Controllable preparation of magnetic polymer microspheres with different morphologies by miniemulsion polymerization

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1 Journal of Colloid and Interface Science 308 (2007) Controllable preparation of magnetic polymer microspheres with different morphologies by miniemulsion polymerization Ying Sun, Biao Wang, Huaping Wang, Jianming Jiang State Key Laboratory for Modification of Chemical Fiber and Polymers, College of Material Science and Engineering, Donghua University, Shanghai , People s Republic of China Received 1 October 2006; accepted 16 December 2006 Available online 6 February 2007 Abstract Magnetic poly(styrene-co-acrylic acid-co-acrylamide) microspheres were prepared by water-in-oil-in-water (W/O/W) miniemulsion polymerization of monomers in the presence of Fe 3 O 4 nanoparticles. The copolymerizable monomers of acrylic acid and acrylamide were used not only to modify the surfaces of the microspheres with functional groups, but also to act as viscosity regulators to control the morphology and size of these microspheres. It was experimentally observed that the surfaces of these microspheres were functionalized with NH 2 groups produced by copolymerization, the morphologies (sphere, ringlike, and one-hole) of the microspheres were controlled by the concentration of copolymerizable monomers, and all samples prepared were superparamagnetic. The possible mechanism of formation of these magnetic microspheres is also discussed Elsevier Inc. All rights reserved. Keywords: W/O/W miniemulsion; Magnetic polymer microspheres; Preparation; Morphology; Particle size; Magnetic properties 1. Introduction Magnetic polymer microspheres are usually composed of magnetic cores to ensure a strong magnetic response and polymeric shells to provide favorable functional groups and protect from particle aggregation. These microspheres exhibit many unique features such as small and uniform size, different shapes and morphologies, and various functional groups on the surface, and hence have received much attention in recent years for wide potential applications such as enzyme immobilization [1,2], cell and protein separations [3], and drug delivery processes [4,5]. Among these applications, it is becoming increasingly apparent that the key issues are surface modification and morphology control. Therefore, synthesis of surfacefunctionalized magnetic microspheres with controllable morphology is particularly important both for fundamental studies and for applications. In the past few years, several methods have been developed for synthesis of these materials, including solvent evap- * Corresponding author. Fax: address: wbiao2000@dhu.edu.cn (B. Wang). oration [6], dispersion or suspension polymerization [7 9], microemulsion polymerization [10], and Ugelstad s two-step swelling method [11 13]. Mostly,Fe 3 O 4 has been used as the magnetic core and polymers such as polystyrene [14], poly(glycidyl methacrylate) [15], and polyvinyl alcohol [16] have been used as the shells. Recently, micro- or miniemulsion polymerization has been widely used due to its easiness to obtain surface-functionalized microspheres with uniform particle dispersion. For example, Liu et al. [17] successfully prepared magnetic microspheres with NH 2 groups located on the surface by microemulsion polymerization. Cui et al. [18] synthesized superparamagnetic nanorings of polystyrene/fe 3 O 4 composite by miniemulsion polymerization, but they did not mention how to control the morphology and their results did not show any functional groups (such as NH 2 ) on the spheres surfaces. According to the published literature, we find that little attention has been paid to controlling the morphology and size of the surface-functionalized magnetic microspheres. Here, we first report a simple one-step method to prepare magnetic polymer microspheres that have both controllable morphologies and NH 2 groups located on their surface. The properties of these /$ see front matter 2007 Elsevier Inc. All rights reserved. doi: /j.jcis

2 Y. Sun et al. / Journal of Colloid and Interface Science 308 (2007) resulting magnetic microspheres are also analyzed and a possible forming mechanism is presented. 2. Experimental 2.1. Materials Styrene (St) (China Medical Chemical Reagents Company, Shanghai, China) and acrylic acid (AA) (Shanghai Lingfeng Chemical Reagents Company, Shanghai, China) were vacuumdistilled before use. All other materials, such as potassium persulfate (KPS), sodium dodecyl benzene sulfonate (SDBS), and acrylamide (AM), were purchased from the Shanghai Chemical Reagents Company (Shanghai, China) and were used without further purification Preparation of magnetic polymer microspheres Fe 3 O 4 magnetic nanoparticles (Ferrofluid) were prepared using the method described in our previous work [14], based on the classical coprecipitation procedure. The diameter of Fe 3 O 4 particles was about 15 nm. The magnetic polymer microspheres were prepared using miniemulsion polymerization. First, 0.4 g Fe 3 O 4 nanoparticles were dispersed in 10 ml 4.8 wt% KPS aqueous solution. The reaction mixture was sonicated for 10 min and then was kept at room temperature for 15 h. Second, a new suspension was obtained by adding 1.0 ml Span80, 1.0 ml liquid paraffin, and 16 ml St to the mixture solution. After that, 0.6 g SDBS, 15 ml ethanol, and different amounts of AM and AA were added into the suspension with stirring. Last, the polymerization was carried out at 70 C under nitrogen for 10 h with stirring (300 rpm). The obtained magnetic polymer microspheres were collected using a magnet and washed with deionized water and ethanol repeatedly. Five samples prepared with different amounts of AM and AA added are shown in Table 1. Transmission electron microscopy (TEM, Hitachi, H-800) and scanning electron microscopy (SEM, JSM-5600LV) were used to study the morphologies of the microspheres. A vibrating-sample magnetometer (LakeShore, VSM7400) and thermogravimetric analysis (TGA, TA, TGA2050) were used to characterize the magnetic properties and thermal stability of the magnetic polymer microspheres, respectively. The TGA measurement was carried out with a heating rate of 20 C/min in the nitrogen flow. A standard Ubbelohde viscometer (φ 0.46 mm) was applied to measure the specific viscosity of the reaction system with different amounts of AA and AM at a temperature of 30 ± 0.1 C. 3. Results and discussion Fig. 1 shows FTIR spectra of Fe 3 O 4 nanoparticles, poly(stco-aa-co-am), and magnetic poly(st-co-aa-co-am) microspheres (sample 1). In Fig. 1a, the characteristic absorption band of Fe 3 O 4 appears at 585 cm 1, and it also appears in the spectrum of magnetic poly(st-co-aa-co-am) microspheres (Fig. 1c). In Fig. 1c, there are many other peaks, most of which are the characteristic absorption bands of poly(st-co-aa-co- AM) (Fig. 1b). For example, the peaks at 750 and 700 cm 1 are the absorption bands of St. The peaks at 1705 and 1451 cm 1 are the absorption bands of carboxyl groups of AA. Furthermore, the absorption band at 1601 cm 1 is assigned to the peaks of the NH 2 group. The FTIR spectra of other samples show the same features as those of sample 1. These results indicate that the functional groups ( NH 2 ) are located on the microspheres surfaces and Fe 3 O 4 nanoparticles disperse in the polymer matrix. Fig. 2 shows the SEM and TEM images of magnetic poly- (St-co-AA-co-AM) microspheres (sample 1). The solid microspheres with perfect sphere-shaped morphologies have a very large average size (about 1000 nm in diameter), which is 2.3. Characterization of poly(st-co-aa-co-am) magnetic microspheres A particle size distribution apparatus (Malvern, HPPS) was used to determine the size and size distribution of the magnetic polymer microspheres. A Fourier transform infrared spectroscope (FTIR, Bruck, EQUINOX55) was used to characterize the organic groups on the surfaces of microspheres. The microspheres were dried, and the powders were mixed with KBr and pressed to a plate for measurement. Table 1 Samples with different amounts of AA and AM added and their average sizes Samples St (ml) AM (g) AA (ml) Average size (nm) a a Diameters of the samples measured by Malvern HPPS particle size distribution apparatus. Fig. 1. FTIR spectra of Fe 3 O 4 (a), poly(st-co-aa-co-am) (b), and magnetic poly(st-co-aa-co-am) microspheres (c).

3 334 Y. Sun et al. / Journal of Colloid and Interface Science 308 (2007) Fig. 3. TGA curves of Fe 3 O 4 (a) and magnetic polymer microspheres: (b) sample 3, (c) sample 1, and (d) sample 5. Fig. 4. Magnetization curve of magnetic polymer microspheres. Fig. 2. SEM (a) and TEM (b) images of magnetic polymer microspheres (sample 1). almost consistent with the size measured using a size distribution apparatus (see Table 1). In Fig. 2b, some of the Fe 3 O 4 particles in the polymer matrix can be observed. Fig. 3 shows the TGA curves of microspheres. As can be seen from Fig. 3, the content of Fe 3 O 4 in the magnetic polymer microspheres is about 25 wt%, which is higher than the results reported by Zheng et al. [19]. The effect of the amount of AA and AM on the microsphere s size is presented in Table 1. The average size decreases with the increase of AA added, which indicates that the size of these magnetic microspheres can be controlled by the quantity of copolymerizable monomers (AM or/and AA). Fig. 4 shows a typical magnetization curve of magnetic poly(stco-aa-co-am) microspheres. The saturation magnetization of these microspheres is about 30.9 emu/g and no hysteresis loop was observed, indicating that these samples are superparamagnetic. Interestingly, the content of copolymerizable monomers (AM and AA) affects not only the size (see Table 1) but also the morphologies of the microspheres. The ring-like microspheres (Figs. 5a and 5b) can be obtained by adding 2.5 g AM and 3 ml AA (sample 3), which is 2 ml more than for sample 1. From Fig. 5, many free magnetic particles that are not contained in the polymer matrix are observed. When 3 ml AA and 1.0 g AM are added to the reactive system, the obtained microspheres (sample 5) show one-hole morphologies (Figs. 6a and 6b). Furthermore, the nanoparticles disperse uniformly in the polymer matrix (Fig. 6b). These results indicate that the morphology of these microspheres could be controlled by adjusting the content of copolymerizable monomers in the reaction system. Although the formation mechanism of these morphologies is still not clear in the present stage, we think that the viscosity of reaction system plays a very important role in the morphology formation. In this work, Span80 and liquid paraffin were used as primary surfactants (lyophilic surfactants) and SDBS was used as secondary surfactant (hydrophilic surfactant). When the W/O/W miniemulsion formed, the internal and external water phases with dissolved AA and AM were separated by an oil phase in which St dissolved. The small internal water droplets with a primary surfactant-stabilizing layer disperse in the oil phase, which, in turn, disperses in the external water phase. The viscosity of the water phases varies with the amounts of AM and AA. Fig. 7 shows the relationship between the viscosity and the AA and AM amounts in the water phases. From Fig. 7,

4 Y. Sun et al. / Journal of Colloid and Interface Science 308 (2007) Fig. 5. SEM (a) and TEM (b) images of magnetic polymer microspheres (sample 3). Fig. 6. SEM (a) and TEM (b) images of magnetic polymer microspheres (sample 5). the viscosity increases with the amount of AA or AM. In the case of lower viscosity, the internal water droplets have tended to move out and combine with the external water phase, since the internal water droplets are under inherent thermodynamic instability in W/O/W miniemulsion systems [20]. A small increase of viscosity could enhance the stability of the internal water droplets and hence make them difficult to move out to the external water phase. As a result, the ringlike or one-hole morphologies could form due to the viscosity change of the water phases. 4. Conclusions In summary, the poly(st-co-aa-co-am) magnetic microspheres with amino groups have been prepared by using a simple one-step method. Fe 3 O 4 nanoparticles used as the magnetic core have been incorporated during polymerization. Solid Fig. 7. Relationship between the viscosity and the AA and AM amounts in water phases.

5 336 Y. Sun et al. / Journal of Colloid and Interface Science 308 (2007) sphere, ringlike, and one-hole structural morphologies of these microspheres are obtained by adjusting the contents of the copolymerizable monomers (AM and AA) in the reaction system. The viscosity of the internal water phases may be the main factor in the formation of the microspheres morphologies. The presence of amino groups can be used to immobilize proteins or enzymes, and the special structural morphologies (e.g., onehole structure) are favorable for drugs to be loaded and released. Therefore, the prepared microspheres may be useful in many areas of high technology, such as targeted drug delivery, affinity separation of biomolecules, and diagnostic applications. Acknowledgments This work was financially supported by the Special Nano Foundation of Shanghai Technology Commission (No nm064) and the Science and Technology Development Foundation of Donghua University. References [1] H.P. Khng, D. Cunliffe, S. Davies, N.A. Turner, E.N. Vulfson, Biotechnol. Bioeng. 60 (4) (1998) 419. [2] P. Dunnil, M.D. Lilly, Biotechnol. Bioeng. 16 (1974) 987. [3] J. Ugelstad, A. Berge, T. Ellingsen, T.N. Nilsen, P.C. Mork, P. Stenstad, E. Hornes, O. Olsvik, Prog. Polym. Sci. 17 (1992) 87. [4] L. Chen, W.J. Yang, C.Z. Yang, J. Mater. Sci. 32 (1997) [5] H. Yu, J.W. Raymonda, T.M. McMahon, A.A. Campagnari, Biosens. Bioelectron. 14 (2000) 829. [6] T. Bahar, S.S. Celibi, J. Appl. Polym. Sci. 72 (1999) 69. [7] D. Horák, J. Boháček, M. Šubrt, J. Polym. Sci. 38 (2000) [8] Z.Y. Ma, Y.P. Guan, X.Q. Liu, H.Z. Liu, J. Appl. Polym. Sci. 96 (2005) [9] J. Ugelstad, A. Berge, T. Ellingsen, T.N. Nilsen, P.C. Mork, P. Stenstad, E. Hornes, O. Olsvik, Prog. Polym. Sci. 17 (1992) 87. [10] Y.H. Deng, L. Wang, W.L. Yang, S.K. Fu, E. Abdelhamid, J. Magn. Magn. Mater. 24 (2003) 920. [11] J. Ugelstad, K.H. Kaggerud, E.K. Hansen, A. Berge, Makromol. Chem. 180 (1979) 737. [12] J. Ugelstad, P.C. Mork, K.H. Kaggerud, T. Ellingsen, A. Berge, Adv. Colloid Interface Sci. 13 (1980) 101. [13] J. Ugelstad, P.C. Mork, I. Nordhuus, H. Mfutakamba, E. Soleimany, T. Ellingsen, A. Berge, A.A. Khan, Makromol. Chem. Suppl. 10 (1985) 215. [14] Y. Sun, B. Wang, C. Hui, H.P. Wang, J.M. Jiang, J. Macromol. Sci. Phys. 45 (2006) 653. [15] D. Horák, N. Benedyk, J. Polym. Sci. A Polym. Chem. 42 (2004) [16] S. Akgől, Y. Kaçar, A. Denizli, M.Y. Arica, Food Chem. 74 (2001) 281. [17] Z.L. Liu, X.B. Yang, K.L. Yao, G.H. Du, Z.S. Liu, J. Magn. Magn. Mater. 302 (2006) 529. [18] L.L. Cui, H.C. Gu, H. Xu, D.L. Shi, Mater. Lett. 60 (2006) [19] W.M. Zheng, F. Gao, H.C. Gu, J. Magn. Magn. Mater. 293 (2005) 199. [20] J.W. Kim, Y.G. Joe, K.D. Suh, Colloid Polym. Sci. 277 (1999) 252.