Langmuir 2008, 24, 5485-5491
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Synthesis of Ellipsoidal Hematite/Silica/Polymer Hybrid Materials and the Corresponding Hollow Polymer Ellipsoids Guangyu Liu,†,‡ Xinlin Yang,*,† and Yongmei Wang‡ Key Laboratory of Functional Polymer Materials, the Ministry of Education, Institute of Polymer Chemistry, Nankai UniVersity, Tianjin 300071, China, and Department of Chemistry, Nankai UniVersity, Tianjin 300071, China ReceiVed NoVember 10, 2007 Ellipsoidal trilayer hematite/silica/poly(divinylbenzene) hybrid particles were prepared by distillation precipitation polymerization of divinyl benzene (DVB) in the presence of hematite/3-(methacryloxy)propyl trimethoxysilane (MPS)modified silica (SiO2) core-shell particles as the seeds. The polymerization of DVB was performed in neat acetonitrile with 2,2′-azobisisobtyronitrile (AIBN) as initiator to coat the hematite/MPS-modified SiO2 seeds through the capture of DVB oligomer radicals with the aid of a vinyl group on the surface of the hematite/MPS-modified silica core-shell particles in the absence of any stabilizer or surfactant. The other hematite/silica/polymer trilayer hybrid particles with different polarity and various functionality, such as hematite/silica/poly(ethylene glycol dimethacrylate) and hematite/ silica/poly(divinyl benzene-co-methacrylic acid) could also be prepared by this procedure. Hematite/silica/poly(N,N′methylenebisacrylamide) composite particles could be prepared with unmodified hematite/silica particles as seeds. Hollow poly(divinyl benzene) (PDVB) and poly(N,N′-methylenebisacrylamide) (PMBAAm) ellipsoids with movable hematite cores were subsequently developed after the selective etching of the silica midlayer in diluted hydrofluoric acid from hematite/silica/PDVB and hematite/silica/PMBAAm trilayer hybrids. Hollow PDVB ellipsoids were obtained by removal of the silica midlayer and hematite core of the trilayer hybrids with concentrated HF solution. The resultant trilayer hybrid particles and hollow polymer ellipsoids were characterized by transmission electron microscopy and vibrating sample magnetometer.
Introduction The development of materials with novel structure has been a fundamental focus of chemical research, which promotes the advancement in both academic and industrial fields. In recent years, organic-inorganic hybrid materials, which combine the properties of inorganic and organic components within a single material, have attracted expanding interest of material scientists due to their advantages of the organic polymer matrix having facile processabilities, flexibilities, and various functional groups, together with characteristics of the inorganic particles in terms of mechanical strength, modulus, and thermal stability.1,2 The organic-inorganic hybrid materials can exhibit novel and excellent properties, such as mechanical, chemical, electrical, rheological, magnetic, optical, and catalytic, by varying the composition, dimensions, and structures, which have proven the diverse applications as drug-delivery system, diagnostic, coating, and catalyst.3–8 Hematite (R-Fe2O3), which is the most stable iron oxide under ambient conditions with n-type semiconducting properties, low * Corresponding author. Tel:+86-22-23502023. Fax: +86-22-23503510. E-mail:
[email protected]. † Institute of Polymer Chemistry. ‡ Department of Chemistry. (1) Sanchez, C.; Soler-Illia, G. J. A. A.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061. (2) Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. Angew. Chem., Int. Ed. 2006, 45, 3216. (3) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Chem. Mater. 2001, 13, 109. (4) Fleming, M. S.; Mandal, T. K.; Walt, D. R. Chem. Mater. 2001, 13, 2210. (5) Zhu, J.; Morgan, A. B.; Lamelas, F. J.; Wilkie, C. A. Chem. Mater. 2001, 13, 3774. (6) Zhou, J.; Zhang, S. W.; Qiao, X. G.; Li, X. Q.; Wu, L. M. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 3202. (7) Qi, L.; Chapel, J.-P.; Castaing, J.-C.; Fresnais, J.; Berret, J.-F. Langmuir 2007, 23, 11996. (8) Hu, Y. Q.; Wu, H. P.; Gonsadves, K. E.; Merhari, L. Microelectron. Eng. 2001, 56, 289.
cost and high resistance to corrosion, has been extensively utilized with great scientific and technical importance in the production of gas sensors, catalysts, pigments, magnetic recording media, anticorrosive agents, and lithium ion batteries.9–15 Many efforts have been devoted to the incorporation of magnetic particles into core-shell structures in order to control the particle’s shape, size, and magnetic properties.16–18 For instance, solvent-free atom transfer radical polymerization has been used for the synthesis of Fe2O3@polystyrene (PS) core-shell nanoparticles with welldefined shape.19 Zeolite capsules encapsulated with Fe2O3 have been prepared by the wet impregnation technique.20 Colloidal polypyrrole-magnetite-silica nanoparticles have been synthesized by aqueous deposition of silica onto ultrafine (5-20 nm) magnetite particles via controlled hydrolysis of sodium silicate with the subsequent oxidant polymerization of pyrrole using various oxidants in the presence of silica-coated magnetite particles.21 The uniform coating of silica onto ultrafine hematite particles has altered most properties of the iron oxide particles, (9) Dimitrov, D. V.; Hadjipanayis, G. C.; Papaefthymiou, V.; Simopoulos, A. J. Magn. Magn. Mater. 1998, 188, 8. (10) Liu, X. Q.; Tao, S. W.; Shen, Y. S. Sens. Actuators, B 1997, 40, 161. (11) Chen, J.; Xu, L. N.; Li, W. Y.; Gou, X. L. AdV. Mater. 2005, 17, 582. (12) Huo, L.; Li, W.; Lu, L.; Cai, H.; Xi, S.; Wang, J.; Zhao, B.; Shen, Y.; Lu, Z. Chem. Mater. 2000, 12, 790. (13) Gondal, A. M. A.; Hameed, A.; Yamani, Z. H.; Suwaiyan, A. Chem. Phys. Lett. 2004, 385, 111. (14) Jiang, J. Z.; Lin, R.; Lin, W.; Nielsen, K.; Mørup, S.; Dam-Johansen, K.; Clasen, R. J. Phys. D: Appl. Phys. 1997, 30, 1459. (15) Sun, H. T.; Cantalini, C.; Faccio, M.; Pelino, M.; Catalano, M.; Tapfer, L. J. Am. Ceram. Soc. 1996, 79, 927. (16) Levy, L.; Sahoo, Y.; Kim, K. S.; Bergey, E. J.; Prasad, P. N. Chem. Mater. 2002, 14, 3715. (17) Sacanna, S.; Rossi, L.; Philipse, A. P. Langmuir 2007, 23, 9974. (18) Xia, A.; Hu, J. H.; Wang, C. C.; Jiang, D. L. Small 2007, 3, 1811. (19) Wang, Y.; Teng, X. W.; Wang, J. S.; Yang, H. Nano Lett. 2003, 3, 789. (20) Dong, A. G.; Wang, Y. J.; Tang, Y.; Ren, N.; Zhang, Y. H.; Gao, Z. Chem. Mater. 2002, 14, 3217. (21) Butterworth, M. D.; Bell, S. A.; Armes, S. P.; Simpson, A. W. J. Colloid Interface Sci. 1996, 183, 91.
10.1021/la8002004 CCC: $40.75 2008 American Chemical Society Published on Web 04/16/2008
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such as the dispersibility in aqueous or nonaqueous media,22 in which the anisometric shape of the hematite cores was preserved with benefits via introduction of other functional materials onto the particles. Hematite/silica/polypyrrole (R-Fe2O3/SiO2/PPy) ellipsoidal sandwich composite spheres as well as SiO2, SiO2/ PPy, PPy hollow capsules, and PPy ellipsoidal hollow capsules with movable hematite cores have been successively afforded from hematite olivary particles through the oxidative polymerization of Py in the presence of FeCl3 as oxidant with subsequent selective removal of the inner cores.23 Polymer hollow microspheres, which have been synthesized by a variety of physical and chemical techniques,24–27 have also attracted much attention for their wide applications, such as encapsulation for controlled release of drugs and enzymes, fillers, pigments, catalysts, and adsorption materials for sound. Nonspherical core-shell particles and the corresponding hollow particles have scarcely been reported in the literature; the suitable nonspherical template (which is difficult to get)23,28 would be more attractive for applications as ordered arrays due to the lower symmetries as compared to the spherical particles. Most of the nonspherical particles and their ordered assembly structures have been forcibly obtained from the original spherical ones under ion irradiation or mechanical pressure.29 It is still a challenge to get nonspherical polymer particles with a well-defined shape and a controllable size via facile polymerization. In our previous work, distillation precipitation polymerization has been successfully developed as a facile and powerful technique for the preparation of monodisperse inorganic/organic core-shell composite/hybrid microspheres.30,31 Here, we introduce distillation precipitation polymerization as a facile method for the synthesis of ellipsoidal hematite/silica/polymer trilayer hybrid materials and further development of the corresponding polymer hollow ellipsoidal particles and polymer hollow ellipsoidal particles with movable hematite particles by complete removal of the inorganic cores under concentrated hydrofluoric acid (40 wt %) aqueous solution and selective removal of the silica midlayer under dilute HF (1 wt %) aqueous solution, respectively.
Experimental Section Chemicals. Ferric chloride (FeCl3·6H2O) and poly(N-vinyl pyrrolidone) (PVP, K30, Mr ) 10 000) were purchased from Tianjin Gangfu Fine Chemical Engineering Institute. Tetraethyl orthosilicate (Si(OEt)4, TEOS) was bought from Aldrich and used without further purification. Ammonia (25%, aqueous solution) was purchased from Tianjin Dongzheng Fine Chemical Reagent Factory, China. 3-(Methacryloxy)propyltrimethoxysilane (MPS) was purchased from Aldrich and distilled under vacuum. Divinyl benzene (DVB, 80% divinylbenzene isomers) was supplied as technical grade by Shengli Chemical Technical Factory, Shandong, China and was washed with 5% aqueous sodium hydroxide and water then dried over anhydrous magnesium sulfate prior to utilization. Ethylene glycol dimethacrylate (EGDMA) was chemical grade available from Heshibi Chemical Eng. Co., Ltd., Shanghai, China and was used without further purification. N,N′-Methylenebisacrylamide (MBAAm, chemical grade, Tianjin Bodi Chemical Engineering Co.) was recrystallized from acetone. Methacrylic acid (Tianjin Reagent Factory I, China) (22) Furusawa, K.; Matsumura, H.; Majima, T. J. Colloid Interface Sci. 2003, 264, 95. (23) Hao, L. Y.; Zhu, C. L.; Jiang, W. Q.; Chen, C. N.; Hu, Y.; Chen, Z. Y. J. Mater. Chem. 2004, 14, 2929. (24) Caruso, F. AdV. Mater. 2001, 13, 11. (25) Gill, I.; Ballesteros, A. J. Am. Chem. Soc. 1998, 120, 8587. (26) Zhang, J.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 7908. (27) Li, G. L.; Yang, X. L.; Bai, F. Polymer 2007, 48, 3074. (28) Gabrielson, L.; Folekes, M. J. J. Mater. Sci. 2001, 36, 1. (29) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2374. (30) Liu, G. Y.; Yang, X. L.; Wang, Y. M. Polymer 2007, 48, 4385. (31) Liu, G. Y.; Zhang, H.; Yang, X. L.; Wang, Y. M. Polymer 2007, 48, 5896.
Liu et al. was purified by vacuum distillation before use. 2,2′-Azobisisobutyronitrile (AIBN) was provided by Chemical Factory of Nankai University and recrystallized in methanol. Hydrofluoric acid (containing 40 wt % of HF) was obtained from Tianjin Chemical Reagent Institute. Acetonitrile (analytical grade, Tianjin Chemical Reagents II Co.) was dried over calcium hydride and purified by distillation before use. All the other reagents were analytical grade and were used without any further treatment. Synthesis of Ellipsoidal Hematite/MPS-Modified Silica Core-Shell Particles. Ellipsoidal hematite/silica (R-Fe2O3/SiO2) core-shell particles were prepared according to the method in the literature.23 Shuttlelike monodisperse hematite particles were obtained via aging the aqueous solution of 2 × 10-2 M FeCl3 and 4 × 10-4 M NaH2PO4 at 95 °C for 3 days. The formation of hematite particles was reflected as the appearance of a brick-reddish color in the hydrolysis system. The resultant hematite particles were centrifuged, decanted, and redispersed in water three times then dried at 50 °C under vacuum until of constant weight. The synthesis of R-Fe2O3/SiO2 core-shell particles was performed by a modified Sto¨ber method via the encapsulation of a silica shell onto R-Fe2O3 seeds with the aid of PVP under mildly basic conditions. A typical procedure for such a hydrolysis was as follows: 0.10 g of shuttlelike hematite particles was dispersed in 50 mL of ethanol containing 0.05 g of PVP (K30) with ultrasound irradiation. The mixture was allowed to stand for 24 h for adequate adsorption of the PVP onto the surface of the hematite particles. Unadsorbed PVP was removed by three cycles of centrifugation/dispersion in ethanol. The PVPmodified R-Fe2O3 particles were used as seeds to be coated with a silica shell by the Sto¨ber method: 30 mg of hematite particles was dispersed in a mixture of 16 mL of water, 80 mL of ethanol, and 2 mL of aqueous ammonia solution (25 wt %), then 0.8 mL of TEOS was dropped with rapid stirring into the above mixture, and the hydrolysis was continued further for 24 h with stirring. MPS (1.0 g, 4. 0 mmol) was introduced into 20 mL of the ethanol suspension of hematite/silica particles (0.5 g) under stirring in a 50-mL round-bottom flask. Coating of hematite/silica particles with MPS was achieved by stirring the mixture of hematite/silica particles and MPS for 48 h at room temperature. The resultant MPS-modified hematite/silica particles were purified by three cycles of centrifugation, decantation, and resuspension in ethanol to remove the excessive MPS. The final MPS-modified hematite/silica particles were dried in a vacuum oven at 50 °C until of constant weight. Preparation of Ellipsoidal Hematite/Silica/PDVB Trilayer Hybrids by Distillation Precipitation Polymerization. A typical procedure for the distillation precipitation polymerization: In a dried, 50-mL, two-necked flask, 0.05 g of MPS-modified hematite/silica particles was suspended in 40 mL of acetonitrile, and the suspension became brick-red. Then DVB (0.2 mL, total as 0.5 vol % of the reaction system) and AIBN (0.004 g, 2 wt % relative to the monomer) were dissolved in the suspension. The two-necked flask, attached to a fractionating column and a Liebig condenser and receiver, was then submerged in a heating mantle. The reaction mixture was heated from ambient temperature to the boiling state within 20 min, and the reaction system was kept under a refluxing state for an additional 10 min. The polymerization was then carried out by distilling the solvent out of the reaction system, and the reaction was ended after 20 mL of acetonitrile was distilled off the reaction mixture within 70 min. After the polymerization, the resultant hematite/silica/PDVB trilayer hybrid materials were purified by repeating centrifugation, decantation, and resuspension in acetone with ultrasonic irradiation three times. The hybrid particles were then dried in a vacuum oven at 50 °C until of constant weight. The other distillation precipitation polymerizations to prepare hematite/silica/PDVB with different polymer shell thicknesses and hematite/silica/poly(ethylene glycol dimethacrylate) (PEGDMA) and hematite/silica/poly(DVB-co-MAA) of various polarities were very similar to that of the typical procedure. The difference consisted of varying either the mass ratio of DVB monomer to MPS-modified hematite/silica particles or substituting EGDMA, DVB, and MAA as the monomers instead of DVB while the amount of AIBN initiator was maintained at 2 wt % relative to the monomer. The treatment
Hematite/Silica/Polymer Hybrid Materials of these trilayer hybrid particles was the same as that for the typical procedure. The reproducibility of the polymerizations was confirmed through several duplicate and triplicate experiments. Preparation of Ellipsoidal Hematite/Silica/PMBAAm Trilayer Hybrids by Distillation Precipitation Polymerization. A typical procedure for the distillation precipitation polymerizationwas as follows: In a dried 50-mL, two-necked flask, 0.5 g of unmodified hematite/silica particles was suspended in 40 mL of acetonitrile, and the suspension became brick-red. Then MBAAm (0.4 g, total as 1 vol % of the reaction system) and AIBN (0.008 g, 2 wt % relative to the monomer) were dissolved in the suspension. The two-necked flask, attached to a fractionating column and a Liebig condenser and receiver, was then submerged in a heating mantle. The reaction mixture was heated from ambient temperature to the boiling state within 20 min, and the reaction system was kept under a refluxing state for an additional 10 min. Then the polymerization was carried out by distilling the solvent out of the reaction system, and the reaction was ended after 20 mL of acetonitrile was distilled off the reaction mixture within 70 min. After the polymerization, the resultant hematite/silica/PMBAAm trilayer hybrid materials were purified by repeating centrifugation, decantation, and resuspension in acetone with ultrasonic irradiation three times. The composite particles were then dried in a vacuum oven at 50 °C until of constant weight. Synthesis of Hollow PDVB and PMBAAm Ellipsoids with Movable Hematite Cores. The resultant hematite/silica/PDVB or hematite/silica/PMBAAm trilayer particles were immersed in 1 wt % hydrofluoric acid aqueous solution for 4 h. (Caution: hydrofluoric acid is very corrosive and should be handled carefully!) Then the excessive HF and the formed SiF4 were expelled from the reaction system. The hollow PDVB or hematite/silica/PMBAAm ellipsoids with movable hematite cores were purified by several centrifugation/ washing cycles in water until reaching a pH of 7. The resultant PDVB or PMBAAm hollow ellipsoids with movable hematite cores were dried in a vacuum oven at 50 °C until of constant weight. Synthesis of Hollow PDVB Ellipsoids. The ellipsoidal hematite/ silica/PDVB trilayer hybrid particles were immersed in 40 wt % HF aqueous solution for 2 h. The resultant hollow PDVB ellipsoids were purified by several centrifugation/washing cycles in water until reaching a pH of 7. The resultant PDVB hollow ellipsoids were dried in a vacuum oven at 50 °C until of constant weight. Characterization. The size and morphology of the hematite, hematite/silica core-shell particles, hematite/silica/polymer trilayer hybrid materials, and the corresponding hollow polymer ellipsoids and hollow polymer ellipsoids with movable hematite cores were determined by transmission electron microscopy (TEM, Technai G2 20-S-TWIN). The magnetic properties of the hematite/silica/ PDVB trilayer hybrids were studied with a vibrating sample magnetometer (9600 VSM, BDJ Electronics Inc., Troy, MI) at room temperature. Fourier-transform infrared spectra (FT-IR) were scanned over the range of 400-4000 cm-1 with a potassium bromide slice on a Bio-Rad FTS135 FT-IR spectrometer.
Results and Discussion It is difficult to directly perform the polymerization of DVB on the surface of hematite nanoparticles for the preparation of hybrids containing R-Fe2O3 and polymer due to a lack of an appropriate interaction between the R-Fe2O3 particles and the polymer component. To overcome such a problem, it is necessary for the insertion an interlayer, which should have a suitable interaction between the hematite nanoparticles and the polymer layer. Coating of hematite nanoparticles with a silica layer is capable of accomplishing such a requirement. The silica layer with active hydroxyl groups on the surface not only occurs as a stable and homogeneous dispersion in a nonaqueous solvent but also behaves as the idea core for the further polymerization to afford the complicated hematite/silica/polymer hybrids. Uniform R-Fe2O3 shuttlelike nanoparticles were prepared by homogeneous hydrolysis from a solution of iron salt (FeCl3) and phosphate anions (NaH2PO4), as shown in Figure 1A with a
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Figure 1. TEM micrographs of ellipsoidal particles: (A) hematite; (B) hematite/silica core-shell particles; (C-E) hematite/silica/PDVB trilayer hybrids obtained with different DVB loadings for the polymerization: (C) 0.20 mL, (D) 0.30 mL, and (E) 0.40 mL; (F) hematite/silica/ PEGDMA; (G) hematite/silica/poly(DVB-co-MAA); and (H) hematite/ silica/PMBAAm.
typical TEM micrograph, which indicated that R-Fe2O3 nanoparticles had an ellipsoidal shape. The mean lengths of the major and minor axes were 390 and 92 nm, respectively. Hematite/ silica core-shell particles were prepared according to the method in the literature.23 An FT-IR spectrum (Figure 2a) proved the formation of the silica shell with a strong peak at 1104 cm-1 and a middle peak at 802 cm-1 corresponding to the symmetrical and asymmetrical stretching vibration of the Si-O-Si bond. The typical TEM micrograph in Figure 1B of hematite/silica nanoparticles demonstrated that the particles have a core-shell structure with a regularly ellipsoidal shape with light contrast silica shells and dark contrast cores of R-Fe2O3. The average lengths of the major and minor axes were 449 and 147 nm, respectively. These results indicated that the silica shell layers
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Figure 2. FT-IR spectra: (a) MPS-modified hematite/silica, (b) hematite/ silica/PDVB trilayer hybrids, (c) hematite/silica/PEGDMA, (d) hematite/ silica/poly(DVB-co-MAA), (e) hematite/silica/PMBAAm, (f) hollow PDVB ellipsoids with movable hematite cores, and (g) hollow PDVB ellipsoids.
with a thickness of ∼29 nm were encapsulated onto hematite nanoparticles with the aid of PVP, in which the amphiphilic and nonionic nature of the PVP as a steric surfactant played an essential role during the coating process to restrain the occurrence of the secondary nucleation of shell particles (silica). As a result, the ellipsoidal hematite particles were successfully coated by a layer of silica with a shell thickness of 29 nm by hydrolysis of TEOS under mild basic conditions, which permitted the modification of the surface with MPS. Preparation of Ellipsoidal Hematite/Silica/PDVB Trilayer Hybrids. Scheme 1 illustrates the synthesis of hematite/silica/ polymer trilayer hybrids by distillation polymerization of different monomers in the presence of hematite/silica core-shell particles as seeds and the further development of the corresponding hollow polymer ellipsoids and hollow polymer ellipsoids with movable hematite cores. PDVB cannot be encapsulated onto the hematite/silica particles directly without modification due to the hydrophobic nature of the DVB component, which lacks efficient interaction with the hydrophilic hematite/silica cores. To solve the problem, hematite/ silica particles were modified by MPS to introduce the reactive vinyl groups onto the surface, which would permit the further encapsulation of the PDVB component onto the inorganic cores via the capture the PDVB during the polymerization. Following the strategy in our previous work,31 MPS-modified hematite/ silica particles were synthesized via condensation of MPS with the reactive hydroxyl groups of hematite/silica ellipsoids to incorporate the reactive vinyl groups onto the surface. The modification of the hematite/silica particles was confirmed further by FT-IR spectra, as shown in Figure 2a, with the presence of the bands at 1635 and 1699 cm-1, respectively. The residual reactive vinyl groups on the surface of the PDVB cores were essential to grow the microspheres further and to obtain monodisperse core-shell functional microspheres by distillation precipitation polymerization in our previous work,32,33 in which the newly formed oligomers and monomers in the reaction system were efficiently captured by the reactive vinyl groups without formation of any second-initiated particles during the furtherstage polymerization. In the present work, the MPS-modified hematite/silica ellipsoidal particles were used as seeds in the distillation precipitation polymerization for the full encapsulation (32) Bai, F.; Yang, X. L.; Huang, W. Q. Macromolecules 2004, 37, 9746. (33) Qi, D. L.; Yang, X. L.; Huang, W. Q. Polym. Int. 2007, 56, 208.
Liu et al.
of the PDVB shell-layer over the inorganic cores, resulting in hematite/silica/PDVB trilayer hybrids as shown in Scheme 1. An important concern for the present work is to synthesize hematite/silica/polymer hybrid ellipsoids and to retain the particles with well-defined shape and adjustable polarity. A series of experiments were then conducted to investigate the effect of the DVB feed on the morphology of the resultant hematite/silica/PDVB trilayer hybrid particles during the distillation precipitation polymerization. TEM micrographs of the resultant hematite/silica/PDVB trilayer hybrid particleswith different DVB loadings are shown in Figure 1C-E. The results indicated that the final hematite/silica/PDVB trilayer hybrid particles had ellipsoidal shapes with slightly rough surfaces in the absence of any second-initiated particles with a DVB feed ranging from 0.20 to 0.40 mL, which demonstrated that the reactive vinyl groups on the surface of MPS-modified hematite/ silica seeds efficiently captured the newly formed DVB oligomers during the encapsulation of the inorganic seeds by distillation precipitation polymerization. The TEM micrographs in Figure 1C-E proved the core-shell structures of the hematite/silica/ PDVB ellipsoids have a light contrast PDVB shell-layer and dark contrast cores of the hematite/silica inorganic component. The above synthetic route is a general strategy and can be applied for the synthesis of other trilayer hematite/silica/polymer ellipsoids. In our previous work, PEGDMA and poly(divinyl benzene-co-methacrylic acid) (poly(DVB-co-MAA)) could be synthesized by distillation polymerization facilely.34,35 In the present work, we synthesized hematite/silica/PEGDMA and hematite/silica/poly(poly(DVB-co-MAA) following the above procedure. TEM images of hematite/silica/PEGDMA and hematite/silica/poly (DVB-co-MAA) are shown in Figure 1F and G, respectively. Our previous report30 showed that silica/poly(N,N′-methylenebisacrylamide) (silica/PMBAAm) could be prepared by distillation polymerization of MBAAm monomer in the presence of unmodified silica particles as seeds. In the present work, with unmodified hematite/silica particles as seeds, hematite/silica/ PMBAAm composite particles could be prepared by distillation precipitation polymerization of MBAAm in neat acetonitrile with 2,2′-azobisisobutyronitrile (AIBN) as the initiator in the absence of any stabilizer or surfactant, in which the hydrogen-bonding interaction between the hydroxyl group on the surface of hematite/ silica particles and the amide unit of MBAAm played a key role during the distillation precipitation polymerization and served as the driving force for the encapsulation of PMBAAm on hematite/silica particles. In the reaction system, MBAAm monomer was first adsorbed onto the surface of hematite/silica particles by hydrogen-bonding interaction, and the reactive vinyl groups were incorporated. Then the adsorbed vinyl groups on the surface of the hematite/silica particles captured the newly formed PMBAAm oligomers for the growth of hematite/silica/ PMBAAm composite particles, which was similar to the role for the vinyl groups of MPS during the distillation precipitation polymerization of DVB with the MPS-modified hematite/silica as seeds. The experimental conditions for distillation precipitation polymerizations of various monomers (with MPS-modified or unmodified hematite/silica particles as seed), sizes, and yields of the resultant hematite/silica/polymer trilayer hybrids are summarized in Table 1. The polymer shell thickness was measured by TEM characterization, which was calculated as one-half of (34) Bai, F.; Yang, X. L.; Huang, W. Q. Eur. Polym. J. 2006, 42, 2088. (35) Bai, F.; Yang, X. L.; Li, R.; Huang, B.; Huang, W. Q. Polymer 2006, 47, 5775.
Hematite/Silica/Polymer Hybrid Materials
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Scheme 1. Preparation of Hematite/Silica/Polymer Trilayer Hybrids and the Corresponding Hollow Polymer Ellipsoidal Particles
Table 1. Reaction Conditions, Sizes, Polymer Shell Thicknesses, Yields, and Magnetic Parameters of the Hematite/Silica Cores and Hybrid Hematite/Silica/Polymer Particles with Different Thicknesses of the Shell Layer
entry
hematite/silica (g)
monomer (mL)
particle size (major/minor axes) [nm]
polymer shell thickness (major/minor axes) [nm]
yield (%)c
coercivity (Hc) [Oe]
saturation magnetization (Ms) [emu/g]
remanent magnetization (Mr) [emu/g]
Aa Ba Ca Da Ea Fa Gb
0 0.05 0.05 0.05 0.05 0.05 0.5
0 DVB 0.2 DVB 0.3 DVB 0.4 EGDMA 0.2 mL DVB 0.1 MAA 0.1 MBAAm 0.04 g
449/147 481/167 501/187 518/205 516/204 501/189 462/159
0/0 16/10 26/20 34/29 33/28 26/21 6/6
0 10 19 21 39 31 93
416.3 406.2 410.9 396.3 / / /
0.2184 0.1676 0.1566 0.1307 / / /
0.08982 0.07319 0.06641 0.06084 / / /
a
MPS-modified hematite/silica particles.
b
Hematite/silica particles.
c
the difference between the average lengths in the major/minor axis directions of the final trilayer hybrids and those of the corresponding MPS-modified or unmodified hematite/silica seed particles. The major/minor axes of the ellipsoidal hybrids increased from 481/167 nm to 518/205 nm with increasing DVB feed from 0.20 to 0.40 mL for the distillation precipitation polymerization. These results mean that the thickness of the PDVB shell layer was in the range of 16/10 to 34/29 nm in the major/minor axis directions with the DVB feed enhancing from 0.20 to 0.40 mL for the polymerization. This indicated that the growth speed of the PDVB shell layer in the major axis was a little higher than that in the minor axis for the encapsulation of PDVB onto the MPS-modified hematite/silica seeds by distillation precipitation polymerization. The difference of reactivity in the major and minor axis directions during the polymerization may result in a slightly rough surface of the resultant hematite/silica/ PDVB trilayer hyrbrids, although the detailed reason for such difference in the reactivity between the major and minor axes is still unclear at present. Yields of the final hematite/silica/polymer trilayer hybrids were listed in Table 1, which indicated that the yield of the hybrid particles (the mass ratio as the amount of PDVB
Yield ) (Mhematite/silica/polymer - Mhematite/silica)/Mmonomer × 100%.
encapsulated onto the inorganic seeds to total DVB monomer for the polymerization) was significantly increased from 10 to 21% with increasing DVB feed from 0.20 to 0.40 mL. As discussed in our previous work,32 the reactive divnyl groups in DVB monomer were connected by the rigid phenyl rings, which resulted in the low yield due to the steric hindrance with monomer loadings (less than 2 wt % of the reaction system). Yields of hematite/silica/PEGDMA, hematite/silica/poly(DVB-co-MAA), and hematite/silica/PMBAAm were higher than that of hematite/ silica/PDVB, indicating that the reactivity of EGDMA, MAA, and MBAAm monomers was higher than that of DVB monomer. The surface modification of the polymer component leading to hematite/silica/polymer trilayer hybrid structure was proved further by FT-IR spectra, as shown in Figure 2b-e. FT-IR spectra (the curve in Figure 2b) of hematite/silica/PDVB particles (with entry C in Table 1 as a sample) had a weak peak at 709 cm-1 that was attributed to the typical adsorption of the phenyl group of the PDVB component. FT-IR spectra (the curve in Figure 2c) of the hematite/silica/PEGDMA particles had a strong peak at 1733 cm-1 corresponding to the vibration of the carbonyl unit in the ester group of the polyEGDMA component. The FT-IR spectra (the curve in Figure 2d) of the hematite/silica/poly(DVB-
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Figure 3. Hysterisis loops for samples at room temperature. (a) Hematite/ silica and (b-d) hematite/silica/PDVB with different PDVB shell thicknesss (minor axis): (b) 20 nm, (c) 27 nm, and (d) 35 nm.
co-MAA) particles had a strong peak at 1700 cm-1 corresponding to the vibration of the carbonyl unit in carboxylic acid group of polyMAA component. FT-IR spectra (the curve in Figure 2e) of the hematite/silica/PMBAAm particles showed strong peaks at 1676 and 1528 cm-1 corresponding to the stretching vibration of carbonyl groups and the bending vibration of an N-H bond due to the encapsulation of polyMBAAm. These results mean that polymer shells with different structures and polarities were successfully encapsulated over modified or unmodified hematite/ silica seeds during the polymerization, resulting in hematite/ silica/polymer trilayer hybrids, which was much different from the results in the literature.23 Furthermore, the thickness of the polymer shell-layer and the functionality were conveniently controlled by the feed of the monomer for the distillation precipitation polymerization. With the incorporation of reactive vinyl groups on the surface of the hematite/silica particles, hematite particles were encapsulated by distillation precipitation polymerization with functional polymer materials as a facile and general way to overcome the heavy flocculation of the hematite nanoparticles, which would find the wide applications in many fields. The Magenetization Property of the Hematite/Silica/PDVB Trilayer Hybrids. Bulk R-Fe2O3 is an antiferromagnetic material below the Morin temperature (TM) of 263 K and behaves with weak ferromagnetic property between 263 K and the Neel temperature (TN) of 955 K. The magnetic properties of hematite/ silica and hematite/silica/PDVB hybrid materials were studied by using a VSM at room temperature. Figure 3 shows the magnetization curves of hematite/silica core-shell particles and hematite/silica/PDVB trilayer hybrids with various PDVB shell thicknesses ranging from 10 to 29 nm (minor axis). Obvious magnetic hysterisis loops were observed for the hematite/silica/ PDVB hybrid from the field-dependent magnetization plots in Figure 3. In other words, the remanence existed when the magnetic field was removed, indicating that all the resultant hematite/ silica/PDVB hybrid materials showed hysteresis features and retained weak ferromagnetic properties originating from R-Fe2O3 ellipsoidal cores at room temperature. The magnetic properties of the hematite/silica/PDVB trilayer hybrids with different PDVB shell thicknesses are summarized in Table 1. The coercivity (Hc), saturation magnetization (Ms), and remanent magnetization (Mr) values for MPS-modified hematite/silica core-shell ellipsoids were 416.3 Oe, 0.218 emu/g, and 0.089 emu/g, respectively. The saturation magnetization of hematite/silica/PDVB trilayer hybrids was slightly decreased from 0.167 to 0.131 emu/g with an
Figure 4. (A, B) TEM micrographs of hollow ellipsoids with PDVB shell thickness of 35 nm (minor axis): (A) PDVB hollow ellipsoids with movable hematite cores, (B) PDVB hollow ellipsoids, and (C) TEM micrograph of hollow PMBAAm ellipsoids with movable hematite cores and PMBAAm shell thickness of 6 nm (minor axis).
increase in the PDVB shell thickness from 10 to 29 nm (minor axis), while the remanent magnetization was simultaneously decreased from 0.0732 to 0.0608 emu/g. These results indicate that the magnetization of the hematite/silica/PDVB trilayer hybrid materials decreased with the increase in the polymer component due to the decrease in the effective mass of the hematite core. Polymer Hollow Ellipsoids. The silica midlayer of the resultant hematite/silica/PDVB or hematite/silica/PMBAAm trilayer hybrids was selectively removed by etching in dilute hydrofluoric acid (1 wt %) to afford PDVB or PMBAAm hollow ellipsoids with movable hematite cores. The driving force for such selective removal was due to the formation of the SiF4 gas, which was given off from the trilayer hybrids during the etching process. The TEM micrograph in Figure 4A and C illustrates the typical PDVB and PMBAAm hollow ellipsoids with movable hematite cores having polymer shell thicknesses (minor axis) of 35 and 6 nm, respectively. It can be seen in Figure 4A and C that the hematite cores are away from the centers of the hybrid particles, whereas the hematite cores in Figure 1 were exactly located in the center of the hybrid hematite/silica/polymer particles. Therefore, the hematite cores cannot be fixed in the middle of the particles according to the hollow polymer ellipsoids. In other words, these results indicated that hollow PDVB and PMBAAm ellipsoids contained the movable hematite shuttlelike particles inside the polymer exterior shells with a cavity in the midlayer of the final nanoparticles. The novel composite microstructures with hollow particles containing movable cores might be interesting for the construction of nano/micrometer-scale devices with tunable functionality originating from the movable inner cores. After the hematite/silica/PDVB trilayer ellipsoids were soaked in concentrated hydrofluoric acid (40 wt %) aqueous solution, not only the silica midlayer but also the hematite cores were etched to get PDVB hollow ellipsoids. The following reactions occurred during the etching process, in which the FeF63- was dissolved in aqueous solution and the formed SiF4 was expelled out of the reaction system.
Hematite/Silica/Polymer Hybrid Materials
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Fe2O3 + 12HF f 2H3FeF6 + 3H2O
(1)
SiO2 + 4HF f SiF4 v + 2H2O
(2)
The typical TEM micrograph of the PDVB hollow ellipsoids with a thickness of 35 nm (minor axis) is shown in Figure 4B, in which the convincing capsules with regularly ellipsoidal shapes and uniform sizes were observed with the presence of a cavity in the interior. All these results further confirmed the successful and complete removal of hematite/silica cores by concentrated hydrofluoric acid during the etching process. The recent simulation and experiment work in three-dimensional systems has shown that maximal jammed packing of nonspherical particles depended in a nontrivial way on the aspect ratio and can become more dense than that of the corresponding spheres.36,37 The hematite/ silica/polymer ellipsoidal hybrids and the corresponding polymer hollow ellipsoids could be potential unique candidates to be assembled into ordered structure in a magnetic field due to the magnetization property originating from the hematite cores.
Conclusion Ellipsoidal hematite/silica/PDVB trilayer hybrids were prepared by distillation precipitation polymerization of DVB in neat acetonitrile with AIBN as the initiator in the presence of MPSmodified hematite/silica core-shell particles as seeds, in which the incorporated reactive vinyl groups on the surface of seeds played a key role in the formation of the ellipsoidal hybrids with regular shape via the capture of the DVB oligomers and monomers (36) Donev, A.; Cisse, I.; Sachs, D.; Variano, E.; Stillinger, F. H.; Connelly, R.; Torquato, S.; Chaikin, P. M. Science 2004, 303, 990. (37) Donev, A.; Stillinger, F. H.; Chaikin, P. M.; Torquato, S. Phys. ReV. Lett. 2004, 92, 255506.
during the polymerization. The thickness of the PDVB shell layer of the resultant hematite/silica/PDVB hybrids was conveniently controlled in the range of 10-29 nm (minor axis) and 16-34 nm (major axis) by altering the DVB monomer feed from 0.20 to 0.40 mL during the polymerization. The magnetization of the hematite/silica/PDVB trilayer hybrid materials decreased with an increase in the polymer component due to a decrease in the effective mass of the hematite. The PDVB hollow ellipsoids with movable hematite cores and hollow PDVB ellipsoids were further developed with the selective removal of the silica midlayer in dilute hydrofluoric acid (1 wt %) and the complete removal of hematite/silica inorganic cores in concentrated HF (40 wt %) aqueous solution, respectively. Not only ellipsoidal hematite/ silica/PDVB trilayer hybrids and hollow ellipsoids with a nonpolar polymer layer but also trilayer hybrids and hollow ellipsoids with various polarities and different functionalities, such as PEGDMA, poly(DVB-co-MAA), and PMBAAm, can be synthesized by this method. The applications of the ellipsoidal trilayer hybrids and hollow polymer ellipsoids with various polarities were studied in our laboratory systematically. Acknowledgment. This work was supported in part by the National Science Foundation of China (Project No. 20504015) and the Opening Research Fund from the State Key Laboratory of Polymer Chemistry and Physics, Chinese Academy of Sciences (Project No. 200613). Supporting Information Available: X-ray diffraction pattern of hematite/silica/PDVB. This material is available free of charge via the Internet at http://pubs.acs.org. LA8002004
