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On the electrochemical and spectroscopic properties of a soluble polyaniline parent copolymer
Electrochimica Acta 47 (2002) 2005 /2011 www.elsevier.com/locate/electacta
On the electrochemical and spectroscopic properties of a soluble polyaniline parent copolymer A.L. Schemid, L.M. Lira, S.I. Co´rdoba de Torresi * Instituto de Quı´mica, Universidade de Sa˜o Paulo, C.P. 26077, 05513-970 Sa˜o Paulo (SP), Brazil Received 13 December 2001
Abstract Copolymers of aniline and 2-ethylaniline can be easily formed by electrochemical or chemical oxidation in acidic medium, but the composition of the polymeric chain differs from the composition of the synthesis solution. Copolymers formed from precursor solutions containing less than 30% (in moles) of 2-ethylaniline show intermediate properties between those of the homopolymers, and these properties vary gradually with the amount of 2-ethylaniline units in the copolymeric chain. The properties of those copolymers have been studied by UV /Vis and resonance Raman spectroscopies, cyclic voltammetry, two-probe conductivity and solubility measurements. Results obtained by these techniques corroborate the formation of a block copolymeric material. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyaniline; Copolymerization; Electrochemistry; Raman; d.c. conductivity
1. Introduction The research in electroactive polymers in the past few years has received considerable attention [1,2], in particular aromatic polymers. Among these polymers, the polyaniline (PANI) has been one of the most widely studied conducting polymers because of its optical and electrochemical properties and due to its chemical and oxidative stability, which can be used in many applications, including rechargeable batteries, corrosion protection, light emitting diodes, molecular sensors, electrochromic devices, etc. [3 /7]. However, as it is common with other conjugated polymers, polyaniline is limited by poor thermal processability and solvent solubility [8], due to the stiffness of its backbone. Consequently, their post-synthesis proces-
* Corresponding author. Tel.: 55-11-3091-2165; fax: 55-113091-3890. E-mail address: [email protected] (S.I. Co´rdoba de Torresi).
sability is quite difficult, and a lot of work has been done recently to overcome this problem [9 /18]. Improved solubility can be achieved by introducing bulky alkyl substituents into the polyaniline backbone, but limitations are then imposed on the conductivity of the polymer produced. The conductivity of polyaniline and the solubility of substituted polyanilines can be achieved by copolymerization. These copolymers of aniline and substituted anilines show improved solvent solubility, while maintaining high electrical conductivity which can be readily tailored by varying the composition of the copolymer [19]. In this paper, we report the preparation, by electrochemical methods, of some soluble copolymers of aniline and 2-ethylaniline and the materials are compared with those prepared by chemical oxidative procedure. By electrochemical methods and Raman spectroscopy we have observed that the composition of the copolymeric chain differs from the precursor solution; copolymers formed from precursor solutions containing less than 30% (in moles) of 2-ethylaniline show intermediate properties between those of the homopolymers, and
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these properties vary gradually with the amount of 2ethylaniline units in the copolymeric chain. This is an evidence of the formation of a new material, and not only a mixture of homopolymers. Those materials can be used for technological applications, due to some of their interesting properties: good conductivity, electrochromism, and solubility in some common organic solvents, like N -methyl-2-pyrrolidinone or chloroform, those properties are also reported in this paper.
2. Experimental 2.1. Electrochemical synthesis Polyaniline (PANI), poly(2-ethylaniline) and poly(aniline-co-2-ethylaniline) films were electropolymerized onto glass/ITO (Delta Technologies, R 5/20 V/I) transparent conducting substrates by applying triangular potential sweeps (0.01 V s 1) in the /0.25 to 0.75 V (PANI films) and /0.25 to 0.85 V (substituted PANI and copolymers) potential ranges. All potentials are referred to the saturated Ag/AgCl electrode. The polymerization was carried out in a 0.5 mol L1 monomer/ 1.0 mol L 1 HCl electrolytic solution. In the case of the synthesis of the copolymers, the concentration of monomer or the mixture of monomers was maintained constant at 0.5 mol L1 and the proportion in moles of 2-ethylaniline/aniline was varied in the following form: 1:9 (f1 /0.1), 3:7 (f1 /0.3), 5:5 (f1 /0.5), 7:3 (f1 /0.7), and 9:1 (f1 /0.9), in order to cover a broad range of solution composition (f1), where f1 is defined as: f1
n2
ethylaniline
ntotal
The amount of electrodeposited material was controlled through the electrochemical charge passed, all films being grown until 4.10 3 C cm 2. 2.2. Chemical synthesis and solubility studies The homopolymers and the copolymers were chemically synthesized by oxidative polymerization with K2S2O8 in acid media at low temperature (273 K), following the common procedure for polyaniline synthesis [20]. A solution containing 100 ml of 0.165 mol l 1 potassium peroxidisulfate was added dropwise in 100 ml of 0.165 mol l 1 monomer or mixture of monomers/ 1.0 mol l 1 HCl solution, then it was stirred and placed in an ice bath. The mixture was allow to stir over a period of 6 h. In the case of the synthesis of the copolymers, the following solutions with different molar fractions of 2-ethylaniline (f1), were added in water to achieve the total volume of 100 ml:
1.83 ml 2-ethylaniline (0.149 mol l1)/0.15 aniline (0.016 mol l1)/4.14 ml HCl (f1 /0.9) 1.42 ml 2-ethylaniline (0.115 mol l1)/0.45 aniline (0.050 mol l1)/ 4.14 ml HCl (f1 /0.7) 1.02 ml 2-ethylaniline (0.083 mol l1)/0.75 aniline (0.083 mol l1)/ 4.14 ml HCl (f1 /0.5) 0.61 ml 2-ethylaniline (0.050 mol l1)/1.05 aniline (0.115 mol l1)/4.14 ml HCl (f1 /0.3) 0.20 ml 2-ethylaniline (0.016 mol l1)/1.35 aniline (0.149 mol l1)/4.14 ml HCl (f1 /0.1)
ml ml ml ml ml
The resulting green precipitate was filtered and dried in air overnight. This procedure leads to the polymer in the doped acid form. Part of this material was dedoped to the basic form by washing the powder with a 1.0 mol l 1 KOH solution under vigorous stirring during 4 h. After that, the polymer was filtered, washed with deionized water and dried again. The solubility studies of homopolymers, copolymers and mechanical mixtures of homopolymers with the same molar composition of the copolymer’s feed solution in NMP, CHCl3 and DMF were performed using the following procedure. The base form of each powder was ground with a mortar and pestle and put through an 80-mesh sieve before solubility testing. An amount of 0.5 g of the powder was added to 10 ml of each solvent and stirred for 1 h before filtering. The filter paper was pre-weighted, and the filtrate was allowed to evaporate at 30 8C and 200 mbar, before weighting the papers again to calculate the solubility of the polymers. 2.3. Spectroelectrochemical measurements Spectroelectrochemical experiments were performed under potentiodynamic conditions using a potentiostat/ galvanostat Autolab PGSTAT30 (Ecochemie) by placing the electrochemical cell in the optical pathway of a digital fiber optic wheel Spectrophotometer (WPIs). After deposition, films were carefully rinsed with purified water and placed in a one-compartment electrochemical cell. The electrolytes were 0.8 mol l1 HCl, HClO4, p -toluene sulfonic acid (PTSA) and camphor sulfonic acid (HCSA) solutions. A platinum wire was used as the counter electrode and all potentials are referred to the saturated Ag/AgCl electrode. All solutions were prepared from A.R. chemicals and purified water (Elga System UHQ), the monomers were distilled under vacuum prior use and HCSA was recrystallized with ethyl acetate for spectroelectrochemical measurements. 2.4. Raman spectroscopy and conductivity measurements Chemically synthesized polymers were analyzed by Raman spectroscopy using a Renishaw Raman Imaging System 3000 coupled with an He /Ne laser (Spectra
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Physics, mod. 127, l0 /632.8 nm), and equipped with an Olympus metallurgical microscope and a CCD detector. Laser power was kept below 0.7 mW at the sample to avoid thermal degradation. Resistivity measurements of copolymer powders were performed by the two point method using an Impedance Analyzer Autolab PGSTAT30 (Ecochemie) with FRA module. A d.c. potential of 0.0 V with a modulation of 5 mV (rms) was imposed in the 10 kHz /10 mHz frequency range.
2007
Electrochemical synthesis of poly(aniline-co-2-ethylaniline) from different precursor solutions produces homogeneous and adherent green films. They show great electroactivity in acidic media and the voltammetric profiles are very similar to the potential values of the current peaks being located at intermediate values of the typical electrochemical responses of the homopolymers. Fig. 1 shows the potentiodynamic j/E profiles of PANI, poly(2-ethylaniline), and copolymers formed from different 2-ethylaniline/aniline ratio polymerization solutions: 1:9, 5:5 and 9:1. It should be observed that the electrochemical response of the copolymers is not the simple addition of those related to PANI and poly(2-ethylaniline); this fact would be an indication of the formation of a new material with differentiated properties. As it was already shown in previous work [21 /23], the presence of an alkyl substituent in the benzene ring provokes the shift of leucoemeraldine/ emeraldine redox couple to more positive potentials and the emeraldine/pernigraniline couple to less positive potentials. This fact can be explained on the basis of steric and electronic effects of the /C2H5 group. As it was already shown in our previous work [22,23], the shift of the redox process depends on the chemical nature of the dopant; in this case that the electrolytic solution is HCl, the two redox processes overlapped in a
large peak in poly(2-ethylaniline) j /E potentiodynamic profile. Conklin et al. [8] have already reported that the chemical formation of copolymers between PANI and its ethylated derivative leads to block copolymerization with long blocks of 2-ethylaniline and short blocks of PANI. The same behavior has been reported for copolymers prepared from PANI and other alkylated derivates, such as o -toluidine, m -toluidine and N butylaniline. This was determined by 1H NMR measurements and the molar fraction of 2-ethylaniline was calculated from the ratio of integrated area of the methylated protons and aromatic ones. In this way, it was found that the reactivity ratio, that is to say the ability of the monomer to enter to the copolymer chain, is higher in 2-ethylaniline than in aniline. Thus, the molar composition of the copolymer is very different from that of the precursor solution. Specifically, in the case of poly(aniline-co-2-ethylaniline), it was found that when the amount of 2-ethylaniline in the feed solution (f1) was greater than 40%, the composition of the copolymer is almost 100% poly(2-ethylaniline). Fig. 2 shows the peak potential values (Ep) of both the first and the second oxidation peaks for copolymers cycled in p -toluene sulfonic acid electrolytic solution. Experiments in other electrolytes (HCl, HClO4 and HCSA) were also performed but they are not shown for simplicity; anyway, the electrochemical responses were similar. As can be seen, the increase of the first potential peak and the decrease of the second one is gradual when f1 varies from 0.0 to 0.4. From this point till f1 /1.0, the difference between the redox processes (DEp) remains constant, indicating that the material is almost the same in all this range of feed solution composition. So, considering voltammetric profiles, copolymers formed by electrochemical methods give the same behavior of those prepared by oxidative polymerization. It is well known that substituted polyanilines present a strong electrochromism independently of the electrolyte used. In the case of poly(2-ethylaniline) [22] and its
Fig. 1. j /E potentiodynamic profiles of the homopolymers PANI ( */) and poly(2-ethylaniline) ( */), and the copolymers prepared from different solution compositions: f1 0.9 (- × / × -), f1 0.5 (---) and f1 0.1 ( × × × × × × ). Electrolytic solution: 1.0 mol l 1 HCl. v 10 mV s 1.
Fig. 2. Peak potential as a function of the composition of feed solution (f1). (m) leucoemeraldine/emeraldine redox couple and (k) emeraldine/pernigraniline redox couple. Electrolytic solution 1 mol l 1 PTSA. B-spline curve was used to link the data points.
3. Results and discussion
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copolymers with aniline they change from transparent yellow to blue with the potential. Both electrochromism and electroactivity are related to the injection/ejection of the charge carriers into/from the polymeric matrix. In this case we can associate the absorbance (A /obc ) with the inserted charge per unit area, as was already done for inorganic and organic materials [24,25]: dA dc jo ob (1) dt dt zF where o is the molar absorptivity of the film, b is the film thickness, c is the concentration of optically active sites on the electrode, j is the current density, z is the number of electrons participating in the redox reaction and F is the Faraday constant. If the reaction responsible for the coloration change of the material is the same producing a redox wave during the voltammetry and all the current is consumed in the electrochromic process, the dA /dt versus E profile must coincide with the j /E profile obtained under the same experimental conditions. This statement is correct if all current related to the electrochromic process has a faradaic nature, separating, in this way, faradaic from capacitive currents. Fig. 3 shows the voltammograms of a poly(aniline-co2-ethylaniline) (f1 /0.1) together with the transmittance variation and the dA /dt curves calculated from the transmittance variation for four different wavelengths in HCl solution. As was shown previously, the voltammetric profile depends strongly on f1 in the 0.0 /0.4 range; specifically, in the case of the copolymer with
Fig. 3. j /E ( */) and %T or dA /dt ( × × × × × ) potentiodynamic profiles recorded at different wavelengths for a poly(aniline-co-2-ethylaniline) (f1 0.1) film in 0.8 mol l 1 HCl electrolytic solution. v 10 mV s 1.
f1 /0.1, the electrochemical response is very unusual and it is possible to distinguish a third peak at higher potentials. Results obtained from NMR technique [8] have shown that the composition of this copolymer corresponds to ca. 50% of the two homopolymers. The left part of Fig. 3 shows the %T versus E profiles obtained simultaneously with the voltammograms. It can be observed that transmittance changes in a different way for each wavelength when the film is oxidized and, considering only these plots, it is clear that it is rather difficult to correlate each redox process to changes in the optical absorption. This difficulty arises by the fact that absorption (or transmittance) are integral quantities while the current is a differential one; that is why it would be better to try to do this relationship by comparing two differential quantities, current and dA /dt. As can be seen in the right part of Fig. 3, the first redox process is associated with chromophore groups absorbing at 420 and 700 nm so the redox process at 0.3 V could be related to the leucoemeraldine/emeraldine. The dA /dt profiles recorded at 520 and 620 nm show that, at these wavelengths, the main absorption is related to the third and second peaks in the voltammograms. This is clearer from Fig. 4 where the UV /Vis spectra as a function of the potential are shown. In 420 nm the major absorbance variation on the spectral bands occurs on the potential range of 0.2 /0.3 V, where the leucoemeraldine/ emeraldine transition occurs. In 520 nm the major absorbance variation occurs in the 0.6 /0.7 V potential range and this variation is related to the emeraldine/ pernigraniline transition. In 620 and 700 nm, the major variation occurs in the 0.4 /0.5 V potential range, but there are intense contributions from other potential ranges which makes difficult the correlation of the voltammogram peaks and the absorption bands. As was already pointed out, the use of the dA /dt method allows a more direct association of the absorption bands and the redox processes because of the comparison of two differential quantities, j and dA /dt . Independently on the copolymer composition, the electrochromic behavior is similar and can be followed by this treatment.
Fig. 4. In situ UV /Vis spectra of a poly(aniline-co-2-ethylaniline) (f1 0.1) film in 0.8 mol l 1 HCl solution.
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As was mentioned above, the combination of good processability and high conductivity of these materials is the main goal of the synthesis and characterization of copolymers. Fig. 5 shows the conductivity of the different homopolymers and copolymers measured by the two points method. As can be seen from the figure, the conductivities vary in a range from 10 S cm 1 (pure PANI) to 1 /10 3 S cm 1 (pure poly(2-ethylaniline)). It is very important to point out that the greater variation occurs in the range of f1 /0.1 /0.3, in good agreement with the fact that the greater variation of copolymer composition takes place in this specific range. Another important feature regarding these results, is the increase of the conductivity of the material in one order of magnitude by the addition of a small proportion of aniline units in the polymeric chains. Higher amounts of aniline will not change the conductivity until a proportion of 70% in the feed solution is reached. The variation of conductivity must be related to solubility tests results performed with the copolymers and mechanical mixtures of the homopolymers, and they are shown in Table 1. As can be seen, improved solubility of the copolymers, compared to polyaniline, is always obtained for the three solvents employed, NMP, CHCl3 and DMF. The measurements were performed following the procedure already described in the literature [18], but with powders obtained by electrochemical polymerization. The higher solubility must be related to the introduction of bulky alkyl chains in the PANI backbone and, again, it can be inferred that for compositions of the feed solutions different from f1 / 0.0 /0.4, the solubility is equal to that corresponding to poly(2-ethylaniline). Raman spectroscopy is a very useful tool for the characterization of conducting polymers giving information about the structure of different segments in the polymer backbone [26,27]. It is well known that the Raman spectra of poly conjugated systems is dependent on the laser’s wavelength, due to resonance effect [28]. In the case of polyanilines, the l0 /632.8 nm excitation line enhances the intensity of the bands corresponding
Fig. 5. Plot of conductivity (s ) vs. co-monomer feed composition (f1). B-spline curve was used to link data points.
2009
to oxidized segments; so that Raman spectra of the base form of polyanilines are well defined at this radiation. Figs. 6 and 7 show the Raman spectra of copolymers prepared from feed solutions of different compositions recorded in two wavenumber regions. In 1995, Berrada [29] has reported the Raman spectra of emeraldine base form of poly(2-methylaniline) at 676 nm, which are very similar to those obtained with the poly(2-ethylaniline) at 632.8 nm. Fig. 6 shows the spectra displayed in the 1000 /1400 cm 1 region. The spectrum of PANI (f1 /0) shows two intense bands at 1164 and 1220 cm 1 corresponding to in-plane C /H bending (d C /H) and C /N stretching (nC /N), respectively [30]. In the case of the poly(2-ethylaniline) (f1 /1.0) these modes are shifted to 1156 and 1223 cm 1, due to the presence of the alkyl substituents in the orto position of the rings. In this spectrum other bands located at 1120 and 1242 cm 1, corresponding to C /C stretching and C /C twisting of the alkyl chains [29], are also depicted. The relative intensity of these bands diminishes when the amount of aniline in the feed solution is increased from 70 to 100% and remains constant for lower values. This fact is in excellent agreement with results obtained by conductivity measurements or electrochemistry, showing that the effective formation of a copolymer occurs in this region (f1 /0.0 /0.4). Raman spectra obtained in the 1400/ 1700 cm 1 range are shown in Fig. 7 and depict similar behavior but related to other vibrational modes. The most intense band in PANI spectrum (f1 /0.0) is located at 1475 cm 1 and is related to C /N stretching. It can be seen that it shifts to 1490 cm 1 for poly(2-ethylaniline) and there is a progressive shift from PANI value to poly(2-ethylaniline)one when f1 changes from 0.0 to 0.5. In this way, Raman spectroscopy data confirms the formation of a new material and reinforces the conclusion that the true composition of the copolymer differs from that of the feed solution from which it was prepared.
4. Conclusions Copolymers of aniline and 2-ethylaniline can be easily synthesized by chemical and electrochemical methods, but the composition of the copolymeric chains differs from the feed solution composition. This papers shows that materials obtained by electrochemical polymerization present the same characteristics of those synthesized by the ordinary chemical method. Cyclic voltammetry and solubility studies, as well as Raman spectroscopy and conductivity measurements have shown that copolymers formed from feed solutions containing less than 30% (in moles) of 2-ethylaniline show intermediate properties between those of the homopolymers, and these properties vary gradually with the amount of 2-ethylaniline units in the copoly-
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Table 1 Solubility (in g/10 ml) of polyaniline, poly(2-ethylaniline) and poly(aniline-co-2-ethylaniline) in NMP, CHCl3, and DMF Polymer (f1)
NMP Copolymer
0.0 0.1 0.3 0.5 0.7 0.9 1.0
0.22 0.30 Complete Complete Complete Complete
CHCl3 Mixture
0.27 0.39 0.44 Complete Complete Complete
DMF
Copolymer
Mixture
0.26 0.26 0.24 0.35 0.35 0.46 0.34 Complete 0.35 Complete 0.44 Complete
Copolymer 0.27 0.34 Complete Complete Complete Complete
Mixture
0.29 0.28 0.40 0.43 Complete Complete
the same as pure polyaniline (11 S cm 1) and has shown increased solubility in DMF and NMP 0.30 g/10 ml, while for the pure polyaniline its solubility is 0.22 g/10 ml. Another copolymer that has shown interesting properties is the f1 /0.9, that has shown a conductivity of 1 /102 S cm 1, one order of magnitude higher than pure poly(2-ethylaniline) (1 /103 S cm 1) and is completely soluble in all the solvents studied. Using copolymers it is possible to combine two important properties for technological applications, the conductivity, solubility in common organic solvents and to tailor the properties of the material depending on the copolymer composition. Fig. 6. Raman spectra of the base form powders of homopolymers and copolymers. Numbers depicted in the left of the figure correspond to f1. (l0 632.8 nm).
Acknowledgements Brazilian agencies CNPq and FAPESP (Proc. No. 98/ 7624-8) are gratefully acknowledged for financial support. A.L.S. and L.M.L. thank FAPESP (Proc. No. 98/ 14944-9) and CNPq, respectively, for fellowships granted. Authors are indebted to Laborato´rio de Espectroscopia Molecular (IQ-USP) for Raman facilities.
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Fig. 7. Raman spectra of the base form powders of homopolymers and copolymers. Numbers depicted in the right of the figure correspond to f1. (l0 632.8 nm).
meric chains. Above 30% (in moles) of 2-ethylaniline the properties of the copolymers are almost the same as pure poly(2-ethylaniline). The base form powders of the copolymers have shown an improved solubility compared to parent polyaniline and mechanical mixture of homopolymers, and an enhanced conductivity compared to poly(2-ethylaniline). For example, a poly(aniline-co-2-ethylaniline) (f1 /0.1) has shown a conductivity of 7 S cm1 almost
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On the electrochemical and spectroscopic properties of a soluble polyaniline parent copolymer A.L. Schemid, L.M. Lira, S.I. Co´rdoba de Torresi * Instituto de Quı´mica, Universidade de Sa˜o Paulo, C.P. 26077, 05513-970 Sa˜o Paulo (SP), Brazil Received 13 December 2001
Abstract Copolymers of aniline and 2-ethylaniline can be easily formed by electrochemical or chemical oxidation in acidic medium, but the composition of the polymeric chain differs from the composition of the synthesis solution. Copolymers formed from precursor solutions containing less than 30% (in moles) of 2-ethylaniline show intermediate properties between those of the homopolymers, and these properties vary gradually with the amount of 2-ethylaniline units in the copolymeric chain. The properties of those copolymers have been studied by UV /Vis and resonance Raman spectroscopies, cyclic voltammetry, two-probe conductivity and solubility measurements. Results obtained by these techniques corroborate the formation of a block copolymeric material. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyaniline; Copolymerization; Electrochemistry; Raman; d.c. conductivity
1. Introduction The research in electroactive polymers in the past few years has received considerable attention [1,2], in particular aromatic polymers. Among these polymers, the polyaniline (PANI) has been one of the most widely studied conducting polymers because of its optical and electrochemical properties and due to its chemical and oxidative stability, which can be used in many applications, including rechargeable batteries, corrosion protection, light emitting diodes, molecular sensors, electrochromic devices, etc. [3 /7]. However, as it is common with other conjugated polymers, polyaniline is limited by poor thermal processability and solvent solubility [8], due to the stiffness of its backbone. Consequently, their post-synthesis proces-
* Corresponding author. Tel.: 55-11-3091-2165; fax: 55-113091-3890. E-mail address: [email protected] (S.I. Co´rdoba de Torresi).
sability is quite difficult, and a lot of work has been done recently to overcome this problem [9 /18]. Improved solubility can be achieved by introducing bulky alkyl substituents into the polyaniline backbone, but limitations are then imposed on the conductivity of the polymer produced. The conductivity of polyaniline and the solubility of substituted polyanilines can be achieved by copolymerization. These copolymers of aniline and substituted anilines show improved solvent solubility, while maintaining high electrical conductivity which can be readily tailored by varying the composition of the copolymer [19]. In this paper, we report the preparation, by electrochemical methods, of some soluble copolymers of aniline and 2-ethylaniline and the materials are compared with those prepared by chemical oxidative procedure. By electrochemical methods and Raman spectroscopy we have observed that the composition of the copolymeric chain differs from the precursor solution; copolymers formed from precursor solutions containing less than 30% (in moles) of 2-ethylaniline show intermediate properties between those of the homopolymers, and
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these properties vary gradually with the amount of 2ethylaniline units in the copolymeric chain. This is an evidence of the formation of a new material, and not only a mixture of homopolymers. Those materials can be used for technological applications, due to some of their interesting properties: good conductivity, electrochromism, and solubility in some common organic solvents, like N -methyl-2-pyrrolidinone or chloroform, those properties are also reported in this paper.
2. Experimental 2.1. Electrochemical synthesis Polyaniline (PANI), poly(2-ethylaniline) and poly(aniline-co-2-ethylaniline) films were electropolymerized onto glass/ITO (Delta Technologies, R 5/20 V/I) transparent conducting substrates by applying triangular potential sweeps (0.01 V s 1) in the /0.25 to 0.75 V (PANI films) and /0.25 to 0.85 V (substituted PANI and copolymers) potential ranges. All potentials are referred to the saturated Ag/AgCl electrode. The polymerization was carried out in a 0.5 mol L1 monomer/ 1.0 mol L 1 HCl electrolytic solution. In the case of the synthesis of the copolymers, the concentration of monomer or the mixture of monomers was maintained constant at 0.5 mol L1 and the proportion in moles of 2-ethylaniline/aniline was varied in the following form: 1:9 (f1 /0.1), 3:7 (f1 /0.3), 5:5 (f1 /0.5), 7:3 (f1 /0.7), and 9:1 (f1 /0.9), in order to cover a broad range of solution composition (f1), where f1 is defined as: f1
n2
ethylaniline
ntotal
The amount of electrodeposited material was controlled through the electrochemical charge passed, all films being grown until 4.10 3 C cm 2. 2.2. Chemical synthesis and solubility studies The homopolymers and the copolymers were chemically synthesized by oxidative polymerization with K2S2O8 in acid media at low temperature (273 K), following the common procedure for polyaniline synthesis [20]. A solution containing 100 ml of 0.165 mol l 1 potassium peroxidisulfate was added dropwise in 100 ml of 0.165 mol l 1 monomer or mixture of monomers/ 1.0 mol l 1 HCl solution, then it was stirred and placed in an ice bath. The mixture was allow to stir over a period of 6 h. In the case of the synthesis of the copolymers, the following solutions with different molar fractions of 2-ethylaniline (f1), were added in water to achieve the total volume of 100 ml:
1.83 ml 2-ethylaniline (0.149 mol l1)/0.15 aniline (0.016 mol l1)/4.14 ml HCl (f1 /0.9) 1.42 ml 2-ethylaniline (0.115 mol l1)/0.45 aniline (0.050 mol l1)/ 4.14 ml HCl (f1 /0.7) 1.02 ml 2-ethylaniline (0.083 mol l1)/0.75 aniline (0.083 mol l1)/ 4.14 ml HCl (f1 /0.5) 0.61 ml 2-ethylaniline (0.050 mol l1)/1.05 aniline (0.115 mol l1)/4.14 ml HCl (f1 /0.3) 0.20 ml 2-ethylaniline (0.016 mol l1)/1.35 aniline (0.149 mol l1)/4.14 ml HCl (f1 /0.1)
ml ml ml ml ml
The resulting green precipitate was filtered and dried in air overnight. This procedure leads to the polymer in the doped acid form. Part of this material was dedoped to the basic form by washing the powder with a 1.0 mol l 1 KOH solution under vigorous stirring during 4 h. After that, the polymer was filtered, washed with deionized water and dried again. The solubility studies of homopolymers, copolymers and mechanical mixtures of homopolymers with the same molar composition of the copolymer’s feed solution in NMP, CHCl3 and DMF were performed using the following procedure. The base form of each powder was ground with a mortar and pestle and put through an 80-mesh sieve before solubility testing. An amount of 0.5 g of the powder was added to 10 ml of each solvent and stirred for 1 h before filtering. The filter paper was pre-weighted, and the filtrate was allowed to evaporate at 30 8C and 200 mbar, before weighting the papers again to calculate the solubility of the polymers. 2.3. Spectroelectrochemical measurements Spectroelectrochemical experiments were performed under potentiodynamic conditions using a potentiostat/ galvanostat Autolab PGSTAT30 (Ecochemie) by placing the electrochemical cell in the optical pathway of a digital fiber optic wheel Spectrophotometer (WPIs). After deposition, films were carefully rinsed with purified water and placed in a one-compartment electrochemical cell. The electrolytes were 0.8 mol l1 HCl, HClO4, p -toluene sulfonic acid (PTSA) and camphor sulfonic acid (HCSA) solutions. A platinum wire was used as the counter electrode and all potentials are referred to the saturated Ag/AgCl electrode. All solutions were prepared from A.R. chemicals and purified water (Elga System UHQ), the monomers were distilled under vacuum prior use and HCSA was recrystallized with ethyl acetate for spectroelectrochemical measurements. 2.4. Raman spectroscopy and conductivity measurements Chemically synthesized polymers were analyzed by Raman spectroscopy using a Renishaw Raman Imaging System 3000 coupled with an He /Ne laser (Spectra
A.L. Schemid et al. / Electrochimica Acta 47 (2002) 2005 /2011
Physics, mod. 127, l0 /632.8 nm), and equipped with an Olympus metallurgical microscope and a CCD detector. Laser power was kept below 0.7 mW at the sample to avoid thermal degradation. Resistivity measurements of copolymer powders were performed by the two point method using an Impedance Analyzer Autolab PGSTAT30 (Ecochemie) with FRA module. A d.c. potential of 0.0 V with a modulation of 5 mV (rms) was imposed in the 10 kHz /10 mHz frequency range.
2007
Electrochemical synthesis of poly(aniline-co-2-ethylaniline) from different precursor solutions produces homogeneous and adherent green films. They show great electroactivity in acidic media and the voltammetric profiles are very similar to the potential values of the current peaks being located at intermediate values of the typical electrochemical responses of the homopolymers. Fig. 1 shows the potentiodynamic j/E profiles of PANI, poly(2-ethylaniline), and copolymers formed from different 2-ethylaniline/aniline ratio polymerization solutions: 1:9, 5:5 and 9:1. It should be observed that the electrochemical response of the copolymers is not the simple addition of those related to PANI and poly(2-ethylaniline); this fact would be an indication of the formation of a new material with differentiated properties. As it was already shown in previous work [21 /23], the presence of an alkyl substituent in the benzene ring provokes the shift of leucoemeraldine/ emeraldine redox couple to more positive potentials and the emeraldine/pernigraniline couple to less positive potentials. This fact can be explained on the basis of steric and electronic effects of the /C2H5 group. As it was already shown in our previous work [22,23], the shift of the redox process depends on the chemical nature of the dopant; in this case that the electrolytic solution is HCl, the two redox processes overlapped in a
large peak in poly(2-ethylaniline) j /E potentiodynamic profile. Conklin et al. [8] have already reported that the chemical formation of copolymers between PANI and its ethylated derivative leads to block copolymerization with long blocks of 2-ethylaniline and short blocks of PANI. The same behavior has been reported for copolymers prepared from PANI and other alkylated derivates, such as o -toluidine, m -toluidine and N butylaniline. This was determined by 1H NMR measurements and the molar fraction of 2-ethylaniline was calculated from the ratio of integrated area of the methylated protons and aromatic ones. In this way, it was found that the reactivity ratio, that is to say the ability of the monomer to enter to the copolymer chain, is higher in 2-ethylaniline than in aniline. Thus, the molar composition of the copolymer is very different from that of the precursor solution. Specifically, in the case of poly(aniline-co-2-ethylaniline), it was found that when the amount of 2-ethylaniline in the feed solution (f1) was greater than 40%, the composition of the copolymer is almost 100% poly(2-ethylaniline). Fig. 2 shows the peak potential values (Ep) of both the first and the second oxidation peaks for copolymers cycled in p -toluene sulfonic acid electrolytic solution. Experiments in other electrolytes (HCl, HClO4 and HCSA) were also performed but they are not shown for simplicity; anyway, the electrochemical responses were similar. As can be seen, the increase of the first potential peak and the decrease of the second one is gradual when f1 varies from 0.0 to 0.4. From this point till f1 /1.0, the difference between the redox processes (DEp) remains constant, indicating that the material is almost the same in all this range of feed solution composition. So, considering voltammetric profiles, copolymers formed by electrochemical methods give the same behavior of those prepared by oxidative polymerization. It is well known that substituted polyanilines present a strong electrochromism independently of the electrolyte used. In the case of poly(2-ethylaniline) [22] and its
Fig. 1. j /E potentiodynamic profiles of the homopolymers PANI ( */) and poly(2-ethylaniline) ( */), and the copolymers prepared from different solution compositions: f1 0.9 (- × / × -), f1 0.5 (---) and f1 0.1 ( × × × × × × ). Electrolytic solution: 1.0 mol l 1 HCl. v 10 mV s 1.
Fig. 2. Peak potential as a function of the composition of feed solution (f1). (m) leucoemeraldine/emeraldine redox couple and (k) emeraldine/pernigraniline redox couple. Electrolytic solution 1 mol l 1 PTSA. B-spline curve was used to link the data points.
3. Results and discussion
2008
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copolymers with aniline they change from transparent yellow to blue with the potential. Both electrochromism and electroactivity are related to the injection/ejection of the charge carriers into/from the polymeric matrix. In this case we can associate the absorbance (A /obc ) with the inserted charge per unit area, as was already done for inorganic and organic materials [24,25]: dA dc jo ob (1) dt dt zF where o is the molar absorptivity of the film, b is the film thickness, c is the concentration of optically active sites on the electrode, j is the current density, z is the number of electrons participating in the redox reaction and F is the Faraday constant. If the reaction responsible for the coloration change of the material is the same producing a redox wave during the voltammetry and all the current is consumed in the electrochromic process, the dA /dt versus E profile must coincide with the j /E profile obtained under the same experimental conditions. This statement is correct if all current related to the electrochromic process has a faradaic nature, separating, in this way, faradaic from capacitive currents. Fig. 3 shows the voltammograms of a poly(aniline-co2-ethylaniline) (f1 /0.1) together with the transmittance variation and the dA /dt curves calculated from the transmittance variation for four different wavelengths in HCl solution. As was shown previously, the voltammetric profile depends strongly on f1 in the 0.0 /0.4 range; specifically, in the case of the copolymer with
Fig. 3. j /E ( */) and %T or dA /dt ( × × × × × ) potentiodynamic profiles recorded at different wavelengths for a poly(aniline-co-2-ethylaniline) (f1 0.1) film in 0.8 mol l 1 HCl electrolytic solution. v 10 mV s 1.
f1 /0.1, the electrochemical response is very unusual and it is possible to distinguish a third peak at higher potentials. Results obtained from NMR technique [8] have shown that the composition of this copolymer corresponds to ca. 50% of the two homopolymers. The left part of Fig. 3 shows the %T versus E profiles obtained simultaneously with the voltammograms. It can be observed that transmittance changes in a different way for each wavelength when the film is oxidized and, considering only these plots, it is clear that it is rather difficult to correlate each redox process to changes in the optical absorption. This difficulty arises by the fact that absorption (or transmittance) are integral quantities while the current is a differential one; that is why it would be better to try to do this relationship by comparing two differential quantities, current and dA /dt. As can be seen in the right part of Fig. 3, the first redox process is associated with chromophore groups absorbing at 420 and 700 nm so the redox process at 0.3 V could be related to the leucoemeraldine/emeraldine. The dA /dt profiles recorded at 520 and 620 nm show that, at these wavelengths, the main absorption is related to the third and second peaks in the voltammograms. This is clearer from Fig. 4 where the UV /Vis spectra as a function of the potential are shown. In 420 nm the major absorbance variation on the spectral bands occurs on the potential range of 0.2 /0.3 V, where the leucoemeraldine/ emeraldine transition occurs. In 520 nm the major absorbance variation occurs in the 0.6 /0.7 V potential range and this variation is related to the emeraldine/ pernigraniline transition. In 620 and 700 nm, the major variation occurs in the 0.4 /0.5 V potential range, but there are intense contributions from other potential ranges which makes difficult the correlation of the voltammogram peaks and the absorption bands. As was already pointed out, the use of the dA /dt method allows a more direct association of the absorption bands and the redox processes because of the comparison of two differential quantities, j and dA /dt . Independently on the copolymer composition, the electrochromic behavior is similar and can be followed by this treatment.
Fig. 4. In situ UV /Vis spectra of a poly(aniline-co-2-ethylaniline) (f1 0.1) film in 0.8 mol l 1 HCl solution.
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As was mentioned above, the combination of good processability and high conductivity of these materials is the main goal of the synthesis and characterization of copolymers. Fig. 5 shows the conductivity of the different homopolymers and copolymers measured by the two points method. As can be seen from the figure, the conductivities vary in a range from 10 S cm 1 (pure PANI) to 1 /10 3 S cm 1 (pure poly(2-ethylaniline)). It is very important to point out that the greater variation occurs in the range of f1 /0.1 /0.3, in good agreement with the fact that the greater variation of copolymer composition takes place in this specific range. Another important feature regarding these results, is the increase of the conductivity of the material in one order of magnitude by the addition of a small proportion of aniline units in the polymeric chains. Higher amounts of aniline will not change the conductivity until a proportion of 70% in the feed solution is reached. The variation of conductivity must be related to solubility tests results performed with the copolymers and mechanical mixtures of the homopolymers, and they are shown in Table 1. As can be seen, improved solubility of the copolymers, compared to polyaniline, is always obtained for the three solvents employed, NMP, CHCl3 and DMF. The measurements were performed following the procedure already described in the literature [18], but with powders obtained by electrochemical polymerization. The higher solubility must be related to the introduction of bulky alkyl chains in the PANI backbone and, again, it can be inferred that for compositions of the feed solutions different from f1 / 0.0 /0.4, the solubility is equal to that corresponding to poly(2-ethylaniline). Raman spectroscopy is a very useful tool for the characterization of conducting polymers giving information about the structure of different segments in the polymer backbone [26,27]. It is well known that the Raman spectra of poly conjugated systems is dependent on the laser’s wavelength, due to resonance effect [28]. In the case of polyanilines, the l0 /632.8 nm excitation line enhances the intensity of the bands corresponding
Fig. 5. Plot of conductivity (s ) vs. co-monomer feed composition (f1). B-spline curve was used to link data points.
2009
to oxidized segments; so that Raman spectra of the base form of polyanilines are well defined at this radiation. Figs. 6 and 7 show the Raman spectra of copolymers prepared from feed solutions of different compositions recorded in two wavenumber regions. In 1995, Berrada [29] has reported the Raman spectra of emeraldine base form of poly(2-methylaniline) at 676 nm, which are very similar to those obtained with the poly(2-ethylaniline) at 632.8 nm. Fig. 6 shows the spectra displayed in the 1000 /1400 cm 1 region. The spectrum of PANI (f1 /0) shows two intense bands at 1164 and 1220 cm 1 corresponding to in-plane C /H bending (d C /H) and C /N stretching (nC /N), respectively [30]. In the case of the poly(2-ethylaniline) (f1 /1.0) these modes are shifted to 1156 and 1223 cm 1, due to the presence of the alkyl substituents in the orto position of the rings. In this spectrum other bands located at 1120 and 1242 cm 1, corresponding to C /C stretching and C /C twisting of the alkyl chains [29], are also depicted. The relative intensity of these bands diminishes when the amount of aniline in the feed solution is increased from 70 to 100% and remains constant for lower values. This fact is in excellent agreement with results obtained by conductivity measurements or electrochemistry, showing that the effective formation of a copolymer occurs in this region (f1 /0.0 /0.4). Raman spectra obtained in the 1400/ 1700 cm 1 range are shown in Fig. 7 and depict similar behavior but related to other vibrational modes. The most intense band in PANI spectrum (f1 /0.0) is located at 1475 cm 1 and is related to C /N stretching. It can be seen that it shifts to 1490 cm 1 for poly(2-ethylaniline) and there is a progressive shift from PANI value to poly(2-ethylaniline)one when f1 changes from 0.0 to 0.5. In this way, Raman spectroscopy data confirms the formation of a new material and reinforces the conclusion that the true composition of the copolymer differs from that of the feed solution from which it was prepared.
4. Conclusions Copolymers of aniline and 2-ethylaniline can be easily synthesized by chemical and electrochemical methods, but the composition of the copolymeric chains differs from the feed solution composition. This papers shows that materials obtained by electrochemical polymerization present the same characteristics of those synthesized by the ordinary chemical method. Cyclic voltammetry and solubility studies, as well as Raman spectroscopy and conductivity measurements have shown that copolymers formed from feed solutions containing less than 30% (in moles) of 2-ethylaniline show intermediate properties between those of the homopolymers, and these properties vary gradually with the amount of 2-ethylaniline units in the copoly-
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2010
Table 1 Solubility (in g/10 ml) of polyaniline, poly(2-ethylaniline) and poly(aniline-co-2-ethylaniline) in NMP, CHCl3, and DMF Polymer (f1)
NMP Copolymer
0.0 0.1 0.3 0.5 0.7 0.9 1.0
0.22 0.30 Complete Complete Complete Complete
CHCl3 Mixture
0.27 0.39 0.44 Complete Complete Complete
DMF
Copolymer
Mixture
0.26 0.26 0.24 0.35 0.35 0.46 0.34 Complete 0.35 Complete 0.44 Complete
Copolymer 0.27 0.34 Complete Complete Complete Complete
Mixture
0.29 0.28 0.40 0.43 Complete Complete
the same as pure polyaniline (11 S cm 1) and has shown increased solubility in DMF and NMP 0.30 g/10 ml, while for the pure polyaniline its solubility is 0.22 g/10 ml. Another copolymer that has shown interesting properties is the f1 /0.9, that has shown a conductivity of 1 /102 S cm 1, one order of magnitude higher than pure poly(2-ethylaniline) (1 /103 S cm 1) and is completely soluble in all the solvents studied. Using copolymers it is possible to combine two important properties for technological applications, the conductivity, solubility in common organic solvents and to tailor the properties of the material depending on the copolymer composition. Fig. 6. Raman spectra of the base form powders of homopolymers and copolymers. Numbers depicted in the left of the figure correspond to f1. (l0 632.8 nm).
Acknowledgements Brazilian agencies CNPq and FAPESP (Proc. No. 98/ 7624-8) are gratefully acknowledged for financial support. A.L.S. and L.M.L. thank FAPESP (Proc. No. 98/ 14944-9) and CNPq, respectively, for fellowships granted. Authors are indebted to Laborato´rio de Espectroscopia Molecular (IQ-USP) for Raman facilities.
References
Fig. 7. Raman spectra of the base form powders of homopolymers and copolymers. Numbers depicted in the right of the figure correspond to f1. (l0 632.8 nm).
meric chains. Above 30% (in moles) of 2-ethylaniline the properties of the copolymers are almost the same as pure poly(2-ethylaniline). The base form powders of the copolymers have shown an improved solubility compared to parent polyaniline and mechanical mixture of homopolymers, and an enhanced conductivity compared to poly(2-ethylaniline). For example, a poly(aniline-co-2-ethylaniline) (f1 /0.1) has shown a conductivity of 7 S cm1 almost
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