Fe-Containing Nanoparticles in Siloxane Rubber Matrices

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1 ISSN , Inorganic Materials, 26, Vol. 42, No. 5, pp Pleiades Publishing, Inc., 26. Original Russian Text G.Yu. Yurkov, D.A. Astaf ev, L.N. Nikitin, Yu.A. Koksharov, N.A. Kataeva, E.V. Shtykova, K.A. Dembo, V.V. Volkov, A.R. Khokhlov, S.P. Gubin, 26, published in Neorganicheskie Materialy, 26, Vol. 42, No. 5, pp Fe-Containing Nanoparticles in Siloxane Rubber Matrices G. Yu. Yurkov a, D. A. Astaf ev a, L. N. Nikitin b, Yu. A. Koksharov c, N. A. Kataeva a, E. V. Shtykova d, K. A. Dembo d, V. V. Volkov d, A. R. Khokhlov b, and S. P. Gubin a a Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, Russia b Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, Russia c Moscow State University, Vorob evy gory 1, Moscow, Russia d Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, Russia gy-yurkov@yandex.ru Received November 25, 25 Abstract Polymer-matrix composites consisting of Fe 3 O 4 Fe 2 O 3 and FeCoB nanoparticles in SKTN and SIEL siloxane rubber matrices are synthesized. The introduction of nanoparticles into siloxane oligomers leads to their polymerization, resulting in polymer-matrix nanomaterials. Transmission electron microscopy and small-angle x-ray scattering are used to determine the size of the Fe-containing nanoparticles. The magnetic properties of the nanocomposites are studied by electron magnetic resonance. DOI: /S INTRODUCTION There is currently considerable interest in the modification and study of the properties of various thermoplastics and elastomers, which find many industrial applications. An important issue in this area of research is the reinforcement of elastomers with fine-particle fillers. In the rubber industry, elastomers are commonly reinforced with silicate and soot fillers [1]. The reinforcement of elastomers depends primarily on the specific surface (particle size) of the filler and the formation of chemical bonds between the elastomer and the filler surface [1, 2]. The reinforcing properties of a filler are highly dependent on its particle size, as discussed in detail by Edwards [1]. The particle size of highly reinforcing silicate and soot fillers is 1 35 nm, and those of reinforcing, semireinforcing, and extending fillers are 35 1, 1 1, and 1 1 nm, respectively. Fine-particle metallic fillers enhance the thermal and electrical conductivity, magnetic susceptibility, heat capacity, and other properties of polymeric materials [3]. Using magnetic fillers, one can obtain magnetoplastics which combine properties of the metallic filler and polymer matrix. Polymer-bonded permanent magnets (PPMs), or magnetoplastics, are fabricated from a mixture of a magnetic powder and polymer binder. The key advantages of PPMs are enhanced flexibility in designing constructions, low forming shrinkage, uniform magnetic properties throughout the material, high electrical resistivity, reduced weight (by a factor of 3 4 compared to sintered magnets), and high corrosion resistance. Pole cores made of polymeric magnets are superior to metallic cores in performance parameters, especially at frequencies on the order of several megahertz. Polymeric magnets are used most widely in the fabrication of rotors, synchronous motors, annular magnets for starting motors, and magnets for electronbeam focusing systems in television picture tubes [3]. At the same time, magnetoplastics are not free of drawbacks. In particular, they are incapable of competing with sintered magnets as to the coercive force, which is determined by the volume fraction of the magnetic material. The content of the magnetic component in PPMs is usually no higher than 6 vol %, against 1% in ordinary magnets. For this reason, composite materials are inferior in magnetic characteristics. Therefore, creating magnetoplastics based on metalcontaining nanoparticles is an important practical issue in eliminating the above drawbacks. Nanoparticles are, as a rule, stabilized using polymer matrices [4]. The physical properties of nanoparticles embedded in polymers, in particular their magnetic characteristics, have been studied in sufficient detail [5 1] and differ markedly from those of bulk analogs: in most cases, nanoparticles have an increased coercive force and magnetization per atom [11]. This suggests that nanoparticles in polymer matrices have great potential for producing high-performance polymeric magnets retaining the advantages of polymers. 496

2 Fe-CONTAINING NANOPARTICLES IN SILOXANE RUBBER MATRICES 497 The objective of this work was to examine the possibility of producing nanoparticles in oligomeric siloxane rubber matrices (SKTN and SIEL), to analyze the effect of the filler on the properties of the elastomer, and to investigate the physical and magnetic properties of the resultant nanomaterials. We studied Fe 3 O 4 Fe 2 O 3 and FeCoB nanoparticles embedded in the above matrices. EXPERIMENTAL Nanomaterials containing Fe x O y and FeCoB nanoparticles stabilized in oligomeric siloxane rubber matrices (SKTN and SIEL) were prepared by two different procedures. Fe x O y nanoparticles were produced by a modified cluspol technique [5]. Iron(III) acetate was introduced directly into a heated oligomer (siloxane rubber), with no oil, since the rubber is a viscous liquid stable up to 2 25 C. In each experiment, we used 1 g of the corresponding siloxane rubber. The main starting reagents were of analytical grade. Fe x O y nanoparticles were obtained via thermal decomposition of iron(iii) acetate tetrahydrate, Fe(CH 3 COO) 3 4H 2 O, in an inert atmosphere, followed by holding in air at the synthesis temperature in order to complete the oxidation. Iron(III) acetate was introduced into heated SIEL and SKTN so as to ensure complete decomposition of the precursor without delivering an additional amount of the starting reagent. This, in turn, enabled the preparation of nanoparticles uniform in both composition and size. In all of the experiments, the flow rate of the inert gas (argon) was adjusted so as to rapidly remove all of the ligands and solvent from the reaction zone. An appropriate amount of the metal-containing precursor was added to heated siloxane rubber with vigorous stirring. In syntheses with SIEL as a stabilizing matrix, we obtained elastic spheres with brown particles on their surfaces. With SKTN, we obtained a brown viscous substance. Both reaction products were insoluble in water and organic solvents (ethanol and hexane). FeCoB nanoparticles were prepared as described by Linderoth et al. [12], by reducing water-soluble iron and cobalt salts with sodium borohydride. Synthesis was run in the two-phase system hexane water in the presence of the sodium dodecylbenzenesulfonate surfactant. After the reaction reached completion, the organic layer was removed by decantation, washed with water, and added to a hexane solution of the polymer (SKTN or SIEL). After mixing for 1 h (room temperature, air), the excess solvent was distilled off, and the resultant material was dried in air. The FeCoB prepared in SKTN had the form of elastic flakes, black on the inside and reddish on the outside, and that in SIEL had the form of an aggregated dark green-brown powder. The starting oligomers SKTN and SIEL were soluble in alcohols, while the reaction products were insoluble in water and organic solvents (ethanol and hexane). The nanoparticle size was determined by transmission electron microscopy (TEM) on a JEOL JEM- 1B. The specimen was dispersed in an alcohol-inwater solution by sonication, and a drop of the dispersion was applied to a carbon-coated copper grid. Electron magnetic resonance (EMR) spectra were measured with a Varian E-4 X-band spectrometer at room temperature. The analogous output signal of the spectrometer was processed using an application program designed by one of us [13]. The signal width H pp and resonance field H r were determined by a peak-topeak method. The particle size distributions of different components in the rubber nanoparticle system were determined by small-angle x-ray scattering (SAXS). To enhance the accuracy of the results, we configured a POLYMIX7 program for direct modeling of polydisperse systems. The results obtained with POLYMIX7 were compared to Fourier analysis data (GNOM program) [14]. SAXS measurements were performed on an AMUR-K bench-scale diffractometer 1 equipped with an OD-3 position-sensitive linear array detector 2 at a fixed wavelength λ =.1542 nm in the Kratky geometry in the range s = nm 1 θ (s = 4πsin --, λ where 2θ is the scattering angle). The receiving slit width was 2 µm. Since the samples differed in thickness, no absolute calibration was performed. The experimental data were normalized by the incident beam intensity and corrected for collimation aberrations. For all of the samples, we evaluated the fractal dimension D f using FRACTAL interactive software, which allows one to select the range where the scattering curve follows a power law, I(s) = const s D f, and to calculate D f. X-ray diffraction patterns of powders and pressed samples were collected on a DRON-3 diffractometer (Cu K α1 radiation, scan rate of 2 /min). Peak positions were determined with an accuracy of ±.5. RESULTS AND DISCUSSION We synthesized polymer-matrix nanocomposites consisting of Fe 2 O 3 Fe 3 O 4 and FeCoB nanoparticles in SKTN and SIEL siloxane rubber matrices: Fe 2 O 3 1 Designed at the Special Design Bureau, Shubnikov Institute of Crystallography, Russian Academy of Sciences. 2 Designed at the Budker Institute of Nuclear Physics, Siberian Division, Russian Academy of Sciences. INORGANIC MATERIALS Vol. 42 No. 5 26

3 498 YURKOV et al. 5 nm Fig. 1. TEM micrograph of Fe 2 O 3 Fe 3 O 4 nanoparticles in SKTN. 5 nm Fig. 2. TEM micrograph of Fe 2 O 3 Fe 3 O 4 nanoparticles in SIEL. Fe 3 O 4 SKTN, Fe 2 O 3 Fe 3 O 4 SIEL, FeCoB SKTN, and FeCoB SIEL. An important point is that, using oligomeric matrices, we obtained polymeric nanomaterials. During treatment of those materials with an appropriate polymerization initiator, the latter showed no weight loss. Therefore, the incorporation of nanoparticles favored polymerization of the siloxane oligomer. It seems likely that the nanoparticles acted as effective cross-linking centers. TEM examination confirmed the presence of nanoparticles in the synthesized materials. Figures 1 3 show typical TEM micrographs of our samples. The size of the Fe 2 O 3 Fe 3 O 4 nanoparticles in SKTN and SIEL was determined to be 6.9 ± 1.1 and 8.9 ± 1.3 nm, respectively. The size of the FeCoB nanoparticles in SKTN was 19.4 ± 5.4 nm. Comparison of the SAXS curves for nanoparticles in SKTN matrices shows that, in the range s =.3 2. nm 1, the slope of the curve for FeCoB nanoparticles is steeper than that for Fe 2 O 3 Fe 3 O 4 nanoparticles. It seems, therefore, likely that the smallest FeCoB nanoparticles are larger than the smallest Fe 2 O 3 Fe 3 O 4 nanoparticles. At the same time, the steeper slope of the scattering curve at s.3 nm 1 indicates that the Fe 2 O 3 Fe 3 O 4 system contains a small fraction of larger particles, which contribute very little to the particle size distribution curve. Moreover, the shape of the scattering curve for Fe 2 O 3 Fe 3 O 4 nanoparticles in SKTN points to a degree of ordering in this system. These nanoparticles are, most likely, more uniform in size than are the FeCoB nanoparticles. Unfortunately, because of the broad size distribution, we failed to determine characteristic dimensions in this system. We assume that the Fe 2 O 3 Fe 3 O 4 nanoparticles in SKTN have the form of thin platelets. Similar results were obtained for the SIEL-matrix nanomaterials. From the initial portion (s 1. nm 1 ) of the experimental SAXS curves, we calculated the distributions of particle volume with respect to particle size, D V (R). The integral equation R max Iq ( ) = ( ρ) 2 D V ( R)m 2 ( R)i ( qr) dr R min (1) 4 nm Fig. 3. TEM micrograph of FeCoB nanoparticles in SKTN. was solved to obtain D V (R) using the GNOM program (indirect Fourier transformation) under the assumption that the nanoparticles were spherical in shape. In Eq. (1), R is the radius of a sphere, i (x) = sin x xcos x is its form factor, and m(r) = R 3 is x π 3 its volume. In calculating D V (R), R min was set to equal zero, and R max was determined in each case as the min- INORGANIC MATERIALS Vol. 42 No. 5 26

4 Fe-CONTAINING NANOPARTICLES IN SILOXANE RUBBER MATRICES 499 D V (R) D V (R) Fe 2 O 3 Fe 3 O 4 FeCoB Fe 2 O 3 Fe 3 O 4 FeCoB R, nm R, nm Fig. 4. Size distributions for nanoparticles in SKTN. Fig. 5. Size distributions for nanoparticles in SIEL. imum value at which the rms deviation of the curve calculated by Eq. (1) from the experimental data was within 1.7 (the theoretically predicted deviation of 1. was unattainable because of the weak systematic intensity deviations comparable to the noise in our measurements). Using POLYMIX7, the experimental SAXS curve was modeled with mixtures of several systems of polydisperse spherical particles by simultaneously determining the average radius and full width at half maximum of the radius distribution for each system. Each size fraction was represented as a set of spherical particles of different radii, with their volume fractions obeying the Schulz law. The overall distribution was then the sum of two distributions with different parameters, which were determined simultaneously by minimizing the sum of the squares of the deviations of the calculated curve from the experimental data. We considered two systems: small and large particles. The overall distribution curves corresponded to the total distribution of the nanoparticles in the samples, and the shape of the curves was in good agreement with the indirect Fourier transformation results obtained with GNOM. The experimental SAXS curves were used to calculate the D V (R) distribution functions (Figs. 4, 5). As would be expected, the size distribution of the FeCoB nanoparticles in both SKTN and SIEL is broader. On the whole, the Fe 2 O 3 Fe 3 O 4 nanoparticles are smaller than the FeCoB nanoparticles. In addition, the distribution curves for the Fe 2 O 3 Fe 3 O 4 nanoparticles have a narrow peak around R = 5 nm and indicate the presence of a small amount of larger nanoparticles, up to 8 nm in radius. The distribution curves show that the amount of FeCoB nanoparticles in SKTN is notably smaller than that of Fe 2 O 3 Fe 3 O 4 nanoparticles: the peak in the distribution curve of the FeCoB nanoparticles is far weaker (Fig. 4). The amounts of Fe 2 O 3 and FeCoB nanoparticles in the SIEL matrices differ little, and the distribution curves in both systems have a maximum at R = 5 nm (Fig. 5). It is well known that nanoparticle sizes evaluated by SAXS and TEM may differ significantly [15] because TEM probes only a small region, while SAXS analyzes the entire nanomaterial. The calculated fractal dimensions (table) indicate that the formation of FeCoB nanoparticles in both matrices was accompanied by the formation of welldefined boundaries or surfaces which differed in elastic density from the matrix. These boundaries are responsible for the scattering at the smallest angles and seem to be the surfaces of the Fe-containing nanoparticles. Since we did not investigated the structure of matrices free of nanoparticles, the detailed nature of the boundaries is still unclear. According to elemental analysis data, the composition of the Fe x O y SKTN material is Fe 6.96, Si 33.51, H 7.21, and C 29.87%, which indicates that the material contains enough oxygen for the formation of stoichiometric Fe 3 O 4, whose content is 8.4 wt %. Calculated fractal dimensions Sample Fe 2 O 3 Fe 3 O 4 SKTN 1.3 FeCoB SKTN 3. Fe 2 é 3 Fe 3 O 4 SIEL 1.8 FeCoB SIEL 3.1 D f INORGANIC MATERIALS Vol. 42 No. 5 26

5 5 YURKOV et al Intensity, arb. units Intensity, arb. units θ, deg θ, deg Fig. 6. XRD pattern of the Fe 2 O 3 SKTN sample. Fig. 7. XRD pattern of the Fe 2 O 3 SIEL sample. The XRD pattern of that sample (Fig. 6) shows reflections at 2θ = and 62.47, attributable to Fe 3 O 4 (JCPDS PDF, no ), and at 2θ = 35.63, corresponding to Fe 2 O 3 (JCPDS PDF, no ). According to elemental analysis data, the composition of the Fe 2 O 3 Fe 3 O 4 SIEL material is Fe 25.52, Si 3.4, H 5.73, and C 23.67, which indicates that the material contains enough oxygen for the formation of stoichiometric Fe 3 O 4, whose content is wt %. The XRD pattern of that sample (Fig. 7) shows weak reflections at 2θ = 29.68, 43.27, and 62.83, attributable to Fe 3 O 4 (JCPDS PDF, no ), and at 2θ = and 57.23, arising most likely from Fe 2 O 3 (JCPDS PDF, no ). The large width of reflections and the small number of strong reflections are typical of nanometer-sized fillers in stabilizing matrices. Unfortunately, this impedes unambiguous determination of the phase composition of nanoparticles by XRD, but the entire set of elemental analysis, EMR, and XRD data allows us to infer the composition of the nanoparticles incorporated in SIEL and SKTN. According to XRD data, the materials containing FeCoB nanoparticles are x-ray amorphous. Figure 8 shows the EMR signals of the samples containing FeCoB nanoparticles. The resonances are seen to be asymmetric and broad ( H pp 248 ka/m), with a significant downfield shift as compared to the resonance field H r 264 ka/m corresponding to the g-factor of free electrons, g = 2.. These features attest to high magnetization of the samples. The same is evidenced by the microwave absorption hysteresis in our samples in low magnetic fields [16]. The iron oxide nanoparticles produced by the cluspol technique are superparamagnets, as evidenced by the lack of microwave absorption hysteresis. The EMR line of the nanoparticles in SIEL is symmetric and is well fitted by two Gaussians (Fig. 9). One of the components is markedly broader than the other, but their resonance fields differ insignificantly. The EMR signal of the iron oxide nanoparticles in SKTN is notably asymmetric and is well fitted by three Gaussians. The two stronger components are close in parameters to the two components in the spectrum of the nanoparticles in SIEL; the third component is markedly weaker and is slightly shifted upfield (Fig. 1). The hysteretic behavior of the samples suggests that, in both matrices, the FeCoB magnetic nanoparticles are in a blocked state [17]. Note that the absorption line in the EMR spectrum of the FeCoB SIEL sample is narrower than that of the FeCoB SKTN sample (by about 8 ka/m). It seems likely that the two Gaussian components in the spectrum of the nanoparticles in SIEL, which also dominate the spectrum of the FeCoB in SKTN FeCoB in SIEL H, ka/m Fig. 8. EMR signals of FeCoB nanoparticles in SKTN and SIEL. INORGANIC MATERIALS Vol. 42 No. 5 26

6 Fe-CONTAINING NANOPARTICLES IN SILOXANE RUBBER MATRICES 51 Fe 2 O 3 nanoparticles range in size from 6 to 9 nm, and the FeCoB nanoparticles, from 19 to 22 nm, in reasonable agreement with SAXS results. EMR data indicate that the nanoparticles produced by the cluspol process are superparamagnets, as evidenced by the lack of microwave absorption hysteresis. The FeCoB nanoparticles in both the SKTN and SIEL matrices are in a blocked state, as evidenced by their hysteretic behavior H, ka/m Fig. 9. EMR spectrum of iron oxide nanoparticles in SIEL. The open squares represent the experimental data, the dashed lines show a fit with two Gaussians, and the solid line is their sum. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (grant nos , , and ), the President s Grants Council (grant no. MK ), and the Russian Academy of Sciences (basic research program Engineering of Novel Materials). REFERENCES H, ka/m Fig. 1. EMR spectrum of iron oxide nanoparticles in SKTN. The open squares represent the experimental data, the dashed lines show a fit with three Gaussians, and the solid line is their sum. nanoparticles in SKTN, correspond to magnetite (broad component) and γ-fe 2 O 3 (narrow component). The weak narrow component in the spectrum of the nanoparticles in SKTN may be due to an iron oxide phase under special conditions, e.g., on the particle surface. CONCLUSIONS Fe 3 O 4 Fe 2 O 3 and FeCoB nanoparticles have been for the first time produced in SIEL and SKTN siloxane rubber matrices. An important point is that, using oligomeric matrices, we obtained polymeric materials and that, during treatment of these materials with an appropriate polymerization initiator, the latter showed no weight loss. According to TEM results, the Fe 3 O 4 1. Edwards, D.C., Review: Polymer Filler Interactions in Rubber Reinforcement, J. Mater. Sci., 199, vol. 25, no. 1, pp Kraus, G., Reinforcement of Elastomers, New York: Interscience, Katz, H.S. and Milewski, J.V., Handbook of Fillers and Reinforcements for Plastics, New York: Van Nostrand Reinhold, Pomogailo, A.D., Rozenberg, A.S., and Uflyand, I.E., Nanochastitsy metallov v polimerakh (Metal Nanoparticles in Polymers), Moscow: Khimiya, Yurkov, G.Yu., Gubin, S.P., Pankratov, D.A., et al., Iron(III) Oxide Nanoparticles in a Polyethylene Matrix, Neorg. Mater., 22, vol. 38, no. 2, pp [Inorg. Mater. (Engl. Transl.), vol. 38, no. 2, pp ]. 6. Gubin, S.P., Spichkin, Yu.I., Yurkov, G.Yu., and Tishin, A.M., Nanomaterials for High Density Magnetic Data Storage, Russ. J. Inorg. Chem., 22, vol. 47, suppl. 1, pp Baraton, M.I., Synthesis, Functionalization, and Surface Treatment of Nanoparticles, Los-Angeles: American Scientific, Koksharov, Yu.A., Gubin, S.P., Kosobudsky, I.D., et al., Electron Paramagnetic Resonance Spectra near the Spin-Glass Transition in Iron Oxide Nanoparticles, Phys. Rev. B: Condens. Matter, 21, vol. 63, no. 17, pp Gubin, S.P., Spichkin, Yu.I., Koksharov, Yu.A., et al., Magnetic and Structural Properties of Co Nanoparticles in Polymeric Matrix, J. Magn. Magn. Mater., 23, vol. 265, pp Gubin, S.P., Yurkov, G.Yu., and Kosobudsky, I.D., Nanomaterials Based on Metal-Containing Nanoparticles in Polyethylene and Other Carbon-Chain Polymers, Int. J. Mater. Prod. Technol., 25, vol. 23, no. 1/2, pp INORGANIC MATERIALS Vol. 42 No. 5 26

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