Fluxible Nanoclusters of Fe3O4 Nanocrystal-Embedded Polyaniline

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Fluxible Nanoclusters of Fe3O4 Nanocrystals Embedded Polyaniline by Macromolecule-Induced Self-Assembly Jing Huang, Qi Li, Yue Wang, Lijie Dong, Haian Xie, Jun Wang, and Chuanxi Xiong Langmuir, Just Accepted Manuscript • DOI: 10.1021/la402367c • Publication Date (Web): 18 Jul 2013 Downloaded from http://pubs.acs.org on July 19, 2013

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Fluxible Nanoclusters of Fe3O4 Nanocrystals Embedded Polyaniline by Macromolecule-Induced Self-Assembly Jing Huang,

†

Qi Li,

*,†

Yue Wang,

†

Lijie Dong,

†

Haian Xie,

†

Jun Wang,

†

and Chuanxi

Xiong*,†,‡ †

School of Materials Science and Engineering, and State Key Laboratory of Advanced

Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, People’s Republic of China ‡

School of Materials Science and Engineering, Wuhan Textile University, Wuhan 430073,

People’s Republic of China

ABSTRACT: We have prepared Fe3O4 nanocrystals embedded polyaniline hybrids with welldefined cluster-like morphology through macromolecule-induced self-assembly. These magnetic and electrically conductive composite nanoclusters show flowability at room temperature in absence of any solvent, which offers great potential in applications such as microwave absorbents and electromagnetic shielding coatings. This macromolecule-induced self-assembly strategy can be readily applied on the fabrication of other ion oxide/conjugated polymer composites to achieve robust multifunctional materials.

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INTRODUCTION Polyaniline (PANi), as an electrically conductive polymer, has been investigated extensively nowadays for its unusual conducting mechanism, extended π-conjugation along the backbone, controllable electrochemical properties, and reversible doping/de-doping process. It thus finds important applications in electrochromic devices, thin film transistors, lightweight batteries, and fuel cells.1 Lately due to its solubility and processability, the combination of PANi with other components is proved to be an energetic strategy to produce advanced multifunctional materials. In particular, PANi incorporated with magnetite nanocrystals, i.e. Fe3O4, have shown great potential in the fabrication of catalyst supports, electromagnetic shielding materials and electrochemical display devices, among others, on account of the combined magnetic and electrical properties.2-6 To date, all the previously reported hybrid systems regarding conductive polymer incorporated with magnetite nanocrystals behave as a solid at room temperature without the aid of any solvent.7-10 In this study, we report on the synthesis of monodisperse Fe3O4 nanocrystals embedded PANi with defined cluster-like morphology (thus denoted as Fe3O4@PANi nanoclusters hereafter) through macromolecule-induced self-assembly, in which the PANi is chemically doped with a flexible long chain protonic acid, and therefore this composite material shows a typical liquid-like behavior in absence of any solvent.11-19 To the best of our knowledge, this is a first example of dry (without any solvent) flowable conductive/magnetic multifunctional material. We anticipate that, with a robust flowability, such magnetic and electrically conductive Fe3O4@PANi nanoclusters are promising candidates for microwave absorbents as well as electromagnetic shielding coatings.


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The magnetite nanocrystals in most of previously reported Fe3O4@PANi composite materials are achieved by employing hydrothermal or co-precipitation methodologies that usually result in a large average particle size and wide size distribution, which inevitably deteriorates the assembly property of the magnetite nanocrystals and eventually decreases the performance of resultant composite materials. Besides, the as-prepared magnetite nanocrystals are terminated with native ligands of oleic acid, making them highly hydrophobic. The water insoluble feature of these particles set big obstacle in combining with PANi moiety since the combination is usually achieved by in situ oxidative polymerization of aniline in aqueous medium.20 Therefore, effective coating of monodisperse magnetite nanocrystals with small particle size (less than 20 nm) and narrow size distribution by PANi molecules remains a challenging task. To address this issue, we designed here an efficient procedure for the synthesis of Fe3O4@PANi nanoclusters, as schematically presented in Scheme 1. Hydrophobic Fe3O4 nanocrystals with a very small size variation were prepared according to a literature method.21 With the aim of enabling the subsequent in situ polymerization of aniline in aqueous medium, a ligand exchange reaction by using degraded gelatin was conducted on the as-prepared hydrophobic Fe3O4 nanocrystals to switch them to hydrophilic. Detailed characterization regarding the inorganic component could be found in the Supporting Information (Figure S1). The preparation of the Fe3O4@PANi nanoclusters could be roughly divided into three steps: First, the hydrophilic Fe3O4 nanocrystals were dispersed in an aqueous solution of poly(vinylpyrrolidone) (PVP). And then, the aniline monomers were added. Since the surface of the iron oxides were densely covered with amine groups from degraded gelatin, they show strong adsorption affinity for aniline monomers.2, 3 Therefore, once the nucleation of aniline generated,


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the polymerization took place preferentially and continuously near the adjacent Fe3O4 particles to generate Fe3O4@PANi nanoclusters rather than forming isolated PANi species. EXPERIMENTAL SECTION Synthesis of hydrophilic Fe3O4 nanocrystals: The oleic acid terminated hydrophobic Fe3O4 nanocrystals were prepared according to a literature method.21 To produce amine groups functionalized Fe3O4 nanocrystals, degraded gelatin solutions (1.0 wt.%) were prepared previously: As-received gelatin (1.0 g, 98%, Aldrich) was dissolved into 100 mL NaHCO3 aqueous solution (8.4 g, pH =8.5), and then the mixture was aged at 40 °C for 120 h. Afterwards, 25 mL of the degraded gelatin aqueous solution was added into 25 mL hexane solution of hydrophobic Fe3O4 nanocrystals before sonicating for 4 h, after which the organic layer used for dissolving hydrophobic Fe3O4 nanocrystals was evaporated while aqueous solution of hydrophilic Fe3O4 nanocrystals was obtained. The resultant aqueous solution was washed repeatedly with fresh hexane to further remove hydrophobic species followed by a 10 min rotary evaporation to eliminate remaining hexane. Subsequently, high-speed centrifugation (10,000 rpm) was employed to remove any aggregated species. Eventually, the Fe3O4 nanocrystals were collected under the external magnetic field.

Synthesis of Fe3O4@PANi: Hydrophilic Fe3O4 nanocrystals (0.02 g) and poly(vinylpyrrolidone) (0.05g) were dissolved in 130 mL deionized water with the aid of a mild sonication at room temperature (20 °C). After 30 min, aniline monomer (0.1 mol) and HCl aqueous solution (5 g, 37 wt.%) was added into the mixture, followed by the addition of 37 g NPES (C9H19C6H4O(CH2CH2O)10CH2CH2CH2-SO3H) dissolved in 92.5 mL deionized water. After 1 h, ammonium persulfate (11.4 g) was added slowly into the above mixture within 30 min. The reaction was allowed to proceed for 20 h in ice bath. The resultant mixture was filtered under a reduced pressure through an ultrafiltration membrane (8,000 molecular weight cut-off). The final product was dried in vacuum for 48 h at 70 °C.

Synthesis of NPES doped PANi: Aniline monomer (0.1 mol) and HCl aqueous solution (5 g, 37 wt.%) was mixed together before the addition of 37 g NPES dissolved in 92.5 mL deionized water. After 1 h, ammonium persulfate (11.4 g) was added slowly into the above mixture within 30 min. The reaction was


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allowed to proceed for 20 h in ice bath. The resultant mixture was filtered under a reduced pressure through an ultrafiltration membrane (8,000 molecular weight cut-off). The final product was dried in vacuum for 48 h at 70 °C.

Fabrication of PVDF/ Fe3O4@PANi composite: Suitable amount of Fe3O4@PANi nanoclusters were blended with PVDF powder. The mixture was then molded at about 190 °C under a pressure of 10 MPa. Diskshaped samples of diameter 15 mm were cut from the molded sheet (2 mm in thickness) and silver-paint electrodes were applied to the samples.

Characterization: TEM (Joel JEM-2001F) images were obtained by dispersing the material in corresponding solvents, placing a few drops of the dispersion on a copper grid, and evaporating them prior to observation. IR measurements were made on a FTIR spectrometer (Thermo Nicolet Nexus) using KBr pellets. XRD patterns were acquired on a D/Max-IIIA (Rigaku) diffractometer using Cu Kα radiation (λ=1.54 Å). TGA measurements were performed with a simultaneous thermal analyzer (Netzsch, STA 499C) under N2 flow. Rheological properties were studied using an ARES-RFS rheometer (TA) at an angular frequency of ω = 1 s-1 and with a strain amplitude of 10% in the temperature range from 20 to 80 °C. UV-vis absorption spectrum was measured on a diode array UV-vis spectrometer (UV-2550, SHIMADZU). The magnetic properties were measured with a Model 4HF VSM (ADE, U.S.). For measurement of magnetoelectric conversion coefficient, a small sine disturbing magnetic field Hac superimposed on the Hdc was applied on the sample and a charge amplifier measured the induced charge.22

RESULTS AND DISCUSSION Morphology and Structure. Transmission electron microscopy (TEM) was applied to reveal the morphology and assembly behavior of magnetite nanocrystals both before and after the coverage of PANi. Figure 1a and 1b exhibit the monodisperse oil-phase and water-phase Fe3O4 nanocrystals, respectively, in a large scale. Images with a higher magnification (inserts) for Figure 1a and 1b demonstrate that the particle size of both the two samples is well below 20 nm.


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It is worth pointing out that the ligand exchange process employed in this study did not alter the assembly behavior of the inorganic particles as comparing the TEM image of hydrophilic nanocrystals to that of pristine hydrophobic ones. By contrast, the resultant Fe3O4@PANi composites assemble as well-defined cluster-like structure with the average dimension of 50~100 nm (Figure 1c and d). On the basis of the previous results, the mechanism could be proposed. As an amphiphilic and nonionic surfactant, PVP plays an important role here that contributes to the formation of Fe3O4@PANi nanoclusters: It serves as a linkage between the PANi chain and the Fe3O4 surface. Due to the presence of pyrrolidone groups, these PVP macromolecules could be attached to the exterior amine functionalities of the Fe3O4 particles via hydrogen bonding. Then owing to the interaction between PVP and the flexible long chain protonic acid (C9H19C6H4O(CH2CH2O)10CH2CH2CH2-SO3H, NPES) which could be readily doped to the backbone of PANi23 (the doping procedure is illustrated in Figure S2), the polymerization was successfully initiated, and terminated on the surface of the Fe3O4 core rather than in solution. As a result, the long chains of PANi connect every isolated iron oxide particle to an entirety, and hence, Fe3O4@PANi core/shell nanoclusters were eventually obtained. Chemical structures of all the intermediate and final products have been investigated by using Fourier transform infrared spectroscopy (FT-IR) and UV-vis spectra. The successful polymerization of aniline onto the Fe3O4 core was confirmed by FT-IR, as shown in Figure 2. It reveals that Fe3O4@PANi composite material has characteristic peaks at around 1538 (C=C stretching deformation of quinoid and benzenoid ring), 1350 (C-N stretching of secondary aromatic amine), 1186, and 832 cm-1 (out-plane deformation of C-H in the 1,4-disubstituted benzene ring), which are similar to that of the NPES doped PANi sample prepared under the same experimental parameters except for the absence of Fe3O4 nanocrystals. The relatively low


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intensity of a band at 596 cm-1 (Fe-O stretching of Fe3O4) indicates the high content of PANi in the Fe3O4@PANi composite. Multi-functionality of Fe3O4@PANi nanoclusters.

Figure 3 shows the UV-vis

spectra of Fe3O4@PANi nanoclusters. For the NPES doped PANi, the characteristic absorption peak at approximately 442 nm is attributable to the transition from the valence band to the antibonding polaron state, indicating that the PANi is in the doped state. When the granular structure of PANi/Fe3O4 nanoclusters was formed, the characteristic peak assigned to the polaron–π transition was slightly red-shifted, suggesting the interaction between the quinoid rings of PANi and Fe3O4. Figure 4 illustrates the corresponding rheological behaviors of the Fe3O4@PANI composite material. We could find clearly that the storage modulus G', the loss modulus G" and the viscosity η decrease gradually with increasing temperature. Remarkably, the composite exhibits typical liquid-like behavior as suggested by the fact that the shear loss modulus G" is higher than the storage modulus G' throughout the measured temperature range (from 20 to 80 oC) (Figure 4a).11 Also, the viscosity is as low as 0.1 Pa·s at 80 oC, making the material flow just like a “real liquid” (Figure 4b inset). It should be pointed out that the composite system reported here is essentially solvent-free without any remaining unreacted NPES molecules, as proved in our previous work regarding the NPES doped PANI.23 Such self-suspending nature makes this gallery of materials promising in applications associated with conventional solution-processing techniques, and moreover, they are green materials considering the zero-vapor-pressure possessed.


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The composites are intrinsically conductive, with the conductivities within the semiconductor region. Figure 5 shows the temperature dependence of the ionic conductivity of the Fe3O4@PANi nanoclusters. The conductivity follows the Vogel-Tammann-Fulcher (VTF) expression, also characteristic of liquid-like behavior.11 Based on the dissimilarity of decomposing temperature of organic and inorganic components, it can be determined from the results of thermal gravimetric analysis that the Fe3O4 nanocrystals take up approximately 8 wt.% proportion in the composite nanoclusters, as shown in Figure 6. Although with such low proportion of magnetic constituent, the composite nanoclusters remain magnetic with a saturation magnetization of 5.07 emu/g that is high enough for the proposed applications of microwave absorbents and electromagnetic shielding coatings (Figure 7). Moreover, taking advantages of both such liquid and magnetic nature, the Fe3O4@PANi nanoclusters can be facilely composited with polyvinylidene fluoride (PVDF) to produce the magnetoelectric (ME) thin film that exhibits interesting response under applied magnetic field. As depicted by Figure 8, both the magnetoelectricity conversion coefficients of the ME composites containing 5 wt.% and 7 wt.% Fe3O4@PANi show almost linear relationship in the applied magnetic field ranging from 0 Oe and 2000 Oe, and come to a plateau above 2500 Oe, which can be therefore exploited for prospective applications including information record and storage, and magnetic field detection and sensing. CONCLUSIONS In conclusion, we have demonstrated a facile and reproducible synthetic method for preparing Fe3O4@PANi composites with well-defined cluster-like structure. During the whole constructing process, the possible forming mechanism of the composite nanoclusters was discussed. Thus, this macromolecule-induced self-assembly technique can be readily applied on


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the fabrication of other ion oxide/conjugated polymer composites to achieve robust multifunctional materials. In addition to the intrinsic magnetic and electrical properties that are from the constituting components, these hybrid materials are shown to be liquid-like and solventfree, which allows them to serve as ideal candidates for microwave absorbents, electromagnetic shielding coatings and other applications associated with conventional solution-processing techniques.

Scheme 1. The synthetic procedure of the Fe3O4@PANi nanoclusters.


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(b)

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Figure 1. TEM images of (a) hydrophobic Fe3O4 nanocrystals (b) hydrophilic Fe3O4 nanocrystals and (c) Fe3O4@PANi nanoclusters. Inserts are corresponding high-magnification images. (d) A typical Fe3O4@PANi nanocluster at high-magnification.


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Figure 2. FT-IR spectra of PANi, hydrophilic Fe3O4 and Fe3O4@PANi nanoclusters.

NPES doped PANi Fe3O4@PANi nanoclusters

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Wavelength /nm Figure 3. UV-vis absorption spectra of PANi and the Fe3O4@PANi nanoclusters.


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Figure 4. The rheological response of the Fe3O4@PANi hybrids. (a) Modulus (G', G") and (b) viscosity (η) versus temperature trace. The insert shows the flowability of the flexible nanoclusters.


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log [conductivity /S⋅cm-1]

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Figure 6. TG trace of the NPES doped PANi and Fe3O4@PANi hybrids.


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hydrophobic Fe3O4 hydrophilic Fe3O4 Fe3O4@PANi

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5% Fe3O4@PANi 7% Fe3O4@PANi

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H /Oe Figure 8. Plots of magnetoelectricity conversion coefficient (dE/dH) of PVDF/Fe3O4@PANi composite as a function of applied magnetic field strength (H).


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ASSOCIATED CONTENT Supporting Information. XRD patterns hydrophobic and hydrophilic Fe3O4 nanocrystals and the doping procedure of NPES to the backbone of PANi molecule are included. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51173139, 51072151) and the 973 Program (No. 2010CB227105).

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