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Processible Optically Transparent Block Copolymer Films Containing Superparamagnetic Iron Oxide Nanoclusters B. H. Sohn† and R. E. Cohen*,‡ Program in Polymer Science and Technology and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received June 13, 1996. Revised Manuscript Received September 10, 1996X
Free-standing block copolymer films containing magnetic iron oxide nanoclusters within the microdomains were produced by static casting. The thickness of the nanocomposite films was easily controlled to make optically transparent thin films. The size distribution of the nanoclusters was relatively narrow and the nanoclusters were uniformly distributed within the films. The spherical nanoclusters, ca. 5 nm in diameter, were identified as γ-Fe2O3 and had a well-defined crystalline structure. Magnetic measurements revealed that the nanocomposite films are superparamagnetic.
Introduction Nanoclusters of metal, semiconductor, and oxide are of great interest because they can have physical and chemical properties that are characteristic of neither the atoms nor the bulk counterparts. Quantum size effects and the large ratio of surface area to volume can contribute to some of the unique properties of nanoclusters.1-5 Such clusters are expected to have novel electrical, optical, magnetic, and catalytic properties.6-10 For example, magnetic nanoclusters can be small enough so that each particle is a single magnetic domain8 and exhibit unusual phenomena such as superparamagnetism.9 Magnetic nanoclusters have been produced in a variety of matrix materials such as silicon oxides,11 aluminum oxides,12 porous glass,13 vesicles,14 and polymers.15-18 Some of the synthetic approaches making nanocomposites mimic biomineralization.14-18 Formation of magnetic nanoclusters in biological organisms has been observed.19-21 Some marine bacteria use chains of intracellular magnetic nanoclusters to navigate the earth’s magnetic field.19 †
Program in Polymer Science and Technology. Department of Chemical Engineering. Abstract published in Advance ACS Abstracts, November 1, 1996. (1) Morse, M. D. Chem. Rev. 1986, 86, 1049. (2) Halperin, W. P. Rev. Mod. Phys. 1986, 58, 533. (3) Henglein, A. Chem. Rev. 1989, 89, 1861. (4) Stucky, G. D.; MacDougall, J. E. Science 1990, 247, 669. (5) Kresin, V. V. Phys. Rep. 1992, 220, 1. (6) Hache, F.; Richard, D.; Flytzanis, C. J. Opt. Soc. Am. B 1986, 3, 1647. (7) Kirkpatrick, S. Rev. Mod. Phys. 1973, 45, 574. (8) Kittel, C. Phys. Rev. 1946, 70, 965. (9) Bean, C. P.; Livingston, J. D. J. Appl. Phys. 1959, 30, 120. (10) Henglein, A. Chem. Rev. 1986, 89, 1861. (11) Shull, R. D.; Ritter, J. J.; Swartzendruder, L. J. J. Appl. Phys. 1991, 69, 5414. (12) Gavrin, A; Chien, C. L. J. Appl. Phys. 1990, 67, 938. (13) Borelli, N. F.; Morse, D. L.; Schreurs, J. W. H. J. Appl. Phys. 1983, 54, 3344. (14) Mann, S.; Hannington, J. P. J. Colloid Interface Sci. 1988, 22, 326. (15) Raymond, L.; Revol, J.-F.; Ryan, D. H.; Marchessault, R. H. J. Appl. Polym. Sci. 1996, 59, 1073. (16) Ziolo, R. F.; Giannelis, E. P.; Weinstein, B. A.; O’Horo, M. P.; Ganguly, B. N.; Mehrotra, V.; Russell, M. W.; Huffman, D. R. Science 1992, 257, 219. (17) Okada, H.; Sakata, K.; Kunitake, T. Chem. Mater. 1990, 2, 89. (18) Sobon, C. A.; Bowen, H. K.; Broad, A.; Calvert, P. D. J. Matter. Sci. Lett. 1987, 6, 901. ‡
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Recently we reported a method of synthesizing metal22-24 and semiconductor25-31 nanoclusters within the microdomains of block copolymers prepared by ringopening metathesis polymerization (ROMP). Our technique to synthesize nanoclusters takes advantage of the self-assembled microdomain structure of block copolymers, inside which nanoclusters can be made in a controlled manner. In this paper we report the synthesis and characterization of magnetic iron oxide nanoclusters within the microdomains of block copolymers. Since cluster formation is localized within the microdomains, the block copolymer approach more closely resembles the complicated biomineralization process19-21 than other nanocomposite synthesis methods. Moreover, the block copolymer approach produces free-standing nanocomposite films, the thickness of which can be controlled to form either optically transparent or opaque films. This approach also produces magnetic nanoclusters uniformly distributed over a large area because the precursor iron species used to synthesize the clusters are selectively sequestered, during film processing, into the microdomains of the self-assembled block copolymer morphology. The morphology of the block copolymers was characterized by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Elec(19) Blackmore, R. Science 1975, 190, 377. (20) Mann, S; Frankel, R. B.; Blackmore, R. P. Nature 1984, 310, 405. (21) Mann, S. Nature 1988, 332, 119. (22) Ng Cheong Chan, Y.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 24. (23) Ng Cheong Chan, Y.; Schrock, R. R.; Cohen, R. E. J. Am. Chem. Soc. 1992, 114, 7295. (24) Ng Cheong Chan, Y.; Craig, G. S.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 885. (25) Sankaran, V.; Cummins, C. C.; Schrock, R. R.; Cohen, R. E.; Silbey, R. J. J. Am. Chem. Soc. 1990, 112, 6858. (26) Sankaran, V.; Cohen, R. E.; Cummins, C. C.; Schrock, R. R. Macromolecules 1991, 24, 6664. (27) Cummins, C. C.; Beachy, M. D.; Schrock, R. R.; Vale, M. G.; Sankaran, V.; Cohen. R. E. Chem. Mater. 1991, 3, 1153. (28) Cummins, C. C.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 27. (29) Sankaran, V.; Yue, J.; Cohen, R. E.; Schrock, R. R.; Silbey, R. J. Chem. Mater. 1993, 5, 1133. (30) Yue, J.; Sankaran, V.; Cohen, R. E.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 4409. (31) Yue, J.; Cohen, R. E. Supramol. Sci. 1995, 1, 117.
© 1997 American Chemical Society
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Figure 1. [NORCOOH]30[MTD]300.
tron diffraction (ED) and X-ray photoelectron spectroscopy (XPS) were used to identify the structure of the iron oxide nanoclusters. Magnetic properties of the films containing iron oxide nanoclusters were investigated to reveal the superparamagnetism. Experimental Section All monomers were prepared as described in the literature.31,32 [NORCOOH]30[MTD]300 (NORCOOH ) 2-norbornene5,6-dicarboxylic acid; MTD ) methyltetracyclododecene) (Figure 1) were synthesized using the Schrock alkylidene initiator (Mo(CHCMe3Ph)(NAr)(O-t-Bu)2 (Ar ) 2,6-diisopropylphenyl)) as described elsewhere.31,32 One equivalent of iron(III) chloride (FeCl3) was introduced per NORCOOH in tetrahydrofuran (THF) solution, i.e., Fe3+:COO- ) 1:3. Films (ca. 100 µm thick) were static cast from a 3 wt % THF solution in Teflon-coated aluminum cups under nitrogen. The THF was allowed to evaporate slowly over a period of 3-5 days. Thin films (ca. 10 µm thick) were also static cast for Fourier transform infrared (FTIR) and ultraviolet-visible (UV-vis) spectroscopy experiments. All films were dried under vacuum for at least 2 days. To make iron oxide nanoclusters,14-18 the films containing FeCl3 were stirred in a 2 M NaOH solution at room temperature for 24 h. Thick films (ca. 100 µm) changed from greenish-black to reddish-black, whereas thin films (ca. 10 µm) changed from green to orange-yellow. To complete the oxidation of iron hydroxides by ambient oxygen as well as to wash out residual NaOH and NaCl, films were stirred in deionized water at room temperature for 24 h and then dried under vacuum. FTIR spectra were recorded from 4000 to 500 cm-1 on a Nicolet Model 510 infrared spectrometer. An Oriel Instaspec Model 250 spectrometer was used for UV-vis spectra. An LKB Ultratome III Model 8800 was used to obtain ultrathin sections (ca. 50 nm thick) for TEM. TEM was performed on a JEOL 200 CX operating at 200 kV. Highresolution TEM was performed on an Akashi EM-002B operating at 200 kV. All samples were lightly carbon coated prior to high-resolution TEM. Selected-area electron diffraction (ED) patterns were obtained on both electron microscopes. Morphology of the block copolymers was also studied by small-angle X-ray scattering (SAXS) using a Rigaku 1.54 Å Cu KR rotating-anode point source, Charles Supper double mirror focusing optics, and a Nicolet two-dimensional detector. X-ray photoelectron spectroscopy (XPS) was conducted on a Perkin-Elmer Physical Electronics Division 5100C with a Mg KR X-ray source operating at 15 keV and 300 W. An Ar+ ion beam was used to etch the film surface for 2 min, to better expose the nanoclusters to the incident X-rays. The sample interrogation area was ca. 3 mm × 3 mm using a standard entrance lens aperture. The binding energy of the polymer C(1s) peak was adjusted to 284.6 eV. 33 Magnetic properties of the films containing iron oxide nanoclusters were studied using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design (32) Saunders, R. S., Ph.D. Thesis, MIT Department of Chemical Engineering, 1992. (33) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1979.
Figure 2. FTIR spectra of (a) [NORCOOH]30[MTD]300, (b) [NORCOOH]30[MTD]300 containing FeCl3, (c) [NORCOOH]30[MTD]300 containing iron after NaOH treatment, and (d) [NORCOOH]30[MTD]300 containing iron after NaOH treatment and washing. Model MPMS) at fields ranging from 0 to 20 kOe and at temperatures ranging from 10 to 300 K.
Results and Discussion Synthesis and Characterization. Both FeCl3 and [NORCOOH]30[MTD]300 dissolved readily in THF. Freestanding films were obtained by static casting. In distinction to other methods using vesicles,14 membranes,15 or ion-exchange resins,16 films were obtained and their thicknesses were easily controllable by varying the solution concentration. To verify the ion exchange between FeCl3 and COOH in the block copolymers, films cast without and with FeCl3 were examined by FTIR. Figures 2 (a) and (b) show the FTIR spectra of [NORCOOH]30[MTD]300 films without and with FeCl3, respectively. Without FeCl3, the carboxylic acid carbonyl peak is located at 1708 cm-1. The FTIR spectrum of [NORCOOH]30[MTD]300 containing FeCl3 showed a new peak at 1593 cm-1, which is associated with asymmetric stretching of carboxylate salts. However, the peak at 1708 cm-1 is still present in Figure 2 (b), indicating that not all of the carboxylic acid groups of [NORCOOH]30[MTD]300 engaged in the ion exchange reaction with FeCl3 during film casting even though one charge equivalent of FeCl3 to COOH (i.e., Fe3+: COO- ) 1:3) was added. Thus, FeCl3 could exist either in [NORCOOH] domains or in [MTD] domains. However, scanning transmission electron microscopy (STEM) results showed that essentially all of the iron was located within [NORCOOH] domains. Presumably iron chloride could form a complex with the carboxylic acid groups. The iron species residing selectively in the carboxylic acid-containing microdomains were sufficient to give contrast in TEM. In Figure 3, [NORCOOH]30[MTD]300 containing FeCl3 showed an interconnected cylindrical
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Figure 3. Transmission electron micrograph of [NORCOOH]30[MTD]300 containing FeCl3.
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Figure 5. Transmission electron micrograph of [NORCOOH]30[MTD]300 containing γ-Fe2O3 nanoclusters.
Figure 6. High-resolution transmission electron micrograph of a γ-Fe2O3 nanocluster. Figure 4. SAXS profile of [NORCOOH]30[MTD]300 containing FeCl3.
morphology. The iron-containing microdomains appear as darker regions. The interconnected cylindrical morphology is a nonequilibrium one, a result often observed in solvent cast block copolymer films.34,35 The characteristic size of the iron-containing microdomains is about 4 nm. SAXS results also show an ordered phase, with the maximum intensity at 0.18 nm-1 (Figure 4). The average domain spacing is therefore 35 nm, which is not easily reconciled from TEM (Figure 3). The morphological characterization shows that the iron species are localized within the microdomains and yet uniformly distributed spatially throughout the films. The uniform distribution of iron species within a matrix material is not easily achieved using diffusion process.14-16 A film of [NORCOOH]30[MTD]300 containing FeCl3 was treated with 2 M NaOH to produce iron oxide nanoclusters.14-18 The interconnected carboxylic acidcontaining domains are highly polar and can be swollen by water. Therefore, the interconnected carboxylic acid(34) Cohen, R. E.; Cheng, P. L.; Douzinas, K.; Kofinas, P.; Berney, C. V. Macromolecules 1990, 23, 324. (35) Bates, F. S.; Cohen, R. E. J. Polym. Sci.: Polym. Phys. Ed. 1980, 18, 2143.
containing domains can provide a good diffusion path for aqueous reagents through the films. NaOH treatment of iron chlorides resulted in unstable iron hydroxides which were easily oxidized to iron oxides by ambient oxygen. After NaOH treatment, the film was stirred in water to help oxidation of iron hydroxides by ambient oxygen, as well as to wash out NaOH and NaCl residues. To accelerate the oxidation, oxygen bubbling or hydrogen peroxide could be used, but fast oxidation can generate goethite (R-FeOOH) rather than iron oxide.14,17 After NaOH treatment, the carboxylic acid carbonyl peak at 1708 cm-1 completely disappeared (Figure 2c), and all carboxylic acid groups were converted to sodium carboxylate salts. After washing with water, however, some carboxylic acid groups were regenerated (Figure 2d). Figure 5 shows that oxide nanoclusters formed predominantly within the interconnected microdomains. The oxide nanoclusters are about 5 nm in diameterslightly larger than the original cylindrical microdomain diameter. Figure 6 shows a high-resolution TEM of a single nanocluster. The nanocluster apparently consists of a single crystal, based on observation of the lattice fringes, and has a roughly spherical shape. Wide-angle X-ray diffraction was attempted to identify the structure of the nanoclusters, but the signal-to-noise ratio was too low to identify the crystal structure.36 Figure 7 shows
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Figure 7. Selected-area electron diffraction pattern of [NORCOOH]30[MTD]300 containing γ-Fe2O3 nanoclusters (camera length ) 82 cm, photo enlargement ) 2×). Table 1. Diffraction Data: d Spacing (Å) ED
XRD (γ-Fe2O3)a
XRD (Fe3O4)a
2.914 2.514b 2.084 1.607 1.470
2.950 2.514b 2.086 1.604 1.474
2.967 2.532b 2.099 1.616 1.485
a Joint Committee on Powder Diffraction Standards. b The strongest intensity.
the selected-area electron diffraction pattern of the film containing nanoclusters, and the measured d spacings are tabulated in Table 1. The d spacings measured by ED correspond very closely to those of both γ-Fe2O3 (maghemite) and Fe3O4 (magnetite). To distinguish whether the nanocluster is maghemite or magnetite, XPS was employed because binding energies for the Fe(2p) and Fe(3p) photoelectrons are quite different for γ-Fe2O3 and Fe3O4. The Fe(2p) and Fe(3p) photoelectron peaks of the nanoclusters are shown in Figure 8a,b, respectively. The photoelectron peak at 51.8 eV in Figure 8b corresponds to Na(2s). The positions of the Fe(2p) and Fe(3p) peaks are 711.0 and 55.7 eV, which are in good agreement with the values reported for γ-Fe2O3 in the literature.37 The ED and XPS results indicate that γ-Fe2O3 nanoclusters are produced within the microdomains of the free-standing block copolymer films. The content of γ-Fe2O3 nanoclusters is 21.9 wt % within the microdomains, and 2.6 wt % for the film overall, assuming that all FeCl3 is completely converted to γ-Fe2O3. The amount of γ-Fe2O3 nanoclusters within the microdomains is comparable to that in the crosslinked sulfonated polystyrene after two cycles of loading as reported by Ziolo et al.16 Magnetic and Optical Properties. Bulk γ-Fe2O3 is ferrimagnetic at room temperature, but below a critical particle size it becomes superparamagnetic and shows no remanence or coercivtiy.16,38-40 Figure 9 shows magnetization as a function of applied magnetic (36) Bergmeister, J, J.; Rancourt, J. D.; Taylor, L. T. Chem. Mater. 1990, 2, 640. (37) McIntyre, N. S.; Zetaruk, D. G. Anal. Chem. 1977, 49, 1521. (38) Vassiliou, J. K.; Mehrotra, V.; Russell, M. W.; Giannelis, E. P.; McMichael, R. D.; Shull, R. D.; Ziolo, R. F. J. Appl. Phys. 1993, 73, 5109. (39) Kneller, E. In Magnetism and Metallurgy; Berkowitz, A. E., Kneller, E., Ed.; Academic Press, Inc.: New York, 1969. (40) Cullity, B. D. Introduction to Magnetic Materials; AddisonWesley: Reading, MA, 1972.
Figure 8. X-ray photoelectron spectra of γ-Fe2O3 nanoclusters in [NORCOOH]30[MTD]300 (a) Fe(2p) (b) Fe(3p).
Figure 9. Magnetization vs applied magnetic field for the [NORCOOH]30[MTD]300 film containing γ-Fe2O3 nanoclusters at 300 K.
field for the film containing γ-Fe2O3 nanoclusters at room temperature. There is no hysteresis at room temperature, which is consistent with superparamagnetic behavior. The magnetic moment is about 0.5 emu/g at 20 kG. Below the blocking temperature, magnetic nanoclusters become magnetically frozen, the magnetic moment of the nanoclusters is fixed, and remanence and coercivity (hysteresis) appear on the plot of magnetization as a function of magnetic field.16,38-40 Magnetization plots at 10 K are shown in Figure 10a,b for the zero-field-cooled (ZFC) and the field-cooled (FC) cases, respectively. There are small hysteresis loops in
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Figure 12. UV-vis spectra of (a) [NORCOOH]30[MTD]300 film, (b) [NORCOOH]30[MTD]300 film containing FeCl3, and (c) [NORCOOH]30[MTD]300 film containing γ-Fe2O3.
Figure 10. Magnetization vs applied magnetic field for the [NORCOOH]30[MTD]300 film containing γ-Fe2O3 nanoclusters at 10 K: (a) zero-field cooled; (b) field cooled (H ) 200 Oe).
creases monotonically with decreasing temperature for the FC case, whereas the ZFC magnetization passes through a maximum at 20 K, which can be associated with the blocking temperature.38 For UV-vis spectra, ca. 10 µm thick films were prepared by static casting. A [NORCOOH]30[MTD]300 film is itself colorless and transparent. The UV-vis spectrum is shown in Figure 12a. The film is essentially transparent throughout the visible range. A film of [NORCOOH]30[MTD]300 containing FeCl3 is greenish and the absorption edge is around 430 nm, as shown in Figure 12b. In the spectrum of the film containing γ-Fe2O3 nanoclusters (Figure 12 (c)), the absorption edge is shifted toward longer wavelengths and the film becomes orange yellow but still quite transparent in the visible range. Conclusions
Figure 11. Magnetization vs temperature for the [NORCOOH]30[MTD]300 film containing γ-Fe2O3 nanoclusters at H ) 200 Oe: (a) zero-field cooled; (b) field cooled (H ) 200 Oe).
both cases. The loops are symmetric about the center regardless of the cooling procedure, also characteristic of a superparamagnetic behavior.38 The loops in Figures 10a,b are superimposable on each other, implying that the nanoclusters are magnetically single-phased.16 Temperature-dependent magnetization plots at 200 Oe for both the ZFC and the FC case are shown in Figure 11. Above 70 K the magnetization decreases with increasing temperature in both the ZFC and the FC case. At lower temperatures the magnetization in-
Free-standing block copolymer films containing FeCl3 within interconnected cylindrical microdomains were obtained via static casting from THF solutions. FeCl3 was well localized within the microdomains and uniformly distributed spatially throughout the films. This arrangement of the iron precursor leads to a relatively narrow size distribution of γ-Fe2O3 nanoclusters and a uniform distribution of γ-Fe2O3 nanoclusters throughout the films after NaOH treatment. The thickness of the free-standing nanocomposite films was easily controllable by varying the concentration of the starting solutions, facilitating the production of optically transparent orange-yellow thin films containing γ-Fe2O3 nanoclusters. From TEM and ED results, γ-Fe2O3 nanoclusters were found to be roughly spherical, ca. 5 nm in diameter, with well-defined crystalline structure. The magnetic measurements revealed that the nanocomposite films are superparamagnetic and showed no remanence or coercivity at room temperature. Further investigation of the magnetic properties of the films containing γ-Fe2O3 nanoclusters by Mo¨ssbauer spectroscopy is in progress. Also, we note that our solvent-based film processing methodologies should allow for the production of conformal coatings and films via dip-coating, spin casting, or web casting, and we are currently engaged in exploratory experimentation in this regard. Optically transparent superparamagnetic
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films may find application, for example, in security papers in which embedded magnetic response patterns cannot be altered or erased by the application of a strong magnetic field. Conformal magnetocaloric films might find application in localized heat transfer problems in electronic devices. Finally we note that the film and cluster forming reported here are not limited to the superparamagnetic regime. Other oxides and zerovalent metal clusters can, in principle, be produced in a size regime40 suitable for maximizing coercivity, leading potentially to transparent polymer films with hard magnetic properties.
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Acknowledgment. This work has been supported by the National Science Foundation (DMR-94-00334) and made use of MRSEC Shared Facilities supported by the National Science Foundation. We thank M. Frongillo and E. Shaw of the CMSE Facilities at MIT for assistance in high-resolution TEM and XPS, respectively. We also thank Dr. R. Ziolo of Xerox Webster Research Center for providing ongoing advice during the course of this research project. CM960339D
