis and Mechanical Properties. Langmuir 2009, 25, 2711−2717. (31) Zhu, G. S.; Qiu, S. L.; Terasaki, O.; Wei, Y. Polystyrene beadassisted self-assembly of microstructured silica hollow spheres in highly alkaline media. J. Am. Chem. Soc. 2001, 123, 7723−7724. (32) Zou, H.; Wu, S. S.; Shen, J. Preparation of silica-coated poly(styrene-co-4-vinylpyridine) particles and hollow particles. Langmuir 2008, 24, 10453−10461. (33) Cauvin, S.; Colver, P. J.; Bon, S. A. F. Pickering stabilized miniemulsion polymerization: Preparation of clay armored latexes. Macromolecules 2005, 38, 7887−7889. (34) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. A general method to coat colloidal particles with silica. Langmuir 2003, 19, 6693−6700. (35) Sertchook, H.; Avnir, D. Submicron silica/polystyrene composite particles prepared by a one-step sol-gel process. Chem. Mater. 2003, 15, 1690−1694. (36) Sertchook, H.; Elimelech, H.; Makarov, C.; Khalfin, R.; Cohen, Y.; Shuster, M.; Babonneau, F.; Avnir, D. Composite particles of polyethylene @ silica. J. Am. Chem. Soc. 2007, 129, 98−108. (37) Sertchook, H.; Elimelech, H.; Avnir, D. Composite particles of silica/poly(dimethylsiloxane). Chem. Mater. 2005, 17, 4711−4716. (38) Lu, W.; Chen, M.; Wu, L. M. One-step synthesis of organicinorganic hybrid asymmetric dimer particles via miniemulsion polymerization and functionalization with silver. J. Colloid Interface Sci. 2008, 328, 98−102. (39) Zhu, X. M.; Jaumann, M.; Peter, K.; Möller, M.; Melian, C.; Adams-Buda, A.; Demco, D. E.; Blümich, B. One-pot synthesis of hyperbranched polyethoxysiloxanes. Macromolecules 2006, 39, 1701− 1708. (40) Wang, H. L.; Agrawal, G.; Tsarkova, L.; Zhu, X. M.; Möller, M. Self-Templating Amphiphilic Polymer Precursors for Fabricating Mesostructured Silica Particles: A Water-Based Facile and Universal Method. Adv. Mater. 2013, 25, 1017−1021. (41) Wang, H. L.; Zhu, X. M.; Tsarkova, L.; Pich, A.; Möller, M. AllSilica Colloidosomes with a Particle-Bilayer Shell. ACS Nano 2011, 5, 3937−3942. (42) Zhao, Y. L.; Li, Y. Q.; Demco, D. E.; Zhu, X. M.; Möller, M. Microencapsulation of Hydrophobic Liquids in Closed All-Silica Colloidosomes. Langmuir 2014, 30, 4253−4261. (43) Zhao, Y. L.; Chen, Z.; Zhu, X. M.; Mö ller, M. Silica nanoparticles catalyse the formation of silica nanocapsules in a surfactant-free emulsion system. J. Mater. Chem. A 2015, 3, 24428− 24436. (44) Asua, J. M. Miniemulsion polymerization. Prog. Polym. Sci. 2002, 27, 1283−1346. (45) Landfester, K. Miniemulsion Polymerization and the Structure of Polymer and Hybrid Nanoparticles. Angew. Chem., Int. Ed. 2009, 48, 4488−4507. (46) Delgado, J.; Elaasser, M. S.; Silebi, C. A.; Vanderhoff, J. W.; Guillot, J. Miniemulsion Copolymerization of Vinyl-Acetate and Butyl Acrylate. 2. Mathematical-Model for the Monomer Transport. J. Polym. Sci., Part B: Polym. Phys. 1988, 26, 1495−1517.

REFERENCES

(1) Chung, D. D. L. Composite Materials: Functional Materials for Modern Technologies; Springer: New York, 2003. (2) Chung, D. D. L. Composite Materials: Science and Applications, 2nd ed.; Springer: New York, 2010. (3) Gay, D. Composite Materials: Design and Applications, 3rd ed.; CRC Press: Boca Raton, FL, 2015. (4) Kalia, S.; Haldorai, Y. Organic-Inorganic Hybrid Nanomaterials; Springer: London, 2015. (5) Chauhan, B. P. S. Hybrid Nanomaterials: Synthesis, Characterization, and Applications; Wiley: Hoboken, NJ, 2011. (6) Zou, H.; Wu, S. S.; Shen, J. Polymer/silica nanocomposites: Preparation, characterization, properties, and applications. Chem. Rev. 2008, 108, 3893−3957. (7) Caruso, F. Nanoengineering of particle surfaces. Adv. Mater. 2001, 13, 11−22. (8) Caruso, R. A.; Antonietti, M. Sol-gel nanocoating: An approach to the preparation of structured materials. Chem. Mater. 2001, 13, 3272−3282. (9) Guerrero-Martinez, A.; Perez-Juste, J.; Liz-Marzan, L. M. Recent Progress on Silica Coating of Nanoparticles and Related Nanomaterials. Adv. Mater. 2010, 22, 1182−1195. (10) Ciriminna, R.; Sciortino, M.; Alonzo, G.; de Schrijver, A.; Pagliaro, M. From Molecules to Systems: Sol-Gel Microencapsulation in Silica-Based Materials. Chem. Rev. 2011, 111, 765−789. (11) Blas, H.; Save, M.; Pasetto, P.; Boissiere, C.; Sanchez, C.; Charleux, B. Elaboration of Monodisperse Spherical Hollow Particles with Ordered Mesoporous Silica Shells via Dual Latex/Surfactant Templating: Radial Orientation of Mesopore Channels. Langmuir 2008, 24, 13132−13137. (12) Caruso, F. Hollow capsule processing through colloidal templating and self-assembly. Chem. - Eur. J. 2000, 6, 413−419. (13) Caruso, F.; Caruso, R. A.; Mohwald, H. Nanoengineering of inorganic and hybrid hollow spheres by colloidal templating. Science 1998, 282, 1111−1114. (14) Caruso, F.; Caruso, R. A.; Mohwald, H. Production of hollow microspheres from nanostructured composite particles. Chem. Mater. 1999, 11, 3309−3314. (15) Chen, M.; Wu, L. M.; Zhou, S. X.; You, B. A method for the fabrication of monodisperse hollow silica spheres. Adv. Mater. 2006, 18, 801−806. (16) Cornelissen, J.; Connor, E. F.; Kim, H. C.; Lee, V. Y.; Magibitang, T.; Rice, P. M.; Volksen, W.; Sundberg, L. K.; Miller, R. D. Versatile synthesis of nanometer sized hollow silica spheres. Chem. Commun. 2003, 1010−1011. (17) Deng, T. S.; Marlow, F. Synthesis of Monodisperse Polystyrene@Vinyl-SiO2 Core-Shell Particles and Hollow SiO2 Spheres. Chem. Mater. 2012, 24, 536−542. (18) Hotta, Y.; Alberius, P. C. A.; Bergstrom, L. Coated polystyrene particles as templates for ordered macroporous silica structures with controlled wall thickness. J. Mater. Chem. 2003, 13, 496−501. (19) Leng, W. G.; Chen, M.; Zhou, S. X.; Wu, L. M. Capillary Force Induced Formation of Monodisperse Polystyrene/Silica OrganicInorganic Hybrid Hollow Spheres. Langmuir 2010, 26, 14271−14275. (20) Li, H. Q.; Ha, C. S.; Kim, I. Facile fabrication of hollow silica and titania microspheres using plasma-treated polystyrene spheres as sacrificial templates. Langmuir 2008, 24, 10552−10556. (21) Li, L.; Ding, J.; Xue, J. M. Macroporous Silica Hollow Microspheres as Nanoparticle Collectors. Chem. Mater. 2009, 21, 3629−3637. (22) Lu, Y.; McLellan, J.; Xia, Y. N. Synthesis and crystallization of hybrid spherical colloids composed of polystyrene cores and silica shells. Langmuir 2004, 20, 3464−3470. (23) Niu, Z. W.; Yang, Z. H.; Hu, Z. B.; Lu, Y. F.; Han, C. C. Polyaniline-silica composite conductive capsules and hollow spheres. Adv. Funct. Mater. 2003, 13, 949−954. (24) Schmid, A.; Fujii, S.; Armes, S. P.; Leite, C. A. P.; Galembeck, F.; Minami, H.; Saito, N.; Okubo, M. Polystyrene-silica colloidal 1561

DOI: 10.1021/acs.macromol.6b00038 Macromolecules 2016, 49, 1552−1562

Article

Macromolecules (47) Landfester, K.; Bechthold, N.; Tiarks, F.; Antonietti, M. Formulation and stability mechanisms of polymerizable miniemulsions. Macromolecules 1999, 32, 5222−5228. (48) Miller, C. M.; Sudol, E. D.; Silebi, C. A.; Elaasser, M. S. Miniemulsion Polymerization of Styrene - Evolution of the ParticleSize Distribution. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 1391− 1408. (49) Rodriguez, V. S.; Elaasser, M. S.; Asua, J. M.; Silebi, C. A. Miniemulsion Copolymerization of Styrene Methyl-Methacrylate. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 3659−3671. (50) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990. (51) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry; Wiley: New York, 1979. (52) Bergna, H. E.; Roberts, W. O. Colloidal Silica Fundamentals and Applications; CRC Press: Boca Raton, FL, 2006.

1562

DOI: 10.1021/acs.macromol.6b00038 Macromolecules 2016, 49, 1552−1562