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Polymerization of w/o Microemulsions for the Preparation of Transparent SiO2/PMMA Nanocomposites R. Palkovits,‡ H. Althues,† A. Rumplecker,‡ B. Tesche,‡ A. Dreier,‡ U. Holle,‡ G. Fink,‡ C. H. Cheng,§ D. F. Shantz,§ and S. Kaskel*,† Inorganic Chemistry Division, Technical University Dresden, Mommsenstr. 13, D-01069 Dresden, Germany, Max-Planck-Institut fu¨ r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu¨ lheim an der Ruhr, Germany, and Department of Chemical Engineering, Texas A&M University, Mail Stop 3122, College Station, Texas 77843-3122 Received March 8, 2005. In Final Form: April 20, 2005 Reverse w/o microemulsions composed of methyl methacrylate (MMA) forming the oil phase, nonionic surfactants, and water are used for the synthesis of transparent SiO2/PMMA nanocomposites. An inorganic precursor, tetraethoxysilane (Si(OEt)4, TEOS), is hydrolyzed in the reverse micelles containing aqueous ammonia. During the hydrolysis of TEOS, polymerization of the continuous MMA phase is initiated using AIBN (azobisisobutyronitrile), and after thermal polymerization at 333 K for 12 h, solid blocks of PMMA are obtained in which nanometer-sized silica particles are trapped in the solid polymer matrix. According to small-angle X-ray and dynamic light scattering experiments, the water droplets in MMA microemulsions are 12 nm (RW ) 13) in diameter, whereas after polymerization of the microemulsion, the SiO2 particles in the transparent SiO2/PMMA composites are 26 nm in diameter. Transmission electron micrographs demonstrate a low degree of agglomeration in the composites. In comparison with materials generated from micelle-free solutions, the particle size distribution is narrow. The reverse micelle-mediated approach produces composites of high transparency comparable with that of pure PMMA.
Introduction Microemulsions have received considerable interest because of their small droplet size and complex structural behavior. The structural features depending on the microemulsion composition can be used to prepare particles a few nanometers in diameter by hydrolytic decomposition of molecular precursors or precipitation of ionic salts.1-3 Some of the more recent examples are the preparation of complex oxides such as BaAl12O19,4 aluminum oxide-hydroxide,5 Ba(Mg1/3Ta2/3)O3,6 iron oxide,7,8 BaTiO3,9,10 ZnO,11 and sulfated zirconia for catalytic applications.12 The adjustable size of the micelles is a valuable tool for tailoring the inorganic particle size. Such highly dispersed oxides have high accessible surface areas and are excellent * Author to whom correspondence should be addressed. E-mail:
[email protected]. Telephone: 49-351-46333632. Fax: 49-351-463-37287. † Technical University Dresden. ‡ Max-Planck-Institut fu ¨ r Kohlenforschung. § Texas A&M University. (1) Kumar, P.; Mittal, K. L. Handbook of Microemulsion Science and Technology; Marcel Dekker: New York, 1999. (2) Texter, J. Reactions and Synthesis in Surfactant Systems; Marcel Dekker: New York, 2001. (3) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (4) Zarur, A. J.; Ying, J. Y. Nature 2000, 403, 65. (5) Berkovich, Y.; Aserin, A.; Wachtel, E.; Garti, N. J. Colloid Interface Sci. 2002, 245, 58. (6) Lee, Y. C.; Liang, M. H.; Hu, C. T.; Lin, I. N. J. Eur. Ceram. Soc. 2001, 21, 2755. (7) Dresco, P. A.; Zaitsev, V. S.; Gambino, R. J.; Chu, B. Langmuir 1999, 15, 1945. (8) Lopez Perez, J. A.; Lopez Quintela, M. A.; Mira, J.; Rivas, J.; Charles, S. W. J. Phys. Chem. B 1997, 101, 8045. (9) Beck, C.; Ha¨rtl, W.; Hempelmann, R. J. Mater. Res. 1998, 13, 3174. (10) Herrig, H.; Hempelmann, R. Nanostruct. Mater. 1997, 9, 241. (11) Hingorani, S.; Pillai, V.; Kumar, P.; Multani, M. S.; Shah, D. O. Mater. Res. Bull. 1993, 28, 1303. (12) Althues, H.; Kaskel, S. Langmuir 2002, 18, 7428.
catalysts. Highly dispersed inorganic particles are also used as additives in polymer-based materials and composites. Hybrid materials composed of inorganic glasses and organic polymers have received considerable interest in recent years.13,14 The addition of silica is used to enhance the abrasion resistance of rubbers. Crystalline inorganic components such as UV-absorbing semiconducting oxides (TiO2) are used as UV-protective fillers. Most of the conventional fillers suffer from broad particle size distributions and particles sizes significantly exceeding 100 nm. They scatter visible light and cannot be used as fillers for transparent polymeric bulk materials. In thin films, broad particle size distributions and agglomeration are tolerable because transparency is guaranteed because of the low thickness. However, in transparent bulk composites, control of the particle size is crucial because the scattering power is proportional to d6 (d ) particle diameter) and to the refractive index difference of the two components. For small particles (d < 50 nm), light scattering is significantly reduced and thus the transparency of the polymeric materials is retained. For the preparation of such composite materials containing inorganic nanoparticles embedded in a solid polymer matrix such as high-molecular-weight poly(methyl methacrylate) (PMMA), new methods are needed avoiding the formation of agglomerates inside the matrix. Industrial methods in which the particles are first synthesized for example by flame pyrolysis, and subsequently isolated and mixed with the polymer, often lead to products with reduced transparency because agglomerates are formed during isolation and strong interfacial forces prevent efficient dispersion inside the hydrophobic matrix. Such agglomerates persist in the composite and scatter visible light, leading to turbidity. Several methods have been developed to overcome agglomeration. They can be divided into two different (13) Novak, B. M. Adv. Mater. 1993, 5, 422. (14) Ellsworth, M. W.; Novak, B. M. Chem. Mater. 1993, 5, 839.
10.1021/la050630k CCC: $30.25 © 2005 American Chemical Society Published on Web 05/25/2005
Transparent SiO2/PMMA Nanocomposites Scheme 1. In Situ Generation of Inorganic Nanoparticles in Reverse Microemulsions for the Manufacture of Polymer Nanocomposites
approaches: (a) in situ generation of the particles inside or in the presence of polymers or monomers using sol-gel methods, and (b) modification of the particle surface using “grafting from” or “grafting to” techniques.15 The advantage of in situ processing is the single-step preparation. Thus, isolation and handling of the particles, often causing agglomeration and aggregation, is unnecessary. However, it is crucial to control the size of the generated particles to avoid gelation and network formation inside the composites to obtain transparent materials with narrow particle size distribution. In the following, we describe a method leading to transparent composite materials with particle diameters below 30 nm avoiding isolation and agglomeration. In this method, reverse microemulsions are used to control the generation of the silica particles with the aid of nonionic surfactants.16 Compared with established microemulsion procedures in the present work, the oil phase is not a saturated hydrocarbon but pure monomer (methyl methacrylate, MMA) (Scheme 1). During hydrolysis of the precursor, the entire oil phase is polymerized by radical polymerization. Thus, particle generation and microemulsion polymerization are combined in a single-step synthesis. In this way, particle agglomeration is significantly reduced because the particles are entrapped inside the polymer matrix. Whereas emulsion polymerization is widely used for the manufacture of latex particles, the polymerization of reverse microemulsions, in which the continuous oil phase is a monomer, is less frequently studied because of the subtle dependence of microemulsion stability on the oil phase composition. The pioneering work on the polymerization of monomer-containing microemulsions was carried out by Gan and Chew.17,18 Transparent polymeric materials without inorganic filler were obtained by the polymerization of bicontinuous microemulsions containing polymerizable surfactants (ω-methoxy-poly(ethylene oxide)40-undecyl-R-methacrylate), 2-hydroxyethyl methacrylate, water, and the cross-linker ethylene glycol dimethacrylate.19 Menger and Tsuno have used polymerizable surfactants for the synthesis of porous polystyrene with functional groups on the inner surface.20 Pavel and Mackay have studied the polymerization of reverse microemulsions containing CdS particles.21 Microemulsions containing aerosol OT (AOT) became opaque after polymerization (15) Kickelbick, G. Prog. Polym. Sci. 2003, 28, 83. (16) Kaskel, S.; Palkovits, R.; Althues, H.; Holle, U. German Patent Application DE-A-10349061.2, PCT 2004002348, 2003. (17) Gan, L. M.; Chew, C. H. J. Dispersion Sci. Technol. 1984, 5, 179. (18) Gan, L. M.; Chew, C. H. J. Dispersion Sci. Technol. 1983, 4, 291. (19) Liu, J.; Gan, L. M.; Chew, C. H.; Teo, W. K.; Gan, L. H. Langmuir 1997, 13, 6421. (20) Menger, F. M.; Tsuno, T. J. Am. Chem. Soc. 1990, 112, 6723. (21) Pavel, F. M.; Mackay, R. A. Langmuir 2000, 16, 8568.
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because of aggregation of the particles, whereas the polymerizable surfactant didecyldimethylammonium methacrylate led to the formation of transparent composites containing nonaggregated CdS particles. Inorganic polymer core-shell particles were prepared by Holzinger using atom-transfer radical polymerization (ATRP) and functionalized alkoxide precursors.22 Cheung has used the polymerization of microemulsions for the fabrication of composites. The monomer was polymerized after gelation (aggregation of the particles forming a network) and the materials were opaque.23 Silica-polymer nanocomposites were obtained by Chow and Gan using polymerizable bicontinuous microemulsions containing large amonunts of acrylonitrile. Functionalization of the particles using methacryloxypropyltrimethoxysilane led to uniform composites, nonfunctionalized particles dispersed unevenly.24 Whereas in the latter, only part of the microemulsion is converted in solution using acetonitrile as the solvent, we have developed the microemulsion method for the in situ generation of bulk nanocomposites using the pure monomer MMA as the oil phase. Thus, the entire continuous phase is converted into a transparent solid block of PMMA with a very low degree of inorganic particle agglomeration and particle sizes below 30 nm. Experimental Section The surfactants were supplied by BASF (Lutensol AO5, AO7, AO11) and SASOL (Marlophen NP4, NP5, NP6, NP10, NP15, Novel EO13.5) and used without further purification. Aerosol OT (AOT, sodium bis(2-ethylhexyl)sulfosuccinate) was available from Fluka. Phase diagrams were recorded by titrating the water to various compositions of the surfactant and MMA. After thorough mixing, the samples were allowed to equilibrate at the respective temperature. The clear-turbid points were used to establish the phase boundary of the microemulsion region. The size of the micelles was measured using dynamic light scattering (Zetasizer Nano Series, Malvern Instruments). For the preparation of the composites, appropriate amounts of surfactant, MMA (Aldrich, 99%), initiator (0.2% AIBN, Aldrich), and aqueous ammonia were mixed at room temperature. Subsequently, tetraethoxysilane was added and the clear microemulsion was sealed in a glass ampule. Polymerization was carried out at 333 K and subsequent hardening at 373 K. The transmission of 5-mm-thick PMMA blocks was measured using a UV-vis spectrometer Cary 5G (Varian). For transmission electron microscopic investigations, the composite material was cut into thin slices by means of an ultramicrotome (thickness of the sections < 100 nm). The slices were supported on holeycarbon-film-covered copper grids and imaged using a Hitachi H-7500 (100 kV, LaB6 emitter). Number-weighted size distributions are based on 50 particles in 6 classes. Small-angle X-ray scattering (SAXS) measurements were performed on a Bruker Nanostar high volume instrument. The composites were measured at short sample-to-detector distances (64 cm) with the corresponding q range observed being 0.01-0.3 Å-1.
Results and Discussion Monomer Microemulsions. To develop the reverse microemulsion-mediated synthesis of inorganic nanoparticles for oil phases composed of pure monomers, it is crucial to evaluate surfactants with respect to their ability to form stable microemulsions with significant amounts of water. For the generation of transparent plastics, methyl methacrylate (MMA) and styrene are used because polymerization leads to highly transparent PMMA and polystyrene. In this work, the ternary phase diagrams of ionic and nonionic surfactants such as AOT, Lutensol AOX, (22) Holzinger, D.; Kickelbick, G. Chem. Mater. 2003, 15, 4944. (23) Mukkamala, R.; Cheung, H. M. Langmuir 1997, 13, 617. (24) Chow, P. Y.; Gan, L. M. J. Nanosci. Nanotechnol. 2004, 4, 197.
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Figure 1. Oil-rich part of the ternary phase diagrams of cyclohexane, water, and various Marlophen NP surfactants (T ) 296 K).
Figure 3. Phase diagrams of methyl methacrylate, water, and the “surfactants” AOT and acrylic acid (T ) 296 K).
Figure 2. Phase diagrams of methyl methacrylate, water, and Lutensol surfactants (T ) 296 K).
Novel, and Marlophen NPX (Lutensol AO, Novel: ethoxylated fatty alcohol; Marlophen NP: ethoxylated nonylphenyl ether, X(average number of EO units) ) 5-15) were studied and compared with microemulsions in which cyclohexane or heptane forms the oil phase. Phase Diagrams. A phase diagram (296 K) containing cyclohexane as the oil phase is shown in Figure 1. For adjusting the particle size with the molar water-surfactant ratio (RW), it is desirable to use microemulsions allowing for a variation of RW within the single phase region.12 The maximum amount of water stabilized by a certain amount of surfactant in a given oil phase depends on the HLB (hydrophilic-lipophilic balance) of the surfactant.1 For cyclohexane, high RW values are realized at 296 K using Marlophen NP6 (HLB ) 11). For surfactants with a higher or lower HLB value, the microemulsion phase boundary is shifted to lower water contents. If the oil phase cyclohexane is replaced by the monomer MMA, surfactants with higher HLB values are needed to solubilize significant amounts of water. Figure 2 shows phase diagrams for surfactants with a higher degree of ethoxylation and HLB values ranging from 10 (Lutensol AO5) up to 16 (Novel EO13.5). The microemulsion region for Lutensol AO5 is small, whereas for the polar surfactant Novel EO13.5, the microemulsion region covers the largest
area. Thus, ethoxylated surfactants with higher HLB are suitable to stabilize microemulsions of methyl methacrylate. Structural Characterization of the Microemulsions. Dynamic light scattering measurements of microemulsions containing Marlophen NP10 (HLB ) 13) are in agreement with the presence of spherical micelles with a droplet diameter of 12 nm for RW ) 13. The micelle diameter increases with RW. The stability of the microemulsion was examined by monitoring the effective micelle diameter over 8 h, but significant changes in the droplet size were not observed. According to small-angle X-ray scattering (SAXS) measurements, the micelle diameter in this microemulsion is approximately 11 nm. At RW values higher than 23, a significant increase in polydispersity is observed by means of dynamic light scattering, indicating a structural transformation of the micelles. For the generation of MMA-rich reverse microemulsions, AOT is also a suitable surfactant (Figure 3). Using AOT, only small amounts of surfactant (below 5%) are necessary to obtain transparent mixtures of MMA and water. However, these microemulsions become turbid after polymerization. Large amounts of water can also be stabilized in MMA using acrylic acid as a comonomer, but our dynamic light scattering results do not indicate the formation of micelles in such ternary mixtures of MMA, acrylic acid, and water. The phase diagrams are very sensitive toward changes in the temperature and the HLB of the surfactant. For example, in the case of the surfactant Novel EO13.5 at higher temperature (323 K), the phase boundary is shifted toward lower RW (Figure 4). Stability of the microemulsions at higher temperature is crucial for the generation of the nanocomposites because, in this work, thermal polymerization at 333 K is used to polymerize the continuous phase of the microemulsion. We have also successfully used mixtures that are turbid (two-phase) at room temperature forming a transparent (single-phase) microemulsion at the polymerization temperature (Marlophen NP10). Styrene-based reverse microemulsions were also studied using nonionic surfactants. However, 30-40% surfactant is necessary to stabilize only a few percent of water. As in the case of MMA, AOT was best suited to stabilize larger amounts of water, but after polymerization, only turbid composites were obtained.
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Figure 5. Transmission electron micrographs of SiO2/PMMA nanocomposites using the inverse micelle-mediated synthesis and the surfactants Marlophen NP15 (A) and Novel EO13.5 (B). Figure 4. Phase diagrams of MMA, water, and Novel EO13.5 at T ) 296 and 323 K.
Microemulsion Polymerization. Whereas all the three types of surfactants allow one to obtain transparent microemulsions, only nonionic surfactants were found to give isotropic, transparent materials after polymerization. We were not able to obtain transparent composites using the anionic surfactant AOT. Considering the sensitivity of microemulsions toward small changes of composition and temperature, it is remarkable that isotropic transparent composites are obtained after polymerization. A first change in composition is introduced by adding the precursor, Si(OEt)4. The latter hydrolyzes and thus reduces the amount of water, but at the same time, ethanol is released acting as a cosolvent. In parallel, the whole mixture is polymerized, and thus the composition of the oil phase changes continuously during the hydrolysis of the precursor. Precursor Reactivity. The most critical factor is the reactivity of the precursor. For zirconium alkoxides, Zr(OR)4 (R ) n-C4H9, n-C8H17, n-C12H25), we were unable to obtain transparent composites. Instead, agglomeration of the particles within seconds causes sedimentation of the inorganic material. For tetraethoxysilane, Si(OEt)4, the condensation rate is typically much lower and can be adjusted using acidic or basic catalysts, for example, ammonia. Whereas in heptane-based microemulsions, turbidity is observed after 40-80 h (using 0-1 mol/L NH3 as catalyst), in the MMA-based microemulsions used in this work (surfactant: Marlophen NP 10), the reaction mixtures typically turn turbid after 6-12 h. For low ammonia concentrations (0.2 mol/l) within a period of 6 h of hydrolysis, the effective diameter determined using dynamic light scattering does not change significantly. In the dynamic light scattering experiment, the effective diameter measured reflects the size of the inverse micelle containing the SiO2 particle within the aqueous domain of the droplet. After 6-7 h, a sudden increase of the effective diameter is observed and the mixture turns turbid. To obtain transparent nanocomposites, it is necessary to achieve a high degree of monomer conversion within this initial period of hydrolysis of the precursor because, at a later stage agglomeration of the micellederived particles, irreversibly precludes the formation of transparent composites. Transparent nanocomposites were obtained after the addition of Si(OEt)4 to the MMA/ Marlophen NP10/water microemulsion and subsequent polymerization for 6 h at 333 K.
Figure 6. Transmission electron micrographs of a SiO2/PMMA nanocomposite prepared from a micelle-free acrylic acidmediated hydrolysis.
Structural Characterization of the Composites. For electron microscopic investigations, the transparent composites were cut into thin slices (d < 100 nm) using an ultramicrotome at 298 K. Figure 5 shows transmission electron micrographs of transparent composites obtained using the surfactants Marlophen NP15 (A) and Novel EO13.5 (B). The particles show a narrow size distribution and, more important, are well separated from each other. In contrast, using mixtures of acrylic acid and MMA without any surfactant, after hydrolysis, irregular shaped particles are obtained. Acrylic acid also allows stabilization of water in mixtures of MMA and acrylic acid but without forming micelles (see above). In situ generation of SiO2 particles in nonmicellar transparent solutions is possible, but a broad particle size distribution and aggregation is observed (Figure 6). Composites obtained using acrylic acid are turbid. In contrast, the inverse micelle approach produces transparent plastics with a narrow particle size distribution (Figure 5A and B). For Marlophen NP15-stabilized particles, the mean diameter is 26 nm (Figure 5A). SAXS measurements are consistent with the TEM data. Over the q range of 0.010.06 Å-1, the scattering curves can be well-described by modeling the system as noninteracting spheres with a normal size distribution (Figure 7). The results of the leastsquares fitting are consistent with a mean particle diameter of 26.2 nm and a standard deviation of 6.2 nm.25
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Figure 8. Photograph of a transparent SiO2/PMMA nanocomposite block (10 mm × 10 mm × 100 mm).
Figure 7. Small-angle X-ray scattering curve of a SiO2/PMMA nanocomposite obtained using Marlophen NP15.
The data at high q are not included as we observe powerlaw scattering typical for polymer systems.26 The mean SiO2 particle diameter in composites synthesized using Novel EO13.5 is 28 nm (Figure 5B). SAXS measurements give a mean particle diameter of 31.0 nm and a standard deviation of 4.4 nm. The size of the particles in the composites exceeds the size of the water droplets in the starting microemulsion by a factor of 2. Whereas the microemulsion is a stable system with a droplet size defined by the composition, the silica-containing microemulsion is not stable and the particle size increases in time because of the change of the hydrophobicityhydrophilicity balance in the progress of the hydrolytic condensation. The collision of micelles and the continuous exchange of inorganic components dissolved in the micelles allows for a steady particle size increase, reaching a terminal size after several days.27 Preliminary results indicate the possibility for tuning the particle diameter in the final composite with Rw in the starting microemulsion. Because polymerization was carried out in substance without using regulators, the PMMA obtained is of high molecular weight and does not dissolve in any solvent. Swelling in polar solvents is negligible. The glasstransition temperature (Tg) of PMMA is 398 K. Microemulsions, polymerized without the addition of TEOS, were also transparent and had a Tg of 397 K, whereas for silica-containing composites, Tg was considerably decreased to 367 K (2.67% SiO2). The decrease in Tg may be attributed to residual ethanol, which is released in the hydrolysis of TEOS and occluded in the composite.28 Mechanical tests show a decreased modulus in comparison with pure PMMA. Optical Properties of the Composites. The reverse microemulsion-assisted in situ generation of inorganic particles described here allows the manufacture of transparent inorganic-organic hybrid materials (Figure 8).16 The transmittance of the polymer blocks (5 mm thickness) was determined using a UV-vis spectrometer (400-800 nm). Four important factors reduce the transmittance: (1) Reflection of the beam at both surfaces, (2) imperfect surfaces (scratches) causing scattering, (3) light absorption (25) Pedersen, J. S. Adv. Coll. Interface Sci. 1997, 70, 171. (26) Pedersen, J. S.; Svaneborg, C. Curr. Opin. Colloid Interface Sci. 2002, 7, 158. (27) Osseo-Asare, K.; Arriagada, F. J. J. Colloid Interface Sci. 1999, 218, 68. (28) Wang, P. P.; Lee, S.; Harmon, J. P. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 1217.
Figure 9. Transmission spectra of pure PMMA (a) and a SiO2/ PMMA nanocomposite containing 1.3% SiO2 (b).
Figure 10. Transmission spectra of PMMA (a), SiO2/PMMA nanocomposites with 4% (c), and 6.67% SiO2 (e), and the corresponding SiO2-free microemulsions after polymerization (b corresponds to c, d corresponds to e).
of the polymer or polymer components, and (4) light scattering by particles. The transmittance of pure PMMA is compared with that of a nanocomposite in Figure 9. Reflection of the beam at the surfaces of PMMA glass typically causes a total loss of 8% (800 nm). The transmittance of the composite (1.33 wt % SiO2) is very close to that of pure PMMA. A significant loss in transparency is detected only below λ ) 500 nm. For higher filler contents, a loss of transmittance is also observed in the visible (Figure 10c, 4% silica, and Figure 10e, 6.67% silica). Materials containing 4% SiO2 appear optically transparent even though the transmittance is only 75% at 600 nm (Figure 10c), whereas at 6.67% filler, the transmittance is only 33% and the products are turbid (Figure 10e). The sharp drop in transmittance is attributed to the instability of the particle-containing microemulsion at higher filler contents and the formation of agglomerates during the polymerization and hydrolysis of the precursor. The reduced transmittance is also observed in MMA-based silica-free microemulsions after polymerization (Figure 10b and d). The microemulsion used for the generation of a 4% SiO2 containing composite has a transmittance of only 78% after polymerization, whereas the polymerized microemulsion corresponding to the 6.67% SiO2-containing composite only has a transmittance of 50% (600 nm). Thus, light scattering from larger domains also reduces the
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transmittance of polymerized microemulsions that do not contain silica. Conclusion Summarizing, we have presented a generic sol-gel approach for the in situ processing of bulk polymer nanocomposites. Polymerization of microemulsions containing MMA as the oil phase produces highly transparent nanocomposites with particle sizes below 30 nm and narrow particle size distributions. The potential applicability to a wide range of monomers and inorganic compounds renders the in situ microemulsion method presented here as a promising technology for the generation of multifunctional composites useful in
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optical applications. Further work is needed to elucidate the structural changes during the polymerization with respect to the mechanisms involved in particle growth as well as the polymerization process. Acknowledgment. This work was funded by the “Young Scientist Nanotechnology Initiative” of the Federal Ministry of Education and Research (BMBF: FK 03X5502). The donation of surfactants by CONDEA and BASF is gratefully acknowledged. SAXS measurements were carried out during a DAAD- and NSF-funded exchange program (D/0247208 and INT-023430). LA050630K
