Titania-Coated Polystyrene Hybrid Microballs Prepared with

Hybrid microballs with polystyrene cores coated by titania nanoparticles were prepared in miniemulsion polymerization. Acrylic acid was used as a como...
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Titania-Coated Polystyrene Hybrid Microballs Prepared with Miniemulsion Polymerization Ming Zhang, Ge Gao, Cheng-Qin Li,† and Feng-Qi Liu* College of Chemistry, Jilin University, Changchun, 130023, People’s Republic of China Received April 28, 2003. In Final Form: October 23, 2003 Hybrid microballs with polystyrene cores coated by titania nanoparticles were prepared in miniemulsion polymerization. Acrylic acid was used as a comonomer to promote locating titania nanoparticles on the polymer’s surface. The addition of a hydrophobic agent effectively prevents monomer diffusing into the aqueous phase. The morphology of hybrid particles was examined with the transmission electron microscope, and its change pattern with reactive conditions was observed. The infrared spectra of hybrid nanoparticles showed that there existed a certain interaction between titania nanoparticles and polymers. The crystallization morphology of hybrid particles before and after calcination was characterized with X-ray diffraction.

Introduction In recent years, considerable effort has been devoted to design and fabrication of organic-inorganic hybrid materials.1-8 The organic-inorganic hybrid materials present the properties of both the inorganic nanoparticles and the polymer by combining thermal stability, mechanical strength, or optical properties with flexibility and the ability to form films. Regarding numerous kinds of hybrid materials, core-shell structural materials show some special properties in optics, electronics, magnetics, and catalysis by adjusting their chemical composition and structure. The core-shell materials, in which the shell is made of polymer, can be prepared mainly by polymerization and chemical linking9-12 or layer-by-layer selfassembly.13,14 In another way, inorganic nanoparticle coatings on polymer latex particles have also been prepared by in situ reaction15-17 or the layer-by-layer selfassembly method.18-22 The design and synthesis of nanoscale objects require a good deal of control over interfaces in order to build up and organize the nanostructures.23 This involves surface * To whom correspondence should be addressed. E-mail: [email protected]. † Present address: Element Department, Liaodong College, Dandong , Liaoning , China 118003. (1) Sanchez, C.; Soler-Illia, G. J. de A. A.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061. (2) Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 17, 5775. (3) Fleming, M. S.; Mandal, T. K.; Walt, D. R. Chem. Mater. 2001, 13, 2210. (4) Schneider, J. J. Adv. Mater. 2001, 13, 529. (5) Aliev, F. G.; Correa-Duarte, M. A.; Mamedov, A.; Ostrander, J. W.; Giersig, M.; Liz-Marzan, L. M.; Kotov, N. A. Adv. Mater. 1999, 11, 1006. (6) Pastoriza-Santos, I.; Koktysh, D. S.; Mamedov, A. A.; Giersig, M.; Kotov, N. A.; Liz-Marza´n, L. M. Langmuir 2000, 16, 2731. (7) Schartl, W. Adv. Mater. 2000, 12, 1899. (8) 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. (9) Sondi, I.; Fedynyshyn, T. H.; Sinta, R.; Matijevic, E. Langmuir 2000, 16, 9031. (10) Landfester, K. Adv. Mater. 2001, 13, 765. (11) Landfester, K.; Rothe, R.; Antonietti, M. Macromolecules 2002, 35, 1658. (12) Zhang, K.; Chen, H.; Chen, X.; Chen, Z.; Cui, Z.; Yang, B. Macromol. Mater. Eng. 2003, 288, 380. (13) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mohwald, H. Macromolecules 1999, 32, 2317. (14) Caruso, F.; Trau, D.; Mo¨hwald, H.; Renneberg, R. Langmuir 2000, 16, 1485.

modification of core or shell, for instance, using a comonomer in polymerization to introduce functional groups onto polymers or treating inorganic particles with a coupling agent.24,25 Armes et al. reported the synthesis of polymer/ silica nanocomposites, in which 4-vinylpyridine was used to generate strong acid-base interactions.26 Caruso et al. described the fabrication of magnetic nanocomposite particles by multistep layering.27 Wang et al. reported a novel templating route involving TiO2 coating on a polyelectrolyte.28 A similar work to ours was performed by Imhof.15 Imhof proposed a one-step method to coat polystyrene with titania by hydrolyzing titanium alkoxide directly in the presence of polystyrene. His results showed that the thickness of the coating was controllable, and the obtained hybrid material with smooth and uniform titania shells was easily turned into hollow spheres by dissolution or calcination, from which would emerge promising applications in many fields. To the best of our knowledge and despite the large number of papers published on the encapsulation of inorganic nanoparticles by using the miniemulsion technique, the combination of the miniemulsion technique with in situ assembly in the fabrication of inorganic nanoparticle coated polymer composite material has not yet been performed. In the present work, we report a one-step route by combining the miniemulsion polymerization of polystyrene-co-poly(acrylic acid) (PStA) latex particles and the (15) Imhof, A. Langmuir 2001, 17, 3579. (16) Chen, C. W.; Chen, M. Q.; Serizawa, T.; Akashi, M. Adv. Mater. 1998, 10, 1122. (17) Chen, C. W.; Serizawa, T.; Akashi, M. Chem. Mater. 1999, 11, 1381. (18) Caruso, F.; Lichtenfeld, H.; Giersig, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 8523. (19) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (20) Wang, M.; Zhang, L. J. Mater. Res. 2001, 16, 765. (21) Valtchev, V. Chem. Mater. 2002, 14, 956. (22) Wang, L.; Sasaki, T.; Ebina, Y.; Kurashima, K.; Watanabe, M. Chem. Mater. 2002, 14, 4827. (23) MacLachlan, M. J.; Manners, I.; Ozin, G. A. Adv. Mater. 2000, 12, 675. (24) Caris, C. H. M.; van Elven, L. P. M.; van Herk, A. M.; German, A. L. Br. Polym. J. 1989, 21, 133. (25) Corcos, F.; Bourgeat-Lami, E.; Novat, C.; Lang, J. Colloid Polym. Sci. 1999, 277, 1142. (26) Percy, M. J.; Barthet, C.; Lobb, J. C.; Khan, M. A.; Lascelles, S. F.; Vamvakaki, M.; Armes, S. P. Langmuir 2000, 16, 6913. (27) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Chem. Mater. 2001, 13, 109. (28) Wang, D.; Caruso, F. Chem. Mater. 2002, 14, 1909.

10.1021/la030183d CCC: $27.50 © 2004 American Chemical Society Published on Web 01/15/2004

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Table 1. Experimental Information titania average sample acrylic colloid surfactant hydrophobic particle no. acid (mL) (g) (g) agenta (g) size (nm) 1 2 3 4 5 6 7 8 9 a

0.2 0.2 0.2 0.4 0.6 0.2 0.2 0.2 0.2

5 5 5 3 3 0 5 3 1

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.00 0.10 0.145 0.05 0.05 0.04 0.04 0.05 0.05

90.33 437.5 76.87 60-80 51.25 111.5 74.9 45.65

Cyclohexane.

coating of titania nanoparticles onto the polymer microspheres. During the process, titania was added to enhance the stability of the system because of its charged and hydrophilic properties. Titania nanoparticles were coated on the surface of the polymer by the interaction between titania and carboxyl groups. The effects of additives on the morphology of the hybrid material are discussed. The crystallization morphology of hybrid particles before and after calcination was characterized with X-ray diffraction. Experimental Section Preparation of TiO2 Nanoparticles. TiO2 colloid particles were prepared with the sol-gel method. Tetrabutyl titanate (10 mL) was dispersed in 30 mL of anhydrous ethanol. Then 8 mL of a mixture of deionized water, HCl (concentration 37%), and ethanol with a ratio of 1:2:5 (v/v/v) was dropwise added to the system with agitation. After 4 h, a little yellow transparent nanometer colloid of TiO2 was obtained. Polystyrene Spheres Coated with Titania. This step was completed with miniemulsion polymerization. The recipe is as follows: 20 mL of deionized water, 0.01 g of the surfactant cetyltrimethylammonium bromide (CTAB), 1-5 g of TiO2 colloid, 0.05 g of azobisisobutyronitrile (AIBN), 0.05 g of hydrophobic agent, 1 mL of styrene, and 0.2-0.6 mL of acrylic acid. All the chemicals were added in the above order while stirring (120 rpm). The reaction was processed at 60 °C for 12 h. The detailed experimental information is listed in Table 1. Analytical Methods. The TiO2 colloid particle size was measured with a Malvern Instruments Zeta Sizer 3000 HSA. The morphologies of TiO2-coated polystyrene spheres were characterized with a Hitachi H-8100 transmission electron microscope (TEM). The chemical composition and structure of the TiO2 colloid, PStA, and TiO2-coated PStA were analyzed with a Nicole Avatar 360 FT-IR. The crystallization of TiO2 calcinated and noncalcinated was characterized with a Rigaku wide-angle X-ray diffractometer (D/max γA, using Cu KR radiation at wavelength λ ) 1.541 Å). The detector was calibrated by using KCl powders as a standard with 2θ being 28.345° (200) and 40.507° (220) under Cu KR radiation.

Figure 1. IR spectra: (a) copolymer (sample 6); (b) TiO2-coated polystyrene (sample 7).

Results and Discussion

Figure 2. TEM photograph of TiO2-coated polystyrene particles without hydrophobic agent (sample 1).

1. Selection of Ingredients. One of the key techniques in preparing coated nanospheres is keeping the stability of the miniemulsion polymerization system. For decreasing interfacial tension and stabilizing droplets, a little amount of an emulsifier has to be added to the system (concentration < critical micelle concentration (cmc) value). Anionic sodium dodecyl benzene sulfonate (SDBS), nonionic PEO-PPO-PEO (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)), and cationic CTAB were chosen as emulsifiers. It was found that anionic SDBS and nonionic PEO-PPO-PEO resulted in agglomeration and precipitation of TiO2, so CTAB was used as the emulsifier in experiments. Water-soluble ammonium persulfate and oil-soluble AIBN were chosen

as initiators. Due to having a better effect on stabilizing the polymerization system, AIBN was used as the initiator in experiments. It is important to select an appropriate hydrophilic monomer to polymerize with hydrophobic monomer for better combination of TiO2 nanoparticles with polymers. Figure 1a,b shows the IR spectra of PStA copolymer and TiO2-coated PStA prepared with miniemulsion polymerization. Benzene ring folding appeared at 698 cm-1, C-H bending of the benzene ring at 756 cm-1, and C-C stretching of the benzene ring at 1492 and 1600 cm-1. All the absorption bands of the benzene ring and TiO2 in the hybrid material remained unchanged, but the CdO bond vibration at 1706 cm-1 deteriorated to the shoulder part

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Figure 3. TEM photographs: (a) TiO2-coated polystyrene particles with 0.1 g of cyclohexane (sample 2); (b) TiO2-coated polystyrene particles with 0.145 g of cyclohexane (sample 3).

Figure 4. TEM photographs: (a) copolymer (sample 6); (b) TiO2-coated polystyrene with 5 g of TiO2 colloid (sample 7); (c) TiO2coated polystyrene with 3 g of TiO2 colloid (sample 8); (d) TiO2-coated polystyrene with 1 g of TiO2 colloid (sample 9).

in hybrid particles.29 The absorption band of COO-Ti appearing at 1416 cm-1 demonstrates certain interactions between carboxyl groups and TiO2.30 Therefore, using acrylic acid as a comonomer is able to improve the combination of TiO2 with the polymer. Because of the tendency for carboxyl groups to gather on the surface of latex particles, it is favorable for “anchoring” TiO2 nanoparticles on the surface of polymer microspheres. 2. Effect of a Hydrophobic Agent. The addition of a hydrophobic agent to the miniemulsion system is aimed at decreasing the osmotic pressure of the hydrophobic monomer and preventing the dispersion of monomer from droplets toward the outside.2 Nucleation in the aqueous phase would occur during the polymerization process, if (29) Wang, S. X.; Wang, M. T.; Lei, Y.; Li, G. H.; Zhang, L. D. J. Inorg. Mater. 2000, 15, 45. (30) Jiang, X. M. Acta Pet. Sin. 2002, 18, 62.

hydrophobic monomer diffused toward the exterior in large amount. The TEM photograph of sample 1 without the hydrophobic agent showed three-layer structures with polymer as the core, TiO2 particles as the internal shell, and polymer as the external shell (Figure 2). The formation of the polymer exterior shell is attributed to the fact that monomers dispersed to the interface or to the aqueous phase, polymerized there, and then deposited onto the sphere surface. The influence of increasing the amount of hydrophobic agent on the morphology of hybrid particles is shown in Figure 3a,b. Sample 2, besides single hybrid microspheres, formed big spheres with a particle size of about 400 nm assembled from numerous tiny spheres (Figure 3a). The introduction of some carboxyl groups in polymer segments and the extension of hydrophilic segments containing carboxyl groups from the surface of hybrid microspheres to the aqueous phase resulted in

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Figure 5. XRD diagrams: (a) noncalcined TiO2 colloid; (b) calcined TiO2 colloid. Figure 7. Electron diffraction diagrams: (a) TiO2 colloid; (b) TiO2-coated polystyrene particles (sample 8).

Figure 6. XRD diagrams: (a) TiO2-coated polystyrene particles noncalcined (sample 9); (b) TiO2-coated polystyrene particles calcined (sample 9).

aggregation. The aggregation of hybrid microspheres could reasonably be attributed to the strong interaction of

carboxyl groups with TiO2 particles located on the surface of other hybrid microspheres, that is, the bridging function of carboxyl groups. With the increase of added amount of hydrophobic agent, the tendency for hydrophilic monomer (acrylic acid) to polymerize on the interface of droplets or in the aqueous phase was quite obvious. Sample 3 formed much bigger aggregates composed of hybrid microspheres, in which TiO2 particles dispersed in polymer showing a net structure (Figure 3b), which may be used as a highly efficient and heterogeneous phase catalyst. 3. Effect of the Amount of Acrylic Acid. It is quite critical to use acrylic acid as a comonomer to take advantage of the interaction of carboxyl groups with TiO2 to locate TiO2 on the polymer surface. TEM experiments demonstrated that when the content of acrylic acid was over 0.4 mL, the microspheres would aggregate. Although increasing the amount of acrylic acid is favorable for the adsorption of TiO2, its linking action with each other would result in aggregation of hybrid microspheres. 4. Effect of the Amount of TiO2. The particle size distribution of TiO2 measured by using a light-scattering particle sizer is rather sharp with a diameter range of 13-18 nm. Figure 4a,b presents TEM photographs of PStA and TiO2-coated PStA particles. It was clearly seen from Figure 4a that PStA had a particle size of about 50 nm, and the particle size of TiO2-coated PStA increased a little with a shell thickness of 18 nm (Figure 4b). There existed some free TiO2 particles dispersed in the system, showing a maximum adsorption amount of TiO2 on PStA under the experimental conditions. When the added amount of TiO2 was 3 g, TiO2 could be coated fully on the surface of PStA, but there still were some free TiO2 particles dispersed in the system (Figure 4c). When the added amount of TiO2 was decreased to 1 g, there were no free

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TiO2 particles to be seen, but the coating was unperfect (Figure 4d). For decreasing free TiO2 particles and forming an effective coating, the optimal amount of TiO2 added should be between 1 and 3 g. 5. Crystallization of TiO2. TiO2 colloid particles were dried into powder under decompression for X-ray diffraction (XRD) analysis. It was obvious that TiO2 colloid particles presented an amorphous structure (Figure 5a). When the particles were calcined at a heating rate of 1 °C/min to 550 °C for 2h, they formed anatase crystallization (Figure 5b). Figure 6a displays the X-ray diffraction result of a TiO2-coated polystyrene powder sample dried at 50 °C. Compared with Figure 5a, it was surprising to find that the crystal diffraction peak of TiO2 in hybrid microspheres became stronger than that of pure TiO2. Figure 6b is the XRD diagram of calcined TiO2 in hybrid particles. The degree of crystallinity of calcined TiO2 in hybrid particles was lower than pure TiO2 powder’s compared with Figure 5b. This fact may be attributed to the aggregation of pure TiO2 during calcination. Weight analysis also showed that the hybrid particles calcined contained 25% polymer residues instead of pure TiO2. The residues may obstruct the coagulation of TiO2 nanoparticles and resulted in the crystallization of TiO2 in hybrid particles being not as perfect as that of pure TiO2. The electron diffraction ring of hybrid particles was much clearer than that of pure TiO2 (Figure 7a,b), which comports with the results of X-ray diffraction. This further confirms that inducing crystallization of TiO2 occurred in some range during the preparation process.

TiO2 nanoparticles on the polymer’s surface. But an excess of acrylic acid would result in the aggregation of hybrid particles. 3. The addition of a hydrophobic agent would effectively prevent monomer diffusing into the aqueous phase. The hybrid particles prepared without hydrophobic agent present three-layer structures of polymer-TiO2-polymer. An excess of hydrophobic agent would promote the tendency for acrylic acid to polymerize on the interface and in the aqueous phase. The carboxyl group’s coupling function to TiO2 located on different hybrid microspheres leads to the aggregation of hybrid particles and the formation of great aggregates in which TiO2 disperses in polymer with a net structure. 4. During the preparation of hybrid particles, TiO2 was induced to crystallize further to a certain extent. TiO2 in hybrid particles calcined would form anatase crystallization. Hybrid particles calcined contain some polymer residues, and their obstructing function makes the crystallization of TiO2 in hybrid particles not as perfect as pure TiO2’s. A highly efficient and heterogeneous phase catalyst can be conveniently developed making use of the high specific surface area and the structure characterization of TiO2 nanoparticle hybrid microspheres. The further assembly and post-treatment of the hybrid microspheres would form a rigid spongy mesoporous nanocage of TiO2. It would be used as a highly efficient catalyst and to develop functional materials applied in advanced technique fields.

Conclusions

Acknowledgment. We thank the National Natural Science Foundation of China (20274014) and the Chinese Ministry of Education through the teaching and research award fund for outstanding young teachers in higher education institutions for financial support.

1. TiO2 nanoparticle coated polymer microspheres can be prepared with the miniemulsion polymerization technique by optimizing the recipe. 2. TiO2 has a strong interaction with carboxyl groups. Using acrylic acid as a comonomer is favorable for locating

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