Polyaniline was prepared by using extra pure and AR grade reagents aniline (99.9%), ammonium persulphate (99.9%), and hydrochloric acid (99.9%) using in situ polymerization method.
Synthesis
Synthesis of PANI and PANI– Tungston Oxide (WO3) composites was carried out by polymerization in-situ.
Preparation Of Pani
Polyaniline was synthesized by using extra pure and AR grade reagents aniline (99.9%), ammonium persulphate (99.9%), and hydrochloric acid (99.9%) using oxidative polymerization method. Ammonium persulphate (0.2 M) was added drop wise to a stirred solution to prevent warming of the aniline (0.1 M) solution dissolved in 1 M of an aqueous solution of hydrochloric acid (1 N, aniline hydrochloride) at a temperature of 5°C. Following this addition, stirring was continued for 2 hrs using a magnetic stirrer to confirm completion of the reaction. The time of the initial coloration of mixing the reactants depends upon the temperature and concentration of the portico acid. During the polymerization reaction, HCl was used as a protic acid and the temperature was maintained at lower temperature by using a freezing mixture. The end product was a green-colored precipitate (emarldine salt). This precipitate was filtered by using Buckner funnel and vacuum pump, washed with deionized water, with acetone in order to remove the oligomers and excess ammonium persulphate, and with 1 N HCl solution to remove the Cl− ions and unreacted aniline until clear filtrate. Finally, the precipitate was dried in hot air oven for 24 hrs at a temperature of room temperature to achieve a constant mass.
Preparation Of Pani–tungsten Oxide (Wo) Composites
Aniline (0.2 M) was dissolved in 1 M HCl and stirred for 2 hrs to form aniline hydrochloride. WO3 was added in the mass fraction to the above solution with vigorous stirring in order to keep the WO3 homogeneously suspended in the solution. To this mixture,1M of ammonium persulphate, which acts as an oxidant was slowly added drop-wise with continuous stirring at room temperature for 8 hrs to completely polymerize the monomer aniline. The precipitate was filtered, washed with demonized water and finally dried in a hot air oven for 24 hrs to achieve a constant mass. In this way, PANI–WO3 composites containing various mass fractions of WO3 (5% and 45%) in PANI were synthesized.
Characterization
The morphology and the structures of synthesized PANI and PANI–WO3 composites were studied using X-ray diffraction (XRD) patterns of the powders were taken using Rigaku- Ultima IV, Japan diffractometer with Cu Kα radiation (λ = 1.54 A˚). The DC conductivity, AC conductivity and dielectric properties of PANI and all the composites with varying WO3 concentration were measured by standard four probe method. Ray Diffraction (XRD) Analysis The X-ray diffraction shape of the pure polyaniline is depicted in the Fig. 2(a). The broad peak is observed at 2θ = 25.630, which clearly indicates complete amorphous nature which stimulates high mobility of the ions within the substance [13]. Figure 2(b-f) indicates the X-ray diffraction configuration of all composites with the varying WO3 concentration. Almost all sharp and large peaks show the presence of WO3 particles in the polyaniline-WO3 composite. The intense sharp peaks seen at 2θ = 23°, 24.1°, 33°, 50° and 56° to (0 0 2), (2 0 0), (2 0 2), (1 2 0), (2 1 2) planes which shows the existence of WO3 particles which are having both monoclinic and orthorhombic phase. All the peaks seen at these 2θ angles were in line with the JCPDS No. 00-024-0747 and also 01-071-0131 which corresponds to monoclinic and orthorhombic phase of WO3. However due to overlapping of peaks of polyaniline with that of WO3 particles is the reason why we are unable to see any peaks corresponding to polyaniline. Here the sharp diffractions peak of all composites indicates strong intensity of crystalline structure of the composites [14].
Dc Conductivity Studies
Figure 3 shows the conductivity of PANI and poyaniline-WO3 composites with varying WO3 content from 5–45%. Here the conductivity was evaluated by applying a four point probe method in the temperature extending from 30–200°C. It can be insured that with rise in concentration of WO3, the conductivity is found to increase. Among all composites, the one with 45% WO3 content was found to have highest conductivity of close to 0.12 S/cm while least conductivity value of 0.06 S/cm was noted in composites with 5% WO3 content. The rise in conductivity is generally attributed to elongated chain length of polyaniline which help in hopping of charge carriers. But unusual behaviour in conductivity was examined when the temperature was extended from 30 to 200°C. It was viewed that there were two temperature ranges, were the conductivity was noticed to rise with the rise in temperature and these were 70 to 110°C and 160 to 200°C. So the highest conductivity was seen at 200°C for all composites with different WO3 concentration. As said earlier with the increase in WO3 substance and rise in temperature the efficiency of charge communication among the polyaniline and WO3 increases [15–16]
Ac Conductivity Studies
Figure 4 analyzes the frequency related AC conductivity of the pure PANI and PANI/ WO3 combination at normal temperature. It is detected that the conductivity of the PANI and compounds are growing as frequency is enlarged in single phase and obtaining nearly constant in higher frequency area i.e, from above 105Hz, the conductivity of PANI and compounds increases by maintaining almost constant value. Doped PANI experiences two types of charged system, one polaron / bipolaron system, which is free to transfer along the chain; the others are attached charges which have restricted mobility. It is noticed from the figure that, as frequency increases, the conductivity increases because of the movement of polarons along shorter displacements in the polymer chain. It is also noticed that there is an increase in the A.C conductivity for PANI/ WO3 (15wt%) when compared with PANI/ WO3 (25 and 35wt%). This is usually due to the increase of isolated polarons and bipolarons and may be due to interfacial polarization. Decrease in conductivity for PANI/ WO3 (25 and 35wt%) when compared to PANI/WO3 (15wt%) might be due to serious pinning of polarons thus restricting their hopping hence reducing their conductivity. Similar observation was observed by L.N.Shubha for PANI/TiO2 [17]
Dielectric Constant
Figure 5 shows the variation of real part of dielectric constant (ε’) with frequency (f) for pure PANI and PANI/WO3 composites. For pure PANI the real dielectric constant has a value of about 4.942⋅102 at 3.2 MHz, which decreases with frequency, reaching a value of 0.409×102 at 5.6 MHz. Such values of real permittivity are related to effects of space charge polarization and electrode polarization. PANI is a semiconducting system with mobile polaron/bipolaron, upon increase in the frequency of the supplied field; the dipoles present in the organization couldn’t reorient themselves quickly in response to the supplied field reducing the dielectric constant [18]. Dielectric constants of PANI/ WO3 composites are dependent on composition, protonation, temperature and delocalization length. Dependence of ε′ with frequency of the compounds can be classified into three stages. In the first stage, ε′ increases with the increase of frequency within the range of 104 to 2.43X106 Hz. In the second stage, from 2.43X106 to 3.57X106 Hz, ε′ is greatly lesser value for the pure PANI (4.942 ×102) and much high measure for 45 wt% WO3 in PANI composite (4.191X103), which attributes to the stronger localization of charge carriers. In the third stage, the decrease of ε′ above 3.57X106 Hz frequency might be assigned to the electrical relaxation processes, i.e., the momentary delay in the dielectric constant of a substance with change in the electric field [19–21].