Preparation and characterization of poly(styrenemethacrylic acid)/mcm-41 core/shell nanocomposite microspheres

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1 e-polymers 2009, no ISSN Preparation and characterization of poly(styrenemethacrylic acid)/mcm-41 core/shell nanocomposite microspheres Yansheng Zhao, * Xingji Ma, Yongmei Liu, Guangwei Yuan, Meijuan Guo, Kai Chen, Yinghua Shen * College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan , China; fax: ; tel.: ; zhaoyansheng@tyut.edu.cn (Received: 30 October, 2008; published: 27 October 2009) Introduction Abstract: In acidic media, poly(styrene-methacrylic acid)/mcm-41 [P(St- MAA)/MCM-41] core/shell microspheres were synthesized using monodisperse P(St-MAA) particles contained in soap-free emulsion and cetyltrimethylammonium bromide as co-templates by adsorption self-assembly method. The effects of P(St- MAA) composition on shell structure of the core/shell microspheres were investigated. The morphology and composition of P(St-MAA)/MCM-41 microspheres were characterized by TEM, XRD and FTIR. The results show that the ordering degree of MCM-41 shells increased as the molar ratio of MAA to St increased. When n(maa)/n(st) is 0.2, the average diameter and the shell thickness of nanocomposite microspheres are about 170 nm and 20 nm, respectively. Keywords: Poly(styrene-methacrylic acid), MCM-41, core/shell nanocomposite microspheres, synthesis, microstructure Core shell particles are composite materials consisting of a core domain covered by a shell domain, which have found various applications in many areas, such as strengthening polymeric materials [1], the stationary phases for chromatography and sensing materials [2]. Up to now, there are many methods adopted to prepare core/shell composite microspheres, such as self-assembling method [3, 4], dispersion polymerization [5], step wise heterocoagulation methods [6] and precipitation process [7]. Caruso et al. deposited silica nanoparticle-polyelectrolyte multilayers on polystyrene (PS) particles by electrostatic self-assembly in alkaline media [3]. Zhu et al. reported microstructured silica hollow spheres by PS bead-assisted electrostatic self-assembly at 42 0 C for 3 weeks [4]. Yang et al. synthesized the mesoporous hollow spheres using PS spheres by a precipitation process [7]. In above researches, the process of separation and redispersion of PS microspheres for core template are rather complicated and all the coating processes were carried out in the alkaline media. Compared with alkaline media, the synthesis of MCM-41 in acid media has advantages of mild reaction conditions and small pore size [8-10]. However, there have been few reports on the synthesis of core/shell microspheres with MCM-41 shells in acid media. In our foregoing work, the poly(styrene-methyl methacrylate)/mcm-41 [P(St-MMA)/MCM-41] core/shell nanocomposite microspheres have been successfully synthesized by adsorption self-assembly 1

2 method in alkaline media [11]. The diameter of P(St-MMA)/MCM-41 nanocomposite microspheres is about 240 nm and the thickness of shell is 20nm more or less. In present work, P(St-MAA)/MCM-41 core/shell nanocomposite microspheres were prepared in acidic media using P(St-MAA) microspheres contained in soap-free emulsion as the core template and cetyltrimethylammonium bromide (CTAB) as template for mesoporous structure. The P(St-MAA)/MCM-41 core/shell microspheres had smaller diameter (about 170nm). Results and discussion Fig.1 shows the FTIR spectra of the P(St-MAA) and P(St-MAA) /MCM-41. Both curve (a) and curve (b) reveal the well-defined bands (700, 758, and 3026cm -1 ) of phenyl group and the band of 2924cm -1 assigned to -CH 2 - group [12]. Additionally, the spectra showed the stretching vibration of O-H ( cm 1 ) and C=O (1701cm - 1 ), which were the characteristic absorption of -COOH of MAA [13]. In addition, curve (b) displays typical silica absorption bands at 1078cm -1 (the asymmetrical stretching of siloxane bond Si-O-Si) and at 966cm -1 (the stretching of the hydroxyl group in Si- OH) [14, 15]. The band at 463cm -1 was assigned to symmetric bending vibration of rocking mode of Si-O-Si. This result confirms that the products were made up of MCM-41 and P(St-MAA). b Transmittance a Wavenumbers(cm -1 ) Fig.1. FTIR spectra of P(St-MAA) microspheres (a) and P(St-MAA)/MCM-41 nanocomposite microspheres (b). Fig. 2 shows the XRD patterns of P(St-MAA)/MCM-41 before and after calcination. The peaks in the region of 2θ from 2 to 3 that can be indexed to a hexagonal lattice are typical of MCM-41 materials [16]. Higher order Bragg reflection was not resolved in the patterns, which can be attributed to the small sizes of particles of samples according to Corma [17]. The (100) peak shifted to a higher 2θ angle for shrinkage of crystal cells of MCM-41 after calcination [16, 18]. 2

3 intensity(cps) intensity(cps) 6000 a b theta (deg.) Fig. 2. XRD patterns of P(St-MAA)/ MCM-41 before (a) and after (b) calcinations d 4000 c b 2000 a theta (deg.) Fig. 3. XRD patterns of P(St-MAA)/ MCM-41 prepared by using the core template with different composition [n(maa)/n(st)= 0 (a); 0.1(b); 0.15 (c) and 0.2 (d)]. Fig. 3 shows the XRD patterns of P(St-MAA)/MCM-41 core/shell nanocomposite microspheres prepared by using the core template with different composition [n(maa)/n(st)= 0, 0.1, 0.15 and 0.2]. It shows that the intensity of the (100) peak increased with the increase of n(maa)/n(st) in the range from 0 to 0.2. Without adding MAA, the structure of MCM-41 shell was not obvious in acidic media (Fig. 3a). 3

4 When n(maa)/n(st) is 0.2, the (100) peak is well-resolved, which indicates that the well ordered shells of MCM-41 were formed. Fig. 4. TEM images of P(St-MAA) (a), P(St-MAA)/MCM-41 (b), hollow spheres obtained by calcination (c) and the detailed texture of MCM-41 shell (d). TEM images of P(St-MAA), P(St-MAA)/MCM-41, hollow spheres obtained by calcination and the detailed texture of MCM-41 shell are shown in Fig. 4. The P(St- MAA) (Fig. 4a) shows the feature of smooth surfaces and uniform spherical shape. The morphology of P(St-MAA)/MCM-41 (Fig. 4b) is different from Fig. 4a in that the particle surface was rough because of the formation of MCM-41 on the surface of P(St-MAA) microspheres. The core/shell structure of P(St-MAA)/MCM-41 (Fig. 4c) was proved by calciniation of the P(St-MAA)/MCM-41 at C in air for 6 h. The average diameter of P(St-MAA)/MCM-41 microspheres was about 170 nm and the thickness of shell was about 20 nm. The detailed texture of MCM-41 shell after calcination (Fig. 4d) indicates that the shells of nanocomposite microspheres have a uniform, well-defined hexagonal mesostructure, which is consistent with the result of XRD. This result indicates that MCM-41 was coated onto the surface of P(St-MAA). Conclusions In acidic media, P(St-MAA)/MCM-41 core/shell nanocomposite microspheres were prepared at 40 0 C for 72 h by directly using monodisperse P(St-MAA) microspheres contained in soap-free emulsion as core template. The ordered hexagonal channels of MCM-41 shells were confirmed by TEM observation. The ordering degree of MCM- 41 shells increased with the increase of n(maa)/n(st). The diameter and the shell thickness of P(St-MAA)/MCM-41 nanocomposite microspheres were about 170 nm and 20 nm, respectively. Experimental part Materials Styrene (St) was purchased from Tianjin Damao Chemical Co and methacrylic acid (MAA) was acquired from Tianjin Guangfu Fine Chemical Research Institute. 4

5 Tetraethylorthosilicate (TEOS) and CTAB were supplied by Tianjin Kemio Chemical Reagent Company. Preparation of P(St-MAA) latex P(St-MAA) latex was synthesized using styrene (St) and methacrylic acid (MAA) as monomers by soap-free emulsion polymerization. The molar composition was St : NaOH : NaHCO 3 : K 2 S 2 O 8 : H 2 O : MAA = 1 : 0.02 : 0.01 : : 115 :X (X=0, 0.1, 0.15 and 0.2). To eliminate oxygen effects, the solution was purged with nitrogen before the whole process was initiated. The reaction was carried out in a 500 ml flask as follows: H 2 O, NaOH and NaHCO 3 were firstly added into the flask, and then the monomer mixture was added into the solution under stirring. Sequentially, the initiator K 2 S 2 O 8 was added dropwise into the solution under stirring at 75 C. After 6 h, the polymerization was finished and the conversion efficiency exceeded 96%, the resulting soap-free emulsion containing P(St-MAA) microspheres was obtained. Preparation of P(St-MAA)/MCM-41 core/shell composite microspheres Firstly, 6 g of CTAB was dissolved into 300 ml of the above soap-free emulsion at 40 C under stirring. The ph of the solution was adjusted to < 2 with hydrochloric acid. Then 13.5 ml of TEOS was added dropwise (1 ml/5 min) into the solution in three batches and the mixture was stirred for 72 h. The centrifugally collected solids were washed with distilled water to remove extra acid and surfactant adsorbed on the surface of the product until the ph became 7. Finally, the product was obtained after drying at 60 0 C for 12 h. Characterization FTIR was carried out using a BIO-RAD FTS165 IR Fourier transform spectrophotometer in potassium bromide discs. The powder XRD patterns were recorded on Rigaku D/MAX-2500 at 40 kv and 100 ma (CuKα radiation). Diffraction data were recorded using continuous scanning with a rate of 1 0 /min. TEM was performed using a JEOL JEM-100CX electron microscope. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No ), and the Youth Foundation of Shanxi (Grant No ). References [1] Guoliang, L.; Yang, X.; Wang, B. et al., Polymer 2008, 49, [2] Pathak, S.; Greci, M.T.; Kwong, R.C. et al., Chem. Mater. 2000,12, [3] Caruso, F.; Lichtenfeld, H.; Giersig, M. et al. J. Am. Chem. Soc. 1998,120, [4] Zhu, G.; Qiu, S.;Osamu Terasaki, et al. J. Am. Chem. Soc. 2001, 123, [5] Zhu, A.; Shi, Z.; Cai, A. et al, Polym. Test. 2008, 27, 540. [6] Okubo, M.; Lu, Y. Colloid Polym. Sci. 1996, 274, [7] Yang, M.; Wang, G.; Yang, Z. Mater. Chem. Phys. 2008, 111, 5. [8] Huo, Q. S.; Ciesla, U.; Margolese, D. et al. Nature, 1994, 368, 317. [9] Wang, L.; Yu, J.; Shi, J. et al. Journal of the Chinese Ceramic Society. 1999, 27, 22 5

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