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Dielectric and magnetic properties of conducting ferromagnetic composite of polyaniline with γ-Fe2O3 nanoparticles
Materials Chemistry and Physics 112 (2008) 651–658
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Dielectric and magnetic properties of conducting ferromagnetic composite of polyaniline with ␥-Fe2 O3 nanoparticles Kuldeep Singh a , Anil Ohlan a , R.K. Kotnala b , A.K. Bakhshi c , S.K. Dhawan a,∗ a
Polymeric & Soft Materials Section, National Physical Laboratory, New Delhi 110012, India Magnetic Standards, National Physical Laboratory, New Delhi 110012, India c Department of Chemistry, University of Delhi, Delhi 110007, India b
a r t i c l e
i n f o
Article history: Received 22 February 2008 Received in revised form 1 May 2008 Accepted 8 June 2008 Keywords: Electrochemical technique Magnetic Materials Dielectric properties Electrical conductivity
a b s t r a c t The present paper reports the synthesis of conducting polyaniline polymer composite with nanoclusters of ferrite (␥-Fe2 O3 ) particles in the presence of dodecylbenzene sulfonic acid in aqueous medium through electrochemical and chemical oxidative polymerization. Different formulations have been prepared to study the effect of ferrite constituent on the electrical and dielectric properties of polyaniline nano-composite. Vibrating sample magnetometer (VSM) studies and electrical conductivity measurements have revealed that conducting polymer composite has a saturation magnetization (Ms ) value of 48.9 emu g−1 and conductivity of the order of 0.13 S cm−1 . The particle size of ␥-Fe2 O3 was found in the range of 8–15 nm as analyzed by transmission electron microscopy (TEM). Fourier transform infrared spectroscopy (FTIR) results have shown the presence of characteristic band stretching of Fe O band at 630 and 558 cm−1 , indicating the presence of ␥-Fe2 O3 in the polyaniline matrix which is in agreement with the electrochemical results. Dielectric measurements have shown decreasing trend of dielectric constant with the increase of ␥-Fe2 O3 particles in the polymer matrix while shielding effective (SE) of −11.2 dB was achieved for the polymer composite in 8.2–12.4 GHz (X-band) frequency range. The characterization of the composite was further carried out by X-ray diffraction, UV–vis and thermal gravimetric analysis (TGA). © 2008 Elsevier B.V. All rights reserved.
1. Introduction Electronically conducting polymers are the novel class of synthetic metals with wide spread application in number of technological devices like EMI shielding and electrostatic charge dissipation [1–5], sensors [6–8], organic light emitting diodes [9–11] and polymer solar cells [12,13]. The prospects of conducting organic magnetic materials have inspired much interest where lightweight, flexibility, moderate conductivity and magnetization are required. Deliberate modifications in chemical and super molecular structure of polymer matrix by incorporating nanoferromagnetic particles can lead to the formation of conducting ferromagnetic materials which can be suitably designed for high tech applications. Among different conducting polymers, polyaniline has been chosen because of its unique structure, containing an alternate arrangement of benzene rings and nitrogen atoms. The polyaniline exists in four forms namely leucoemeraldine (fully reduced form), emeraldine base (50% oxidized and 50% reduced
∗ Corresponding author. Fax: +91 11 25726938. E-mail address: [email protected] (S.K. Dhawan). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.06.026
form), pernigraniline (fully oxidized form) and conductive emeraldine salt. In recent years much research attention has been paid to the conducting polymer composites with one or more magnetic materials so that polymer possesses both electrical as well as magnetic properties. For the absorption of electromagnetic radiations, ferrites are incorporated in the polymers as they possess high magnetization values which make them useful at higher frequencies [14–16]. Many attempts to produce the colloidal polyaniline composite containing the ferrite have been made using the different charge carriers for doping the polymer [17–23]. However, the resultant polymer composites lose its conductivity and have low magnetization value. Nanostructures of polyaniline-Fe3 O4 nanoparticle composites were also prepared in the presence of -naphthalene sulfonic acid as a dopant that shows a magnetization value of 6 emu g−1 [24]. US Patent 6,764,617 claims a formation of conductive ferromagnetic composition comprising sulfonated lignin or a sulfonated polyflavonid or derivatives thereof and ferromagnetic iron oxide particles [25]. The present work deals with electrochemical and chemical oxidative polymerization of the aniline with nanosized ␥-Fe2 O3 particles with dodecyl benzene sulfonic acid (DBSA) as dopant and
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reports the effect of ␥-Fe2 O3 on the electrical, magnetic, dielectric and shielding properties of the resultant conducting polyaniline␥-Fe2 O3 nano-composite. Electrochemical studies were carried out using cyclic voltammetric technique in order to see the incorporation of ␥-Fe2 O3 nanoparticles in the conducting polyaniline matrix. Beside this, characterization of the polymer composite has been carried out by FTIR spectroscopy, transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) 2. Experimental 2.1. Synthesis of the -Fe2 O3 nanoparticles The magnetic nanoparticles of ␥-Fe2 O3 were synthesized by coprecipitaion method. The aqueous solution of 1.0 M FeCl2 ·4H2 O and 2.0 M FeCl3 were mixed together and precipitated by adding ammonium hydroxide solution with continuous stirring for 2–3 h by maintaining the pH at 10–11 [26]. The precipitated ferrite particles are filtered out and washed thoroughly with distilled water. ␥-Fe2 O3 particles so obtained are dried at 120 ± 1 ◦ C in vacuum oven. The formation of ␥-Fe2 O3 nanoparticles were confirmed by X-ray diffraction pattern. 2.2. Synthesis of polyaniline composite with -Fe2 O3 Chemical oxidative polymerization of aniline was carried out in the presence of nanoferrite particles in aqueous medium. 0.3 M DBSA and ␥-Fe2 O3 were homogenized by using the ART-Miccra D-8 (N0 -10956) homogenizer at 10500 rpm for 2–3 h. To this 0.1 M of aniline (An) was added and supersonic stirring was continued for 1 h to form an emulsion. The oxidant ammonium peroxidisulfate (0.1 M) was added drop-by-drop keeping the temperature of the reactor at −2.0 ◦ C with vigorous stirring for 5–6 h to the above emulsion. The green polymer precipitate so obtained was treated with isopropyl alcohol under vigorous stirring for 2–3 h. The resulting precipitate was then filtered and washed thoroughly and dried at 60–65 ◦ C in a vacuum oven. Several composition of the polymer composite having different weight ratio of monomer to ferrite An:␥-Fe2 O3 ::2:1(PC21), 1:1(PC11), 1:1.5(PC115), 1:2(PC12) are also synthesized in DBSA medium to check the effect of ferrite constituents in the polymer matrix. Beside this, for comparison of results, polyaniline doped with DBSA (PD13) without ferrite particles is also synthesized using emulsion polymerization.
polyaniline involves protonation as well as ingress of counter anions in the polymer matrix to maintain charge neutrality. Protonation and electron transfer in polyaniline leads to formation of radical cations by an internal redox reaction, which causes the reorganization of electronic structure to give two semiquinone radical cations. In the doping process, ingress of anions occurs to maintain charge neutrality in the resultant doped polyaniline matrix. In situ emulsion polymerization of aniline in the presence of ␥-Fe2 O3 constituents resulted in the formation of ferromagnetic conducting polymer. In order to avoid phase segregation, the ␥-Fe2 O3 nanoparticles were functionalized with the surfactant DBSA that ensure its compatibility with the polymer. The electrochemical polymerization of aniline with DBSA in aqueous medium was carried out using cyclic potential sweep method by switching the potential from −0.20 to 0.95 V vs. SCE at a scan rate of 20 mV s−1 . The rise in current value at 0.78 V in the first cycle corresponds to the oxidation of aniline leading to generation of anilinium radical cations (Fig. 1). In the subsequent cycles, new oxidation peaks appear which indicates that these radical cations undergo further coupling to form benzenoid structure and combination of benzenoid and quinoid structure. The peak current increases continuously with successive potential scans to build up electroactive polyaniline on the electrode surface.
2.3. Electrochemical polymerization The electrochemical polymerization of 0.1 M aniline in 0.3 M DBSA was carried out at 0.8 V on platinum electrode vs. SCE reference electrode. The polymer film growth was also studied by cycling the potential between −0.20 and 0.95 V on Pt electrode at a scan rate of 20 mV s−1 . Prior to polymerization, the solution was deoxygenated by passing argon gas through the reaction solution for 30 min. Electrochemical growth study of aniline in the presence of ␥-Fe2 O3 particles were also studied on platinum electrode in DBSA medium. 2.4. Characterization The conductivity of the powder pellet of the sample polyaniline-␥-Fe2 O3 composite was measured by four-probe method using Keithley programmable current source and nanovoltmeter attached to digital temperature controller and APD Cryo cooler. The magnetic measurements of the ferrite as well as conducting PANI–␥Fe2 O3 composite were carried out using vibrating sample magnetometer (VSM), Model 7304, Lakeshore Cryotronics Inc., USA. Thermogravimetric analysis of the polymer and composite were carried on a Mettler Toledo TGA 851e. FTIR spectra were recorded on Nicolet 5700 and UV–vis absorption studies were carried on Shimadzu 1601 Spectrophotometer. Three-electrode cell geometry was used in all the electrochemical experiments, where Pt was used as working electrode as well as counter electrode and SCE was used as reference electrode. An Auto lab PGSTAT30 (Ecochemie, Utrecht, The Netherlands) potentiostat/ galvanostat interfaced with a personal computer was used in all the electrochemical measurements. The particle size and the morphology were examined using a Transmission electron microscopy (TEM, JEOL JEM 1011) and the samples were deposited on carbon coated nickel grids. Permittivity and dielectric loss measurements were carried out on an Agilent E8362B Vector Network Analyzer in a microwave range of 8.2–12.4 GHz (X-band), using 15.8 mm × 7.9 mm × 6 mm copper sample holder connected between the waveguide flanges. To avoid air gap the above sample holder is modified with a groove of 1.5 mm on each side having 3 mm depth.
3. Result and discussion The emulsion polymerization of aniline to polyaniline in the presence of ␥-Fe2 O3 particles may bring certain changes in the properties of polyaniline because conduction mechanism in
Fig. 1. Electrochemical growth behavior of aniline in DBSA medium (PD13) and aniline in DBSA medium containing ferrite particles (PC11) on cycling the potential between −0.2 and 0.95 V, taking eight successive scans, on platinum electrode vs. SCE at a scan rate of 20 mV s−1 .
K. Singh et al. / Materials Chemistry and Physics 112 (2008) 651–658
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Scheme 1. Mechanism for radical cations and dications formation in polyaniline.
However when polymerization of aniline was carried out in the presence of ␥-Fe2 O3 particles entrapped in the surfactant medium, electrochemical growth behavior shows shifting of peak potential values, which indicates the incorporation of ␥-Fe2 O3 in the polymer backbone. Cyclic voltammogram of polyaniline film obtained by potential sweeping technique in blank DBSA medium shows characteristic peaks at 0.11, 0.44, 0.54 and 0.77 V (Fig. 2) due to the generation of radical cations at 1st peak potential values which were subsequently oxidized to dications [27] and represented in the mechanism (Scheme 1). However, the cyclic voltammogram of polyaniline embedded with ␥-Fe2 O3 particle in DBSA medium shows characteristic peaks at 0.14, 0.48, 0.56 and 0.86 V (Fig. 2). The shifting of characteristic redox peaks can be assigned to the incorporation of ␥-Fe2 O3 particles in the polyaniline matrix. Fig. 3 shows the FTIR spectra of polyaniline doped with DBSA, PC12 and ␥-Fe2 O3 .The main characteristics bands for the polyaniline doped with DBSA are found at 1515 and 1458 cm−1 which are assigned for the C C bond stretching of quinoids and benzenoid ring, respectively. Bands at 1257 and 1164 cm−1 are due to C N stretching and in-plane bending of the C H bond while peak at 1024 cm−1 is due to −SO3 H group. The peak at 1640 cm−1 may be due to the formation of the carbonyl group arising due to the over oxidation of the alkyl chain of the DBSA. In case of PC12, the main peaks are observed at 1034, 1123.1294, 1400, 1455, 1633, 630 and 558 cm−1 . The presence of the peaks at 630 cm−1 and 558 cm−1 which are the characteristic band of Fe–O band stretching (Fig. 3, curve a) clearly indicate the presence of ␥-Fe2 O3 in the polymer
matrix while the shifting in the main peaks arises due to the interaction of the Fe2 O3 with N-atom of the aniline ring. Fig. 4 shows the UV absorption spectra of the different samples of polyaniline and its composite with ␥-Fe2 O3 . The main peaks and calculated energy bands are shown in Table 1. Emeraldine base form of polyaniline in N-methyl pyrrolidione (NMP) shows two characteristic bands at 326 and 630 nm while the conductive form of polyaniline doped with DBSA has shown the red shift to 353 nm and 739 nm which were assigned to the –* transition of the benzenoid ring and polaronic transition respectively. But in case of polyaniline composite, two changes were observed. First a blue shift was observed for the band from 739 to 726 nm, which was ascribed to polaronic transition. The reason behind this shifting may be the possible interaction of the ␥-Fe2 O3 with polyaniline ring leading to the formation of ferromagnetic composite. Second when the ␥Fe2 O3 content increases in different samples, absorption spectra shows the bathochromic shift for the band 353 to 349 nm. The optical band energy of the polymer was obtained from the given relation [28] ˛h = (h − Eg )
1/2
(1)
where ˛ is the absorption coefficient, h is the photon energy and Eg is the optical band gap. The band gaps calculated by using the above Eq. (1) varies from 1.45 to 1.49 eV and 2.87 to 2.75 eV for the polaronic transitions and –* transition of the benzenoid ring respectively.
Table 1 Conductivity, magnetization, UV–vis bands and dielectric properties of polyaniline and its composite with ␥-Fe2 O3 Sample name
PD13 PC21 PC115 PC12 ␥-Fe2 O3
(S cm−1 )
2.11 1.80 0.80 0.13 10−7
Ms (emu g−1 )
– 1.19 15.2 48.9 69.8
Microwave properties at 10.2 GHz
Band gap (eV)
Dielectric constant εr
Dielectric loss εr
–*
*-polaron
– 12.78 11.02 10.16 6.32
– 9.13 7.82 6.77 0.29
2.82 2.78 2.75 2.75 –
1.45 1.49 1.49 1.47 –
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Fig. 4. UV/Visible spectra of (䊉) EB, () PD13, () PC21, () PC115 and () PC12. Inset shows the calculation of band gap plots in (˛h)1/2 vs. photon energy (h).
Fig. 2. Cyclic voltammogram of polyaniline film in DBSA medium (PD13) and PANIferrite composite in DBSA medium (PC11) on Pt electrode vs. SCE at a scan rate of 20 mV s−1 .
Fig. 5 shows the X-ray diffraction patterns of ␥-Fe2 O3 , polyaniline doped with DBSA and it composite with different compositions of ␥-Fe2 O3 . The main peaks for ␥-Fe2 O3 are observed at 2 = 30.281 (d = 2.949 Å), 35.699 (d = 2.513 Å), 43.435 (d = 2.081 Å), 53.805 (d = 1.702 Å), 57.437 (d = 1.603 Å), 63.0460 (d = 1.473 Å) corresponding to the (2 0 6), (1 1 9), (0 0 12), (2 2 12), (1 1 15), (4 4 1) reflections which matches with the standard XRD pattern of ␥-Fe2 O3 (PDF No. 25-1402). The peaks present in ␥-Fe2 O3 were also observed in all the compositions of polyaniline composite with ␥-Fe2 O3 which indicates the presence of ferrite particles in the polymer matrix and the intensity of peaks increase with the increase in the ratio of iron oxide. While the presence of polyaniline and its semicrystalline nature is confirmed by the broad peaks at 2 = 19.795 (d = 4.481 Å) and 25.154 (d = 3.537 Å) [29,30] and it is also observed that the intensity of these peaks increases with the decrease in iron oxide composite. The line broadening of the peaks in the entire patterns of polyaniline composite indicates about the small dimensions of the iron oxide particles. The crystallite size of ␥-Fe2 O3 particle can
Fig. 3. FTIR spectra of (a) ␥-Fe2 O3 , (b) polyaniline-␥-Fe2 O3 composite and (c) polyaniline doped with DBSA in KBr pellet.
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Fig. 5. X-ray diffraction plots of ␥-Fe2 O3 (a), PC12 (b), PC11(c), PC21 (d) and PD13 (e) with arbitrary no of counts vs. 2.
be calculated by line broadening using Scherer’s formula k D= ˇ cos
(2)
where D is the crystallite size for individual peak, is the X-ray wavelength, k the shape factor, D is the crystallite size for the individual peak of the crystal in angstroms, the Bragg angle in degrees, and ˇ is the line broadening measured by half-height in radians. The value of k is often assigned a value of 0.89, which depends on several factors, including the Miller index of the reflecting plane and the shape of the crystal. The average size of ␥-Fe2 O3 particles was calculated using above equation and estimated as 8.99 nm for pure ␥-Fe2 O3 and 9.87 nm for polyaniline composite with iron oxide having aniline: ␥-Fe2 O3 ::1:2 (PC12) which is in accordance with the TEM analysis (Fig. 6) which shows the uniformly dispersed iron oxide particles of 8–15 nm and the agglomerated polymer composite containing 8–13 nm size particles of iron oxide. Thermogravimetric analysis of the polyaniline doped with DBSA and polyaniline composite was carried out in order to see the effect
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Fig. 7. Thermal gravimetric analysis of polyaniline doped with DBSA and polyaniline composite with ␥-Fe2 O3 with increasing content of ␥-Fe2 O3 ; (䊉) PC12, () PC115, () PC11, () PC21 and () PD13.
of the ␥-Fe2 O3 content on the thermal stability of the composite (Fig. 7). Thermogram of different samples shows three major weight losses, first at 100 ◦ C due to the loss of water contents, second in the range of 230–380 ◦ C due to the loss of the dopant from the polymer matrix and the third major loss from 380 to 700 ◦ C was attributed to the destruction of polymeric backbone. Polyaniline doped with DBSA (PD13) is thermally stable up to 230 ◦ C. However, when conducting polymer was synthesized by incorporating ferric oxide moieties in the reaction system along with the surfactant, it has been observed that the thermal stability of the polymer has increased to 260 ◦ C. This shows that in situ polymerization of aniline in the presence of ferrite particles leads to a better thermally stable conducting polymer. The approximate amount of iron oxide for different polymer ferrite composites was calculated by subtracting the residual weight of the blank polymer from residual weight of composite at 700 ◦ C. It is observed that the for different compositions, PC21; PC11; PC115 and PC12 the weight percent of ␥-Fe2 O3 is estimated to be 10.7%, 16.1%, 29.9% and 42.1% which is in accordance with the amount taken during synthesis.
Fig. 6. TEM image of ␥-Fe2 O3 (a) and PC12 (b) having 8–15 nm size ␥-Fe2 O3 particles.
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Fig. 8. Temperature dependence of conductivity of (a) PD13 (b) PC11 (c) PC12 along with temperature from 50 to 300 K and (d) variation of conductivity with different An/␥-Fe2 O3 ratio at 300 K.
The temperature dependent DC conductivity of the polyaniline␥-Fe2 O3 composite having different weight ratio of ferric oxide contents were measured at temperature ranging from 50 to 300 K. The DC conductivity follows the semiconducting behavior and decreases with the decrease of temperature. The room temperature conductivity of the samples is shown in Table 1. Several models were used to explain the conductivity behavior in polymer like Arrhenius law and Mott’s equation. But it is observed that for low temperature range of 300–1.8 K the conductivity studies are best studies by the VRH model which follows the Mott’s equation [31–34].
1/ TO
(T ) = O exp −
T
(3)
where TO is the Mott characteristic temperature and can be expressed as TO = 8˛/kB N(EF )Z
(4)
and O is the conductivity at T = ∞ R = [˛ /kB TN(EF )]
−1/2
(5)
where ˛−1 is the localization length, which can be, determined from the magneto resistance data. From the observed values of TO and O , one can calculate N (EF ) density of states, R, the average hopping distance with the use of Eqs. (4) and (5). Exponent in Eq. (3) is the dimensionality factor having values 2, 3, and 4 for 1-, 2- and 3-dimensional conduction mechanisms, respectively. In this paper, we have plotted ln vs. temperature with different
values of as 2, 3 and 4. It was observed that the conductivity data fits for the one dimensional VRH model with = 2 with linearity factor of 0.9996, 0.9997 and 0.9992 for the polyaniline (PD13), polyaniline-ferrite composite PC11 and PC12, respectively (Fig. 8) and the corresponding value for 3D-VRH mechanism are 0.9933, 0.9962, 0.9956, respectively. The calculated values of the conductivity () at room temperature are given in Table 1. The plot of conductivity vs. the An/␥-Fe2 O3 (Fig. 8d) ratio shows that the conductivity decreases with the increase of ferrite constituent due to the insulating nature of the ␥-Fe2 O3 which hinder the flow of charge by blocking the conduction path in the polymer matrix. Thus from the above data, it is observed that 1D–VRH model is suitable for the conduction mechanism of the polyaniline-␥-Fe2 O3 composite and the conductivity decreases with the increase of ferrite content. The magnetic properties of the polyaniline-␥-Fe2 O3 composite and ␥-Fe2 O3 were explained by using the M–H curve (Fig. 9). The saturation magnetization (Ms ) value of the ␥-Fe2 O3 was found to 69.77 emu g−1 at an external field of 10 kOe having small value of coercivity and negligible retentivity with no hysteresis loop, indicating the super paramagnetic nature. When these nanoferrite particles are incorporated in the polyaniline matrix in weight ratio of 1:1(PC11) the magnetization saturation (Ms ) value was found 4.13 emu g−1 . However, on changing the weight composition of An/␥-Fe2 O3 to 1:2, the Ms value was drastically increased from 4.13 to 48.9 emu g−1 , keeping the external applied field at 10 kOe. The Ms values of different polyaniline-␥-Fe2 O3 composites was measured and given in the Table 1. In all the cases very small coercivity is observed with negligible retentivity which indicates the ferrimagnetic nature. Ms value increases due to high poly-dispersivity of the
K. Singh et al. / Materials Chemistry and Physics 112 (2008) 651–658
Fig. 9. Magnetization curves of () ␥-Fe2 O3 , (䊉) PC12, () PC115, () PC11 and () PC21 showing the decrease in saturation magnetization with the decrease in ␥-Fe2 O3 content.
␥-Fe2 O3 in polyaniline matrix that arises due to the functionalization of nanoferrite particles with the surfactant DBSA. The variation of complex permittivity of different samples of polyaniline-␥-Fe2 O3 composite in the frequency range of
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Fig. 11. EMI shielding effectiveness (SE) of ␥-Fe2 O3 (), PC12 (䊉), PC115 (), PC21 () measured in X-band (8.2–12.4 GHz).
8.2–12.4 GHz (X-band) are shown in Fig. 10. The real and imaginary part of permittivity (εr and εr ) of PC12 decreases from 12.1 to 9.7 and 7.6 to 4.5, respectively, with the increase in frequency. The minimum values of the dielectric losses propose polyaniline-␥Fe2 O3 composite to be good shielding material [35]. The dielectric properties are mainly due to interfacial polarization and intrinsic electric dipole polarization which are partially attributed by the disordered motion of the charge carrier along the back bone of conducting polymer chain. The electromagnetic shielding effectiveness (SE) of the ␥ Fe2 O3 and polymer composites was also measured from S-parameters as given below SE = −10 log
PT ET = −20 log = −20 log S21 PI EI
(6)
where PI (EI ) and PT (ET ) are the power (electric field) of incident and transmitted EM waves and |S21 |2 is the transmission coefficient, respectively. The maximum SE of −11.2 dB was recorded for the sample PC21 while SE of −2.5 dB was observed for ␥-Fe2 O3 (Fig. 11) proving that the polymer composite is better EMI shielding material. 4. Conclusion
Fig. 10. Real ( ) and imaginary ( ) part of permeability of () ␥-Fe2 O3 , () PC12, (䊉) PC115, () PC21 measured in X-band (8.2–12.4 GHz).
Emulsion polymerization of the aniline with ␥-Fe2 O3 in aqueous medium of DBSA leads to the formation of conducting super paramagnetic polyaniline ␥-Fe2 O3 composite with an Ms of 48.9 emu g−1 and moderate conductivity of 0.13 S cm−1 . Maximum dielectric constant value of 15.1 and shielding effectiveness of −11.2 dB was observed for the polymer composite for its application in EMI shielding and found to decrease with the increase in frequency. It was observed that the variation of conductivity with temperature for polyaniline and polyaniline-ferrite composite follows onedimensional VRH model with linearity factor of 0.9996 and 0.9997. The presence of ␥-Fe2 O3 in the polyaniline matrix is confirmed by the cyclic voltametry, as peak potential values of polyaniline shifts by incorporation of ferrite particles in the polymer matrix. FTIR result has also shown the presence of characteristic band stretching of Fe O band at 630 and 558 cm−1 which clearly indicate the presence of ␥-Fe2 O3 in the polymer matrix. Thermal stability of PANI-␥-Fe2 O3 composite is higher than the thermal stability of PANI-DBSA synthesized without ␥-Fe2 O3. The enhancement in the thermal stability arises due to the presence of ␥-Fe2 O3 particles in the polymer matrix.
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Dielectric and magnetic properties of conducting ferromagnetic composite of polyaniline with ␥-Fe2 O3 nanoparticles Kuldeep Singh a , Anil Ohlan a , R.K. Kotnala b , A.K. Bakhshi c , S.K. Dhawan a,∗ a
Polymeric & Soft Materials Section, National Physical Laboratory, New Delhi 110012, India Magnetic Standards, National Physical Laboratory, New Delhi 110012, India c Department of Chemistry, University of Delhi, Delhi 110007, India b
a r t i c l e
i n f o
Article history: Received 22 February 2008 Received in revised form 1 May 2008 Accepted 8 June 2008 Keywords: Electrochemical technique Magnetic Materials Dielectric properties Electrical conductivity
a b s t r a c t The present paper reports the synthesis of conducting polyaniline polymer composite with nanoclusters of ferrite (␥-Fe2 O3 ) particles in the presence of dodecylbenzene sulfonic acid in aqueous medium through electrochemical and chemical oxidative polymerization. Different formulations have been prepared to study the effect of ferrite constituent on the electrical and dielectric properties of polyaniline nano-composite. Vibrating sample magnetometer (VSM) studies and electrical conductivity measurements have revealed that conducting polymer composite has a saturation magnetization (Ms ) value of 48.9 emu g−1 and conductivity of the order of 0.13 S cm−1 . The particle size of ␥-Fe2 O3 was found in the range of 8–15 nm as analyzed by transmission electron microscopy (TEM). Fourier transform infrared spectroscopy (FTIR) results have shown the presence of characteristic band stretching of Fe O band at 630 and 558 cm−1 , indicating the presence of ␥-Fe2 O3 in the polyaniline matrix which is in agreement with the electrochemical results. Dielectric measurements have shown decreasing trend of dielectric constant with the increase of ␥-Fe2 O3 particles in the polymer matrix while shielding effective (SE) of −11.2 dB was achieved for the polymer composite in 8.2–12.4 GHz (X-band) frequency range. The characterization of the composite was further carried out by X-ray diffraction, UV–vis and thermal gravimetric analysis (TGA). © 2008 Elsevier B.V. All rights reserved.
1. Introduction Electronically conducting polymers are the novel class of synthetic metals with wide spread application in number of technological devices like EMI shielding and electrostatic charge dissipation [1–5], sensors [6–8], organic light emitting diodes [9–11] and polymer solar cells [12,13]. The prospects of conducting organic magnetic materials have inspired much interest where lightweight, flexibility, moderate conductivity and magnetization are required. Deliberate modifications in chemical and super molecular structure of polymer matrix by incorporating nanoferromagnetic particles can lead to the formation of conducting ferromagnetic materials which can be suitably designed for high tech applications. Among different conducting polymers, polyaniline has been chosen because of its unique structure, containing an alternate arrangement of benzene rings and nitrogen atoms. The polyaniline exists in four forms namely leucoemeraldine (fully reduced form), emeraldine base (50% oxidized and 50% reduced
∗ Corresponding author. Fax: +91 11 25726938. E-mail address: [email protected] (S.K. Dhawan). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.06.026
form), pernigraniline (fully oxidized form) and conductive emeraldine salt. In recent years much research attention has been paid to the conducting polymer composites with one or more magnetic materials so that polymer possesses both electrical as well as magnetic properties. For the absorption of electromagnetic radiations, ferrites are incorporated in the polymers as they possess high magnetization values which make them useful at higher frequencies [14–16]. Many attempts to produce the colloidal polyaniline composite containing the ferrite have been made using the different charge carriers for doping the polymer [17–23]. However, the resultant polymer composites lose its conductivity and have low magnetization value. Nanostructures of polyaniline-Fe3 O4 nanoparticle composites were also prepared in the presence of -naphthalene sulfonic acid as a dopant that shows a magnetization value of 6 emu g−1 [24]. US Patent 6,764,617 claims a formation of conductive ferromagnetic composition comprising sulfonated lignin or a sulfonated polyflavonid or derivatives thereof and ferromagnetic iron oxide particles [25]. The present work deals with electrochemical and chemical oxidative polymerization of the aniline with nanosized ␥-Fe2 O3 particles with dodecyl benzene sulfonic acid (DBSA) as dopant and
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reports the effect of ␥-Fe2 O3 on the electrical, magnetic, dielectric and shielding properties of the resultant conducting polyaniline␥-Fe2 O3 nano-composite. Electrochemical studies were carried out using cyclic voltammetric technique in order to see the incorporation of ␥-Fe2 O3 nanoparticles in the conducting polyaniline matrix. Beside this, characterization of the polymer composite has been carried out by FTIR spectroscopy, transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) 2. Experimental 2.1. Synthesis of the -Fe2 O3 nanoparticles The magnetic nanoparticles of ␥-Fe2 O3 were synthesized by coprecipitaion method. The aqueous solution of 1.0 M FeCl2 ·4H2 O and 2.0 M FeCl3 were mixed together and precipitated by adding ammonium hydroxide solution with continuous stirring for 2–3 h by maintaining the pH at 10–11 [26]. The precipitated ferrite particles are filtered out and washed thoroughly with distilled water. ␥-Fe2 O3 particles so obtained are dried at 120 ± 1 ◦ C in vacuum oven. The formation of ␥-Fe2 O3 nanoparticles were confirmed by X-ray diffraction pattern. 2.2. Synthesis of polyaniline composite with -Fe2 O3 Chemical oxidative polymerization of aniline was carried out in the presence of nanoferrite particles in aqueous medium. 0.3 M DBSA and ␥-Fe2 O3 were homogenized by using the ART-Miccra D-8 (N0 -10956) homogenizer at 10500 rpm for 2–3 h. To this 0.1 M of aniline (An) was added and supersonic stirring was continued for 1 h to form an emulsion. The oxidant ammonium peroxidisulfate (0.1 M) was added drop-by-drop keeping the temperature of the reactor at −2.0 ◦ C with vigorous stirring for 5–6 h to the above emulsion. The green polymer precipitate so obtained was treated with isopropyl alcohol under vigorous stirring for 2–3 h. The resulting precipitate was then filtered and washed thoroughly and dried at 60–65 ◦ C in a vacuum oven. Several composition of the polymer composite having different weight ratio of monomer to ferrite An:␥-Fe2 O3 ::2:1(PC21), 1:1(PC11), 1:1.5(PC115), 1:2(PC12) are also synthesized in DBSA medium to check the effect of ferrite constituents in the polymer matrix. Beside this, for comparison of results, polyaniline doped with DBSA (PD13) without ferrite particles is also synthesized using emulsion polymerization.
polyaniline involves protonation as well as ingress of counter anions in the polymer matrix to maintain charge neutrality. Protonation and electron transfer in polyaniline leads to formation of radical cations by an internal redox reaction, which causes the reorganization of electronic structure to give two semiquinone radical cations. In the doping process, ingress of anions occurs to maintain charge neutrality in the resultant doped polyaniline matrix. In situ emulsion polymerization of aniline in the presence of ␥-Fe2 O3 constituents resulted in the formation of ferromagnetic conducting polymer. In order to avoid phase segregation, the ␥-Fe2 O3 nanoparticles were functionalized with the surfactant DBSA that ensure its compatibility with the polymer. The electrochemical polymerization of aniline with DBSA in aqueous medium was carried out using cyclic potential sweep method by switching the potential from −0.20 to 0.95 V vs. SCE at a scan rate of 20 mV s−1 . The rise in current value at 0.78 V in the first cycle corresponds to the oxidation of aniline leading to generation of anilinium radical cations (Fig. 1). In the subsequent cycles, new oxidation peaks appear which indicates that these radical cations undergo further coupling to form benzenoid structure and combination of benzenoid and quinoid structure. The peak current increases continuously with successive potential scans to build up electroactive polyaniline on the electrode surface.
2.3. Electrochemical polymerization The electrochemical polymerization of 0.1 M aniline in 0.3 M DBSA was carried out at 0.8 V on platinum electrode vs. SCE reference electrode. The polymer film growth was also studied by cycling the potential between −0.20 and 0.95 V on Pt electrode at a scan rate of 20 mV s−1 . Prior to polymerization, the solution was deoxygenated by passing argon gas through the reaction solution for 30 min. Electrochemical growth study of aniline in the presence of ␥-Fe2 O3 particles were also studied on platinum electrode in DBSA medium. 2.4. Characterization The conductivity of the powder pellet of the sample polyaniline-␥-Fe2 O3 composite was measured by four-probe method using Keithley programmable current source and nanovoltmeter attached to digital temperature controller and APD Cryo cooler. The magnetic measurements of the ferrite as well as conducting PANI–␥Fe2 O3 composite were carried out using vibrating sample magnetometer (VSM), Model 7304, Lakeshore Cryotronics Inc., USA. Thermogravimetric analysis of the polymer and composite were carried on a Mettler Toledo TGA 851e. FTIR spectra were recorded on Nicolet 5700 and UV–vis absorption studies were carried on Shimadzu 1601 Spectrophotometer. Three-electrode cell geometry was used in all the electrochemical experiments, where Pt was used as working electrode as well as counter electrode and SCE was used as reference electrode. An Auto lab PGSTAT30 (Ecochemie, Utrecht, The Netherlands) potentiostat/ galvanostat interfaced with a personal computer was used in all the electrochemical measurements. The particle size and the morphology were examined using a Transmission electron microscopy (TEM, JEOL JEM 1011) and the samples were deposited on carbon coated nickel grids. Permittivity and dielectric loss measurements were carried out on an Agilent E8362B Vector Network Analyzer in a microwave range of 8.2–12.4 GHz (X-band), using 15.8 mm × 7.9 mm × 6 mm copper sample holder connected between the waveguide flanges. To avoid air gap the above sample holder is modified with a groove of 1.5 mm on each side having 3 mm depth.
3. Result and discussion The emulsion polymerization of aniline to polyaniline in the presence of ␥-Fe2 O3 particles may bring certain changes in the properties of polyaniline because conduction mechanism in
Fig. 1. Electrochemical growth behavior of aniline in DBSA medium (PD13) and aniline in DBSA medium containing ferrite particles (PC11) on cycling the potential between −0.2 and 0.95 V, taking eight successive scans, on platinum electrode vs. SCE at a scan rate of 20 mV s−1 .
K. Singh et al. / Materials Chemistry and Physics 112 (2008) 651–658
653
Scheme 1. Mechanism for radical cations and dications formation in polyaniline.
However when polymerization of aniline was carried out in the presence of ␥-Fe2 O3 particles entrapped in the surfactant medium, electrochemical growth behavior shows shifting of peak potential values, which indicates the incorporation of ␥-Fe2 O3 in the polymer backbone. Cyclic voltammogram of polyaniline film obtained by potential sweeping technique in blank DBSA medium shows characteristic peaks at 0.11, 0.44, 0.54 and 0.77 V (Fig. 2) due to the generation of radical cations at 1st peak potential values which were subsequently oxidized to dications [27] and represented in the mechanism (Scheme 1). However, the cyclic voltammogram of polyaniline embedded with ␥-Fe2 O3 particle in DBSA medium shows characteristic peaks at 0.14, 0.48, 0.56 and 0.86 V (Fig. 2). The shifting of characteristic redox peaks can be assigned to the incorporation of ␥-Fe2 O3 particles in the polyaniline matrix. Fig. 3 shows the FTIR spectra of polyaniline doped with DBSA, PC12 and ␥-Fe2 O3 .The main characteristics bands for the polyaniline doped with DBSA are found at 1515 and 1458 cm−1 which are assigned for the C C bond stretching of quinoids and benzenoid ring, respectively. Bands at 1257 and 1164 cm−1 are due to C N stretching and in-plane bending of the C H bond while peak at 1024 cm−1 is due to −SO3 H group. The peak at 1640 cm−1 may be due to the formation of the carbonyl group arising due to the over oxidation of the alkyl chain of the DBSA. In case of PC12, the main peaks are observed at 1034, 1123.1294, 1400, 1455, 1633, 630 and 558 cm−1 . The presence of the peaks at 630 cm−1 and 558 cm−1 which are the characteristic band of Fe–O band stretching (Fig. 3, curve a) clearly indicate the presence of ␥-Fe2 O3 in the polymer
matrix while the shifting in the main peaks arises due to the interaction of the Fe2 O3 with N-atom of the aniline ring. Fig. 4 shows the UV absorption spectra of the different samples of polyaniline and its composite with ␥-Fe2 O3 . The main peaks and calculated energy bands are shown in Table 1. Emeraldine base form of polyaniline in N-methyl pyrrolidione (NMP) shows two characteristic bands at 326 and 630 nm while the conductive form of polyaniline doped with DBSA has shown the red shift to 353 nm and 739 nm which were assigned to the –* transition of the benzenoid ring and polaronic transition respectively. But in case of polyaniline composite, two changes were observed. First a blue shift was observed for the band from 739 to 726 nm, which was ascribed to polaronic transition. The reason behind this shifting may be the possible interaction of the ␥-Fe2 O3 with polyaniline ring leading to the formation of ferromagnetic composite. Second when the ␥Fe2 O3 content increases in different samples, absorption spectra shows the bathochromic shift for the band 353 to 349 nm. The optical band energy of the polymer was obtained from the given relation [28] ˛h = (h − Eg )
1/2
(1)
where ˛ is the absorption coefficient, h is the photon energy and Eg is the optical band gap. The band gaps calculated by using the above Eq. (1) varies from 1.45 to 1.49 eV and 2.87 to 2.75 eV for the polaronic transitions and –* transition of the benzenoid ring respectively.
Table 1 Conductivity, magnetization, UV–vis bands and dielectric properties of polyaniline and its composite with ␥-Fe2 O3 Sample name
PD13 PC21 PC115 PC12 ␥-Fe2 O3
(S cm−1 )
2.11 1.80 0.80 0.13 10−7
Ms (emu g−1 )
– 1.19 15.2 48.9 69.8
Microwave properties at 10.2 GHz
Band gap (eV)
Dielectric constant εr
Dielectric loss εr
–*
*-polaron
– 12.78 11.02 10.16 6.32
– 9.13 7.82 6.77 0.29
2.82 2.78 2.75 2.75 –
1.45 1.49 1.49 1.47 –
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Fig. 4. UV/Visible spectra of (䊉) EB, () PD13, () PC21, () PC115 and () PC12. Inset shows the calculation of band gap plots in (˛h)1/2 vs. photon energy (h).
Fig. 2. Cyclic voltammogram of polyaniline film in DBSA medium (PD13) and PANIferrite composite in DBSA medium (PC11) on Pt electrode vs. SCE at a scan rate of 20 mV s−1 .
Fig. 5 shows the X-ray diffraction patterns of ␥-Fe2 O3 , polyaniline doped with DBSA and it composite with different compositions of ␥-Fe2 O3 . The main peaks for ␥-Fe2 O3 are observed at 2 = 30.281 (d = 2.949 Å), 35.699 (d = 2.513 Å), 43.435 (d = 2.081 Å), 53.805 (d = 1.702 Å), 57.437 (d = 1.603 Å), 63.0460 (d = 1.473 Å) corresponding to the (2 0 6), (1 1 9), (0 0 12), (2 2 12), (1 1 15), (4 4 1) reflections which matches with the standard XRD pattern of ␥-Fe2 O3 (PDF No. 25-1402). The peaks present in ␥-Fe2 O3 were also observed in all the compositions of polyaniline composite with ␥-Fe2 O3 which indicates the presence of ferrite particles in the polymer matrix and the intensity of peaks increase with the increase in the ratio of iron oxide. While the presence of polyaniline and its semicrystalline nature is confirmed by the broad peaks at 2 = 19.795 (d = 4.481 Å) and 25.154 (d = 3.537 Å) [29,30] and it is also observed that the intensity of these peaks increases with the decrease in iron oxide composite. The line broadening of the peaks in the entire patterns of polyaniline composite indicates about the small dimensions of the iron oxide particles. The crystallite size of ␥-Fe2 O3 particle can
Fig. 3. FTIR spectra of (a) ␥-Fe2 O3 , (b) polyaniline-␥-Fe2 O3 composite and (c) polyaniline doped with DBSA in KBr pellet.
K. Singh et al. / Materials Chemistry and Physics 112 (2008) 651–658
Fig. 5. X-ray diffraction plots of ␥-Fe2 O3 (a), PC12 (b), PC11(c), PC21 (d) and PD13 (e) with arbitrary no of counts vs. 2.
be calculated by line broadening using Scherer’s formula k D= ˇ cos
(2)
where D is the crystallite size for individual peak, is the X-ray wavelength, k the shape factor, D is the crystallite size for the individual peak of the crystal in angstroms, the Bragg angle in degrees, and ˇ is the line broadening measured by half-height in radians. The value of k is often assigned a value of 0.89, which depends on several factors, including the Miller index of the reflecting plane and the shape of the crystal. The average size of ␥-Fe2 O3 particles was calculated using above equation and estimated as 8.99 nm for pure ␥-Fe2 O3 and 9.87 nm for polyaniline composite with iron oxide having aniline: ␥-Fe2 O3 ::1:2 (PC12) which is in accordance with the TEM analysis (Fig. 6) which shows the uniformly dispersed iron oxide particles of 8–15 nm and the agglomerated polymer composite containing 8–13 nm size particles of iron oxide. Thermogravimetric analysis of the polyaniline doped with DBSA and polyaniline composite was carried out in order to see the effect
655
Fig. 7. Thermal gravimetric analysis of polyaniline doped with DBSA and polyaniline composite with ␥-Fe2 O3 with increasing content of ␥-Fe2 O3 ; (䊉) PC12, () PC115, () PC11, () PC21 and () PD13.
of the ␥-Fe2 O3 content on the thermal stability of the composite (Fig. 7). Thermogram of different samples shows three major weight losses, first at 100 ◦ C due to the loss of water contents, second in the range of 230–380 ◦ C due to the loss of the dopant from the polymer matrix and the third major loss from 380 to 700 ◦ C was attributed to the destruction of polymeric backbone. Polyaniline doped with DBSA (PD13) is thermally stable up to 230 ◦ C. However, when conducting polymer was synthesized by incorporating ferric oxide moieties in the reaction system along with the surfactant, it has been observed that the thermal stability of the polymer has increased to 260 ◦ C. This shows that in situ polymerization of aniline in the presence of ferrite particles leads to a better thermally stable conducting polymer. The approximate amount of iron oxide for different polymer ferrite composites was calculated by subtracting the residual weight of the blank polymer from residual weight of composite at 700 ◦ C. It is observed that the for different compositions, PC21; PC11; PC115 and PC12 the weight percent of ␥-Fe2 O3 is estimated to be 10.7%, 16.1%, 29.9% and 42.1% which is in accordance with the amount taken during synthesis.
Fig. 6. TEM image of ␥-Fe2 O3 (a) and PC12 (b) having 8–15 nm size ␥-Fe2 O3 particles.
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Fig. 8. Temperature dependence of conductivity of (a) PD13 (b) PC11 (c) PC12 along with temperature from 50 to 300 K and (d) variation of conductivity with different An/␥-Fe2 O3 ratio at 300 K.
The temperature dependent DC conductivity of the polyaniline␥-Fe2 O3 composite having different weight ratio of ferric oxide contents were measured at temperature ranging from 50 to 300 K. The DC conductivity follows the semiconducting behavior and decreases with the decrease of temperature. The room temperature conductivity of the samples is shown in Table 1. Several models were used to explain the conductivity behavior in polymer like Arrhenius law and Mott’s equation. But it is observed that for low temperature range of 300–1.8 K the conductivity studies are best studies by the VRH model which follows the Mott’s equation [31–34].
1/ TO
(T ) = O exp −
T
(3)
where TO is the Mott characteristic temperature and can be expressed as TO = 8˛/kB N(EF )Z
(4)
and O is the conductivity at T = ∞ R = [˛ /kB TN(EF )]
−1/2
(5)
where ˛−1 is the localization length, which can be, determined from the magneto resistance data. From the observed values of TO and O , one can calculate N (EF ) density of states, R, the average hopping distance with the use of Eqs. (4) and (5). Exponent in Eq. (3) is the dimensionality factor having values 2, 3, and 4 for 1-, 2- and 3-dimensional conduction mechanisms, respectively. In this paper, we have plotted ln vs. temperature with different
values of as 2, 3 and 4. It was observed that the conductivity data fits for the one dimensional VRH model with = 2 with linearity factor of 0.9996, 0.9997 and 0.9992 for the polyaniline (PD13), polyaniline-ferrite composite PC11 and PC12, respectively (Fig. 8) and the corresponding value for 3D-VRH mechanism are 0.9933, 0.9962, 0.9956, respectively. The calculated values of the conductivity () at room temperature are given in Table 1. The plot of conductivity vs. the An/␥-Fe2 O3 (Fig. 8d) ratio shows that the conductivity decreases with the increase of ferrite constituent due to the insulating nature of the ␥-Fe2 O3 which hinder the flow of charge by blocking the conduction path in the polymer matrix. Thus from the above data, it is observed that 1D–VRH model is suitable for the conduction mechanism of the polyaniline-␥-Fe2 O3 composite and the conductivity decreases with the increase of ferrite content. The magnetic properties of the polyaniline-␥-Fe2 O3 composite and ␥-Fe2 O3 were explained by using the M–H curve (Fig. 9). The saturation magnetization (Ms ) value of the ␥-Fe2 O3 was found to 69.77 emu g−1 at an external field of 10 kOe having small value of coercivity and negligible retentivity with no hysteresis loop, indicating the super paramagnetic nature. When these nanoferrite particles are incorporated in the polyaniline matrix in weight ratio of 1:1(PC11) the magnetization saturation (Ms ) value was found 4.13 emu g−1 . However, on changing the weight composition of An/␥-Fe2 O3 to 1:2, the Ms value was drastically increased from 4.13 to 48.9 emu g−1 , keeping the external applied field at 10 kOe. The Ms values of different polyaniline-␥-Fe2 O3 composites was measured and given in the Table 1. In all the cases very small coercivity is observed with negligible retentivity which indicates the ferrimagnetic nature. Ms value increases due to high poly-dispersivity of the
K. Singh et al. / Materials Chemistry and Physics 112 (2008) 651–658
Fig. 9. Magnetization curves of () ␥-Fe2 O3 , (䊉) PC12, () PC115, () PC11 and () PC21 showing the decrease in saturation magnetization with the decrease in ␥-Fe2 O3 content.
␥-Fe2 O3 in polyaniline matrix that arises due to the functionalization of nanoferrite particles with the surfactant DBSA. The variation of complex permittivity of different samples of polyaniline-␥-Fe2 O3 composite in the frequency range of
657
Fig. 11. EMI shielding effectiveness (SE) of ␥-Fe2 O3 (), PC12 (䊉), PC115 (), PC21 () measured in X-band (8.2–12.4 GHz).
8.2–12.4 GHz (X-band) are shown in Fig. 10. The real and imaginary part of permittivity (εr and εr ) of PC12 decreases from 12.1 to 9.7 and 7.6 to 4.5, respectively, with the increase in frequency. The minimum values of the dielectric losses propose polyaniline-␥Fe2 O3 composite to be good shielding material [35]. The dielectric properties are mainly due to interfacial polarization and intrinsic electric dipole polarization which are partially attributed by the disordered motion of the charge carrier along the back bone of conducting polymer chain. The electromagnetic shielding effectiveness (SE) of the ␥ Fe2 O3 and polymer composites was also measured from S-parameters as given below SE = −10 log
PT ET = −20 log = −20 log S21 PI EI
(6)
where PI (EI ) and PT (ET ) are the power (electric field) of incident and transmitted EM waves and |S21 |2 is the transmission coefficient, respectively. The maximum SE of −11.2 dB was recorded for the sample PC21 while SE of −2.5 dB was observed for ␥-Fe2 O3 (Fig. 11) proving that the polymer composite is better EMI shielding material. 4. Conclusion
Fig. 10. Real ( ) and imaginary ( ) part of permeability of () ␥-Fe2 O3 , () PC12, (䊉) PC115, () PC21 measured in X-band (8.2–12.4 GHz).
Emulsion polymerization of the aniline with ␥-Fe2 O3 in aqueous medium of DBSA leads to the formation of conducting super paramagnetic polyaniline ␥-Fe2 O3 composite with an Ms of 48.9 emu g−1 and moderate conductivity of 0.13 S cm−1 . Maximum dielectric constant value of 15.1 and shielding effectiveness of −11.2 dB was observed for the polymer composite for its application in EMI shielding and found to decrease with the increase in frequency. It was observed that the variation of conductivity with temperature for polyaniline and polyaniline-ferrite composite follows onedimensional VRH model with linearity factor of 0.9996 and 0.9997. The presence of ␥-Fe2 O3 in the polyaniline matrix is confirmed by the cyclic voltametry, as peak potential values of polyaniline shifts by incorporation of ferrite particles in the polymer matrix. FTIR result has also shown the presence of characteristic band stretching of Fe O band at 630 and 558 cm−1 which clearly indicate the presence of ␥-Fe2 O3 in the polymer matrix. Thermal stability of PANI-␥-Fe2 O3 composite is higher than the thermal stability of PANI-DBSA synthesized without ␥-Fe2 O3. The enhancement in the thermal stability arises due to the presence of ␥-Fe2 O3 particles in the polymer matrix.
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