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Preparation of magnetic nanoparticles modified by amphiphilic copolymers
Materials Letters 60 (2006) 2167 – 2170 www.elsevier.com/locate/matlet
Preparation of magnetic nanoparticles modified by amphiphilic copolymers Huiling Bao a,⁎, Zhiming Chen b , Lin Kang a , Peiheng Wu a , Juzheng Liu b a
Research Institute of Superconductor Electronics (RISE), Department of Electronic Science and Engineering, Nanjing University, Nanjing 210093, P. R. China b Department of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096, P. R. China Received 21 September 2005; accepted 22 December 2005 Available online 13 January 2006
Abstract Coordination method was used to prepare magnetic nanoparticles. Magnetic hysteresis loop studies showed that the obtained nanoparticles are categorized as soft magnetic materials. Microspheres prepared by the coordination method had PBMA core and hydroxylated PGMA shell bonded with Fe ion. The magnetic content and response decreased with the adding rate of the ferrous salt, while the size and size distribution of the microspheres increased. The magnetic response increased with the concentration of the ferrous salt. Also, the influence of the polymer content was studied, which indicated the particle size decreased with the polymer concentration and its distribution increased. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Magnetic; Amphiphilic
1. Introduction Promoted by increasing interest of developing functional polymers, a lot of work has been done to prepare polymeric magnetic nanoparticles (MNPs) [1,2]. The excellent superparamagnetism of polymeric MNPs has been attracting much attention for their potential as tools in biological and medical research [3,4]. A great variety of polymers have been used to prepare magnetic nanoparticles. Polymers with carboxylate [5–8], carboxyl [9–12], styrene [13–17] and vinyl alcohol [18–21] groups were usually adopted recently. And also, more surface modification reactions were carried on to obtain functionalized microspheres [14,15,22–24]. For instance, to enhance the hydrophilic quality, Liu et al. modified magnetic nanoparticles with hydroxyl group [23]. Another interesting bifunctional polymer layer, without layer for steric stabilization and interior layer for solubilization, was obtained by Moeser et al. [24]. Increasing well-defined di- or tri-block copolymers [14,23] were designed to yield polymers with prospective and narrow dispersed polymer chains in which
⁎ Corresponding author. Tel.: +86 25 83592933; fax: +86 25 83592933. E-mail address: [email protected] (H. Bao). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.092
hydrophilic or hydrophobic property can be controlled better. Nicholas et al. [14] synthesized poly(styrene-b-tetraethylenepentamine) (PS-b-TEPA) block copolymers by the controlled radical polymerization routine, in which PS chains connected at one end to single TEPA segments. Then the copolymers would have a polar head–hydrophobic tail structure by this way. On the side of natural polymers such as albumin [16], cellulose [25], and chitosan [26], relative less work has been done. Coating or encapsulation of magnetic particles with polymers is the oldest and simplest method to prepare magnetic microspheres [27]. Other methods include suspending polymerization [28], dispersion polymerization [29], emulsion polymerization [30], etc. The size distribution of particles by these methods is, however, not always satisfactory because of the inherent inhomogeneity of particle size. This does not permit uniform behavior of the particles in a solution or a magnetic field. Ugelstad [31] developed a two-step swelling method that is more suitable in this respect. But the complicated preparation process for nanoparticles makes this method very expensive. The purpose of the present study is to prepare magnetic nanoparticles by coordination between transition metal and functional groups of amphiphilic copolymers. The distribution of particle size is determined by the molecular weight
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evaporation, and the product was dried at 60 °C under vacuum. The block copolymerization was carried out by the successive addition technique. The second monomer (GMA) was directly added until the first monomer had reached the desired conversion. 0.7821 g sample of P(BMA-b-GMA) was dissolved with 10 ml THF, and 3 ml H2O containing 10 drops of HClconc was added dropwise with magnetic stirring. After 30 min, hydroxylated P(BMA-b-GMA) was produced. The copolymer was finally isolated by rotary evaporation. Fig. 1. TEM micrograph of magnetic polymer particles prepared by the coordination method.
distribution (MWD) of the copolymer. A novel living radical polymerization (ATRP) was used to synthesize the copolymer, which would narrow the distribution of particle size effectively. Moreover, compared with other controlled polymerizations such as the anionic polymerization, ATRP is more economic due to the mechanism of radical polymerizations. Since magnetic particles can be easily collected in a magnetic field, coupling of appropriate ligands to above magnetic nanopaticles provides an effective tool to achieve rapid, simple and specific separation. Such magnetic materials can also be applied to cell labeling [32], target medicines [33], enzyme immobility [7,12], and so forth. 2. Experimental 2.1. Materials n-butyl methacrylate (BMA), glycidyl methacrylate (GMA) and 4-methyl-2-pentanone solvent were distilled over MgSO4 under vacuum just before use. Phenanthroline (Phen) was recrystallized twice from acetone and dried under argon. All the other reagents were commercially available and used without further treatment.
2.3. Preparation of magnetic nanoparticles The hydroxylated copolymer was dissolved in 10 ml THF, and 100 ml Fe2+ solution was added. The reaction was continued for 4 h with magnetic stirring at 70 °C. Thereafter, the reaction mixture was cooled to room temperature and the latex was separated from the medium by centrifugation. Then the obtained latex was filtered with methanol, and the precipitation was washed with water and methanol three times in turn. After vacuum dried at 60 °C, the final product was obtained. 2.4. Characterization A drop of latex was placed on a carbon film supported by a copper grid, and the solvent was evaporated naturally at normal temperature. TEM observation was then carried out with a JEOL-2000EX transmission electron microscope, at a voltage of 120 kV. The particle size and size distribution were obtained by the statistical treatment on the TEM micrograph. The weight percentage of the residue remaining from a dried latex sample after thermal analysis up to 600 °C was given as the magnetite content, assuming that the residue is the pure magnetite. The magnetic response was obtained by measuring the attracted altitude for magnetic particles in the presence of a magnetic field. Magnetic hysteresis loop measurements were
2.2. Preparation of hydroxylated P(BMA-b-GMA) by ATRP Polymerization was carried out using schlenk techniques under argon atmosphere. To a dry, 10 ml, round-bottom schlenk flask, with a magnetic stir bar, ligand (4 equivalent for Phen), Cu (4 equivalent) and CuCl2 (2 equivalent) were added. The flask was closed with a stopcock. Then the contents of the flask were placed under vacuum and the flask was backfilled with argon(3×) to remove oxygen. The degassed monomer (BMA) and solvent were then added by syringe technique. After the mixture was allowed to stir at room temperature until it was homogeneous, the initiator was added and the flask was immersed in an oil bath kept at the desired temperature with magnetic stirring. After a certain time, the flask was removed from the oil bath and the mixture was diluted with THF. The solution was passed over a column with neutral alumina to remove the catalyst. Then the rest of the solution was concentrated by rotary
Fig. 2. Histograms of magnetic particles at various particle sizes from 8 to 18 nm.
H. Bao et al. / Materials Letters 60 (2006) 2167–2170
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Table 1 The variation of the properties of magnetic polymer particles with the adding rate of Fe2+
Fig. 3. The magnetic hysteresis loop of magnetic polymer particles prepared by the coordination method.
obtained using a vibrating sample magnetometer at room temperature (VSM; EG&G, Inc., USA). 3. Results and discussion The typical FTIR spectra of the block copolymer before and after the hydrolysis showed that the peaks characteristic of oxirane group located at 1239.46 cm− 1 (symmetric stretching vibration of the ring), 970.40 cm− 1 (asymmetric stretching vibration of the ring) and 765.92 cm− 1 (C–H bending vibration) disappeared completely after the hydrolysis. Instead, a broad and strong peak located at 3446.21 cm− 1 characteristic of hydroxyl group appeared. So it can be confirmed that the oxirane group have been completely hydrolyzed to the hydroxy group. Fig. 1 is a transmission electron micrograph of the magnetic hydroxylated [P(BMA-b-GMA)]Fe particles which are nearly spherical. The dark regions correspond to the magnetic segments where the electron dense is higher. And the light regions correspond to the polymer segments where the electron dense is lower. In the previous work of Wang and Pan [11] on the fine copolymer particles prepared by emulsifier-free emulsion polymerization, they reported that the copolymerization of hydrophilic monomers resulted in the formation of a water soluble polymer layer on the surface of the particles. From this result, we can imagine the presence of flexible polymer chains
Fig. 4. The variation of the magnetic response with the Fe2+ concentration.
Appending rate Magnetic of Fe2+ content (mg/g)
Magnetic response (mm)
Particle size (nm)
Size distribution
0.01 ml/s 0.05 ml/s 1.0 ml/s
1.8 ± 0.2 1.2 ± 0.1 0.8 ± 0.1
11.0 ± 0.3 11.6 ± 0.1 12.8 ± 0.7
11.4 ± 0.9% 13.2 ± 0.2% 14.7 ± 0.5%
53.2 ± 0.6 50.6 ± 0.4 48.3 ± 0.5
protruding from the surface of particles. With the flexibility of the polymer chains increasing, the rigidity of microspheres decreased. So, their shape became more irregular compared to the blank microspheres. The TEM micrograph seems not very clear maybe caused by a relative low resolution of our transmission electron microscope for a magnification of 10,0000× around. The aggregation behavior of particles observed here might be because of competition between interparticle magnetostatic interactions [11,34] and electrostatic repulsion with each other. The possible functional groups such as hydroxyl and carboxyl can bring more remanent charges onto the surface of particles, which produces the electrostatic repulsion between particles and separates them apart from each other. On the other hand, the magnetostatic interactions existing in the magnetic particles may induce undesired aggregation above a threshold concentration of particles. The nature of all those involved factors (e.g. density of charges, value of pH, concentration of particles) is still under our further investigation. Fig. 2 shows histograms of magnetic particles of various particle sizes from 8 to 18 nm that was calculated based on the sizes of 50 particles in different regions of TEM micrograph. It can be seen that the size distribution of particles is narrow, presumably owing to the narrow MWD of polymers synthesized by ATRP. Ferromagnetic substances are classified according to the value of the coercive force (Hc). One class is the soft magnetic material, whose coercive force is small (b 102 Oe). The remanent magnetic induction is small and the magnetic hysteresis loop is long and narrow. Another class is the hard magnetic materials, whose coercive force is large (102∼104 Oe). The remanent magnetic induction is large and the magnetic hysteresis loop is short and wide. It can be inferred from Fig. 3 that the magnetic polymer microspheres can be classified as soft magnetic materials. The coercive force was 32 Oe, the saturation magnetization was 1.19 emu/g, and the shape of the magnetic hysteresis loop was long and narrow.
Fig. 5. The variation of the particle size and size distribution with the polymer concentration: (A) Particle size (B) Size distribution.
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Fig. 4 depicts the variation of the magnetic response of particles with the Fe2+ concentration. As expected, the magnetic response of particles increased with Fe2+ concentration. But the increasing extent had a downtrend because the maximum coordination between Fe2+ and the hydroxyl group would have been reached. As shown in Table 1, the magnetic content and response of products increased with a lower adding rate of Fe2+ due to more sufficient reaction between the polymer and Fe2+. And with the increase of adding rate, Fe2+ would not have adequate time to be dispersed well in the polymer, for which particles were easier to agglomerate. Therefore, the particle size and size distribution increased. The influence of the polymer concentration on the size and size distribution of particles was also investigated. In Fig. 5(a), a decrease in the particle size with the polymer concentration is observed. This is because the amphiphilic polymer not only acted as a component of the particle, but also made the whole system more stable. So, by increasing the content of the amphiphilic polymer, particles would be dispersed into smaller particles instead of agglomerating, and the size of single particle became smaller. Meanwhile, dissociative polymers might exist with increasing the content of polymers, which broadened the distribution of the particle size ( Fig. 5(b) ).
4. Conclusion Magnetic nanoparticles with various sizes from 8 to 18 nm were prepared by the coordination method. The particles can be classified as soft magnetic materials with coercive force of 32 Oe and saturation magnetization of 1.19 emu/g. The magnetic response of particles increased with the Fe2+ concentration. With increasing the adding rate of Fe2+, the magnetic content and response decreased, while the particle size and size distribution increased. A decrease in the particle size with the polymer concentration was observed, and the distribution of the particle size broadened. Acknowledgement The authors acknowledge the excellent technical contribution of Professor Guangjun Shen and Li Shao. References [1] D.K. Kim, Y. Zhang, W. Voit, et al., J. Magn. Magn. Mater. 225 (2001) 30. [2] S.M. Montemayor, L.A. Garcia-Cerda, J.R. Torres-Lubián, Mater. Lett. 59 (2005) 1056.
[3] A. S˘ panová, B. Rittich, D. Horák, et al., J. Chromatogr., A 1009 (2003) 215. [4] D. Thapa, V.R. Palkar, M.B. Kurup, et al., Mater. Lett. 58 (2004) 2692. [5] W.M. Zheng, F. Gao, H.C. Gu, J. Magn. Magn. Mater. 293 (2005) 199. [6] A. Španová, D. Horák, E. Soudková, B. Rittich, J. Chromatogr., B 800 (2004) 27. [7] Z. Guo, S. Bai, Y. Sun, Enzyme Microb. Technol. 32 (2003) 776. [8] M.Y. Arica, H. Yavuz, S. Patir, et al., J. Mol. Catal., B Enzym. 11 (2000) 127. [9] Z.L. Liu, Z.H. Ding, K.L. Yao, et al., J. Magn. Magn. Mater. 265 (2003) 98. [10] A. Ditsch, P.E. Laibinis, D.I.C. Wang, et al., Langmuir 21 (2005) 6006. [11] Y.M. Wang, C.Y. Pan, Eur. Polym. J. 37 (2001) 699. [12] H. Lei, W. Wang, L.L. Chen, et al., Enzyme Microb. Technol. 35 (2004) 15. [13] W.M. Zheng, F. Gao, H.C. Gu, J. Magn. Magn. Mater. 288 (2005) 403. [14] N.A.D. Burke, H.D.H. Stöver, F.P. Dawson, Chem. Mater. 14 (2002) 4752. [15] F. Sauzedde, A. Elaïssari, C. Pichot, Colloid Polym. Sci. 277 (1999) 846. [16] J. Chatterjee, Y. Haik, C.J. Chen, J. Magn. Magn. Mater. 225 (2001) 21. [17] L.C. de S. Maria, M.C.A.M. Leite, M.A.S. Costa, et al., Mater. Lett. 58 (2004) 3001. [18] S. Sindhu, S. Jegadesan, A. Parthiban, et al., J. Magn. Magn. Mater. 296 (2005) 104. [19] A.A. Novakova, V.Y. Lanchinskaya, A.V. Volkov, J. Magn. Magn. Mater. 258 (2003) 354. [20] D. Müller-Schulter, H. Brunner, J. Chromatogr. A 711 (1995) 53. [21] S. Akgöl, Y. Kaçar, A. Denizli, et al., Food Chem. 74 (2001) 281. [22] X.Q. Liu, Y.P. Guan, Z. Ma, et al., Langmuir 20 (2004) 10278. [23] X.H. Yan, G.J. Liu, M. Haeussler, et al., Chem. Mater. 17 (2005) 6053. [24] G.D. Moeser, W.H. Green, P.E. Laibinis, et al., Langmuir 20 (2004) 5223. [25] Z. Bílková, M. Slováková, A. Lyèka, et al., J. Chromatogr., B 770 (2002) 25. [26] E.B. Denkbaş, E. Kiliçay, C. Birlikseven, et al., React. Funct. Polym. 50 (2002) 225. [27] A. Voigt, N. Buske, G.B. Sukhorukov, et al., J. Magn. Magn. Mater. 225 (2001) 59. [28] Z. Guo, S. Bai, Y. Sun, Enzyme Microb. Technol. 32 (2003) 776. [29] X.Y. Liu, X.B. Ding, Z.H. Zheng, et al., J. Appl. Polym. Sci. 90 (2003) 1879. [30] G. Xie, Q.Y. Zhang, Z.P. Luo, et al., J. Appl. Polym. Sci. 87 (2003) 1733. [31] J. Ugelstad, T. Ellirgsen, A. Berge, et al. (1988) USP 4,774,265. [32] N. Wedemeyer, T. Potter, Clin. Genet. 60 (2001) 1. [33] S.C. Goodwin, C.A. Bittner, C.L. Peterson, et al., Toxicol Sci. 60 (2001) 177. [34] A.K. Boal, B.L. Frankamp, O. Uzun, et al., Chem. Mater. 16 (2004) 3252.
Preparation of magnetic nanoparticles modified by amphiphilic copolymers Huiling Bao a,⁎, Zhiming Chen b , Lin Kang a , Peiheng Wu a , Juzheng Liu b a
Research Institute of Superconductor Electronics (RISE), Department of Electronic Science and Engineering, Nanjing University, Nanjing 210093, P. R. China b Department of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096, P. R. China Received 21 September 2005; accepted 22 December 2005 Available online 13 January 2006
Abstract Coordination method was used to prepare magnetic nanoparticles. Magnetic hysteresis loop studies showed that the obtained nanoparticles are categorized as soft magnetic materials. Microspheres prepared by the coordination method had PBMA core and hydroxylated PGMA shell bonded with Fe ion. The magnetic content and response decreased with the adding rate of the ferrous salt, while the size and size distribution of the microspheres increased. The magnetic response increased with the concentration of the ferrous salt. Also, the influence of the polymer content was studied, which indicated the particle size decreased with the polymer concentration and its distribution increased. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Magnetic; Amphiphilic
1. Introduction Promoted by increasing interest of developing functional polymers, a lot of work has been done to prepare polymeric magnetic nanoparticles (MNPs) [1,2]. The excellent superparamagnetism of polymeric MNPs has been attracting much attention for their potential as tools in biological and medical research [3,4]. A great variety of polymers have been used to prepare magnetic nanoparticles. Polymers with carboxylate [5–8], carboxyl [9–12], styrene [13–17] and vinyl alcohol [18–21] groups were usually adopted recently. And also, more surface modification reactions were carried on to obtain functionalized microspheres [14,15,22–24]. For instance, to enhance the hydrophilic quality, Liu et al. modified magnetic nanoparticles with hydroxyl group [23]. Another interesting bifunctional polymer layer, without layer for steric stabilization and interior layer for solubilization, was obtained by Moeser et al. [24]. Increasing well-defined di- or tri-block copolymers [14,23] were designed to yield polymers with prospective and narrow dispersed polymer chains in which
⁎ Corresponding author. Tel.: +86 25 83592933; fax: +86 25 83592933. E-mail address: [email protected] (H. Bao). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.092
hydrophilic or hydrophobic property can be controlled better. Nicholas et al. [14] synthesized poly(styrene-b-tetraethylenepentamine) (PS-b-TEPA) block copolymers by the controlled radical polymerization routine, in which PS chains connected at one end to single TEPA segments. Then the copolymers would have a polar head–hydrophobic tail structure by this way. On the side of natural polymers such as albumin [16], cellulose [25], and chitosan [26], relative less work has been done. Coating or encapsulation of magnetic particles with polymers is the oldest and simplest method to prepare magnetic microspheres [27]. Other methods include suspending polymerization [28], dispersion polymerization [29], emulsion polymerization [30], etc. The size distribution of particles by these methods is, however, not always satisfactory because of the inherent inhomogeneity of particle size. This does not permit uniform behavior of the particles in a solution or a magnetic field. Ugelstad [31] developed a two-step swelling method that is more suitable in this respect. But the complicated preparation process for nanoparticles makes this method very expensive. The purpose of the present study is to prepare magnetic nanoparticles by coordination between transition metal and functional groups of amphiphilic copolymers. The distribution of particle size is determined by the molecular weight
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evaporation, and the product was dried at 60 °C under vacuum. The block copolymerization was carried out by the successive addition technique. The second monomer (GMA) was directly added until the first monomer had reached the desired conversion. 0.7821 g sample of P(BMA-b-GMA) was dissolved with 10 ml THF, and 3 ml H2O containing 10 drops of HClconc was added dropwise with magnetic stirring. After 30 min, hydroxylated P(BMA-b-GMA) was produced. The copolymer was finally isolated by rotary evaporation. Fig. 1. TEM micrograph of magnetic polymer particles prepared by the coordination method.
distribution (MWD) of the copolymer. A novel living radical polymerization (ATRP) was used to synthesize the copolymer, which would narrow the distribution of particle size effectively. Moreover, compared with other controlled polymerizations such as the anionic polymerization, ATRP is more economic due to the mechanism of radical polymerizations. Since magnetic particles can be easily collected in a magnetic field, coupling of appropriate ligands to above magnetic nanopaticles provides an effective tool to achieve rapid, simple and specific separation. Such magnetic materials can also be applied to cell labeling [32], target medicines [33], enzyme immobility [7,12], and so forth. 2. Experimental 2.1. Materials n-butyl methacrylate (BMA), glycidyl methacrylate (GMA) and 4-methyl-2-pentanone solvent were distilled over MgSO4 under vacuum just before use. Phenanthroline (Phen) was recrystallized twice from acetone and dried under argon. All the other reagents were commercially available and used without further treatment.
2.3. Preparation of magnetic nanoparticles The hydroxylated copolymer was dissolved in 10 ml THF, and 100 ml Fe2+ solution was added. The reaction was continued for 4 h with magnetic stirring at 70 °C. Thereafter, the reaction mixture was cooled to room temperature and the latex was separated from the medium by centrifugation. Then the obtained latex was filtered with methanol, and the precipitation was washed with water and methanol three times in turn. After vacuum dried at 60 °C, the final product was obtained. 2.4. Characterization A drop of latex was placed on a carbon film supported by a copper grid, and the solvent was evaporated naturally at normal temperature. TEM observation was then carried out with a JEOL-2000EX transmission electron microscope, at a voltage of 120 kV. The particle size and size distribution were obtained by the statistical treatment on the TEM micrograph. The weight percentage of the residue remaining from a dried latex sample after thermal analysis up to 600 °C was given as the magnetite content, assuming that the residue is the pure magnetite. The magnetic response was obtained by measuring the attracted altitude for magnetic particles in the presence of a magnetic field. Magnetic hysteresis loop measurements were
2.2. Preparation of hydroxylated P(BMA-b-GMA) by ATRP Polymerization was carried out using schlenk techniques under argon atmosphere. To a dry, 10 ml, round-bottom schlenk flask, with a magnetic stir bar, ligand (4 equivalent for Phen), Cu (4 equivalent) and CuCl2 (2 equivalent) were added. The flask was closed with a stopcock. Then the contents of the flask were placed under vacuum and the flask was backfilled with argon(3×) to remove oxygen. The degassed monomer (BMA) and solvent were then added by syringe technique. After the mixture was allowed to stir at room temperature until it was homogeneous, the initiator was added and the flask was immersed in an oil bath kept at the desired temperature with magnetic stirring. After a certain time, the flask was removed from the oil bath and the mixture was diluted with THF. The solution was passed over a column with neutral alumina to remove the catalyst. Then the rest of the solution was concentrated by rotary
Fig. 2. Histograms of magnetic particles at various particle sizes from 8 to 18 nm.
H. Bao et al. / Materials Letters 60 (2006) 2167–2170
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Table 1 The variation of the properties of magnetic polymer particles with the adding rate of Fe2+
Fig. 3. The magnetic hysteresis loop of magnetic polymer particles prepared by the coordination method.
obtained using a vibrating sample magnetometer at room temperature (VSM; EG&G, Inc., USA). 3. Results and discussion The typical FTIR spectra of the block copolymer before and after the hydrolysis showed that the peaks characteristic of oxirane group located at 1239.46 cm− 1 (symmetric stretching vibration of the ring), 970.40 cm− 1 (asymmetric stretching vibration of the ring) and 765.92 cm− 1 (C–H bending vibration) disappeared completely after the hydrolysis. Instead, a broad and strong peak located at 3446.21 cm− 1 characteristic of hydroxyl group appeared. So it can be confirmed that the oxirane group have been completely hydrolyzed to the hydroxy group. Fig. 1 is a transmission electron micrograph of the magnetic hydroxylated [P(BMA-b-GMA)]Fe particles which are nearly spherical. The dark regions correspond to the magnetic segments where the electron dense is higher. And the light regions correspond to the polymer segments where the electron dense is lower. In the previous work of Wang and Pan [11] on the fine copolymer particles prepared by emulsifier-free emulsion polymerization, they reported that the copolymerization of hydrophilic monomers resulted in the formation of a water soluble polymer layer on the surface of the particles. From this result, we can imagine the presence of flexible polymer chains
Fig. 4. The variation of the magnetic response with the Fe2+ concentration.
Appending rate Magnetic of Fe2+ content (mg/g)
Magnetic response (mm)
Particle size (nm)
Size distribution
0.01 ml/s 0.05 ml/s 1.0 ml/s
1.8 ± 0.2 1.2 ± 0.1 0.8 ± 0.1
11.0 ± 0.3 11.6 ± 0.1 12.8 ± 0.7
11.4 ± 0.9% 13.2 ± 0.2% 14.7 ± 0.5%
53.2 ± 0.6 50.6 ± 0.4 48.3 ± 0.5
protruding from the surface of particles. With the flexibility of the polymer chains increasing, the rigidity of microspheres decreased. So, their shape became more irregular compared to the blank microspheres. The TEM micrograph seems not very clear maybe caused by a relative low resolution of our transmission electron microscope for a magnification of 10,0000× around. The aggregation behavior of particles observed here might be because of competition between interparticle magnetostatic interactions [11,34] and electrostatic repulsion with each other. The possible functional groups such as hydroxyl and carboxyl can bring more remanent charges onto the surface of particles, which produces the electrostatic repulsion between particles and separates them apart from each other. On the other hand, the magnetostatic interactions existing in the magnetic particles may induce undesired aggregation above a threshold concentration of particles. The nature of all those involved factors (e.g. density of charges, value of pH, concentration of particles) is still under our further investigation. Fig. 2 shows histograms of magnetic particles of various particle sizes from 8 to 18 nm that was calculated based on the sizes of 50 particles in different regions of TEM micrograph. It can be seen that the size distribution of particles is narrow, presumably owing to the narrow MWD of polymers synthesized by ATRP. Ferromagnetic substances are classified according to the value of the coercive force (Hc). One class is the soft magnetic material, whose coercive force is small (b 102 Oe). The remanent magnetic induction is small and the magnetic hysteresis loop is long and narrow. Another class is the hard magnetic materials, whose coercive force is large (102∼104 Oe). The remanent magnetic induction is large and the magnetic hysteresis loop is short and wide. It can be inferred from Fig. 3 that the magnetic polymer microspheres can be classified as soft magnetic materials. The coercive force was 32 Oe, the saturation magnetization was 1.19 emu/g, and the shape of the magnetic hysteresis loop was long and narrow.
Fig. 5. The variation of the particle size and size distribution with the polymer concentration: (A) Particle size (B) Size distribution.
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Fig. 4 depicts the variation of the magnetic response of particles with the Fe2+ concentration. As expected, the magnetic response of particles increased with Fe2+ concentration. But the increasing extent had a downtrend because the maximum coordination between Fe2+ and the hydroxyl group would have been reached. As shown in Table 1, the magnetic content and response of products increased with a lower adding rate of Fe2+ due to more sufficient reaction between the polymer and Fe2+. And with the increase of adding rate, Fe2+ would not have adequate time to be dispersed well in the polymer, for which particles were easier to agglomerate. Therefore, the particle size and size distribution increased. The influence of the polymer concentration on the size and size distribution of particles was also investigated. In Fig. 5(a), a decrease in the particle size with the polymer concentration is observed. This is because the amphiphilic polymer not only acted as a component of the particle, but also made the whole system more stable. So, by increasing the content of the amphiphilic polymer, particles would be dispersed into smaller particles instead of agglomerating, and the size of single particle became smaller. Meanwhile, dissociative polymers might exist with increasing the content of polymers, which broadened the distribution of the particle size ( Fig. 5(b) ).
4. Conclusion Magnetic nanoparticles with various sizes from 8 to 18 nm were prepared by the coordination method. The particles can be classified as soft magnetic materials with coercive force of 32 Oe and saturation magnetization of 1.19 emu/g. The magnetic response of particles increased with the Fe2+ concentration. With increasing the adding rate of Fe2+, the magnetic content and response decreased, while the particle size and size distribution increased. A decrease in the particle size with the polymer concentration was observed, and the distribution of the particle size broadened. Acknowledgement The authors acknowledge the excellent technical contribution of Professor Guangjun Shen and Li Shao. References [1] D.K. Kim, Y. Zhang, W. Voit, et al., J. Magn. Magn. Mater. 225 (2001) 30. [2] S.M. Montemayor, L.A. Garcia-Cerda, J.R. Torres-Lubián, Mater. Lett. 59 (2005) 1056.
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