A.C electrical properties of pure and doped polyaniline salt prepared by electro-chemical polymerization.
Polymers are known, in general as insulating materials but the conjugated polymers have interesting electrical properties. PANI is the most attractive conducting polymer because of its low cost, high stability, good electrical conductivity and good for many applications such as molecular electronics. Pure Polyaniline salt, and protonation PANI by HCl were synthesized by electro-chemical oxidative polymerization of aniline with ammonium peroxy disulphate in acidic medium. The solution was prepared in reaction temperature equal 291 K, the acidity of aqueous solution was 1M and the a played voltage was (0.5, 1 , 1.5 and 2) mV . The produced polymer was diagnosed by many ways like Fourier Transformation Infra-Red (FT-IR) and the morphology of prepared PANI was studying by Atomic Force Microscope (AFM), A.C. conductivity was studied in the frequency range (40Hz-15MHz) at room temperature. The shape of grain is circular and uniform, when increasing the applied voltage the grain arranged and the average diameter increase indicate the increase in average diameter with increase [alpha][omega.sup.s] ), the results of conductivity was (applied voltage because of increase polymer chain growth. conductivity changing to the relation [sigma] indicate the increase of conductivity due to the increasing polarons concentration, value of exponent S was found and values was in range (0.0536-0.579),these value indicated hoping process that's mean (polaron hoping between polymer chains). The Cole-Cole diagram was study by imaginary impedance as a function of real impedance from the figure was estimate equivalent circuit for each sample, polarizability and relaxation time. The results indicate to increasing of polarizability with increasing of applied voltage and decreasing in relaxation time with increasing of applied voltage,
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One fundamental property which normally distinguishes polymers from metals is electrical conductivity. The value of electrical conductivity for metals is very high and is generally in the order of [10.sup.4]--[10.sup.6] S cm (good conductors such as copper and silver have conductivities close to [10.sup.6] S cm ) while for polymers which are generally insulators this value does not exceed [10.sup.-14] S [cm.sup.-1] (good insulators such as Teflon and polystyrene have conductivity value close to [10.sup.-18] S [cm.sup.-1]) . Though the low electrical conductivity of polymers has found its immense use in the manufacture of insulators and dielectric substances. During the last two decades, the researchers, through the simple modification of ordinary organic conjugated polymers, have succeeded in preparing polymers with high electrical conductivity. Called electrically conducting polymers or synthetic metals .Polyaniline (PANI) has been known for more than one hundred years in its 'aniline black' form, an undesirable black deposit formed on the anode during electrolysis involving aniline. Among the conducting polymers, polyaniline (PANI) is the most promising polymer due to its simple synthesis, controllable electrical conductivity, and good environmental stability. PANI is a typical phenylene-base polymer having a chemically flexible--NH--group in the polymer chain flanked on either side by a phenylene ring. The protonation and deprotonation and various other physico-chemical properties of PANI can be traced to the presence of the--NH--group . It is well known that PANI exists in three different oxidation states (leucoemeraldine, emeraldine, and pernigraniline); only polyemeraldine is electrically conductive. The electronic transport properties of PANI can be changed by doping. 
The aspect which attracted our interest is the protonated structure of polyaniline in the acidic media (Fig.1) Polyaniline in its sulfonated form shows a close structure with the ion conducting polymers, where the inorganic anions are represented by HS[O.sub.4.sup.-] species.
In the emeraldine salt (ES), the HS[O.sub.4.sup.-]- species are ionically bonded with the -NH groups presented in the polymer chain. Actually, these kinds of bonds are very weak and can be removed very easy by changing the system's pH. By supplementary doping of polyaniline in sulfuric acid media, actually we introduced multiple chargecarriers which determine an improvement of the electrical conduction along the polymer chain. Moreover,it was assume that the total conduction in SPANI is given by summing the electrical conduction and ionicconduction that occurs by sulfonic groups on the aromatic rings and transition of electrons as shown in Fig2.
A number of studies have been reported on the electrical properties of conjugated polymer specially PANI, as well as polyaniline /ZnO nano composite . And polyaniline /ferrofluid nanocomposites. or polypyrrole /TiO2 nano composite. The electrical properties of these samples are sensitive to preparation condition, particle size, inter--particle interaction and temperature.
Generally, experimental set-up for electrochemical synthesis of electro conducting polymers in laboratory conditions is simple. It involves, in majority of cases, standard three-electrode electrochemical cell, although in some cases of galvano static polymerization, two electrode cell can be used . The polymer obtained by this procedure is deposited directly on the electrode. Novel experimental set-up, enabling electrochemical generation of polyaniline colloids, using flow-through electrochemical cell, was also reported . In this paper electrochemical cell, anode was separated from two cathodes by ion exchange membrane the anodic electrode was Graphite rod and the two cathodic electrodes was plates of stainless steel. The anodic and cathodic electrolytes were passed through electrode compartments at specified flow, while polymerization was achieved at constant potential as shown in Fig. 3.
Preparation of PANI:
Pure polyaniline was prepared by dissolving of (2.59 g) of aniline hydrochloride in (50 ml) of distilled water and mixed with (5.71 g) from ammonium peroxy disulfate that dissolved in (50 ml) of distilled water in the Container of electrochemical cell at room temperature and the doped sample was prepared in the same way but both of aniline hydrochloride and ammonium peroxy disulfate was dissolved in (1 M) of [H.sub.2]S[O.sub.4] for protonation PANI by [H.sub.2]S[O.sub.4]and dissolved in (1 M) of HCl for protonation PANI by HCl and then constant voltage was applied for a certain sample for two hours. The same process was repeated for (0.5, 1, 1.5, and 2) volt. The deposited Polyaniline has been collected from stainless steel electrode and washed with distilled water. Polyaniline (emeraldine) hydrochloride powder is dried in air for about one hour then in vacuum oven about (80 [degrees]C) for (6 hours) the average yield is (2.29g).
The A.C electrical measurements are used to investigate polyaniline samples doped and pure during polymerization with various voltages. The polyaniline powder was thoroughly grounded in a mortar to obtain very fine particles, and then it was compressed under a pressure of (3 tone) in the form of a pellet. The resulting pellet has a diameter of (10 mm and thickness of 1.68 -1.98mm). To improve the electrical contact the faces of the pellet were coated with aluminum by thermal evaporation. LCR-Meter was used for the A.C measurements. The sample was placed in a holder specially designed to minimize stray capacitance. The frequency range was (40Hz-15MHz) for electric field.The total conductivity [sigma.sub.tot] at a certain frequency and temperature is defined as
[sigma.sub.tot]= [sigma.sub.ac]([omega]) + [sigma.sub.dc] (1)
[sigma.sub.a.c] is the A.C. conductivity,[sigma.sub.d.c] is the D.C. conductivity, then the empirical relation for the frequency dependence A.C conductivity is given by :
[sigma.sub.a.c] ([omega]) = [A.sub.1] [omega.sub.s] (2)
[A.sub.1] is constant parameter, and s is an exponential factor. Its value is 0<s<1 
The exponent (s) is a function of frequency and is determined from the slope of a plot ln [sigma.sub.a.c] ([omega]) versus ln ([omega]) then,
S= Ln[sigma.sub.a.c] ([omega]) / Ln ([omega]) (3)
And the dielectric constant, [epsilon.sub.1], is calculated from the equation:
Where [epsilon.sub.o] is the permittivity of free space, L is the thickness, C is the capacitance and A is the cross section area. The dielectric loss, [epsilon.sub.2], is calculated from the equation:
[epsilon.sub.2] = [epsilon.sub.1] tan[delta] (5)
Where [delta] =90--[phi], [phi] is the phase different angle.
The Cole-Cole impedance model comprises three hypothetical circuit elements: a low-frequency resistor [R.sub.0], a high-frequency resistor [R.sub.1] and a constant phase element (CPE),The CPE is also known as the fractionalcapacitor , and its impedance is [Z.SUB.CPE] = 1/ (j[omega]C)[.sup.alpha] where C is the capacitance and a is its order (0 < a [less than or equal to] 1). The Cole-Cole impedance is given by:
Z= [R.sub.infinity] +([R.sub.o]- [R.sub.infinity/1] +(j[omega]C)[.sup.alpha]) = Z'+jZ' (6)
RESULTS AND DISCUSSIONS
The Surface Morphology AFM studies:
Atomic Force Microscopy (AFM) or Surface Morphology Analysis has been used to characterize the external Surface roughness which has been regarded as one of the most important surface properties. The results of the Surface Morphology Analysis of pure PANI are shown in 3D view in Fig.4. It reveals the formation of conducting PANI nanostructures distributed almost uniformly and the diameter of spherical PANI nanoparticles was estimated to be in the range of (50-150) nm. The micrograph also some micro or Nano tubes made up from aggregates of many PANI nanoparticles by increasing applied voltage. The average diameter for PANI nanoparticles is about 101.07 nm and roughness surface about 2.56 nm.
AFM analysis for protonation PANI by HCl at different applied Voltage as shown in Figures 5,6 and 7. The surface of 0.5V sample consisted of small cluster with average diameter in the range of 79.46 nm, and the average roughness of the surface was 1.66 nm and diameter would increase with increasing of applied voltage and roughness decrease with increasing of applied voltage 6.98 nm for 1V and 2.2 nm for 2V as shown in figures 5 and 6. By increasing of applied voltage the particles of PANI arranged and consist tube i.e the Figures shows micro or nanotube of arranged nanoparticles of PANI. In the general the protonation samples has average diameter more than the pure PANI.
2. Fourier Transform Infrared (FT-IR) Study:
FTIR spectroscopy is a highly useful measurement for studying the functional groups of the structure. Fig.8shows FTIR of pure PANI.
The characteristic peaks of PANI from the Fig.8 were observed at 476.374 [cm.sup.-1] ( naphthalenes Out of plane ring bending), 800 [cm.sup.-1] (N-H out of plane bending absorption), 970 [cm.sup.-1] (C=C bending), 1150 [cm.sup.-1] (C-H bending mode of Q ring, 1296 [cm.sup.-1]C-N stretching of secondary aromatic amine),1494 [cm.sup.-1](C=C stretching/C=N asymmetric stretching/C-H bending modes of benzenoid ring), 1560 [cm.sup.-1] (C=C stretching/C-H bending modes of quinoid ring)[15,16].
In protonation PANI by HCl, the same characteristic peaks are observed, but plane ring deformation (1020) [cm.sup.-1] was not observed at (1V protonation PANI by HCl) as shown in Fig. 9 b
From figures 8 and 9c we can see that the quinoid peaks was shifted to higher wave number (1560.54and 1566.09) [cm.sup.-1] respectively and benzenoid peaks was shifted to lower wave number (1494.14, and 1471.59) [cm.sup.-1] respectively.
3. A.C. conductivity measurements:
The A.C. conductivity ([sigma]A.C. ([omega])) was measured for pure and protonationPANI by HCl to study the difference in the mechanism of conductivity. The frequency depended of the measured total conductivity [sigma.sub.t] at different applied voltage. Fig.10shows the A.C conductivity HCl protonation at different voltage. The behavior of [sigma.sub.t].([omega]) with the frequency can be explained in terms of polarization effect and hopping i.e., polarization effect in low frequency region is slightly changed and [sigma.sub.D.C.] is dominating, but at higher frequencies the polarization is increased gradually, and the conductivity here can be related to the hopping of the charge carrier over a small barrier height. The conductivity in PANI increases because of contribution of polaron, which is moving along smaller and smaller distances in polymer chain. With increasing the applied voltage the behavior was not systematic, [sigma.sub.ac],is obtained by subtracting the d.c conductivity, from the measured total conductivity according to Eq.2.The variation of conductivity with frequency revels that the charge transport mechanism can be considered to be semiconductor. 
As shown in fig.10 that the transition frequency ([omega.sub.c]) shifted to lower frequencies with increasing of applied voltage.
The frequency exponent s, can be calculated from the slope of the straight lines in fig.11, the value of exponent s is listed in table 2.
From table 2, the exponent s was found to be increase with increasing of applied voltage except the 1V sample.Fig.12 shows total conductivity of pure and protonation PANI by HCl at 2V it's clear from this figure that the conductivity of protonation PANI by HCl is higher than that of pure
Also the frequency exponent s for pure PANI, can be calculated from the slope of the straight lines in Fig.13, the value of exponent s for pure PANI was 0.1423
4. Cole-Cole diagram:
The oxidation structures of PANI were strongly dependent on the applied voltage and dopant type, so the impedance curves of the samples were measured at different applied voltage and different dopant type. The Cole-Cole plots obtained at 2V for pure PANI and doped samples at voltages of (0.5, 1and 2) V, are given in Figures (14. a-c) and (15). All impedance curves had a semicircle in the high-frequency region; this can be described by the double layer capacitance in parallel with the ionic charge-transfer resistance (Rct). The high-frequency intercept of the semi-circle on the real axis yielded the ohmic resistance (Rs).Polarizability ([alpha]) is a measure of the distribution of relaxation times. The parameter a can be determined from the location of the center of the Cole-Col circles, of which only an arc lies above Z-axis. Figures (14.a-c) and (15) depicts a Cole-Cole plot between Z' and Z for pure and protonation PANI.It's evident from this plot that relaxation process differs from the monodispersive Debye process (for which [alpha]=0). The parameter [alpha], was determined from the angle subtended by the radius of the circle with the Z-axis passing through the origin of Z'-axis is listed in table 3 the value of a between 0 and 1, describes the broad distribution of relaxation times in system .The Cole-Cole plot confirms the effect of applied voltage, As the results show that the polarizability increases with increasing of applied voltage.Relaxation time [TAU] also was calculated from top of semicircle and listed in table 3.
AC equivalent circuits for pure and protonation PANI, which represent the effect of applied voltage and type of dopant on the electrical conductivity and polarization of the material have been proposed.In general, for a given data set there exists more than one equivalent circuit which gives a reasonable fitting. The choice between these ones has to be based both in simplicity and consistency with the known physical and chemical processes which take place in the system . The best fittings were reached after several attempts. All the complex impedance plots obtained, Cole-Cole, were modeled, obeying the above criteria, by the equivalent circuit shown in Figures 16 and 17.
From FTIR spectroscopes, HCl are reacted physically with PANI causing a shift in the quinoid ring and benzoid ring peaks and the increasing in applied voltage cause increasing in the intensity.The AFM images detection shows the particles shape and size, (average diameter for pure PANI was 101.07 nm, and for protonation PANI by HCl was 116.23 nm) at 2 V. The average diameter increase with increasing of applied voltage, this increasing because of chain growth. From the measurement of A.C. conductivity. The increase applied voltage indicating dominates hopping process. The conductivity increased three orders with increasing of applied voltage, The charge transport mechanism can be considered to be semiconductor with effect of applied voltage and show aohmic behavior. The frequency exponent, S, was less than unity. From Cole-Cole diagram; polarizability and relaxation time were calculated the results show that the polarizability increase with increase applied voltage and relaxation time decrease with increase applied voltage. From semi-circle of Cole-Cole diagram equivalent circuits was estimated.
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Table 1: General list of FTIR band positions and proposed assignments for pure PANI and protonation PANI Samples Pure 0.5V PANI-HCl 1V PANI-HCl 2V PANI-HCl 1560.54 1564.16 1579.59 1566.09 Band range 1494.14 1485.09 1485.09 1471.59 ([cm.sup.-1]) 1296.1 1296.08 1303.79 1301.86 1150 ____ ____ ____ ____ 1105.14 1120.56 1114.78 ____ ____ ____ ____ 970 1029.92 ____ 1035.7 800 885.508 877.55 881.41 467.374 468.67 455.17 433.95 Assignments C=C stretching/C-H bending modes of quinoid ring Band range C=C stretching/ C=N asymmetric stretching/C-H ([cm.sup.-1]) bending modes of benzenoid ring C-N stretching of secondary aromatic amine C=N Imines bending C-H bending mode of Q ring S-O bond stretching of S[O.sub.3.sup.-] C-H and N-H in-plane ring deformation C-H out-of-plane ring deformation vibration naphthalenes Out of plane ring bending Table 2: list of the value S for pure and protonation PANI. Samples (S )exponent factor Pure PANI 0.1423 0.5V Protonation PANI by HCl 0.3247 1V Protonation PANI by HCl 0.0536 1.5 V Protonation PANI by HCl 0.428 2V Protonation PANI by HCl 0.579 Table 3: polarizability and Relaxation time data for doped samples. Sample polarizability Relaxation time [mu] Sec Pure 0.175 0.338908 0.5 V HCl 0.188 0.0546342 1 V HCl 0.211 0.0431128 2 V HCl 0.2472 0.0150621
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|Author:||Obaid, Mustafa N.; Hassan, Salma M.|
|Publication:||Advances in Environmental Biology|
|Date:||Feb 1, 2017|
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