Langmuir 2005, 21, 1175-1179
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Characterization of the Nanomorphology of Polymer-Silica Colloidal Nanocomposites Using Electron Spectroscopy Imaging J. I. Amalvy,† M. J. Percy, and S. P. Armes*,‡ Department of Chemistry, School of Life Sciences, University of Sussex, Falmer, Brighton, BN1 9QJ, United Kingdom
C. A. P. Leite and F. Galembeck* Universidade Estadual de Campinas, Instituto de Quı´mica, P.O. Box 6154, Campinas SP, Saˇ o Paulo, 13083-862, Brazil Received October 6, 2004. In Final Form: December 7, 2004 The internal nanomorphologies of two types of vinyl polymer-silica colloidal nanocomposites were assessed using electron spectroscopy imaging (ESI). This technique enables the spatial location and concentration of the ultrafine silica sol within the nanocomposite particles to be determined. The ESI data confirmed that the ultrafine silica sol was distributed uniformly throughout the poly(4-vinylpyridine)/ silica nanocomposite particles, which is consistent with the “currant bun” morphology previously used to describe this system. In contrast, the polystyrene/silica particles had a pronounced “core-shell” morphology, with the silica sol forming a well-defined monolayer surrounding the nanocomposite cores. Thus these ESI results provide direct verification of the two types of nanocomposite morphologies that were previously only inferred on the basis of X-ray photoelectron spectroscopy and aqueous electrophoresis studies. Moreover, ESI also allows the unambiguous identification of a minor population of polystyrene/silica nanocomposite particles that are not encapsulated by silica shells. The existence of this second morphology was hitherto unsuspected, but it is understandable given the conditions employed to synthesize these nanocomposites. It appears that ESI is a powerful technique for the characterization of colloidal nanocomposite particles.
Introduction In recent years, there has been increasing interest in the synthesis of particulate or colloidal organic/inorganic hybrids.1 For example, Huang and Brittain reported the synthesis of poly(methyl methacrylate)-clay nanocomposites using both suspension and emulsion polymerization techniques.2 Using wide-angle X-ray diffraction, these authors reported that exfoliated nanostructures were initially produced from both synthetic routes, with a mixture of intercalated and exfoliated nanostructures being obtained after melt processing. Bourgeat-Lami and Lang reported the synthesis of polystyrene/silica composite particles by the nonaqueous dispersion polymerization of styrene in alcoholic media in the presence of surfacefunctionalized silica particles using polymeric stabilizers such as poly(N-vinyl pyrrolidone).3,4 The silica particles ranged from 13 nm to more than 600 nm in diameter and contained polymerizable methacrylate groups which had been surface-modified using siloxane chemistry. Smaller silica sols led to the formation of nanocomposite particles, whereas larger silica sols produced core-shell morphologies (silica cores, polystyrene shells). In related work, Furusawa et al. utilized hydropropylcellulose as a polymeric binder/stabilizer in the aqueous emulsion polymerization of styrene in order to promote the formation of * To whom correspondence should be addressed. E-mail:
[email protected];
[email protected]. † Member of CIC, Buenos Aires, Argentina. Permanent address: CIDEPINT, Av. 52, entre 121 y 122 s/n and INIFTA (Facultad de Ciencias Exactas, Universidad Nacional de La Plata), Diag. 113 y 64 CC 16 Suc. 4 (1900) La Plata, Argentina. ‡ Present address: Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, S3 7HF, U.K. (1) Bourgeat-Lami, E. J. Nanosci. Nanotechnol. 2002, 2, 1. (2) Huang, X.; Brittain, W. J. Macromolecules 2001, 34, 3255. (3) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1998, 197, 293. (4) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1999, 210, 281.
polystyrene/silica composite particles.5 More recently, Bourgeat-Lami and co-workers6,7 described the preparation of poly(methyl methacrylate)/silica composite particles using an electrostatically adsorbed cationic initiator to promote polymerization at the surface of an anionic silica sol. Either framboidal or core-shell particle morphologies were obtained, depending on the size and nature of the silica seed particles. The same group prepared micrometersized polystyrene/silica particles by using an adsorbed poly(ethylene glycol)-based macromonomer to enhance adhesion between the hydrophobic polystyrene and the hydrophilic silica surface.8 Arkhireeva and Hay9 prepared surface-derivatized silica sols using both the classical Stober method and also a nonhydrolytic sol-gel route in order to produce hybrid organic/inorganic nanoparticles with a range of nanostructures. Sertchook and Avnir10 synthesized polystyrene-silica particles using a different sol-gel approach: a surfactant-stabilized emulsion of polystyrene in tetraethoxysilane was dispersed in an alkaline ethanolic solution in order to prepare bicontinuous composite particles. In 1999 we reported that vinyl polymer-silica nanocomposite particles of colloidal dimensions are readily prepared by the (co)polymerization of an auxiliary monomer, 4-vinylpyridine (4VP), in the presence of an ultrafine silica sol.11,12 Particle diameters were typically 100-200 (5) Furusawa, K.; Kimura, Y.; Tagawa, T. J. Colloid Interface Sci. 1986, 109, 69. (6) Luna-Xavier, J. L.; Bourgeat-Lami, E.; Guyot, A. Colloid Polym. Sci. 2001, 279, 947. (7) Luna-Xavier, J. L.; Guyot, A.; Bourgeat-Lami, E. J. Colloid Interface Sci. 2002, 250, 82. (8) Reculusa, S.; Poncet-Legrand, C.; Ravaine, S.; Mingotaud, C.; Duguet, E.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, 2354. (9) Arkhireeva, A.; Hay, J. N. Polym. Polym. Compos. 2004, 12, 101. (10) Sertchook, H.; Avnir, D. Chem. Mater. 2003, 15, 1690. (11) Barthet, C.; Hickey, A. J.; Cairns, D. B.; Armes, S. P. Adv. Mater. 1999, 11, 408. (12) Percy, M. J.; Barthet, C.; Lobb, J. C.; Khan, M. A.; Lascelles, S. F.; Vamvakaki, M.; Armes, S. P. Langmuir 2000, 16, 6913.
10.1021/la047535g CCC: $30.25 © 2005 American Chemical Society Published on Web 01/08/2005
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Table 1. Summary of the Silica Type, Silica Sol Diameter, Synthesis Temperature, Particle Size, Silica Content, and Particle Density Data for the Two Types of Vinyl Polymer-Silica Nanocomposites Studied in This Work monomer type
silica sol type
silica sol diameter (nm)
synthesis temp (°C)
particlea diameter (nm)
silicab content (%)
particle densityc (g cm-3)
styrene 4-vinylpyridine
Nissan IPA-STd Nyacol 2040e
13 20
25 60
180 220
48 38
1.40 1.49
a Determined from dynamic light scattering studies. b Determined by thermogravimetry. c Measured by helium pycnometry. d Supplied by Nissan Chemicals, U.S., as a 32 wt % dispersion in 2-propanol. e Supplied by Eka Chemicals, Sweden, as a 40 wt % aqueous dispersion.
nm, with silica contents varying from 10 to 50% silica by mass. Transparent, abrasion-resistant nanocomposite coatings were obtained with film-forming comonomers such as n-butyl acrylate.13 These nanocomposite syntheses were carried out in the absence of any added surfactants in aqueous media, and as little as 10 mol % of the 4VP auxiliary was required to ensure successful nanocomposite formation. Antonietti and co-workers subsequently modified this 4VP auxiliary protocol in order to synthesize colloidally stable nanocomposite particles using miniemulsion polymerization. This approach required the addition of both surfactant and cosurfactant but had the great advantage of dramatically increasing the silica aggregation efficiency.14 Nevertheless, it would be preferable if such nanocomposite particles could be prepared without recourse to auxiliary comonomers. In a recent communication,15 we showed that replacing the aqueous silica sol with a commercially available alcoholic silica sol enabled poly(methyl methacrylate)/silica nanocomposite particles to be prepared in the absence of any auxiliary comonomers. This initial work was extended16 to include styrene, several acrylates, and other methacrylic monomers, and also two further commercial silica sols, thus providing access to a wide range of vinyl polymer-silica nanocomposite particles. These inorganic/organic hybrid particles can be readily prepared with reasonably uniform size distributions in aqueous alcoholic media at ambient temperature. In this paper, electron spectroscopy imaging (ESI) was used to characterize the nanomorphology of these colloidal nanocomposite particles. This technique has been previously used to characterize conventional colloidal latex particles prepared by both surfactant-free17 and surfactant-stabilized18 emulsion copolymerization. However, as far as we are aware, this is the first time that ESI has been used to study colloidal nanocomposites. The nanomorphologies indicated by the ESI studies are compared to those previously inferred for the same nanocomposite particles by a combination of X-ray photoelectron spectroscopy (XPS) and aqueous electrophoresis studies.19 Experimental Section Nanocomposite Synthesis. The polystyrene/silica nanocomposite particles were prepared by the protocol described by Percy and co-workers16 using a 13 nm Nissan IPA-ST silica sol dispersed in 2-propanol (32 wt %), supplied by Nissan Chemicals (U.S.). The synthesis was conducted at 25 °C using 5.0 mL of monomer and 4.0 g of silica sol in 50 mL of an aqueous alcohol solution, together with 1.0 wt % (based on monomer) of a 1:1 ammonium persulfate/N,N,N′,N′-tetramethylethylenediamine complex. (13) Amalvy, J. I.; Percy, M. J.; Armes, S. P.; Wiese, H. Langmuir 2001, 17, 4770. (14) Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 17, 5775. (15) Percy, M. J.; Armes, S. P. Langmuir 2002, 18, 4562. (16) Percy, M. J.; Amalvy, J. I.; Randall, D. P.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 2004, 20, 2184. (17) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1998, 14, 3187. (18) Amalvy, J. I.; Asua, J. M.; Leite, C. A. P.; Galembeck, F. Polymer 2001, 42, 2479. (19) 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.
The poly(4-vinylpyridine)/silica nanocomposite particles were prepared by the protocol described by Percy and co-workers12 using a 20 nm Nyacol 2040 silica sol supplied as a 40 wt % aqueous dispersion by Eka Chemicals (Bohus, Sweden). The synthesis was conducted at 60 °C using 5.0 mL of 4-vinylpyridine monomer and 8.0 g of silica in 100 mL of an aqueous solution at approximately pH 10 using ammonium persulfate initiator (1 wt % based on monomer). After purification via several centrifugation-redispersion cycles to remove the excess nonaggregated silica sol, the mean silica contents, particle densities, and intensity-average diameters of the polystyrene/silica and poly(4-vinylpyridine)/silica nanocomposites were determined by thermogravimetric analysis, helium pycnometry, and dynamic light scattering studies, respectively. These data are summarized in Table 1. Chemical Composition. Thermogravimetric analyses were performed using a Perkin-Elmer TGA-7 instrument. Nanocomposite dispersions were dried at 50 °C overnight to yield white powders, which were heated to 800 °C in air at a scan rate of 20 °C min-1. The observed mass loss was attributed to the quantitative pyrolysis of the (co)polymer component. The incombustible residues remaining after pyrolysis were assumed to be pure silica (SiO2), and the inorganic contents of the nanocomposites were calculated after correcting for loss of surface moisture of the silica sol at elevated temperature. CHN microanalyses were carried out at an independent external analytical laboratory (Medac Ltd., Egham, Surrey, U.K.). ESI Analysis. The nanomorphology and the spatial elemental distribution within the vinyl polymer-silica nanocomposite particles were assessed by ESI using a Carl Zeiss CEM 902 transmission electron microscope equipped with a CastaingHenry-Ottensmeyer filter spectrometer within the column. One drop of each purified nanocomposite dispersion (1% solids content) was deposited in turn onto carbon-coated 300-mesh copper grids (Ted Pella). Images were acquired using electrons with zero-loss energy, low loss energy (between 20 and 50 eV), and elementspecific threshold energies, recorded with a slow-scan CCD camera (Proscan) and processed using the AnalySis 3.0 software. When the electron beam passes through the sample, interaction with electrons of different elements results in element-characteristic energy losses. A prism-mirror system deflects electrons with different energies through different angles, so that only electrons with a well-defined energy are selected. If only elastic electrons are chosen (∆E ) 0 eV), a transmission image with reduced chromatic aberration is obtained, which greatly improves the image contrast and gives an unusually well resolved beam energy (80 kV). When monochromatic inelastically scattered electrons are selected, electron spectroscopy images are produced, in which the contrast is dependent on the local energy-loss spectrum and thus on the local concentrations of the chosen element. Clear areas in the elemental distribution maps correspond to element-rich domains. ESI is meaningful only if multiple electron scattering is avoided, which is usually achieved using thin samples or sufficiently small particles.17 Appropriate conditions can be verified by changing the energy loss of the electron beam from 0 eV to approximately 25-30 eV, which should cause contrast inversion. Second, the CEM-902 instrument has a built-in software check (“R-mapping”) for the adequacy of the images used, based on the actual beam intensity transmitted by the sample. Both checks were performed in the present study in order to validate the elemental maps shown in this paper. Elemental images were obtained for the relevant elements of interest using monochromatic electrons corresponding to the carbon K-edge, the oxygen K-edge, and the silicon K-edge with an energy-selecting slit width of 15 eV. The energy-selecting slit
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was set at 278 ( 6 eV for carbon, 532 ( 6 eV for oxygen, and 100 ( 6 eV for silicon. Images were recorded with a slow-scan CCD camera (Proscan) and processed using the AnalySis 3.0 software. The three-window technique20 was used to perform the background subtraction for each elemental image. For each element of interest, three images were acquired from the area under examination. The first image (A) was taken at an energy above the absorption threshold that is characteristic of the element of interest, and the two other images (B and C) were recorded at energies lower than this absorption threshold. These latter two images (B and C) were used to obtain a fourth image (D) by extrapolation to the same energy used for image A (such image extrapolation consists of calculating the gray-level value for each pixel, using the values measured from images B and C and the well-known exponential decay for the energy-loss spectrum background). Finally, a difference image was obtained by subtracting image D from image A. This is known as the elemental map. Image processing and densitometer double-line profiling of a given width (for noise averaging) was performed with a PC, using the Image-Pro Plus 3.0 image analyzer program (Media Cybernetics). The double-line scans are plots of pixel gray level as a function of position, and the gray-level scale used varies from 0 (full black) to 255 (full white).
Results and Discussion The synthesis conditions, particle size, silica content, and particle density for both vinyl polymer-silica colloidal nanocomposites are summarized in Table 1. The polystyrene/silica nanocomposite was prepared using a 13 nm diameter alcoholic silica sol at 25 °C, whereas the poly(4-vinylpyridine)/silica nanocomposite was prepared using an aqueous 20 nm silica sol at 60 °C, as described previously.11-13 In both cases, significant quantities of nonaggregated excess silica sol are obtained in addition to the desired nanocomposite particles, but this contaminant can be readily removed by multiple centrifugation-redispersion cycles. It is also perhaps noteworthy that these two types of silica sols cannot be interchanged: colloidally stable nanocomposite dispersions are not obtained if 4-vinylpyridine is homopolymerized in the presence of the alcoholic silica sol or if styrene is homopolymerized in the presence of the aqueous silica sol. This is presumably related to the differing modes of molecular interaction between the polymer and inorganic phases for these two types of nanocomposite particles.21 Dynamic light scattering was used to determine the particle size data shown in Table 1. For particle size distributions of finite width, such intensity-average particle diameters are always larger than the number-average diameters that are observed by electron microscopy. The polystyrene-silica particles contained 48% silica and had a mean density of 1.40 g cm-3. Although the poly(4-vinylpyridine)/silica particles contained only 38% silica, the density of this latter nanocomposite is higher due to the higher density of its polymer component [the density of polystyrene is 1.05 g cm-3, whereas the density of poly(4-vinylpyridine) is 1.20 g cm-3; the densities of the aqueous and alcoholic silica sols are 2.17 and 2.13 g cm-3, respectively]. Polystyrene/Silica Nanocomposite Particles. A typical bright-field image of the polystyrene/silica nanocomposite particles is shown in Figure 1. This micrograph reveals that these particles are approximately spherical, have significant surface roughness and relatively uniform diameters. (20) Reimer, L.; Zepke, U.; Moesch, J.; Schulze-Hillert, St.; RossMessemer, M.; Probst, W.; Weimer, E. EELS Spectroscopy: A Reference Handbook of Standard Data for Identification and Interpretation of Electron Energy Loss Spectra and for Generation of Electron Spectroscopic Images; Carl Zeiss: Oberkochen, Germany, 1992. (21) Agarwal, G. K.; Titman, J. J.; Percy, M. J.; Armes, S. P. J. Phys. Chem. B 2003, 107, 12947.
Figure 1. Bright-field transmission electron micrograph of polystyrene/silica nanocomposite particles (scale bar ) 100 nm).
Figure 2. Bright-field, energy-filtered (25 eV) transmission electron micrographs for the same sample field of polystyrene/ silica nanocomposite particles, before (a,c) and after (b,d) the acquisition of the elemental distribution maps (scale bar ) 100 nm).
These nanocomposite particles are formed by heteroflocculation of the original ultrafine silica sol by the precipitating polystyrene chains. The bright-field and energyfiltered transmission electron microscopy (EFTEM) images shown in Figure 2 were recorded before and after the acquisition of the elemental mapping studies, so as to evaluate the stability of these particles toward electron beam damage. The 17-particle aggregate shrank by approximately 6% in its largest dimension. Otherwise, no change in the morphology of this aggregate was observed. Close inspection indicates that two particles are qualitatively different from the others (see Figures 2c and 2d). Although of similar size, their surfaces are relatively smooth and they appear much brighter than the other particles. These two particles also underwent a more
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marked contraction of around 15% during image mapping. Under EFTEM conditions, brighter (whiter) particles indicate the presence of elements of lower atomic number, since lighter elements are less opaque to the electron beam. Thus these two white particles differ significantly in their elemental compositions compared to the other particles. More specific elemental information is obtained from Figure 3. In the carbon map, most of the particles have a uniform gray appearance with a diffuse halo. However, bright halos with darker particle interiors are observed in the silicon and oxygen maps. These images are consistent with a “core-shell” type particle morphology, in which each nanocomposite particle is surrounded by a thin shell of ultrafine silica (SiO2) particles. Moreover, the carbon map also indicates that the two anomalous particles contain significantly higher carbon contents than the other particles within the aggregate. On the other hand, the silicon and oxygen maps indicate weak silicon and oxygen intensities (and hence lower silica contents) and there is certainly no evidence for a coreshell particle morphology. The existence of a minor population of such “low silica content” nanocomposite particles within the main population of silica-coated nanocomposite particles had not been appreciated prior to this ESI study. Careful examination of other lowmagnification EFTEM images obtained for the same sample (not shown) suggests that this minor population comprised only around 1% of the total number of particles. So-called “surfactant-free” emulsion polymerization can produce colloidally stable charge-stabilized polystyrene latex in water-alcohol mixtures in the absence of any added surfactant (or silica sol). Thus, in retrospect, it is not difficult to explain the formation of a minor fraction of such low silica content nanocomposite particles. Their presence may well indicate more than one locus of polymerization, at least for this particular set of conditions. Closer examination suggests that there is a very thin layer of carbon-rich material located at the outer surface of the nanocomposite particles. In aqueous solution, a silica-rich hydrophilic nanocomposite surface is thermodynamically favorable, but under high-vacuum conditions the nanocomposite particle/vacuum interface is very hydrophobic. The silica sol is relatively immobile, especially given the high Tg of the polystyrene, but migration of polystyrene oligomers to the surface of the nanocomposite particles can occur in order to minimize the surface free energy. Thus the thin layer of carbon-rich material that is observed under vacuum is most likely not present in aqueous solution. The silica-coated nanocomposite particles do not appear to show any signs of significant morphological changes during drying caused by interparticle capillary adhesion, whereas this effect is observed for the low silica content nanocomposite particles. Thus, the silica shells seem to make these polystyrene/silica nanocomposite particles tougher and more resistant to deformation. Figure 4 shows the double-line densitometry profile obtained for polystyrene/silica nanocomposite particles, using the silicon elemental map of the image shown in Figure 3c. The pixel intensity distribution confirms the silicon-rich surface and the well-defined core-shell morphology of these particles. The shell thickness corresponds approximately to the mean diameter of the ultrafine silica sol (13 nm), suggesting that a monolayer of silica particles is adsorbed at the surface of the nanocomposite particles. Poly(4-vinylpyridine)/Silica Nanocomposite Particles. Typical bright-field and corresponding EFTEM images for these particles are shown in Figure 5. The
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Figure 3. Elemental distribution maps obtained for carbon (a), silicon (b), and oxygen (c) from the polystyrene/silica nanocomposite particles (scale bar ) 100 nm).
particle size distribution is not quite as well controlled as that obtained for the polystyrene/silica nanocomposite particles, but it is still reasonably narrow. Inspection of Figure 6 suggests that the spatial distribution of the ultrafine silica sol within the poly(4-
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Figure 4. Double-line densitometric profile of pixel intensity across a silicon elemental map of the image shown in Figure 3c (scale bar ) 100 nm).
Figure 5. Typical bright-field (a,c) and energy-filtered (25 eV; b,d) transmission electron micrographs obtained for the poly(4-vinylpyridine)/silica nanocomposite particles (scale bar ) 100 nm).
Figure 6. Double-line densitometric profiles of pixel intensities across an energy-filtered (25 eV) micrograph of poly(4-vinylpyridine)-silica nanocomposite particles. Double-line scans were recorded for two particles (scale bar ) 40 nm).
vinylpyridine)/silica particles is rather uniform. In striking contrast to the polystyrene/silica nanocomposite particles, there is no evidence for a core-shell particle morphology in this case. This hypothesis is supported by the doubleline densitometry results, which indicate high and fairly
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constant pixel intensities for cross-sections through the two selected nanocomposite particles. This so-called “currant bun” particle morphology is fully consistent with our earlier investigations. For example, aqueous electrophoresis measurements11,12 had revealed an isoelectric point at around pH 6. Since the original aqueous 20 nm silica sol remained anionic across a wide pH range (pH 2-10), this indicated that the basic poly(4-vinylpyridine) component made a contribution to the electrophoretic mobility and was therefore located at or very near the surface shear plane. Moreover, XPS studies19 had confirmed that the surface Si/N atomic ratios obtained for a series of poly(4-vinylpyridine)/silica nanocomposites were always comparable (within experimental error) to the corresponding Si/N bulk atomic ratios calculated from thermogravimetric analysis and nitrogen microanalysis. Finally, the EFTEM images also reveal some evidence for a continuous thin layer of polymeric material present at the surface of the nanocomposite particles. Composite particles often exhibit nonspherical morphologies, suggesting that they are sufficiently rigid not to be affected by surface forces, which usually favor spherical morphologies. Additional evidence for the rigidity of these particles can be obtained by examining the very dark superimposed particle domains that can be observed in Figure 5d. The appearance of these dark domains is consistent with that expected for rigid particles, rather than that for soft, deformable particles. Conclusions The ESI data, together with densitometry profiling, confirm that the ultrafine silica sol is distributed uniformly throughout the poly(4-vinylpyridine)/silica nanocomposite particles, which is consistent with the currant bun morphology previously used to describe this system.11,12,19 In contrast, the majority of the polystyrene/silica nanocomposite particles clearly have a distinctive core-shell morphology, with the silica sol adsorbed as a well-defined monolayer around the nanocomposite particles.16 Overall, these ESI results are consistent with the nanocomposite morphologies inferred from previous X-ray photoelectron spectroscopy and aqueous electrophoresis studies. Moreover, ESI also allows the unambiguous identification of a minor population of nanocomposite particles with no silica shells that contaminate the major population of core-shell polystyrene-silica nanocomposite particles. The existence of these low silica content nanocomposite particles was hitherto unsuspected, but it is understandable given the conditions employed in these syntheses. It is clear that ESI is a powerful characterization technique for assessing the internal nanostructure of vinyl polymer-silica colloidal nanocomposites. Acknowledgment. S.P.A. acknowledges EPSRC for a ROPA postdoctoral research grant (GR/R79814) to support J.I.A. The Comisio´n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires (CICPBA, Argentina) is thanked for allowing J.I.A. a one-year secondment at the University of Sussex, where the nanocomposite syntheses were carried out. J.I.A. also acknowledges the support of CICPBA, CONICET (PIP 0208), and ANPCyT (PICT 1408709) from Argentina and the Instituto de Quı´micaUNICAMP (PADCT/CNPq/MCT (Millenium Institute Program) from Brasil. F.G. acknowledges the support of CNPq, Pronex/Finep/MCT, and PADCT/CNPq/MCT (Millenium Institute Program). Eka Chemicals (Bohus, Sweden) and Nissan Chemicals (U.S.) are thanked for the donation of the two ultrafine silica sols. LA047535G
