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PPy core–shell composite microspheres with conductivity and superparamagnetic behaviors
Synthetic Metals 161 (2011) 1921–1927
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Synthesis of P(St-MAA)-Fe3 O4 /PPy core–shell composite microspheres with conductivity and superparamagnetic behaviors Huiqiang Chen, Wei Wang, Guoliang Li, Chao Li, Ying Zhang ∗ Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education); School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an, Shaanxi 710062, China
a r t i c l e
i n f o
Article history: Received 25 April 2011 Received in revised form 24 June 2011 Accepted 29 June 2011 Available online 26 July 2011 Keywords: Magnetic particles Polypyrrole Core–shell structure Composite materials
a b s t r a c t Styrene (St) and methacrylic acid (MAA) copolymers were synthesized by emulsion polymerization using Fe3 O4 particles modified by oleic acid as central components. P(St-MAA)-Fe3 O4 /PPy (polypyrrole) core–shell composite microspheres with conducting and supermagnetic behaviors were obtained via in situ chemical oxidative polymerization on the surface of the P(St-MAA)-Fe3 O4 magnetic composite microspheres. The –COOH groups in the composites acted as co-dopants in the formation of PPy and adsorbed the pyrrole monomers onto the surface of P(St-MAA)-Fe3 O4 . The P(St-MAA)-Fe3 O4 /PPy composite microspheres have been extensively characterized by transmission electron microscope (TEM), Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The Fe3 O4 particles were coated within P(St-MAA) polymers when the volume ratio of MAA to St was maintained at 1:5 during the co-polymerization process. The P(St-MAA)-Fe3 O4 /PPy multi-component composite microspheres show excellent superparamagnetism. The electrical conductivity of the composite microspheres largely depended on the PPy loading amount in the outer layer. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Intrinsically conducting polymers with nano-sized composites have recently drawn extensive attention because of their low density, diversified structure, and unique physical and chemical properties [1–3]. However, the intrinsically conducting polymers cannot be used for many practical applications due to their low processability and low solubility in common solvents. A usual approach to improving the processability and performance of intrinsically conducting polymers is to form composite conducting polymers through the combination of intrinsically conducting polymers with other organic or inorganic materials having different functionalities. Because of their excellent electrical conductivity, processability, magnetic, mechanical, and environmental sensitivity, these composite materials have been widely used in some important fields, such as electromagnetic shielding, biosensors, electrochromism, and corrosion resistance [4–10].
∗ Corresponding author at: Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education); School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an, Shaanxi 710062, PR China. Tel.: +86 29 85300932; fax: +86 29 85307609. E-mail address: [email protected] (Y. Zhang). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.06.036
Among the various intrinsically conducting polymers, polypyrrole (PPy) is one of the important conducting polymers because of its high conductivity, good environmental stability and biocompatibility [11–14]. Usually, in order to form blends or composites, polystyrene (PS), poly(methylmethacrylate) (PMMA) and poly(N-isopropylacrylamide) (PNIPAM), and other polymers that possess good workability were combined with PPy to form nano-composites [15–23]. Many researchers have focused on the preparation of PS and PPy composite microspheres because PS is readily available and easy to cast into desired forms while maintaining the mechanical integrity of the matrix [15–19]. For example, Armes and his co-workers [17] have synthesized near-monodisperse, micrometer-sized poly(N-vinylpyrrolidone) stabilized polystyrene latexes coated with PPy by in situ deposition of the conducting polymer from aqueous solution. Rais et al. [19] prepared core–shell particles comprised of a PS-poly(ethylene glycol) monomethacrylate (PS-PEGMA) core and a PPy shell. The thickness of the PPy shell is controlled by the amount of pyrrole loading, the overall template surface area and the polymerization kinetics of pyrrole in the presence of different oxidants. Inorganic nano-composites have been widely applied in optical, electronic, chemical and biological sensors because of their high surface area, quantum size effect and other unique properties [23–25]. Composite conducting polymer materials possess both the excellent conductivity and the physical and chemical properties of
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the inorganic nano-particles. A variety of nano-composite materials have been synthesized from metal and PPy composites [13,26–29], oxide and PPy composites [30–41] and carbon nanotubes and PPy [42,43]. Conducting polymer-based composites that contain magnetic nano-particles aroused particular interest because of their unique electromagnetic properties and applications in electrochromic devices, non-linear optical systems and electromagnetic interference shielding. For example, Wu’s group [34] has successfully synthesized PPy/iron oxide nano-composites using an in situ chemical oxidative polymerization technique with the monodispersed magnetite nano-particles as core components. Compared to pure PPy in the absence of magnetite nano-particles, the electrical conductivity of PPy/magnetite nano-composites was enhanced. The magnetic properties of 24 and 36 wt% nano-composites demonstrate ferromagnetic and supermagnetic behavior, respectively. Herrasti et al. [35] have developed magnetic conducting composites based on polypyrrole and iron oxide nanoparticles via electrochemistry technique. The amount of magnetic material was optimized to 10% with respect to the monomer to obtain a composite with high electrical conductivity and magnetic properties. Mangeney et al. [37] synthesized a new type of magnetic Fe2 O3 /PS/PPy core/shell latex particles. The shell layer was made of reactive N-carboxylic acid-functionalized polypyrrole (PPyCOOH). The multi-component composites have good superparamagnetic and electroactive properties. Luo et al. [38] have obtained Fe3 O4 /PPy/P(MAA-co-AAm) electromagnetic composite microspheres with trilayer core–shell structure by an oxidation polymerization method and a miniemulsion polymerization approach in sequence. The resulting composites exhibit ferromagnetic, electric and pH response. In the preparation of conducting polymer composites containing PPy and Fe3 O4 nanoparticles, the PPy could not be deposited directly onto the surface of the Fe3 O4 particles because of flocculation and the hydrophilic surfaces of the Fe3 O4 particles. The poor coating and polydispersion of the obtained composite materials limit their practical application [44]. In the present study, we propose a simple route for the synthesis of P(St-MAA)-Fe3 O4 /PPy composite materials with excellent conducting and superparamagnetic behaviors. The styrene and methacrylic acid copolymer coatings with Fe3 O4 nanoparticles were synthesized by emulsion polymerization, using Fe3 O4 nanoparticles as the central components. P(St-MAA)-Fe3 O4 /PPy core–shell composite microspheres with conducting and superparamagnetic properties were obtained via in situ chemical oxidative polymerization on the surface of the P(St-MAA)-Fe3 O4 magnetic microspheres. One of the advantages of this method is that the surfaces of the P(St-MAA)-Fe3 O4 microspheres do not need to be modified. The –COOH group in the P(St-MAA) copolymers acts as the co-dopant in the formation of PPy and adsorbs the pyrrole monomers onto the surface of P(St-MAA)Fe3 O4 composite microspheres. The electrical conductivity of the composite microspheres depends on the amount of PPy loading in the outside layer. 2. Experimental 2.1. Materials Pyrrole (Py), styrene (St) and methacrylic acid (MAA) were purified by distillation under reduced pressure prior to polymerization. The FeCl3 ·6H2 O, FeCl2 ·4H2 O, ammonia (NH3 ·H2 O), oleic acid (OA), K2 S2 O8 (KPS) and ethanol (EtOH) were of analytical grade. The sodium dodecyl sulfate (SDS) was of chemical grade. All of the chemicals except for the pyrrole, styrene and methacrylic acid monomers were used without further purification. The water used in the experiment was double distilled.
2.2. Preparation of Fe3 O4 magnetic particles The magnetic Fe3 O4 nanoparticles modified by oleic acid were prepared according to the method described by Khan [45]. Briefly, 4.80 g of FeCl2 ·4H2 O and 13.0 g of FeCl3 ·6H2 O were dissolved in 70 mL of double distilled water and transferred into a 150 mL threeneck flask under a nitrogen atmosphere with vigorous stirring at room temperature. The reaction mixture solution immediately turned black after the rapid addition of 30 mL of an aqueous solution containing 25–28 wt% ammonia. After 30 min, an 8 mL oleic acid solution in ethanol (1:1 in volume) was dropped into the three-neck flask. The solution was placed in an 80 ◦ C water bath and stirred at 400 RPM for 30 min. The resulting Fe3 O4 magnetic nano-particles modified by oleic acid were separated by magnetic decantation and washed with alternating doubly distilled water and anhydrous ethanol 3–5 times. The as-prepared black Fe3 O4 magnetic nanoparticles were dried under vacuum at 45 ◦ C for 24 h and then kept sealed in a refrigerator. 2.3. Preparation of P(St-MAA)-Fe3 O4 composite microspheres The P(St-MAA)-Fe3 O4 composite microspheres were synthesized by emulsion polymerization [46]. In the procedure, 1.0 g of Fe3 O4 particles modified by oleic acid and 2.0 mL of St monomer were mixed and formed an oil phase in a beaker. An aqueous phase solution composed of 100 mL of doubly distilled water and 0.090 g of SDS in a three-neck flask. The oil phase was mixed with the aqueous phase solution, and an oil-in-water mixture was formed by ultrasound dispersion in an ice bath for 20 min. The three-neck flask was removed from the ice-water bath and warmed to room temperature. Then, 0.10 g of KPS (dissolved in 5 mL doubly distilled water) and 0.40 mL of MAA were added to the suspension system. The volume ratio of MAA to St was set at 1:10, 1:5, or 2:5, respectively. The polymerization was carried out at 70 ◦ C for 6 h under a nitrogen atmosphere, and the stirring rate was maintained at 400 RPM. The brown P(St-MAA)-Fe3 O4 composites were washed with alternating doubly distilled water and anhydrous ethanol 3–4 times and dried in air. 2.4. Preparation of P(St-MAA)-Fe3 O4 /PPy core–shell composite microspheres P(St-MAA)-Fe3 O4 /PPy composite microspheres with a core–shell structure were synthesized by chemical oxidative polymerization using P(St-MAA)-Fe3 O4 microspheres as the core materials. Firstly, 0.50 g of dried P(St-MAA)-Fe3 O4 microspheres was dispersed into 100 mL of doubly distilled water solution containing 0.18 g of SDS. The dispersal was carried out by ultrasound treatment in a beaker for about 20 min. The suspension was transferred into a three-neck flask, and 0.090 mL of pyrrole monomer was added into the suspension and dispersed for 20 min at a constant stirring rate of 400 RPM. Finally, the oxidant FeCl3 was dissolved in 5.0 mL of doubly distilled water (the molar ratio of the FeCl3 ·6H2 O to the pyrrole monomer was 1:1) [47] and added into the suspension system. The polymerization reaction was carried out for 20 h at 25 ◦ C. The prepared black P(St-MAA)-Fe3 O4 /PPy composites were separated by magnetic decantation and washed with alternating doubly distilled water and anhydrous ethanol 3–4 times and then dried under vacuum at 40 ◦ C for 24 h. The schematic illustration of the preparation of composites was shown in Fig. 1. 2.5. Characterization The morphology of the composite microspheres was observed on transmission electron microscopy (TEM, JEM-2100) at an accel-
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for 24 h. Electrical conductivity measurements of samples were performed using the high-accuracy resistivity-temperature characteristic tester (RT109A, Northwestern Polytechnical University of China) at room temperature. The dried samples were pressed pellets at 10 MPa before measuring.
3. Results and discussion 3.1. Morphology and structure of P(St-MAA)-Fe3 O4 /PPy composite microspheres
Fig. 1. Schematic representation procedure for the preparation of P(St-MAA)Fe3 O4 /PPy composite microspheres with core–shell structures.
erating voltage of 200 kV. The samples used for TEM were prepared by dropping the diluted suspension on the carbon coated copper grid and drying in air. Fourier transform infrared (FT-IR) analysis was performed with an Avatar 360 FT-IR spectrometer (Germany Brucher), using the KBr pellet technique. X-ray powder diffraction (XRD) analysis was carried out by using D/Max-3C automatic X-ray diffractometer (Japan Rigalcu) at accelerating voltage of 35 kV and pipe flow of 40 mA. A scan rate of 0.02◦ /s was applied to record the pattern in the 2 range of 10–80. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo-VGESCALABK␣ instrument equipped with a monochromatic Al K␣ X-ray as the excitation source. Thermogravimetric analyses (TGA) were performed using a Perkin-Elmer TGA-7 instrument. The measurements were conducted at a heating rate of 20 ◦ C/min under high-purity N2 atmosphere. The samples for TGA were dried at 50 ◦ C for 12 h before measuring. The magnetization in the applied magnetic field was performed by model vibrating sample magnetometer (VSM-7303, Lake Shore) at room temperature, and the samples were dried in vacuum
Fig. 2 shows the TEM micrographs of Fe3 O4 nano-particles modified by oleic acid, P(St-MAA)-Fe3 O4 composite microspheres and P(St-MAA)-Fe3 O4 /PPy core–shell composite microspheres. The Fe3 O4 particles modified by oleic acid are shaped like irregular spheres, and their diameters ranged from 5 to 20 nm (Fig. 2a). The particles show good dispersion and have no obvious aggregation. After compositing of the P(St-MAA) copolymers, the P(St-MAA)Fe3 O4 composite microspheres appeared spherical in shape, and the Fe3 O4 particles were fully encapsulated by the P(St-MAA) polymer layer (Fig. 2b). The average size of the P(St-MAA)-Fe3 O4 composite microspheres ranged from 100 to 250 nm, and the P(St-MAA)-Fe3 O4 /PPy composite microspheres became larger to a certain extent (Fig. 2c). To determine the functional surface on the Fe3 O4 particles, we selected methacrylic acid (MAA) as a co-monomer in the polymerization of polystyrene. The carboxyl on the surface of the P(St-MAA)-Fe3 O4 can absorb pyrrole monomers and simultaneously act as a co-dopant in PPy by conducting polymeric particles [48]. When the volume ratio of MAA to St was about 1:10, the Fe3 O4 particles show a low coating content and an uneven distribution in the interior of the P(St-MAA) polymer microspheres (Fig. 3a). When the volume ratio of MAA to St was increased to 1:5, the Fe3 O4 particles were dispersed homogeneously inside of the P(St-MAA) microspheres with the higher content of Fe3 O4 (Fig. 3b). When the MAA to St ratio was fixed at 2:5, the Fe3 O4 particles remained well dispersed inside of the P(St-MAA) microspheres (Fig. 3c), but the P(St-MAA)-Fe3 O4 particles aggregated because the methacrylic acid easily polymerizes in the aqueous phase. Based on
Fig. 2. TEM images of Fe3 O4 particles modified by oleic acid (a), P(St-MAA)-Fe3 O4 composite microspheres (b), and P(St-MAA)-Fe3 O4 /PPy multiple component composite microspheres (c).
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Fig. 3. TEM images of P(St-MAA)-Fe3 O4 composite microspheres prepared under the different volume ratio of MAA to St (a) 1:10, (b) 1:5, and (c) 2:5.
these results, the volume ratio of MAA to St was kept at about 1:5 during the polymerization of styrene and MAA. Fig. 4 shows the FT-IR spectra of the P(St-MAA) microspheres, P(St-MAA)-Fe3 O4 composite microspheres and P(StMAA)-Fe3 O4 /PPy core–shell composite microspheres. The characteristic absorption peaks at 1710 and 1450 cm−1 in Fig. 4a are ascribed to the stretching vibration of the C O bond and the –CH3 deformation stretching in the MAA, respectively. The absorption peaks at 1490 and 1603 cm−1 correspond to the benzene ring group, and the absorption peaks at 700 and 760 cm−1 are caused by the bending vibration of the C–H on the benzene ring [49]. The FT-IR spectrum of the P(St-MAA)-Fe3 O4 composite microspheres (Fig. 4b) includes the characteristic peaks of P(St-MAA) and confirms the formation of P(St-MAA)-Fe3 O4 composites. The characteristic peak appears at 590 cm−1 , corresponding to the Fe–O bond. This result confirms the existence of Fe3 O4 in the P(St-MAA)-Fe3 O4 nanocomposites [50]. The stretching vibration of the C O bond at 1710 cm−1 was blue-shifted because of the interaction between the carboxylic group and the magnetic particles [51]. The absorptions of the P(St-MAA)-Fe3 O4 /PPy composite microspheres (Fig. 4c) centered at
1545 cm−1 and 1314 cm−1 correspond to the pyrrole rings vibration and the C N stretching vibrations. The bands at 1034, 1198, 850 and 922 cm−1 were assigned to the C–H in-plane vibration and outof-plane vibration on the pyrrole rings, respectively [36]. The FT-IR analysis indicates that the PPy was deposited on the surface of the P(St-MAA)-Fe3 O4 microspheres. The crystalline structures of the different microspheres were investigated by XRD. For the Fe3 O4 particles (Fig. 5a), the diffraction peaks at 2 of 30.1◦ , 35.4◦ , 43.1◦ , 53.4◦ , 56.9◦ and 62.5◦ , correspond to the spinel structure of Fe3 O4 [35]. These peaks can be ascribed to the diffractions of the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) faces of the crystals, respectively. The XRD patterns of the P(St-MAA)-Fe3 O4 and P(St-MAA)-Fe3 O4 /PPy composite microspheres (Fig. 5b and 5c) indicate that the crystalline structure of the Fe3 O4 particles was retained after deposition of the polymer layers. The broad diffraction peak in the range of 2 between 20◦ and 30◦ can be attributed to the amorphous polymer of the composite microspheres. The XRD results suggest that the Fe3 O4 particles were successfully coated with polymers.
Fig. 4. FT-IR spectra of P(St-MAA) microspheres (a), P(St-MAA)-Fe3 O4 composite microspheres (b), and P(St-MAA)-Fe3 O4 /PPy composite microspheres (c).
Fig. 5. XRD patterns of Fe3 O4 particles (a), P(St-MAA)-Fe3 O4 composite microspheres (b), and P(St-MAA)-Fe3 O4 /PPy composite microspheres (c).
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Fig. 6. XPS spectra of P(St-MAA)-Fe3 O4 composite microspheres (a), and P(St-MAA)Fe3 O4 /PPy composite microspheres (b). XPS spectra of Fe3 O4 nanoparticles as shown in inset.
XPS measurements provide the information for the surface composition of P(St-MAA)-Fe3 O4 and P(St-MAA)-Fe3 O4 /PPy composite microspheres. Comparing with those of the core components of P(St-MAA)-Fe3 O4 particles (Fig. 6a), the XPS patterns of P(St-MAA)Fe3 O4 /PPy core–shell structure composites (Fig. 6b) exhibited a new peak located at 399.7 eV, assigned to N1s energy level, and the peak of Fe2p3/2 and Fe2p1/2 disappeared. It may provide the experimental proof of PPy layer coating on the P(St-MAA)-Fe3 O4 particles [37]. In order to provide the experimental evidence of P(StMAA)-Fe3 O4 and PPy are not simple blends, we analyze the high-resolution C1s spectra of XPS for the uncoated and PPy-coated P(St-MAA)-Fe3 O4 particles. For P(St-MAA)-Fe3 O4 particles (Fig. 7a), the C1s region shows the two main components centered at 285 and 289 eV, respectively. The lower binding energy component can be assigned to the C–C/C–H groups, while the high binding energy component at 289 eV is corresponding to the carboxyl groups in the methacrylic acid group [37] However, it is worth noting that XPS spectrum of C1s from P(St-MAA)-Fe3 O4 /PPy composite particles (Fig. 7b), the binding energy component at 289 eV is disappeared due to the polypyrrole overlayers are coated on the surface of P(St-MAA)-Fe3 O4 . Furthermore, P(St-MAA)-Fe3 O4 /PPy composite particles exhibit a C1s narrow region that is very distinct from that of P(St-MAA)-Fe3 O4 composites. The components centered at 285 eV can be assigned to the ˇ carbon of pyrrole and underlying polystyrene of P(St-MAA), and a new minor peak at 286.7 eV exhibits the existence of ␣ carbon of pyrrole and surface oxidation of carbon atoms from the overlayers. The XPS results are agreement with the literature 37. Therefore, the results of XPS is a strong supporting evidence that PPy is coated on the surface of P(St-MAA)Fe3 O4 particles, as well as the similar results can be confirmed by the magnetic property of the prepared composite materials P(StMAA)-Fe3 O4 /PPy.
Fig. 7. XPS spectra of C1s from P(St-MAA)-Fe3 O4 (a) and P(St-MAA)-Fe3 O4 /PPy composite particles.
ticles modified by oleic acid, which is nearly close to the Ms of 65 emu/g of Fe3 O4 microparticles reported by Wan’s group [52]. The magnetic saturation value decreased to about 47.1 emu/g and 38.4 emu/g for the P(St-MAA)-Fe3 O4 and P(St-MAA)-Fe3 O4 /PPy composite microspheres, respectively. The Ms values of composite materials are considerably lower than that of the pure Fe3 O4 nanoparticles resulting from the non-magnetic P(St-MAA) and PPy coating contribution to the total magnetization [50]. Moreover, no pronounced hysteresis loop was found, indicating that both the retentivity and the coercivity of the composite microspheres are about zero. The results of the measured magnetic properties show
3.2. Properties of the P(St-MAA)-Fe3 O4 /PPy composite microspheres 3.2.1. Magnetic properties The magnetic properties of different samples were determined by a vibrating sample magnetometer at room temperature. Fig. 8 shows the magnetization curves of the Fe3 O4 particles, the P(St-MAA)-Fe3 O4 composite microspheres and the P(St-MAA)Fe3 O4 /PPy composite microspheres, respectively. The magnetic saturation value reached about 62.4 emu/g for the Fe3 O4 micropar-
Fig. 8. The magnetic hysteresis loops of Fe3 O4 particles (a), P(St-MAA)-Fe3 O4 composite microspheres (b), and P(St-MAA)-Fe3 O4 /PPy composite microspheres (c).
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Fig. 9. Photographs of the separation process of P(St-MAA)-Fe3 O4 /PPy composite microspheres without external magnetic field (a), and with external magnetic field (b).
Fig. 11. Dependence of the electrical conductivity of P(St-MAA)-Fe3 O4 /PPy composite microspheres on the pyrrole content.
that the prepared P(St-MAA)-Fe3 O4 /PPy composite microspheres are superparamagnetic. From the results of the magnetic properties and XPS analysis, it can be further proved that the presence of a PPy shell around the P(St-MAA)-Fe3 O4 particles [38]. Fig. 9 illustrates the separation of the prepared composite magnetic particles using an external magnetic field. The black dispersion solution of the Fe3 O4 /P(St-MAA)/PPy magnetic composite microspheres was enriched and became transparent in a very short time (about 30 s) when the external magnetic field was applied.
the P(St-MAA)-Fe3 O4 composite microspheres (Fig. 10c). The TGA analysis also suggests that the prepared products included both inorganic and organic composites.
3.2.2. Thermal stability TGA curves of the Fe3 O4 particles, P(St-MAA)-Fe3 O4 composite microspheres and P(St-MAA)-Fe3 O4 /PPy composite microspheres are shown in Fig. 10. The initial weight loss step of the three samples below 200 ◦ C can be attributed to the elimination of the residual water or solvent in the samples. For the Fe3 O4 particles (Fig. 10a), the one-step mass loss of about 20% between 200 and 460 ◦ C indicates that the oleic acid adsorbed on the Fe3 O4 particles could not be washed out. The thermal decomposition process of the pure Fe3 O4 particles differed from that of the Fe3 O4 particles coated with P(StMAA) (Fig. 10b). The pronounced weight loss (about 20%) in the temperature range from 260 ◦ C to 440 ◦ C was caused by the thermal degradation of the P(St-MAA) polymers. About 66.5% of the material remaining at 800 ◦ C was Fe3 O4 . The thermal decomposition process of the P(St-MAA)-Fe3 O4 /PPy composites was slower than that of
3.2.3. Conductivity To study the effect of the PPy concentration on the electrical conductivity of the P(St-MAA)-Fe3 O4 /PPy composite microspheres, we varied the pyrrole content from 5 wt% to 30 wt%. The conductivity improved with increasing pyrrole content in the composites (Fig. 11). At lower pyrrole concentrations (<10 wt%), a very small change of the pyrrole content in the composites increased the electrical conductivity significantly. The electrical conductivity of the composites increased from 4.87 × 10−5 S/cm to 3.60 × 10−3 S/cm when the pyrrole content was increased from 5 wt% to 10 wt% and from 3.60 × 10−3 S/cm to 1.36 × 10−2 S/cm when the pyrrole content was increased from 10 wt% to 30 wt%. The electrical conductivity was very weak at lower pyrrole concentrations because the insulating surface of the P(St-MAA)-Fe3 O4 microspheres was not completely coated with PPy and was exposed to air. This incomplete coverage is due an insufficient concentration of pyrrole monomers in the reaction medium. The conductivity of the composite materials increased rapidly with increasing PPy content in the composite particles because the PPy domains provide an excellent coating layer. The surface of the P(St-MAA)-Fe3 O4 microspheres may have been completely coated with PPy when the pyrrole content reached 10 wt%, allowing the charge in the composite materials to be transported effectively. The electrical conductivity of the composites tends to become more stable with increasing pyrrole content [16].
4. Conclusion
Fig. 10. TGA curves of Fe3 O4 particles (a), P(St-MAA)-Fe3 O4 composite microspheres (b), and P(St-MAA)-Fe3 O4 /PPy composite microspheres (c).
A facile method for preparing P(St-MAA)-Fe3 O4 /PPy composite microspheres with core–shell structures was proposed, through in situ polymerization of pyrrole monomers on a template of P(StMAA)-Fe3 O4 composites. The as-prepared P(St-MAA)-Fe3 O4 /PPy composites exhibit excellent superparamagnetic behavior with various saturation magnetizations. The electrical conductivity of the composite microspheres directly depends on the PPy loading amount in the outside layer. P(St-MAA)-Fe3 O4 /PPy composite microspheres with controllable magnetic and electrical behaviors have potential applications in fields such as electromagnetic shielding, microwave absorbing, biological separation, and enzyme immobilization.
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Synthesis of P(St-MAA)-Fe3 O4 /PPy core–shell composite microspheres with conductivity and superparamagnetic behaviors Huiqiang Chen, Wei Wang, Guoliang Li, Chao Li, Ying Zhang ∗ Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education); School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an, Shaanxi 710062, China
a r t i c l e
i n f o
Article history: Received 25 April 2011 Received in revised form 24 June 2011 Accepted 29 June 2011 Available online 26 July 2011 Keywords: Magnetic particles Polypyrrole Core–shell structure Composite materials
a b s t r a c t Styrene (St) and methacrylic acid (MAA) copolymers were synthesized by emulsion polymerization using Fe3 O4 particles modified by oleic acid as central components. P(St-MAA)-Fe3 O4 /PPy (polypyrrole) core–shell composite microspheres with conducting and supermagnetic behaviors were obtained via in situ chemical oxidative polymerization on the surface of the P(St-MAA)-Fe3 O4 magnetic composite microspheres. The –COOH groups in the composites acted as co-dopants in the formation of PPy and adsorbed the pyrrole monomers onto the surface of P(St-MAA)-Fe3 O4 . The P(St-MAA)-Fe3 O4 /PPy composite microspheres have been extensively characterized by transmission electron microscope (TEM), Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The Fe3 O4 particles were coated within P(St-MAA) polymers when the volume ratio of MAA to St was maintained at 1:5 during the co-polymerization process. The P(St-MAA)-Fe3 O4 /PPy multi-component composite microspheres show excellent superparamagnetism. The electrical conductivity of the composite microspheres largely depended on the PPy loading amount in the outer layer. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Intrinsically conducting polymers with nano-sized composites have recently drawn extensive attention because of their low density, diversified structure, and unique physical and chemical properties [1–3]. However, the intrinsically conducting polymers cannot be used for many practical applications due to their low processability and low solubility in common solvents. A usual approach to improving the processability and performance of intrinsically conducting polymers is to form composite conducting polymers through the combination of intrinsically conducting polymers with other organic or inorganic materials having different functionalities. Because of their excellent electrical conductivity, processability, magnetic, mechanical, and environmental sensitivity, these composite materials have been widely used in some important fields, such as electromagnetic shielding, biosensors, electrochromism, and corrosion resistance [4–10].
∗ Corresponding author at: Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education); School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an, Shaanxi 710062, PR China. Tel.: +86 29 85300932; fax: +86 29 85307609. E-mail address: [email protected] (Y. Zhang). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.06.036
Among the various intrinsically conducting polymers, polypyrrole (PPy) is one of the important conducting polymers because of its high conductivity, good environmental stability and biocompatibility [11–14]. Usually, in order to form blends or composites, polystyrene (PS), poly(methylmethacrylate) (PMMA) and poly(N-isopropylacrylamide) (PNIPAM), and other polymers that possess good workability were combined with PPy to form nano-composites [15–23]. Many researchers have focused on the preparation of PS and PPy composite microspheres because PS is readily available and easy to cast into desired forms while maintaining the mechanical integrity of the matrix [15–19]. For example, Armes and his co-workers [17] have synthesized near-monodisperse, micrometer-sized poly(N-vinylpyrrolidone) stabilized polystyrene latexes coated with PPy by in situ deposition of the conducting polymer from aqueous solution. Rais et al. [19] prepared core–shell particles comprised of a PS-poly(ethylene glycol) monomethacrylate (PS-PEGMA) core and a PPy shell. The thickness of the PPy shell is controlled by the amount of pyrrole loading, the overall template surface area and the polymerization kinetics of pyrrole in the presence of different oxidants. Inorganic nano-composites have been widely applied in optical, electronic, chemical and biological sensors because of their high surface area, quantum size effect and other unique properties [23–25]. Composite conducting polymer materials possess both the excellent conductivity and the physical and chemical properties of
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the inorganic nano-particles. A variety of nano-composite materials have been synthesized from metal and PPy composites [13,26–29], oxide and PPy composites [30–41] and carbon nanotubes and PPy [42,43]. Conducting polymer-based composites that contain magnetic nano-particles aroused particular interest because of their unique electromagnetic properties and applications in electrochromic devices, non-linear optical systems and electromagnetic interference shielding. For example, Wu’s group [34] has successfully synthesized PPy/iron oxide nano-composites using an in situ chemical oxidative polymerization technique with the monodispersed magnetite nano-particles as core components. Compared to pure PPy in the absence of magnetite nano-particles, the electrical conductivity of PPy/magnetite nano-composites was enhanced. The magnetic properties of 24 and 36 wt% nano-composites demonstrate ferromagnetic and supermagnetic behavior, respectively. Herrasti et al. [35] have developed magnetic conducting composites based on polypyrrole and iron oxide nanoparticles via electrochemistry technique. The amount of magnetic material was optimized to 10% with respect to the monomer to obtain a composite with high electrical conductivity and magnetic properties. Mangeney et al. [37] synthesized a new type of magnetic Fe2 O3 /PS/PPy core/shell latex particles. The shell layer was made of reactive N-carboxylic acid-functionalized polypyrrole (PPyCOOH). The multi-component composites have good superparamagnetic and electroactive properties. Luo et al. [38] have obtained Fe3 O4 /PPy/P(MAA-co-AAm) electromagnetic composite microspheres with trilayer core–shell structure by an oxidation polymerization method and a miniemulsion polymerization approach in sequence. The resulting composites exhibit ferromagnetic, electric and pH response. In the preparation of conducting polymer composites containing PPy and Fe3 O4 nanoparticles, the PPy could not be deposited directly onto the surface of the Fe3 O4 particles because of flocculation and the hydrophilic surfaces of the Fe3 O4 particles. The poor coating and polydispersion of the obtained composite materials limit their practical application [44]. In the present study, we propose a simple route for the synthesis of P(St-MAA)-Fe3 O4 /PPy composite materials with excellent conducting and superparamagnetic behaviors. The styrene and methacrylic acid copolymer coatings with Fe3 O4 nanoparticles were synthesized by emulsion polymerization, using Fe3 O4 nanoparticles as the central components. P(St-MAA)-Fe3 O4 /PPy core–shell composite microspheres with conducting and superparamagnetic properties were obtained via in situ chemical oxidative polymerization on the surface of the P(St-MAA)-Fe3 O4 magnetic microspheres. One of the advantages of this method is that the surfaces of the P(St-MAA)-Fe3 O4 microspheres do not need to be modified. The –COOH group in the P(St-MAA) copolymers acts as the co-dopant in the formation of PPy and adsorbs the pyrrole monomers onto the surface of P(St-MAA)Fe3 O4 composite microspheres. The electrical conductivity of the composite microspheres depends on the amount of PPy loading in the outside layer. 2. Experimental 2.1. Materials Pyrrole (Py), styrene (St) and methacrylic acid (MAA) were purified by distillation under reduced pressure prior to polymerization. The FeCl3 ·6H2 O, FeCl2 ·4H2 O, ammonia (NH3 ·H2 O), oleic acid (OA), K2 S2 O8 (KPS) and ethanol (EtOH) were of analytical grade. The sodium dodecyl sulfate (SDS) was of chemical grade. All of the chemicals except for the pyrrole, styrene and methacrylic acid monomers were used without further purification. The water used in the experiment was double distilled.
2.2. Preparation of Fe3 O4 magnetic particles The magnetic Fe3 O4 nanoparticles modified by oleic acid were prepared according to the method described by Khan [45]. Briefly, 4.80 g of FeCl2 ·4H2 O and 13.0 g of FeCl3 ·6H2 O were dissolved in 70 mL of double distilled water and transferred into a 150 mL threeneck flask under a nitrogen atmosphere with vigorous stirring at room temperature. The reaction mixture solution immediately turned black after the rapid addition of 30 mL of an aqueous solution containing 25–28 wt% ammonia. After 30 min, an 8 mL oleic acid solution in ethanol (1:1 in volume) was dropped into the three-neck flask. The solution was placed in an 80 ◦ C water bath and stirred at 400 RPM for 30 min. The resulting Fe3 O4 magnetic nano-particles modified by oleic acid were separated by magnetic decantation and washed with alternating doubly distilled water and anhydrous ethanol 3–5 times. The as-prepared black Fe3 O4 magnetic nanoparticles were dried under vacuum at 45 ◦ C for 24 h and then kept sealed in a refrigerator. 2.3. Preparation of P(St-MAA)-Fe3 O4 composite microspheres The P(St-MAA)-Fe3 O4 composite microspheres were synthesized by emulsion polymerization [46]. In the procedure, 1.0 g of Fe3 O4 particles modified by oleic acid and 2.0 mL of St monomer were mixed and formed an oil phase in a beaker. An aqueous phase solution composed of 100 mL of doubly distilled water and 0.090 g of SDS in a three-neck flask. The oil phase was mixed with the aqueous phase solution, and an oil-in-water mixture was formed by ultrasound dispersion in an ice bath for 20 min. The three-neck flask was removed from the ice-water bath and warmed to room temperature. Then, 0.10 g of KPS (dissolved in 5 mL doubly distilled water) and 0.40 mL of MAA were added to the suspension system. The volume ratio of MAA to St was set at 1:10, 1:5, or 2:5, respectively. The polymerization was carried out at 70 ◦ C for 6 h under a nitrogen atmosphere, and the stirring rate was maintained at 400 RPM. The brown P(St-MAA)-Fe3 O4 composites were washed with alternating doubly distilled water and anhydrous ethanol 3–4 times and dried in air. 2.4. Preparation of P(St-MAA)-Fe3 O4 /PPy core–shell composite microspheres P(St-MAA)-Fe3 O4 /PPy composite microspheres with a core–shell structure were synthesized by chemical oxidative polymerization using P(St-MAA)-Fe3 O4 microspheres as the core materials. Firstly, 0.50 g of dried P(St-MAA)-Fe3 O4 microspheres was dispersed into 100 mL of doubly distilled water solution containing 0.18 g of SDS. The dispersal was carried out by ultrasound treatment in a beaker for about 20 min. The suspension was transferred into a three-neck flask, and 0.090 mL of pyrrole monomer was added into the suspension and dispersed for 20 min at a constant stirring rate of 400 RPM. Finally, the oxidant FeCl3 was dissolved in 5.0 mL of doubly distilled water (the molar ratio of the FeCl3 ·6H2 O to the pyrrole monomer was 1:1) [47] and added into the suspension system. The polymerization reaction was carried out for 20 h at 25 ◦ C. The prepared black P(St-MAA)-Fe3 O4 /PPy composites were separated by magnetic decantation and washed with alternating doubly distilled water and anhydrous ethanol 3–4 times and then dried under vacuum at 40 ◦ C for 24 h. The schematic illustration of the preparation of composites was shown in Fig. 1. 2.5. Characterization The morphology of the composite microspheres was observed on transmission electron microscopy (TEM, JEM-2100) at an accel-
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for 24 h. Electrical conductivity measurements of samples were performed using the high-accuracy resistivity-temperature characteristic tester (RT109A, Northwestern Polytechnical University of China) at room temperature. The dried samples were pressed pellets at 10 MPa before measuring.
3. Results and discussion 3.1. Morphology and structure of P(St-MAA)-Fe3 O4 /PPy composite microspheres
Fig. 1. Schematic representation procedure for the preparation of P(St-MAA)Fe3 O4 /PPy composite microspheres with core–shell structures.
erating voltage of 200 kV. The samples used for TEM were prepared by dropping the diluted suspension on the carbon coated copper grid and drying in air. Fourier transform infrared (FT-IR) analysis was performed with an Avatar 360 FT-IR spectrometer (Germany Brucher), using the KBr pellet technique. X-ray powder diffraction (XRD) analysis was carried out by using D/Max-3C automatic X-ray diffractometer (Japan Rigalcu) at accelerating voltage of 35 kV and pipe flow of 40 mA. A scan rate of 0.02◦ /s was applied to record the pattern in the 2 range of 10–80. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo-VGESCALABK␣ instrument equipped with a monochromatic Al K␣ X-ray as the excitation source. Thermogravimetric analyses (TGA) were performed using a Perkin-Elmer TGA-7 instrument. The measurements were conducted at a heating rate of 20 ◦ C/min under high-purity N2 atmosphere. The samples for TGA were dried at 50 ◦ C for 12 h before measuring. The magnetization in the applied magnetic field was performed by model vibrating sample magnetometer (VSM-7303, Lake Shore) at room temperature, and the samples were dried in vacuum
Fig. 2 shows the TEM micrographs of Fe3 O4 nano-particles modified by oleic acid, P(St-MAA)-Fe3 O4 composite microspheres and P(St-MAA)-Fe3 O4 /PPy core–shell composite microspheres. The Fe3 O4 particles modified by oleic acid are shaped like irregular spheres, and their diameters ranged from 5 to 20 nm (Fig. 2a). The particles show good dispersion and have no obvious aggregation. After compositing of the P(St-MAA) copolymers, the P(St-MAA)Fe3 O4 composite microspheres appeared spherical in shape, and the Fe3 O4 particles were fully encapsulated by the P(St-MAA) polymer layer (Fig. 2b). The average size of the P(St-MAA)-Fe3 O4 composite microspheres ranged from 100 to 250 nm, and the P(St-MAA)-Fe3 O4 /PPy composite microspheres became larger to a certain extent (Fig. 2c). To determine the functional surface on the Fe3 O4 particles, we selected methacrylic acid (MAA) as a co-monomer in the polymerization of polystyrene. The carboxyl on the surface of the P(St-MAA)-Fe3 O4 can absorb pyrrole monomers and simultaneously act as a co-dopant in PPy by conducting polymeric particles [48]. When the volume ratio of MAA to St was about 1:10, the Fe3 O4 particles show a low coating content and an uneven distribution in the interior of the P(St-MAA) polymer microspheres (Fig. 3a). When the volume ratio of MAA to St was increased to 1:5, the Fe3 O4 particles were dispersed homogeneously inside of the P(St-MAA) microspheres with the higher content of Fe3 O4 (Fig. 3b). When the MAA to St ratio was fixed at 2:5, the Fe3 O4 particles remained well dispersed inside of the P(St-MAA) microspheres (Fig. 3c), but the P(St-MAA)-Fe3 O4 particles aggregated because the methacrylic acid easily polymerizes in the aqueous phase. Based on
Fig. 2. TEM images of Fe3 O4 particles modified by oleic acid (a), P(St-MAA)-Fe3 O4 composite microspheres (b), and P(St-MAA)-Fe3 O4 /PPy multiple component composite microspheres (c).
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Fig. 3. TEM images of P(St-MAA)-Fe3 O4 composite microspheres prepared under the different volume ratio of MAA to St (a) 1:10, (b) 1:5, and (c) 2:5.
these results, the volume ratio of MAA to St was kept at about 1:5 during the polymerization of styrene and MAA. Fig. 4 shows the FT-IR spectra of the P(St-MAA) microspheres, P(St-MAA)-Fe3 O4 composite microspheres and P(StMAA)-Fe3 O4 /PPy core–shell composite microspheres. The characteristic absorption peaks at 1710 and 1450 cm−1 in Fig. 4a are ascribed to the stretching vibration of the C O bond and the –CH3 deformation stretching in the MAA, respectively. The absorption peaks at 1490 and 1603 cm−1 correspond to the benzene ring group, and the absorption peaks at 700 and 760 cm−1 are caused by the bending vibration of the C–H on the benzene ring [49]. The FT-IR spectrum of the P(St-MAA)-Fe3 O4 composite microspheres (Fig. 4b) includes the characteristic peaks of P(St-MAA) and confirms the formation of P(St-MAA)-Fe3 O4 composites. The characteristic peak appears at 590 cm−1 , corresponding to the Fe–O bond. This result confirms the existence of Fe3 O4 in the P(St-MAA)-Fe3 O4 nanocomposites [50]. The stretching vibration of the C O bond at 1710 cm−1 was blue-shifted because of the interaction between the carboxylic group and the magnetic particles [51]. The absorptions of the P(St-MAA)-Fe3 O4 /PPy composite microspheres (Fig. 4c) centered at
1545 cm−1 and 1314 cm−1 correspond to the pyrrole rings vibration and the C N stretching vibrations. The bands at 1034, 1198, 850 and 922 cm−1 were assigned to the C–H in-plane vibration and outof-plane vibration on the pyrrole rings, respectively [36]. The FT-IR analysis indicates that the PPy was deposited on the surface of the P(St-MAA)-Fe3 O4 microspheres. The crystalline structures of the different microspheres were investigated by XRD. For the Fe3 O4 particles (Fig. 5a), the diffraction peaks at 2 of 30.1◦ , 35.4◦ , 43.1◦ , 53.4◦ , 56.9◦ and 62.5◦ , correspond to the spinel structure of Fe3 O4 [35]. These peaks can be ascribed to the diffractions of the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) faces of the crystals, respectively. The XRD patterns of the P(St-MAA)-Fe3 O4 and P(St-MAA)-Fe3 O4 /PPy composite microspheres (Fig. 5b and 5c) indicate that the crystalline structure of the Fe3 O4 particles was retained after deposition of the polymer layers. The broad diffraction peak in the range of 2 between 20◦ and 30◦ can be attributed to the amorphous polymer of the composite microspheres. The XRD results suggest that the Fe3 O4 particles were successfully coated with polymers.
Fig. 4. FT-IR spectra of P(St-MAA) microspheres (a), P(St-MAA)-Fe3 O4 composite microspheres (b), and P(St-MAA)-Fe3 O4 /PPy composite microspheres (c).
Fig. 5. XRD patterns of Fe3 O4 particles (a), P(St-MAA)-Fe3 O4 composite microspheres (b), and P(St-MAA)-Fe3 O4 /PPy composite microspheres (c).
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Fig. 6. XPS spectra of P(St-MAA)-Fe3 O4 composite microspheres (a), and P(St-MAA)Fe3 O4 /PPy composite microspheres (b). XPS spectra of Fe3 O4 nanoparticles as shown in inset.
XPS measurements provide the information for the surface composition of P(St-MAA)-Fe3 O4 and P(St-MAA)-Fe3 O4 /PPy composite microspheres. Comparing with those of the core components of P(St-MAA)-Fe3 O4 particles (Fig. 6a), the XPS patterns of P(St-MAA)Fe3 O4 /PPy core–shell structure composites (Fig. 6b) exhibited a new peak located at 399.7 eV, assigned to N1s energy level, and the peak of Fe2p3/2 and Fe2p1/2 disappeared. It may provide the experimental proof of PPy layer coating on the P(St-MAA)-Fe3 O4 particles [37]. In order to provide the experimental evidence of P(StMAA)-Fe3 O4 and PPy are not simple blends, we analyze the high-resolution C1s spectra of XPS for the uncoated and PPy-coated P(St-MAA)-Fe3 O4 particles. For P(St-MAA)-Fe3 O4 particles (Fig. 7a), the C1s region shows the two main components centered at 285 and 289 eV, respectively. The lower binding energy component can be assigned to the C–C/C–H groups, while the high binding energy component at 289 eV is corresponding to the carboxyl groups in the methacrylic acid group [37] However, it is worth noting that XPS spectrum of C1s from P(St-MAA)-Fe3 O4 /PPy composite particles (Fig. 7b), the binding energy component at 289 eV is disappeared due to the polypyrrole overlayers are coated on the surface of P(St-MAA)-Fe3 O4 . Furthermore, P(St-MAA)-Fe3 O4 /PPy composite particles exhibit a C1s narrow region that is very distinct from that of P(St-MAA)-Fe3 O4 composites. The components centered at 285 eV can be assigned to the ˇ carbon of pyrrole and underlying polystyrene of P(St-MAA), and a new minor peak at 286.7 eV exhibits the existence of ␣ carbon of pyrrole and surface oxidation of carbon atoms from the overlayers. The XPS results are agreement with the literature 37. Therefore, the results of XPS is a strong supporting evidence that PPy is coated on the surface of P(St-MAA)Fe3 O4 particles, as well as the similar results can be confirmed by the magnetic property of the prepared composite materials P(StMAA)-Fe3 O4 /PPy.
Fig. 7. XPS spectra of C1s from P(St-MAA)-Fe3 O4 (a) and P(St-MAA)-Fe3 O4 /PPy composite particles.
ticles modified by oleic acid, which is nearly close to the Ms of 65 emu/g of Fe3 O4 microparticles reported by Wan’s group [52]. The magnetic saturation value decreased to about 47.1 emu/g and 38.4 emu/g for the P(St-MAA)-Fe3 O4 and P(St-MAA)-Fe3 O4 /PPy composite microspheres, respectively. The Ms values of composite materials are considerably lower than that of the pure Fe3 O4 nanoparticles resulting from the non-magnetic P(St-MAA) and PPy coating contribution to the total magnetization [50]. Moreover, no pronounced hysteresis loop was found, indicating that both the retentivity and the coercivity of the composite microspheres are about zero. The results of the measured magnetic properties show
3.2. Properties of the P(St-MAA)-Fe3 O4 /PPy composite microspheres 3.2.1. Magnetic properties The magnetic properties of different samples were determined by a vibrating sample magnetometer at room temperature. Fig. 8 shows the magnetization curves of the Fe3 O4 particles, the P(St-MAA)-Fe3 O4 composite microspheres and the P(St-MAA)Fe3 O4 /PPy composite microspheres, respectively. The magnetic saturation value reached about 62.4 emu/g for the Fe3 O4 micropar-
Fig. 8. The magnetic hysteresis loops of Fe3 O4 particles (a), P(St-MAA)-Fe3 O4 composite microspheres (b), and P(St-MAA)-Fe3 O4 /PPy composite microspheres (c).
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Fig. 9. Photographs of the separation process of P(St-MAA)-Fe3 O4 /PPy composite microspheres without external magnetic field (a), and with external magnetic field (b).
Fig. 11. Dependence of the electrical conductivity of P(St-MAA)-Fe3 O4 /PPy composite microspheres on the pyrrole content.
that the prepared P(St-MAA)-Fe3 O4 /PPy composite microspheres are superparamagnetic. From the results of the magnetic properties and XPS analysis, it can be further proved that the presence of a PPy shell around the P(St-MAA)-Fe3 O4 particles [38]. Fig. 9 illustrates the separation of the prepared composite magnetic particles using an external magnetic field. The black dispersion solution of the Fe3 O4 /P(St-MAA)/PPy magnetic composite microspheres was enriched and became transparent in a very short time (about 30 s) when the external magnetic field was applied.
the P(St-MAA)-Fe3 O4 composite microspheres (Fig. 10c). The TGA analysis also suggests that the prepared products included both inorganic and organic composites.
3.2.2. Thermal stability TGA curves of the Fe3 O4 particles, P(St-MAA)-Fe3 O4 composite microspheres and P(St-MAA)-Fe3 O4 /PPy composite microspheres are shown in Fig. 10. The initial weight loss step of the three samples below 200 ◦ C can be attributed to the elimination of the residual water or solvent in the samples. For the Fe3 O4 particles (Fig. 10a), the one-step mass loss of about 20% between 200 and 460 ◦ C indicates that the oleic acid adsorbed on the Fe3 O4 particles could not be washed out. The thermal decomposition process of the pure Fe3 O4 particles differed from that of the Fe3 O4 particles coated with P(StMAA) (Fig. 10b). The pronounced weight loss (about 20%) in the temperature range from 260 ◦ C to 440 ◦ C was caused by the thermal degradation of the P(St-MAA) polymers. About 66.5% of the material remaining at 800 ◦ C was Fe3 O4 . The thermal decomposition process of the P(St-MAA)-Fe3 O4 /PPy composites was slower than that of
3.2.3. Conductivity To study the effect of the PPy concentration on the electrical conductivity of the P(St-MAA)-Fe3 O4 /PPy composite microspheres, we varied the pyrrole content from 5 wt% to 30 wt%. The conductivity improved with increasing pyrrole content in the composites (Fig. 11). At lower pyrrole concentrations (<10 wt%), a very small change of the pyrrole content in the composites increased the electrical conductivity significantly. The electrical conductivity of the composites increased from 4.87 × 10−5 S/cm to 3.60 × 10−3 S/cm when the pyrrole content was increased from 5 wt% to 10 wt% and from 3.60 × 10−3 S/cm to 1.36 × 10−2 S/cm when the pyrrole content was increased from 10 wt% to 30 wt%. The electrical conductivity was very weak at lower pyrrole concentrations because the insulating surface of the P(St-MAA)-Fe3 O4 microspheres was not completely coated with PPy and was exposed to air. This incomplete coverage is due an insufficient concentration of pyrrole monomers in the reaction medium. The conductivity of the composite materials increased rapidly with increasing PPy content in the composite particles because the PPy domains provide an excellent coating layer. The surface of the P(St-MAA)-Fe3 O4 microspheres may have been completely coated with PPy when the pyrrole content reached 10 wt%, allowing the charge in the composite materials to be transported effectively. The electrical conductivity of the composites tends to become more stable with increasing pyrrole content [16].
4. Conclusion
Fig. 10. TGA curves of Fe3 O4 particles (a), P(St-MAA)-Fe3 O4 composite microspheres (b), and P(St-MAA)-Fe3 O4 /PPy composite microspheres (c).
A facile method for preparing P(St-MAA)-Fe3 O4 /PPy composite microspheres with core–shell structures was proposed, through in situ polymerization of pyrrole monomers on a template of P(StMAA)-Fe3 O4 composites. The as-prepared P(St-MAA)-Fe3 O4 /PPy composites exhibit excellent superparamagnetic behavior with various saturation magnetizations. The electrical conductivity of the composite microspheres directly depends on the PPy loading amount in the outside layer. P(St-MAA)-Fe3 O4 /PPy composite microspheres with controllable magnetic and electrical behaviors have potential applications in fields such as electromagnetic shielding, microwave absorbing, biological separation, and enzyme immobilization.
H. Chen et al. / Synthetic Metals 161 (2011) 1921–1927
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