activated carbon composites and preparation of conductive films

activated carbon composites and preparation of conductive films

Materials Chemistry and Physics 120 (2010) 46–53 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.els...

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Materials Chemistry and Physics 120 (2010) 46–53

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Synthesis and characterization of polyaniline/activated carbon composites and preparation of conductive films Huseyin Zengin ∗ , Güllü Kalaycı Department of Chemistry, Faculty of Science and Literature, Kahramanmaras Sutcu Imam University, Kahramanmaras 46100, Turkey

a r t i c l e

i n f o

Article history: Received 8 April 2009 Received in revised form 30 September 2009 Accepted 18 October 2009 Keywords: Composite materials Electrical conductivity Photoluminescence spectroscopy Electron microscopy (SEM)

a b s t r a c t Polyaniline was synthesized via polyaniline/activated carbon (PANI/AC) composites by in situ polymerization and ex situ solution mixing. PANI and PANI/AC composite films were prepared by drop-by-drop and spin coating methods. The electrical conductivities of HCl doped PANI film and PANI/AC composite films were measured according to the standard four-point-probe technique. The composite films exhibited an increase in electrical conductivity over neat PANI. PANI and PANI/AC composites were investigated by spectroscopic methods including UV–vis, FTIR and photoluminescence. UV–vis and FTIR studies showed that AC particles affect the quinoid units along the polymer backbone and indicate strong interactions between AC particles and quinoidal sites of PANI. The photoluminescence properties of PANI and PANI/AC composites were studied and the photoluminescence intensity of PANI/AC composites was higher than that of neat PANI. The increase of conductivity of PANI/AC composites may be partially due to the doping or impurity effect of AC, where the AC competes with chloride ions. The amount of weight loss and the thermostability of PANI and PANI/AC composites were determined from thermogravimetric analysis. The morphology of particles and films were examined by a scanning electron microscope (SEM). SEM measurements indicated that the AC particles were well dispersed and isolated in composite films. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Conjugated polymers have an alternating single and double bond framework between the atoms, especially carbon atoms, sometimes carbon–nitrogen atoms [1] and result in electrically conducting materials. Conducting polymers have drawn much attention over the last decades due to their unique properties, such as, mechanical strength, electrical conductivity, corrosion stability and the possibility of chemical and electrochemical synthesis [2]. Thus, conducting polymers have been considered for a wide area of practical applications, such as, for solar cells, lightweight batteries, light emitting diodes, energy storage, sensors, molecular electronic devices, electrochemical power sources, electromagnetic interference shielding and for the inhibition of corrosion [3]. Polyaniline (PANI) is an important conducting polymer because of its high electrical conductivity, ease of producibility and its environmental stability [4]. PANI has received considerable interest in recent years because of its applications in a variety of technological fields, such as for electronic and electrochromic devices [5], chemical sensors [6], charge storage systems [7] and the protection against corrosion [8]. PANI is considered as an organic metal even though its specific conductivity and its temperature depen-

∗ Corresponding author. E-mail address: [email protected] (H. Zengin). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.10.019

dence of conductivity are semi-metallic. Other properties, such as, its thermo power are classified as metallic. PANI can be used as an electronic material because of its reversible proton dopability, redox recyclability, environmental stability, electrical conductivity, low cost and ease of synthesis [9]. PANI is invaluable for microelectronic technology, in particular, for the manufacture of recordable optical disks [10] and sensors [11]. As polymers have promising commercial applications, work on polymer composite forms in the fields of aerospace, automotive, marine, infrastructure and the military has been encouraged [12]. The performance of composite materials dictates the eventual outcome of products during their use in outdoor applications. The durability of polymeric materials is an important feature of these materials as their respective lifetimes, maintenance and replacements become factors that need to be considered. The deterioration of these materials depends on the duration and extent of interaction with the environment. Activated carbon (AC) particles possess high surface area due to their outstanding porous structure, however, they are difficult to process and are insoluble in most solvents. Recently polymers have been used to wrap AC and render them soluble in water or organic solvents. Also, it has been found that AC particle-filled polymer composites often exhibit remarkable improvements of mechanical, thermal and physicochemical properties compared with the pure polymer [13]. These AC particles have attracted the attention of researchers because of their low cost, abundance and high surface area, which give greater possibil-

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ity of energy transfer from one phase to another [14]. Introducing AC particles into semiconducting organic material is expected to improve the performance of electronic devices, such as, for field effect transistors. A number of studies have been dedicated to composite materials prepared by mixing different additives or fillers with PANI. For example, an order of magnitude increase in electrical conductivity of CNT/PANI over that of neat PANI was measured, and the resulting interactions were probed by Raman spectroscopy [15]. Also, doping effects of carbon nanotubes in PANI/CNT composite have been investigated by different spectroscopic methods [16]. Ramamurthy et al. [17] reported the use of PANI/CNT composites in organic electronic devices. They reported improvements in material consistency and a reduction in defect densities making these composites suitable for use in the fabrication of organic electronic devices. Encapsulation of inorganic nanomaterials inside the shell of PANI has also become a popular and interesting aspect of nanocomposite synthesis. To date, a number of methods have been described for generating PANI coated inorganic nanomaterials, such as, nanoparticles [18], nanotubes [19], nanobelts [20] and silicate clays [21] by in situ oxidative polymerization, gamma radiation-induced chemical polymerization or electropolymerization. In situ polymerization of PANI in the presence of AC particles give composites with AC “cores” and polymer “shells”. The aim of this study was to report on the fabrication of highly conductive PANI/AC composite materials by in situ polymerization and ex situ solution mixing and to enhance the conductivity of PANI by using a new dopant. Characterizations using various spectroscopic methods were presented. These fine materials revealed good dispersity of AC particles in composite films. The electrical conductivities of HCl doped PANI film and PANI/AC composite films were measured by the four point-probe method. The thermal behavior and stability of PANI and PANI/AC composite were investigated by Thermogravimetric Analysis (TGA). PANI/AC composites showed enhanced electric conductivity and thermal stability. We attribute the enhancement tentatively to the charge transfer interaction between the external PANI shell layer and the internal AC particles. 2. Experimental 2.1. Materials Aniline (ACS grade) and ammonium peroxydisulfate (APS) (ACS grade) were purchased from Acros and used as received. 1-Methyl-2-pyrrolidinone (NMP) (99%, HPLC grade, and spectrophotometric grade), lithium chloride (99%, ACS reagent), hydrochloric acid (37%) and ammonium hydroxide solution (29%, reagent grade), 9,10-diphenylantracene, potassium bromide, acetone, methanol, ethanol, diethyl ether, Pall Model acrodisc syringe filters (PTFE syringe filters, 0.45 ␮m) and AC particles were purchased from Aldrich Chem. Co. and Merck and used as received.

2.2. Instrumentation Conformation of the EB oxidation state of polymer and the PANI/AC composites were examined in spectrophotometric grade N-methylpyrrolidinone (NMP) solvent. The UV–vis spectra were recorded from 190 to 850 nm using a Shimadzu UV-160A scanning spectrophotometer on 0.05 wt./vol. sample solutions, prepared in spectrophotometric grade NMP, and contained in 1 cm optical path quartz cuvette. The photoluminescent properties of PANI and PANI/AC composites were studied using a Perkin Elmer LS55 Model Luminescence Spectrometer. All the samples were prepared in NMP and analyzed in a 1 cm optical path quartz cuvette. PANI and PANI/AC samples were excited at 278 nm wavelength for different sample concentrations. The photoluminescence quantum efficiencies of the polymer were calculated using 9,10-diphenylantracene as the standard [22,23]. FTIR spectra of PANI, EB powders, and PANI/AC composites were obtained at room temperature using a Shimadzu 8300 FTIR spectrometer at a resolution of 1 cm−1 over 64 scans. FTIR spectra were obtained using samples prepared as KBr pellets, where samples were dry-ground and well dispersed in a mortar, using a pestle and then pressed into transparent pellets. The amount of weight loss and the thermostability of PANI and PANI/AC composites were determined from TGA. The TGA measurements were performed in a Mettler-Toledo TGA/SDTA851 instrument and an alumina sample pan was used for measurements.

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AC particle suspensions and reaction mixtures were sonicated and homogenized by a Bandelin Sonopuls Model Ultrasonic Homogenizer. PANI and PANI/AC composite films were prepared by spin coating method using a Chemat Technology Model Spin-Coater (KW-4A). All polymer solution samples were filtered with 0.45 ␮m Pall Model acrodisc syringe filters before use. 2.3. In situ polymerization of aniline and ex situ preparation of PANI/AC composites In situ composites were synthesized by polymerization of aniline with ammonium peroxydisulfate (APS) as the oxidant in the presence of AC particles (various wt.%). A solution of HCl (1 M, 60 mL) containing AC (0.0224 g), was sonicated at room temperature to disperse the AC particles for 3 days. Aniline monomer (0.2235 g) was then added to the AC suspension and stirred for 30 min. A solution of HCl (1 M, 60 mL) containing ammonium peroxydisulfate (APS) (0.5439 g) was added dropwise to the well-stirred reaction mixture over 1 h. After a few minutes, the dark suspension became green, indicating polymerization of aniline; polymerization was carried out at 0 ◦ C, under a dynamic nitrogen flow while stirring for 24 h. The composite was obtained by filtering and washing the reaction mixtures with deionized water, resulting in the conductive emeraldine salt (ES) form of the PANI/AC composite. The composite solid was stirred with excess 3 wt.% ammonium hydroxide (60 mL), under nitrogen for 48 h. The EB composite was isolated by vacuum filtration and washed with deionized water (50 mL), and then with methanol (20 mL) in order to remove oligomers. The precipitate was again washed with deionized water until pH ∼ 7, and dried under vacuum at 55 ◦ C for 72 h. Ex situ PANI/AC composite film was prepared by first adding AC (10.0 mg) to N-methylpyrrolidinone (NMP) (10.0 mL) and sonicating at room temperature for 2 days to disperse the AC particles. The previously synthesized EB form of PANI (90.0 mg) was dissolved completely in NMP (20.0 mL). Then PANI solution and AC suspension were mixed and sonicated at room temperature for another 2 days. The composite film was cast on a glass substrate or silicon wafer by drop-by-drop or spin coating methods. The film was washed with methanol, and dried in a vacuum oven at 100 ◦ C for 24 h. 2.4. Doping of film samples Film doping was carried out by immersing EB films in an excess amount of doping solution at a fixed concentration of 1 M HCl for 24 h; weight ratios of solutionto-film were greater than 50. The doped films were then washed with methanol or acetone to remove absorbed doping solution. The films were then dried in a vacuum oven at 70 ◦ C for 24 h. 2.5. Conductivity measurement Film conductivities were measured according to the standard four point probe technique (ASTM F84). The probe used was a Signatone Pro4 equipped with a tip spacing of 0.04”, spring force of 40–70 g per tip, and probe tip radii of 0.002”, where films were mounted onto the Signatone Pro4 contact probe station. An electrochemical analyzer, Keithley 2400 Model Source Meter, was used as the current source; the operating mode was set to time base (TB) and output potential was controlled to provide currents in the range of 1–10 mA. The potential difference between the middle two tips was measured by a Keithley 2400 Model Source Meter. 2.6. Morphology Surface morphology and dimensionality of unsonicated and sonicated AC particles, and the surface morphology and cross-sections of PANI and PANI/AC composite films were examined by scanning electron microscopy (SEM). Film thickness’ were obtained by SEM measurements of PANI and PANI/AC composite film cross-sections. Samples were attached to carbon tape for surface morphology and cross-section analyses. These samples were then placed onto aluminum stubs and then after coating with platinum, were used for measurements. The micrographs were taken at an acceleration voltage of 15 kV in a Hitachi S-4700 Field Emission Scanning Electron Microscope.

3. Results and discussion 3.1. In situ polymerization of aniline and ex situ preparation of PANI/AC composites PANI can be obtained using both electrochemical and chemical oxidative synthesis. Many studies have been carried out for electrochemical polymerization [24]. Chemical polymerization process is especially important since this route is most feasible for large scale production of PANI [25]. Chemical synthesis of PANI can be carried out by oxidation of aniline with ammonium peroxydisulfate in dilute (1 M) aqueous

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hydrochloric acid solutions, provided that the temperature does not exceed room temperature [26]. In this method PANI is obtained in the ES form and can be deprotonated to the processable EB form of the polymer. PANI occurs in four separate oxidation states, emeraldine salt (ES), leucoemeraldine base (LEB), emeraldine base (EB), and pernigraniline base (PN) [26,27] of which only ES is conducting in nature, while the rest are insulating. In its protonated form, PANI is a salt with the positive charge delocalized over the polymer chain interacting negatively charged counter ions. When ES is treated with alkali ammonium hydroxide, it changes to insulating EB. EB contains alternating amine and imine repeat units and when doped with protonic acid, protonation occurs at the imine nitrogen sites to yield polysemiquinone in which the polaron delocalizes along the chain [25]. The polymerization of aniline and the formation of ES and EB forms of PANI are illustrated in Fig. 1(a). High molecular weights may be obtained with polymerization reactions carried out at temperatures below 0 ◦ C, as previously reported [26]. Subzero polymerization reactions necessitate the use of lithium chloride to prevent the aqueous reaction solution from freezing. Aqueous polymerization reactions result in the salt form of PANI. Though the ES form of PANI is not soluble, dedoping it with ammonium hydroxide yields soluble EB form. PANI synthesized, either electrochemically or chemically, is not a mechanically strong material. In order to

use it as a membrane or thick coating material, it is necessary to process the polymer in different ways. A simple way to achieve mechanical strength is to prepare a mixture of PANI with some other substances. Fig. 1(b) illustrates the in situ polymerization of aniline in the presence of AC particles. The PANI macromolecules formed are located on the surface of AC pores, and therefore many PANI polymers cover the surface of the AC particles. 3.2. Ultraviolet–visible spectrophotometry (UV–vis) studies Fig. 2 shows the UV–vis spectra of the EB form of PANI and PANI/AC composites. In non-protonating solvents, such as NMP, two absorptions can be distinguished at 330 and 634 nm for the EB form of PANI [28]. The absorption at 330 nm is ascribed to a ␲–␲* transition in the benzenoid ring. The 634 nm absorption describes the benzenoid to quinoid ring excitonic transition. Thus the presence of two peaks in NMP solution of PANI base is indicative of the presence of two types of chemically nonequivalent rings in the polymer chain, namely the benzenoid and the quinoid rings [29]. Fig. 2 indicates that the PANI polymer is in the EB form due to the two absorption bands observed with max at 329 and 633 nm and is consistent with reported observations [30]. From the spectra of the composites, it can be seen that the intensity of ␲–␲* transition peak and excitonic transition peak increased as the percentage amount

Fig. 1. Polymerization of aniline and formation of ES and EB forms (a); and in situ polymerization and proposed composite interaction (b).

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of AC content increased in the composites, compared to that of neat PANI. However, this increase was quite high in the excitonic transition peak and rather low in the ␲–␲* transition peak. This showed that AC particles had interactions with quinoid rings on the polymer and this effects the excitonic transition peak. Also, the ␲–␲* transition peaks of the polymer composites shifted to higher wavelengths as the percentage amount of AC content in the composites increased. These results indicated that AC particles can cause interactions with polymer molecules to result in complexation. Thus these interactions can reduce the band gap of ␲–␲* transition of rings which exist in polymer structure, and electron transitions can occur with lower energy. The excitonic transition peaks of polymer composites slightly shifted to lower wavelengths as the percentage amount of AC content in composites increased and as a result the electron transitions occurred with higher energy. PANI/AC in NMP showed the individual component absorptions; the excitonic peak increased while the ␲–␲* transition peak of the polymer composites slightly decreased. This suggests that there are interactions between PANI and AC, which increase the relative number of quinoid units in the EB form in solution and result in transitions between the rings of the polymer. This indicates that AC particles can interact with polymer molecules to result in complexation. 3.3. Photoluminescence (PL) studies The absorption and photoluminescence of PANI polymer solutions, excited at 278 nm, were studied. The most striking feature was that the PANI polymer gave an intense emission upon irradiation by UV light. The photoluminescence spectrum of PANI in NMP is shown in Fig. 3(a) where maximum luminescent intensity was observed at 337 nm and the full width at half maximum was 69 nm. PANI exhibited a photoluminescence quantum yield of 27% and a long excited-state lifetime of 2.56 ns. The reason for high quantum yield is that the ␲-electron’s extensive delocalization form a large conjugated system in the polymer structure. Also, Fig. 3 shows the comparison of excitation and emission spectra of the EB form of PANI and PANI/AC composites in NMP. The photoluminescence property of the composites in NMP was enhanced with the addition of AC particles after being well sonicated. The composite samples were excited at 278 nm. Neat PANI polymer gave an intense emission with max at 337 nm. However, the composite samples showed more intense emission peaks at max of 340 nm as compared to that of neat PANI, as the percent-

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Fig. 3. Photoluminescent spectra of PANI and PANI/AC composites in the solvent NMP; (a) PANI-Em, (b) PANI/1% AC-Em, (c) PANI/3% AC-Em, (d) PANI/5% AC-Em, (e) PANI/10% AC-Em, (f) PANI/20% AC-Em, (g) PANI/30% AC-Em, (h) PANI-Ex, (i) PANI/1% AC-Ex, (k) PANI/3% AC-Ex, (l) PANI/5% AC-Ex, (m) PANI/10% AC-Ex, (n) PANI/20% AC-Ex, and (o) PANI/30% AC-Ex. Samples were excited at 278 nm.

age amount of AC content in composites increased. The increase of intensity of emission peaks of the composite indicated an improved photoluminescence quantum yield for these composite samples. Also, PANI/AC composites showed three large shoulder peaks at 351, 389 and 415 nm, in addition to the main peak at 340 nm. This suggests that there are strong interactions between PANI and AC particles, and indicate that the AC particles can interact with polymer molecules to result in complexation. The photoluminescence intensities and quantum yields of composites increased with respect to that of PANI polymer upon the formation of the composites. This increase of emission intensities was due to the complexation of the N-atom on the polymer with the AC particles. These polymer complexes make possible the energy transfer from the excited state of the polymer to the AC particles. A decrease in the non-radiated transition of the polymer excited state and an increase in the fluorescence emission was observed. The photoluminescence data for PANI polymer and PANI/AC composites are summarized in Table 1. 3.4. Fourier transform infrared spectroscopy (FTIR) studies Fig. 4 shows the FTIR spectra for dedoped PANI and dedoped in situ prepared PANI/AC composites. The FTIR spectrum of pure AC particles is shown in Fig. 4(a). A characteristic peak was not observed for the AC particles. However, the broad and strong peak at 3471.6 cm−1 is due to adsorbed water on AC particles. The base form of PANI has several important peaks in the FTIR (Fig. 4(b)). The peak at 3380 cm−1 belongs to the N–H stretching in Table 1 Photoluminescence data for PANI polymer and PANI/AC composites.

Fig. 2. UV–vis spectra of PANI and PANI/AC composites; (a) PANI-EB, (b) PANI/1% AC, (c) PANI/3% AC, (d) PANI/5% AC, (e) PANI/10% AC, (f) PANI/20% AC, and (g) PANI/30% AC.

Entity

max Ex (nm)

In Ex

max Em (nm)

In Em

f (%)

 f (ns)

PANI PANI/%1 AC PANI/%3 AC PANI/%5 AC PANI/%10 AC PANI/%20 AC PANI/%30 AC

283 284 284 284 285 285 285

526 571 607 656 719 768 835

337 338 339 339 340 340 340

524 568 603 652 715 765 831

27 29 31 32 35 36 38

2.56 2.74 2.87 3.03 3.23 3.35 3.54

max Ex: maximum excitation wavelength; In Ex: maximum excitation intensity; max Em: maximum emission wavelength; In Em: maximum emission intensity; f : quantum yield;  f : excited-state lifetime.

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Fig. 5. Thermogravimetric curves; (a) PANI-EB, (b) PANI/1% AC, (c) PANI/3% AC, (d) PANI/5% AC, (e) PANI/10% AC, (f) PANI/20% AC, and (g) PANI/30% AC. Fig. 4. FTIR spectra of PANI and PANI/AC composites; (a) AC, (b) PANI-EB, (c) PANI/1% AC, (d) PANI/5% AC, (e) PANI/10% AC, (f) PANI/20% AC, and (g) PANI/30% AC.

the benzenoid–N–H–benzenoid units (B–NH–B). The broad band at 3310 cm−1 belongs to hydrogen bonded N–H, and that at 3170 cm−1 to the terminal quinoid–N–H stretching. The modes at about 1500 and 1600 cm−1 are associated with aromatic ring stretching; the peak at about 1500 cm−1 with the benzenoid ring and the absorption at 1600 cm−1 is the quinoid ring stretching. These spectra exhibit the presence of benzenoid and quinoid ring vibrations at 1500 and 1600 cm−1 , respectively. This is indicative of the oxidation state of EB PANI [31,32]. The very weak and broad band near 3400 cm−1 is assigned to the N–H stretching mode. The strong band at 1150 cm−1 was described by MacDiarmid et al. as the “electroniclike band”. This is considered to be a measure of the degree of delocalization of electrons, and thus it is the characteristic peak for PANI conductivity [31]. Intrinsic PANI also shows three peaks; there is medium absorption at 1300 cm−1 and weak ones at 1380 and 1240 cm−1 . The peak at about 800 cm−1 is from C–H out-ofplane bending modes. The band at about 528 cm−1 is an aromatic ring deformation peak. The FTIR spectra of PANI/AC composites are shown in Fig. 4(c–g). The characteristic peaks of PANI can be seen in all these composite spectra. Also, it can be seen from respective spectra, as the percentage amount of AC content in the composites increased, the intensity of the peaks increased, becoming either more sharp or broad. The most striking feature was that the FTIR spectra of the PANI/AC composites revealed a new peak at 1728 cm−1 . It can be seen from respective spectra, as the percentage amount of AC content in composites increased, the intensity of the peak at 1728 cm−1 increased significantly. This peak may originate from the formation of a new covalent bond between the C atoms of AC particles, and the C atoms of aromatic rings on polymer backbone structure or N atoms on the exterior aromatic rings of the polymer chains. Also, addition of AC particles caused significant reductions in the intensity of the aromatic ring deformation peak at 528 cm−1 . This indicates that the aromatic rings on the polymer structure have become more stable. 3.5. Thermogravimetric analysis The amount of weight loss and the thermal stability of PANI and PANI/AC composites were determined using TGA at a heating rate of 10 ◦ C min−1 , in a nitrogen atmosphere. TGA (%) weight loss data was obtained between 50 and 1000 ◦ C. The TGA traces

of EB and PANI/AC composites are shown in Fig. 5. The onset of the decomposition for thermogram was 425 ◦ C. Most of the weight losses occurred between 465 and 640 ◦ C. Comparison of the PANI and PANI/AC composites showed that PANI was less stable than the PANI/AC composites. As shown in their respective thermogram, 46% weight was observed loss for EB and 47, 48, 50, 53, 59, and 65% weight loss for 1, 3, 5, 10, 20, and 30% respectively, for the AC containing composites. Also, comparison of the PANI/AC composites showed that the thermal stability increased from 1% AC containing composite to 30% AC containing composites, which is from less AC containing to more AC containing composites. The weight losses between 65–215 ◦ C are thought to be due to the presence of unremoved water molecules as moisture in polymers and PANI/AC composites. 3.6. Conductivity measurement The electrical conductivities of neat and composite films were measured using an ASTM F84 model according to the standard fourpoint-probe technique. Throughout all measurements, a current (1–10 ␮A) was applied to two outer probes and the voltage between two inner probes was measured. The films were found to be Ohmic over the region studied (0.0056–0.7225 mV). The resistivity of HCl doped PANI film and PANI/AC (10 wt.% AC) composite film was measured to be 0.299253 and 0.062448  cm; the conductivity for each was calculated to be 3.342 and 16.013 S cm−1 , respectively. The resistivity of the composite films at room temperature decreased compared to that of the PANI films, thus the conductivity for the composite films increased. This enhanced conductivity is perhaps due to the dopant effect and/or impurity contribution and/or charge transfer from the quinoid unit of PANI to the AC particles. PANI can be considered as a fairly good electron donor [33]. The resistivity, conductivity and film thickness of HCl doped PANI film and PANI/AC composite films are summarized in Table 2. The conductivity of PANI/AC composite films is plotted against the increase of percentage amount of AC content and is shown in Fig. 6. The inset plot shows the resistivity of PANI/AC composite films versus the increase of the percentage amount of AC content. As can be seen in Fig. 6, the conductivity of PANI/AC composite films greatly increased with increasing percentage amount of AC content. The conductivity of PANI/AC composite films reached maximum conductivity at 10 wt.% AC content. Percentage amounts AC greater than this caused the conductivity to gradually decline. The reason for this reduction in conductivity of the PANI/AC composite films

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Table 2 Results for conductivity measurements and film thicknesses. Films AC PANI %1 AC %3 AC %5 AC %10 AC %20 AC %30 AC

% AC content

Conductivity (S cm−1 )

Resistivity ( cm)

Film thickness (␮m)

0 1 3 5 10 20 30

0.2510 3.3416 6.6893 10.9793 13.5961 16.0132 15.7763 15.2374

3.98406 0.29925 0.14949 0.09108 0.07355 0.06245 0.06339 0.06563

11 11 11 12 12 13 13

may be due to the large numbers of AC particles, proving to be being more than necessary, or an aggregation of AC particles in composite films or the formation of micro-cracking on the surfaces of the composite films. These aggregates of AC particles in composite films and/or formation of micro-cracking on the surfaces of composite films can be seen in the SEM micrographs. The SEM micrographs show the formation of poor quality microfilm structures. Thus, in order to obtain more quality film structures, the percentage amount of AC content in composites should be maintained at about 10 wt.%. Similarly, the same discussion can be made for the results of resistivity changes as shown in inset plot of Fig. 6. Also, due to the large surface area of AC particles, they may serve as “conducting bridges”, connecting PANI conducting domains and increasing the effective percolation; typical dopants primarily depend on existing connectivity and do little to enhance charge mobility. Further, typical dopants often hinder the processability of conducting polymers, whereas AC at these 10 wt.% loadings appear to impart little detrimental effect, and may in fact enhance the dissolution of EB PANI. 3.7. Morphology of AC particles and PANI and PANI/AC composite films The morphology, porosity and the particle size of the unsonicated and sonicated AC particles were examined. The micrographs showing the morphology of the unsonicated and sonicated AC particles are shown in Fig. 7, where the bar at the bottom of each respective micrograph was used for particle size measurements. The micrographs depicting the unsonicated AC particles revealed that the particles were plate shape, condensed, unhomogeneous, and of a not so well-ordered structure and shape. The micrographs

Fig. 6. Conductivity of PANI and PANI/AC (wt.% 1, 3, 5, 10, 20, 30 AC) composite films. Inset plot is resistivity of PANI and PANI/AC (wt.% 1, 3, 5, 10, 20, 30 AC) composite films.

Fig. 7. SEM of (a) unsonicated and (b) sonicated AC particles.

depicting sonicated AC particles revealed that the particles were not wholly spherical, but closer to a granular shape. The micrographs clearly showed that the particle size of the sonicated AC particles significantly decreased compared to the particle size of the unsonicated AC particles. The morphology of the neat and composite films and the qualitative dispersion of the AC particles in the composites were examined by SEM. The micrographs showing the morphology of neat and composite films are shown in Fig. 8. The micrograph depicting the morphology of the neat PANI film revealed strong, plain, homogeneous and condense film surfaces (not shown). The micrographs for the HCl doped PANI film and the PANI/AC composite film revealed smooth film surfaces. The micrographs of the composite films clearly indicated that the AC particles were well dispersed in the composite films (Fig. 8a–c). This suggests that the interaction between polymer molecules and AC particles overcomes the van der Waals interaction between AC particles, which would otherwise result in AC aggregates. The SEM micrograph of composite films also showed that there were no apparent aggregations of AC particles in the PANI film matrices. This indicates that there are strong interactions between polymer molecules and AC particles. These interactions may be due to charge transfer from the nitrogen atom of the quinoid units of the polymer to AC particles. The micrographs depicting the surface morphology of PANI/1% AC, PANI/10%

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Fig. 8. SEM of (a) PANI/1% AC, (b) PANI/10% AC, (c) PANI/30% AC composite film and (d) cross-section of PANI film.

AC, and PANI/30% AC composite films are shown in Fig. 8a–c. It can be seen from the micrographs that as the percentage amount of AC content in composites increased, the ratio and density of AC particles in composite films increased. Therefore, AC particles were homogeneously well dispersed in the polymer film matrices. The dispersity and density of the particles increased gradually from 1 to 3% AC content in the composite films as can be seen from the SEM micrographs. The conductivity measurement results showed that the conductivity of these films gradually increased up until 10% AC content. Further, when the percentage amount of AC content was 30 wt.%, the density of AC particles in the composites reached a maximum at which saturation occurred. The micrograph for PANI/30% AC composite films showed aggregation of AC particles in various regions of the PANI film matrix. The reduction of conductivities of PANI/20% AC and PANI/30% AC composite films, having greater than 10% AC, may be due to the formation of fractures and/or aggregates of AC particles in the PANI film matrices. The morphology and the cross-section of neat and composite films were examined by SEM. The micrograph depicting the morphology of the cross-section of all the films revealed condense, homogeneous and plain structures. All the thickness’ of prepared films were calculated using the cross-section measurements of neat and composite films from their respective micrographs. These film thickness’ (Table 2) were subsequently used for the conductivity calculations. The micrographs depicting cross-section morphology and higher magnification micrographs depicting cross-section morphology of neat PANI, PANI/1% AC, PANI/3% AC, PANI/5% AC,

PANI/10% AC, PANI/20% AC and PANI/30% AC composite films were examined. The SEM of cross-sections of neat PANI is shown in Fig. 8d; the bar at the bottom of each respective micrograph was used for film thickness’ measurements. 4. Conclusions PANI was synthesized using optimized conditions and PANI/AC composites were prepared by in situ polymerization and ex situ solution mixing. The composite films exhibited an increase in electrical conductivity over neat PANI. The increase of conductivity of PANI/AC composites may be partially due to the doping or impurity effect of AC, where AC particles compete with chloride ions. The conductivity of PANI/AC composite films reached optimal conductivity at 10 wt.% AC content; percentage amounts greater than this led to a decline in conductivity. UV–vis and Infrared spectroscopic studies showed that AC particles affected the quinoid units along the polymer backbone and indicated strong interactions, otherwise known as the doping effect between AC particles and the quinoidal sites of PANI. The photoluminescence properties of PANI and PANI/AC composites were studied where PANI/AC composites showed an increase in intensity. TGA studies showed that the thermal stability increased from PANI to PANI/AC. SEM measurements indicated that AC particles were well dispersed and isolated in the composite films. When high percentage amounts of AC was used poor quality microfilm structures resulted as revealed by SEM micrographs. Thus, in order to obtain better quality film structures,

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