pubs.acs.org/Langmuir © 2009 American Chemical Society
Synthesis and Characterization of Magnetic Nanoparticle/Block Copolymer Composites Chieh-Tsung Lo* and Chun-Jung Chao Department of Chemical Engineering, National Cheng Kung University, No. 1, University Road, Tainan City 701, Taiwan, ROC Received September 2, 2009. Revised Manuscript Received October 7, 2009 A simple strategy to improve the particle dispersion and structural ordering of magnetic nanoparticle/block copolymer composites is demonstrated. This method consists of manipulating both the block copolymer structure in a solvent and the relative interactions of particle/solvent and polymer/solvent to facilitate the self-assembly of particles in the preferred domain of the block copolymer. In a neutral solvent, the freely expanded structure of the polymer and the slightly weaker affinity between particle and solvent than between particle and polymer promotes the selection of particles in the preferred domain of the block copolymer. Furthermore, more particles are allowed to be incorporated into the polymer matrix while still obtaining the nicely ordered structure when these particles exhibit smaller magnetization.
Introduction Magnetic nanoparticles have received much attention in nanotechnology as data storage media,1,2 spin-dependent electron transport devices,3 and therapeutic or diagnostic medical functions.4,5 For such applications, it is necessary to fabricate a stable single domain of magnetic particles with uniform distribution. Significant effort has been devoted to synthesizing magnetic nanoparticles. However, not much success is achieved owing to the agglomeration of particles.6 It has recently been suggested that block copolymers with their rich diversity of structures on nanometer length scales may provide an effective means for controlling the magnetic particle location and patterns.7-12 The idea is to anchor particles preferably to one moiety of the block copolymer and let the phase behavior of the block copolymer do the required work in ordering the nanoparticles in the ordered microphases of the polymer matrix. *To whom all correspondence should be addressed. E-mail: tsunglo@ mail.ncku.edu.tw. Tel: þ886-6-2757575 ext 62647. Fax: þ886-6-2344496. (1) Kryder, M. H. Proc. Electrochem. Soc. 2002, 2002-27, 3. (2) Plumer, M. L.; van Ek, J.; Weller, D. The Physics of Ultra-High-Density Magnetic Recording; Springer: New York, 2001; p 249. (3) Black, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S. Science 2000, 290, 1131. (4) Gu, H.; Ho, P.-L.; Tsang, K. W. T.; Wang, L.; Xu, B. J. Am. Chem. Soc. 2003, 125, 15702. (5) Huber, D. L. Small 2005, 1, 482. (6) Grigorova, M.; Blythe, H. J.; Blaskov, V.; Rusanov, V.; Petkov, V.; Masheva, V.; Nihtianova, D.; Martinez, L. M.; Munoz, J. S.; Nikhov, N. J. Magn. Magn. Mater. 1998, 183, 163. (7) Darling, S. B.; Yufa, N. A.; Cisse, A. L.; Bader, S. D.; Sibener, S. J. Adv. Mater. 2005, 17, 2446. (8) Darling, S. B.; Bader, S. B. J. Mater. Chem. 2005, 15, 4189. (9) Park, M. J.; Char, K.; Park, J.; Hyeon, T. Langmuir 2006, 22, 1375. (10) Park, M. J.; Park, J.; Hyeon, T.; Char, K. J. Polym. Sci., Polym. Phys. Ed. 2006, 44, 3571. (11) Char, K.; Park, M. J. React. Funct. Polym. 2009, 69, 546. (12) Xu, C.; Ohno, K.; Ladmiral, V.; Composto, R. J. Polymer 2008, 49, 3568. (13) Chiu, J. J.; Kim, B. J.; Kramer, E. J.; Pine, D. J. J. Am. Chem. Soc. 2005, 127, 5036. (14) Kim, B. J.; Chiu, J. J.; Yi, G.-R.; Pine, D. J.; Kramer, E. J. Adv. Mater. 2005, 17, 2618. (15) Bockstaller, M. R.; Thomas, E. L. Phys. Rev. Lett. 2004, 93, 166106. (16) Lo, C.-T.; Lee, B.; Dietz-Rago, N. L.; Winans, R. E.; Thiyagarajan, P. Macromol. Rapid Commun. 2007, 28, 1607.
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In the literature, a few approaches have been proposed for the incorporation of noble13-17 and semiconductor nanoparticles18-20 and quantum dots21,22 selectively into the preferred domain of block copolymers to create a 2D or 3D spatial distribution of particles in polymer matrices. These initial studies have demonstrated the effectiveness of leveraging the rich phase behavior of block copolymers for ordering nonmagnetic particles in polymer matrices. For magnetic nanoparticles, the challenge is to disperse particles owing to the magnetic dipole-dipole interaction, resulting in a poor selection of particles in the copolymer. In addition, particles tend to aggregate and phase separate from polymer matrices. This causes the disordering of polymer structure. In this letter, we demonstrate a simple approach of using a neutral solvent to improve the particle dispersion and polymer ordering of magnetic particle/block copolymer composites. The basis for this approach is that the freely expanded chains of copolymer in a neutral solvent can provide a good possibility for surfactant-anchored magnetic particles to incorporate into the preferred domain of a block copolymer. In addition, particle/ solvent and particle/polymer interactions also play an important role in self-assembly of this composite. Furthermore, the morphology of composites as a function of particle magnetization and volume fraction was investigated. The results revealed that the order-disorder transition of composites lies in the particle magnetization and composites with particles having less saturation magnetization can accommodate higher particle concentration and still form good particle dispersion in the registry. The knowledge gained to organize magnetic particle/block copolymer composites with long-range order holds great potential in the (17) Lee, B.; Lo, C.-T.; Seifert, S.; Dietz-Rago, N. L.; Winans, R. E.; Thiyagarajan, P. Macromolecules 2007, 40, 4235. (18) Lin, Y.; Boker, A.; He, J.; Sill, K.; Xiang, H.; Abetz, C.; Li, X.; Wang, J.; Emrick, T.; Long, S.; Wang, Q.; Balazs, A.; Russell, T. P. Nature 2005, 434, 55. (19) Zhang, Q.; Gupta, S.; Emrick, T.; Russell, T. P. J. Am. Chem. Soc. 2006, 128, 3898. (20) Yeh, S.-W.; Wei, K.-H.; Sung, Y. S.; Jeng, U.-S.; Liang, K. S. Macromolecules 2005, 38, 6559. (21) Yeh, S.-W.; Wei, K.-H.; Sun, Y.-S.; Jeng, U.-S.; Liang, K. S. Macromolecules 2003, 36, 7903. (22) Li, C.-P.; Wei, K.-H.; Huang, J. Y. Angew. Chem., Int. Ed. 2006, 45, 1449.
Published on Web 10/15/2009
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manipulation and dispersion of magnetic materials in a hierarchical order.
Experimental Section The synthesis of spherical iron nanoparticles was presented elsewhere.23 Briefly, 0.6 mL of Fe(CO)5 (>97%, Sigma-Aldrich) and 5.0 g of trioctylphosphine oxide (TOPO, 99%, SigmaAldrich) were mixed at 70 °C. The resulting solution was added to preheated TOPO (10.0 g) at 340 °C under an argon atmosphere and aged for 30 min at 320 °C to ensure the complete thermal decomposition of Fe(CO)5. As-prepared nanoparticles were purified by the addition of excess acetone. Nanoparticles were mixed with polystyrene-b-poly(2-vinylpyridine) (PS-PVP, Mn values of PS and PVP are 40 500 and 40 000, respectively, PDI=1.10, Polymer Source) in a solvent to prepare particle/PS-PVP composites. The composites were embedded in epoxy and dried in vacuum for 1 day. Samples were annealed at 150 °C for 3 days and then microtomed to ∼80 nm thickness. The specimens were stained using iodine to enhance the contrast between PS and PVP. The structure of the composites was then analyzed using JEOL JEM-1400 and Hitachi H7500 microscopes.
Results and Discussion Figure 1a shows the TEM image of TOPO-tethered nanoparticles, and the average size of these particles is ∼6.1 ( 1.0 nm. Some particles exhibited core-shell structure, which was caused by the oxidation of iron particles during synthesis, forming an Fe core and iron oxide layer. The XRD (Rigaku, RINT-2000) pattern of these particles is shown in Figure 1b. Because magnetite and maghemite have similar XRD patterns, it is difficult to exclude the existence of maghemite in the sample. Therefore, X-ray photon spectroscopy (XPS, Kratos AXIS Ultra DLD) was performed to determine the magnetite and maghemite ratios as shown in Figure 1c. The result of deconvoluted peaks gives Fe3þ/Fe2þ = 0.478:0.522. In Figure 1d, the field dependence of the magnetization for the particles was obtained using a superconducting quantum interface device magnetometer (SQUID, Quantum Design, MPMS-XL) operated at 10 and 300 K. At both temperatures, the magnetization shows a small amount of hysteresis. The saturation magnetization obtained from the M-H curves is 78 emu/g at 10 K and 67 emu/g at 300 K. To synthesize particle/block copolymer composites, the conventional method is to purify and dry particles completely before mixing with polymer. In the system of magnetic particle/block copolymer composites, this approach is not appropriate because magnetic particles tend to aggregate when they are dried as a result of the strong dipole-dipole interaction. This makes it difficult to redisperse particles in a solvent. To improve the particle dispersion in the polymer matrix, we modified the synthesis route to assemble particles with polymer. This approach involved wetting particles with a solvent before they were completely dried during purification. Polymer was then dissolved in the same solvent and mixed with particles to form a composite. We investigated the effect of the solvency on the self-assembly of magnetic particle/PS-PVP composites. Figure 2a shows the composite prepared in toluene, which is a selective solvent for PS. It was found that particles could not be incorporated into any domain of PS-PVP, and the morphology of this composite showed macrophase separation. A previous study on TOPOtethered nonmagnetic particles in PS-PVP revealed that particles prefer to locate in the PVP domains as as result of the stronger (23) Park, S.-J.; Kim, S.; Lee, S.; Khim, Z. G.; Char, K.; Hyeon, T. J. Am .Chem. Soc. 2000, 122, 8581.
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Figure 1. (a) TEM image of iron oxide nanoparticles. (b) XRD of iron oxide nanoparticles. (c) XPS of iron oxide nanoparticles. (d) Corresponding variation of magnetization as a function of the magnetic field for iron oxide nanoparticles. The inset shows an enlargement of the magnetization data. Langmuir 2009, 25(22), 12865–12869
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Figure 2. (a) Magnetic particle/PS-PVP composite prepared in toluene. (b) Schematic model of this composite in toluene. (c) Magnetic particle/PS-PVP composite prepared in pyridine. (d) Schematic model of this composite in pyridine. (e) Magnetic particle/PS-PVP composite prepared in THF. (f) Schematic model of this composite in THF.
affinity of TOPO to PVP.18 When TOPO-tethered particles were mixed with PS-PVP in the PS-selective solvent, PS-PVP formed micelles with PVP in the core and PS in the shell (Figure 2b). This structure causes particles to be unable to assemble into the PVP domains of PS-PVP. In addition, because toluene is also not a good solvent for TOPO-tethered particles, particles were poorly dispersed in toluene and partially formed aggregates. Both effects led to the macrophase separation of the particle/PS-PVP composite. Figure 2c shows the TEM image of the magnetic particle/ PS-PVP composite prepared in pyridine. Because pyridine is a Langmuir 2009, 25(22), 12865–12869
selective solvent for PVP, when PS-PVP was dissolved in pyridine it formed micelles with PS in the core and PVP in the shell (Figure 2d). This structure makes it easier for TOPO-tethered particles to assemble into PVP domains. However, TOPOtethered particles also have the same affinity for pyridine solvent molecules. These corresponding intermolecular interactions of particle/solvent and particle/PVP make the particles exhibit no preference for contact with solvent molecules or PVP chains. However, for particles to be tailored into the polymer, the polymer has to change its conformation, which reduces the entropy of the system. Thus, particles prefer to disperse in DOI: 10.1021/la9032833
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Figure 4. Morphology of magnetic particle/PS-PVP composites as a function of particle volume fraction and magnetization.
Figure 3. Magnetic particle/PS-PVP composites with Ms = 55 emu/g and (a) φ=1.2%, (b) φ=2.4%, and (c) φ=4.5%.
pyridine rather than to attach to the PVP domains. When this composite was deposited on a substrate, particles were separated from the polymer and kinetically trapped. Further annealing could not provide enough driving force to move particles to the PVP domains, and thus particles and PS-PVP macrophase separated. In Figure 2e, the morphology of the magnetic particle/PS-PVP composite prepared in THF is shown. THF is a neutral solvent for PS-PVP, and when PS-PVP was dissolved in THF, the polymer chains expanded to increase the contact with solvent molecules. This promotes the possibility to incorporate particles into PVP domains. Unlike the behavior of particles and PS-PVP in pyridine, THF does not exhibit a strong affinity for the TOPOtethered particles. Therefore, particles tend to come into contact with PVP chains to reduce the enthalpy of the system (Figure 2f). This behavior causes particles to locate in PVP domains. With further annealing, the particle/PS-PVP composite formed a wellordered structure with particles dispersed in PVP domains. We further investigated the effect of particle magnetization on the morphology of magnetic particle/PS-PVP composites. In this 12868 DOI: 10.1021/la9032833
study, particles with different saturation magnetization (Ms) were prepared at different flow rates of Ar gas. The synthesized particles had similar diameters between 6 and 12 nm, and their Ms values were 67, 55, and 15 emu/g. Figure 3 shows the morphology of composites with an Ms value for the particles of 55 emu/g. When the particle volume fractions in the composite (φ) were 1.2% (Figure 3a) and 2.4% (Figure 3b), particles were located in the PVP domains and the morphology of the composites exhibited ordered lamellae. With an increase in φ to 4.5% (Figure 3c), particle aggregation occurred, and that induced an order-disorder transition in polymer morphology. The morphology of magnetic particle/PS-PVP composites as a function of φ and Ms is shown in Figure 4. It was found that the order-disorder transition of composites depends strongly on the Ms of the particles. When the Ms of the particles is high, the morphology of the composites is disordered with even a small addition of particles. In contrast, when a block copolymer is mixed with particles that exhibit small Ms, the composites are able to accommodate more particles and still form an ordered structure. The morphology of magnetic particle/PS-PVP composites depends on the free energy of mixing of particles and PS-PVP. In addition, van der Waals and magnetic dipole-dipole interactions also contribute to the self-assembly of composites. The free energy of mixing is a function of the volume fraction of each component, the molecular weights of both PS and PVP, the size of particles, and temperature. The van der Waals interaction depends on the particle size and the separation distance between particles. In our systems, the only parameter varied was the particle magnetization, which was independent of both the free energy of mixing and the van der Waals interaction. Thus, the discrepancy in the order-disorder transition of these composites is attributed to the magnetic dipoledipole interaction between particles. When the interaction of particles is high, particles tend to aggregate, resulting in a disordered composite. When the interaction of particles is low, particles prefer to disperse in the polymer matrix to reduce the conformational entropy of the system. This leads to the dispersion of particles in the preferred domain of the block copolymer. Therefore, the critical volume fraction of particles in inducing an order-disorder transition of composites increases with decreasing particle magnetization. This behavior can also be confirmed by the study on nonmagnetic nanoparticle/block copolymer composites where the critical volume fraction of particles needed to induce an order-disorder transition of composites is much higher than that in magnetic particle/block copolymer composites.18 Langmuir 2009, 25(22), 12865–12869
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Conclusions In summary, we have developed a simple approach to improve the dispersion of magnetic particles in the ordered domains of block copolymer. This method relies on the manipulation of both block polymer structure in a solvent and the relative interactions of particle/solvent and polymer/solvent to facilitate the selfassembly of particles in the preferred domain of block copolymer. We also investigated the effect of particle magnetization on the morphology of particle/block copolymer composites. The results showed that the critical particle volume fraction needed to induce the order-disorder transition of composites depends strongly on the particle magnetization and the addition of particles with a smaller saturation magnetization enables the composites to tailor
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the higher particle concentration while obtaining a good dispersion of nanoparticles in the polymer matrix. We believe that our accomplishments in tailoring magnetic nanoparticles into arrays in an ordered polymer phase will provide exciting new possibilities on the materials front. Acknowledgment. This work was funded by National Science Council of the Republic of China under grant no. NSC 96-2218E-006-290. Supporting Information Available: FTIR of neat TOPO and TOPO-tethered nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.
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