Molecular Design of Fe3O4-Containing Polyimide as a Route to

Ind. Eng. Chem. Res. 2008, 47, 4123–4130

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MATERIALS AND INTERFACES Molecular Design of Fe3O4-Containing Polyimide as a Route to Nanomagnetic Materials Su¨leyman Ko¨ytepe and Turgay Sec¸kin* Chemistry Department, Inonu UniVersity, 44280 Malatya, Turkey

γ-Fe3O4 doped polyimide films with different weight percentages (1, 5, and 10 wt % of Fe3O4) have been prepared and the effect of nonmagnetic γ-Fe3O4 content on the structural and magnetic properties has been studied. X-ray diffraction (XRD) studies revealed that the nanopowders obtained are magnetite. The calculated grain sizes from XRD data have been verified using scanning electron microscopy (SEM). SEM micrographs show that the powders consist of nanometer-sized grains. Magnetic hysteresis loops were measured at room temperature with a maximum applied magnetic field. As the γ-Fe3O4 content increases, the measured magnetic hysteresis curves become more and more narrow and the saturation magnetization and remanent magnetization both decreased. The prepared nanocomposites were identified in terms of their structure, morphology, and magnetic and thermal properties. These studies showed that the particles seem to be dispersed randomly, although the particles do appear to form aggregates at increasing particle loadings. Introduction Magnetic nanoparticles are of great interest for researchers from a wide range of disciplines, including magnetic fluids, catalysis, biotechnology/biomedicine, magnetic resonance imaging, data storage, and environmental remediation.1 Recently, simple and reproducible methods to synthesize magnetic nanocrystals with desired sizes, shapes, and self-assembly have drawn much attention, because of their unique size-dependent properties, such as magnetic, optical, electronic, and surface reactivity.1–10 Insights into these unique properties not only are important for fundamental understanding but they also have technological importance. Among the nanocrystals, magnetic nanoparticles and their dispersions have been used in various fields, such as biomedical,11 optical,12 electronics,13 chemical,14 and mechanical15 applications. In addition, these nanoparticles serve as ideal systems for fundamental studies such as superparamagnetism, magnetic dipolar interactions,16–18 to understand molecular interactions at the emulsion droplet interface.19,20 However, an unavoidable problem that is associated with particles in this size range is their intrinsic instability over longer periods of time. Such small particles have a tendency to form agglomerates to reduce the energy associated with the high surface-area-tovolume ratio of the nanosized particles. Polymers are often employed to passivate the surface of the nanoparticles during or after the synthesis, to avoid agglomeration. Among these materials, polyacrylamide magnetic iron oxide nanocomposites, which are used as control agents for molecular imaging, using magnetic resonance imaging (MRI), is reported.21 Other appealing features have been observed when the polyimide has been loaded with magnetic materials,22 making them polymer magnetic composites, and these materials are supported to have applications as memory devices, magnetic fluids, magnetic sensors, etc.23 The urgent market demand to produce higherperformance electronic devices with smaller size, lightweight, and better-quality developing polyimide (PI) films with a low coefficient of thermal expansion (CTE) has become one of the * To whom correspondence should be addressed. E-mail: tseckin@ inonu.edu.tr.

most important issues.24,25 The reason for this behavior is that polyimide overcomes the thermal concentration and associated reliability problems produced by the mismatch between polymers. Metals and ceramics that comprise electronic devices enable polyimides to meet some ultimate requirement in the demanding applications. To date, one of the best ways to reduce the CTE is to incorporate inorganic materials such as γ-Fe2O3 into the polyimide matrix.25 Earlier work on γ-Fe2O3-polymer composites 27–39 to understand the effect of the loading of magnetic oxides and other additives in the polymer matrices showed interesting morphology and thermal and electrical behavior of the synthesized polymer composite films. In this study, novel polyimide-Fe3O4 hybrid nanocomposite films (PI-γ-Fe3O4) have been developed from the poly(amic acid) of 2,6-diaminopyridine with different weight percentages (1, 5, and 10 wt %) of Fe3O4, using N-methyl-2-pyrrolidone (NMP) as an aprotic solvent. The prepared PI-γ-Fe3O4 nanocomposite films were characterized for their structure, morphology, and thermal behavior, using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermal analysis (TGA/DSC) techniques. Magnetic properties of the nanomagnetic Fe3O4 particles coated with polyimide were measured at room temperature, using a vibration sensing magnetometer (VSM). These studies showed the homogeneous dispersion of γ-Fe3O4 in the polyimide matrix with an increase in the thermal steadiness of the composite films on γ-Fe3O4 loadings. Magnetization measurements (magnetic hysteresis traces) have shown very high values of coercive force, indicating their possible use in memory devices and in other related applications. The magnetic hysteresis results were interesting and also are consistent with the SEM images. Experimental Section The chemicals used for the synthesis–FeSO4.7H2O, FeCl3 · 6H2O, 25% aqueous ammonia, oleic acid, 35% hydrochloric acid, NMP, hexane (HPLC grade), acetone, etc.–were procured from Merck. All these chemicals were

10.1021/ie701690w CCC: $40.75  2008 American Chemical Society Published on Web 05/17/2008

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reagent-grade (RG) (except hexane) and were used without any further purification. The water used in all of the experiments was triply distilled and filtered from a 0.22 µm size filter. The synthesis of magnetite nanoparticles was performed by precipitating iron salts, FeSO4 · 7H2O and FeCl3 · 6H2O, with molar ratio of 1:2 in an alkaline medium, on constant stirring. For all the experiments, the initial pH of the iron salts mixture was adjusted to a particular value (0.9 to 6.9) and was digested for 30 min. The pH then was increased rapidly (within 20 s) to 10 ((0.1), by adding 25% of aqueous ammonia at a constant rate. After 30 min of the digestion at the same pH and stirring speed, oleic acid coating was performed dynamically at 70 °C for 30 min. After surfactant coating, these particles were initially washed with triply distilled water at 60 °C, to remove the ionic impurities. Later, the precipitate was washed with an acetone/hexane mixture, to remove excess surfactant that was present in the precipitate. After washings, the aforementioned obtained surfactant-coated nanoparticles were dried at 35 °C for 48 h in an argon atmosphere, and, later, dried particles were redispersed in hexane. To understand the initial pH effect on the nucleated particle size and composition, the iron salts mixture pH values are adjusted to different values of 0.9 (( 0.1), 1.79 (( 0.1), 3.9 (( 0.1), 4.9 (( 0.1), 5.9 (( 0.1) and 6.9 (( 0.1). To understand the effect of temperature, the iron salts solution temperatures are adjusted to 30 or 60 °C, before the initiation of the magnetite precipitation. The samples were characterized by X-ray diffraction (XRD) for the crystal structure, average particle size, and the concentration of impurity compounds present. A Rigaku model Rad B-Dmax II powder X-ray diffractometer was used to obtain XRD patterns of these samples. The 2θ values were taken from 20° to 110°, with a step size of 0.04° 2θ, using Cu KR´ radiation (λ ) 2.2897 Å). The dried samples were dusted on to plates with low background. A small quantity of 30 (( 2) mg spread over 5 cm2 area used to minimize error in peak location and also the broadening of peaks due to thickness of the sample is reduced. These data illustrate the crystal structure of the particles and also provide the interplanar space (d). The broadening of the peak was related to the average diameter (L) of the particle, according to Scherrer’s formula,40 i.e., L ) 0.9λ/(∆ cos θ), where λ is the X-ray wavelength, ∆ the line broadening measured at half-height, and θ the Bragg angle of the particles. Infrared (IR) spectra were recorded on KBr pellets in the range of 4000-650 cm-1 on an ATI UNICAM Systems 2000 Fourier transform spectrometer. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed with Shimadzu model DSC-60 and model TGA-50 thermal analyzers, respectively. Thermal analysis was assessed by TGA and DSC at a heating rate of 10 °C/min in nitrogen (N2). Chemical composition analysis by energy-dispersive X-ray (EDX) study was performed with an EDX system (Ro¨nteck xflash detector analyzer, associated with scanning electron microscopy (SEM) equipment (model Leo-Evo 40xVP). Incident electron beam energies from 3 keV to 30 keV had been used. In all cases, the beam was oriented at normal incidence to the sample surface, and the measurement time was 100 s. All the EDX spectra were corrected using the ZAF correction, which takes the influence of the matrix material on the obtained spectra into consideration. A Cryogenics vibrating sample magnetometer (VSM) (model Q 3398) was used for magnetization measurements. These measurements were taken from 0 kOe fields to (7 kOe fields. From these fields, relative to the magnetization

Figure 1. Preparation route for the PI-Fe3O4 nanocomposite films.

curve patterns, saturation magnetization values of the samples were measured. Preparation of Polyimide-γ-Fe3O4 (PI-γ-Fe3O4) Nanocomposite Films PI-γ-Fe3O4 nanocomposites were prepared with different weight percentages of γ-Fe3O4 (1, 5, 10 wt %). The details of the method are as follows. One mole of 2,6-diaminopyridine (DAP) and 1 mol of benzophenone tetracarboxylic dianhydride (BTDA) in 25 mL NMP gave a viscous gel of poly(amic acid), which is used for the experiment. Different weight percents of γ-Fe3O4 (1, 5, 10 wt %) were sonicated for 1 h in an oleic acid solution and were then added to the weighed poly(amic acid), and the suspension was stirred for 2 h at room temperature under the flow of nitrogen. The composites films were then cast from the suspension placed on a glass plate that was kept in a humidityfree chamber. The films were then cured to obtain PI-Fe3O4 films, as shown in Figure 1. Results and Discussion Nanoparticle Characterization. The sample that was prepared with an initial pH of 0.9 exhibits a smaller particle size than the other samples. The driving force for the dissolution-crystallization process during growth is interfacial tension, which leads to an increase in particle growth or a decrease in specific surface area. The excess ions in the reaction mixture reduce the interfacial tension. Therefore, because of higher ionic strength in the solutions where the initial pH was 0.9, the particle growth is less, compared to those prepared with a high initial pH. This explains smaller particles obtained at pH 0.9. Compared to pH 0.9, the ionic strength of the mixture at pH 1.79 is less and, hence, a higher interfacial tension, which is the contributing parameter for the increasing particle size. At pH 3.9 and 4.9, particles are nucleated after the formation of a Fe2+-ferrihydrite complex. In comparison to that observed at pH 1.79, the reaction rate

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Figure 2. X-ray diffraction (XRD) pattern of the polyimide-γ-Fe3O4 hybrid nanocomposite films (PI-Fe3O4).

in this case is less for the formation of a Fe2+-ferrihydrite complex. Earlier studies show that, at high reaction rates, more initial nuclei are formed and it would result in a large number of particles with smaller size. The solutions digested at an initial pH of >4.9 show the impurity of goethite, because of the oxidation of ferrous (Fe2+) ions to the ferric (Fe3+) state, which leads to excess Fe3+ ions in the system. Because of the depletion of Fe2+ ions in the iron salt mixture, the Fe3+/(Fe3+ + Fe2+) ratio shifts toward a value of 1, where the required ratio for magnetite formation should be (within a range of 0.6 and 0.66). This could lead to the formation of nonmagnetic impurities during coprecipitation. The spinel structure and the average iron oxide particle size (10 nm) were estimated from the XRD peak line width analysis. Because of the strongly broadened X-ray line width of the films, the XRD method seemed to be problematic, in regard to distinguishing between the γ-Fe2O3 and Fe3O4 phases. Figure 2 shows the XRD patterns of samples that have been prepared from iron salt solutions. The XRD patterns obtained for samples confirm the cubic inverse spinal structure with its characteristic peaks. Quantitative analysis on the percentage of goethite impurities has been performed, using the intensity of the peaks of XRD patterns. The calibration curve has been obtained using a series of mixtures with varying amounts of pure goethite and magnetite41 in the ratio of 20:80, 40:60, 55:45, and 80:20, respectively. For these standard mixtures, the areas under a prominent peak of goethite (1 1 0), IG and magnetite26 (4 0 0), IM are measured and a plot of IG/(IG + IM) vs A/(A + B) is constructed (here, A is the amount of goethite and B is the amount of magnetite used to obtain the XRD pattern). Although the prominent peak in the case of magnetite was (3 1 1), it was not taken into consideration due to the overlap of (3 1 1) peaks with the (1 1 1), (0 2 1) and (1 3 0) peak of goethite. The peak intensities of the unknown samples are compared with the intensity of the same peak in a calibration curve to estimate the amount of goethite and magnetite. The estimated percentage of goethite and magnetite are given in Figure 3, which shows that shows the the goethite content with increasing pH. The existence of any impurity compound other than magnetite is not observed. The characteristic peaks of magnetite are (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and

Figure 3. Estimated percentage of goethite and magnetite for the samples prepared under different pH conditions, and the magnetization, as a function of weight percentage.

Figure 4. FTIR spectrum of the PI-γ-Fe3O4 composite.

(4 4 0). The low intensity peaks were not observed in any of our samples, because the percentage of magnetite impurities is very low. The unit-cell parameter of the magnetite samples is varied from ∼0.836 nm to ∼0.837 nm, indicating the partially oxidized magnetite. The typical unit-cell parameter reported for magnetite is ∼0.839 nm. The XRD patterns of the samples prepared at the initial pH values of 5.9 and 6.9 confirms the existence of goethite. The crystal structure of goethite is orthorhombic, with cell dimensions of a ) 0.4608 nm, b ) 0.9956 nm, and c ) 0.30215 nm. Goethite consists of double bands of edge-sharing FeO3(OH)3 octahedral. The double bands are linked by corner-sharing in such a way that 2 × 1 octahedral tunnels are crossed by hydrogen bridges.22 These patterns confirm the increasing goethite impurity with increasing initial pH. The FTIR spectrum of the pure PI is shown in Figure 4a. This figure shows the presence of aliphatic stretching frequencies at 2850-2890 cm-1, a symmetric imide frequency (CdO) at 1720-1730 cm-1, asymmetric frequencies (CdO) stretching at 1790-1765 cm-1, CsN bending at 730-760 cm-1, and a typical poly(amic acid) absorption band at 3400-2900 cm-1 that corresponds to amide (sNHs) and

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Figure 5. SEM images of the (a) polyamide (PI) and PI-γ-Fe3O4 composites ((b) 1%, (c) 5%, and (d) 10% PI).

Figure 7. DSC second heating curves thermograms of (a) PI and PI-γFe3O4 composites ((b) 1%, (c) 5%, and (d) 10% PI).

Figure 6. DSC thermograms of (a) PI and PI-γ- Fe3O4 composites ((b) 1%, (c) 5%, and (d) 10% PI).

Figure 8. TGA thermograms of PI (a) and PI-γ-Fe3O4 composites ((b) 1%, (c) 5%, and (d) 10% PI).

acid (OH) stretching. The (CdO) stretching frequencies of carboxylic acid at 1720 cm-1 and the (CsN) stretching frequencies of amide at 1540 cm-1 were not observed, indicating the formation of polymer. The IR spectrum of the PI-γ-Fe3O4 composite shown in Figure 4b had all the frequencies of Figure 4a with additional frequencies at 650 cm-1, which correspond to the metal-oxygen vibration frequencies of pure Fe3O4. There was a slight blue shift (,5 cm-1) in these two peaks, when compared to that of pure γ-Fe2O3 (not shown in the figure).42,43 Scanning Electron Micrograph Study. Figures 5a-d show SEM images of the pure polyimide and PI-γ-Fe3O4 composites under low and high resolution. Figure 5a shows the SEM image of pure PI under low and high resolution. The film shows some pores, which are usually observed due to some deformation of polymer film during casting. The SEM images of PI-γ-Fe3O4 (1 wt % γ-Fe3O4), observed in Figure 5b, showed particle agglomerates under both resolutions. The agglomerates of the iron oxide particles seem to be submicrometer in size, whereas the average particles of the Fe3O4, as mentioned previously, were nanosized. The low concentration of the oxide might have contributed to the particle agglomeration during film formation. Also, the dispersion of the oxide particles is not uniform, which again has been caused by its low concentration. Figure 5c shows the SEM image of PI-γ-Fe3O4 (5 wt % Fe3O4). The dispersion appears to be more dense, when compared with its predecessor (5 wt %); however,

some submicrometer-sized agglomerates are also noticed, along with some pores of the original polymer film. The SEM image of PI-γ-Fe3O4 (10 wt % Fe3O4) shown in Figure 5d had a uniform dispersion of the iron oxide particles in the polymer, even covering the pores observed in the polyimide film. The films appear to be more homogeneous; however, particle agglomerates also cannot be ruled out in this case. From the SEM images, it may be concluded that the higher percentage of Fe3O4 (up to 10 wt % loading) shows better homogeneity and, therefore, may possess higher thermal stability. Figure 6 shows the differential scanning calorimetry (DSC) traces for pure PI and DSC traces for 1 to 10 wt % Fe3O4 loaded PI-γ-Fe3O4. Figure 6a shows a broad endothermic peak at 100 °C, which is due to the removal of hydrated water from the polymer. The glass-transition temperature (Tg) is evidenced by a step that exhibits onset, inflection, and end points. Figure 6b for the composite shows a broad endothermic peak at 98 °C that corresponds to the removal of hydrated water in the composite. The Tg was not clearly visible on the DSC trace; however, upon close observation, the Tg was predicted at 162.8 °C for 10% loading, whereas values of 141.6, 150.4, and 152.1 °C were obtained for pure PI , 1% PI-γ-Fe3O4, and 5% PI-γ-Fe3O4, respectively. The increase in the Tg value from 141.6 °C to 162.8 °C shows an increase in the γ-Fe3O4 loadings in the composite films. Differential scanning calorimetry (DSC) second heating curves of PI-Fe3O4 is reported in Figure 7 when the compound has

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Figure 9. Magnetic hysteresis loop trace and M-T of PI-γ-Fe3O4 composite film: (a and c) 5% PI and (b and d) 10% PI.

been heated at a rate of 10 °C/min. The Tg value for PI-Fe3O4 composites increases up to 10 °C for 10 wt % Fe3O4 and then decreases for additions above 10 wt % POSS. In the case of PI-Fe3O4 composites, however, there is a continuous reduction in the Tg with incorporation of Fe3O4, which leads to a monotonic decrease in Tg with an increasing level of additive filled. The Tg of PI was observed to decrease as much as 16 °C for the composites that contain 10 wt % Fe3O4. TGA analysis of virgin PI (curve a in Figures 7 and 8), and the PI-γ-Fe3O4 is given in Figure 8a-d. Their thermal decomposition temperatures (Td, which denotes the temperature at 5% mass loss) were measured via TGA and showed that the introduction of Fe3O4 into polymer backbones increased thermal stability. We found that all materials had an extremely impressive thermal stability (Td ) 390-400 °C); however, in our studies, we found that Fe3O4 generally reduces interactions between polymer backbones. It is interesting to note that decreased interchain interactions normally decrease the Tg of a polymer; however, the rigid three-dimensional nature of the Fe3O4 seems to increase the rigidities of the polymer chain, increasing Tg. The geometric freedom of polymer chains is also well-known to greatly impact thermal properties. These effects will greatly impact the development of thermal properties in polymer nanocomposites. This confinement effect on the mobility has been related to the depression of Tg in amorphous polymer nanocomposites.44–47 The chemistry of the nanoscale filler influences two primary properties that are related to the polymer/nanofiller interaction. First, the chemistry of the Fe3O4 contributes to the enthalpic interaction with the polyimide chain. The enthalpic interactions play a large role in the efficiency of stress transfer across the Fe3O4/polyimide interface. These

enthalpic interactions can be defined by the van der Waals interactions between the nanoparticle and polyimide chains. The strength of these interactions strongly influences the morphology of a polymer nanocomposite. The van der Waals interaction between neighboring filler particles is also strongly influenced by the filler chemistry. These interparticle interactions are important at high volume fractions of the filler in the nanocomposite and in the strength of the filler aggregates. In terms of this latter effect, nanofillers can phase separate and form domains that are rich in the nanofiller and poor in the polymer. In these domains, if the interparticle interactions are highly attractive, the aggregate will behave as a large filler particle rather than independent nanoscale fillers. Conversely, if the interparticle attractions are weak, then the deformation processes related to the aggregate may play a large role in the storage and loss of applied mechanical energy. These deformation processes will greatly influence the development of thermal properties in the polymer nanocomposite. Magnetic Hysteresis Study. Figures 9a and 9b show the magnetic hysteresis loop at room temperature for all the different weight-percentage compositions of γ-Fe3O4 loaded into the polyimide matrix. The presence of magnetic hysteresis confirms the magnetic nature of the composites. The saturation magnetization values are higher for the 5 and 10 wt % samples, whereas the coercivity is higher for the 10 wt % sample. These interesting results may be explained as follows. The amount of Fe3O4 loadings is dominating, which has a pronounced effect on the coercivity (i.e., freely oriented in loadings of 5 wt % Fe3O4 (Figure 9a) and 10 wt % Fe3O4 (Figure 9b)), giving rise to higher Ms values. The increase in the amount of loading of γ-Fe3O4 particles might have

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Figure 10. Magnetization curve at room temperature for the sample prepared with iron salt solutions of initial pH at 0.9 at 25 °C (1% PI-γ-Fe3O4 composite film).

contributed to this effect; however, the 1 wt % sample shows a soft hysteresis, an interesting feature (similar values of Mr and Ms), thereby indicating that this sample may have application in bubble memory devices. Possible reason for the higher value in this sample is attributed to the chemical homogeneity of the dispersed γ-Fe3O4 particles achieved in this film. It may also be mentioned here that the effect of particle size on magnetic behavior cannot be ruled out completely, although it is minimum in our case, as we have used the Fe3O4 prepared in a single lot that has an almost similar morphology throughout. The coercivity values (Hc) also collaborate with the Ms values, as a ferromagnetic material generally increases as the hysteresis trace broadens. Also note that the magnetic hysteresis traces and SEM images for our samples augment each other. On the basis of the magnetic hysteresis traces, we may conclude that the polymer matrix has hindered the free rotation of the easy axis of magnetization of γ-Fe3O4 particles in the composite film, thereby increasing these Ms values, when compared to that of pure γ-Fe3O4. Figure 10 shows the magnetization curve at room temperature for the sample that was prepared with iron salt solutions with an initial pH value of 0.9 at 25 °C (1% PI-γ-Fe3O4 composite film). The saturation magnetization values increases from 44 emu/g to 58 emu/g when the pH of the solution has increased from 0.9 to 6.9. Upon further increases in pH, Ms decreases. From the XRD analysis, it is evident that there was no goethite formation up to an initial pH of 4.9. In fact, when the initial pH of the solution was increased from 0.9 to 4.9 the particle size has increased from 60 to 90 nm, which was also evident from the increase in Ms values observed in Figure 10. These results are consistent with the earlier reports where a decrease in Ms with decreasing crystallite size is observed. Therefore, the reason for the increase in Ms is clearly due to an increase in particle size at these pH values. Above 4.9 pH, we have already seen that the amount of goethite is increasing. Therefore, the reduction in Ms above pH 4.9 is expected to be due to the presence of goethite. The goethite is known to be antiferromagnetic in nature, with spins parallel to the c-axis, which is evident from the low magnetization values observed in our samples. The magnetic moments is expected to be either due to an unbalanced population of magnetic secondary lattices in nanomaterials with consists of few magnetic atoms or a disturbance of surface atomic layers which consists of atoms with the same spin orientation. The magnetic properties such

Figure 11. EDX of PI-γ-Fe3O4 composites: (a) 1%, (b) 5%, and (c) 10% PI.

as saturation magnetization, hysteresis, eminence and coercivity show strong grain size dependence due to the influence of magnetic domain state of the samples on the grain size. Domain states change from single domain to multidomain as the grain size increases. Energy-dispersive X-ray analysis (EDX) studies (see Figures 11 and 12) demonstrated that γ-Fe3O4 nanoparticles seem to be dispersed randomly, although the particles do appear to form aggregates at increasing particle loadings. Iron oxide nanoparticles showed mosaic nanopatterns and nanoparticles surrounded by polyimides. Controlling more than a single type of nanoparticle in specific positions can provide an opportunity to utilize the unique properties of each type of nanoparticle. Thus, the methodology demonstrated in this study can be a good example of how different types of functional nanometer-sized building blocks can be organized in specific arrangements by physical and chemical assembling procedures on structured templates.

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Figure 12. EDX mapping of the PI-γ- Fe3O4 composites: (a) 1% PI and (b) 5% PI.

The size of the filler is at the heart of the “nano” effect on mechanical properties in nanocomposites. Closely coupled with the size of the filler is the shape of the nanofiller. Generally, we can consider the filler size and shape as dictating two important contributions to the overall polymer nanocomposite properties. Conclusions In this paper, the magnetic properties of polymer nanocomposites that contain nanoparticles of magnetite in a polyimide (PI) matrix were investigated. The compositions were synthesized by means of an original method “in situ” preparation, which allows one to obtain the PI films and different concentrations of iron oxide particles in the PI matrix. The methods used in the present study to prepare PI-Fe3O4 magnetic nanocomposites possess different morphologies for different compositions. An increase in the glasstransition temperature (Tg) of the nanocomposites when compared with the pure polymer (polyimide) indicated better dimensional and thermal stability of the films. The magnetic hysteresis results were interesting and also collaborate with the SEM images. Acknowledgment Authors would like to thank the TUBITAK-TBAG Turkish Scientific Research Council for the financial support (under Project No. TBAG-105T386). Literature Cited (1) Parvin, K.; Ma, J.; Ly, J.; Sun, X. C.; Nikles, D. E.; Sun, K.; Wang, L. M. Synthesis and magnetic properties of monodisperse Fe3O4 nanoparticles. J. Appl. Phys. 2004, 95, 7121. (2) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Colloidal nanocrystal shape and size control: The case of cobalt. Science 2001, 291 (5511), 2115. (3) Chen, J.; Xu, L.; Li, W.; Gou, X. R-Fe2O3 nanotubes in gas sensor and lithium-ion battery applications. AdV. Mater. 2005, 17 (5), 582–586. (4) Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. Monodisperse MFe2O4 (M ) Fe, Co, Mn) Nanoparticles. J. Am. Chem. Soc. 2004, 126 (1), 273. (5) Park, J.; An, K.; Hwang, Y.; Park, J. E. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hyeon, T. Ultra-large-scale syntheses of monodisperse nanocrystals. Nat. Mater. 2004, 3 (12), 891. (6) Jiang, L.; Sun, W.; Kim, J. Preparation and characterization of ω-functionalized polystyrene-magnetite nanocomposites. Mater. Chem. Phys. 2007, 101 (2-3), 291. (7) Satyanarayana, L.; Reddy, K. M.; Manorama, S. V. Nanosized spinel NiFe2O4: A novel material for the detection of liquefied petroleum gas in air. Mater. Chem. Phys. 2003, 82, 21. (8) Gudiksen, M. S.; Wang, J.; Lieber, C. M. Synthetic control of the diameter and length of single crystal semiconductor nanowires. J. Phys. Chem. B 2001, 105 (19), 4062. (9) Wang, J.; Chen, Q.; Zeng, C.; Hou, B. Single-crystalline nanowires of Fe3O4. AdV. Mater. 2004, 16, 137.

(10) Pathmamanoharan, C.; Philipse, A. P. Preparation and properties of monodisperse magnetic cobalt colloids grafted with polyisobutene. J. Colloid Interface Sci. 1998, 205 (2), 340. (11) Okawa, K.; Sekine, M.; Maeda, M.; Tada, M.; Abe, M.; Matsushita, N.; Nishio, K.; Handa, H. Heating ability of magnetite nanobeads with various sizes for magnetic hyperthermia at 120 kHz, a noninvasive frequency. J. Appl. Phys. 2006, 99, 08H102. (12) Philip, J.; Jaykumar, T.; Kalyanasundaram, P.; Raj, B. A tunable optical filter. Meas. Sci. Technol. 2003, 14 (8), 1289. (13) Ban, S.; Korenivski, V. Pattern storage and recognition using ferrofluids. J. Appl. Phys. 2006, 99, 08R907. (14) Vander, W. R. L.; Ticich, T. M. Comparative flame and furnace synthesis of single-walled carbon nanotubes. Chem. Phys. Lett. 2001, 336 (1-2), 24. (15) Raj, K.; Moskowitz, B.; Casciari, R. Advances in ferrofluid technology. J. Magn. Magn. Mater. 1995, 149 (1-2), 174. (16) Elmore, W. C. Ferromagnetic Colloid for Studying Magnetic Structures. Phys. ReV. 1938, 54, 309. (17) Luo, W.; Nagel, S. R.; Rosenbaum, T. F.; Rosensweig, R. E. Dipole interactions with random anisotropy in a frozen ferrofluid. Phys. ReV. Lett. 1991, 67 (19), 2721. (18) Butter, K.; Bomans, P. H. H.; Frederik, P. M.; Vroege, G. J.; Philipse, A. P. Direct observation of dipolar chains in iron ferrofluids by cryogenic electron microscopy. Nat. Mater. 2003, 2 (2), 88. (19) Philip, J.; Prakash, G. G.; Jayakumar, T.; Sundaram, P. K.; Raj, B. Stretching and Collapse of Neutral Polymer Layers under Association with Ionic Surfactants. Phys. ReV. Lett. 2002, 89, 268301. (20) Philip, J.; Gnanaprakash, G.; Jayakumar, T.; Kalyanasundaram, P.; Raj, B. Three Distinct Scenarios under Polymer, Surfactant, and Colloidal Interaction. Macromolecules 2003, 36 (24), 9230. (21) Zhang, Y.; Kohler, N.; Zhang, M. Surface modification of superparamagnetic magnetite nanoparticles and their intracellular uptake. Biomaterials 2002, 23 (7), 1553. (22) Schwertmann, U.; Cornell, R. M. Iron Oxides in the Laboratory; VCH Verlagsgesellschaft mbH: Germany, 1991. (23) Jolivet, P.; Chane´ac, C. ; Tronc, E. Iron Oxide Chemistry. From Molecular Clusters to Extended Solid Networks. Chem. Commun. 2004, 5, 481. (24) Tronc, E.; Belleville, P.; Jolivet, J. P.; Livage, J. Transformation of ferric hydroxide into spinel by Fe(II) adsorption. Langmuir 1992, 8 (1), 313. (25) Brydson, J. A. Plastics Materials, 7th Ed.; Butterworth-Heinemann: Boston, MA, 1999. (26) Cullity, B. D. Introduction to Magnetic Materials; AddisonWesley: Reading, MA,1972. (27) Parodi, F. ComprehensiVe Polymer Science Series, Vol. 5; Geoffrey, A., Ed.; Oxford University Press: New York, 1989. (28) Mark, H. F.; Bikales, N. M.; Overberger, C. G.; Crosch-witz, J. I. M. Encyclopedia of Polymer Science and Engineering, Vol. 12; Wiley: New York, 1988; p 363. (29) Hallensleben, M. L. In Handbook of Polymer Synthesis, 2nd Ed.; Nuyken, O. H. R., Swift, G., Eds.; CRC Press: Boca Raton, FL, 2004. (30) Seckin, T.; Koytepe, S.; Cetinkaya, B.; Ozdemir, I.; Yigit, B. Synthesis and properties of novel polyimides from dichloro(1,3-p-dimethylaminobenzylimidazolidine-2-ylidene) p-cymene ruthenium (II). Des. Monomers Polym. 2003, 6 (2), 175. (31) Mittal, K. L. , Ed. Polyimides: Synthesis, Characterization and Application, Vols. 1 and 2; Plenum Press: New York, 1984. (32) Feger, C.; Khojasteh, M. M.; Htoo, M. S. AdVances in Polyimide Science and Technology; Technomic Publishing: Lancaster, PA, 1993. (33) Hasegawa, M.; Horie, K. Photophysics, photochemistry, and optical properties of polymides. Prog. Polym. Sci. 2001, 26, 259. (34) Mallkpour,; S, E.; Hajipour, A. R.; Fard, R. R. Synthesis of novel heterocyclic polyimides containing trimellitimide acid moieties. J. Appl. Polym. Sci. 2001, 79, 1716. (35) Koo, O. M.; Rubinstein, I.; Onyuksel, H. Role of nanotechnology in targeted drug delivery and imaging. Nanomed. Nanotechnol. Biol. Med. 2005, 1, 193. (36) Vijayanand, H. V.; Arunkumar, L.; Gurubasawaraj, P. M.; Sharma, P. M. V.; Basavaraja, S.; Saleem, A.; Venkataraman, A.; Ghanwat, A.; Maldar, N. N. Synthesis and characterization of polyimide-γ-Fe2O3 nanocomposites. J. Appl. Polym. Sci. 2007, 103, 834. (37) Moffat, B. A.; Reddy, G. R.; McConville, P.; Hall, D. E.; Chenevert, T. L.; Kopelman, R. R.; Philbert, M.; Weissleder, R.; Rehemtulla, A.; Ross, B. D. A novel polyacrylamide magnetic nanoparticle contrast agent for molecular imaging using MRI. Mol. Imaging 2003, 2, 324. (38) Chung, Y.; Lim, K. S.; Kim, C. K.; Kim, Y. H.; Yoon, C. S. Synthesis of γ-Fe2O3 nanoparticles embedded in polyimide. J. Magn. Magn. Mater. 2004, E1167, 272.

4130 Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008 (39) Lim, S. K.; Chung, K. J.; Kim, Y. H.; Kim, C. K.; Yoon, C. S. Synthesis of iron oxide nanoparticles in a polyimide matrix. J. Colloid Interface Sci. 2004, 273, 517. (40) Collity, B. D. Publisher’s Note. In Elements of X-ray Diffraction; Addison-Wesley: Reading, MA, 1998. (41) Oh, C.-Y.; Oh, J.-H.; Ko, T. The microstructure and characteristics of magnetite thin films prepared by ultrasound-enhanced ferrite plating. IEEE Trans. Magn. 2002, 38 (5), 3018. (42) Vijayanand, H. V.; Arunkumar, L.; Gurubasawaraj, P. M.; Veeresha, P. M.; Basavaraja, S.; Saleem, A.; Venkataraman, A.; Ghanwat, A; Maldar, N. N. Synthesis and characterization of polyimide-γ-Fe2O3 nanocomposites. J. Appl. Polym. Sci. 2006, 103, 834. (43) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Size-dependent depression of the glass-transition temperature in polymer-films. Europhys. Lett. 1994, 27, 1–59. (44) Bansal, A.; Yang, H. C.; Li, C. Z.; Cho, K. W.; Benicewicz, B. C.; Kumar, S. K.; Schadler, L. S. Quantitative equivalence between polymer nanocomposites and thin polymer films. Nat. Mater. 2005, 4, 693.

(45) Ash, B. J.; Rogers, D. F.; Wiegand, C. J.; Schadler, L. S.; Siegel, R. W.; Benicewicz, B. C.; Apple, T. Mechanical properties of Al2O3/ polymethylmethacrylate nanocomposites. Polym. Compos. 2002, 23, 1014. (46) Zhang, Q. L.; Gupta, S.; Emrick, T.; Russell, T. P. Surfacefunctionalized CdSe nanorods for assembly in diblock copolymer templates. J. Am. Chem. Soc. 2006, 128, 3898. (47) Lin, Y.; Boker, A.; He, J. B.; Sill, K.; Xiang, H. Q.; Abetz, C.; Li, X. F.; Wang, J.; Emrick, T.; Long, S.; Wang, Q.; Balazs, A.; Russell, T. P. Self-directed self-assembly of nanoparticle/copolymer mixtures. Nature 2005, 434, 55.

ReceiVed for reView December 11, 2007 ReVised manuscript receiVed March 16, 2008 Accepted March 26, 2008 IE701690W