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Synthesis and Electromagnetic Interference Shielding Properties of Iron Oxide/Polypyrrole Nanocomposites

J. Azadmanjiri,1 P. Hojati-Talemi,1 G.P. Simon,1 K. Suzuki,1 C. Selomulya2 1 Department of Materials Engineering, Monash University, Clayton 3800, Australia 2

Department of Chemical Engineering, Monash University, Clayton 3800, Australia

Magnetic nanocomposites were prepared by an in situ oxidative polymerization method to encapsulate different loadings of iron oxide nanoparticles (MNP) by a conductive polymer, polypyrrole (PPy), and were blended into an epoxy resin matrix. The morphology, DC conductivity, magnetic, and electromagnetic interference (EMI) shielding behaviors of samples dispersed in the resin were characterized, the latter by use of a vector network analyzer in a frequency range of 0.1–18 GHz. Nanocomposites based on the use of MNP/PPy composite nanoparticles in which the magnetic and conducting phases coexist in intimate contact showed a marked increase in the absorption 10.10 dB at the maximum frequency limit (17–18 GHz) of the instrument, in comparison with the absorption bands for PPy particles only (7.5 dB) or MNP only (2.6 dB) or physical blends of MNP and PPy particles (3.6 dB) in the resin. The mechanism of this enhancement is discussed based on electromagnetic theory. POLYM. ENG. SCI., 51:247–253, 2011. ª 2010 Society of Plastics Engineers

INTRODUCTION Electromagnetic interference (EMI) shielding is a process of limiting the flow of electromagnetic fields between two locations by reflection and/or absorption of electromagnetic waves by use of a shielding material against the penetration of incoming radiation. High-frequency radiation from equipment such as mobile phones can be highly detrimental to the performance of electronics devices, e.g., the central processing units in computers. Because of the strong requirement of today’s society for dependable electronic devices, and the rapid growth in the number of radio frequency radiation sources, enhanced materials with better EMI shielding characteristics against the radiation sources are needed [1]. Polymers with conductive properties represent such materials, because of their low density, good electrical properties, and resistance to degCorrespondence to: George Simon; e-mail: [email protected] DOI 10.1002/pen.21813 Published online in Wiley Online Library (wileyonlinelibrary.com). C 2010 Society of Plastics Engineers V

POLYMER ENGINEERING AND SCIENCE—-2011

radation by corrosion, and are also much easier to process than metals. However, a high level of shielding efficiency can generally not be obtained using conductive polymers alone [2]. Conductive materials are able to shield EM waves from electrical sources, but if they come from a magnetic source they are best be shielded by magnetic materials [2]. Recent studies on magnetic nanocomposites show that magnetic nanoparticles can be used to absorb electromagnetic waves [3–9]. The surface effects of small particles with a few nanometers diameters are also likely important for absorption due to the surface and shape magnetic anisotropies and surface damping, all of which increase with decreasing diameter [3]. The combination of conducting and nanomagnetic components are thus of much interest in the development of new EMI shielding materials [10]. Polypyrrole (PPy) is a widely used conductive polymer for commercial applications because of its good conductivity and durability and thus potential to act as a shielding material. One of the drawbacks of using polypyrrole alone in EMI shielding is its lack of processibility and low adhesion to surfaces. Because of its chemical structure the polymer can, however, be readily dispersed in other thermosetting resins such as epoxy resins, which offer better processibility and can subsequently be applied as a coating with good adhesion on different surfaces. In this work, the EMI shielding properties of conductive polypyrrole combined with nanomagnetic materials were investigated to understand synergistic effects of nanomagnetic and conductive materials in these composite nanoparticles. Several approaches to synthesizing nanocomposites consisting of magnetic nanoparticles and polypyrrole have been reported [11–15]. However, these investigations have mainly focused on the synthesis conditions or processing methods, and microstructure, electrical, and magnetic properties of the composites, with their microwave absorbing properties remaining little reported [16–20]. In this work, composite binary iron oxide/polypyrrole (M/P) nanoparticles were prepared by an ultrasonic-based processing technique to obtain nanocomposites with both

high electrical conductivity and high saturation magnetization. To synthesis these composite nanoparticles, the iron oxide nanoparticles were first dispersed in solution using ultrasonic processing, followed by polymerization of pyrrole by the addition of an oxidant in that same solution. We demonstrate that the resulting polypyrrole was deposited on the surface of the nanoparticles, leading to polypyrrole-encapsulated magnetite particles. These composite nanoparticles were subsequently added to an epoxy resin to produce a nanocomposite material appropriate for EMI shielding applications. The morphological, electrical, magnetic, dielectric, and EMI shielding properties of these nanocomposite materials were investigated and compared with the cases where polypyrrole or magnetite particles were used separately or when both components were simultaneously physically blended into the epoxy matrix. This latter configuration, compared with the binary nanoparticles, allowed the usefulness (or otherwise) of having the coexisting magnetic and conducting phase in direct contact to be assessed.

drical sample to be made (which also allowed bubbles to be easily excluded), and cured at 758C for 24 h and postcured at 1258C for 3 h. The sample was then cut into a ring shape with an outer diameter of 7 mm, an inner diameter of 3 mm and a thickness of 2 mm, which was appropriate for measuring microwave absorption. The electrical conductivities of pressed pellets of PPy and PPy/MNP composite nanoparticles were measured using the four-probe technique [21]. Magnetization curves were obtained using a Vibrating Sample Magnetometer (VSM) in magnetic fields up to 15 kOe at room temperature. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images of the composite nanoparticles were obtained on a JEOL JSM 6300F FEG and a JEOL 2011, respectively. The powders were also characterized by X-ray diffraction (XRD), using CuKa radiation (k ¼ 0.154183 nm). Mo¨ssbauer spectroscopy was performed on the commercial iron oxide nanoparticles to confirm the phase. EMI shielding properties were measured using an 8510C vector network analyzer over a frequency rage of 0.1–18 GHz.

EXPERIMENTAL Pyrrole monomer (98% purity) and iron oxide (99% purity) nanoparticles (\50 nm) were purchased from Sigma Aldrich. Before use, the pyrrole was purified using an activated alumina column. The composite nanoparticles (magnetic nanoparticles (MNP) and polypyrrole) were made in the following manner. The desired concentration of MNP were added to distilled water and sonicated for 30 min using an ultrasonic probe (pulsed) on a MISONIX Sonicator 3000, with power ¼ 9 (w). The required amount of purified pyrrole was then added to the slurry, and 2 molar ammonium persulfate ([98%) ((NH4)2S2O8) oxidant solution added dropwise during sonication, with the sonication continued for 20 min after addition. The product was then rinsed with ethanol and water twice and the composite nanoparticles separated by centrifuging and were dried under vacuum at 758C for 24 h. The following method was used to make the nanocomposites in the epoxy resin. A range of samples were made including epoxy nanocomposites incorporating MNP alone, PPY particles alone, mixtures of MNP and PPy, and the inclusion of the PPy/MNP. In the case of mixtures of MNP and PPy, relative component concentrations similar to the PPy/MNP composite nanoparticles were used. In each case, the appropriate amounts of epoxy and acetone solvent were mixed together (Table 1) and the appropriate nanoparticles added. The suspension was weighed at this point, to allow the degree of evaporation to be monitored. This mixture was further agitated by the ultrasonic probe for 1 h (pulsed mode) until homogenously mixed and the solvent subsequently evaporated. Jeffamine D-230 (Polyoxypropylene) with the molar ratio of 1:1 stoichiometric amine:epoxy groups were added as the curing agent before casting. The mold was a plastic syringe, allowing an appropriate, readily removable cylin248 POLYMER ENGINEERING AND SCIENCE—-2011

RESULTS AND DISCUSSION SEM images of a range of materials are shown in Fig. 1, including pure PPy particles (a), and composite PPy/MNP nanoparticles, containing 10% (b), 30% (c), and 50% (d) magnetic nanoparticles (hereafter the samples are referred to as PPy, M10/P, M30/P, and M50/ P). The PPy particles alone (Fig. 1a) appear to be granular, spherical in shape with sizes ranging from 100 to 1000 nm. With respect to the composite nanoparticles, the relatively spherical morphology of the iron oxide causes the nanoparticles act to nucleate PPy polymerization, the intrinsic granular morphology of composite PPy/MNP nanoparticles prepared in aqueous phase remaining [22]. The PPy/MNP composite nanoparticles show average diameters ranging from 150 to 500 nm, with the increase in average size with decreasing amounts of MNP. Increasing the relative concentration of nanoparticle nuclei thus limits the possible growth TABLE 1. Concentrations of components in epoxy samples.

Name

Composite nanoparticles (g)

Epoxy resin (g)

Amine (g)

5.00 5.00 5.00 5.00 5.00

1.50 1.50 1.50 1.50 1.50

MNP (g)

Epoxy resin (g)

Amine (g)

0.195

5.00

1.50

Iron oxide/polypyrrole nanocomposite samples PPy 0.65 PPy-10 wt% MNP (M10/P) 0.65 PPy-30 wt% MNP (M30/P) 0.65 PPy-50 wt% MNP (M50/P) 0.65 MNP 0.65 Name

PPy (g) Iron oxide/polypyrrole-blended sample 70 wt% PPy þ 30 wt% MNP 0.455

DOI 10.1002/pen

FIG. 1. SEM images of M/P nanocomposites with different concentrations of MNP: (a) 0%, (b) 10%, (c) 30%, and (d) 50%.

of polypyrrole layer [23], leading to a smaller diameter of the composite nanoparticles. In addition, because of the lower surface energy of polypyrrole in comparison with water, polymerized particles (in PPy/M nanocomposites) were prone to agglomerate to reduce their surface energy and formed larger particles, with a number of magnetic nanoparticles imbedded within the structure. Figure 2 shows the Mo¨ssbauer spectrum acquired at room temperature from commercial iron oxide nanoparticles. A sextet with a hyperfine field (Bhf) value of 43.0 T and a subspectral area (Ra) of 0.20 in addition to a larger sextet with Bhf ¼ 47.5 T and Ra ¼ 0.80 were necessary to fit the spectrum satisfactory. These hyperfine parameters are in good agreement with those reported for the tetrahedral and octahedral sites in maghemite (c-Fe2O3) [24]. Figure 3 shows the XRD patterns for PPy, MNP, and M30/P composite nanoparticles. No crystalline reflection peak is seen on the pattern of the pure PPy sample, indicating that this sample is amorphous, whereas Fig. 3b also shows reflection peaks for iron oxide nanoparticles. The XRD patterns of M30/P composite nanoparticles show DOI 10.1002/pen

clear reflection peaks in the 308–708 range, along with a broad background, showing that the M30/P sample is composed of an amorphous polymer and MNP phases. The DC conductivity and magnetic properties of M/P composite nanoparticles for different weight fractions at room temperature are shown in Table 2. The polypyrrole is a conductive polymer, and thus, the composite nanoparticle

FIG. 2. Room temperature Mo¨ssbauer spectra of magnetic nanoparticles.

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FIG. 4. M-H hysteresis loops for M/P nanocomposites at room temperature with different concentrations of MNP: 10%, 30%, 50%, and 100%.

FIG. 3. The XRD pattern of PPy, MNP, and M30/P composite nanoparticles.

conductivity is reduced for higher weight fractions of the insulating MNPs, whereas the saturation magnetization (rs) increases (Fig. 4). The higher content of MNP in the composite particles leads naturally to greater values of rs (Fig. 4). This decrease can be attributed to a reduction in the conductive pathways in the PPy due to the imbedded MNPs. TEM images were taken from the ultramicrotomed slices of M30/P physically blended mixtures and M30/P composite nanoparticles in epoxy samples (Fig. 5). It can be seen from these TEM images that the nanoparticles of iron oxide are of the order of 40 nm in dimension. Figure 5a shows that the aggregates of iron oxide nanoparticles and PPy particles tend to segregate if directly blended in epoxy. However, by coating the magnetite nanoparticles with polypyrrole via in situ polymerization, better dispersion and contact between the magnetic and conductive elements can be achieved in the resin (Fig. 5b), potentially contributing to the improved shielding properties (discussed below). High-resolution transmission electron microscopy (HRTEM) was used to further reveal the morphology of TABLE 2. The conductivity and magnetic properties of M/P nanocomposites with different concentrations of MNP.

No. 1 2 3 4 5

Name

Conductivity (S/m)

rs (emu/g)

Hc (Oe)

PPy PPy-10 wt% MNP (M10/P) PPy-30 wt% MNP (M30/P) PPy-50 wt% MNP (M50/P) MNP

1.04 0.13 0.10 0.04 —

— 1.3 19.9 25.3 69.1

— 154 147 150 124

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M/P composite nanoparticles. Figure 6 shows a HRTEM image of the M/P composite nanoparticles where the MNP phase encapsulated by PPy can be clearly observed. The electromagnetic properties of polymer composites of epoxy resins with such M/P composite nanoparticles should benefit from this uniformity of particle dispersion within the PPy phase (and thus ultimately the final epoxy composite). In Figs. 7 and 8, we show the real and imaginary part of permittivity and permeability measured for 10 wt% PPy, 10 wt% MNP, 10 wt% M30/P blended and 10% M30/P composite nanoparticles. The 10 wt% PPy/epoxy composite shows a higher (real permittivity) than the other components in the 0.1–18 GHz range, which implies that the capacitive behavior of the 10 wt% PPy is the highest of the composite nanoparticles, whereas the imaginary component of this composite between 1 and 18 GHz is low and can be ascribed to the low conductivity of PPy. It is known that the electric field induces both the conduction and displacement currents within the materials. The former arises from free electrons and will lead to electric loss (imaginary permittivity), whereas the latter comes from the bond charges, i.e., polarization effect (real permittivity) [25, 26]. The increase of the real part of complex permittivity can thus chiefly be attributed to dielectric relaxation and the space charge polarization effect, whereas the increase of the imaginary part of complex permittivity can be assigned to the improved electrical conductivity of the composites. Figure 8 shows the complex permeability (real part in (a) and imaginary part in (b)) for the pure PPy, Fe3O4, M30/P physical mixtures, and M30/P composite nanoparticles, all blended into epoxy resin, in the frequency range from 0.1 to 18 GHz. The real part of permeability values (l0 ) for 10 wt% PPy, 10 wt% MNP and 10 wt% M30/P blended samples are relatively constant for all frequencies, whereas of the 10 wt% M30/P composite nanoparticles maDOI 10.1002/pen

FIG. 6. High-resolution TEM image of M30/P composite nanoparticles.

the speed of light, is the frequency of microwave and d is the thickness of the absorber. The calculated results are shown in Fig. 9. The first sample containing 10 wt% PPy nanoparticles with the conductivity of 1.04 S/m shows that the maximum reflection loss reaches 7.48 dB at the frequency of 17.9 GHz, and the mechanism of microwave

FIG. 5. TEM images from the ultramicrotomed slices of (a) M30/P physical mixtures and (b) M30/P composite nanoparticles epoxy samples.

terial is initially flat in the 0.5–14 GHz range and then increases slightly at higher frequencies, as shown in Fig. 8a. Figure 8b also shows the frequency dependence of the imaginary part of permeability (l00 ) for the nanocomposites. The frequency dependence of microwave absorption is a key criterion in selecting materials for EMI shielding applications, and the reflection loss of the four different epoxy nanocomposite samples in the frequency region of 0.1–18 GHz were calculated using Eqs. 1 and 2: rffiffiffiffiffi pffiffiffiffiffiffiffiffi 2pfd mr er mr Zin ¼ tanh j c er

(1)

Zin 1 RLðdBÞ ¼ 20 log Zin þ 1

(2)

where mr and er are relative complex permeability and permittivity of the composite medium, respectively, c is DOI 10.1002/pen

FIG. 7. Real part (a) and imaginary part (b) of the complex permittivity of 10 wt% MNP, 10 wt% PPy, 10 wt% (PPy/30% MNP) nanocomposites, and 10 wt% (PPy/30% MNP) blended in epoxy.


absorption for this sample can be ascribed to dielectric loss that occurs when the relaxation time and the frequency of the applied field are similar, so that a phase lag occurs and energy is absorbed. The second sample containing 10 wt% MNPs exhibits a narrow absorption band in the 0.1–18 GHz range with a maximum reflection loss of 2.64 at 16.9 GHz and is due to magnetic loss (mr00 ). The tight absorption bandwidth of these samples limit their use in broadband engineering applications to overcome EMI, because only dielectric loss contributes to the energy loss of electromagnetic wave for PPy-based composites, whereas the effect of magnetic loss is dominant for MNPs in epoxy resin [27]. In the case of the third and fourth samples, we prepared composites by blending the same ratio of polypyrrole and MNPs in epoxy resin (Table 1 [iron oxide/polypyrrole-blended sample]) and also made a sample containing 10 wt% (PPy/30% MNP) nanocomposite. The absorption peak can be observed at the frequency of 17.90 GHz (Fig. 9) for both samples, with reflection loss of about 3.2 dB and 10.10 dB, respectively. As expected, the microwave absorption is significantly enhanced for the 10 wt% (PPy/30% MNP) nanocomposites sample. This may be attributed to the synergistic effects of using both conductive and magnetic components in direct connectivity with each other. Wu and Chung [28] showed that the combined use of magnetic and conductive fillers in a polymer matrix led to a composite material that was more effective for electromagnetic interference shielding than the use of magnetic

FIG. 9. Reflection loss of 10 wt% MNP, 10 wt% PPy, 10 wt% (PPy/ 30% MNP) nanocomposites, and 10 wt% (PPy/30% MNP) blended in epoxy with a layer thickness of 2 mm.

or conductive filler alone, although in their work the magnetic and electrically conductive particles were not attached, as they are in this work. The combination of magnetic and electrically conductive materials is of interest for shielding, because of the electrical conductivity permitting the flow of eddy current induced by the magnetic field that is revealed by the magnetic component, providing a mechanism for the absorption of the radiation.

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FIG. 8. Real part (a) and imaginary part (b) of the complex permeability of 10 wt% MNP, 10 wt% PPy, 10 wt% (PPy/30% MNP) nanocomposites, and 10 wt% (PPy/30% MNP) blended in epoxy.

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