J. Phys. Chem. B 2010, 114, 16687–16693
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Influence of Potential and Temperature on the ESR Spectra of Polyaniline Synthesized Using the Interface Polymerization Fengmin Zhang†,‡ and Shaolin Mu*,† Department of Chemistry, Yangzhou UniVersity, Yangzhou 225002, Jiangsu ProVince, China and Laboratory Center of Yangzhou UniVersity, Yangzhou 225002, Jiangsu ProVince, China ReceiVed: July 28, 2010; ReVised Manuscript ReceiVed: October 28, 2010
In situ ESR-electrochemical measurements indicate that the distinct redox properties of polyaniline synthesized using the interface polymerization method (labeled IP-polyaniline) are strongly related to its unpaired spin density. IP-polyaniline in a 1.0 M NaCl solution of pH 5.5 still holds stronger ESR signals at a wide potential range, which results in its high redox activity in this solution. The influence of pH on the potential range for the formation of polaron is detected. Also, some unusual phenomena are observed in the measurements of ESR signal intensity as a function of applied potential, for example, the ESR signal intensity of IP-polyaniline in 0.20 M HCl solution decreases with increasing potential from 0.30 to 0.80 V accompanied with the peakto-peak line width ∆Hpp of the ESR signal increasing from 0.30 to 0.60 V, and then, however, ∆Hpp decreases pronouncedly as the potential increases further. The results from measurements for the ESR susceptibility of IP-polyaniline as a function of temperature demonstrate the presence of the conversion of the temperaturedependent Curie susceptibility to the temperature-independent Pauli susceptibility at the temperature range 135-335 K; however, the ESR susceptibility of IP-polyaniline increases again from 335 to 375 K. The ∆Hpp value increases very obviously from 135 to 195 K and then decreases with increasing temperature up to 375 K. 1. Introduction Free radicals exist in polyaniline, which give it novel properties different from general organic polymers. Polyaniline can be oxidized from lower oxidation levels to higher oxidation levers and vice versa for the reduction process. From the point of view of electrochemistry, the unpaired spin density of polyaniline mainly depends on its oxidation levels, which can be carried out by using applied potentials. The relationship between the unpaired spin density and the applied potentials for conventional polyaniline prepared using electrochemical polymerization has been investigated using the in situ electron spin resonance (ESR)-electrochemical technique.1-8 The unpaired spin density is closely related to the peak-to-peak line width ∆Hpp of the ESR signal. This is because if the ESR line width is too wide, the ESR signal cannot be detected, indicating the disappearance of ESR signal, i.e., no unpaired spins exist in the measured species. From the point of view of protonation, polyaniline can be protonated and deprotonated via altering the pH value of the solution. A high proton activity diminishes the tendency to lose protons from cation radicals, which thus may have reasonable stability under strongly acidic conditions. The free radicals existing in polyaniline carry positive charges, i.e., cation radicals. Therefore, the unpaired spin density of polyaniline at a given potential is also dependent on pH values. From the point of view of physics, the unpaired spin density of polyaniline can be changed by temperature, which is mainly used to investigate its conduction mechanisms.9-23 The change in the ESR signal intensity with temperature can provide a * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemistry, Yangzhou University. ‡ Laboratory Center of Yangzhou University.
wealth of information on the peak-to-peak line width ∆Hpp, the unpaired spin density, and the ESR susceptibility. The conventional polyanilines synthesized using chemical and electrochemical oxidative polymerizations of aniline are essentially electrochemically inactive at pH 6.24,25 Recently, we found that polyaniline synthesized using the solid-liquid interface chemical oxidative polymerization of aniline has a distinct redox activity in a wide pH range from pH < 1 to pH 9.0 and wider potential range; in addition, the pH dependence of the conductivity of polyaniline synthesized using the interface polymerization method (labeled IP-polyaniline) is also improved.26 This indicates that polyaniline synthesized in this manner has a better resistance to deprotonation. Both the protonation and the deprotonation of polyaniline are related to the unpaired spin density and line width ∆Hpp. In such a case, we used the in situ ESR-electrochemical technique to study the influence of pH and applied potential on the ESR signal of IP-polyaniline, and also we determined the relationship between the ESR susceptibility and temperature of IP-polyaniline. The purpose for this study is to gain a better understanding of the influence of the unpaired spin density on the electrochemical activity and conduction mechanism of polyaniline. 2. Experimental Section The chemicals used were of analytical regent grade. Aniline was distilled under reduced pressure before use. Doubly distilled water was used to prepare solutions. The pH values of the solutions were determined with a PXD-12 pH meter. The electrochemical experiments were performed on a CHI 407 electrochemical workstation. A conventional three-electrode system with a platinum foil counter electrode was used to determine cyclic voltammograms of polyaniline. The scan rate was set at 60 mV s-1.
10.1021/jp107041m 2010 American Chemical Society Published on Web 12/01/2010
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SCHEME 1: Schematic Diagram for the in Situ ESR-Electrochemical Measurement
Figure 1. Cyclic voltammograms of IP-polyaniline in (1) 0.20 M HCl solution and (2) 1.0 M NaCl solution of pH 5.5 at a scan rate of 60 mV s-1.
Polyaniline was synthesized via solid-liquid interface polymerization,26 i.e., a given amount of the solid oxidant, ammonium peroxydisulfate, was very quickly added into a solution containing 0.20 M aniline and 1.0 M HCl. The resulting solution approximately consisted of 0.20 M aniline, 1.0 M HCl, and 0.25 M (NH4)2S2O8. Before reaction, the aniline solution was cooled with an ice bath and stirred with a stirring magnetic bar, a platinum wire or a platinum foil was immersed into the aniline solution, and then the solid oxidant was quickly added into the stirred aniline solution.26 After addition of the solid oxidant, chemical polymerization of aniline started to take place at the surface of the solid oxidant particles. For simplicity, this reaction is called the interface polymerization. The reaction time was controlled for 2 h. During the chemical polymerization process, part of polyaniline deposited on a platinum wire and a platinum foil that were used as working electrodes after rinsing with 0.05 M HCl solution and then distilled water, whereas most of the product precipitated in the bottom of a beaker was washed with 0.05 M HCl solution until the filtrate was colorless and then dried at 104 °C. The ESR measurements were carried out using a Bruker A300 spectrometer operating in X-band (9.862 GHz). To determine the change in the ESR signal of IP-polyaniline, its potential must be controlled using a potentiostat. Therefore, an in situ ESR-electrochemical technique was used here (Scheme 1). A cell for the in situ ESR-electrochemical experiment consisted of a platinum wire electrode with IP-polyaniline, a platinum wire counter electrode, and a reference electrode of Ag/AgCl with saturated KCl solution, which were inserted into a glass capillary to construct the cell. The microwave power for the in situ ESR-electrochemical measurements of IP-polyaniline was set at 2.0 mW for 0.20 M HCl solution and 20.1 mW for 1.0 M NaCl solution of pH 5.5; the changes in the ESR signal intensity of IP-polyaniline with potential were determined between -0.10 and 0.80 V with an interval of 0.10 V between two measurements. The potential was maintained at each step for 5 min until spectral changes ceased. IP-Polyaniline powder of 10.0 mg was used for measurements of the temperature dependence of the ESR signal intensity. The microwave power for this experiment was as low as 2 × 10-3 mW in order to avoid saturation of ESR signals. The temperature was controlled by flowing cold nitrogen. The changes in the ESR signal intensity of IP-polyaniline with temperature were determined between 135 and 375 K with an interval of 20 K. The time at each measuring point was maintained for 5 min. The modulation amplitude was set at 1.0
G. The Bruker Co. provides a g-factor marker with S3/2. Its g factor is 1.9800 ( 0.0006. 3. Results and Discussion 3.1. Cyclic Voltammograms of IP-Polyaniline. Both pH and potential can affect the redox activity of polyaniline, which is related to the unpaired spin density of polyaniline. To gain a better understanding of the influence of pH and potential on the unpaired spin density of IP-polyaniline, the redox activity of IP-polyaniline in solutions with different pH values was characterized using cyclic voltammetry. Curve 1 in Figure 1 shows the cyclic voltammogram of IP-polyaniline in 0.20 M HCl in which there are two pairs of redox peaks. This electrochemical behavior is the same as that of the conventional polyaniline in 1 M HCl solution.24 The first oxidation peak occurs at 0.23 V (vs SCE), and its corresponding reduction peak is at -0.05 V (vs SCE). The oxidation peak at 0.23 V is attributed to oxidation of the fully reduced polyaniline (leucoemeraldine) to the cation radicals (emeraldine salt)27
In this reaction there is no exchange of protons but accompanied by the exchange of anions between polyaniline and the solution.28 It is obvious that this electrochemical reaction is a free radical reaction, which results in formation of the cation radicals, i.e., polarons. The second oxidation peak at 0.73 V is attributed to oxidation of the cation radicals (emeraldine salt) to pernigraniline, accompanied by exchange of protons between polyaniline and the solution.24,28
This reaction makes the cation radicals turn gradually into spinless dications (bipolarons) as the potential increases from 0.23 to 0.73 V. The above electrochemical reactions indicate that formation of the free cation radicals, i.e., polarons, is caused
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Figure 2. Influence of applied potentials on the ESR signal of IP-polyaniline in 0.20 M HCl solution, curves: (1) -0.10, (2) 0.00, (3) 0.10, (4) 0.20, (5) 0.30, (6) 0.40, (7) 0.50, (8) 0.60, (9) 0.70, and (10) 0.80 V.
by the first oxidation step of IP-polyaniline at a less positive potential range and formation of the spinless dications, i.e., bipolarons, is caused by the second oxidation step at more positive potentials. The above electrochemical reactions are reversible. The reason for the interconversion between polaron and bipolaron in the above potential range will be examined using in situ ESR-electrochemical measurements. As the applied potential is continuously raised, the dications can be oxidized to form the completely oxidized polymer,29 accompanied by loss of two protons.
Curve 2 in Figure 1 shows the cyclic voltammogram of IPpolyaniline in 1.0 M NaCl solution of pH 5.5. There are still two pairs of redox peaks on the cyclic voltammogram, indicating that IP-polyaniline still holds a good redox activity at pH 5.5, which is much better than the conventional polyaniline.24 This result indicates that IP-polyaniline has a better resistance to deprotonation compared to the conventional polyaniline, which would be related to the structure of IP-polyaniline. It was found that the 1H NMR spectrum of IP-polyaniline is different from that of the conventional polyaniline,26 which indicates that there is a structural difference between IP-polyaniline and conventional polyaniline. However, we do not have enough experimental evidence to support establishment of a new structure of IP-polyaniline. Figure 1 demonstrates that the redox activity of IP-polyaniline at pH 5.5 is lower than that in 0.20 M HCl solution, indicating that the redox activity of IP-polyaniline is affected by the proton concentration. 3.2. Influence of Applied Potential and pH on ESR Signals of IP-Polyaniline. Figure 2 shows the change in the ESR signal intensity of IP-polyaniline in 0.20 M HCl solution
with the applied potential. The ESR signal intensity is proportional to the unpaired spin density because the weight and volume of IP-polyaniline on the platinum wire electrode were kept unchanged in the whole experiment. Each ESR spectrum consists of a symmetric signal. As can be seen in Figure 2, the ESR signal intensity increases with an increasing potential from -0.10 to 0.30 V and then decreases as the potential increases until 0.80 V (vs Ag/AgCl with a saturated KCl solution). The former is caused by an increase in the unpaired spin density due to the increase in the oxidation level. This is because the first oxidation peak of IP-polyaniline appears at 0.23 V (vs SCE) on curve 1 in Figure 1, which is near 0.30 V (vs Ag/AgCl with saturated KCl solution) in Figure 2, at which the maximum ESR signal intensity appears. Clearly, leucoemeraldine is oxidized to form radical cations in this potential range as explained above. The potential for the appearance of the maximum ESR signal in Figure 2 is more positive than that of conventional polyaniline in acidic solutions by 0.10 V.1,30 This potential difference demonstrates that the potential range of IP-polyaniline for the formation of polaron is wider than that of the conventional polyaniline in acidic solutions.1,30 A possible reason for this is that IP-polyaniline is more resistant to biopolaron formation in 0.20 M HCl solution during the oxidation process compared with the conventional polyaniline, which allows the presence of the radical cations of IP-polyaniline in a wider potential range. As the potential moves from 0.30 to 0.80 V, the ESR signal intensity decreases with increasing potential because the polymer is oxidized further to form gradually the spinless bipolarons. The change in the ESR signal intensity with the applied potential in Figure 2 is obtained based on the peak heights of the ESR signals at different potentials. However, the width of the ESR signal would be changed by the applied potential, which will affect the peak height of the ESR signal. Therefore, the ESR susceptibility is generally used as a measurement of the
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Figure 3. (A) Potential dependence of the relative ESR susceptibility χV/χ0.8V of IP-polyaniline in 0.2 M HCl solution. (B) Plot of the peak-to-peak line width of IP-polyaniline as a function of potential in 0.20 M HCl solution.
ESR intensity. The ESR susceptibility is measured by doubly integrating the ESR signal, which is also proportional to the unpaired spin density for a given weight and volume of IPpolyaniline in our case. Figure 3A shows the potential dependence of the relative ESR susceptibility χV/χ0.8V of IP-polyaniline in 0.20 M HCl solution. It is clear that there is a small difference between Figure 2 and Figure 3A because of the strong ESR signal and narrow line width ∆Hpp of IP-polyaniline in 0.20 M HCl solution, which make the measuring errors between the height of the ESR peak and the area of the doubly integrating ESR signal small. Figure 3B shows the ∆Hpp value of IP-polyaniline in 0.20 M HCl solution as a function of the applied potential. Clearly, the ∆Hpp value decreases with increasing potential from -0.10 to 0.30 V that is caused by an increase in the ESR signal intensity and then increases from 0.30 to 0.60 V due to a decrease in the ESR signal intensity shown in Figure 3A. This experimental phenomenon is generally observed in conventional polyaniline,18,30 self-doped polyaniline,31 and poly(3-methylthiophene).11 It is well known that an infinitely wide peak-to-peak line width ∆Hpp of the ESR signal demonstrates no unpaired spins in the measured species. Therefore, the ∆Hpp value generally decreases with increasing unpaired spin density. As discussed above, in this experiment the ESR signal intensity of IP-polyaniline is proportional to the unpaired spin density. Therefore, the ∆Hpp value of IP-polyaniline decreases with increasing ESR intensity and vice versa. As the applied potential increases from 0.60 to 0.80 V, the ∆Hpp value decreases again. As can be seen from curve 1 in Figure 1, the second oxidation peak occurs at 0.73 V and its corresponding reduction peak is at 0.55 V (vs SCE), indicating that IP-polyaniline is oxidized further from 0.55 to 0.73 V, which is accompanied with a pronounced proton loss in IP-polyaniline. This would be a reason for conversion of Curie spin behavior into Pauli spin behavior for conventional polyaniline in the acid solution at the high doping level,17 which is caused by high oxidation potentials. The higher Pauli spin susceptibility in the polyaniline system may suggest that there are more delocalized polarons or mobile Pauli spins in polyaniline.32 Therefore, the decrease in the ∆Hpp value of IPpolyaniline from 0.60 to 0.80 V would be caused by formation of the delocalized polarons due to the convention of the spin nature from Curie spins to Pauli spins with increasing potential in the highly acidic solution. The g value of IP-polyaniline is 2.0056, which is not affected by the potential in the range from -0.10 to 0.80 V. Figure 4 shows the influence of the applied potential on the ESR signal intensity of IP-polyaniline in 1.0 M NaCl solution of pH 5.5. The microwave power in this experiment was set at
20.1 mW as mentioned previously, which is much higher than that in 0.20 M HCl solution. This is because the ESR signal intensity of IP-polyaniline at pH 5.5 is significantly weaker than that in 0.20 M HCl solution, indicating that the unpaired spin density is strongly dependent on the pH value that affects the protonation level of IP-polyaniline. As can be seen in Figure 4, the tendency to the change in the ESR signal intensity with potential from -0.10 to 0.40 V is very similar to that in Figure 2, only the potential for the maximum ESR signal intensity is at 0.10 V in Figure 4, which is less positive than that in Figure 2. As a result, the potential range for the formation of polarons in 1.0 M NaCl solution of pH 5.5 is narrower than that in 0.20 M HCl solution. This result is expected because the first oxidation peak of IP-polyaniline in 1.0 M NaCl solution of pH 5.5 occurs at 0.15 V, which is less positive than that in 0.20 M HCl solution (Figure 1), and the redox activity of IP-polyaniline in 1.0 M NaCl solution of pH 5.5 is lower than that in 0.20 M HCl solution. However, the ESR signal intensity increases as the potential increases again from 0.40 to 0.60 V in Figure 4, even though this increment is not too large compared to that of potential increasing from -0.10 to 0.10 V in this solution. The reason for the ESR signal intensities at 0.50 and 0.60 V being higher than that at 0.40 V will be discussed later. Finally, the ESR signal intensity decreases quickly with increasing potential further. The result in Figure 4 demonstrates that IP-polyaniline still holds stronger ESR signals at pH 5.5 and in a wide potential region. Thus, the novel redox activity of IP-polyaniline at pH 5.5 is reasonable to be attributed to the stable free radicals in IP-polyaniline. Figure 5A shows the potential dependence of the relative ESR susceptibility χV/χ0.8V of IP-polyaniline in 1.0 M NaCl of pH 5.5 in which the maximum ESR signal occurs at 0.20 V and the χV/χ0.8V decreases continuously from 0.20 to 0.80 V. Therefore, the result in Figure 5A is different from that in Figure 4. However, the ESR signal intensities at 0.50 and 0.60 V are a little higher than that at 0.40 V in Figure 4, which is caused by the weak ESR signal and broad ∆Hpp of IP-polyaniline in 1.0 M NaCl solution of pH 5.5. Both of them affect the peak heights of the ESR signals. Figure 5B shows the change in the ∆Hpp value with applied potential for IP-polyaniline in 1.0 M NaCl solution of pH 5.5, which is similar to that in Figure 3B. The difference between Figure 3B and Figure 5B is that the potential for the minimum ∆Hpp value shifts from 0.30 V in Figure 3B to 0.10 V in Figure 5B. This is due to the shift of the potential for the maximum ESR signal intensity to 0.10 V in 1.0 M NaCl solution of pH 5.5 in Figure 4. The maximum ∆Hpp value occurs at 0.40 V in Figure 5B. The results from Figures 3B and 5B demonstrate
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Figure 4. Influence of applied potentials on the ESR signal of IP-polyaniline in 1.0 M NaCl solution of pH 5.5, curves: (1) -0.10, (2) 0.00, (3) 0.10, (4) 0.20, (5) 0.30, (6) 0.40, (7) 0.50, (8) 0.60, (9) 0.70, and (10) 0.80 V.
Figure 5. (A) Potential dependence of the relative ESR susceptibility χV/χ0.8V of IP-polyaniline in 1.0 M NaCl of pH 5.5. (B) Plot of the peakto-peak line width of IP-polyaniline as a function of potential in 1.0 M NaCl solution of pH 5.5.
that the ∆Hpp value of IP-polyaniline is dependent on the ESR intensity when the oxidation potential is e0.60 V in 0.20 M HCl solution whereas is e0.40 V in 1.0 M NaCl solution of pH 5.5, and the ∆Hpp value of IP-polyaniline in 0.20 M HCl solution is smaller than that at pH 5.5. This difference is related to the electrochemical redox activity of IP-polyaniline, which is caused by different protonation levels. The g value of IPpolyaniline in 1.0 M NaCl solution of pH 5.5 is between 2.0056 and 2.0060 in the potential range from -0.10 to 0.80 V. 3.3. ESR Measurements of IP-Polyaniline at Different Temperatures. The total electronic paramagnetic susceptibility measured in the disordered conducting polymers is usually expressed as a sum of Curie and Pauli components
χtot ) χPauli + χCurie ) χPauli + C/T This indicates that Curie susceptibility decreases with increasing temperature, i.e., is dependent on temperature. Figure 6A shows
the temperature dependence of the relative ESR susceptibility χT/χ375 for IP-polyaniline. As can be seen in Figure 6A, the relative ESR susceptibility decreases quickly from 135 to 235 K and then decreases a little up to 335 K. This result indicates that the temperature-dependent Curie susceptibility turns into the temperature-independent Pauli susceptibility, which is similar to those of the conventional polyaline15,22 and poly(aniline-com-aminophenol).23 The low-temperature Curie contribution to the susceptibility arises from single occupancy of a localized state near EF; when the Fermi energy is close to the mobility edge and much greater than the average on-site interaction, the temperature dependence of the spin susceptibility gradually changes from Curie-law behavior to temperature-independent Pauli-type behavior with increasing temperature.15 However, the relative ESR susceptibility increases pronouncedly from 335 to 375 K. This phenomenon has not been reported before. In previous reports, the experimental temperature for measurements of the temperature dependence of susceptibility of polyaniline was controlled below 310 K.15,22,32-34 Therefore, no experimental
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Figure 6. (A) Temperature dependence of the relative ESR susceptibility χT/χ375 for IP-polyaniline. (B) Relative ESR susceptibility χT/χ375 vs 1/T for IP-polyaniline.
from 335 to 375 K, which is attributed to formation of the unpaired spins because of the increase in the susceptibility with increasing temperature. The g value of IP-polyaniline is 2.0055 in the temperature range 135-375 K, i.e., temperature does not affect the g value. 4. Conclusions
Figure 7. Plot of the peak-to-peak line width as a function of temperature for IP-polyaniline.
results above 310 K were reported there. However, the experimental temperature for measurements of the relationship between temperature and susceptibility of poly(aniline-co-maminophenol) was controlled at 136-356 K, in which the relative susceptibility does not increase with increasing temperature from 330 to 256 K.23 Therefore, the increase in the relative susceptibility with increasing temperature from 335 to 375 K may be caused by the increase in the unpaired spin density in IP-polyaniline, i.e., higher temperatures initiate formation of the unpaired spins. Figure 6B shows the relative susceptibility as a function of the reciprocal of temperature. It is obvious that the temperaturedependent Curie susceptibility is detected at the temperature range 135-235 K and the temperature-independent susceptibility is at the temperature range 235-335 K. This result is analogous to those of the conventional polyaniline15,32 and poly(aniline-co-m-aminophenol).23 However, the susceptibility of IP-polyaniline increases with increasing temperature from 335 to 375 K. It is obvious that this phenomenon cannot be attributed to Curie spin behavior. A possible reason is that this unusual phenomenon is caused by formation of the unpaired spins at higher temperatures as discussed previously. Figure 7 shows the ∆Hpp as a function of temperature. The ∆Hpp value increases pronouncedly as the temperature increases from 135 to 195 K. In this temperature range, the polymer exhibits Curie-like behavior as explained above. The Curielike contribution arises from unpaired localized spins. Therefore, the broadening of the line width of the ESR signal gives strong support for the formation of the localized spins. As can be seen in Figure 7, the ∆Hpp value decreases with increasing temperature from 195 to 335 K, indicating formation of delocalized spins in the polymer. The ∆Hpp value decreases continuously
The in situ ESR-electrochemical measurements of polyaniline synthesized using the interface polymerization method were performed in solutions with different pH values at a wide potential range. The ESR signal intensity increases from -0.10 to 0.30 V and -0.10 to 0.10 V in 0.20 M HCl solution and 1.0 M NaCl solution of pH 5.5, respectively, and then decreases with increasing potential up to 0.80 V (vs Ag/AgCl with saturated KCl solution). The above experimental results indicate that the potential range for the formation of the unpaired spins of IP-polyaniline is dependent on the pH values, which affects its redox activity. An unusual relationship between the peakto-peak line width ∆Hpp of the ESR signal and the applied potential is detected in 0.20 M HCl solution, in which the ∆Hpp value decreases obviously with increasing potential from 0.60 to 0.80 V. This would be attributed to formation of delocalized polarons. The temperature dependence of the ESR susceptibility of IP-polyaniline was determined between 135 and 375 K; the results demonstrate the presence of the convention of temperature-dependent Curie behavior to the temperature-independent Pauli behavior at the temperature range 135-335 K. However, the susceptibility of IP-polyaniline increases again from 335 to 375 K, which would be caused by the increase in the unpaired spin density at higher temperatures. The ∆Hpp value increases obviously from 135 to 195 K, which supports formation of the localized spins, and the decrease in the ∆Hpp value is observed from 195 to 375 K. The g value of IP-polyaniline is hardly altered by the potential, pH, and temperature in our experimental conditions, indicating that the electronic structure of IPpolyaniline is not changed. References and Notes (1) Glarum, S. H.; Marshall, J. H. J. Phys. Chem. 1986, 90, 6076– 6077. (2) Glarum, S. H.; Marshall, J. H. J. Electrochem. Soc. 1987, 134, 2160–2165. (3) Ohsawa, T.; Kabata, T.; Kimura, Q.; Yoshino, K. Synth. Met. 1989, 29, E203-210. (4) Łapkowski, M. Synth. Met. 1990, 35, 183–194. (5) Łapkowski, M.; Genies, E. M. J. Electroanal. Chem. 1990, 279, 157–168. (6) Lippe, J.; Holze, R. Synth. Met. 1991, 41-43, 2927–2930. (7) Yang, S. M.; Li, C. P. Synth. Met. 1993, 55-57, 636–641.
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