ESR Studies of Poly(aniline-co-m

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J. Phys. Chem. B 2007, 111, 6998-7002

ARTICLES ESR Studies of Poly(aniline-co-m-aminophenol) in the Solid State and Nonaqueous Solution Shaolin Mu* Department of Chemistry, Yangzhou UniVersity, Yangzhou 225002, Jiangsu ProVince, China

Chong Chen Laboratory Center of Yangzhou UniVersity, Yangzhou 225002, Jiangsu ProVince, China ReceiVed: January 23, 2007; In Final Form: April 21, 2007

The copolymers, poly(aniline-co-m-aminophenol)s, used for the ESR studies were synthesized chemically in the solutions consisting of different concentration ratios of m-aminophenol to aniline. On the basis of the ESR measurements, the unpaired spin (polaron) densities of the copolymers were calculated to be 1.14 × 1019 spins per gram for copolymer-A with the conductivity of 7.02 × 10-6 S cm-1 and 2.03 × 1020 spins per gram for copolymer-C with the conductivity of 2.34 S cm-1. The ESR measurements of the copolymers in the solid states show that the peak-to-peak line width ∆Hpp decreases with a decreasing concentration ratio of m-aminophenol to aniline, but the g-value hardly changes. A conversion of Curie spins to Pauli spins for the copolymers is observed as the temperature changes in going from low temperature to high temperature between 136 and 356 K. The ESR studies of the copolymers in a nonaqueous solution first reveal that there are two free radicals in the copolymer, and the unpaired spins in the copolymers arise from nitrogen nuclei.

1. Introduction Electron spin resonance (ESR) measurements of polyaniline in the solid state provide a wealth of information on the g-value, the peak-to-peak line width ∆Hpp, the unpaired spin density, and the change in the ESR susceptibility with temperature. These results are very useful in elucidating the conduction mechanism of polyaniline. There are two theoretical conduction models, that is, bipolaron and polaron lattice ones, to describe the conduction mechanism of polyaniline. Polyaniline exists in different redox states.1 From the point of view of electrochemistry, polyaniline can be oxidized from lower oxidation levels to higher levels, accompanied with doping. ESR measurements revealed evidence for the generation of polarons at an early oxidation state. As polyaniline is further oxidized to higher doping levels, the measurements of ESR and conductivity show that a decrease in unpaired spin density with concomitant increase in conductivity is found. On the basis of these facts, some research groups2-8 proposed that at higher doping levels polarons are further oxidized to bipolarons, which are spinless and act as the majority carriers. On the contrary, other research groups9-13 suggested a polaron lattice model owing to a change in spin nature from Curie spins to Pauli spins with increasing doping level or increasing potential. On the basis of the measurement of paramagnetic susceptibility of highly conducting polyaniline, Heeger and co-workers14 drew a conclusion that the model assuming transport by spinless bipolarons is not appropriate for the conducing emeraldine salt of polyaniline. According to the results from the in-situ cyclic-voltammetrydependent EPR spectra and DC conductivity, Wei and Epstein15 * Corresponding author. E-mail: [email protected].

also concluded that the conduction of polyaniline is via the polaron lattice. Later, Patil and co-workers16 determined the mobilities and ESR signal intensities of polyaniline at various potentials and provided evidence for the transformation between Curie and Pauli spins. Zhou and co-wokers17 reported in-situ variable-temperature ESR measurements. Their results showed the conversion of the spin nature from typical Curie spins to typical Pauli spins with increasing potential, which also strongly supports the polaron lattice model. This indicates that the conversion of the Curie spins to Pauli spins is related to the insulator-metal transition for the conducting polymers of polyaniline type, which depends on the doping levels. In addition to the above reports on the ESR measurements of polyaniline, the ESR measurements of the solid polyaniline at different conditions were studied.18-25 These results afford important insights into the nature of polyaniline. Especially, Kahol and co-workers reported the ESR measurements of the emeraldine base polyaniline. They presented a spin-pairs model appropriate to a disordered material, which provides a natural interpretation for the observed behavior in both the undoped and the doped states of the emeraldine base.24,25 Recently, Long and coworkers26 reported magnetic susceptibility measurements on conducting polyaniline and polypyrrole nanostructures. They consider that the susceptibility data cannot be simply described as Curie-like susceptibility at lower temperatures and temperature-independent Pauli-like susceptibility at higher temperatures; the transitions between Pauli and Curie spins suggest the coexistence of paramagnetic polarons and spinless bipolarons and the possible formation of bipolarons (or polarons) with changes in doping level and temperature. It is clear that the ESR measurements of the polyaniline in

10.1021/jp0705976 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007

ESR Studies of Poly(aniline-co-m-aminophenol) the solid state have showed a great success for explaining the conduction mechanism of polyaniline and but have only detected a single resonance line. Therefore, the origin of the spin cannot be gained via the ESR measurements of the solid polyaniline samples. MacDiarmid and co-workers27 reported the ESR spectra of the pernigraniline base and emeraldine base of polyaniline in nonaqueous solutions. The obtained ESR spectra revealed direct evidence that unpaired spins reside on a nitrogen atom in polyaniline, which is very significant for the understanding of the polymerization mechanism of aniline and the polyaniline structure. The conducting copolymer, poly(aniline-co-m-aminophenol), synthesized electrochemically under optimum conditions has a good redox activity from more acid solutions to pH 11.0 and has a rather high conductivity with a small pH dependence.28 This indicates that the pH dependence of the redox activity and the conductivity of poly(aniline-co-m-aminophenol) were improved markedly, compared with those of polyaniline. Therefore, poly(aniline-co-m-aminophenol) has been used for a cathodic material in a rechargeable battery.29 To meet needs of a large amount of poly(aniline-co-m-aminophenol), recently we synthesized poly(aniline-co-m-aminophenol) by means of chemical copolymerization. It was found that the electrical properties of the copolymer prepared chemically are even better than those of one synthesized electrochemically. However, the ESR studies on poly(aniline-co-m-aminophenol) were not reported. In this case, we focused our attention on the effect of the concentration ratio of m-aminophenol to aniline in an electrolytic mixture on the ESR behavior of the copolymers in the solid state and ESR characteristics of the copolymers in nonaqueous solution, because the electrical properties of poly(aniline-co-m-aminophenol) are pronouncedly affected by the monomer concentration ratio in the electrolytic solution used for electrochemical copolymerization. 2. Experimental Section Chemicals used were of analytical reagent grade. Aniline was distilled under reduced pressure before use. Mixtures containing different concentration ratios of m-aminophenol to aniline were used to synthesize the copolymers. A mixture containing 0.34 M aniline, 0.012 M m-aminophenol, 0.47 M ammonium peroxydisulfate, and 2 M H2SO4 was found to be an optimum mixture for preparing poly(aniline-co-m-aminophenol).30 The mixture was stirred with a stirring magnetic bar for 15 h at room temperature. The product was filtered and washed with 0.02 M HCl solution until the filtrate was colorless, and then it was dried under dynamic vacuum at 90 °C for 15 h. On the basis of the structure of the emeraldine salt of polyaniline in the oxidation state1 and the FTIR and 1H NMR spectra of poly(aniline-com-aminophenol),30 the structure of the emeraldine salt of poly(aniline-co-m-aminophenol) is suggested as follows:

where A- is an anion. This structure is similar to that of the polaron lattice model of polyaniline proposed by Epstein and co-workers.9 The amount of m-aminophenol in the copolymer depends on the monomer concentration ratio in the electrolytic solution used for copolymerization. The ESR measurements were carried out using a Bruker A300 spectrometer operating in X-band (9.862 GHz). The microwave power for the measurements of the copolymers in the solid state

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Figure 1. ESR spectra, (1) copolymer-A, (2) copolymer-B, (3) copolymer-C, (4) copolymer-D.

was as low as 2 × 10-3 mW in order to avoid saturation of ESR signals; and a modulation amplitude was set at 1.0 G. The microwave power for the measurements of the copolymer solutions was 20 mW, and modulation amplitude was set at 2.0 G. The temperature dependence of the ESR signal intensity of the copolymers in the solid states was measured between 136 and 356 K, which was controlled by flowing cold nitrogen. The Bruker Company provides a g-factor marker with S3/2, its g-value is 1.9800 ( 0.0006. To determine the spin densities of the copolymers prepared under different conditions, each copolymer and DPPH were weighed. DPPH (1,1′-diphenyl-2picrylhydrazil) with known spin density was used as a reference standard. The copolymers used in the ESR measurements are denoted as copolymers-A, -B, -C, and -D, respectively. Copolymer-A was prepared in a solution consisting of 0.34 M aniline, 0.34 M (NH4)2S2O8, 2 M H2SO4, and 0.085 M m-aminophenol; copolymer-B was prepared in the same solution, but containing 0.023 M m-aminophenol. Copolymer-C was prepared in a mixture containing 0.34 M aniline, 0.47 M (NH4)2S2O8, 2 M H2SO4, and 0.012 M m-aminophenol. Copolymer-D was obtained from copolymer-C, which was immersed in 0.1 M NH4OH solution with stirring for 2 h. The nonaqueous solutions of copolymers-A through -D were prepared using dimethyl sulfoxide (DMSO) as a solvent. 3. Results and Discussion 3.1. ESR Measurements of the Copolymers in the Solid State. Curves 1-4 in Figure 1 are the ESR spectra of copolymers-A through -D, respectively, which were measured at room temperature. Each spectrum line consists of a symmetric signal. The values of the peak-to-peak line width ∆Hpp of copolymers-A through -C are 8.97 (curve 1), 1.75 (curve 2), and 1.52G (curve 3), respectively. The change of the line width is caused by the nature of the copolymer itself. The nature of the copolymer is strongly dependent on the concentration ratio of m-aminophenol to aniline in a reaction mixture, which was used for the preparation of the copolymers. The ESR spectra in Figure 1 show that the narrowing of the line width resulted from decreasing the concentration ratio of m-aminophenol to aniline in a reaction mixture, d[M1]/d[M2]. The symbol d[M1]/d[M2] indicates the ratio of the change in concentration in each respective species. This ratio is the same as the ratio of the numbers of each kind of repeat unit in the polymer formed from the solution containing M1 and M2 at the concentrations [M1] and [M2], respectively.31 On the basis of conductivity measurements,30 the conductivities of copolymers-A through -C are 7.02 × 10-6, 2.22, and 2.34 S cm-1, respectively. Comparison of the ∆Hpp values and conductivities shows that the narrowing

7000 J. Phys. Chem. B, Vol. 111, No. 25, 2007 of the line width of the ESR signals is accompanied with an increase in the conductivity. The ∆Hpp value of copolymer-D is 2.68 G that is larger than that of copolymer-C owing to copolymer-D being in the emeraldine base form as mentioned in the experimental section. It is obvious that this difference is caused by the deprotonation of the copolymer. Also, the conductivity of copolymer-D (0.57 S cm-1) is lower than that of copolymer-C. These results give a clue that if the conductivity of the copolymer decreases further, it will result in a further broadening of the line width until the disappearance of the ESR signal. The conductivity of the copolymer results from carriers in one. Thus, the ∆Hpp value is related to the unpaired spin density in the copolymer. The values of the g-factor of copolymer-A and copolymers-B through -D are 2.0034 and 2.0030, respectively. Therefore, their values are almost the same as each other, and are very close to the value of 2.0023 for a free electron. The g-value depends on the electronic structure of the species because the applied field has to be able to move the electron through the molecule. Thus, the same g-values demonstrate that the electronic structures of copolymers-A through -D are the same. In order to determine the unpaired spin densities of the copolymers, DPPH with a known density was used as a reference standard. Its spin density is 1.53 × 1021 spins per gram.32 The ESR measurement of DPPH was carried out under exactly the same conditions as those of the ESR measurements of the copolymers, but their measurements were separately carried out without using a dual-sample cavity cell. This is because the resonance of electron spins of the copolymer and DPPH takes place at very close applied fields. For example, the center of the spectrum line of copolymer-C lies at 3517.3 G, and that of DPPH lies at 3516.0 G. In this case, the area of the spectrum line of the copolymer obtained by using the double integration of the first derivative ESR spectrum will be influenced by the spectrum line of DPPH, which will give rise to an error for calculating the unpaired spin density of the copolymer. On the basis of the double integration of the firstderivative ESR spectra, the known weight of each copolymer and DPPH, and the known spin density of DPPH, the unpaired spin densities of copolymers-A through -C were evaluated to be 1.08 × 1019, 1.78 × 1020, and 1.93 × 1020 spins per gram, respectively. The unpaired spin densities obtained here are more or less affected by the individual measurements of ESR spectra of the copolymer and DPPH as described above. The above results show that the unpaired spin density of the copolymer increases with a decrease in the amount of m-aminophenol unit in the copolymer chain. Although the increase in the unpaired spin density of the copolymer is accompanied with increasing conductivity, no directly proportional relationship between the unpaired spin density and the conductivity was found. For example, the unpaired spin density of copolymer-B is about 17 times as high as that of copolymer-A; however, the conductivity of copolymer-B is 6 orders of magnitude as high as that of copolymer-A. A possible explanation is that the unpaired spins in copolymer-A are localized. The motion of the localized unpaired spins is confined, which have a low contribution to the electric conduction. However, as the decrease in the amount of m-aminophenol unit in the copolymer chain, the localized spins or localized polarons are transited to a metallic polaron lattice in copolymer-B. The delocalized polarons have a large contribution to the conductivity of copolymer-B and copolymerC. The unpaired spin density of copolymer-D is 1.28 × 1020 spins per gram lower than that of copolymer-C. This result is expected, because copolymer-D was in the deprotonated state,

Shaolin and Chen

Figure 2. Plot of the peak-to-peak line width as a function of temperature for copolymer-A (b) and copolymer-B (4).

Figure 3. Temperature dependence of the relative ESR susceptibility χT/χ356K for copolymer-A(b) and copolymer-C (2).

which results in decreases in the unpaired spin density and the conductivity. Figure 2 shows the ∆Hpp as a function of temperature for copolymer-A and copolymer-B, respectively. The ∆Hpp of copolymer-A is about 4 times as large as that of copolymer-B, which is caused by the high amount of maminophenol in copolymer-A. The broad signal of copolymer-A also gives strong support for the formation of localized polarons or localized spins.16 The difference in ∆Hpp between two copolymers is attributed to most of the unpaired spins in copolymer-A being localized; however, the unpaired spins in copolymer-B are delocalized as discussed above. The increase in the electron localization leads to reduced motional narrowing or spin diffusion.16,33 The ∆Hpp of copolymer-A increases slowly with decreasing temperature, which is also attributed to the increase in the electron localization with decreasing temperature. However, the ∆Hpp of copolymer-B is more pronouncedly affected by temperature, compared with that of copolymer-A. A feature for the ∆Hpp temperature dependence of copolymer-B is that there are three temperature regions with different ∆Hpp values in the experimental temperature range; and the ∆Hpp remained unchanged in any temperature region. The reason for this is not clear. The changes in ∆Hpp with temperature for copolymer-C and copolymer-D are very similar to that of copolymer-B. Even though ∆Hpp values of the copolymers change with temperature, the g-values of copolymers-A through -D remained almost unchanged in the temperature range 136 to 356 K. The changes in the ESR signal intensities of copolymers-A through -D with temperature were determined between 136 and 336 K with an interval of 5 min between two measurements. Figure 3 shows the temperature dependence of the relative ESR susceptibility χT/χ356K for copolymer-A and copolymer-C. The ESR signal intensity was measured by doubly integrating the ESR signal. Figure 3 indicates that the relative ESR susceptibil-

ESR Studies of Poly(aniline-co-m-aminophenol)

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Figure 5. ESR spectrum of copolymer-B in DMSO solution. Figure 4. The relative ESR susceptibility χT/χ356K vs 1/T for copolymer-A (b) and copolymer-C (4).

ity of copolymer-A decreases with increasing temperature up to 316 K, and then turns into the temperature-independent Pauli susceptibility. Therefore, a temperature-dependent Curie susceptibility of copolymer-A is obviously observed between 136 and 316 K. The Curie-like contribution arises from unpaired localized spins. Also, the temperature dependence of the relative ESR susceptibility χT/χ356K for copolymer-C is similar to that of copolymer-A, but the relative ESR susceptibility of copolymer-C hardly changes at T > 276 K. This means that the temperature range of Pauli susceptibility of copolymer-C is larger than that of copolymer-A. This is caused by the conductivity of copolymer-C being much higher than that of copolymer-A. 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 The Curie susceptibility arises from unpaired localized spins and is dependent on temperature; the Pauli susceptibility is characteristic of the presence of delocalized spins and independent of temperature.14,24 Figure 4 shows the relative ESR susceptibility χT/χ356K as a function of the reciprocal of temperature for copolymer-A and copolymer-C. It is obvious that the temperature-dependent Curie susceptibility of copolymer-A dominates in the tested temperature range, and the Pauli term takes over only above 316 K. Also, the temperaturedependent Curie susceptibility of copolymer-C is detected at the temperature range 136-276 K. The slopes of both straight lines in Figure 4 show that the temperature dependence of the ESR susceptibility for copolymer-C is smaller than that of copolymer-A, indicating that the copolymer with high conductivity has a weak temperature-dependent Curie susceptibility, compared with the copolymer with low conductivity. This result is analogous to that of polyaniline with high electrical conductivities (100-400 S cm-1), which exhibits temperatureindependent magnetic susceptibility at lower temperatures.14,34 It is clear that the susceptibility of copolymer-C or polyaniline with highly electrical conductivity approaches or belongs to Pauli susceptibility, respectively. And the DC conductivity is proportional to the number of Pauli carriers.15 This is a reason why the copolymer with high conductivity has a weak temperature-dependent susceptibility. 3.2. ESR Spectra of the Copolymers in the Solutions. The ESR spectra of the solutions of copolymers-A through -D were carried out at room temperature. Figure 5 shows the ESR spectrum of copolymer-B dissolved in DMSO. It was found that the ESR spectra of the solutions of other copolymers are identical in shape to that in Figure 5. There are six main signals

in the magnetic field range of 3265-3765 G. The ESR signal intensity in Figure 5 decreases gradually with an increase of the magnetic field strength. This result is different from the ESR spectrum of the pernigraniline base of polyaniline in dioxane, in which the ESR signal is split into a triplet in the magnetic field range 3440-3490 G,27 due to the 14N nucleus with unit spin. Therefore, the ESR spectrum in Figure 5 is attributable to two free radicals on two nitrogen nuclei. This result is first to reveal direct evidence that two free radicals exist in a conducting polymer of the polyaniline type, which is an unusual feature because only one free radical was detected in polyaniline. The significance of the ESR spectrum shown in Figure 5 provides experimental evidence for the theoretical conduction model of the conducting polymers. The appearance of the separated ESR signals in Figure 5 is owing to a greater space between two free radicals in the copolymer, whose structure is suggested in the experimental section. In such a case, the interaction between two free radicals is very weak. So the ESR spectrum of two isolated free radicals or polaron lattice in a conducting polymer solution can be detected. A doublet of unequally intense ESR absorption lines is observable for each main ESR signal in Figure 5. This is caused by a proton attached on a nitrogen nucleus, which leads to the hyperfine splitting of the spectrum. It is obvious that the ESR spectrum of copolymer-B in the solution gives valuable evidence that two isolated •NH free radicals exist in the copolymer, which is consistent with the structure of the emeraldine salt of the copolymer suggested; this result is also new evidence for the polaron lattice model of the polyaniline proposed by Epstein and co-workers.9 On the basis of the isotropic coupling constant of 55.2 mT (552 G) for a 2s electron in nitrogen35 and the experimental data of the isotropic coupling constant of a nitrogen radical, the unpaired spin spent in the nitrogen 2s-orbital can be calculated.35 The isotropic coupling constant for 14N increases from 90 to 100 G in going from the low field to high field in Figure 5. We take 95 G (9.5 mT) as an average value, so the unpaired spin of 17% spends in the nitrogen 2s-orbital of copolymer-B. 4. Conclusions The conductivity, electrochemical activity, and pH dependence of the electrical properties of the copolymer, poly(anilineco-m-aminophenol), strongly depend on the concentration ratio of m-aminophenol to aniline in a reaction mixture or the ratio of the numbers of each kind of repeat unit in the copolymer. The ESR measurements of the copolymers confirm that the peak-to-peak line width ∆Hpp of the ESR signal and unpaired spin density are also affected by the monomer concentration ratio in a reaction mixture, but the g-value is almost unaffected.

7002 J. Phys. Chem. B, Vol. 111, No. 25, 2007 The copolymer synthesized at optimum conditions has a high conductivity, good electrochemical property, highly unpaired spin density, and a minimum ∆Hpp value. The results from the ESR measurements of the solid copolymers at various temperatures show the conversion between Curie spins and Pauli spins, but this conversion is dependent on the monomer concentration ratio, and is complicated as with polyanilines synthesized under different conditions. The latter is discussed in the introduction section. The ESR measurements of the copolymers in nonaqueous solutions first revealed direct evidence that unpaired spins reside on two nitrogen atoms to form two free radicals in the copolymers. References and Notes (1) Huang, W. S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2385-2400. (2) Lippe, J.; Holze, R. Synth. Met. 1991, 41, 2927-2930. (3) Christensen, P. A; Hamnett, A. Electrochim. Acta 1991, 36, 12631286. (4) Iida, M.; Asajl, T.; Inoue, M.; Grijalva, H.; Inoue, M. B.; Nakamura, D. Bull. Chem. Soc. Jpn. 1991, 64, 1509-1513. (5) Xie, S.-J.; Mei, L.-M.; Lin, D. L. Phys. ReV. B 1994, 50, 1336413370. (6) Kudelski, A.; Bukowska, J.; Jackowska, K. Synth. Met. 1998, 95, 87-91. (7) Harima, Y.; Eguchi, T.; Yamashita, K.; Kojima, K.; Shiotani, M. Synth. Met. 1999, 105, 121-128. (8) Neoh, K. G.; Sampanthar, J. T.; Kang, E. T. J. Phys. Chem. B 2001, 105, 5618-5625. (9) Epstein, A. J.; Ginder, J. M.; Zuo, F.; Bigelow, R. W.; Woo, H. S.; Tanner, D. B.; Richter, A. F.; Huang, W. S.; MacDiarmid, A. G. Synth. Met. 1987, 18, 303-309. (10) Javadi, H. H. S.; Laversanne, R.; Epstein, A. J.; Kohli, R. K.; Scherr, E. M.; MacDiarmid, A. G. Synth. Met. 1989, 29, E439-E444. (11) Lapkowski, M.; Genies, E. M. J. Electroanal. Chem. 1990, 279, 157-168. (12) Yang, S. M.; Li, C. P. Synth. Met. 1993, 55, 636-641.

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