epoxidized natural rubber nanocomposites

Journal of Alloys and Compounds 561 (2013) 40–47

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Synthesis, characterization and impedance spectroscopy study of magnetite/epoxidized natural rubber nanocomposites W.L. Tan, M. Abu Bakar ⇑ Nanoscience Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia

a r t i c l e

i n f o

Article history: Received 4 June 2012 Received in revised form 30 November 2012 Accepted 20 January 2013 Available online 10 February 2013 Keywords: Magnetite Epoxidized natural rubber Nanocomposite Impedance spectroscopy Conductivity

a b s t r a c t The magnetite (Fe3O4) particles were synthesized in situ in the presence of epoxidized natural rubber (ENR) to produce the various Fe3O4/ENR nanocomposites. The X-ray diffraction (XRD) analysis confirmed the existence of Fe3O4 particles in the composites. The FTIR and DSC studies suggested that no chemical interactions between the particles and the matrix. The SEM and X-mapping micrographs revealed that the Fe3O4 particles were distributed within the ENR matrix. The ENR matrix exerts control on the Fe3O4 particles with a size of <20 nm in the composites. The Fe3O4 particles also affect the electrical properties of the composites. Impedance spectroscopy studies show that the electrical conductivity of the nanocomposites increases with the increase in Fe3O4 loading in the composite. The equivalent circuit for the Fe3O4/ENR nanocomposites is proposed and discussed. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Magnetite (Fe3O4) is an interesting type of iron oxide and has been extensively studied. Fe3O4 differs from other iron oxides as it contains both divalent Fe2+ and trivalent Fe3+ ions in the interstitial tetrahedral and octahedral sites [1]. This facilitates electron(s) migration within the Fe3O4 lattice and thus exhibits high electronic conductivity. The band gap of Fe3O4 semiconductor is 0.1 eV. This makes it almost a metal-like semiconductor [1]. Fe3O4 also displays other unique properties including optical, magnetic, catalytic and thermal properties. Due to these interesting properties, Fe3O4 has been used in a broad range of industrial applications including catalysis, magnetic storage devices, colorants, and ferro-fluids [1,2]. In the recent decade, polymer–inorganic nanocomposites have become one of the emerging classes of advanced materials. Inorganic nanoparticles such as transition metal oxides have been incorporated into polymers affording various multifunctional materials [3] with adopted properties of the respective constituents. Numerous reports on the Fe3O4/polymer composites are found in literatures. The Fe3O4 particles have been combined with suitable polymers of desired properties to produce a composite that holds potential applications. Thus, Fe3O4 particles has been incorporated into non-toxic, biocompatible and biodegradable polymers such as poly(D,L-lactide) (PDLLA) [4], alginic acid [5], ⇑ Corresponding author. Tel.: +60 4 653 3888; fax: +60 4 657 4854. E-mail address: [email protected] (M. Abu Bakar). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.135

L-lysine [6], poly(N-vinyl pyrrolidone) (PVP) [7] and poly(lacticco-glycolic acid) (PLGA) [8]. This has resulted in the utilization of the nanocomposites in several biomedical fields including magnetic resonance imaging (MRI), drug delivery, and hyperthermia as well as cell separation. Other polymers such as polyurethane (PU) [9], polyaniline (PANI) [10], polypyrrole (Ppy) [11], polyethylene oxide (PEO) [12] and natural rubber [13], have also been used to prepare the respective Fe3O4/polymer composites. These composites have potential use in batteries, capacitors, fuel and solar cells, electromagnetic wave absorbers and magnetic materials. The electrical characteristics of Fe3O4/polymer nanocomposites have also attracted the interest of researchers recently due to their potential applications particularly in electronic packaging. Bakyal and co-workers [5,6,14–18] have reported the electrical conductivity characteristics of several corresponding Fe3O4/polymer nanocomposites. In most of the cases, they found that the composites exhibit high conductivity at high temperature and the composite’s conductivity was less dependent on frequency. Epoxidized natural rubber (ENR) is a modified natural rubber with epoxide and alkene groups randomly distributed in its backbone [19]. It possesses low glass transition temperature (Tg), high polarity and flexibility, good elastomeric and adhesion properties. Due to these characteristics, it has therefore been applied in gas membrane [20], lithium cell [21,22] and so on. Our group has successfully prepared ENR-metal (Au [23,24], Pt [24] and Ag [25]) nanocomposites recently. The role of ENR as stabilizing agent to control the size and dispersion of metal nanoparticles in the composite was evident by these works.

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To the best of our knowledge, there is scarcely any report on the Fe3O4/ENR nanocomposites. In these composites, it is believed that the ENR matrix affects the Fe3O4 particle size while the particles also exert electrical, magnetic or optical properties. Hence, these composites may have potential applications in electronic devices, sensors, radiation absorbers, recording media and the likes. Thus, in this article, we report a simple in situ synthesis of Fe3O4/ENR nanocomposites and their electrical properties as studied via impedance spectroscopy.

Table 1 The Tg, exponent (n), pre-exponential factor (A) and linear regression (r) values for ENR and the various Fe3O4/ENR nanocomposites.

a

Base

Fe3O4 contenta (wt.%)

Tg (°C)

n

A

r

ENR

0 2.8 3.9 5.1 9.4 16.3

19 16.8 18.4 17.6 17.3 17.3

0.97 0.88 0.87 0.79 0.78 0.78

1.75  1012 7.98  1012 1.73  1011 2.93  1011 3.66  1010 3.03  1010

0.9995 0.9998 0.9997 0.9996 0.9996 0.9986

Determined using AAS.

2. Experimental 2.1. Materials All chemicals were used without further purification unless otherwise stated. The following chemicals were purchased; tetrahydrofuran, THF (J.T. Baker, USA), toluene (Fisher Chemicals, UK), iron (II) sulfate heptahydrate, FeSO4.7H2O (99.5%) and potassium hydroxide, KOH (both from R & M Chemicals, UK) and ethanol (Systerm, Malaysia). Epoxidized natural rubber with 50% epoxidation (ENR-50) was supplied by Guthrie Polymer (Malaysia) Sdn. Bhd. and was purified according to literature before use [26].

2.2. Synthesis of Fe3O4/ENR nanocomposites The route to obtain the Fe3O4 nanoparticles was achieved according to our previous work [27] while the synthesis of the Fe3O4/ENR nanocomposites was carried out in a medium of toluene/water/ethanol according to Wen et al. [28]. In a typical one-pot preparation, a certain amount of FeSO4 7H2O was dissolved in 3 mL of deionized water in a round-bottomed flask. Then 5 mL of 50 mg mL1 ENR in toluene was added slowly with vigorously stirring. This was followed by the addition of 1 mL ethanol and the mixture was left to stir for an hour. The mixture was then heated to 75 °C. After attaining the temperature, 0.5 mL of 2.5 M KOH was added slowly into the mixture. Upon completion, the reaction mixture was heated for another 2 h while vigorously stirring. Upon cooling, the solvent was evaporated using a rotary evaporator. The residue was rinsed several times with distilled water prior to redispersion of the residue in THF. A film with a thickness of 1 mm was obtained by solvent casting the redispersed residue and drying under vacuum at 50 °C overnight. The other Fe3O4/ENR nanocomposites of various amount of Fe3O4 were prepared accordingly.

2.3. Characterizations The FTIR spectra were recorded using a Thermo Nicolet IR200 spectrophotometer. The sample was prepared by drop casting the redispersed residue as film onto a KRS-5 window. The spectra were collected in the region of 4000– 400 cm1. The micrographs of the samples were obtained using a Philip CM 12 transmission electron microscope (TEM) operating at 80 kV. A drop of the colloid was placed on a 400 mesh carbon coated copper grid and the solvent was evaporated off. The particle sizes were measured from the TEM micrographs and analyzed using a computer software ‘‘analySis Docu’’ Version 3.2 (Soft Imaging System GmbH, Munster, Germany). The average particle size and size distribution were obtained from P300 particles. The Leo Supra 50VP Field Emission Scanning Electron Microscopy (FE-SEM) equipped with X-mapping was utilized to determine the morphology and the distribution of components in the samples. The samples were coated with a layer of gold prior to the analysis. Sample purity and crystallinity were characterized using a SIEMENS D5000 X-ray diffractometer (XRD) equipped with a monochromatic Cu Ka radiation filter. The samples were scan in the 2h range of 0–100°. The glass transition temperature (Tg) of the samples were obtained using a Perkin Elmer Pyris Differential Scanning Calorimeter (DSC). About 10 mg of the sample was sealed in an aluminium pan and heated from room temperature to 120 °C at a heating rate of 20 °C min1. This was held for 3 min at 120 °C and then quenched to 50 °C at a cooling rate of 100 °C min1. The temperature was again held for another 3 min at 50 °C. The sample was then heated to 120 °C at a heating rate of 20 °C min1. The electrical conductivity of the samples was studied using EIS, GAMRY instrument at ambient conditions. The samples were sandwiched between two stainless steel disks that function as working and counter electrode [11] and scanned over a frequency range of 1 Hz–1 MHz at 10 mV amplitude. The samples were placed in a vacuum oven at 50 °C for 2 h prior to the EIS analyses. The Nyquist plot was fitted using a ZSimDemo 3.22d software to obtain the equivalent circuit. The Fe content in the composite samples was determined using a Perkin–Elmer Analyst 200 atomic absorption spectroscopy (AAS). Typically, 20 mg of each of the samples was acid digested prior to AAS analyses. The AAS results were expressed in terms of wt.% of Fe3O4 as tabulated in Table 1.

3. Results and discussion 3.1. Synthesis and characterizations An in situ synthesis of Fe3O4 particles in the ENR matrix was carried out in a medium of water–toluene–ethanol. The addition of ethanol into an immiscible mixture of ENR in toluene and aqueous salt solution afforded a homogenous milky white solution. The ethanol has a mutual solubility in water and toluene solvent. Upon addition of KOH, the milky solution gradually changed to green and finally to black in color. This corresponds to the typical transition of the transient iron hydroxide to magnetite particles as previously reported [27]. The conversion of FeSO4 to Fe3O4 is according to Eq. (1). A black flexible film of the composite was obtained via solvent casting method

6FeSO4 þ 12KOH þ O2 ! 2Fe3 O4 þ 6H2 O þ 6K2 SO4

ð1Þ

XRD analysis confirmed the existence of Fe3O4 particles. Fig. 1 shows the typical XRD pattern of the composite. The observed 2h values at 30.3°, 35.7°, 43.1°, 53.6°, 57.4°, 62.6° and 90.2° are indexed to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3) reflection planes attributed to the magnetite crystals (JCPDS File No. 19-629) [29]. The average size of Fe3O4 particles is about 16 nm based on the Scherrer analysis [30]. Fig. 2 depicts the FTIR spectra of ENR, typical Fe3O4/ENR nanocomposite, pristine Fe3O4 and ENR treated with KOH (ENR/KOH). The peaks and their corresponding assignments are shown in Table 2. The FTIR spectrum of ENR (Fig. 2a) shows characteristic peaks similar to that reported in several literatures [31,32]. The peaks at 876 and 1253 cm1 are assigned to the respective asymmetrical and symmetrical stretching of the epoxide ring (

).

Whereas the carbon–carbon double bond (C@C) stretching and the @CAH bending peaks are situated at 1659 and 836 cm1 respectively. The intense peaks at 2965, 2924 and 2862 cm1 are attributed to CAH stretching vibrations. The CAH bending peaks are located at 1377 and 1453 cm1. In addition, a broad peak also appears at 3490 cm1 which may be due to the OAH of epoxide ring opened derivative of ENR as well as the adventitious moisture. The typical FTIR spectrum of the composite is shown in Fig. 2b. The composite still inherits the characteristic peaks of ENR and the peak positions were similar to that of ENR as shown in Table 2. An additional peak however, appears at 574 cm1 in the spectrum of the composite. This peak is attributed to FeAO stretching [27] and is similar to the FeAO stretching in pristine Fe3O4 as shown by the spectrum in Fig. 2c. In order to understand the effect of KOH on ENR, an experiment was carried out under similar conditions and procedure but with the exclusion of Fe salt. The obtained spectrum is shown in Fig. 2d. The main difference between spectra of Fig. 2a and d is the broadening and intensifying of the OAH peak for the later. This may be attributed to the presence of diol resulting from the ring opening of epoxide in the ENR under alkaline condition [33,34] as well as the inter-chain hydrogen bondings that occur between

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Fig. 1. Typical XRD pattern of Fe3O4/ENR nanocomposite and the corresponding reference peak profile (JCPDS File No. 19-629).

Table 2 FTIR peak positions and assignments for ENR, typical Fe3O4/ENR nanocomposite, Fe3O4 and ENR reacted with KOH. Assignment

tas (CAH) ts (CAH) CAH bending

ts (

)

tas tas (C@C) @CAH bending OAH t (FeAO)

Fig. 2. Typical FTIR spectra of (a) ENR, (b) Fe3O4/ENR nanocomposite, (c) pristine Fe3O4 and (d) ENR reacted with KOH.

the diol derivatives of ENR. Other peaks including CAH stretching and bending, (

) stretching, C@C stretching and @CAH bending

remain similar to those of ENR. To further evaluate the extent of ring opening in the samples, the relative peak height of tas(

) or OAH to the @CAH bending

was monitored [34]. The @CAH bending peak was selected as reference because it does not involve in the ring opening reaction. The ratio of [tas(

)/@CAH bending] and [OAH/@CAH bending] for

Peak position ENR

Fe3O4/ENR

Pristine Fe3O4

ENR/KOH

2965, 2924 2862 1377, 1453 1253

2964, 2925 2861 1379, 1453 1255

– – – –

2962, 2923 2860 1379, 1447 1256

876

877

–

877

1659 836 3489 –

1657 837 3490 574

– – – 577

1652 838 3400 –

ENR is 1.18 and 0.43 respectively. These are respectively 1.12 and 0.74 in the Fe3O4/ENR composite and 1.02 and 1.18 in the ENR/KOH sample. Obviously, some epoxide ring opening reactions have taken place in the composite and the ENR/KOH sample. Nevertheless, the extent of the epoxide ring opening reaction in the Fe3O4/ENR was less than that in ENR/KOH sample. This may mean that most of the KOH was involved in the formation of Fe3O4 particles instead of acting as catalyst for the epoxide ring opening reaction of the ENR in the composite. Therefore, FTIR results suggest that, in the Fe3O4/ENR composite, there were occurrence of epoxide ring opening in the ENR and there is no evidence of chemical interactions between ENR and Fe3O4 particles. The surface morphology of ENR and Fe3O4/ENR nanocomposites were investigated by SEM and the micrographs are shown in Fig. 3a–c. From Fig. 3a, ENR shows a smooth and clean surface. The surface is however, rougher for Fe3O4/ENR nanocomposites due to the distribution of Fe3O4 particles even in the composite containing low wt.% of Fe3O4 as shown by the composite in Fig. 3b. This surface roughness was more pronounced for composites containing a higher wt.% Fe3O4 as shown in Fig. 3c. X-mapping analysis was performed in order to determine the distribution of Fe3O4 particles in the composite. The micrographs of the surface and cross section of the Fe3O4/ENR nanocomposite as well as the respective distribution of Fe are presented in Fig. 4a and b correspondingly. It is clearly seen that the Fe of

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43

Fig. 3. SEM micrographs (350 magnification) of (a) ENR and the ENR/Fe3O4 composites containing (b) 2.8 wt.% and (c) 9.4 wt.% Fe3O4 and the TEM micrographs of (d) Fe3O4 particles and the ENR/Fe3O4 composites containing (e) 2.8 wt.% and (f) 9.4 wt.% Fe3O4.

Fe3O4 particles which are presented by the brighter area are well distributed in the entire matrix. Even so, from Fig. 4a(ii), some particle agglomerates are also visible at high magnification. The size and morphology of Fe3O4 particles in the various Fe3O4/ ENR nanocomposites as well as the bare Fe3O4 particles were investigated. The TEM micrographs are shown in Fig. 3d–f. Without the protection of ENR, the bare Fe3O4 particles were highly agglomerated as shown in Fig. 3d(i). These particles have an average size of 33.3 ± 21.9 nm with a broad size distribution exhibiting various geometries. Some of these particles show distinctive outlines like circle, square, rod or hexagon as shown in Fig. 3d(ii). This observation is similar to previous reported work and is attributed to the un-inhibited crystal growth [35]. The TEM micrographs of the representative Fe3O4/ENR nanocomposites are shown in Fig. 3e–f. As inferred from Fig. 3e(i), the distribution of Fe3O4 particles in the composite with lower wt.% (ca. 2.8 wt.%) were isolated congregates. However, as the Fe3O4 particles loading is increased (ca. 9.4 wt.%) in the composite, the

particles were more dispersed (Fig. 3f(i)). From Fig. 3e(ii) and f(ii), the particles are mostly sphere-like. Especially at high wt.% of Fe3O4 loading some rod-like and cube shape particles are also observed. The average particle size is 4.0 ± 1.0 nm and 4.1 ± 1.2 nm for composites with Fe3O4 loadings of 2.8 and 3.9 wt.% respectively. These values increased slightly to 4.9 ± 1.0 nm, 5.3 ± 1.3 nm and 4.8 ± 2.1 nm for the respective composites containing 5.1, 9.4 and 16.3 wt.% of Fe3O4. The particle size and size distribution for Fe3O4 particles synthesized in the presence of ENR were much smaller compare to the bare Fe3O4. The reduction in particle size of Fe3O4 when incorporated in the ENR matrix is explainable as follows. During synthesis, the metal precursors are enclosed within the nanovoids formed within the entanglement of the polymer chains. These nanovoids serve as nanoreactors for the formation of Fe3O4 particles [36]. The confinement within the cavity of the voids inhibits the growth of particles and simultaneously protects the particles from serious aggregation. This inevitably governs the size of the as-formed Fe3O4 particles [24,25].

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Fig. 4. SEM and X-mapping micrographs of ENR composited with 5.1 wt.% Fe3O4 showing the (a) surface at (i) low (100) and (ii) high (2500) magnification and (b) low (100) magnification cross sectional profiles.

Thus size control of Fe3O4 particles can be achieved using ENR as the matrix where the particles in the composites are in the size range of 3–6 nm based on TEM observations. It was also noted that there is a discrepancy in the average particle size obtained based on XRD and TEM analyses. The former is based on the peak width of the most intense diffraction plane while the later is based on direct visual measurement from micrograph images. In the former, there are some factors that may contribute to the inaccuracy including shape factor, strain, broad size distribution and polycrystallinity [37,38]. According to Calvin and co-workers [38], TEM is the most accurate method to estimate the size of the particles provided that sufficient data is available, while XRD method tends to encompass the largest crystal size in the sample.

The Tg of ENR and ENR treated with KOH is 19 °C [24] and 16.4 °C respectively. A decrement in the number of epoxide groups in the ENR treated with KOH and the simultaneous formation of inter-chain hydrogen bondings by diol derivatives of ENR have resulted in higher Tg. As shown in Table 1, the Tg for the composites fall within the range of 18.4 to 16.8 °C. This increment was marginal (ca. <2 °C) as compared to ENR. The addition of inorganic fillers into a polymer matrix reduces the free volume of the polymer and this tends to restrict the flexibility or movement of the polymer segments that consequently resulted in an increase in Tg of the polymer [39]. Any shiftment in Tg of a composite with respect to their parent components can be used as an indication to evaluate the extent of interaction [40]. In this current study, the

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-3 0 wt% 2.8 wt% 3.9 wt% 5.1 wt% 9.4 wt% 16.3 wt%

-5 -6 -7 -8 -9

-10 -11

1.00E-05

σDC (Scm -1)

log ( σAC)/ Scm -1

-4

1.00E-06 1.00E-07 1.00E-08 1.00E-09 1.00E-10

0

1

2

3

4

log (w)/ s

5

6

7

2

4

6

8

10

12

14

16

18

Fe 3O4 content (wt%)

Fig. 5. Plot of AC conductivity versus angular frequency for ENR and Fe3O4/ENR nanocomposites with various wt.% of Fe3O4.

small difference in Tg values between the ENR of the composites and ENR suggests that, in the composite, there is no chemical interaction between the Fe3O4 particles and the ENR matrix. This is supported by the FTIR results discussed previously where the difference in the wavenumber due to the FeAO stretching in bare and composited Fe3O4 is <5 cm1. Similar attribute has also been reported previously [40–42]. For instance, Kuljanin et al. [41,42] has incorporated lead sulfide (PbS) and cadmium sulfide (CdS) nanoparticles into polyvinyl alcohol (PVA) and polystyrene (PS) respectively. The FTIR and DSC techniques were employed to determine the possible interactions between the particle and polymer. They found that there is no difference between the FTIR spectra of the pristine polymer and the composite while the change in Tg is <2 °C. They conclude that the interaction between the particle and polymer in the respective composites as insignificant. 3.2. Electrical conductivity The correlation between the alternating current (AC) conductivity (rAC) and the direct current (DC) conductivity (rDC) at a constant temperature can be expressed via the following [39]:

rAC ¼ rDC þ AðxÞn

0

-1

ð2Þ

In Eq. (2), A and n are parameters dependent on temperature and filler content. Thus the rAC is a summation of actual ohmic conductivity (rDC) and the dielectric dispersion [29,43]. The latter is mainly due to the polarization and dielectric relaxation in the system. The rDC is related to frequency-independent conductivity. It is characterized by the movement of the charge carriers and is limited by the conducting path provided in the system [29]. The rAC of ENR and ENR filled with various amount of Fe3O4 was studied as a function of angular frequency and the results are presented in Fig. 5. Generally, the conductivity of ENR steadily increases with the increase in frequency and is in the order of 1011–107 S cm1. This increase is also commonly observed for organic materials like L-lysine [6], poly(2-thiophen-3-yl-malonic acid) [16] and poly(1-vinyl-1,2,4-triazole) (PVTri) [17]. The Fe3O4/ENR nanocomposites however, exhibit a conductivity plateau at low frequency but the conductivity increases at high frequency. The range of frequency encompass by the plateau increases upon increasing in Fe3O4 content in the composite. For example, the ENR composite with 2.8 and 3.9 wt.% Fe3O4 showed a plateau ranged between 0.8 and 1.7 s1 while the plateau for ENR with 16.3 wt.% Fe3O4 is from 0.8 to 5.1 s1. Furthermore, increasing the wt.% of Fe3O4 in the ENR has resulted in the rAC of the composites to increase. For instance, the rAC of the ENR composites with 2.8 and 16.3 wt.% Fe3O4 is in the order of 1010–107 and 106–104 S cm1 respectively. The rAC curves at higher frequencies were fitted according to Eq. (2), and the best fitted curves are shown as (red1) solid lines in Fig. 5. 1 For interpretation of color in ‘Figs. 5 and 7’, the reader is referred to the web version of this article.

Fig. 6. The plot of rDC conductivity against the wt.% of Fe3O4 in the composites.

The n and A as well as their linear regression (r) values extracted from the fitted curves are tabulated in Table 1. The Fe3O4/ENR composites exhibit lower n (i.e. 0.78–0.87) compare to ENR (0.97) and this falls within the range of 0.5 < n < 0.9. This is typical for carrier transport in disordered materials [44]. The rDC of the Fe3O4/ENR nanocomposites was derived from the plateau region of Fig. 5 [14,15,17,29]. The plot of rDC of the composites versus wt.% of Fe3O4 in the composites is depicted in Fig. 6. The rDC of the Fe3O4/ENR nanocomposites increases with the increase in wt.% Fe3O4. Those composites with 2.8 and 3.9 wt.% Fe3O4 demonstrate lower conductivity at 2.2  1010 and 4.5  1010 S cm1 respectively. This increases to 1.0  108 S cm1 for composite with 5.1 wt.% Fe3O4 and 1.1  107 S cm1 for composite with 9.4 wt.% Fe3O4. The highest rDC was shown by the composite with 16.3 wt.% Fe3O4 and is at 1.4  106 S cm1. This rDC behavior of the Fe3O4/ENR nanocomposites is probably due to the grain and/or grain boundary charge carriers migration [15]. The composites with low wt.% of Fe3O4 have particles mostly in isolated congregates and thus conduction of these composites were relatively low. In the composites with high wt.% of Fe3O4, the distance between the particles was closer and this facilitates the conduction across the polymer and increase the conductivity in the composite [15]. 3.3. Equivalent circuit The Nyquist plots for the various Fe3O4/ENR nanocomposites and the fitted circuit are presented in Fig. 7. The respective red and green markers represent the experimental and calculated values. The plot demonstrates an incomplete semicircle for composites containing low wt.% of Fe3O4 (Fig. 7a and b). This was however, transformed into a slightly distorted semicircle as the amount of Fe3O4 was increased to 5.1 wt.% (Fig. 7c). Further increment of Fe3O4 in the composite (ca. to 9.4 and 16.3 wt.% as shown in Fig. 7d and e respectively) caused a small or shrinking semicircle with a straight line tail. Although the feature of Nyquist plots was transformed upon varying the amount of added Fe3O4 in the composite, they are nevertheless, best-fitted into an equivalent circuit as shown in Fig. 7f. As seen, an equivalent circuit with a combination of constant phase element, Qm that is parallel with another Qp in series with resistor, Rm was fitted to the experimental data. The Fe3O4 particles behave both as resistor and capacitor due to its respective semiconducting and polarizing nature [1]. The intraparticle (grain boundary) as well as the inter-particle (grain) conduction dictates the Rm in the circuit. At the same time, Fe3O4 particles can also be polarized where the charge carrier(s) migration between the Fe2+ and Fe3+ respond to the direction of the applied field. In this case, Fe3O4 particles give a capacitive behavior, Qm to the circuit. The ENR matrix around the Fe3O4 particles may further contribute capacitive behavior, Qp, in the composite [15]. This is because the polymer may undergo polarization and relaxation in an applied electrical field [45]. Hence a Qp is in parallel to Qm in the circuit. It should be noted that a non-ideal capacitor (viz. constant phase element) is used to present the capacitance

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Fig. 7. Nyquist plot for Fe3O4/ENR nanocomposites containing (a) 2.8 wt.%, (b) 3.9 wt.%, (c) 5.1 wt.%, (d) 9.4 wt.%, (e) 16.3 wt.% Fe3O4 and (f) the equivalent circuit.

element in the system instead of an ideal capacitor (C). This may be due to the range of particle sizes as well as distribution of Fe3O4 particles within the composite. Even so, in the case of composites with higher Fe3O4 content (i.e. 9.4 and 16.3 wt.%), a Warburg element (Zw) may also be added in series to the (QRQ) circuit to suit the low frequency spike feature of the plots. This contribution accounts the ionic diffusion along the Fe3O4 particle paths and at the electrode interfaces [46]. 4. Conclusion The Fe3O4 particles were successfully synthesized in situ in the presence of ENR. XRD results support the formation of Fe3O4 crystals and no trace of impurities in the composites was observed. The calculated particle size was around 16 nm based on Scherrer anal-

ysis. The FTIR spectra of ENR and Fe3O4/ENR nanocomposites show no significant change or shift in peak wavenumber. This suggests that there was no chemical interaction between ENR and Fe3O4 particles. This is supported by the Tg value of nanocomposites that only marginally shifted (i.e. 2 °C) as compared to ENR. The average particle size of the Fe3O4 particles in the composites was within 3–6 nm based on TEM results. Therefore, the ENR matrix was able to control the particle size at ca. <20 nm. SEM and X-mapping images demonstrate that the Fe3O4 particles were distributed throughout the ENR matrix. The electrical conductivity of Fe3O4/ ENR nanocomposites shows a plateau at low frequency but increased steadily at high frequency. Incorporating and increasing the amount of Fe3O4 particles in the composite increased the conductivity of the composite as more conduction paths are provided for the migration of charge carrier(s).

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