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Langmuir 2007, 23, 3062-3066
Sol-Gel Reaction in Acrylic Polymer Emulsions: The Effect of Particle Surface Charge Mitsuru Watanabe* and Toshiyuki Tamai Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Jyoto-ku, Osaka 536-8553, Japan ReceiVed October 11, 2006. In Final Form: December 26, 2006 Acrylic polymer-silica hybrid emulsions were synthesized from both anionic and cationic polymer emulsions by simple post-addition of tetraethoxysilane as a silica precursor. Solvent resistance of the films from the hybrid emulsions and the ζ-potential of the hybrid emulsions suggested the different forms of silica components in each hybrid emulsion. Thermal gravimetric analysis, 29Si NMR measurements, and transmission electron microscope observations revealed that the hybrid emulsion from the anionic polymer emulsion was a mixture of anionic polymer particles and homogeneously dissolved silicate oligomer-polymer. On the contrary, the hybrid emulsion from cationic polymer emulsion consisted of polymer core-silica shell particles. The electrostatic interaction between the cationic polymer particle surface and the silicate would be responsible for the accumulation of the silicate onto the particle surface, leading to the silica shell layer formation. The sol-gel condensation reaction of silicate in the acidic emulsion phase was revealed to be controllable by the surface charge of the coexisting particles.
Introduction Organic-inorganic hybrid (nanocomposite) particles have attracted much interest in various fields of material science such as coatings, catalysts, microelectronics, and biotechnology. A sol-gel reaction using a metal alkoxide as a precursor of inorganic components is a versatile method of obtaining organic-inorganic hybrid materials.1 Although polymer emulsion chemistry can provide easy access for extensive amounts and varieties in the polymer nanoparticle synthesis,2 the sol-gel reaction was not widely applied in polymer emulsion chemistry.3-10 In some of those investigations,3-7 the sol-gel reaction was used to form an inorganic-shell layer on the polymer core by a basic catalyst in organic solvent (i.e., the Sto¨ber method-like11 conditions). To achieve efficient inorganic shell layer formation, the functional groups expressed on the surface of the core particle were controlled, such as the alkoxysilyl groups6 and the cationic charged groups.4,5,7 Some authors stressed the importance of controlling the polymer particle surface, of which the property determines the affinity between inorganic components and polymer components.4 Similarly, it is well-known that particle-to-polyelec* Corresponding author. Tel.: +81-6-6963-8029. Fax: +81-6-6963-8040. E-mail:
[email protected]. (1) (a) Innocenzi, P.; Brusatin, G.; Licoccia, S.; Di Vona, M. L.; Babonneau, F.; Alonso, B. Chem. Mater. 2003, 15, 4790. (b) Schottner, G. Chem. Mater. 2001, 13, 3422. (c) Corriu, R. J. P.; Leclercq, D. Angew. Chem., Int. Ed. Engl. 1996, 35, 1420. (2) Lovell, P. A.; El-Aasser, M. S. Emulsion Polymerization and Emulsion Polymers; John Wiley and Sons: New York, 1997. (3) Yang, J.; Hu, D.; Fnag, Y.; Bai, C.; Wang, H. Chem. Mater. 2006, 18, 4902. (4) Lu, Y.; McLellan, J.; Xia, Y. Langmuir 2004, 20, 3464. (5) Sertchook, H.; Avnir, D. Chem. Mater. 2003, 15, 1690. (6) (a) Tissot, I.; Reymond, J. P.; Lefebvre, F.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, 1325. (b) Tissot, I.; Novat, C.; Lefebvre, F.; Bourgeat-Lami, E. Macromolecules 2001, 34, 5737. (7) (a) Zhang, M.; Gao, G.; Li, C.-Q.; Liu, F.-Q. Langmuir 2004, 20, 1420. (b) Imhof, A. Langmuir 2001, 17, 3579. (8) (a) Watanabe, M.; Tamai, T. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4763. (b) Tamai, T.; Watanabe, M. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 273. (9) (a) Tong, X.; Tang, T.; Feng, Z.; Huang, B. J. Appl. Polym. Sci. 2002, 86, 3532. (b) Tong, X.; Tang, T.; Zhang, Q.; Feng, Z.; Huang, B. J. Appl. Polym. Sci. 2002, 83, 446. (10) Kan, C. Y.; Liu, D. S.; Kong, X. Z.; Zhu, X. L. J. Appl. Polym. Sci. 2001, 82, 3194. (11) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.
trolyte12 and particle-to-particle13 electrostatic interactions form shell layers on the particles. Silicate oligomer-to-surfactant electrostatic interactions resulted in mesoporous silica formation.14,15 Thus, the interactions through organic-inorganic interfaces are important for the formation of organic-inorganic hybrids. A few groups investigated polymer-silica hybrid emulsions for film forming materials prepared by sol-gel reactions.8-10 We have synthesized acrylic polymer- silica organic-inorganic hybrid emulsions by an acidic condition sol-gel reaction using a silane coupling agent containing acrylic polymer emulsion and tetraethoxysilane (TEOS).8 Although the hybrid emulsion did not seem to have an affinity between the silica and the particles, the film obtained from the hybrid emulsion showed excellent solvent resistance. The film had a honeycomb-like nanometer scale phase separated structure, suggesting that the silica was located in the space between the packed polymer particles. Tong et al. also reported similar nanometer scale phase separated films by adding pre-hydrolyzed silica precursor solution into polymer emulsions.9 It is well-known that the character of the catalyst affects the product of the sol-gel reaction because of the reaction mechanism differences. In general, the acidic catalyzed sol-gel reaction gives linear and less branched silicate oligomers, and the basic catalyzed sol-gel reaction gives dense crosslinked silica particles. A basic catalyst was used for the synthesis of the silica shell on the polymer particle.3-7 However, the constituent of the organicinorganic hybrid emulsions synthesized by acidic sol-gel reactions was not extensively investigated, whereas the micro(12) (a) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317. (b) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202. (13) (a) Lawrie, G.; Grøndahl, L.; Battersby, B.; Keen, I.; Lorentzen, M.; Surawski, P.; Trau, M. Langmuir 2006, 22, 497. (b) Cho, Y.-S.; Yi, G.-R.; Lim, J.-M.; Kim, S.-H.; Manoharan, V. N.; Pine, D. J.; Yang, S.-M. J. Am. Chem. Soc. 2005, 127, 15968. (14) (a) Frasch, J.; Lebeau, B.; Soulard, M.; Patarin, J.; Zana, R. Langmuir 2000, 16, 9049. (b) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (c) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (15) Hentze, H.-P.; Raghavan, S. R.; McKelvey, C. A.; Kaler, E. W. Langmuir 2003, 19, 1069.
10.1021/la062999v CCC: $37.00 © 2007 American Chemical Society Published on Web 02/13/2007
Sol-Gel Reaction in Acrylic Polymer Emulsions
structures of their dried films were known.8-10 Furthermore, an attempt to apply attractive interactions between polymer particle and silica in acidic condition sol-gel reactions is worth studying. In this paper, sol-gel reactions in polymer emulsions containing anionic and cationic charged particles under acidic conditions were studied. The surface charge on the polymer particles was expected to affect the interaction between the silica component and the polymer surface.4 The amounts of the silica component in the hybrid emulsions after a few purification treatments were measured to study the form of the silica component in the hybrid emulsions. Also, measurements of ζ-potentials, 29Si NMR, and transmission electron microscopy (TEM) gave information about the surface charge of the particles, the chemical status of the silicon atom, and the microstructures of the particles, respectively, in the hybrid emulsions. Experimental Procedures Materials and Instruments. n-Butyl methacrylate (BMA, Nacalai) and n-butyl acrylate (BA, Nacalai) were distilled under reduced pressure prior to use. Methacryloxypropyltriethoxysilane (MPTES, Shinetsu), tetraethoxysilane (TEOS, Nacalai), potassium persulfate (KPS, Nacalai), sodium dodecylsulfate (SDS, Nacalai), 2,2′-azobis(2-methylpropionamidine)dihydrochloride (AIBA, Wako), and cetyltrimethylammonium bromide (CTMAB, Nacalai) were used as received. Particle sizes and size distributions were measured by dynamic light scattering (DLS) employing DLS-6000HLC (Otsuka Electronics). ζ-Potentials were measured by laser-Doppler electrophoretic light scattering (ELS) using ELS-6000 (Otsuka Electronics). Thermal gravimetric analyses (TGA) of hybrid samples were done by TG/ DTA 320 (Seiko Instruments). Silicon NMR spectra were determined on a JEOL JNM-EX270 spectrometer (54 MHz) with a TMS/CDCl3 external reference. Transmission electron microscopy was observed with a JEOL JEM-1200EX microscope. Acrylic Emulsions 1 and 2. A 1 L four-necked flask equipped with a condenser and a mechanical stirrer was filled with deionized water (410 mL), BMA (17.8 g), BA (1.80 g), MPTES (150 mg), and SDS (1.40 g). The mixture was purged with nitrogen gas for 15 min and then heated to 80 °C. An aqueous solution (10 mL) of KPS (420 mg) was added to the flask, and the mixture was kept at 80 °C for 2 h with vigorous stirring. The obtained seed emulsion (solid content 5.0 wt % by gravimetric analysis) contained particles with an average diameter (DLS) of 47 nm and had a monodispersed size distribution (polydispersity index ) 0.025). In the second stage of polymerization, a portion of the seed emulsion (30 g) was introduced into the reaction flask and purged with nitrogen gas for 15 min. The flask was heated to 80 °C, and two solutions were then fed into the flask at a constant rate over 8 h with vigorous stirring. One solution contained BMA (11.0 g), BA (1.1 g), and MPTES (92 mg). Another solution contained SDS (200 mg) and KPS (24 mg) in water (9 mL). After an additional 12 h of stirring, emulsion 1 was obtained. The obtained emulsion 1 (solid content 27 wt % by gravimetric analysis) contained particles with an average diameter (DLS) of 93 nm and had a monodispersed size distribution (polydispersity index ) 0.030). Cationic emulsion 2 was synthesized in a similar manner described previously using CTMAB as a surfactant and AIBA as an initiator. Typical Procedure of Preparing Hybrid Emulsion (1a, 1b, and 1c). Emulsion 1 was diluted to 5 wt % solid concentration by deionized water. The pH of the emulsion was adjusted to around 3 by adding a small amount of acid. TEOS (345 mg) was added to the emulsion (10 mL) in 10 portions every 30 min under magnetic stirring at room temperature. After an additional 2 h of stirring, hybrid emulsion 1a was obtained. Hybrid emulsions were aged for a few days at room temperature before use unless otherwise noted. Hybrid emulsions 1b and 1c were also prepared by adding 690 and 1725 mg of TEOS, respectively. If all added TEOS was converted to silica, the amounts of silica were around 20, 40, and 100 wt % for 1, 1b, and 1c, respectively, against the acrylic polymer component.
Langmuir, Vol. 23, No. 6, 2007 3063 Evaluating the Solvent Resistance of the Hybrid Films. Films were prepared by casting of the emulsion onto a Pyrex glass plate at 30 °C and annealing at 80 °C for 1 h to remove water inside the film. The solvent resistance of the film was characterized by solvent extraction and swelling measurements. Film specimens (initial weight: W0 ) ca. 55 mg) were immersed in 1,4-dioxane for 24 h at room temperature to attain swelling and dissolution equilibrium. The remaining film specimens were removed from the solvent, and the weight of the film (W1) was measured. The film was then dried to a constant weight (W2) at room temperature. The gel content (wt %) and swelling ratio were calculated with the following formulas: gel content (wt %) ) W2/W0 × 100 swelling ratio ) W1/W2 Measurement of ζ-Potential (mV). The mobility of the polymer particles was measured in 1 mM KCl solution with a small amount of acetic acid (1 mg/mL), affording a pH of around 3. The values of the particle ζ-potentials (mV) were calculated from the mobility by Smoluchowski’s equation. Measurement of Silica Content (wt %). Emulsion samples were dried well, and ca. 10 mg of specimen was afforded to combust in an aluminum pan by the TGA instrument. The residual weight after combustion at 600 °C indicates the weight of the total minerals in the sample. The silica contents against a unit polymer weight were calculated by the following formula: silica (wt % vs polymer) ) residual weight/(initial weight - residual weight) × 100
Results and Discussion Syntheses of Hybrid Emulsions and Measurement of Solvent Resistance of the Hybrid Films and ζ-Potentials of the Particles. Acrylic emulsions 1 and 2 were synthesized by conventional feed-emulsion polymerization. Emulsion 1 was synthesized using SDS as a surfactant and KPS as an initiator. Emulsion 2 was synthesized using CTMAB and AIBA instead. Each emulsion had a negative and positive ζ-potential, respectively, derived from the charge of the surfactant and initiator. Copolymer P[BMA-co-BA] (90:10) was chosen as a base polymer for acrylic polymer emulsions because it had a glass transition temperature below room temperature and was able to form transparent films under ambient conditions. A silane coupling agent monomer, MPTES (0.4 mol %), was added to give rise to good solvent resistant films.8,10,16 Properties of emulsions 1 and 2 are listed in Table 1. Acrylic polymer-silica hybrid emulsions 1a, 1b, and 1c and 2a, 2b, and 2c were synthesized by post-addition of TEOS to 1 and 2 under weak acidic conditions (pH ∼3). Hybrid emulsions 1a, 1b, and 1c contained silica components of 20, 40, and 100 wt % against the acrylic polymer component, respectively, if all of the added TEOS was converted into silica. Emulsions 1 and 2 and hybrid emulsions 1a and 2a formed transparent films via drying on a glass plate. Gel contents and swelling ratios of the films were evaluated to obtain the solvent resistance (Table 2). Both the films from 1 and 2 showed moderate solvent resistance, suggesting that copolymerized MPTES worked as a reactive crosslinking agent during the film formation process.17 The film from hybrid emulsion 1a showed a significantly higher gel content and lower swelling ratio as compared to those of 1, indicating the improvement of the solvent resistance of the film by the (16) (a) Ni, K. F.; Shan, G. R.; Weng, Z. X.; Sheibat-Othman, N.; Fevotte, G.; Lefebvre, F.; Bourgeat-Lami, E. Macromolecules 2005, 38, 7321. (b) Ni, K. F.; Sheibat-Othman, N.; Shan, G. R.; Fevotte, G.; Bourgeat-Lami, E. Macromolecules 2005, 38, 9100. (17) Marcu, I.; Daniels, E. S.; Dimonie, V. L.; Hagiopol, C.; Roberts, J. E.; El-Aasser, M. S. Macromolecules 2003, 36, 328.
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Table 1. Properties and Emulsion Polymerization Conditions of Emulsions and Hybrid Emulsions emulsion-hybrid emulsion seed of 1 1 1a 1b 1c seed of 2 2 2a 2b 2c a
BMA (mol ratio)
BA (mol ratio)
MPTES (mol ratio)
90 90
10 10
0.4 0.4
surfactant (mol ratio) SDS (3.5) SDS (0.8)
initiator (mol ratio)
TEOS (mg)a
solid content (wt %)
diameter (nm)b
5 27
47 93
5 26
48 96
KPS (1) KPS (0.1) 345 690 1725
90 90
10 10
0.4 0.4
CTMAB (2.7) CTMAB (0.6)
AIBA (1) AIBA (0.1) 345 690 1725
Amount of TEOS addition into 10 mL, 5 wt % emulsions. b Measured by DLS analysis.
Table 2. Solvent Resistances (vs 1,4-Dioxane) of Hybrid Films and ζ-Potentials of Hybrid Emulsions emulsion 1 1a 1b 2 2a 2b
gel content (wt %)a
swelling ratiob
74 99 d 87 75 d
4.9 1.4 d 4.5 3.1 d
ζ-potential (mV)c -41 -32 -36 +58 +20 +6
a Where W ) initial weight of a film, W ) weight of a solvent 0 1 swelled film, W2 ) weight of a dried film after swelling, and W2/W0 × 100 gives gel content (wt %). b W1/W2 to give swelling ratio. c Measured by ELS analysis in 1 mM KCl with 1 mg/mL acetic acid. d Not evaluated due to poor film formability.
addition of TEOS. However, the gel content of the film from 2a was less than 2, and the improvement of the swelling ratio was less than that of 1 and 1a.18 These results suggested that structurally different films had been formed from 1a and 2a. ζ-Potentials of the hybrid emulsions are also shown in Table 2. As the amount of TEOS addition increased (TEOS amount: 1a < 1b and 2a < 2b), the ζ-potential of the cationic particle in emulsion 2 significantly decreased, although the ζ-potential of the anionic particle in emulsion 1 did not vary so much. These results suggested that the silica component in the hybrid emulsions strongly affected the surface of cationic polymer particle but rarely affected the anionic surface. These contrasting results showed significantly different effects of anionic and cationic particle surfaces for the properties of acrylic polymer-silica hybrid emulsions. To make the effect of the particle surface charge clear, reactions that occurred in the hybrid emulsion will be discussed in the following sections. Classification of Silica Component in the Hybrid Emulsions. The silica component that was formed from TEOS added into the emulsion would be classified in four possible forms (Figure 1). A: Silic acid or its alkoxy substituted monomer and low molecular weight silicate oligomer, which are dissolved in water homogeneously; B: relatively high molecular weight silicate polymers, which are dissolved in water homogeneously; C: silica particles; and D: silica or silicate adsorbed on the polymer particle surface. With the aim of measuring the amount of each component, two purification methods, which are often used for the purification of polymer emulsions, were carried out for the hybrid emulsions. A is expected to be removed after dialysis and B-D would remain in the hybrid emulsion. Similarly, A and B would be (18) Immersion of the film from 2a to 1,4-dioxane caused many small clacks on the film. Some pieces from the film were dispersed into the solvent, and some amount of the solvent remained between the clacks when the weight of the swelled film was measured. This might be because of underestimation of the gel content and overestimation of the swelling ratio. Although this result did not mean that the film from 2a was soluble in the solvent, the film was easily affected by the solvent. Thus, the film from 2a was less solvent resistant.
Figure 1. Schematic view of the possible four forms of the silica component in the acrylic polymer-silica hybrid emulsion. Table 3. Silica Contents of Hybrid Emulsions Before and After Purification emulsiona
silica content (wt %)
constituent
1a dialysis centrifugation 2a dialysis centrifugation
20 7 1 22 21 20
A+B+C+D B+C+D C+D A+B+C+D B+C+D C+D
a
Emulsions were aged for a few days at room temperature before
use.
removed by a centrifugation-redispersion cycle, so that C and D would remain. Thus, both amounts of A and B could be measured. Hybrid emulsions 1a and 2a were treated with dialysis and centrifugation. The silica contents of each sample, which were evaluated by TGA, are shown in Table 3. In the case of 1a, about two-thirds of the silica was lost by dialysis,19 and almost no silica was found from the precipitate after centrifugation, indicating that no silica particles formed and that none of the silica component was adsorbed on the anionic polymer particles. On the contrary, the silica contents of 2a remained constant after either procedure of purification, suggesting that all of the added silica component condensed into silica particles or adsorbed on the cationic surface of the acrylic polymer particles. As a result, the silica component in 1a only consisted of A and B and that of 2a consisted of C and/or D. 29Si NMR and TEM Measurement of the Hybrid Emulsions. We next investigated 29Si NMR and TEM observations to obtain further information on the silica component in the hybrid (19) Rejection molecular weight of the cellulose tube used was 12 000 to 14 000. The degree of polymerization was calculated to be about 200mer for the MW 12 000 to 14 000 polymerized silicate.
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Figure 2. 29Si NMR spectra of (a) fresh 1a, (b) aged 1a, (c) fresh 2a, and (d) aged 2a.
emulsions. By the measurement of both freshly prepared 1a and 2a (Figure 2a,c) were observed similar spectra, from which Q0 (silic acid), Q1 (silicate dimer or end group of the chain), and Q2 (linear polymer or cyclic oligomer) silicon could be identified. In the spectrum of 1a after aging for a few days (Figure 2b), Q2 and Q3 (bridgehead of the branch) silicon were dominantly observed, but there was no Q0 and only a trace amount of Q1. The silicate condensation reaction proceeded to some extent in 1a and produced a relatively low molecular weight silicate oligomer. In contrast, no Q0 to Q3 silicon was observed in the spectrum of 2a after aging for a few days (Figure 2d), suggesting the formation of Q4 (silica) in the emulsion.20 This result indicated different forms of the silica component in each hybrid emulsion and agreed well with the result of the silica contents shown in Table 3. The hybrid emulsions 1c and 2c21 were freeze-dried and embedded in epoxy resin. The blocks of epoxy resin were sectioned on ultra-microtome, and the sections were observed by TEM (Figure 3). It could be clearly seen that there were no silica particles larger than ∼5 nm either in 1c or in 2c. Figure 3a shows that the silica component was dispersed between anionic polymer particles uniformly and was not adsorbed or accumulated on the surface of the particles. For cationic emulsion 2c (Figure 3b), the silica component was accumulated on the surface of the polymer particles. Polymer core-silica shell particles, whose shell layer was up to 10 nm thick, were formed. From this contrasting result of TEM and all other results shown previously, the constituents of the silica component in the hybrid emulsions derived from 2 were revealed to be only D. Constituents of the Hybrid Emulsions and Their Film Formation Mechanisms. The constituents of the silica component in hybrid emulsions was revealed as shown in Figure 4a by the silica content measurement, 29Si NMR, and TEM observations. In the hybrid emulsion derived from 1, the silica component existed as a silicate oligomer and polymer dissolved in the water phase homogeneously. The weight ratio of the silicate oligomer and polymer was estimated to be around 2:1 (Table 3). In contrast, all silica components in the hybrid emulsion derived (20) Q4 silicon (silica) could not be observed because its signal was hidden by the broad and strong background peak of the NMR sample tube glass. The absence of Q0 to Q3 silicon indicated a full conversion of silicate to silica. (21) Observations of 1a and 2a were not successful. High silica content was needed to achieve good contrast. However, such high silica content hybrid emulsions were not stable and caused gelation for several days.
Figure 3. Transmission electron micrographs of freeze-dried sample of (a) 1c and (b) 2c.
Figure 4. (a) Constituent of the silica component in acrylic polymersilica hybrid emulsions. (b) Film formation mechanisms.
from 2 was adsorbed on the polymer particle surface and formed acrylic polymer core-silica shell particles. Anionic charged silicate oligomers or polymers would interact with the cationic charged polymer particle of 2.12 The silicate that is adsorbed and
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accumulated on the surface would give rise to the progress of the sol-gel condensation that forms the silica shell layer on the particle within a few days. The decrease of the ζ-potential also could be explained by considering the core-shell structure (i.e., neutralization of the surface cationic charge by the anionic silica layer). On the other hand, the silicate condensation proceeded only partly when there was no affinity between dissolved silicate and polymer particles such as in 1a, 1b, and 1c. The weak acidic conditions would prevent the progress of the condensation reaction of the dissolved silicate. When the hybrid emulsions were cast and dried to form the film (Figure 4b), silicate oligomers in 1a would be concentrated into the pocket between polymer particle cells and would condense. The polymer particles were in contact with each other and were fused strongly by the intercellular polymer diffusion across the interface.22 The silane coupling agent (MPTES) copolymerized in acrylic polymer would not only co-condense with itself to make polymer crosslinkages17 but also would condense to make a bond between the silica component and the polymer component.8 Thus, a nanometer scale dispersed polymersilica hybrid film was formed, which showed a high solvent resistance. In the case of 2a, the silica shell prevented polymer diffusion and adhesion of the core particles. MPTES worked as a reactive crosslinking agent only inside the shell. The remaining boundary of the particles in the film would be responsible for the less solvent resistance of the film.22,23
Conclusion Acrylic polymer-silica hybrid emulsions were synthesized from both anionic and cationic polymer emulsions 1 and 2, respectively, by the post-addition of a silica precursor (TEOS) under weak acidic conditions (pH ∼3). Although sol-gel reaction conditions were identical other than the polymer emulsion used, the hybrid emulsions derived from 1 and 2 showed significantly different properties, such as ζ-potential and solvent resistance (22) (a) Pe´rez, E.; Lang, J. Langmuir 2000, 16, 1874. (b) Tamai, T.; Pinenq, P.; Winnik, M. A. Macromolecules 1999, 32, 6102. (23) (a) Aradian, A.; Raphae¨l, E.; de Gennes, P.-G. Macromolecules 2002, 35, 4036. (b) Aradian, A.; Raphae¨l, E.; de Gennes, P.-G. Macromolecules 2000, 33, 9444.
Watanabe and Tamai
of the film. The solvent resistance of the film from 2a was much less than that of 1a. Investigating the form of the silica component in the hybrid emulsions revealed different sol-gel reaction products corresponding to the starting emulsions 1 or 2. Silica components in 1a, 1b, and 1c homogeneously dissolved in the water phase as forms of the silicate oligomer and polymer and rarely interacted with the anionic polymer particles. On the other hand, an acrylic polymer core-silica shell particle was produced in 2a, 2b, and 2c. The surface charge of the polymer particle controlled not only the location of the silica component but also the form of the silica component. A driving force of the silica formation at the polymer particle surface would be the concentrating effect of the silicate oligomer onto the surface in a similar manner to the particle-to-polyelectrolyte electrostatic interaction.12 When silicate oligomer was accumulated on the polymer particle surface, the condensation proceeded until the silica shell layer formed even under weak acidic conditions, while basic conditions were generally preferred for silica formation from TEOS.3-6,11 In the method described in this article, TEOS was simply added to a water-borne emulsion under weak acidic conditions. Although the degree of polymerization of silicate tends to be low under acidic conditions in general, the condensation reaction of silicate was revealed to be controllable by the surface charge of the coexisting particles. This finding will open a new possibility to applying sol-gel reactions for polymer emulsion chemistry. The mixture of acrylic polymer particles and homogeneously dissolved silicate oligomer-polymer (1a, 1b, and 1c) would be suitable for film forming materials due to its reactivity during the film formation. The silica shell of 2a, 2b, and 2c had enough strength to prevent polymer diffusion and adhesion. This property would potentially be applied as a protecting layer of functional materials encapsulated inside the particle with a mild condition of shell layer formation. Acknowledgment. The authors thank Drs. S. Watase and Y. Hatanaka in the Osaka Municipal Technical Research Institute for the 29Si NMR measurements and for the transmission electron microscopy observations, respectively. LA062999V
