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Langmuir 2009, 25, 2486-2494
Efficient Preparation of Polystyrene/Silica Colloidal Nanocomposite Particles by Emulsion Polymerization Using a Glycerol-Functionalized Silica Sol Andreas Schmid and Steven P. Armes* Department of Chemistry, Dainton Building, The UniVersity of Sheffield, Brook Hill, Sheffield S3 7HF, United Kingdom
Carlos A. P. Leite and Fernando Galembeck UniVersidade Estadual de Campinas, Instituto de Quı´mica, P.O. Box 6154, Campinas SP, Sa˘o Paulo 13083-862, Brazil ReceiVed October 27, 2008. ReVised Manuscript ReceiVed December 5, 2008 Colloidally stable polystyrene/silica nanocomposite particles of around 200-400 nm diameter and containing 22-28 wt % silica can be readily prepared by aqueous emulsion polymerization at 60 °C using a cationic azo initiator in combination with a commercially available glycerol-functionalized ultrafine aqueous silica sol in the absence of any surfactant, auxiliary comonomer, or nonaqueous cosolvent. Optimization of the initial silica sol concentration allows relatively high silica aggregation efficiencies (up to 95%) to be achieved. Control experiments confirm the importance of selecting a cationic initiator, since nanocomposite particles were not formed when using an anionic persulfate initiator. Similarly, the glycerol groups on the silica sol surface were also shown to be essential for successful nanocomposite particle formation: use of an unfunctionalized ultrafine silica sol in control experiments invariably led to polystyrene latex coexisting with the silica nanoparticles, rather than efficient nanocomposite formation. Electron spectroscopy imaging transmission electron microscopy studies of ultramicrotomed polystyrene/silica nanocomposite particles indicate well-defined “core-shell” particle morphologies, which is consistent with both X-ray photoelectron spectroscopy and aqueous electrophoresis studies.
Introduction It is well-known that in situ polymerization of various types of vinyl monomers in the presence of commercial ultrafine silica sols in either aqueous solution,1-19 alcohol/water mixtures,20-24 * To whom correspondence should be addressed. E-mail: s.p.armes@ sheffield.ac.uk. (1) Gill, M.; Mykytiuk, J.; Armes, S. P.; Edwards, J. L.; Yeates, T.; Moreland, P. J.; Mollett, C. J. Chem. Soc., Chem. Commun. 1992, 108. (2) Maeda, S.; Armes, S. P. J. Colloid Interface Sci. 1993, 159, 257. (3) Han, M. G.; Armes, S. P. Langmuir 2003, 19, 4523. (4) Barthet, C.; Hickey, A. J.; Cairns, D. B.; Armes, S. P. AdV. Mater. 1999, 11, 408. (5) Percy, M. J.; Barthet, C.; Lobb, J. C.; Khan, M. A.; Lascelles, S. F.; Vamvakaki, M.; Armes, S. P. Langmuir 2000, 16, 6913. (6) Amalvy, J. I.; Percy, M. J.; Armes, S. P.; Wiese, H. Langmuir 2001, 17, 4770. (7) Percy, M. J.; Michailidou, V.; Armes, S. P.; Perruchot, C.; Watts, J. F.; Greaves, S. J. Langmuir 2003, 19, 2072. (8) Maeda, S.; Gill, M.; Armes, S. P.; Fletcher, I. W. Langmuir 1995, 11, 1899. (9) Butterworth, M. D.; Corradi, R.; Johal, J.; Lascelles, S. F.; Maeda, S.; Armes, S. P. J. Colloid Interface Sci. 1995, 174, 510. (10) Percy, M. J.; Amalvy, J. I.; Barthet, C.; Armes, S. P.; Greaves, S. J.; Watts, J. F.; Wiese, H. J. Mater. Chem. 2002, 12, 697. (11) Agarwal, G. K.; Titman, J. J.; Percy, M. J.; Armes, S. P. J. Phys. Chem. B 2003, 107, 12497. (12) Amalvy, J. I.; Percy, M. J.; Armes, S. P.; Leite, C. A. P.; Galembeck, F. Langmuir 2005, 21, 1175. (13) Bourgeat-Lami, E. J. Nanosci. Nanotechnol. 2002, 2, 1. (14) Luna-Xavier, J.-L.; Bourgeat-Lami, E.; Guyot, A. Colloid Polym. Sci. 2001, 279, 947. (15) Luna-Xavier, J.-L.; Guyot, A.; Bourgeat-Lami, E. J. Colloid Interface Sci. 2002, 250, 82. (16) Reculusa, S.; Mingotaud, C.; Bourgeat-Lami, E.; Duguet, E.; Ravaine, S. Nano Lett. 2004, 4, 1677. (17) Chen, M.; Wu, L.; Zhou, S.; You, B. Macromolecules 2004, 37, 9613. (18) Chen, M.; Zhou, S.; You, B.; Wu, L. Macromolecules 2005, 38, 6411. (19) Cheng, C. M.; Micale, F. J.; Vanderhoff, J. W.; El -Aasser, M. S. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 235. (20) Percy, M. J.; Armes, S. P. Langmuir 2002, 18, 4562.
or purely alcoholic media25-28 (e.g., methanol or 2-propanol) can lead to the formation of vinyl polymer/silica nanocomposite particles. Such colloidal particles have various potential applications, ranging from transparent, scratch-resistant coatings and durable exterior fac¸ade paints29,30 to synthetic mimics for silicate-based micrometeorites31 to new pH-responsive Pickering emulsifiers.32,33 However, a significant generic problem for this route is the relatively low silica aggregation efficiency that is typically achieved. For example, in the conducting polymer/ silica nanocomposite syntheses originally reported by our group, around 50% of the silica sol typically remains in solution.1-3,8,9 Similar aggregation efficiencies were also obtained for the preparation of poly(4-vinylpyridine)/silica nanocomposite particles.4-7,32,33 This is an important problem, because this excess silica sol is known to compromise the performance of the nanocomposite particles in certain applications, for example, as stimulus-responsive Pickering emulsifiers.33 Moreover, deter(21) Percy, M. J.; Amalvy, J. I.; Randall, D. P.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 2004, 20, 2184. (22) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1998, 197, 293. (23) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1999, 210, 281. (24) Bourgeat-Lami, E.; Lang, J. Macromol. Symp. 2000, 151, 377. (25) Yoshinaga, K.; Yokoyama, T.; Sugawa, Y.; Karakawa, H.; Enomoto, N.; Nishida, H.; Komatsu, M. Polym. Bull. 1992, 28, 663. (26) Schmid, A.; Fujii, S.; Armes, S. P. Langmuir 2006, 22, 4923. (27) Schmid, A.; Fujii, S.; Armes, S. P. Langmuir 2005, 21, 8103. (28) Schmid, A.; Fujii, S.; Armes, S. P.; Leite, C. A. P.; Galembeck, F.; Minami, H.; Saito, N.; Okubo, M. Chem. Mater. 2007, 19, 2435. (29) Leuninger, J.; Tiarks, F.; Wiese, H.; Schuler, B. Farbe Lack 2004, 110, 30. (30) Xue, Z.; Wiese, H. Int. Patent WO 03000760, 2003. (31) Burchell, M. J.; Willis, M. J.; Armes, S. P.; Khan, M. A.; Percy, M. J.; Perruchot, C. Planet. Space Sci. 2002, 50, 1025. (32) Fujii, S.; Armes, S. P. AdV. Mater. 2005, 17, 1014. (33) Fujii, S.; Armes, S. P.; Binks, B. P.; Murakami, R. Langmuir 2006, 22, 6818.
10.1021/la803544w CCC: $40.75 2009 American Chemical Society Published on Web 01/13/2009
Colloidally Stable PS/SiO2 Nanocomposite Particles
mination of the mean silica contents of the nanocomposite particles cannot be undertaken until the contaminating silica sol has been removed by time-consuming centrifugation/redispersion cycles. One elegant solution to this problem was reported by Tiarks and co-workers, who utilized a miniemulsion polymerization technique to ensure very high silica aggregation efficiencies.34 Unfortunately, such formulations usually require specialist equipment and are generally not widely used, at least when compared to aqueous emulsion polymerization. Recently, we reported that very high silica aggregation efficiencies could be obtained when (co)polymerizing 2-vinylpyridine in the presence of an ultrafine anionic silica sol using a cationic azo initiator.35 However, 2-vinylpyridine is a relatively expensive and malodorous auxiliary comonomer, and only high Tg nanocomposite particles could be obtained using this approach. As far as we are aware, there is only one commodity product based on colloidal vinyl polymer/silica nanocomposite particles that has been commercialized. BASF manufactures acrylic copolymer/silica nanocomposite particles known as col.9,36 which are then formulated by Akzo Nobel to produce a high performance exterior fac¸ade paint marketed as Herbol-Symbiotec. In the current work, an improved nanocomposite formulation based on a new glycerol-functionalized aqueous silica sol is reported. Colloidal polystyrene/silica (PS/SiO2) nanocomposite particles are prepared with remarkably high silica aggregation efficiencies by the surfactant-free emulsion polymerization of styrene using a cationic azo initiator in the absence of any auxiliary comonomer. The influence of various synthesis parameters such as the initial silica sol concentration, the temperature, and the pH have been investigated, and the resulting nanocomposite particles have been characterized with regard to their particle size, silica content, silica incorporation efficiency, and morphology.
Experimental Section Materials. Styrene (Aldrich) was passed through a column of basic alumina to remove its inhibitor and then stored at -20 °C prior to use. 2,2′-Azobis(isobutyramidine) dihydrochloride (AIBA) and ammonium persulfate (APS) were used as received from Aldrich. Two commercial glycerol-modified, aqueous silica sols were obtained from Eka Chemicals, Bohus, Sweden, Bindzil CC40 (manufacturer specifications: 12 nm diameter aqueous silica sol at 40 w/w %) and Bindzil CC30 (manufacturer specifications: 7 nm diameter aqueous silica sol at 30 w/w %); both sols were used as received. However, our in-house TEM studies suggested that these sols actually had mean particle diameters of 19 and 12 nm, respectively. In certain control experiments, an unfunctionalized aqueous silica sol (Bindzil 2040, 20 nm nominal diameter at 40 w/w %; also obtained from Eka Chemicals) was used. Particle Synthesis. A typical nanocomposite synthesis was conducted as follows. The appropriate amount of the aqueous silica sol (5.4 g of aqueous dispersion, equivalent to 2.0 g of dry silica) and 37.6 g of deionized water were added in turn to a round-bottomed flask containing a magnetic stir bar, and then styrene monomer (5.0 g) was added. The mixture was degassed by five evacuation/nitrogen purge cycles and subsequently heated up to 60 °C using an oil bath. The AIBA initiator (50.0 mg; 1.0 wt % based on styrene) was dissolved in degassed deionized water (4.0 g) and added to the reaction solution, giving a total water content of 45 g. Each polymerization was allowed to continue for 24 h. The resulting milky-white colloidal dispersions were purified by repeated centrifugation redispersion cycles (3000-5000 rpm for 30 min), with each successive supernatant being carefully decanted and replaced (34) Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 17, 5775. (35) Dupin, D.; Schmid, A.; Balmer, J. A.; Armes, S. P. Langmuir 2007, 23, 11812. (36) BASF-Aktiengesellschaft, www.col9.com (accessed Feb 3, 2008).
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Figure 1. Schematic representation of the synthesis of polystyrene/ silica nanocomposite particles by aqueous emulsion polymerization of styrene at 60 °C using a cationic AIBA initiator in the presence of a commercial 19 nm glycerol-modified aqueous silica sol (Bindzil CC40) as the sole stabilizing agent.
with deionized water. This was repeated, if required, until transmission electron microscopy studies confirmed that any excess silica sol had been removed by this purification protocol. Excessive centrifugation rates (>8000 rpm) and times (>1 h) were avoided, since these would otherwise result in sedimentation of the excess silica sol and also make redispersion of the sedimented particles more difficult. Characterization. Dynamic light scattering (DLS; Malvern Zetasizer NanoZS instrument) was used to obtain intensity-average hydrodynamic particle diameters by analyzing the scattered light at 173°. Disk centrifuge photosedimentometry (DCP; Brookhaven Instruments) was used to obtain the weight-average particle size distributions of the purified aqueous latexes. Solid-state particle densities were measured by helium pycnometry (Micromeritics AccuPyc 1330 instrument). Thermogravimetric analyses (TGA) were conducted using a Perkin-Elmer Pyris 1 TGA instrument. Dried samples were heated in air to 800 °C at a heating rate of 10 °C min-1, and the observed mass loss was attributed to the quantitative degradation of the polystyrene, with the remaining incombustible residues being assumed to be that of pure silica (SiO2). Transmission electron microscopy (TEM) samples were prepared by drying a drop of a dilute aqueous dispersion onto a carbon-coated copper grid, followed by analysis using a Phillips CM100 instrument operating at 100 kV. Particle growth and monomer conversion curves were determined by periodic sampling of the mechanically stirred polymerizing solution conducted on a larger scale. Quenching of the styrene polymerization was achieved by exposure to air. Solids contents were determined gravimetrically with an Ohaus M45 moisture analyzer at a drying temperature of 150 °C, and DLS particle diameters were determined after dilution with deionized water. Aqueous electrophoresis measurements were conducted using a Malvern Zetasizer NanoZS instrument. Measurements were conducted in the presence of 1.0 mM KCl background electrolyte. Typically, the solution was initially adjusted to approximately pH 11 using KOH and then titrated to pH 1.5 using HCl. However, for the determination of zeta potentials of AIBA-coated Bindzil CC40 (see Figure 4), no added salt was used. This accounts for the minor difference (-29 mV vs -40 mV; compare Figures 4 and 5) observed for the pristine Bindzil CC40 silica sol at pH 8.9. An adsorption isotherm of the cationic AIBA initiator on the 19 nm glycerol-modified aqueous silica sol (Bindzil CC40) was determined using the depletion method. Mixtures of 1.08 g of this aqueous silica sol (corresponding to 400 mg of dry silica) in water (40 mL) containing various amounts of AIBA initiator were equilibrated at room temperature for 60 min and then centrifuged at 20 000 rpm and 20 °C for 4 h. The initiator concentration in each of these supernatants was determined by UV-visible spectroscopy using the AIBA absorption peak at 368 nm and a previously constructed calibration curve (see the Supporting Information). X-ray photoelectron spectra (XPS) were acquired on a Kratos Axis ULTRA “DLD” X-ray photoelectron spectrometer equipped with a monochromatic Al KR X-ray source (hν ) 1486.6 eV) and operating with a base pressure in the range 10-8-10-10 mbar. Dried
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Figure 2. Representative TEM images of control latexes (a, b, c) prepared under conditions identical to those used for the nanocomposite formulations: (a) using a non-functionalized 20 nm Bindzil 2040 silica sol instead of a glycerol-modified Bindzil CC40 sol; (b) an anionic APS initiator instead of the cationic AIBA initiator; (c) emulsion polymerization of styrene initiated with AIBA in the absence of any silica sol. In contrast, using the cationic AIBA initiator with the glycerolfunctionalized silica sol produces well-defined polystyrene/silica nanocomposite particles (d, e, f) using either the 12 nm Bindzil CC30 silica sol (entry 8 in Table 1) or the 19 nm Bindzil CC40 silica sol (entries 1 and 2 in Table 1), respectively.
particles were spread on an indium plate with a spatula and stored under reduced pressure before XPS measurements. Electron spectroscopy imaging transmission electron microscopy (ESI/TEM) studies were conducted on ultramicrotomed nanocomposite particles in order to assess the morphology and spatial distribution of the silica within the PS/SiO2 nanocomposite particles using a Carl Zeiss CEM 902 TEM instrument fitted with a Castaing-Henry-Ottensmeyer filter spectrometer within its column. Aqueous dispersions of the nanocomposite particles were dried at room temperature, and the resulting solids were embedded in a soft epoxy resin that was cured at 60 °C. The resin blocks were trimmed and cut using an EM FC6 Leica ultramicrotome equipped with a Diatome diamond knife. Ultrathin sections of approximately 40 nm thickness were placed over copper grids for ESI/TEM analysis. Images were acquired using electrons with element-specific threshold energies (e.g., 101 eV for silicon and 284 eV for carbon), recorded using a slow-scan CCD camera (Proscan), and processed using AnalySis 3.0 software.
Results and Discussion The recent commercial availability of a new glycerolfunctionalized aqueous silica sol (Bindzil CC 40 or CC30) allows the efficient synthesis of PS/SiO2 colloidal nanocomposite
Figure 3. Representative TEM images of polystyrene/silica nanocomposite particles prepared at (a) 70 °C and (b) 80 °C (entries 2 and 3 in Table 2), at different solution pH (c) pH 10 and (d) pH 5 (entries 5 and 9 in Table 2) and using various AIBA/silica mass ratios of (e) 2 mg g-1, (f) 7 mg g-1 and (g) 25 mg g-1 (see entries 1-3 in Table 3).
particles in purely aqueous media. This formulation does not require auxiliary comonomers, added surfactants, or the use of alcoholic silica sols (see Figure 1). According to its manufacturer, this glycerol-functionalized silica sol is prepared using a proprietary protocol described in the patent literature.37 A conventional aqueous silica sol (Bindzil 2040) is treated with either γ-glycidoxypropyl trimethoxysilane or γ-glycidoxypropyl methyldiethoxysilane in the presence of water. Hydrolysis of the alkoxy groups leads to surface grafting of the silane species, with concomitant ring-opening of the epoxide ring producing the hydrophilic glycerol functionality. Our XPS studies of a dried sample of the Bindzil CC40 silica sol indicate a surface carbon content of around 12%, compared to a bulk carbon content of 3.7% estimated from carbon microanalysis. In particular, peak fitting of C1s core-line spectra using commonly used chemical (37) Greenwood, P.; Lagnemo, H. Int. Patent WO2004/035474A1, 2004.
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wide range of conditions. TEM images of PS/SiO2 particles prepared with the Bindzil CC40 silica sol using various initial silica sol concentrations are shown in Figure 2e and f. These images clearly show the presence of silica particles at the surface of the nanocomposite particles. Relatively narrow particle size distributions were indicated by disk centrifuge photosedimentometry: mean particle diameters of 256 ( 35 nm and 285 ( 38 nm were obtained for nanocomposite particles prepared using the 12 and 19 nm Bindzil silica sols (CC30 and CC40), respectively. Solid-state densities of 1.21 and 1.22 g cm-3 were determined by helium pycnometry for these nanocomposite particles, while thermogravimetry indicated very similar silica contents of 23 and 24 wt %, respectively. To assess the silica sol incorporation efficiency, the monomer conversion was determined gravimetrically by measuring the solids content of the reaction solution after drying to constant weight at 150 °C using a moisture analyzer. The styrene conversion was calculated as the difference between the experimental solids content after polymerization and the theoretical solids content at zero conversion normalized to the difference between the theoretical solids content at complete conversion and the theoretical solids content at zero conversion. The silica sol incorporation efficiency was determined using this styrene conversion together with the silica content of the purified nanocomposite particles obtained from thermogravimetry using the following equation:
(
)
mmonomerc - mmonomerc 1-s x) × 100% msilica Figure 4. (a) Langmuir-type isotherm obtained for the electrostatic adsorption of the cationic AIBA initiator onto 19 nm Bindzil CC40 silica sol at pH 8.9 and 20 °C. (b) Variation of zeta potential with AIBA /silica mass ratio at pH 8.9 and 20 °C. Note the initial marked reduction in the magnitude of the zeta potential from -29 mV to around -11 mV up to approximately 20 mg of AIBA per gram of silica, with a much weaker concentration dependence being observed thereafter.
shifts38 is consistent with the presence of glycerol surface groups (see Figures S1 and S2 in the Supporting Information). According to its manufacturer, the Bindzil CC40 sol contains approximately 800 surface glycerol groups per silica particle. Influence of the Silica Sol Type. Our preliminary control experiments confirmed that stable nanocomposite particles cannot be obtained if an unfunctionalized aqueous anionic silica sol (e.g., Bindzil 2040) is used in the emulsion polymerization of styrene using a cationic AIBA initiator. Instead, TEM studies (see Figure 2a) suggest that polydisperse, charge-stabilized latex particles and the original silica sol are both present, with little or no interaction between these two species. Similarly, the emulsion polymerization of styrene conducted in the presence of the glycerol-modified Bindzil CC silica sol leads to coagulation when an anionic APS initiator is employed (see Figure 2b). If the emulsion polymerization of styrene is conducted in the absence of any silica sol using the cationic AIBA initiator, then polydisperse charge-stabilized latex particles are obtained (see Figure 2c). However, polymerizing styrene in the presence of either the 12 nm Bindzil CC30 (see Figure 2d and entry 8 in Table 1) or the 19 nm Bindzil CC40 silica sol (see entry 3 in Table 1) using the cationic AIBA initiator invariably led to the formation of colloidally stable nanocomposite particles over a (38) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers; John Wiley & Sons: Chichester, 1992.
(1)
where x is the silica sol incorporation efficiency, mmonomer and msilica are the initial masses of monomer and silica, respectively, c is the styrene conversion, and s is the fractional silica content of the purified nanocomposite particles as determined by thermogravimetry. For the two nanocomposites described above (prepared using the CC30 and CC40 silica sols), this approach leads to estimated silica sol incorporation efficiencies of 63 and 73%, respectively. These silica incorporation efficiencies are significantly higher than those of PS/SiO2 nanocomposites obtained previously using alcoholic silica sols;21,26,28 thus, this new formulation seemed rather promising. It is emphasized here that only the combination of the glycerol-functionalized silica sol and the cationic azo initiator leads to colloidally stable nanocomposite particles. Influence of Silica Sol Concentration. Having identified reaction conditions that gave colloidally stable PS/SiO2 particles, the initial silica concentration was then systematically varied in order to improve the silica incorporation efficiency. More specifically, nanocomposite syntheses were conducted using 1.0-8.0 g of the 19 nm Bindzil CC40 silica sol at a solution pH of 8.9 in a fixed 50 mL reaction volume at 60 °C. Representative TEM images of PS/SiO2 particles obtained after centrifugal purification to remove any excess are shown in Figure 2e and f. Lowering the initial silica mass from 4.0 to 1.0 g leads to larger mean particle diameters, as judged by DCP. At the lowest initial silica concentration of 1.0 g (entry 1 in Table 1 and Figure 2f), a relatively broad size distribution is obtained and DLS studies suggest that incipient flocculation occurs. The optimum amount of silica appears to be 1.5 g (see entry 2): under these conditions, the styrene conversion is 84%, a relatively narrow particle size distribution is obtained, and the silica aggregation efficiency is estimated to be around 95%. Higher amounts of silica lead to smaller PS/SiO2 particles but significantly lower silica aggregation
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Table 1. Summary of the Effect of Varying the Silica Sol Type and Initial Bindzil CC40 Silica Concentration on Various Nanocomposite Particle Properties for the Emulsion Polymerization of Styrene Using the Cationic AIBA Initiator at 60°C for 24 h entry initial silica monomer particle density silica silica incorporation hydrodynamic particle weight-average no. mass (g) conversion (%) (g cm-3)a content (wt %)b efficiency (%) diameter (nm)c particle diameter (nm)d 1 2 3 4 5 6 7 8e
1.0 1.5 2.0 3.0 4.0 6.0 8.0 2.0
86 84 92 81 83 82 84 84
1.19 1.24 1.22 1.25 1.23 1.19 1.22 1.21
18 25 24 27 26 22 24 23
97 95 73 50 37 20 17 63
504 (0.123) 408 (0.012) 333 (0.057) 330 (0.060) 308 (0.055) 297 (0.057) 283 (0.104) 305 (0.026)
379 ( 133 325 ( 44 285 ( 38 281 ( 38 269 ( 43 278 ( 40 280 ( 48 256 ( 35
a As determined by helium pycnometry. b Determined by thermogravimetry (10 °C per min in air). c As measured by dynamic light scattering; polydispersities are given in brackets. d As measured by disk centrifuge photosedimentometry. e Using the 12 nm Bindzil CC30 silica sol instead of the 19 nm Bindzil CC40 silica sol.
Table 2. Summary of the Effect of Varying the Reaction Temperature and the Solution pH on the Synthesis of Polystyrene/Silica Nanocomposite Particles in the Polymerization of Styrene with Cationic AIBA in the Presence of a 19 nm Commercial Aqueous Silica Sol (Bindzil CC40)a entry silica T solution monomer particle density silica silica incorporation e hydrodynamic particle weight-average no. (g) (°C) pH conversion (%) (g cm-3)b content (wt %)c fficiency (%) diameter (nm)d particle diameter (nm)e 1 2 3 4
1.5 1.5 1.5 1.5
60 70 80 90
8.9 8.9 8.9 8.9
84 79 75 69
1.24 1.23 1.21 1.23
25 26 22 24
95 92 71 73
408 (0.012) 435 (0.058) 472 (0.189) 514 (0.201)
325 ( 44 336 ( 57 321 ( 50 320 ( 35
5 6 7 8 9
2.0 2.0 2.0 2.0 2.0
60 60 60 60 60
10.0 8.9 8.0 7.0 5.0
82 92 75 79 81
1.19 1.22 1.26 1.26 1.26
20 24 28 27 28
51 73 73 75 80
401 (0.027) 333 (0.057) 426 (0.179) 370 (0.084) 445 (0.211)
304 ( 42 285 ( 38 280 ( 33 275 ( 43 283 ( 37
a The amount of styrene used in each synthesis was 5.0 g and each polymerization was run for 24 h (N.B. entry 6 in this table is identical to entry 3 in Table 1.) b As determined by helium pycnometry. c Determined by thermogravimetry (10 °C per min in air). d As measured by dynamic light scattering; polydispersities are given in brackets. e As measured by disk centrifuge photosedimentometry.
efficiencies (see entries 3-7). Thus, the majority of additional silica is wasted, since it is not incorporated within the PS/SiO2 particles. The silica contents of these nanocomposite particles are relatively constant at around 22-27% by mass as judged by thermogravimetry and confirmed by carbon microanalyses. There is a reasonably good correlation between these silica contents and the mean particle densities determined by helium pycnometry, which varied from 1.19 to 1.25 g cm-3. In summary, lower amounts of silica are beneficial for achieving higher silica incorporation efficiencies but colloidal stability can be compromised below a certain minimum silica concentration. Influence of Polymerization Temperature. The influence of polymerization temperature was studied under the same conditions as described above using the optimized silica mass of 1.5 g. The polymerization temperature was systematically increased in 10 °C increments from 60 °C up to 90 °C. Inspecting Table 2, it is clear that lower conversions are obtained at higher temperatures. Presumably this is because the initiator half-life of 420 min at 60 °C is comparable to the polymerization time, whereas significantly shorter half-lives are obtained at higher temperatures. According to its manufacturer, the AIBA initiator has a half-life of 420 min at 60 °C, 200 min at 65 °C, 125 min at 70 °C, 30 min at 80 °C, and 1.6 min at 90 °C. The mean particle diameters determined by DCP are relatively constant, while the DLS data indicate the presence of larger, more polydisperse particles; this suggests higher degrees of flocculation of the primary nanocomposite particles. The mean silica contents of the PS/SiO2 particles are approximately the same within experimental error, but there is a systematic decrease in the silica aggregation efficiency. Thus, there appear to be no obvious advantages in conducting these nanocomposite syntheses at temperatures above 60 °C. Nevertheless, it seems that the highest silica incorporation efficiencies and lowest DLS polydispersity index (corresponding to good colloidal stability) are obtained at 60 °C, rather than at higher temperatures (see entry 1 in Table 2).
Influence of Solution pH. A second polymerization parameter examined was the solution pH. Preliminary nanocomposite syntheses were conducted at pH 8.9, which is the “native” pH obtained on diluting the concentrated aqueous BindzilCC40 silica sol with water. Literature examples of nanocomposite syntheses involving aqueous silica sols are typically conducted at pH 9-10.5-7 At this basic pH, the silica sol is highly anionic, which ensures good colloidal stability. The solution pH was systematically varied from pH 5 to pH 10, and the results are summarized in Table 2. At pH 10, the silica incorporation efficiency is reduced to only 51% and the mean silica content of the PS/SiO2 particles is also somewhat lower (20 wt %). Significantly higher silica incorporation efficiencies are obtained on lowering the solution pH, which also leads to higher silica contents. On the other hand, the apparent hydrodynamic particle diameter also increases, which may indicate some incipient flocculation. This poorer colloidal stability is understandable given that the anionic charge density of the silica nanoparticles on the surface of the PS/SiO2 nanocomposite particles is reduced at lower pH. AIBA Adsorption. It has been shown previously that the cationic AIBA initiator electrostatically adsorbs onto various anionic silica sols.14,27,28,35 Following our previously reported protocol,28,35 a Langmuir-type isotherm was constructed for AIBA adsorption onto the Bindzil CC40 silica sol (see Figure 4a). Briefly, aqueous dispersions containing identical amounts of silica sol were mixed with varying amounts of the AIBA initiator, which was allowed to adsorb electrostatically at 20 °C for 60 min. Thereafter, these AIBA-coated silica sols were centrifuged at 20 000 rpm for 4 h. This rather high centrifugation rate was required to ensure efficient sedimentation; care was taken to maintain the centrifuge temperature at 20 °C to prevent thermal decomposition of the initiator. A calibration curve was constructed using aqueous solutions of known AIBA concentration using its characteristic absorbance at 368 nm (see Figure S3 in the Supporting Information). After centrifugation, the UV-visible
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Table 3. Effect of Varying the AIBA Initiator/Silica Mass Ratio on Polystyrene/Silica Nanocomposite Properties for the Emulsion Polymerization of Styrene at 60°C for 24 h Using Fixed Amounts of Silica (2.0 g) and Styrene (5.0 g) entry no. 1 2 3
number-average weight-average hydrodynamic initiator AIBA/SiO2 monomer silica particle mass incorporation particle particle mass ratio conversion particle density silica (mg) (%) (g cm-3)a (mg/g) Content (wt %)b efficiency (%) diameter (nm)d diameter (nm)e diameter (nm)c 4 14 50
2 7 25
82 94 92
1.19 1.24 1.22
23 26 24
61 83 73
248 ( 62 163 ( 43 283 ( 41
249 ( 172 164 ( 29 285 ( 38
208 (0.025) 333 (0.057)
a As determined by helium pycnometry. b Determined by thermogravimetry (10 °C min-1 in air). c As measured by dynamic light scattering; polydispersities are given in brackets. d As measured from TEM images. e As measured by disk centrifuge photosedimentometry.
Figure 5. Zeta potential versus pH curves for polystyrene/silica nanocomposite particles (entries 3 and 8 in Table 1) and the corresponding silica sols revealing negative zeta potentials over the whole pH range investigated for both silica sols and polystyrene/silica nanocomposite particles.
spectrum of each supernatant was recorded. The adsorbed amount of AIBA was then determined by subtracting the amount of AIBA remaining in solution after adsorption from the initial amount of AIBA (depletion method). These adsorbed amounts were plotted against the equilibrium AIBA concentration, resulting in the adsorption isotherm shown in Figure 4. This isotherm suggests that the maximum amount of AIBA, Γ, adsorbed onto the Bindzil CC40 silica sol is around 6.8 mg g-1, or 0.045 mg m-2. This latter value is significantly lower than the Γ of 0.25 mg m-2 found for AIBA adsorbed onto an unfunctionalized Nyacol 2040 silica sol,35 which suggests that glycerol functionalization of the surface of the Bindzil CC40 silica sol necessarily reduces the number of silanol groups and hence its anionic surface charge density. Nevertheless, the reduced Γ of 0.045 mg m-2 still corresponds to approximately 114 AIBA molecules per Bindzil CC40 silica particle. Having determined this AIBA adsorption isotherm, three nanocomposite syntheses were conducted for which the amounts of styrene and silica sol were fixed and the AIBA concentration was varied (see Table 3). Thus, the three AIBA/silica mass ratios of 2, 7, and 25 mg g-1 correspond to (i) submonolayer coverage of the silica particles with little or no AIBA remaining in the reaction solution (i.e., below the knee of the isotherm); (ii) monolayer coverage with little or no AIBA remaining in the reaction solution (i.e., approximately at the knee of the isotherm); and (iii) monolayer coverage with a substantial amount of excess AIBA present in the reaction solution (i.e., above the knee of the isotherm). TEM images of the resulting nanocomposite particles are shown in Figure 3e (2 mg g-1), 3f (7 mg g-1), and 3g (25 mg g-1). It is perhaps noteworthy that the highest silica incorporation efficiency of 83% was obtained under conditions that correspond to the knee of the adsorption isotherm (see entry 2 in Table 3).
In this particular formulation, essentially all of the AIBA initiator is present at the surface of the silica particles prior to styrene polymerization, so it is tempting to suggest that surface polymerization may be important for successful nanocomposite formation. Moreover, the PS/SiO2 particles prepared at the lowest initiator/silica mass ratio appear to be only rather sparsely coated with silica particles compared to the PS/SiO2 particles produced with the other two formulations (compare Figure 3f with panels e and g). Indeed, it seems that some of the former so-called “nanocomposite” particles barely contain any silica at all (see arrow inset in Figure 3e). In this context, it is perhaps noteworthy that we have previously observed a minor population of particles that contained very little silica (i.e., latex-type particles) when examining PS/SiO2 nanocomposite particles prepared with an alcoholic silica sol using ESI/TEM.12 Surprisingly, high incorporation efficiencies were also obtained for the 25 mg g-1 formulation, in which a monolayer of adsorbed AIBA coats the silica particles but the majority of the initiator remains in aqueous solution. Of course, these adsorption isotherm data were obtained at 20 °C and hence should be treated with due caution: it is conceivable that the adsorption of AIBA initiator, or indeed its decomposition products, onto silica may be significantly different at 60 °C (which is the polymerization temperature used in these nanocomposite syntheses). Unfortunately, it is simply not possible to determine the AIBA adsorption isotherm directly at 60 °C, since the initiator thermally decomposes at this temperature. Nevertheless, given that the cationic AIBA initiator does adsorb onto the anionic silica sol, it is pertinent to ask how this affects the electrophoretic mobility of the AIBA-coated particles. For example, does either charge compensation or charge reversal occur? To address this question, the zeta potential of the silica sol was determined as a function of added AIBA initiator (see Figure 4b). These measurements indicate a systematic reduction in zeta potential from initially -30 mV to approximately -11 mV on adding up to 20 mg of AIBA per gram of silica. Further addition of AIBA only led to a minor additional reduction, and the zeta potential always remained negative. These experiments suggest that an AIBA/silica mass ratio of around 20 mg g-1 corresponds to a plateau value or “knee”. This perhaps explains why an initiator/silica mass ratio of 25 mg g-1 can also lead to high silica incorporation efficiencies: the reduction in zeta potential should favor silica adsorption onto the surface of the polystyrene particles by minimizing repulsive forces between neighboring silica particles. Surface Characterization. The surface composition of the nanocomposite particles was assessed by both aqueous electrophoresis and XPS. Zeta potential measurements indicate that there is little difference in electrophoretic behavior between the pristine silica sols and various PS/SiO2 particles prepared at various initial silica sol concentrations or initiator/silica mass ratios. In all cases, negative zeta potentials were observed over the whole pH range investigated (see Figure 5). This close correspondence suggests a silica-rich surface for the PS/SiO2 particles, which is consistent with the TEM images shown in
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Figures 2 and 3. Similar conclusions were drawn by Butterworth et al.,9 who analyzed the electrophoretic behavior of various polypyrrole/silica nanocomposite particles with “raspberry” morphologies. However, as we shall see later, nanocomposite particle morphologies should not be inferred on the basis of aqueous electrophoretic data alone. XPS is a highly surface-specific technique with a typical sampling depth of 2-10 nm.10,38 XPS survey spectra reveal the presence of Si2s and Si2p signals, which act as unique elemental markers for the silica nanoparticles (see the Supporting Information) and provide further evidence that this component is present either at or very near the surface of the nanocomposite particles. The presence of a relatively intense C1s signal (compared to the weak C1s signal observed for the pristine Bindzil CC40 silica sol) confirms that the polystyrene component is also present at (or near) the nanocomposite particle surface. Moreover, the C1s signal obtained for the PS/SiO2 nanocomposite particles has an associated π-π* shakeup satellite at around 292 eV (data not shown) which indicates aromatic character, as expected for polystyrene. This feature is absent in the weaker C1s signal observed for the pristine Bindzil CC40 silica sol, which confirms the wholly aliphatic nature of the carbon atoms in this case. The surface composition can be assessed by comparing the surface Si atom % data determined by XPS to the bulk Si atom % values calculated from thermogravimetry (see Table S1 in the Supporting Information). This approach indicates that the surface of the nanocomposite particles contains approximately five times more silicon than their corresponding bulk compositions, which is consistent with either a “core-shell” or a “raspberry” particle morphology.10 In view of their silica contents, it was considered more likely that these PS/SiO2 particles would have a “core-shell” morphology (with a polystyrene core and a silica shell) rather than a “raspberry” morphology. This hypothesis is examined in the following section. Selective Removal of the Silica and Polystyrene Components. Either the silica or the polystyrene component was selectively removed from the PS/SiO2 particles. Thus, calcination at 550 °C leads to complete pyrolysis of the polystyrene component, leaving the thermally stable silica component unaffected. This thermal treatment led to the formation of hollow silica capsules consisting of either 19 or 12 nm silica particles.39 Removal of the silica component was achieved by treating the same PS/SiO2 sample with 50 wt % NaOH. This chemical treatment led to digestion of the surface-adsorbed silica particles, leaving the polystyrene component unaffected. TEM studies confirmed that the initially rough nanocomposite particle surface became noticeably smoother due to removal of the nanosized silica particles. Moreover, aqueous electrophoresis measurements conducted on these PS/ SiO2 particles before and after etching of the silica revealed a significant change in the isoelectric point from around pH 2 to pH 8.4 (see Figure 6). The pronounced cationic character of the silica-etched PS/SiO2 particles presumably arises from cationic initiator fragments located at the polystyrene latex surface that were previously obscured by the surface layer of silica nanoparticles. Hence, both experiments suggest that the PS/SiO2 particles possess a “core-shell” particle morphology. ESI/TEM Studies. Direct experimental evidence for the “core-shell” morphology of these PS/SiO2 nanocomposite particles was verified using ESI/TEM, which provides elementspecific information at high spatial resolution. This technique was used to examine nanocomposite particles embedded in an epoxy resin and subjected to ultramicrotomy to produce sectioned (39) A preliminary account of our results has already been published (see Schmid, A.; Tonnar, J.; Armes, S. P. AdV. Mater. 2008, 20, 3331.).
Schmid et al.
Figure 6. Aqueous electrophoresis curves obtained for polystyrene/ silica nanocomposite particles (lower curve) and the polystyrene latex particles produced after treatment of these nanocomposite particles with 50% NaOH (upper curve). The latex particles exhibit significant cationic character (appearance of an isoelectric point at pH 8.4) due to the AIBA initiator fragments located at the surface.
particles. In the carbon map shown in Figure 7, the sectioned PS/SiO2 particles (entry 3 in Table 1) are easily visualized within the diffuse gray epoxy resin matrix. Bright halos with dark interiors within the gray matrix were also observed in the silicon map. These images confirm that each nanocomposite particle has a well-defined “core-shell” morphology, consisting of a purely polystyrene core surrounded by a thin shell of ultrafine silica particles. Moreover, the shell thickness corresponds to a monolayer of silica particles rather than multilayers. It is also noteworthy that apparently smaller particles that appear to be “filled” with silica are not due to a secondary population but simply represent particles that have been sectioned off-center. Polymerization Kinetics. Both the evolution of particle diameter and the extent of polymerization were simultaneously monitored during a scaled-up nanocomposite synthesis conducted under the conditions used for entry 2 in Table 1. The former parameter was assessed by periodic extraction of aliquots from the reaction solution, followed by dilution (and quenching) in water and analysis of the nascent particles by dynamic light scattering. Monomer conversions were calculated from the difference between the solids content determined gravimetrically (from the same extracted aliquots) and the theoretical solids content at 100% conversion, taking into account the presence of the nonvolatile silica sol. These data are shown in Figure 8. Clearly, there is a reasonably good correlation between the increase in mean intensity-average particle diameter and the extent of styrene polymerization. After approximately 6-7 h, the final particle diameter of approximately 400 nm was attained at a monomer conversion of over 95%. The polydispersity index decreased rapidly within the first hour and remained below 0.1 thereafter. In this particular experiment more than 95% monomer conversion was achieved, but most other formulations had somewhat lower conversions (see Tables 1-3). This difference is believed to be due to the more efficient mechanical stirring employed in the scaled-up syntheses required for periodic sampling (only magnetic stirring was used for the small-scale syntheses summarized in Tables 1-3). Particle Formation Mechanism. In view of the well-defined “core-shell” particle morphology, and bearing in mind the presence of the cationic initiator fragments on the latex surface after NaOH etching of the outer shell of silica nanoparticles, it is tempting to suggest the following mechanism for nanocom-
Colloidally Stable PS/SiO2 Nanocomposite Particles
Figure 7. TEM images of ultramicrotomed polystyrene/silica nanocomposite particles prepared using the glycerol-modified 19 nm Bindzil CC40 silica sol (entry 2 in Table 1). The bright-field image (top) and the two elemental maps for silicon (middle) and carbon (bottom) clearly reveal a “core-shell” particle morphology, whereby the core consists of polystyrene and the silica particles form a contiguous outer shell.
posite particle formation. In the absence of any added surfactant, the initial styrene polymerization occurs under surfactant-free emulsion polymerization conditions. Thus, the AIBA initiator generates cationic surfactant-like styrene oligomers. These oligomers then self-assemble in aqueous solution to form micelles with hydrophobic cores and cationic head groups. Further polymerization within these styrene-swollen micelles via micellar nucleation leads to the formation of near-monodisperse cationic polystyrene latex particles, whose growth is eventually arrested by the subsequent electrostatic adsorption of a monolayer of anionic silica particles. This hypothetical mechanism is consistent with the formation of colloidally stable, albeit polydisperse, cationic polystyrene latex in the absence of any silica sol (see Figure 2c). However, similar behavior might be expected for a conventional unfunctionalized silica sol such as Bindzil 2040, yet our control experiments confirm that PS/SiO2 nanocomposite particles are not formed in this case (see Figure 2a). Moreover, in our optimized formulations, essentially all of the AIBA initiator is adsorbed onto the surface of the silica sol, thus little or no initiator is available for polymerization within the micelles. Thus, this rather simplistic particle formation mechanism must be discounted. One reviewer of this manuscript has suggested that the mechanism of particle formation probably involves “Pickering emulsion polymerization.” This possibility can also be discounted,
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because there is no correspondence between the polydisperse silica-stabilized styrene monomer droplets (with diameters of the order of tens of micrometers) and the near-monodisperse PS/SiO2 nanocomposite particles of approximately 300 nm diameter. Instead, our tentative proposed mechanism39 for the formation of these nanocomposite particles is as follows. The cationic AIBA initiator that is initially electrostatically adsorbed28 onto the silica nanoparticles thermally decomposes, generating radicals. This leads to surface polymerization of the styrene, which produces hydrophobic patches on the silica nanoparticles, leading to their in situ aggregation to produce nuclei, which quickly become monomer-swollen (since styrene is a good solvent for polystyrene). As further polymerization proceeds, these nascent particles grow in size and the fractional silica surface coverage is reduced. This growth inevitably leads to a “patchy” silica overlayer. The anionic silica nanoparticles remaining in solution eventually adsorb electrostatically onto the bare cationic patches to form the final well-defined “core-shell” nanocomposite particles, but this process lags behind the rate of styrene polymerization due to slow mass transport of the relatively massive silica nanoparticles. In addition, there is the possibility of chain transfer from growing polystyrene radicals to the surface glycerol groups, leading to in situ chemical grafting of the polystyrene chains to at least some of the silica nanoparticles. In a typical nanocomposite synthesis, we estimate that the molar ratio of styrene monomer to surface glycerol groups is approximately 30:1, which suggests that such chain transfer is quite feasible, particularly given that the chain transfer constants for styrene to water, ethylene glycol, and 1,2-propanediol at 60 °C are 0.6 × 10-6, 1.36 × 10-4, and 2.08 × 10-4, respectively.40 In contrast, for an unfunctionalized silica sol (e.g., Bindzil 2040), such chain transfer is not possible, which is consistent with the unsuccessful nanocomposite syntheses obtained under these conditions. Clearly, further mechanistic studies of the formation of these colloidal nanocomposite particles are required. This work is ongoing and will be reported in full in due course.
Conclusions The judicious combination of a new commercial glycerolfunctionalized ultrafine silica sol with a cationic azo initiator allows the facile preparation of polystyrene/silica nanocomposite particles via aqueous emulsion polymerization in the absence of any added surfactant, cosolvents, or auxiliary comonomers. Such syntheses seem to be quite robust: colloidally stable nanocomposite particles are obtained over a wide range of initial silica sol concentration, polymerization temperature, and solution pH, albeit with varying silica aggregation efficiencies. Silica contents are typically around 22-28 wt %, with intensity-average particle diameters ranging between approximately 200 and 400 nm. If the initial silica sol concentration is too low, the colloid stability is compromised. Conversely, relatively high silica sol concentrations lead to significant quantities of excess nonaggregated silica particles. However, optimized silica sol concentrations for a given set of conditions allow relatively high silica aggregation efficiencies to be obtained. A polymerization temperature of 60 °C is preferred, as higher temperatures seem to lead to incipient particle flocculation. Particle growth and monomer conversion curves conducted at 60 °C correlate very well, revealing that a polymerization time of around 6-7 h is sufficient to achieve both high monomer conversion (>95%) and the final particle (40) Polymer Handbook, 2nd ed.; Brandrup, J., Immergut, E. H., Eds.; WileyInterscience: New York, 1975.
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Figure 8. Monomer conversion and particle growth curves for the synthesis of polystyrene/silica nanocomposite particles. Note the strong correlation between particle diameter and monomer conversion, both reaching their final values after 6-7 h at 60 °C. Low polydispersities were obtained within 1 h. Conditions: 50 g of styrene and 15 g of Bindzil CC40 silica sol in 450 g of water, initiated with 500 mg of AIBA initiator and mechanically stirred at 250 rpm.
diameter. Particle surface characterization by both aqueous electrophoresis measurements and XPS indicate a silica-rich nanocomposite particle surface. Well-defined “core-shell” particle morphologies were confirmed by ESI/TEM studies combined with ultramicrotomy. Calcination of these polystyrene/ silica nanocomposites pyrolyzes the organic component, leading to the formation of hollow silica capsules. This observation suggests that the silica nanoparticles form contiguous shells in the original nanocomposite particles. Adsorption isotherm studies conducted at 20 °C indicate that the adsorbed amount of the cationic azo initiator on the glycerol-functionalized silica sol is lower than that for an analogous unfunctionalized silica sol. However, it is difficult to draw further conclusions, since the adsorption isotherm at the actual polymerization temperature of 60 °C cannot be determined due to thermal decomposition of the initiator. Control experiments conducted using either an unfunctionalized silica sol or an anionic persulfate initiator do not result in colloidally stable particles; if the polymerization is
conducted in the absence of any silica sol, a charge-stabilized, cationic polystyrene latex of relatively high polydispersity is obtained. Acknowledgment. A.S. thanks The University of Sheffield for a Ph.D. studentship. S.P.A. is the recipient of a five-year Royal Society-Wolfson Research Merit Award. Dr. P. Greenwood of Eka Chemicals (Bohus, Sweden) is thanked for providing samples of Bindzil CC40, Bindzil CC30, and Bindzil 2040 silica sols. Supporting Information Available: Reaction scheme for the synthesis of glycerol-functionalized silica sol; X-ray photoelectron spectra for glycerol-functionalized silica sol; calibration curve for initiator adsorption assay; X-ray photoelectron survey spectra; and summary table for polystyrene/silica nanocomposite particles plus reference materials. This material is available free of charge via the Internet at http://pubs.acs.org. LA803544W
