Nanostructured Multifunctional Electromagnetic Materials from the

Feb 5, 2010 - A nanostructured electromagnetic polyaniline- polyhydroxy iron-clay composite (PPIC) was prepared by oxidative radical emulsion ...
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Nanostructured Multifunctional Electromagnetic Materials from the Guest-Host Inorganic-Organic Hybrid Ternary System of a Polyaniline-Clay-Polyhydroxy Iron Composite: Preparation and Properties Viswan L. Reena,‡ Chorappan Pavithran,‡ Vivek Verma,§ and Janardhanan D. Sudha*,† Chemical Sciences and Technology DiVision, and Materials and Minerals DiVision, National Institute for Interdisciplinary Science and Technology, CSIR, ThiruVananthapuram 695019, India, and Magnetic Standards, National Physical Laboratory, CSIR, Dr. K. S. Krishnan Marg, New Delhi 110012, India ReceiVed: August 12, 2009; ReVised Manuscript ReceiVed: NoVember 19, 2009

A nanostructured electromagnetic polyaniline- polyhydroxy iron-clay composite (PPIC) was prepared by oxidative radical emulsion polymerization of aniline in the presence of polyhydroxy iron cation (PIC) intercalated clays. Morphological observation through SEM, TEM, and AFM suggested the formation of self-assembled nanospheres of PIC with self-assembled PANI engulfed over PIC, and the presence of iron in PPIC was confirmed by the EDS analysis. XRD studies revealed that PPIC are comprised of exfoliated clay layers with PIC in the distorted spinel structure. Magnetic property measurements showed that saturation magnetization increased from 7.3 × 10-3 to 2.5 emu/g upon varying the amount of PHIC content from 0 to 10%. Electrical conductivity measurements with the same composition were observed to be in the range of 3.0 × 10-2 to 1.1 S/cm. Thermal stability studies using TGA in combination with DTG suggested that PPICs were thermally stable up to 350 °C. The interaction among clay layers, PIC, and PANI chains in PPIC were manifested from the studies made by FTIR and DSC analysis. The prospects for the direct application of this material are developing low-cost chemical sensors and also processable electromagnetic interference shielding materials for high technological applications. 1. Introduction Nanostructured multifunctional conducting polymers have attracted a great deal of attention for their potential applications in various fields such as electromagnetic interference (EMI) shielding, antistatic coatings, chemical sensors, transducers, electrodes, and corrosion protection coatings.1-6 EMI shielding essentially depends on the magnetic and dielectric properties of the materials for various applications. It is known that the conducting material can effectively shield electromagnetic waves generated from an electric source, whereas only magnetic materials can shield effectively the electromagnetic waves originating from a magnetic source. The ideal combination would be to have a magnetically and electrically tunable microwave material. Thus, if materials having both magnetic and electric components are used as EMI shielding materials, good shielding effectiveness can be expected. Due to these factors, we have motivation to synthesize materials having both conductive and magnetic properties suitable for shielding applications. Apart from electromagnetic properties, high thermomechanical properties are also expected for electromagnetic materials for high technological applications. In this context, preparation of a polyaniline-polyhydroxyiron-clay composite (PPIC) is important since it is a novel guest-host system consisting of exfoliated nanoclay layers having a high aspect ratio dispersed in nanomagnets (PIC) encapsulated by conducting polyaniline (PANI). Thus, the resulting system is expected * To whom correspondence should be addressed. Tel: +91471-2515316. Fax: 0091-471-2491712. E-mail: [email protected]. † Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology. ‡ Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology. § National Physical Laboratory.

to provide materials having electromagnetic properties endowed with excellent mechanical strength, thermal stability, and good processability, and also, the redox couple present in Fe2+/ Fe3+can act as an excellent chemical sensor. Smectite clays are interesting inorganic-host-layered materials used to prepare functional materials due to their small particle size and capacity to swell and exchange cations for intercalation.7 They have thin layers of aluminum silicate that organize themselves in a parallel fashion to form stacks with a regular van der Waals gap in between them called interlayer spacing or gallery. Substituting ions of lower charge for those of higher charge in the octahedral layer (e.g., Mg2+ replacing Al3+) produces surface negative charge for the clay. The variability of the c-axis dimension permits the intercalation of a large variety of inorganic and organic interlayer cations by the cation exchange process, and desirable physical properties can be engineered into this material.8 In the interlayer region, there exists Na+ and Ca2+, which can be replaced with polyoxocations of Al,9-11 Zr,12 and Cr,13,14 and these have been studied for their utility as selective catalysts. Recently, nanostructured iron oxide intercalated clays are potentially interesting for applications in different areas such as catalysis, magnetic properties, and biotechnology.15 Magnetite clays are prepared by intercalating the polyoxo iron cation between the clay galleries by the cation exchange process. These cations have discrete structures of definite sizes and shapes belonging to the well-known structural Keggin type. Bradley and Kydd16 reported the intercalation of a Fe polyoxo cation [FeO4Fe12(OH)24(H2O)12]7+ into the smectite clays. They are of great importance for their catalytic applications, as sensors due to the easily accessible Fe2+/Fe3+ redox couple, and for magnetic properties arising from the large number of unpaired electrons in high-spin Fe2+/Fe3+, combined

10.1021/jp907778g  2010 American Chemical Society Published on Web 02/05/2010

Polyaniline-Clay-Polyhydroxy Iron Composites with the possibility of forming spin-crossover magnetic molecular materials.17 Among the multifunctionalised conducting polymers, PANI is unique due to the wide range of associated electrical, electrochemical, and optical properties, coupled with good environmental stability, which makes it attractive for potential applications in diverse fields.18 Literature showed that conducting polymers incorporated with ferrite particles have been successfully prepared by various methods.19 Wan et al.20a reported electromagnetic functional nanotubes of PANI containing magnetic Fe3O4 nanoparticles prepared by a template-free method. They have also reported the synthesis of nanoneedles of conducting PANI-coated γ-Fe2O3 using γ-Fe2O3 as the template.20b These nanostructured composites exhibited good magnetic properties but poor room-temperature electrical conductivity. It is therefore necessary to improve the electrical property of these composites to satisfy the requirements of technology application. Herein, we report the preparation of a nano/microstructured multifunctional electromagnetic polyaniline-polyhydroxy iron-clay composite material by emulsion polymerization of aniline in the presence of PIC intercalated clays at room temperature. Then structure-property evaluation of these nanocomposites and also the effect of the compositional change of aniline/clay/PIC on the electrical conductivity, magnetic property, thermal stability, and so forth are also reported. 2. Experimental Section 2.1. Materials. Aniline monomer (99.5% pure, Ranbaxy Chemicals, Bombay) was distilled under reduced pressure. Ammonium persulphate (APS), methyl alcohol, and ferric chloride hexahydrate were purchased from S.D. Fine Chem Limited, Bombay, India and were used without further purification. Sodium carbonate was purchased from Ranbaxy Fine Chemicals Limited. Na+ cloisite with a cation exchange capacity of 92.6 meq/100 g and a mean chemical formula of (Na,Ca)0.33(Al1.67Mg0.33)Si4O10(OH) · nH2O was purchased from Loba Chemie, Bombay, India. 2.2. Synthesis Procedure. 2.2.1. Preparation of the Polyhydroxy Iron Cation (PIC). The iron polyhydroxy cation solution was prepared by the hydrolysis of a 0.2 M ferric chloride hexahydrate solution with sodium carbonate. The typical procedure is as follows; 0.82 g (0.003 mol) of ferric chloride hexahydrate was dissolved in 15 mL of 0.1 M HCl and stirred vigorously for 20 min at room temperature. Then, 0.32 g (0.003 mol) of anhydrous sodium carbonate powder was added to the stirred solution in small lots. The addition of Na2CO3 was controlled in such a way as to prevent precipitation. It was then flushed with nitrogen to facilitate the removal of carbon dioxide that evolved during the hydrolysis and was aged for 1 day at room temperature to allow the growth of the polycation. The hydrolysis condition produced a pH of 1.8. At this pH, highly condensed species of polynuclear Fe-oxohydroxide complexes were present for incorporation into the clay gallery space.21,22 2.2.2. Preparation of PIC Intercalated Clay Precursors (PHIC). Clay (0.25 g) in 25 mL of water (1 wt %) was added slowly to a vigorously stirred solution of 11.5 mL of 0.2 M PIC. The ratio of the cation to clay was 90 mmol/meq for the synthesis. Upon the complete addition of clay to the hydrolyzed Fe3+ solution, the reaction mixture was stirred for an additional 2 h. The product was washed to remove excess salt by the process of subsequent centrifugation and decantation. The resulting material was then dried in a vacuum oven at 80 °C for 1 day and made into a powder.

J. Phys. Chem. B, Vol. 114, No. 8, 2010 2579 TABLE 1: Details of Elemental Analysis calculated

observed

sample

C

H

N

Fe

C

H

N

Fe

PANI PANICN PPC PPIC1 PPIC2 PPIC3 PPIC4 PPIC5

78.28 71 66 61 61.5 62 62.5 63

6.5 3.2 4.79 3 3.2 3.5 3.8 4.2

15.21 9.9 10.9 9.8 10 10.2 10.6 11

0 0 10 2 4 6 8 10

72.36 67.97 62.53 58.6 59 60 60.6 62

5.34 5 5.13 3.8 3 3.2 3 4

14.37 9.47 10.35 10 10.3 10.8 10.1 10

0 0 7 1 3 5 7 8

TABLE 2: Details of Electrical and Magnetic Properties sample

PHIC/Ani

PANI PPIC1 PPIC2 PPIC3 PPIC4 PPIC5

0:100 2:98 4:96 6:94 8:92 10:90

conductivity (S/cm)

3.0 × 10-2 1.2 × 10-2 3.5 × 10-2 2.0 × 10-1 1.0 1.1

saturation magnetization (emu/g)

7.3 × 10-3 5.0 × 10-1 1.0 1.5 2.0 2.5

2.2.3. Preparation of Polyaniline-Polyhydroxy Iron-Clay Composites (PPIC). Clay (0.25 g) in 25 mL of water (1 wt %) was added slowly to a vigorously stirred solution of 11.5 mL of PIC (0.2M). It was stirred for 0.5 h and 1 g (0.01 mol) of aniline in 100 mL of water was added. Then pH of the solution was adjusted to 2 by the dropwise addition of conc. HCl. Stirring continued for 0.5 h, and the system was kept at 5 °C and polymerized by the dropwise addition of 2.85 g (0.012 mol) of 1.25 M APS. After the addition of APS, polymerization was continued for 6 h to achieve high molecular weight PANI species. The green emerdine salt of PPIC formed was isolated by precipitation from methyl alcohol. It was then filtered and washed a number of times with distilled water and finally with methanol. The product was dried under vacuum at 80 °C for 1 day. Nanostructured conductive materials with different aniline/ PIC ratios were prepared, and their abbreviations and details are given in Table 2. The solubility of PPIC in many solvents like CHCl3, NMP, DMAc, and DMF was checked, and it was found to be soluble. Polyaniline- and polyaniline-clay composites without PIC were also prepared, as reported elsewhere,35a and they were designated as PANI and PANICN, respectively. 2.3. Measurements and Instruments. UV-vis absorption spectra of the samples were studied by dispersing the sample in distilled water and recording the spectra using a UV-vis spectrophotometer (Shimadzu model 2100) in the range of 300-1100 nm. FTIR measurements were made with a fully computerized Nicolet impact 400D FTIR spectrophotometer. Polymers were mixed thoroughly with potassium bromide and compressed into pellets before recording. All spectra were corrected for the presence of moisture and carbon dioxide in the optical path. The molecular weights of the samples were measured using a MALDI TOF mass spectrometer (AXIMA CFR, Shimadzu) equipped with a nitrogen laser emitting at 337 nm and R-cyano-4-hydroxy cinnamic acid as the matrix. Atomic absorption spectroscopy was done using a Perkin-Elmer model AAnalyst100flameatomicabsorptionspectrometer(Perkin-Elmer Life and Analytical Sciences, Shelton, CT). Elemental analysis was performed using a Perkin-Elmer elemental analyzer. X-ray diffraction studies were done with an X-ray diffractometer (Philips X’pert Pro) with Cu KR radiation (λ ≈ 0.154 nm) employing an X’celarator detector and a monochromator at the diffraction beam side. Powder samples were used by employing a standard sample holder. The d spacing of the nanocomposite

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SCHEME 1: Preparation of PIC and Multifunctionalised Nanostructured Electromagnetic Materials

was calculated from the angular positions 2θ of the observed d001 reflection peaks based on the Bragg’s formula nλ ) 2d sin θ, where λ is the wavelength of the X-ray beam and θ is the diffraction angle. An averaged 2θ was used with the 2θ resolution of 0.002° from 2 to 70°. For conductivity measurements, the samples were pressed into a 13 mm diameter disk and were measured by a standard four-probe conductivity meter using a Keithley 6881 programmable current source and a 2128A nanovoltmeter at 30 °C. The resistivity of the samples was measured at five different positions, and at least two pellets were measured for each sample. The average of 10 readings was used for conductivity calculations. PLM photographs were taken in an Olympus BX 51 microscope after drop casting the solution of the sample onto a clean dry glass plate. For SEM measurements, samples were subjected for thin gold coating using a JEOL JFC-1200 fine coater. The probing side was inserted into a JEOL JSM- 5600 LV scanning electron microscope to take photographs. Atomic force microscopy (AFM) images were recorded under ambient conditions using a Ntegra multimode Nanoscope IV operating in the tapping mode regime. Microfabricated silicon cantilever tips (MPP-11100-10) with a resonance frequency of 284-299 kHz and a spring constant of 20-80 N m1- were used. The scan rate varied from 0.5 to 1.5 Hz. Transmission electroscopy was performed in an FEI, TEC NAI 30G2 S-TWIN microscope with an accelerating voltage of 100 kV. For TEM measurements, the sample solutions were prepared by dispersion under an ultrasonic vibrator. They were then deposited on a Formvar coated copper grid and dried in vacuum at room temperature before observation. Thermal stability measurements were performed at a heating rate of 10 °C/ min in a nitrogen atmosphere using Schimadzu, DTG-60 equipment. Surface area measurements were carried out in a Perkin-Elmer Shell sorptometer using N2 as the adsorbate at liquid N2 temperature. DSC scans were performed using a Dupont DSC 2010 differential scanning calorimeter attached to a Thermal Analyst 2100 data solution under a nitrogen atmosphere at a heating rate of 10 °C/min. Magnetic property measurements were made with a vibrating sample magnetometer

(VSM). Powder samples were used to collect the hysteresis behavior of nanoparticles at room temperature. The basis of the method is that a flux change is induced when a vibrating magnetic sample is placed within a uniform magnetic field. The detection coils measure a voltage, which is compared with the voltage of the reference sample. The difference between the two voltages is proportional to the sample’s magnetic moment. 3. Results and Discussion 3.1. Preparation of Polyhydroxy Cation (PIC), PIC Intercalated Clays (PHIC), and Polyaniline-Polyhydroxy Iron Cation-Clay Nanocomposite (PPIC). The polyhydroxy iron cation was prepared by the acidic hydrolysis of ferric salt, as shown in Scheme 1. The mechanism involves the formation of hexa-aquo (hydrated) ions,23a which then condense to form dimers and trimers via deprotonation, having the structure with Fe octahedra sharing hydroxo and oxo bridges, and it finally forms the Fe13 polyiron cation, as shown in Scheme 1.23b The structure of PIC is still ambiguous and is dependent on the pH and temperature of the reaction conditions. The PIC has the general structural formula [FeIIO4FeIII12(OH)24(H2O)12]7+, where 12 Fe2+ of octahedral coordination surround the 13th Fe3+ of tetrahedral coordination.24 Intercalation of PIC in clay was conducted by the dropwise addition of a dilute aqueous dispersion of clay into the PIC solution. The PLM micrograph of PIC exhibited a micropine structure and was observed to be nucleating via a cubic structure that symmetrically cleaves and grows into these micropine structures (Figure 1A). A similar micropine structure for R-Fe2O3 was reported earlier by Wang et al.25 Polyhydroxy cation intercalated clay (PHIC) was prepared by the addition of clay into PIC. Under PLM, it exhibited an extended growth of PIC throughout the matrix with a dendritic crystalline structure. (Figure 1B). PIC intercalation in clay was carried out in different mmol/meq of iron/clay (50, 70, 90, 100, and 110), and the BET surface area was measured. A maximum BET surface area of 269 m2/g was observed for

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Figure 1. PLM picture of (A) PIC, (B) PHIC, and (C) PPIC.

SCHEME 2: Redox Property of Fe2+/Fe3+ Exploited for the Polymerization of Aniline

batches with 90 mmol/mequiv of iron/clay. Therefore, this ratio was fixed for our further experiments. Nanostructured electromagnetic PPICs were prepared by the strategy mentioned in the Experimental Section, and the formation the PPIC nanocomposite is shown in Scheme 1. It was observed that during the addition of aniline to PIC and before the addition of the external oxidant APS, there was a change in color to emeraldine green, revealing the catalytic activity of the redox couple present in the PIC, and the mechanism of the reaction is as shown in Scheme 2. Similar observations were made by Aphesteguy et al.26 during the preparation of PANI-Fe3O4 nanocomposite in the presence of Fe(II) and Fe(III) salts in the absence of external oxidant. The molecular weight of PPIC was measured using MALDI-TOF (see Supporting Information). The molecular weight of PPIC was observed to be in the range of 21000 Da. The morphologies of the nanocomposites were studied by observation under PLM, SEM, AFM, and TEM. The PLM

picture of drop-casted PPIC showed uniform spherical particles connected by nano/microwires (Figure 1C). The SEM pictures of PPC (polyaniline-polyhydroxy iron composite) prepared under two different ratios of PIC/Ani (5:95 and 10:90) are shown in Figure 2A and B, respectively. SEM micrographs of these samples showed a chain of nano/microspherical particles having a core-shell morphology at the lower concentration of PIC (Figure 2A) and capsule-like morphology (Figure 2B) with an increased amount of PIC. Here, the anilinium ion present in the system is adsorbed on the surface of the charged PIC ions through a static interaction.1a The redox couple Fe2+/Fe3+ present in PIC is endowed with high electron density and can therefore act as a catalyst and template during the polymerization of aniline, and it forms a conducting PANI shell outside of the magnetic particle. These nano/microspheres can self-assemble to form a chain of nano/microspherical electromagnetic particles of size ∼100 nm, shown in the inset of Figure 2C. It was also observed from SEM that these nanospheres undergo end/edge

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Figure 2. SEM images of (A, B) PPC, (C) PPIC1, and (D) EDS and (E) TEM images of PPIC.

on collision to form larger nano/microspheres and capsules. The AFM image also supports these observations. These particles and exfoliated clay layers (see XRD discussion) were circumvented by self-assembled PANI nano/microwires These PANI nanowires were formed by an elongation process after the selfassembly. These observations were remniscent with the studies made by other researchers.20 Figure 2D represents the EDS spectra of PPIC, which reveals the existence of elements like aluminum, silicon, oxygen, iron, and so forth and hence confirmed the presence of exfoliated clay layers and iron in the nanocomposite. The presence of iron in the PPIC was further confirmed by iron content analysis by atomic absorption spectroscopy. The presence of carbon, hydrogen, and nitrogen in the nanocomposite was estimated by elemental analysis, which is given in Table 1. The observed values were found to be in close agreement with the calculated values. Figure 2E represents the TEM image of PPIC, and it exhibits bright shades containing nano/microwires of PANI with the dark shade containing self-assembled PIC circumvented by PANI chains. Clay layers are not visible since self-assembled nanowires of the PANI circumvent clay and iron particles. The AFM image of PPIC with the section analysis is shown in Figure 3A and B. The AFM image also shows self-assembled iron spherical

particles, and PANI is formed around these particles. The height profile of the AFM analysis showed that particles are in the ∼100 nm range. 3.2. Structural Characterization. The protonic state and the interaction among different moieties in the composites were studied by UV-vis spectra and FTIR spectroscopy, respectively. The UV-vis spectra of PANICN, PANI, and PPIC are shown in Figure 4A-C, respectively. The UV-vis spectra of PANI exhibited polaron band peaks at ∼420 and 750 nm.29,30a In PANICN, the second absorption maximum was observed as a free carrier tail with a red shift to 800 nm. This shift observed in PANICN compared to PANI is due to the presence of delocalized polarons with the electrons arising from the extended confirmation of PANI chains in the confined environment of nanoclay layers.27 In PPIC, the free carrier tail is shifted to 830 nm. The broad absorption peak observed at ∼420 nm in PANICN and PPIC is due to the merging of two peaks, (i) 340 nm due to the π-π* transition of the benzenoid amine of PANI and (ii) 430 nm corresponding to the π-to-polaron band in the PANI chain,28 revealing a high level of doping in PANICN and PPIC.30b The red shift observed for the peak at 420 nm in PPIC, compared to that in PANI, is due to the interaction between metal ions and PANI chains, which decreases the energy for the π-π* transition.31 The interaction between the

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Figure 3. (A) AFM image and (B) section analysis of PPIC.

Figure 4. UV-vis spectra of (A) PANICN, (B) PANI, and (C) PPIC.

Figure 5. XRD diffractogram of (A) PHIC, (B) PPIC, and (C) Na+ cloisite.

magnetic nanoparticles and polymer backbone is further supported by FTIR analysis. The characteristic bands in FTIR (see Supporting Information) corresponding to Na+ cloisite are 1023 cm-1 (Si-O-Si) str., 911 cm-1 (Al-OH) str., and 525 cm-1 (Si-O-Al) str.32 The absorption bands at 1635 cm-1 and the broad band at 3430 cm-1 have been assigned to bending and stretching modes of adsorbed water, respectively.33a PHIC exhibited an additional peak compared to Na+ cloisite at 590 cm-1, corresponding to Fe-O str. The absorption peaks at around 1560 and 1490 cm-1 observed in PANICN and PPIC are attributed to the CdC str. of quinoid and benzenoid, respectively.33b The peak at 1295 cm-1 is assigned to the C-N stretching vibration of the secondary aromatic amine present in PANI.34 The peak at 3257 cm-1 can be attributed to the N-H stretching modes.33a The peak at 1119 cm-1 originate from the aromatic in-plane bending vibration, also called as electron state band peak.35 The peak at 823 cm-1 is attributed to the out-of-plane deformation of C-H in the 1,4-disubstituted benzene ring. The peak at 1298 cm-1 corresponding to C-N str. in PANI is shifted to 1302 cm-1 in PPIC. The peak observed in PANICN at 1559 cm-1 (CdC) is shifted to 1546 cm-1 in the PPIC composite, and the peak at 590 cm-1 corresponding to Fe-O str. in PHIC shifts to 594 cm-1 in PPIC. The lone pair of electrons present in the nitrogen atom of the PANI chain interacts with the 3d orbitals of the Fe atom to form a coordinate bond and thereby the shift.35b The peak at 1030 cm-1 in clay corresponding to the Si-O-Si str. shifted to 1040 cm-1 in PPIC due to the hydrogen bonding between PANI chains and the basal surface of clay and the polyhydroxy iron cation.36 All of these observed spectral shifts in PPIC when compared to the charasteristic peaks of clay (Si-O-Si), PIC (Fe-O), and PANI (CdC and C-N) suggest the existence of noncovalent/hydrogen bond interaction among these moeties.

The wide-angle X-ray diffraction patterns of PHIC, PPIC, and Na+ cloisite are shown in Figure 5A-C, respectively, and the details of their d spacings with diffraction angles are listed in Table 3. All of the patterns exhibited two distinct types of reflections, general and basal. The general reflections are called the hk bands, which are asymmetrical lines with characteristic “sawtooth”-type reflections.37 Such reflections are caused by the structure of the smectite layers themselves and are independent of external condition. The basal reflections (hkl) on the other hand have symmetrical peaks, whose positions vary with the separation between the layers. This separation depends on the nature of the cation present in the interlayer space, the amount of water, and so forth. From the XRD pattern, it is clear that the materials have a similar level of ordering. The diffraction pattern of clay (Figure 5C) showed reflections at 2θ ) 7.2 (d001), 18.7 (d003), 21.9 (d004), 28.6 (d005), 34.9 (d007), 54.2 (d31, d15, d24), and 61.8° (d33, d06). The basal reflection peak at 2θ ) 7.2° with a d spacing of 12.1 Å corresponds to the d001 basal spacing of the clay. After intercalation of the PIC in clay, the diffractogram (Figure 5A) showed a shift in the d001 reflection to 15 Å, with an enhancement in the interlayer distance of 5.5 Å. This increased distance is equal to the dimension of the hydrated PIC. The diffractogram also exhibited additional reflections at 2θ ) 30.1, 36, 44.6, 54.3, 61.8, and 65.1° characteristic of the crystalline spinel structure of iron oxide.38 The diffractogram of PPIC (Figure 5B) showed a silent reflection for the d001 peak of clay, confirming an exfoliated state of nanoclay layers. In PPICs, the characteristic reflections of PIC were observed at 2θ ) 29.6 (3.01 Å), 35.29 (2.54 Å), 43.25 (2.09 Å), 54.1 (1.69 Å), and 61.97° (1.49 Å). The small discrepancy observed in the d values of PPICs, compared to the observed value for PIC may be due to the partially collapsed phase of PIC in the presence of clay and protonated PANI chains. Thus, it is

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TABLE 3: Details of XRD Studies sample +

Na cloisite PHIC Fe3O4 PPIC

diffraction angle 2θ

d spacing Å

7.2, 19.8, 21.9, 28.6, 34.9, 54.2, 61.8 5.8, 19.7, 28.4, 30.1, 36, 44.6, 54.3, 61.8, 65.1 30.1, 35.42, 43.20, 53.48, 57.94 25.29, 37.73, 44.67, 53.85, 57.33, 62, 72.9

12.1, 4.8, 4, 3.1, 2.6, 1.7, 1.4 15.2, 4.5, 3.1, 2.97, 2.5, 2,1.7, 1.5, 1.4 1.62, 1.71, 2.09, 2.53, 2.97 3.5, 2.38, 2.02, 1.6, 1.49, 1.3

reasonable to believe that spinel structure of PIC present in PPIC, similar to that of Fe3O4, is responsible for the ferromagnetic properties exhibited by the composite.39 3.3. Conductivity Measurements. The room-temperature electrical conductivity (σdc) of PPICs was measured using a fourprobe conductivity meter, and the details of the measurement are given in Table 2. Bulk PANI exhibited a conductivity of 3 × 10-2 S/cm. The effect of PHIC on the electrical conductivity measurement showed that, upon increasing the amount of PHIC in the nancomposite, initially there is a decrease in conductivity. Later, the conductivity was observed as increasing, and it finally appeared as a plateau (1.1 S/cm).3 The decrease in the conductivity upon addition of PHIC might arise from two factors. The first is due to the partial blockage of the conductivity path by the magnetic PIC particles and insulating clay, and the second factor is due to the neutralization of charges of PANI in the presence of charged iron particles. A further increase can be explained like this. Usually, in disordered electromagnetic systems, the conductivity depends on two aspects, microscopic conductivity and macroscopic conductivity. Microscopic conductivity depends on the doping level, conjugation, chain length, and so forth, while the macroscopic conductivity depends on some external factors like the compactness of the sample.40 In PPICs, the compactness and molecular orientation are varied significantly, depending on the PHIC content in the composite. With an increase in the amount of PHIC, there is an increase in the PANI chain growth on their surface, which leads to an increase in the compactness of the material, thereby improving the conductivity. In the polymerization process, the oligomeric species of aniline is adsorbed on the magnetite surface, and the crystalline boundaries become the primary nucleation centers; these centers will have a structuredirecting effect on the further propagation of PANI chains and hence show an increase in the conductivty. 3.4. Thermal Stability. The thermal stability of PPICs was studied by thermogravimetry at a heating rate of 10 °C/min in a nitrogen atmosphere to minimize the mass loss due to iron oxidation. TGA curves of Na+ cloisite, PANI, and PPIC are shown in Figure 6A-C, respectively. The TGA curve of PANI (Figure 6C) showed two decomposition stages. The first loss at around 80 °C can be ascribed to the expulsion of water and volatile impurities, and the second loss starting from 250 °C may be due to the decomposition of the PANI backbone. The

TG curve of PPIC (Figure 6B) showed an enhancement in the second-stage decomposition temperature to 400 °C. The enhanced thermal stability exhibited by the PPIC nanocomposites compared to bulk PANI can be due to the synergetic contribution of insulative clay layers and PIC particles that are present in the former. The temperature at which maximum mass loss occurred was observed from differential thermogravimetric analysis (see Supporting Information). In DTG, PANI exhibited a decomposition peak (Tmax) at around 260 °C, while PPIC showed decomposition at around 380 °C, revealing higher thermal stability for PPIC. Thermal phase transition changes of PANI and PPIC were studied by DSC at a heating rate of 10 °C /min in a nitrogen atmosphere (see Supporting Information). PANI exhibited a transition temperature endothermic peak at 118 °C, while PPIC exhibited two endothermic peaks centered at 88 and 150 °C. Upon heating, PPIC undergoes a change in energy that may arise from two factors. The first transition arises from the conformational change arising from the hydrogen-bonded interaction among the protonated PANI-PANI chains. The second transition might be from the guest-host interaction between protonated PANI chains, PIC, and the clay.41,42 Thus, the noncovalent interaction among PANI-PANI, PIC-PANI, and clay-PANI could be manifested from the studies made by DSC analysis. 3.5. Magnetic Property. The magnetic properties of the nanostructured electromagnetic PPIC and PANI were measured at room temperature using a vibrating sample magnetometer. The magnetic property measurements of the conducting polymers can provide the details of charge-carrying species and unpaired spins. The magnetization of nanostructured PPIC exhibits clear hysteresis behavior in the M-H curve, showing its ferromagnetic behavior. Application of a magnetic field will align the magnetic moment of the nanoparticle in the field direction, and magnetization increases with an increasing field until a saturation value is reached. The magnetic property measurement was done with nanocomposites containing a PHIC content from 2 to 10%. Details of the results are shown in Table 2, and a typical hysterisis curve showing the magnetic property measurement with a 5% PHIC content (PPIC2) is shown in Figure 7. It exhibits a saturation magnetization, Ms, of 1 emu/

Figure 6. TGA plot of (A) Na+ cloisite, (B) PPIC, and (C) PANI.

Figure 7. Hysterisis curve of (A) PPIC2 and (B) PANI.

Polyaniline-Clay-Polyhydroxy Iron Composites g, a coercivity, Hc, of 530 Oe, and a retentivity, Mr, of 0.3 emu/ g. The coercivity observed is almost similar to that of bulk magnetite (500-800 Oe).43,44 PANI exhibits a Ms of 7.26 memu/ g, Hc of 10.9 Oe, and Mr of 206 µemu/g. With an increasing amount of PIC from 2 to 10%, the saturation magnetization was observed to increase from 0.5 to 2.5 emu/g. Aphesteguy et al.45 reported a saturation magnetization of 1 emu/g with 8% loading of magnetic Fe3O4 in the PANI-Fe3O4 composite, and Wan et al.39 reported a saturation magnetization of 3.45 emu/g with 20 wt % Fe3O4 in the PANI-Fe3O4 composite. The saturation magnetization in the nanoparticles is greatly affected by the surface and size effects.46,47 Moreover, the saturation magnetization of the nanocomposites depends on the volume fraction of the magnetic particles. Thus, with an increasing amount of PHIC, the volume fraction of magnetic material will increase in the nanocomposite and exhibit an increase in the saturation magnetization, and the healing defect may be weakened and lead to the increase in coercivity.48 These materials can be used for the fabrication of EMI shielding materials, where the intensity of the interfering electromagnetic radiation is in a low range of frequency. 4. Conclusions In conclusion, a nanostructured multifunctionalized electromagnetic polyaniline-polyhydroxy iron-clay composite comprising a guest-host system is prepared by in situ emulsion polymerization of aniline in the presence of polyhydroxy iron cation intercalated clay at room temperature. Morphology studies reveal the formation of nano/microspherical particles embedded in the exfoliated nanoclay layers that are engulfed with selfassembled conducting PANIs. These electromagnetic materials exhibit excellent electrical conductivity, reasonable magnetic properties, and good thermal stability and are soluble in most of the organic solvents. These attractive properties make them excellent candidates for high advanced technological applications in the field of EMI shielding materials, chemical sensors, and so forth. A future prospective result of this work is the development of water-dispersible, nanosized, uniform-sized particles with excellent electromagnetic properties for application in areas like electromagnetic interference shielding, optical devices, and chemical sensors. Acknowledgment. We thank the Indian Space Research Organisation for their financial support from project GAP 109439 and the CSIR network project (NW 004). We are also thankful to Dr. P. Prabhakar Rao, Mr. M. R. Chandran, Mr Robert Philip, and Mr. P. Guruswamy, NIIST Trivandrum, for SEM, TEM, and WXRD analyses. V.L.R. thanks CSIR, New Delhi, India, for the senior research fellowship. Supporting Information Available: MALDI-TOF spectra of PPIC, FTIR spectra of PHIC, PANICN, PPIC1, PANI, and Na+ cloisite, and DTG and DSC of PANI and PPIC. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Li, X.; Wan, M.; Wei, Y.; Shen, J.; Chen, Z. J. Phys. Chem. B 2006, 110, 14623. (b) Wang, Y. Y.; Jing, X. L. Polym. AdV. Technol. 2005, 16, 344. (2) Xu, P.; Han, X.; Wang, C.; Zhou, D.; Lv, Z.; Wen, A.; Wang, X.; Zhang, B. J. Phys. Chem. B 2008, 112, 10443. (3) Ding, H.; Liu, X. M.; Wan, M.; Fu, S. Y. J. Phys. Chem. B 2008, 112, 9289. (4) Sudha, J. D.; Sivakala, S.; Prasanth, R.; Reena, V. L.; Radhakrishnan Nair, P. Compos. Sci. Technol. 2009, 69, 358.

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