Dielectric Constant and Transport Mechanism of ... - ACS Publications

Article pubs.acs.org/IECR

Dielectric Constant and Transport Mechanism of Percolated Polyaniline Nanoclay Composites. Mini Vellakkat, Archana Kamath, S. Raghu, Sharanappa Chapi, and Devendrappa Hundekal* Department of Physics, Mangalore University, Mangalagangothri 574199, India ABSTRACT: We report the dielectric constant and transport mechanism of intercalated nanoclay−polyaniline composite, an industrially ready to use novel nanocomposite, which is prepared by a simple mechanochemical method. The effects of clay concentration on structure and structure variations on properties were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and impedance spectroscopy (20 Hz−1 MHz and temperature range from 300−380 K). The phenomenon of percolation was observed in these composites. The values of Mott’s temperature, density of states at the Fermi level, hopping distance, and barrier height for polyaniline−nanoclay (PANC) composites were calculated. By applying Mott’s theory, it is found that the PANC composites obey the one-dimensional variable range hopping mechanism. This type of percolated sample can be used as a nanocapacitor in many devices because of its enhanced transport properties.

1. INTRODUCTION Conductive polymer nanocomposites consisting of intercalates with nanosized fillers have created smart and novel materials with significant characteristics like environmental stability, biocompatibility, flexibility, easy processability, low cost, high yield, and stable chemical compatibility with other materials. In recent years, the production of such nanocomposite materials has received much attention due to a wide range of potential applications to it. They show high dielectric constants and high conductivity due to the high mobility of the charge carriers.1 Polyaniline (PANI) is the most attractive conducting polymer because of the presence of the reactive amine group in the polymer chain flanked by a phenyl ring on either side,2 which offers interaction with nanomaterial constituents. PANI’s electrical conductivity can be varied from insulator to conductor by the redox process. Because of these properties, it is used in various applications, for example, in batteries, solar cells, electromagnetic interference (EMI) shielding, corrosion protection, sensors, and electrochromic devices.1,2 It is important to analyze the transport properties of these composites to identify the influence of dispersant on the properties of the conducting polymer matrix.3 To obtain high conductivity and other enhanced properties in these materials, a critical quantity of conductive filler should be added to the polymer. At this critical concentration, the electrical percolation threshold,4 depending on the packing density, particles start to form conducting chains (percolating clusters) that influence the total conductivity and the dielectric properties.5 Alignment of the filler also plays a major role in electrical percolation. When the platelets are aligned in the matrix at relatively low concentrations, fewer contacts are developed between them, and thus the percolation threshold would be expected to increase.6 The filler selected is natural montmorillonite (MMT), which is a layered silicate. PANI−MMT nanocomposite is an attractive and interesting material because of the special properties as well as the wide uses of PANI,7 the nature, abundance, low cost, and attractive © 2014 American Chemical Society

features such as a large aspect ratio and the ion−exchange properties of MMT.8 Intercalated or exfoliated MMT−PANI nanocomposites can be prepared by emulsion intercalation,9−11 the electrochemical method,12,13 inverse emulsion polymerization,14 in situ intercalation,15,16 and the mechanochemical intercalation method.17,18 The conductivity of these nanocomposites show lower values than does pristine PANI9,10,16,19 in many cases; the reason could be the lack of connectivity between intercalated PANI chains15,16 or a change in the nature of its polymeric chains in clay galleries. Mechanochemical synthesis is a known and powerful technique to synthesize nanocrystalline materials;20 however, it was used successfully to synthesize polymer nanocomposites only very recently. In this method, the process was carried out as a one-pot procedure. Purity of the composite can be ensured, and no further purification is necessary. This versatile synthesis technique is convenient and gives also the benefits of mechanochemistry when compared to other mentioned methods. The device structure is simple, handling is easy, and the energy consumption is relatively low. This has obvious advantages over the other conventional techniques since it is a room temperature process. Systematic studies showing the dependence of structure on the clay concentration and the effect of structure variation on the thermal, electrical, and dielectric properties of the composite are rather scarce and show diverse results. The composite can be synthesized with or without a solvent. Few drops of acetone were added to decrease the viscosity of the preparing mixture and to help in the distribution of nanoclay in the PANI matrix. Since the boiling point of acetone is 56 °C, it can be removed from the composite easily. Acetone Received: Revised: Accepted: Published: 16873

July 24, 2014 September 25, 2014 September 30, 2014 September 30, 2014 dx.doi.org/10.1021/ie502922b | Ind. Eng. Chem. Res. 2014, 53, 16873−16882

Industrial & Engineering Chemistry Research

Article

provides an extended path for conduction in the composite. Dry clay and PANI prepared powder were mechanically mixed (for 4 h) in the presence of acetone and formed into a pellet using a hydraulic press at a pressure of 10 Tons. The clay amount is increased in steps of 20, 30, 40, and 50% of PANI and repeated, and they are named PANC10, PANC20, PANC30, PANC40, and PANC50, respectively. 2.4. Characterization Methods. The chemical changes were analyzed by FTIR (Thermo Nicolet, Avatar 370, spectral range of 4000−400 cm−1) using KBr pellets and by X-ray diffraction (XRD) with a Rigaku miniflex-600 benchtop diffractometer with Cu target. The samples were scanned for 2θ, 0−60° at the scanning rate of 3° per minute at room temperature. Electrical conductivity and dielectric studies of PANI and PANC nanocomposites were done by the Wayne Kerr 6500 B Impedance analyzer; scanning electron microscopy (SEM) was done by the JEOL Model JSM-6390LV; TGA was done by the PerkinElmer, Diamond thermogravimetric/differential thermal analysis (TG/DTA) at SAIF Cochin University, Kerala, India; and transmission electron microscopy (TEM) (JEOL, JEM-2100) was done at the Transmission Electron Microscope Facility, Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kerala, India.

treatment resulted in more physical changes than chemical changes, which is clear from the Fourier transform infrared (FTIR) spectra and thermogravimetric analysis (TGA) results. We added acetone to the mixture, and milling was done in a conventional mixer for 4 h to modify the clay and to achieve proper intercalation. To the best of our knowledge, there were no previous studies on PANI−clay composites prepared in acetone medium in this way. Percolation is an effective way to know the electrical transport behavior of composites.21−23 It showed a high conductivity of 2.143 S cm−1. Very high dielectric constants up to 3.7 × 103, 2 × 103, and 2.4 × 106 in different PANI composites were reported.24−26 Our result was found to be 1.83 × 107 for percolated composite, which is one order greater than these reported results. It is difficult to compare the results because values may vary based on the oxidant, synthesis, and nanoparticle type and the conditions in which it is prepared. Thus, the objective of this work was to prepare a novel conductive bio nanocomposite of PANI with nanoclay in an economically attractive, wasteless method and to study the synergic effect in electrical, thermal, dielectric, and modulus properties of the polyaniline−nanoclay (PANC) composites in the vicinity of the percolation threshold. Our study shows that hopping conduction at lower temperatures is a quasi-onedimensional (quasi-1D) variable-range hopping (VRH) model. In percolated PANC composite, the closer approaches of hopping sites enhance conductivity, and such studies can lead not only to a better understanding of the physics of dielectrics, but also the improvement in the design of tailored synthetic materials like new “nanodielectrics,” “supercapacitors,” or “nanoconductors” without expensive attempts.

3. RESULTS AND DISCUSSION 3.1. FTIR Spectra Analysis. FTIR spectra of pure PANI, nanoclay, and PANI−nanoclay composites are given in Figure 1. The main characteristic band of PANI, assigned at 3451

2. MATERIALS AND METHODS 2.1. Materials. Aniline (Sigma-Aldrich) is distilled under reduced pressure, and nanoclay−alumino silicate (MMT) (Sigma-Aldrich, USA) is used as received. Ammonium peroxydisulfate (NH 4 ) 2 S 2 O 8 (mol wt, 228.20 g/mol) (Merck), hydrochloric acid (mol wt, 36.46 g/mol), and all other chemicals used were of analytical reagent (AR) grade. 2.2. Synthesis of Polyaniline. The conducting PANI was synthesized by the oxidative chemical reaction method. The aniline monomer was doubly distilled prior to use. The 0.22 M aniline monomer was dissolved in 240 mL of 1 M HCl, 0.8 M (NH4)2S2O8 (ammonium persulfate, APS) was dissolved in 80 mL of 1 M HCl solution, and the two solutions were precooled to ∼0°C using ice bath. APS solution was added to the aniline solution dropwise over a period of 2 h with constant stirring to ensure thorough mixing. The mixture was continuously stirred for 24 h, and the resulting dark greenish precipitate was filtered and washed with deionized water. Finally, the resultant precipitate was dried in an oven at 50 °C for 24 h to achieve emeraldine salt powder (PANI).27,28 2.3. Preparation of PANC Nanocomposite. In the mechanochemical mixing technique,29 the composite obtained by intimately mixing PANI and clay attained an ordered structure. The layered nanosilicate can easily be dispersed in a solvent like acetone because of the weak forces that stalk the layers. The PANI adsorbs onto the delaminated sheets, and when the solvent is evaporated, the sheets reassemble, which sandwiches the polymer to form a conformational state. A combination of high shear force and the use of swelling agents is the most effective method for intercalation or exfoliation.30 The orientational flexibility of clay molecules in the solid state

Figure 1. FTIR spectra of (a) pure PANI, (b) PANC50, and (c) nanoclay.

cm−1, is attributed to the amine (−NH−) stretching mode; the bands at 1566 and 1483 cm−1 are attributed to the (CN) quinonoid and (CC) benzenoid rings stretching modes, respectively (Figure 1a). The band at 1302 cm−1 is attributed to the (C−N) stretching mode for the benzenoid ring.31−35 The FTIR spectra of silica is dominated by the band in curve c that is centered on 1050 cm−1 due to the asymmetric stretching vibration of the Si−O−Si bond with a very high intensity. The band at 917 cm−1 is due to Al−OH, and the band at 524 cm−1 is due to Si−O−Al. Curve b in Figure 1 indicates that the main characteristic bands of PANI and silica all appear in the PANC nanocomposite, but the stretching mode of N−H is shifted to a lower wavenumber, 3437 cm−1. At 2925, 2364 cm−1 (aliphatic 16874

dx.doi.org/10.1021/ie502922b | Ind. Eng. Chem. Res. 2014, 53, 16873−16882


Article

4.996° and 19.97° in the modified clay confirms the crystallinity of nanoclay. For pure MMT, this first plane (100) of the clay sample is at a 2θ value of 8.9°.41 For PANC nanocomposite, there is an increase in the intensity of most of the nanoclay peaks with an increase in concentration. The intensity of the characteristic peak of PANI at 25.19° ((200) crystal plane) and its d-spacing decreased up to PANC30 and then increased. At the same time, the intensity of the (020) peak of PANI increased because of a similar peak of clay near the same 2θ angle, which shows the intercalation of clay and PANI. The 2θ value shows a maximum right shift in PANC30 at the (020) and (200) peaks of PANI but a maximum left shift at clay peak (100), which shows that intercalation is maximum in PANC30. Intercalation of PANI between the clay layers is evident from the shift of this peak toward a 2θ value of 6.25°. The d-spacing of pure MMT, which is approximately 8.90 Å, increased to a maximum value of 14.71 Å in PANC30,42,43 which confirms that the intercalation of PANI to clay layers successfully occurred. The PANI peak’s dspacing reduced and clay peak’s d-spacing enlarged in PANC30. The enhanced properties of this composite may be due to this reason. The degree of crystallinity, crystallite size (D), and average intercrystallite separation (R) are calculated and listed in Table 1. The result clearly shows an increase in the degree of crystallinity in the composite and a maximum in PANC30 with respect to PANI. Crystalline size D is minimum and R is maximum in PANC30, which shows that the interfacial interaction between PANI and clay is maximum in this composite. These results indicate the modifications of the crystalline phase in the polymer composite with the incorporation of clay. It was observed that with the increase in crystallinity, the conductivity increased, because the structure becomes more organized.44 3.3. SEM Study. This morphological study gives the shape, size, and dispersion of the nanoparticles in the nanocomposite. Figure 3 shows the SEM micrographs of nanoclay and the pure PANI, PANC30, and PANC50 nanocomposites. Knowledge of the morphology helps in the tuning of the composite to the required application. SEM reveals the typical spherical granular morphology of PANI (Figure 3a) and the layered flaky structure of clay (Figure 3d). The higher percolation threshold at Xc = 30% can be explained by the shape of the PANI particles. It is expected that spheres in a matrix will provoke a higher critical concentration in percolated composites. From Figure 3, panels b and c, the formation of composite is clearly seen. If we compare the composites, they also show differences; PANC30 is flakier, or more plate-like than is PANC50. Compared to pristine MMT (Figure 3d), the flake sheets of the composite are much looser. When the content of MMT reaches 50%, the particles are closely stacked together and become globular. It is dense in nature, and it can be seen that the surface of clay is covered with uniformly dispersed spherical PANI particles. PANI particles can not only present on the surface of clay, but also can be occupied throughout the interior of the clay filling cavities.45,46 At high proportions of clay, the surface effect of clay can be easily observed, which supports the conductivity and XRD data observations. 3.4. TEM Images. XRD patterns give a tentative explanation for the structure of the nanocomposites, whereas TEM gives a qualitative description of the internal structure. Figure 4, panels a and b show both larger views, which show the

C−H stretching and aromatic stretching vibrations of nanosilica), and 1056 cm−1 (influence of Si−O−Si peak of nanosilica), new peaks are developed. The bands at 1566 and 1483 cm−1 of PANI are shifted to higher wave numbers, 1594 and 1489 cm−1. The band at 1133 cm−1 that is assigned to a plane bending vibration of C−H mode, which is formed during protonation,36 is shifted to 1127 cm−1 because of the incorporation of nanoclay.37 Presence of this peak shows high conductivity.38,39 These changes in the composites may be associated with the interaction of Si and Al with the nitrogen atom in PANI. Hydrogen bonding between oxygen of nanoclay particles and the PANI molecule can also contribute to the shift of the bands.40 The intercalation of PANI with nanoclay environment is depicted in Scheme 1. Initial intercalation of PANI into the interlayer spacing of the clay may be driven by the exchange of cations like Na+, Fe2+, Fe3+, etc. Scheme 1. Molecular Model of PANI, MMT Clay (Clay Layers Are Separated by a Layer of Cations), and PANC Composite. PANI Entered into the Clay Gallery by Replacing the Cations and Water Molecules in the Composite

3.2. XRD Study. The diffraction patterns of the PANI, nanoclay, and PANC nanocomposite are shown in Figure 2. It is observed that broad peaks appeared at 2θ = 15°, 21°, and 25°, which correspond to the (011), (020), and (200) crystal planes of PANI emeraldine salt. A sharp peak observed at 2θ =

Figure 2. XRD patterns of PANI, PANC composites, and modified nanoclay. 16875



Article

Table 1. Peak Positions, d-Spacing, Full Width at Half Maxium (FWHM), Crystallite Size (D), Intercrystallite Separation (R), and Degree of Crystallinity for PANI and Composites sample

peak position 2θ (clay)

d-spacing (Å) (clay)

peak position 2θ (PANI)

d-spacing (Å) (PANI)

PANI PANC10 PANC20 PANC30 PANC40 PANC50

6.25 6.09 6.01 6.06 6.05

14.20 14.51 14.71 14.59 14.62

25.20 25.30 25.49 25.50 25.31 25.40

3.53 3.52 3.50 3.49 3.51 3.50

FWHM D (Å) 1.87 1.55 1.83 2.09 1.65 1.31

0.76 0.90 0.75 0.66 0.89 1.06

R (Å)

degree of crystallinity (Xc (%))

3.43 1.77 1.81 1.84 1.82 1.83

21.03 29.66 35.25 39.61 37.21 36.12

Figure 5. TGA of PANI and the PANC10, PANC30, and PANC50 composites.

done. In PANI, the large weight loss in the 50−150 °C region was due to the removal of the physically adsorbed water molecules present. The second stage between 150−250 °C is caused by the loss of the doping acid.47,48 The third stage arises from the degradation of PANI. At higher concentrations, the weight loss was more gradual. In PANC, three stages of weight loss were observed. The weight loss below 100 °C was due to the loss of moisture, dopant HCl loss, and removal of chemisorbed water that occurs in the range 200−300 °C and the polymer backbone chain breakage that occurs in the range of 400−600 °C. The weight loss corresponding to the dehydroxylation of the clay sheet was found to occur in a temperature range of 425−650 °C. Degradation temperatures of the PANC composites can be seen to shift to higher temperatures. This shift of degradation temperature toward a higher level is because of the attractive coulomb interaction between the positive nitrogen of the PANI layer and the negatively charged surface of the clay layer. The residual mass is found to be too high in composites (50−55%) compared to in PANI (28%); thus, the nanocomposite formation and the thermal stability of the polymer inside the matrix are evident. PANC30 shows the highest thermal stability, and the results are in good agreement with other studies. This improved thermal stability in PANC30 in the second temperature range onward is caused by the increased crystallinity as shown in the XRD analysis. In other words, the second weight loss stage is associated with the loss of doping acid. It would need more energy for the acids to be removed from the polymer chains if the PANI chains doped with acid are well arranged in a more crystalline structure; thus, the second weight loss stage of samples with higher crystallinity will shift to higher temperature and overlap with the third weight loss stage. With a further increase in temperature, the third weight loss

Figure 3. SEM micrographs of (a) PANI, (b) PANC30, (c) PANC50, and (d) nanoclay.

Figure 4. TEM photographs of PANC30 (a) showing intercalation of the clay within the PANI matrix and the (b) HR-TEM showing discrete clay layers to which PANI is inserted.

dispersion of clay within the PANI matrix, and a high resolution TEM (HR-TEM), which permits the observation of discrete clay layers to which PANI is inserted. The silicate layers are densely dispersed into the matrix. In such a highly intercalated nanocomposite, the insertion of the polymer matrix into layered silicate structure occurs in a crystallographically regular manner. The polymer−clay system with limited interlayer space, and which preserves a layered structure, is the perfect way to structure PANI and creates a percolating path. 3.5. TGA Studies. To study the thermal stability, a TGA (curves shown in Figure 5) of PANI as well as the composites is 16876



Article

stage, ranging from 350−600 °C, is observed with a rapid weight loss in PANI and a comparatively slow weight loss in all samples, which is due to the breakdown of the backbone.49 3.6. Dielectric Property. 3.6.1. Dielectric Constants. Dielectric properties are normally examined with a complex dielectric constant that can be calculated by the equation ε* = ε′ − i ε″, where ε′ and ε″ are the real and imaginary parts, respectively. They are related to the stored and lost energy under an external field’s presence. By using the capacitance and dielectric loss factor (cp, tan δ) of PANI and PANC, the real and imaginary parts were calculated with equations ε′ = cpd/(ε0 A) and ε″ = ε′ tan δ. The ac-conductivity (σac = ω cp d tan δ/A) was calculated, where d is the thickness of the sample, A is the electrode area, ε0 is the dielectric permittivity in vacuum (8.85 × 10−12 Fm−1), and ω is the angular frequency for all of the equations mentioned above.50−54 The plots of frequency-dependent dielectric constants (ε′ and ε″) at room temperature are shown in Figures 6 and 7. The insets show the dielectric constants as a function of weight percent of nanoclay at different frequencies.

Figure 7. Variation of ε″ as a function of frequency (20 Hz−1 MHz) for PANI and composites PANC10, PANC20, PANC30, PANC40, and PANC50 at 303 K and (inset) variation of ε″ versus wt % of nanoclay in PANI.

wt % of nanoclay in PANI. ε′ and ε″ of PANC30 are approximately two orders greater than those of PANI and more than 107 at higher temperatures, which is very high compared to many reported values. The dielectric constant and loss tangent decrease with an increase of frequency of the applied field, which is mainly attributed to the mismatch of interfacial polarization of composites to the external electric field at higher frequencies.58 At the percolation threshold, the electrical property varies greatly; therefore, the study of such conducting composites in the vicinity of the percolation threshold is an effective way to know the electrical transport behavior of composites. The results can be explained in light of dielectric polarization, which is similar to the conduction phenomenon. Temperature-dependent variations of the dielectric parameters are also calculated, which shows that all of the values increase with temperature and at a particular temperature their values are maximum for PANC30. Corresponding graphs of PANC30 are shown in Figure 8. The main source of the loss ε″ is the Maxwell−Wagner interfacial polarization, and therefore it increases slowly with nanoclay content. Because of percolation and the accompanying growth of the dc-conductivity tail, this loss increases much more rapidly for sample PANC30. The tan δ, log f plots of PANI and PANC composites that were calculated at 303 K and with temperature variation in PANC30 showed the same trend as that of the dielectric parameters. We can see a large separation after 343 K, which may be due to the removal of absorbed water as seen in the TGA study or a change in the type of conductivity in the composite; this requires further study. The dielectric studies show that the new composite is a good dielectric material with attractive dielectric constant values. 3.6.2. Modulus Study. To understand the dielectric process more clearly and accurately, the complex electric moduli, derived from the complex permittivity, were analyzed as a function of frequency. This can be successfully applied in cases where interfacial polarization causes large variations in dielectric permittivity at the low frequency range. According to the relation described in ref 59, the electric modulus is defined as the electric analogue of the dynamical mechanical modulus and is related to the complex permittivity M = M′ + i M″. The real (M′) and imaginary (M″) parts of the complex electric modulus

Figure 6. Variation of ε′ as a function of frequency (20 Hz−1 MHz) for PANI and composites PANC10, PANC20, PANC30, PANC40, and PANC50 at 303 K and (inset) variation of ε′ versus wt % of nanoclay in PANI.

The dielectric constant and dielectric loss values are found to decrease with an increase in frequency. This falls off rapidly after the onset frequency of the ac-conductivity. At low frequencies, less than 100 Hz, a sharp increase was observed in dielectric permittivity for all PANC nanocomposites, which indicates that the ionic conduction mechanism is dominant in this frequency range. As shown in Figure 6, a high dielectric constant (nearly 105) was obtained for the PANI synthesized in this work, which is much higher than that reported in the literature.55,56 This may be due to the presence of nanocrystalline domains rather than microdomains present in the PANI, which was due to mechanical shear applied on it. The observed high dielectric response for the nanocrystalline PANC at low frequencies (100 Hz−1 kHz) may be correlated to the hyperelectronic polarization and a strong polaron delocalization.55,57 The dielectric parameters (ε′, ε″, tan δ, and σac) show their maximum values for the 30% nanoclay composite. The dielectric constant and dielectric loss in this composite depend on the content of nanoclay with a percolation threshold at 30 16877



Article

Figure 8. Variation of (a) ε′ and (b) ε″ as a function of log f (frequency 20 Hz−1 MHz) for various temperatures for PANC30 composites. Variation of tan δ as a function of log f for (c) PANI and PANC composites and (d) with temperature variation in PANC30.

Figure 9. Variation of (a) M′ and (b) M″ as a function of log f (frequency 20 Hz−1 MHz) for PANI, PANC30, and PANC50 composites.

can be calculated from ε′ and ε″ values using the relations, M′ = ε′/[(ε′)2 + (ε″)2] and M″ = ε″/[(ε′)2 + (ε″)2]. Figure 9, panel (a) shows the variation of the real part of the electric modulus M′ of PANI and PANC as a function of frequency in the range of 20 Hz−1 MHz. M′ approaches zero at low frequency, which indicates the suppression of the electrode polarization. The observed value of M′ increases with the frequency, which may be due to a lack of the required amount of restoring force that governs the mobility of charge carriers under the action of an induced electric field. M″ reaches a maximum value corresponding to M∞ = (ε∞)−1, which may be attributed to the conduction phenomenon due to short-

range mobility of charge carriers.60 Figure 9, panel b shows the variation of M″ of the electric modulus with the same frequency range. It can be found that the M″ peak is in the upperfrequency region, which means an increase in dc-conductivity. This is up to PANC30 (shows an increase in the mobility of the charge carriers with the increase of clay content) and after that the peak shifts to the left and shows a decrease in dcconductivity, given by the following expression, σdc = ε0/(M∞ τ), where ε0 is the free space permittivity, M∞ is the frequency at which the peak maximum occurs, and τ is the relaxation time calculated by the expression τ = 1/2π f max.61,53 16878



Article

The calculated τ values are 9.72 × 10−7 s, 2.42 × 10−7 s, and 3.88 × 10−6 s for PANI, PANC30, and PANC50, respectively, where PANC30 shows minimum value. The σdc values for PANI, PANC30, and PANC50 are 3.338 × 10−2 S cm−1, 7.12 × 10−2 S cm−1, and 6.73 × 10−3 S cm−1, respectively, and PANC30 shows maximum value, which is in agreement with dielectric and conductivity data. This reflects the increased chain order in the polymer due to increased intercalation in percolated composites, which is in agreement with the XRD and TEM results. 3.7. Transport Property. 3.7.1. Electrical Conductivity. The experimental value demonstrates that PANC30 forms a conducting path and hopping of electrons to the maximum at low frequencies. The conductivity of the low frequency region shows that it is almost independent of frequency. The frequency-independent plateau at the dc region, as in Figure 10, is attributed to the long-range translational motion of ions

composite, polymer chain length may be more flexible, and polarons may receive sufficient energy to hop between favorable sites. 3.7.2. Temperature-Dependent Electrical Conductivity. To investigate the charge-transport mechanism of the PANI and PANC composites, we considered the temperature range of 303−383 K. The conductivity of each sample increases exponentially with an increase in temperature in the studied temperature range. This indicates that PANC composites show semiconducting behavior. Two regions with different activation energies in the ln σ versus T−1 graph (Figure 11) can be seen in all of the composites and PANI, but they are more prominent in the graph of PANC30.

Figure 11. Variation of ln σ as a function of l000/T for PANI and PANC composites.

Two regions in PANC indicate two possible electrical conduction mechanisms in the studied temperature region: (1) hopping mechanisms and (2) thermionic emission. The band conduction tends to occur at high temperatures above a threshold. For temperatures below this, the 1D hopping mechanism becomes dominant. The increased conductivity of this state is explained by the increased PANI-to-PANI contact after the percolating network has formed, which facilitates electron transfer. TGA studies also show that PANC30 is showing the highest thermal stability over PANI and PANC50. The room temperature Mott’s parameters and activation energies of the samples are listed in Table 2. At low temperatures, the thermally-activated VRH mechanism can be used as the dominant charge-transport mechanism for PANC composites. In VRH, charge carriers jump from one localized energy level in the band gap to another, with the emission or absorption of a phonon. The conductivity can be written as σ = σ0 exp (−T0/T)1/(n + 1), where n shows the dimensionality, σ0 is the pre-exponential factor, T0 = 16/k N(EF)L3, where k is the Boltzmann constant, and N(EF) is the density of states at Fermi energy level. The other two hopping parameters, the average hopping distance, RHOP, and the average hopping energy, WHOP, can be written as

Figure 10. Variation of log ac as a function of log f (frequency 20 Hz− 1 MHz) for all of the composites at 303 K, showing dc plateau region.

contributing to dc-conductivity. This type of dc-conductivity was explained by Funke,62 but at higher frequencies, it shows a greater dependence. The conductivity shows a decrease at mid frequencies (1 kHz−10 kHz) in PANC30, which may be due to the competitive effect of ac- and dc-conductivities in a percolated sample at these frequencies. PANC10, PANC20, and PANC30 show increase in dcconductivity over that of PANI. These values can be attributed to the uncoiling of polymeric chains due to strong interfacial interactions between clay crystallites and PANI caused by mechanical grinding. We observed that the slope of the straight line of the log−log plot at lower frequency side for PANC30 is approximately equal to zero, which is the natural result of the frequency-independent conduction, while the value of the slope at higher frequencies lies between zero and one. The frequencyindependent plateau in the 20 Hz−1 kHz range indicates conductivity relaxation across the interface found from the above impedance spectroscopy analysis, and thereby a corresponding f max value is observed in the modulus study. A conductivity of 2.143 S cm−1 is observed for PANC30, which is two orders greater than the maximum conductivity of PANI and is also a very high value compared to reported results. This may be due to variable range hopping of charge carriers and fast mobility of ions through the backbone polymer chain and electronic conductivity through extended states as can be seen in the activation energy graph of PANC30. In this

RHOP =

1/4 3 ⎡ T0 ⎤ ⎢⎣ ⎥⎦ 8 T

(1)

1/4 1 ⎡ T0 ⎤ kT ⎢ ⎥ 4 ⎣T ⎦

(2)

and WHOP = 16879



Article

Table 2. Calculated Mott Parameters and Activation Energies for the Samples PANI, PANC10, PANC20, PANC30, PANC40, and PANC50 sample

T0 (K)

PANI PANC10 PANC20 PANC30 PANC40 PANC50

× × × × × ×

4.19 1.79 6.40 3.19 2.68 2.46

5

10 104 104 103 105 104

activation energy (eV)

N(EF) (eV−1 cm−3)

RHOP (A•)

WHOP (eV)

0.032 0.007 0.006 0.003 0.017 0.016

× × × × × ×

6.86 3.12 4.20 2.03 6.14 3.37

0.390 0.018 0.020 0.012 0.036 0.019

1.64 3.83 1.07 2.15 2.56 2.80

For the sample PANI and PANC composites, the best linear fit of ln σ versus T−1 graph was obtained for n = 1, which indicates 1D hopping. In general, the temperature dependence of σ (dc) for PANI is known to follow the quasi-1D VRH model.63,64 Minimum T0 was obtained for PANC30 and due to this, it has the highest conductivity. In percolated materials, localized states at low temperatures and extended states at high temperatures contribute to the electrical conductivity. As observed from Table 2, the density of localized energy N(EF) around Fermi level increases, and the hopping energy, WHOP, range of hopping, RHOP, and activation energy decrease in PANC30. This could be an indication of the structural modification in the composite structure that results in the decreasing values, which may be attributed to the decreasing of the localization length or, in other words, the closer approaches of hopping sites enhance the conductivity.

10 1029 1029 1030 1028 1029

BSR research fellowship. The authors would also like to thank Dr. M. Revanasiddappa (Department of Engineering Chemistry, PESIT, Bangalore South Campus, Bangalore 560100, India) for the support and guidance given to pursue the work.

■

REFERENCES

(1) Umare, S. S.; Shambharkar, B. H.; Ningthoujam, R. S. Synthesis and Characterization of Polyaniline-Fe3O4 Nanocomposite: Electrical Conductivity, Magnetic, Electrochemical Studies. Synth. Met. 2010, 160, 1815−1821. (2) Ameen, S.; Akhtar, M. S.; Ansari, S. G.; Yang, O.; Shin, S. Electrophoretically Deposited Polyaniline/ZnO Nanoparticles for P− N Heterostructure Diodes, Superlattices. Microstruct. 2009, 46, 872− 880. (3) Cochet, M.; Maser, W. K.; Benito, A. M.; Calle Jas, M. A.; Martinez, M. T.; Benoit, J. M.; Schreiber, J.; Chauvet, O. Synthesis of a New Polyaniline/Nanotube Composite: In Situ Polymerization and Charge Transfer through Site Selective Interaction. Chem. Commun. 2001, 16, 1450−1451. (4) Al-Saleh, M. H.; Sundararaj, U. A Review of Vapor Grown Carbon Nanofiber/Polymer Conductive Composites. Carbon 2009, 47, 2−22. (5) Clerc, J. P.; Giraud, G. J.; Laugier, M.; Luck, J. M. The Electrical Conductivity of Binary Disordered Systems, Percolation Clusters, Fractals, and Related Models. Adv. Phys. 1990, 39, 191−308. (6) Haggenmueller, R.; Gommans, H. H.; Rinzler, A. G.; Fischer, J. E.; Winey, K. I. Aligned Single-Wall Carbon Nanotubes in Composites by Melt Processing Methods. Chem. Phys. Lett. 2000, 330, 219−225. (7) Bedre, M. D.; Basavaraja, S.; Salwe, B. D.; Shivakumar, V.; Arunkumar, L.; Venkataraman, A. Preparation and Characterization of Pani and Pani−Ag Nanocomposites via Inter-Facial Polymerization. Polym. Compos. 2009, 30, 1668−1677. (8) Yoshimoto, S.; Ohashi, F.; Ohnihi, Y.; Nonami, T. Synthesis of Polyaniline−Montmorilonite Nanocomposites by the Mechanochemical Intercalation Method. Synth. Met. 2004, 145, 265−270. (9) Kim, B. H.; Jung, J. H.; Hong, S. H.; Joo, J.; Epstein, A. J.; Mizoguchi, K.; Kim, J. W.; Choi, H. J. Nanocomposite of Polyaniline and Na-Montmorillonite Clay. Macromolecules 2002, 35, 1419−1423. (10) Kim, B. H.; Jung, J. H.; Hong, S. H.; Kim, J. W.; Choi, H. J.; Joo, J. Physical Characterization of Emulsion Intercalated Polyaniline−Clay Nanocomposite. Curr. Appl. Phys. 2001, 1, 112−115. (11) Song, D. H.; Lee, H. M.; Lee, K. H.; Choi, H. J. Intercalated Conducting Polyaniline−Clay Nanocomposites and Their Electrical Characteristics. J. Phys. Chem. Solids 2008, 69, 1383−1385. (12) Chang, K. C.; Jang, G. W.; Peng, J. C. W.; Lin, C. Y.; Shieh, J. C.; Yeh, J. M.; Yang, J. C.; Li, W. T. Comparatively Electro-Chemical Studies at Different Operational Temperatures for the Effect of Nanoclay Platelets on the Anticorrosion Efficiency of DBSA-Doped Polyaniline/Na+−MMT Clay Nanocomposite Coatings. Electrochim. Acta 2007, 52, 5191−5200. (13) do Nascimento, G. M.; Padilha, A. C. M.; Constantino, V. R. L.; Temperini, M. L. A. Oxidation of Anilinium Ions Intercalated in Montmorillonite Clay by Electrochemical Route. Colloids Surf., A 2008, 318, 245−253. (14) Sun, F.; Pan, Y. J.; Wang, J.; Wang, Z.; Hu, C. P.; Dong, Q. Z. Synthesis of Conducting Polyaniline Montmorillonite Nanocompo-

4. CONCLUSION We successfully prepared a series of biodegradable PANC nanocomposites by a simple method of mechanically grinding PANI with nanoclay powder in acetone medium. The structure and properties of the nanocomposite are dependent on the nature and amount of clay used. The FTIR studies suggest the occurrence of chemical interaction between PANI and nanoclay crystallites, while the thermal studies indicate complete enhancement of the thermal properties of PANI in PANC, especially in PANC30. The XRD and TEM studies suggest modification in the structure of PANI due to its interfacial interactions with nanoclay crystallites and high intercalation of clay. The morphological changes are observed in the SEM analysis. In modulus studies, onset of percolating path is seen in PANC30 and for this reason, its thermal, dielectric, and transport properties (ε′, ε″, M′, M″, and σ by varying temperature and frequency) were unusually enhanced. The temperature-dependent conductivity studies suggest a quasi-1D VRH model conduction in pure PANI and its composites at low temperatures. Altogether, this study gives a complete picture of a percolated composite. These nanocomposites can be used for various applications in industry as good capacitor or supercapacitor materials and as tunable electrical and dielectric materials.

■

28

AUTHOR INFORMATION

Corresponding Author

*Tel: 08242888707. Fax: (+91)-0824-2287289. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank the University Grant Commission, New Delhi, for selecting Mini.V for the UGC16880



Article

sites via Inverse Emulsion Polymerization in Supercritical Carbon Dioxide. Polym. Compos. 2010, 31, 163−172. (15) do Nascimento, G. M.; Constantino, V. R. L.; Temperini, M. L. A. Spectroscopic Characterisation of a New Type of Conducting Polymer−Clay Nanocomposite. Macromolecules 2002, 35, 7535−7537. (16) Wu, Q.; Xue, Z.; Qi, Z.; Wang, F. Synthesis and Characterization of PAn/Clay Nanocomposite with Extended Chain Conformation of Polyaniline. Polymer 2000, 41, 2029−2032. (17) Yoshimoto, S.; Ohashi, F.; Ohnishi, Y.; Nonami, T. Synthesis of Polyaniline−Montmorillonite Nanocomposites by the Mechanochemical Intercalation Method. Synth. Met. 2007, 145, 265−270. (18) Bekri-Abbes, I.; Srasra, E. Characterization and Ac Conductivity of Polyaniline−Montmorillonite Nanocomposites Synthesized by Mechanical/Chemical Reaction. React. Funct. Polym. 2010, 70, 11−18. (19) Bober, P.; Stejskal, J.; Spirkova, M.; Trchová, M.; Varga, M.; Prokes, J. Conducting Polyaniline−Montmorillonite Composites. Synth. Met. 2010, 160, 2596−2604. (20) Wang, X. L.; Liu, Z. Q.; Marion, A.; Kalceff, S.; Riesen, H. Mechanochemical Preparation of Nanocrystalline BaFCl Doped with Samarium in the 2+ Oxidation State. Inorg. Chem. 2014, 53, 8839− 8841. (21) Faisal, M.; Khasim, S. Electrical Conductivity, Dielectric Behavior, and EMI Shielding Effectiveness of Polyaniline−Yttrium Oxide Composites. Bull. Korean Chem. Soc. 2013, 34, 99−106. (22) Fattouma, A.; Othman, Z. B.; Arous, M. Dc and Ac Conductivity of Polyaniline/Poly(methyl methacrylate) Blends below the Percolation Threshold. Mater. Chem. Phys. 2012, 135, 117−122. (23) Pant, H. C.; Patra, M. K.; Negi, S. C.; Bhatia, A.; Vadera, S. R.; Kumar, N. Studies on Conductivity and Dielectric Properties of Polyaniline−Zinc Sulphide Composite. Bull. Mater. Sci. 2006, 29, 379−384. (24) Dey, A.; De, S.; De, A.; De, S. K. Characterization and Dielectric Properties of Polyaniline−TiO2 Nanocomposites. Nanotechnology 2004, 15, 1277. (25) Thomas, P.; Satapathy, S.; Dwarakanath, K.; Varma, K. B. R. Dielectric Properties of Poly(vinylidene fluoride)/CaCu3Ti4O12 Nanocrystal Composite Thick Films. eXPRESS Polym. Lett. 2010, 4, 632−643. (26) Thomas, P.; Dwarakanatha, K.; Varma, K. B. R. In Situ Synthesis and Characterization of Polyaniline CaCu3Ti4012 Nanocrystal Composites. Synth. Met. 2009, 159, 2128−2134. (27) Morgan, H.; Foot, P. J. S.; Brooks, N. W. The Effects of Composition and Processing Variables on the Properties of Thermoplastic Polyaniline Blends and Composites. J. Mater. Sci. 2001, 36, 5369−5377. (28) Ono, T.; Yoshida, S.; Esashi, M. Electrical Modification of a Conductive Polymer Using a Scanning Probe Microscope. Nanotechnology 2003, 14, 1051−1054. (29) Vaia, R. A.; Giannelis, E. P. Polymer Melt Intercalation in Organically-Modified Layered Silicates: Model Predictions and Experiment. Macromolecules 1997, 30, 8000−8009. (30) Hussain, F.; Hojjati, M.; Okamoto, M.; Russell, E. G. Review Article: Polymer-Matrix Nanocomposites, Processing, Manufacturing, and Application: An Overview. J. Compos. Mater. 2006, 40, 1511− 1575. (31) Verma, D.; Dutta, V. Novel Microstructure in Spin Coated Polyaniline Thin Films. J. Phys.: Condens. Matter 2007, 19, 186212. (32) El-Ghaffar, M. A. A.; Mohamed, N. A.; Ghoneim, A. A.; Shaffei, K. A. Characterization and Electrical Properties of Some Hydrochloric Acid-Doped Aniline and 2-Amino-Pyridine Homopolymers and Copolymers. Polym.-Plast. Technol. Eng. 2006, 45, 1327−1338. (33) Mathai, C. J.; Saravanan, S.; Anantharaman, M. R.; Venkitachalam, S.; Jayalekshmi, S. Characterization of Low Dielectric Constant Polyaniline Thin Film Synthesized by Ac Plasma Polymerization Technique. J. Phys. D: Appl. Phys. 2002, 35, 241. (34) Kalasad, M. N.; Rabinal, M. K. Tunnelling Conductivity in Conducting Polymer Composites: A Manifestation of Chemical Interaction. J. Phys. D: Appl. Phys. 2009, 42, 065414.

(35) Kang, E. T.; Neoh, K. G.; Tan, K. L. Polyaniline: A Polymer with Many Interesting Intrinsic Redox States. Prog. Polym. Sci. 1998, 23, 277−324. (36) Liu, Y. D.; Fang, F. F.; Choi, H. J.; Seo, Y. Fabrication of Semiconducting Polyaniline/Nano-Silica Nanocomposite Particles and Their Enhanced Electrorheological and Dielectric Characteristics. Colloids Surf., A 2011, 381, 17−22. (37) Berzina, T.; Erokhin, V.; Fontana, M. P. Spectroscopic Investigation of an Electrochemically Controlled Conducting Polymer−Solid Electrolyte Junction. J. Appl. Phys. 2007, 101, 024501− 024506. (38) Brugnerotto, J.; Lizard, J.; Goycoolea, F. M.; Monal, W. A.; Desbrières, J.; Rinaudo, M. An Infrared Investigation in Relation with Chitin and Chitosan Characterization. Polym. J. 2001, 42, 3569−3580. (39) Zheng, W.; Angelopoulos, M.; Epstein, A. J.; MacDiarmid, A. G. Concentration Dependence of Aggregation of Polyaniline in NMP Solution and Properties of Resulting Cast Films. Macromolecules 1997, 30, 7634−7637. (40) Lia, X.; Wanga, G.; Lib, X.; Luc, D. Surface Properties of Polyaniline/Nano-TiO2 Composites. Appl. Surf. Sci. 2004, 229, 395− 401. (41) Kim, B. H.; Jung, J. H.; Kim, J. W.; Choi, H. J.; Joo, J. Nanocomposite Intercalated by Emulsion Polymerization. Synth. Met. 2001, 117, 115−118. (42) Guinier, A. X-ray Diffraction: In Crystals, Imperfect Crystals, and Amorphous Bodies; Dover Publications, Inc.: New York, 1994. (43) Sui, X.; Chu, Y.; Xing, S.; Liu, C. Synthesis of PANI/AgCl, PANI/BaSO4, and PANI/TiO2 Nanocomposites in CTAB/Hexanol/ Water Reverse Micelle. Mater. Lett. 2004, 25, 1255−1259. (44) Zhou, Q.; Wang, J.; Ma, Y.; Cong, C.; Wang, F. The Relationship of Conductivity to the Morphology and Crystallinity of Polyaniline Controlled by Water Content via Reverse Microemulsion. Colloid Polym. Sci. 2007, 285, 405−411. (45) Ray, S. S.; Biswas, B. Water-Dispersible Conducting Nanocomposites of Polyaniline and Poly(N-vinylcarbozole) with Nanodimensional Zirconium Dioxide. Synth. Met. 2000, 108, 231−236. (46) Kazim, S.; Ahmad, S.; Pfleger, J.; Plestil, J.; Joshi, Y. M. Polyaniline−sodium Montmorillonite Clay Nanocomposites: Effect of Clay Concentration on Thermal, Structural, and Electrical Properties. J. Mater. Sci. 2012, 47, 420−428. (47) Ahmed, S. M. Preparation and Degradation of Highly Conducting Polyaniline Doped with Picric Acid. Eur. Polym. J. 2002, 38, 1151−1158. (48) Palaniappan, S.; Narayana, B. H. Conducting Polyaniline Salts: Thermogravimetric and Differential Thermal Analysis. Thermochim. Acta 1994, 237, 91−97. (49) Lu, X.; Ng, H. Y.; Xu, J.; He, C. Electrical Conductivity of Polyaniline−Dodecyl Benzene Sulphonic Acid Complex: Thermal Degradation and Its Mechanism. Synth. Met. 2002, 128, 167−178. (50) Marins, J. A.; Soares, B. G. A Facile and Inexpensive Method for the Preparation of Conducting Polyaniline−Clay Composite Nanofibers. Synth. Met. 2012, 162, 2067−2094. (51) Sarkar, A.; Ghosh, P.; Meikap, A. K.; Chattopadhyay, S. K.; Chatterjee, S. K.; Chowdhury, P.; Roy, K.; Saha, B. ElectricalTransport Properties of Iodine-Doped Conducting Polyaniline. J. Appl. Polym. Sci. 2008, 108, 2312−2320. (52) Gupta, K.; Chakraborty, G.; Ghatak, S.; Jana, P. C.; Meikap, A. K.; Babu, R. Synthesis, Magnetic, Optical, and Electrical Transport Properties of the Nanocomposites of Polyaniline with Some Rare Earth Chlorides. J. Appl. Phys. 2010, 108, 073701−073710. (53) Ho, C. H.; Liu, C. D.; Hsieh, C. H.; Hsieh, K. H.; Lee, S. N. High Dielectric Constant Polyaniline/Poly(acrylic acid) Composites Prepared by in Situ Polymerization. Synth. Met. 2008, 158, 630−637. (54) De, S.; De, A.; Das, A.; De, S. K. Transport and Dielectric Properties of α-Zirconium Phosphate−Polyaniline Composite. Mater. Chem. Phys. 2005, 91, 477−483. (55) Joo, J.; Long, S. M.; Pouget, J. P.; Oh, E. J.; MacDiarmid, A. G.; Epstein, A. J. Charge Transport of the Mesoscopic Metallic State in Partially Crystalline Polyanilines. Phys. Rev. B 1998, 57, 9567−9580. 16881



Article

(56) Yan, X. Z.; Goodson, T. High Dielectric Hyper Branched Polyaniline Materials. J. Phys. Chem. B 2006, 110, 14667−14672. (57) Huang, C.; Zhang, Q. M.; Su, J. High-Dielectric-Constant AllPolymer Percolative Composites. Appl. Phys. Lett. 2003, 82, 3502− 3504. (58) Meyer, J. Glass Transition Temperature As a Guide to Selection of Polymers Suitable for PTC Materials. Polym. Eng. Sci. 1973, 13, 462−468. (59) Macedo, P. B.; Moynihan, C. T.; Bose, R. The Role of Ionic Diffusion in Polarization in Vitreous Ionic Conductors. Phys. Chem. Glasses 1972, 13, 171−179. (60) Dang, T. T. N.; Mahapatra, S. P.; Sridhar, V.; Kim, J. K.; Kim, K. J.; Kwak, H. Dielectric Properties of Nanotube Reinforced Butyl Elastomer Composites. J. Appl. Polym. Sci. 2009, 113, 1690−1700. (61) Han, M. G.; Im, S. S. Dielectric Spectroscopy of Conductive Polyaniline Salt Films. J. .Appl. Polym. Sci. 2001, 82, 2760−2769. (62) Funke, K. Jump Relaxation in Solid Electrolytes. Prog. Solid State Chem. 1993, 22, 111−195. (63) Wang, Z. H.; Li, C.; Scherr, E. M.; MacDiarmid, A. G.; Epstein, A. J. Three Dimensionality of Metallic States in Conducting Polymers: Polyaniline. Phys. Rev. Lett. 1991, 66, 1745−1748. (64) Wang, Z. H.; Scherr, E. M.; MacDiarmid, A. G.; Epstein, A. J. Transport and EPR Studies of Polyaniline: A Quasi-One-Dimensional Conductor with Three-Dimensional Metallic States. Phys. Rev. B 1992, 45, 4190−4202.

16882


Dielectric Constant and Transport Mechanism of ... - ACS Publications

Recommend Documents