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Articles Synthesis of SiO2/Polystyrene Nanocomposite Particles via Miniemulsion Polymerization Sheng-Wen Zhang, Shu-Xue Zhou, Yu-Ming Weng, and Li-Min Wu* Department of Materials Science, The Advanced Coatings Research Center of China Educational Ministry, Fudan University, Shanghai 200433, People’s Republic of China Received September 20, 2004. In Final Form: December 2, 2004 The SiO2/polystyrene nanocomposite particles were synthesized through miniemulsion polymerization by using sodium lauryl sulfate surfactant (SLS), hexadecane costabilizer in the presence of silica particles coated with methacryloxy(propyl)trimethoxysilane. Core-shell or other interesting morphology composite particles were obtained depending on the size of the silica particles and the surfactant concentration employed. By adjusting these parameters, it was possible to control the size and morphology of the composite particles.
Introduction In recent years, considerable efforts have been devoted to the design and controlled fabrication of nanocomposite particles with different interesting morphologies,1-6 since these particles have various structures and can usually exhibit some special performances. For example, coreshell structure particles show some special properties in optics, electronics, magnetics, and catalysis by adjusting their size, chemical composition, and structure order and hence are potentially useful in the areas of coatings, electronics, catalysis, and diagnostics. Nanocomposite particles can be prepared through heterophase polymerization,7,8 heterocoagulation,9,10 and layer-by-layer self-assembly method.11,12 Among these methods, heterophase polymerization is by far the most frequently used technique. In this method, the nanocomposite particles are prepared by carrying out an aqueous phase polymerization in the presence of inorganic particles via suspension, dispersion, emulsion, and miniemulsion polymerization. For instance, composite particles containing several inorganic particles were produced by dispersion polymerization.13,14 Composite particles with “currant * Corresponding author:
[email protected]. (1) Sanchez, C.; Soler-Illia, G. J. de A. A.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061. (2) Caruso, F. Adv. Mater. 2001, 13, 11. (3) Fleming, M. S.; Mandal, T. K.; Walt, D. R. Chem. Mater. 2001, 13, 2210. (4) Reculusa, S. R.; Poncet-Legrand, C.; Ravaine, S.; Mingotaud, C.; Duguet, E.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, 2354 (5) Percy, M. J.; Amalvy, J. I.; Randall, D. P.; Armes, S. P. Langmuir 2004, 20, 2184. (6) Lu, y.; McLellan, J.; Xia, Y. Langmuir 2004, 20, 3464. (7) Barthet, C.; Hickey, A. J.; Cairns, D. B.; Armes, S. P. Adv. Mater. 1999, 11, 408. (8) Sondi, I.; Fedynyshyn, T. H.; Sinta, R.; Matijevic, E. Langmuir 2000, 16, 9031. (9) Tangboriboonrat, P.; Buranabunya, U. Colloid Polym. Sci. 2001, 279, 615. (10) Luiz, J.; Xavier, L.; Guyot, A.; Bourgeat-Lami, E. Polym. Int. 2004, 53, 609. (11) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mohwald, H. Macromolecules 1999, 32, 2317. (12) Caruso, F.; Trau, D.; Mo¨hwald, H.; Renneberg, R. Langmuir 2000, 16, 1485.
bun”-like7 and raspberry-like15,16 morphology were obtained by “surfactant-free” heterophase polymerization. By emulsion polymerization, composite particles with core-shell,17-20 raspberry-like,21 daisy-shape, and multipod-like22 morphology were obtained depending on the surface chemistry and the size of the inorganic particles. In fact, emulsion polymerization has become the most important method for the synthesis of composite particles. But the complexity of particle nucleation mechanism and the kinetic process in emulsion polymerization heavily complicate the structure formation and morphology control of the nanohybrid particles. Recently, miniemulsion polymerization has been found to be a particularly attractive way to obtain nanocomposite particles.23-27 In miniemulsion polymerization, the particle nucleation occurs primarily within the submicrometer monomer droplets.28 If the inorganic particles could be dispersed in monomer phase followed by miniemulsification, then each miniemulsion droplet could indeed be (13) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1998, 197, 293. (14) Percy, M. J.; Michailidou, V.; Armes, S. P. Langmuir 2003, 19, 2072. (15) Stejskal, J.; Kratochvil, P.; Armes, S. P.; Lascelles, S. F.; Riede, A.; Helmstadt, M.; Prokes, J.; Krivka, I. Macromolecules 1996, 29, 6814. (16) Percy, M. J.; Barthet, C.; Lobb, J. C.; Khan, M. A.; Lascelles, S. F.; Vamvakaki, M.; Armes, S. P. Langmuir 2000, 16, 6913. (17) Caris, C. H. M.; V.Elven, L.P. M.; v. Herk, A. M.; German, A. L. Br. Polym. J. 1988, 21, 133. (18) Furusawa, K.; Kimura, Y.; Tagawa, T. J. Colloid Interface Sci. 1986, 109, 69. (19) Hofman-Caris, C. H. M. New J. Chem. 1994, 18, 1087. (20) Quaroni, L.; Chumanov, G. J. Am. Chem. Soc.1999, 121, 10642. (21) Luiz, J.; Xavier, L.; Guyot, A.; Bourgeat-Lami, E. J. Colloid Interface Sci. 2002, 250, 82. (22) Reculusa, S.; Mingotaud, C.; Bourgeat-Lami, E.; Duguet, E.; Ravaine, S. Nano Lett. 2004, 4, 1677-1682. (23) Landfester, K. Adv. Mater 2001, 13, 765. (24) Landfeste, K.; Tiarks, F.; Hentze, H.; Antonietti, M.Macromol. Chem. Phys. 2000, 201, 1. (25) Tiarks, F.; Landfeste, K.; Antonietti, M. Langmuir 2001, 17, 5775. (26) Lelu, S.; Novat, C.; Guyot, A.; Bourgeat-Lami. Polym. Int. 2003, 52, 542. (27) Zhang, M.; Gao, G.; Li, C.; Liu, F. Langmuir 2004, 20, 1420. (28) Ugelsted, J.; EI-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Polym. Lett. 1973, 11, 503.
10.1021/la047652b CCC: $30.25 © 2005 American Chemical Society Published on Web 02/05/2005
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treated as a small nanoreactor, thus resulting in nanocomposite particles with various morphology. To incorporate inorganic particles into the latex particles, many approaches were used to make the inorganic particles well dispersed in the monomer phase. El-Aasser et al.29 encapsulated titanium dioxide by using stabilizer OLOA 370(polybutene-succinimide pentamine). Landfester et al.30 encapsulated calcium carbonate by its specific interaction with oleic acid. Hoffmann et al.31 encapsulated magnetite nanoparticles using oleoyl sarlosine acid or oleic acid as the first surfactant to disperse the magnetite particles into a styrene phase. However, it seemed that much research work was focused on the dispersion and encapsulation of the inorganic particles and very little work was involved in the size and morphology control of the nanocomposite particles by miniemulsion polymerization. Herein, we reported a controlled synthesis of SiO2/ polystyrene nanocomposite particles with core-shell morphology. Our method involved miniemulsion polymerization of styrene in the presence of surfactant, costabilizer, an initiator, and silica particles coated with methacryloxy(propyl)trimethoxysilane (MPS). The effects of the particle size and surface chemistry of silica, as well as of the surfactant concentration on the size and morphology of the composite particles were studied. Experimental Section Materials. Tetraethyl orthosilicate (TEOS), absolute ethanol, ammonium hydroxide, and toluene (Shanghai Chemistry Reagent Co.) were analytical grade and used as received. The silica coupling agents propyltrimethoxysilane (PTS), methyltrimethoxysilane (MTS), vinyltrimethoxysilane (VTS), methacryloxy(propyl)trimethoxysilane (MPS) (Degussa-Hu¨ls AG) were used as received. Styrene (Shanghai Chemistry Reagent Co.) was purified upon distillation under reduced pressure and kept refrigerated until use. Sodium lauryl sulfate (SLS) surfactant and sodium bicarbonate (NaHCO3) (Fisher Scientific) and hexadecane costabilizer (Fluka) were used as received. Potassium persulfate (KPS) (Shanghai Chemistry Reagent Co.) was analytical grade and used as received. Deionized water was applied for all polymerization and treatment processes. Preparation of the MPS-Silica Particles. The colloidal silica particles with 45 nm were synthesized according to the well-known Sto¨ber method.32 A 790 mL portion of absolute ethanol, 39 mL of ammonia, and 14 mL of water were introduced in a 1000-mL, three-neck, round-bottom flask equipped with a refrigerating system and heated to 50 °C under stirring, then 29 mL of TEOS was added into the solution and stirred at 50 °C for 24 h. Silica particles (90 and 200 nm) were synthesized through a similar procedure. Coating of the colloidal silica with MPS was according to the Philipse and Vrij method.33 It started with stirring a mixture of silica sol and MPS for 30 min at room temperature, then some solvents were slowly distilled off during a period of 2 h. During the distillation, the mixture was diluted with some toluene. The dispersion of MPS-modified silica was purified from free MPS and possibly some water or ammonia by centrifugation and redispersion in absolute ethanol (at 15 000 rpm for 30 min at 4 °C using Hitachi Himac CR 22E). MPSmodified silica powder was obtained by drying the purified dispersions in an oven at 70 °C for 24 h. Preparation of the Silica/Polystyrene Nanocomposite Particles. The composite particles were prepared according to the following procedure using the recipe given in Table 1. A 0.3 (29) Erdem, B.; Sudol, E. D.; Dimonie, V. L.; El-Aasser, M. S. J. Polym. Sci., Polym. Chem. 2000, 38, 4431. (30) Bechthold, N.; Tiarks, F.; Willert, M.; Landfester, K.; Antonietti, M. Macromol. Symp. 2000, 151, 549. (31) Hoffmann, D.; Landfeste, K.; Antonietti, M. Megnetohydrodynamics 2001, 37, 217. (32) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (33) Philipse, A. P.; Vrij, A. J. Colloid Interface Sci. 1989, 128, 121.
Langmuir, Vol. 21, No. 6, 2005 2125 Table 1. Recipe Used for the Preparation of Various Miniemulsions components
amount (g)
concentration
styrene MPS-silica SLS hexadecane deionized water KPS NaHCO3
10 0.3 0.116-0.580 0.400 39-40 0.015 0.004-0.008
20 wt %a 3.0 wt %b 10-50 mMc 4 wt %b 78-79%a 1.33 mMc 1.33-2.66 mMc
a Based on total recipe. b Based on monomer. c Based on aqueous phase.
g portion of MPS silica powder was first dispersed into the mixture of 10 g of styrene and 0.4 g of hexadecane with the aid of ultrasound, then the dispersion was introduced into mixture of 0.33 g of SLS, 0.004 g of sodium bicarbonate, and 39 g of water. After the mixture was stirred for 1 h, the miniemulsions were prepared by ultrasonifying the emulsion for 10 min with a 4710 series ultrasonic homogenizer (model CP600, Cole-Palmer Instruments) at a powder level of 8 and a duty cycle of 70%. The resulting miniemulsions were purged with nitrogen for 10 min. The polymerization started with 15 mg of KPS initiator at 70 °C and finished within 3 h. After polymerization, the free latex particles were separated from the composite particles by centrifugation and redispersion into water to obtain the pure colloidal composite particles. The operation was repeated until the serum was no longer turbid. The absence of residual free latex particles was confirmed by transmission electron microscopy (TEM). Characterization of Silica Particles and MPS-Silica Particles. The silica particle size was determined by dynamic light scattering (DLS) using a Beckman Coulter N4Plus submicrometer particle size analyzer working at a fixed angle of 90°. The specific surface area was determined by nitrogen adsorption at 77 K using a Micromeritics Tristar 3000 according to the Brunauer-Emmett-Teller method. The presence of MPS on the silica surface was confirmed by FTIR spectroscopy. It was measured in the wavenumber range from 4000 to 400 cm-1 at a resolution of 4 cm-1 using a Nicolet Nexus 470 FTIR spectrophotometer. The amount of the grafted MPS on the silica surface was determined by elemental analyses using an Elementar Vario EL. The actual coverage of the silica surface by MPS was calculated according to the Berendsen equation.34 Samples for elemental analysis and IR spectroscopy were ground to a fine powder and dried at 60 °C under vacuum for 24 h. Characterization of the Silica/Polystyrene Nanocomposite Particles. The droplet sizes of the miniemulsions were determined by DLS as described above. Droplet size was recorded immediately after sonication by diluting the sample with a saturated SDS aqueous solution. The sizes and morphologies of the silica/polystyrene composite particles and the pure polystyrene latex were characterized by TEM, which could also determine the thickness of the shells for core-shell particles. TEM measurements were performed on JEOL Jem-2011 microscope at an accelerator voltage of 200 kV. One drop of the suspension was diluted into water and placed on a 400-mesh carbon-coated copper grid and dried in air before observation.
Results and Discussion Modification and Dispersion of Silica Particles. A successful synthesis of nanocomposite particles via miniemulsion polymerization required the inorganic particles should be well dispersed in the monomer prior to emulsification.35 However, since the silica particles were hydrophilic and not easily dispersible in organic media, they usually needed to be modified by coating the surface with octadecyl alcohol,36,37 organosilanes,38,39 or poly(34) Berendsen. G. E. De Galan. R. J. Liq. Chromatogr. 1978, 1561. (35) Erdem, B.; Sudol, E. D.; Dimonie, V. L.; El-Aasser, M. S. J. Polym. Sci., Polym. Chem. 2000, 38, 4419. (36) Van Blaaderen, A.; Vrij, A. Langmuir 1992, 8, 2921.
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Table 2. Amount of MPS Grafted on the Silica Surface D Ca Ha Cb Hb specific grafted MPS (nm) (wt %) (wt %) (wt %) (wt %) area (m2/g) (molecules/nm2) 45 108 a
1.67 1.91
1.85 1.55
7.6 5.58
2.60 1.93
60.5 27.3
8.7 10.8
Determined before grafting. b Determined after grafting. Table 3. Effect of the MPS-Silica Particles on the Droplet Size of Miniemulsion
runs
[SLS] (mM)
MPS-silicaa (nm)
droplet size (nm)
PIb
1 2 3 4 5 6 7 8 9 10 11 12
10 10 10 20 20 20 30 30 30 40 40 40
0 45 90 0 45 90 0 45 90 0 45 90
214 276 256 176 210 194 154 163 171 126 143 155
0.451 0.398 0.799 0.665 0.453 0.480 0.342 0.194 0.289 0.456 0.473 0.365
mers.40,41 Here a series of silane coupling agents were used to hydrophobize the silica particles. Theoretically any surface silylation could provide the silica particles with an organic layer, thus achieving a considerable organophilation and compatibility with styrene monomer. But our experiments found that only MPS-modified silica and VTS-modified silica could be well dispersed in styrene, whereas PTS-modified silica and MTS-modifierd silica highly agglomerated in styrene. So, the MPS-modified silica was chosen for the following experiments since it could be more easily dispersed in styrene and copolymerized with styrene compared to VTS-modified silica. The bands at 1630, 1720, and 2890-2950 cm-1 from the FTIR spectrum were assigned to CdC, CdO, and CH stretching vibrations, respectively, indicating MPS had grafted onto the silica surface. The amount of the MPS grafted onto the silica surface was calculated according to the Berendsen equation33 and is summarized in Table 2. It was found that more than eight MPS molecules were grafted per nanometer square silica surface, higher than about 7 Si-OH/nm2 for Sto¨ber silica,42 which excluded the possibility of a single monolayer of MPS on the silica surface. Bourgeat-Lami and Lang13 confirmed that the grafted MPS formed oligomers on the silica surface using NMR analysis. Effect of the Silica Particle Size and Surfactant Concentration on Miniemulsion Droplets Size. Table 3 summarized the effects of silica particle sizes and surfactant concentrations on the droplet sizes of the miniemulsions. It could be clearly seen that the droplet sizes of the miniemulsions in the absence of silica particles significantly decreased from 214 to 126 nm when the SLS increased from 10 to 40 mM. Addition of 3 wt % MPSmodified silica obviously increased the droplet sizes of the corresponding miniemulsions, and the smaller the sizes of silica particles were, the bigger the sizes of (37) Van Blaaderen, A.; Jansen, J.W.; Vrij, A. J. Colloid Interface Sci. 1981, 354. (38) Philipse, A. P.; Vrij, A. J. Colloid Interface Sci. 1989, 128 (1), 121. (39) Van Blaaderen, A.; Vrij, A. J. Colloid Interface Sci. 1993, 156, 1. (40) Ben Quada, H.; Hommel, H.; Legrand, A. P.; Balard, H.; Papirer, E. J. Colloid Interface Sci. 1988, 122, 441. (41) Pathmamanoharan, C. Colloids Surf. 1990, 50, 1. (42) Iler, R. K. the Chemistry of Silica; Wiley: New York, 1979; p 622.
Figure 1. TEM photographs of SiO2/polystyrene composite particle using 20 mM SLS in the presence of silica particles with different size: (a) with 45 nm silica particles; (b) with 90 nm silica particles; (c) with 200 nm silica particles.
miniemulsion droplets were due to incorporation of more MPS-modified silica particles within each droplet, which would be further confirmed by the following TEM. Effect of the Silica Particle Size on the Size and Morphology of the Nanocomposite Particles. Figure 1 illustrated the TEM micrographs of the composite latex particles obtained using 20 mM SLS in the presence of 45, 90, and 200 nm silica beads. Obviously, the sizes of the silica beads had a drastic impact on the size and morphology of the composite particles formed. For 45 nm silica particles, several silica beads were embedded inside each latex particle and no free silica beads were present, as seen in Figure 1a. The average composite particle size was around 200 nm. Further experiments showed that the smaller the size of the silica particles, the greater the numbers of silica particles per composite particles and the larger the sizes of the composite particles (not presented here). For 90 nm silica beads, the composite particles had slightly smaller size (around 170 nm), as illustrated in Figure 1b, and a core-shell morphology was observed with a uniform 30-40 nm thick polymer layer surrounding the silica particles. DLS and TEM analysis indicated that the silica particles had been successfully entrapped inside the latex particles, no free silica particles were found in the nanocomposite particles with 45 or 90 nm silica, giving experimental evidence of a monomer droplet nucleation mechanism. The reason there was no free silica particle could be explained as follows: The grafting density of MPS on the silica particles was above 8 MPS/nm2. The dispersion experiments showed that the silica particles were highly agglomerated in an aqueous solution of sodium lauryl sulfate. Instead, the hydrophobized silica particles were well dispersed in monomer and contained within the monomer droplets. In the miniemusification process, these monomer droplets containing silica particles were dispersed in aqueous phase. Most of the surfactants were adsorbed on the surface of the droplets and stabilized the droplets. So it was very hard for silica particles to escape into the aqueous phase and form free particles. For 200 nm silica beads, most of the silica beads could not be fully encapsulated by a polymer layer. Instead, some polymer latex particles were adsorbed
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Figure 2. SEM photograph of SiO2/polystyrene composite particle using 20 mM SLS in the presence of 200 nm silica particles.
Figure 4. TEM photographs of SiO2/polystyrene composite particle using different SLS concentrations in the presence of 45 nm silica: (a) 20 mM; (b, c) 30 mM; (d) 40 mM.
Figure 3. TEM photographs of pure polystyrene latex particle using different SLS concentration: (a) 20 mM; (b) 40 mM.
onto the surfaces of silica particles, forming “raspberrylike” morphology, as shown in Figure 1c, which could be further confirmed by the scan electron micrograph, as shown in Figure 2. Effect of Surface Chemistry of the Silica Particles on the Size and Morphology of the Nanocomposite Particles. The colloidal silica (45 nm) was modified with MPS according to the Philipse and Vrij method32 and the Bourgeat-Lami method,13 respectively, to achieve 3.6 and 8.7 MPS/nm2 at the silica surface. The obtained MPSmodified silica particles were employed for the synthesis of nanocomposite particles using 20 mM SLS concentration. TEM micrographs of the nanocomposite particles, the same as in Figure 1a, showed that the graft density of MPS on the silica surface had almost no influence on the size and morphology of the nanocomposite particles (no more pictures presented here). Therefore, it could be concluded that the size and morphology of the nanocomposite particles strongly depended upon the size of silica particles regardless of their surface chemistry. This was completely different from emulsion polymerization due to a different nucleation mechanism. In emulsion polymerization, the surface chemistry of the silica particles, especially the density of MPS grafted on the silica surface, strongly influenced the final morphology of nanocomposite particles.22 Effect of SLS Concentrations on the Size and Morphology of the Nanocomposite Particles. The effect of surfactant concentration on the miniemulsion droplet size had been discussed above based on Table 3. Figure 3 further demonstrated TEM micrographs of pure latex particles with diameters around 105 and 68 nm using 20 and 40 mM SLS, respectively. The influence of SLS concentration on the size and morphology of nanocomposite particles prepared from runs 5, 8, and 11 in Table 3 is displayed in Figure 4.
Figure 5. TEM photographs of SiO2/polystyrene composite particle using 40 mM SLS and 90 nm silica (a) and uncoated 90 nm silica particles (b).
For SLS concentration at 20 mM, see Figure 4a, the composite particles had the same morphology as that of Figure 1a, and each composite particle essentially contained several silica particles. The composite particles size was around 200 nm. When SLS concentration increased to 30 mM, see parts b and c of Figure 4, each composite particle contained one or two silica particles which tried to locate at the center of the composite particles. The composite particles decreased to around 110 nm. When SLS concentration continued to increase to 40 mM, a true core-shell morphology composite particle was observed with a uniform 10-20 nm thick polymer layer surrounding the silica particles; see Figure 4d. The composite particles further decreased to around 80 nm. However, if 90 nm silica particles were embedded, for 20 mM SLS, only one silica bead was encapsulated into each composite particle, forming irregular core-shell morphology as illustrated in Figure 1b. The composite latex particle size was about 180 nm. But for 40 mM SLS, a true core-shell structure morphology was observed with a uniform 10-15 nm thick polymer layer surrounding the silica beads as shown in Figure 5a. The composite latex particle size decreased to about 130 nm. The TEM micrographs of the uncoated silica particles are also presented in Figure 5b for the sake of comparison. Thus, the size and morphology of the nano-
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composite particles could be controlled by adjusting the SLS concentration and the size of silica particles. Conclusion This paper presented a controlled synthesis of coreshell and other interesting SiO2/polystyrene morphology nanocomposite particles through miniemulsion polymerization. The size and morphology of nanocomposite particles could be tuned by adjusting the silica particle size and surfactant concentration. For 45 nm silica particles, the size of the nanocomposite particles decreased from 200 to 80 nm with increasing surfactant concentration from 20 to 40 mM, and the numbers of silica particles entrapped into each polymer particle gradually decreased and finally formed core-shell morphology. For 90 nm silica particles, the size of the nanocomposite particles also
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decreased from 180 to 130 nm with increasing surfactant concentration from 20 to 40 mM, but the core-shell morphology was kept unchanged. For 200 nm silica particles, some “raspberry-like” morphology was observed. The effects of the size and morphology of the composite particles on the properties of nanocomposite materials will be further studied. Acknowledgment. We thank Shanghai Special Nano Foundation, Shanghai Economic commission Industrialization Foundation, the Doctoral Foundation of University, Trans-century Outstanding Talented Person Foundation of China Educational Ministry, and Key Project of China Educational Ministry for the financial support for this research. LA047652B
