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Characterization of poly(N-alkylanilines) by Raman spectroscopy
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Synthetic Metals 157 (2007) 974–983
Characterization of poly(N-alkylanilines) by Raman spectroscopy Maija Blomquist a,b , Tom Lindfors a,∗ , Ari Ivaska a b
a Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Abo ˚ Akademi University, Biskopsgatan 8, 20500 Turku/Abo, ˚ Finland The Finnish National Graduate School in Nanoscience (NGS-NANO), Nanoscience Center, P.O. Box 35, 40014 University of Jyv¨askyl¨a, Finland
Received 26 January 2007; received in revised form 9 August 2007; accepted 3 October 2007 Available online 19 November 2007
Abstract Thin films of poly(N-alkylaniline) were synthesized in acidic aqueous solution and in mixtures of aqueous and organic solvents. The polymer films (alkyl = methyl, ethyl, propyl and butyl) were characterized by Raman spectroscopy with the excitation wavelengths of 514.5, 632.8 and 780 nm. The main Raman bands have been characterized for the leucoemeraldine, emeraldine and pernigraniline oxidation states between −0.2 and 0.8 V (vs. Ag|AgCl). This fundamental study shows that the structure of the half-oxidized emeraldine form contains quinoid units, which supports the commonly accepted oxidation and reduction scheme of poly(N-alkylanilines). © 2007 Elsevier B.V. All rights reserved. Keywords: Poly(N-alkylanilines); Electropolymerization; Raman and UV–vis spectroscopy; Cyclic voltammetry
1. Introduction Poly(N-alkylanilines) (PNANIs) are interesting materials because the addition of side groups to polyaniline (PANI) backbone results in improved solubility and processability [1,2]. PNANIs have despite of this been studied much less than PANI. Poly(N-methylaniline) (PNMA) has so far gained most attention among PNANIs. In earlier works, both electrochemically and chemically synthesized PNMA, poly(N-ethylaniline) (PNEA), poly(N-propylaniline) (PNPA) and poly(N-butylaniline) (PNBA) have been reported [1,3–32]. The electrochemistry of PNANIs is generally agreed to follow Scheme 1, which shows the leucoemeraldine (LE), emeraldine (E) and pernigraniline (PN) oxidation states of PNANIs [12,15]. The emeraldine form is the only electrically conducting form of the PNANIs. In the literature, PNANIs has not been systematically characterized with Raman spectroscopy. Only handful of papers, to the best of our knowledge, deals with Raman spectroscopy of substituted polyanilines. Quillard et al. compared Raman spectra of PANI and poly(o-toluidine) [33]. Also photoinduced spectra of poly(o-ethylaniline) and PANI were compared
∗
Corresponding author. Tel.: +358 2 2154422; fax: +358 2 2154479. E-mail address: [email protected] (T. Lindfors).
0379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2007.10.001
in their work. The substituted polymers have a change in symmetry compared with PANI, which leads to new peaks in the spectra in comparison to the PANI spectrum. Malinauskas et al. have characterized PNMA and a copolymer of N-methylaniline and N(3-sulphopropyl)-aniline by Raman spectroscopy [34]. Kilmartin and Wright reported a Raman study of four substituted polyanilines: PNMA, poly(m-methylaniline), poly(o-methoxyaniline) and poly(o-ethoxyaniline) [15]. The spectra showed a recognizable PANI pattern. Raman spectra of the conducting form of ring-sulphonated polyaniline (SPAN) and PANI were compared by Niaura et al. [35]. Some of the Raman bands shifted to higher frequencies in the Raman spectra of SPAN compared to PANI, which indicate structural differences between these two polymers. Structural and electronic characteristics of sulphonated polyanilines and poly(o-methoxyanilines) were compared with Raman spectroscopy by Bernard et al. [36]. They concluded that the substituent could modify the polaronic nature to become more random. Furthermore, Wei et al. have studied the Raman spectra of PNMA electropolymerized in an organic solvent [8]. In this paper, PNMA, PNEA, PNPA and PNBA films were polymerized in acidic aqueous solutions and characterized by Raman spectroscopy. The PNMA and PNBA were for comparison also polymerized in presence of 10% (v/v) acetonitrile (ACN) or dimethyl sulfoxide (DMSO). After the polymerization, the polymer films were systematically studied in the
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with 0.3 m Al2 O3 powder and rinsed with deionized water prior to the film preparation. The potential was controlled with an Autolab (PGSTAT 20 or PGSTAT 100) potentiostat. Prior to all measurements the polymerization and characterization solutions were purged with nitrogen for 20 min. The solutions were also blanketed with nitrogen before they were transferred to the electrochemical cell and during the experiments. After the polymerization and prior to the Raman measurements, the films were conditioned overnight in the cell filled with 1.0 M HClO4 . 2.3. Raman spectroscopy
Scheme 1. The redox mechanism of poly(N-alkylanilines) showing the leucoemeraldine (LE), emeraldine (E) and pernigraniline (PN) forms. X and A− are the alkyl chain and the mobile anion, respectively.
potential range of the leucoemeraldine, emeraldine and pernigraniline transitions. The Raman spectra were measured with the excitation wavelengths of 514.5, 632.8 and 780 nm. This paper is a continuation of our earlier work [25] on PNMA and PNBA films synthesized in mixtures of aqueous and organic solvents. One of the main objectives of this work is to study how the length of the N-substituted alkyl chain affects the Raman spectra of PNANIs. 2. Experimental 2.1. Chemicals N-Methylaniline (NMA) was obtained from Fluka, Nethylaniline (NEA) and N-butylaniline (NBA) from Aldrich, and N-propylaniline (NPA) was obtained from TCI Europe nv. ACN and DMSO were obtained from Riedel-de Ha¨en. All chemicals were used as received. 2.2. Electropolymerization in a flow cell The electropolymerization was performed in a flow cell described in our earlier work [37]. The electropolymerization of 0.1 M NMA, NEA, NPA or NBA was done in the flow cell in 1.0 M HClO4 or in a solvent mixture containing 10% (v/v) organic solvent by cycling the potential between −0.2 and 0.85 V (50 cycles; 50 mV/s). The polymer films formed on the GC disc working electrode (A = 0.07 cm2 ) were characterized by cyclic voltammetry in the flow cell in aqueous monomer free solutions of 1.0 M HClO4 (10 cycles; 50 mV/s). The polymerization and characterization solutions passed continuously through the cell with a flow rate of approximately 0.3 ml/min. The solutions were transferred to the cell with a peristaltic pump. A Pt wire served as the counter electrode and Ag|AgCl wire as the reference electrode. The GC disk electrode was always polished
The flow cell including the working electrode was filled with 1.0 M HClO4 and placed in a 90◦ angle relative to the incoming laser beam and the scattering was therefore collected at a 180◦ configuration. The Raman measurements were done with the 514.5 nm (LaserPhysics, Ar ion laser), 632.8 nm (Renishaw, HeNe laser) and 780 nm (Renishaw, NIR diode laser) excitation wavelengths for PNMA and PNBA films polymerized in aqueous solution. The 780 nm laser was used for the PNEA and PNPA films, and for the PNMA and PNBA films, which were polymerized in the mixtures of aqueous and organic solvents. The Raman spectra were measured with potential intervals of 0.1 V from −0.2 to 0.8 V by stepwise increasing the potential. The films were first reduced at −0.2 V for 15 min before the Raman spectrum was recorded at this potential. Thereafter, the potential was always kept 6 min at the other potentials before the spectra were measured. The laser light was switched on just prior to the Raman measurements in order to avoid unnecessary irradiation of films. All measurements were conducted with a Renishaw Raman imaging microscope (with WireTM v1.3 Raman software) connected to a Leica DMLM microscope. The spectrometer was always calibrated against a Si-standard (520.0 cm−1 ) before starting the Raman measurements. The Raman measurements were conducted with 50% of the maximum laser power in order to avoid degradation of the polymer film. The full power of all laser wavelengths was ∼20 mW, but after passing through the Raman and Leica microscope, the power of the laser beams were reduced approximately to 4 mW (514.5 nm), 1.5 mW (632.8 nm) and 5 mW (780 nm). The detector time used was 30 s (632.8 and 780 nm) and 2 × 30 s (514.5 nm). 3. Results and discussion 3.1. Polymerization of poly(N-alkylanilines) in the flow cell The cyclic voltammograms (CVs) recorded during polymerization of NMA, NEA, NPA and NBA are shown in Fig. 1. The redox peak currents observed during polymerization of PNPA and PNBA are lower than for PNMA and PNEA. This was also observed in our earlier studies [25,38]. The decrease in the redox peak currents of PNPA and PNBA are probably due to the increase of the alkyl chain length, which influences the polymerization process resulting in the formation of less polymeric material on the electrode surface. The longer alkyl chains increase the ring torsion and lower the conductivity of the formed film.
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Fig. 1. Polymerization of 0.1 M (a) NMA, (b) NEA, (c) NPA, (d) NBA in an aqueous solution of 1.0 M HClO4 solution. Every 10th cycle is shown and the 50th cycle (last cycle) is indicated with 50; v = 50 mV/s.
The CVs recorded during characterization of the PNMA, PNEA, PNPA and PNBA films are shown in Fig. 2a. For PNMA the LE/E and the E/PN transitions can be observed at Ep,a1 = 305 mV/Ep,c1 ≈ 265 mV and Ep,a2 = 499 mV/Ep,c2 = 455 mV, respectively. For PNEA the same transitions are located at Ep,a = 521 mV/Ep,c = 452 mV, for PNPA at Ep,a = 646 mV/Ep,c = 492 mV, and for PNBA at Ep,a = 708 mV/Ep,c ≈ 290 mV. The one broad oxidation peak for all the other PNANIs expect for PNMA are due to overlapping of the LE/E and E/PN transitions. The shift
of the LE/E transition to higher potentials and the E/PN transition to lower potentials with an increase of the alkyl chain length is in good accordance with other studies [12,39]. The potential intervals of the LE/E and E/PN transitions observed in the CVs of the PNMA and PNBA films (Fig. 1, curves 1 and 4) are in good accordance with the potential intervals of the same transitions, which were previously observed in electrochemical in situ UV–vis measurements of these films [25]. The UV–vis spectra also indicate that the laser excitation wavelengths used in this study enhance the vibrations
Fig. 2. Characterization of the PNANI films in 1.0 M HClO4 (5th cycle; v = 50 mV/s). (a) (1) PNMA, (2) PNEA, (3) PNPA, (4) PNBA; (b) PNMA films prepared in 1 M HClO4 containing 10% (v/v) (1) ACN, (2) DMSO and PNBA films prepared with 10% (v/v) (3) ACN and (4) DMSO.
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Fig. 3. Raman spectra of (A) PNMA and (B) PNBA film. Excitation wavelength: 514.5 nm. The potentials applied are indicated with (a) −0.2 V, (b) 0 V, (c) 0.2 V, (d) 0.4 V, (e) 0.6 V and (f) 0.8 V. The spectra were measured with 0.1 V intervals.
originating from the quinoid structures in the following order: 780 nm > 632.8 nm > 514.5 nm. The CVs of PNMA and PNBA polymerized in aqueous 1.0 M HClO4 in presence of 10% ACN or 10% DMSO are shown in Fig. 2b. For PNMA prepared in a mixed solvent containing 10% ACN, the LE/E and the E/PN transitions can be observed at Ep,a1 ≈ 440 mV/Ep,c1 ≈ 270 mV and Ep,a2 = 573 mV/Ep,c2 = 466 mV, respectively. For PNMA prepared in a solution containing 10% DMSO, the anodic LE/E and the E/PN transitions can be observed at Ep,a1 = 294 mV/Ep,c1 ≈ 250 mV and Ep,a2 = 536 mV/Ep,c = 565 mV. A third oxidation peak is observed at ∼540 mV, which is probably due to degradation products of PNMA. PNBA films polymerized in presence of 10% (v/v) ACN or DMSO have only one broad oxidation and reduction peak. The overlapping LE/E and E/PN transitions can be observed for the PNBA (10% ACN) film at Ep,a = 668 mV/Ep,c ≈ 375 mV and for the PNBA (10% DMSO) film at Ep,a = 705 mV/Ep,c = 477 mV, respectively. The organic solvents, ACN and DMSO, were chosen for this study on basis of the results from our previous work [25]. It was found that the addition of ACN into the polymerization solution increased the oxidation and reduction currents of PNMA. On the other hand, the addition of DMSO decreased the redox currents. The films polymerized in mixtures of aqueous and organic solvents are hereafter referred to as PNMA (10% ACN), PNMA (10% DMSO), PNBA (10% ACN) and PNBA (10% DMSO). Films polymerized in acidic aqueous solutions are notified as PNMA, PNEA, PNPA and PNBA.
3.2. Raman measurement The Raman spectra were measured from −0.2 to 0.8 V by increasing the potential stepwise with 0.1 V. The UV–vis spectra, which were reported earlier by us [25], show that the 632.8 and 780 nm laser excitation wavelengths mostly enhance vibrational modes of the quinoid units. The vibrations related to the nitrogen atoms are expected to be mainly affected by the Nsubstitution compared to unsubstituted PANI [15]. According to our earlier reported UV–vis spectra of PNMA [25], the vibrational modes of the quinoid units are only slightly enhanced by the 514.5 nm laser compared to the benzenoid units. In the case of PNBA, the 514.5 nm laser is expected to enhance rather equally vibrations originating from both quinoid and benzenoid units [33]. In this study, the main vibrational bands in the Raman spectra of poly(N-alkylanilines) are interpreted. 3.2.1. Excitation wavelength of 514.5 nm 3.2.1.1. Poly(N-methylaniline). The Raman spectra of PNMA and PNBA measured with the 514.5 nm laser excitation wavelength are shown in Fig. 3. The spectra have been separated from each other for the sake of clarity. The assignments of the main vibrational bands of the Raman spectra are given in Table 1 for PNMA and in Table 2 for PNBA. The Raman band indicated with an asterisk in Figs. 4–6 originates from HClO4 . All references to the Raman bands, which were interpreted in this study, are given in Table 1. The PNMA spectrum measured at −0.2 V (Fig. 3A, spectra a) shows strong vibrational bands at 1618, 1358 and 1185 cm−1 .
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Table 1 Assignments of the Raman bands of PNMA obtained with the laser excitation wavelengths of 514.5, 632.8 and 780 nm
The benzenoid and quinoid units are denoted by B and Q, respectively. The electrically conducting emeraldine form is abbreviated as E. a Shoulder.
At this potential, the PNMA film is in the fully reduced LE form. The bands at 1618 and 1185 cm−1 are assigned to C C stretching and C H in-plane bending of benzenoid units, respectively (Table 1). The band at 1618 cm−1 shifts to 1625 as the potential is increased and more quinoid structures are formed. A weak shoulder to the peak at 1618 cm−1 appears in the Raman spectra at ∼1590 cm−1 when the potential is increased to approximately 0.2 and 0.3 V. This peak is assigned to the C C ring stretching vibrations of the quinoid structures in the emeraldine form of PNMA. The appearance of the quinoid vibrations in the Raman spectra is in good accordance with the CV of PNMA (Fig. 2a, curve 1), which shows that the LE to E transition takes place approximately between 0.1 and 0.4 V. The potential interval of the E to PN transition is approximately at 0.45–0.6 V. When the potential is increased to 0.8 V, the C C ring stretching band at
∼1590 cm−1 can clearly be distinguished in the Raman spectra since the number of quinoid units in the PNMA film is increased. Also the CH CH and C N stretching bands of the quinoid units at 1410 and 1440 cm−1 , respectively, can clearly be distinguished in the spectra. The C C ring stretching band of the quinoid units at ∼1515 cm−1 becomes also slightly more visible in the Raman spectra at higher potentials. The quite sharp and narrow Raman band at ∼1185 cm−1 (Fig. 3A, spectra a) is assigned to C H inplane bending of the benzenoid units, which are the dominant species in the polymer structure at −0.2 V. This band becomes broader at 0.2–0.3 V and splits into two overlapping bands at 0.8 V, which are related to the C H in-plane bending of the benzenoid (1193 cm−1 ) and quinoid (1175 cm−1 ) units. This is in good accordance with the formation of quinoid structures
Table 2 Assignments of the Raman bands of PNBA obtained with the laser excitation wavelengths of 514.5, 632.8 and 780 nm
The references for the bands are listed in Table 1. The benzenoid and quinoid units are denoted by B and Q, respectively. The electrically conducting emeraldine form is abbreviated as E. a Shoulder.
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Fig. 4. Raman spectra of (A) PNMA and (B) PNBA film. Excitation wavelength: 632.8 nm. The peak originating from HClO4 is marked with an asterisk. The potentials applied are indicated with (a) −0.2 V, (b) 0 V, (c) 0.2 V, (d) 0.4 V, (e) 0.6 V and (f) 0.8 V. The spectra were measured with 0.1 V intervals.
Fig. 5. Raman spectra of (A) PNMA and (B) PNBA film. Excitation wavelength: 780 nm. The peak originating from HClO4 is marked with an asterisk. The potentials applied are indicated with (a) −0.2 V, (b) 0 V, (c) 0.2 V, (d) 0.4 V, (e) 0.6 V and (f) 0.8 V. The spectra were measured with 0.1 V intervals.
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Fig. 6. Raman spectra of (A) PNEA and (B) PNPA film. Excitation wavelength: 780 nm. The peak originating from HClO4 is marked with an asterisk. The potentials applied are indicated with (a) −0.2 V, (b) 0 V, (c) 0.2 V, (d) 0.4 V, (e) 0.6 V and (f) 0.8 V. The spectra were measured with 0.1 V intervals.
at higher oxidation potentials, which results in the decrease in the intensity of the benzenoid bands. It is expected from the UV–vis spectra of PNMA [25] that the 514.5 nm laser only slightly enhances the vibrations originating from the quinoid units. The excitation wavelength of 514.5 nm should therefore be more suitable than the 632.8 and 780 nm lasers to follow the vibrations of benzenoid units at different oxidation potentials. It should be noted, that a shoulder to the band at 1618 cm−1 can be clearly seen at ∼1640 cm−1 in the Raman spectra measured at −0.2 V. It almost completely disappears from the spectra of the conducting E form at 0.2 and 0.3 V, but grows again in intensity in the potential range of 0.4–0.6 V. A new band at 1681 cm−1 appears in the spectra at E > 0.6 V, which has been previously related to cyclic N-containing species formed by crosslinking [40]. This band was also observed by Arsov et al. [41] and Liu et al. [42], when PANI was studied with the 514.5 nm laser excitation wavelength. It is therefore possible that the band at ∼1640 cm−1 is also related to crosslinked structures and shifts to higher wavenumbers at E > 0.6 V. The semiquinone radical units (CN•+ ) are the only species, which give rise to vibrational bands of PANI in the wavenumber region of 1300–1400 cm−1 . It is expected that the CN-vibrations are mostly affected by the N-substitution in poly(N-alkylanilines). Surprisingly, a strong Raman band at 1358 cm−1 is observed in the spectrum measured at −0.2 V (Fig. 3, spectrum a). The CV (Fig. 1, curve 1) and UV–vis spectra [25] indicate that the PNMA film is mainly in the fully reduced LE form at this potential. The UV–vis spectra indicate, however, that the vibrations in quinoid structures are slightly enhanced
by the 514.5 nm laser, which could be one possible reason for the strong Raman band at ∼1358 cm−1 [25]. However, the full interpretation of this Raman band may be more complex. A slight shift of the band at 1358 cm−1 to higher wavenumbers (1370–1375 cm−1 ) is observed in the potential interval of the LE to E conversion (0.2–0.4 V). Simultaneously, a new Raman band at ∼1335 cm−1 appears as a shoulder to the ∼1370 cm−1 band (0.4 V) and is clearly seen in the spectrum at higher potentials. It seems that mainly one type of CN bond dominates in the reduced LE form, when the PNMA backbone consists only of benzenoid units. The conversion of the LE to the E form is connected with a shift of the 1358 cm−1 band to 1370 cm−1 (0.4 V) and simultaneous appearance of the 1335 cm−1 band. The intensity of the band at ∼1370 cm−1 has its maximum at ∼0.3 V and decreases at higher potentials due to formation of the electrically non-conducting PN form. It is therefore characteristic of the LE to E conversion of PNMA that the 1358 cm−1 band splits into two bands at 0.2–0.4 V. At this potential, the PNMA backbone consists probably of two different types of CN bonds, which appear at different wavenumbers in the Raman spectra. One possible reason for the vibrational band at ∼1335 cm−1 could be dealkylation of the PNMA backbone resulting in the formation of semiquinone type radical structures (CN•+ ), which are usually observed in PANI at 1330–1335 cm−1 . The Raman bands of PNMA in the wavenumber region of 1300–1400 cm−1 are assigned to the conducting E form, but it should be stressed that the exact interpretation of these bands is still unclear. The bands in the fingerprint region (not shown in Fig. 3) keep their intensity constant in the whole potential range of −0.2 to 0.8 V.
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3.2.1.2. Poly(N-butylaniline). The Raman spectra of the reduced LE form of the PNBA film show principally the same vibrational features as the PNMA film (Fig. 3B, spectra a). However, CH CH and C N stretching vibrational bands at 1415 and 1440 cm−1 , respectively, indicate that quinoid structures are present in the polymer structure at −0.2 V. The CN vibrational bands at ∼1335 and 1362 cm−1 , which are assigned to the conducting E form, also confirm the presence of small fractions of quinoid structures in the PNBA film at −0.2 V. Two new quinoid bands at ∼1184 and 1589 cm−1 appear in the Raman spectra as the PNBA film is transformed from the LE to E form at approximately 0.4–0.5 V. The potential interval of the LE to E transition is in good accordance with the CV (Fig. 2a, curve 4) and UV–vis spectra [25] of PNBA. For PANI, the Raman band at ∼1250 cm−1 is usually assigned to semiquinone radical cations. The presence of this band together with the band at ∼1290 cm−1 may possibly indicate a partial dealkylation of the butyl groups from the polymer backbone. This results in PANI type segments included in the PNBA film. The N-substituted alkyl groups (aliphatic primary amines) should give rise to vibrational bands at 760–795 and 810–850 cm−1 [43]. The intensities of all vibrational bands in these wavenumber regions are, however, very weak. No specific vibrational bands can therefore be assigned to primary aliphatic amines. It should be noted, that the Raman bands of the Nsubstituted alkyl groups should not be influenced to any greater extent by the oxidation state of the polymer films (Scheme 1). For both PNMA and PNBA, the transition from the LE to E form is accompanied by the formation of quinoid structures, which supports the validity of the commonly accepted oxidation and reduction mechanism of poly(N-alkylanilines) (Scheme 1). 3.2.2. Excitation wavelength of 632.8 and 780 nm 3.2.2.1. Poly(N-methylaniline). The Raman spectra of PNMA measured with the 632.8 and 780 nm laser excitation wavelength are shown in Figs. 4A and 5A. The vibrational bands of the Raman spectra are listed in Table 1. Previously reported UV–vis spectra of PNMA show, that both laser excitation wavelengths enhance vibrations originating from quinoid units [25]. The PNMA spectrum at −0.2 V has a broad band at 1572 cm−1 (632.8 nm) and 1570 cm−1 (780 nm), which is mainly due to C C ring stretching of quinoid units. The intensity of this band increases significantly at higher potentials and shifts to 1579 cm−1 (632.8 nm) and 1581 cm−1 (780 nm) at 0.8 V. At lower potentials, the C C stretching band is probably overlapping with the C C ring stretching band of the benzenoid units. A clear shoulder at ∼1606 cm−1 (632.8 nm) and ∼1600 cm−1 (780 nm), which belongs to the C C stretching band, becomes visible in the Raman spectra at ∼0.3 V. At low potentials (<0.2 V), only a very weak band due to the C H in-plane bending of quinoid units is observed at ∼1170 cm−1 (−0.2 V) for the 780 nm laser. On the other hand, for the 632.8 nm laser, this band becomes visible in the Raman spectra and starts to grow in intensity in the potential interval of 0.1–0.4 V, where the LE to E transition takes place. A broad band is observed at 0.8 V, consisting of both C H in-plane bending vibrations of both quinoid (1155 cm−1 ) and benzenoid
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units (1190 cm−1 ). The 780 nm laser shows only one rather narrow band at ∼1169 cm−1 , which shows that the vibrations of the quinoid units are mostly enhanced with this laser excitation wavelength. Simultaneously with the growth of the C H in-plane bending vibrations, a vibrational band of the C N stretching at ∼1470 cm−1 (632.8 nm) appears in the Raman spectra in the potential interval of the LE to E transition. This band is already present at −0.2 V (∼1480 cm−1 ) in the Raman spectra measured with the 780 nm laser excitation wavelength (Fig. 5A, spectrum a). In the potential interval of 0.1–0.4 V, the Raman bands at 1310 cm−1 (632.8 nm) and 1298 cm−1 (780 nm) are shifted to 1345 cm−1 . This shift is associated with the LE to E transition. At higher potentials this band is shifted further to 1355 cm−1 . A very weak Raman band at 1250 cm−1 is also observed at E ≥ 0.4 V (Fig. 5A). In PANI, this band is associated with the conducting semiquinone radical cation structure. A Raman band at ∼780 cm−1 , which is assigned to the ring in-plane deformation of quinoid units, grows also in intensity at E ≥ 0.3 V. 3.2.2.2. Poly(N-butylaniline). The Raman spectra of PNBA measured with the 632.8 and 780 nm laser show basically the same features as the PNMA spectra (Figs. 4B and 5B). The LE to E transition takes place approximately between 0.3 and 0.5 V. In this potential interval, the Raman bands at ∼1325 cm−1 (632.8 nm) and ∼1380 cm−1 (780 nm) are shifted to ∼1385 cm−1 (632.8 nm) and ∼1373 cm−1 (780 nm), respectively. Simultaneously, a weak shoulder to these bands appears in the spectra at ∼1335 cm−1 and becomes more pronounced at higher potentials. As with the 514.5 nm laser excitation wavelength, the vibrational band at ∼1335 cm−1 may possibly be assigned to CN vibrations of dealkylated polyaniline type segments in the PNBA backbone. The C N stretching vibration at ∼1480 cm−1 (Fig. 5B) indicate that quinoid structures are present in the PNBA film at −0.2 V. 3.2.2.3. Poly(N-ethylaniline) and poly(N-propylaniline). The Raman spectra of PNEA and PNPA were studied only with the 780 nm laser excitation wavelength (Fig. 6 and Table 3). The spectra of PNEA and PNPA are very similar to the Raman spectra of PNBA (Fig. 5B). It seems that an alkyl group longer than ethyl has no bigger influence on the Raman spectra. All substituted poly(N-alkylanilines) with ethyl, propyl and butyl substituents are expected to be more easily dealkylated than PNMA. This is reflected in the CN vibrational band at ∼1335 cm−1 . According to the Raman spectra of PNEA and PNPA, the LE to E transition takes place approximately between 0.1 and 0.4 V, which is in a quite good accordance with the CVs of PNEA and PNPA (Fig. 2a). At 0.7 V (PNEA) and 0.8 V (PNPA), the PN form should be the dominant oxidation state within the PNEA and PNPA films. 3.2.2.4. PNMA and PNBA polymerized in mixed solvents. The Raman bands of PNMA and PNBA films, which were polymerized in mixed aqueous solvents containing 10% ACN or 10% DMSO, were studied with the 780 nm laser excitation
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Table 3 Assignments of the Raman bands of PNEA and PNPA Wavenumber (cm−1 )
Assignments
PNEA
PNPA
−0.2 V
0.4 V
0.8 V
−0.2 V
0.4 V
∼1605 1570 ∼1485 ∼1373 ∼1293 – ∼1160 ∼775
∼1605 1570 ∼1475 1377 ∼1325 ∼1229 ∼1170 ∼780
∼1605 1577 1488 1379 ∼1331 ∼1247 1179 ∼782
– 1572 ∼1478 ∼1375 ∼1310 – ∼1175 ∼785
– 1570 ∼1480 ∼1374 – ∼1250 1170 ∼780
0.8 V ∼1605 1576 1489 ∼1380 ∼1330 1258 1180 780
C C C C
C ring stretching (B) C ring stretching (Q) N stretching (Q) N stretching of the E form
C N stretching of the E form C H in-plane bending (Q) Ring in-plane deformation (Q)
The Raman measurements were conducted with the 780 nm laser excitation wavelength. The references for the bands are listed in Table 1. The benzenoid and quinoid units are denoted by B and Q, respectively. The electrically conducting emeraldine form is abbreviated as E.
wavelength. The Raman spectra of the PNMA (10% ACN) and PNMA (10% DMSO) films show the same characteristic features as the PNMA film (Fig. 5A) and are therefore not shown here. Compared to the PNMA spectrum, an additional CN vibrational band was observed at ∼1252 cm−1 for both PNMA (10% ACN) and PNMA (10% DMSO) films. The Raman spectra of PNMA (10% DMSO) show a broad band at ∼1284 cm−1 in the potential interval of −0.2 to 0.1 V. As for PNMA, this band shifts to ∼1355 cm−1 in the potential interval of the LE to E transition. A similar behavior was observed for PMNA (10% ACN). The close similarity of the Raman spectra of PNMA, PNMA (10% ACN) and PNMA (10% DMSO) indicate only small differences in the chemical structure of the polymer backbone of these materials. The same conclusion was made for the PNBA films in this study due to the great similarity of the Raman spectra of PNBA, PNBA (10% ACN) and PNBA (10% DMSO).
The Raman spectra of PNEA and PNPA films are very similar to PNBA indicating that the alkyl substituents longer than ethyl have only a minor influence on the Raman spectra of these materials. The spectra of PNMA films polymerized in a mixture of aqueous and organic solvent indicate also that the chemical structure of these films are quite similar in comparison to the PNMA film, which was polymerized in pure aqueous solution of HClO4 . Acknowledgements ˚ Akademi ProThis work is part of the activities of the Abo cess Chemistry Centre within the Finnish centre of Excellence Program (2000–2011) sponsored by the Academy of Finland. Maija Blomquist gratefully acknowledges the financial support of Svenska Tekniska Vetenskapsakademien and the Finnish National Graduate School of Nanoscience.
4. Conclusions References A fundamental Raman characterization of four different poly(N-alkylanilines) was conducted with the laser excitation wavelengths of 514.5, 632.8 and 780 nm. In contrast to the conducting emeraldine salt form of PANI, the results obtained in this study show that the half-oxidized conducting emeraldine form of PNMA, PNEA, PNPA and PNBA consists of both benzenoid and quinoid structures. This supports the validity of the commonly used oxidation and reduction scheme of PNANIs (Scheme 1). In their reduced state at −0.2 V, all PNANIs studied showed usually one CN vibrational band in the Raman spectra in the wavenumber region of 1300–1400 cm−1 . A shift of this Raman band was observed in the potential interval of the transition from the fully reduced non-conducting leucoemeraldine form to the electrically conducting emeraldine form. This transition is often, especially in PNBA, accompanied with the appearance of a CN vibrational band at ∼1335 cm−1 , which may indicate the presence of polyaniline type segments in the polymer structure. The formation of these segments may possibly be due to partial dealkylation of the N-substituted alkyl groups.
[1] J.W. Chevalier, J.Y. Bergeron, L.H. Dao, Macromolecules 25 (1992) 3325. [2] R. Holze, in: H.S. Nalwa (Ed.), Advanced Functional Molecules and Polymers, vol. 2, Gordon and Breach Science Publishers, 2001, pp. 171–221 (Chapter 7). [3] N. Comisso, S. Dahlio, G. Mengoli, R. Salmaso, S. Zecchin, G. Zotti, J. Electroanal. Chem. 255 (1988) 97. [4] T. Lindfors, A. Ivaska, J. Electroanal. Chem. 535 (2002) 65. [5] A. Malinauskas, R. Holze, Ber. Bunsenges. Phys. Chem. 101 (1997) 1859. [6] A. Malinauskas, R. Holze, Electrochim. Acta 44 (1999) 2613. [7] C. Sivakumar, A. Gopalan, T. Vasuvedan, T.-C. Wen, Synth. Met. 126 (2002) 123. [8] D. Wei, T. Lindfors, C. Kvarnstr¨om, L. Kronberg, R. Sj¨oholm, A. Ivaska, J. Electroanal. Chem. 575 (2005) 19. [9] A. Watanabe, K. Mori, A. Iwabuchi, Y. Iwasaki, Y. Nakamura, Macromolecules 22 (1989) 3521. [10] J. Langer, Synth. Met. 35 (1990) 295. [11] L.H. Dao, J.Y. Bergeron, J.W. Chevalier, M.T. Nguyen, R. Paynter, Synth. Met. 41–43 (1991) 655. [12] C. Barbero, M.C. Miras, O. Haas, R. K¨otz, J. Electroanal. Chem. 310 (1991) 437. [13] J. Yano, M. Kokura, K. Ogura, J. Appl. Electrochem. 24 (1994) 1164. [14] A. Lian, S. Besner, L.H. Dao, Synth. Met. 74 (1995) 21. [15] P.A. Kilmartin, G.A. Wright, Synth. Met. 104 (1999) 145.
M. Blomquist et al. / Synthetic Metals 157 (2007) 974–983 [16] A.A. Athawale, B. Deore, M. Vedpathak, S.K. Kulkarni, J. Appl. Polym. Sci. 74 (1999) 1286. [17] G.A. Planes, M.C. Miras, C. Barbero, Polym. Int. 51 (2002) 429. [18] R. Siwakumar, R. Saraswathi, J. Power Sources 104 (2002) 226. [19] R. Siwakumar, R. Saraswathi, Synth. Met. 138 (2003) 381. [20] J. Yano, Y. Ota, A. Kitani, Mater. Lett. 58 (2004) 1934. [21] A. Falcou, A. Duchˆene, P. Hourquebie, D. Marsacq, A. Balland-Longeau, Synth. Met. 149 (2005) 115. [22] A. Lian, S. Besner, N.E. Cherid, J.Y. Bergeron, L.H. Dao, Proc. Polym. Mater.: Sci. Eng. 70 (1994) 445. [23] G.-E. Hwang, K.-Y. Wu, M.-Y. Hua, H.-T. Lee, S.-A. Chen, Synth. Met. 92 (1998) 39. [24] P. Syed Abthagir, R. Saraswathi, S. Sivakolunthu, Thermochim. Acta 411 (2004) 109. [25] M. Blomquist, T. Lindfors, L. V¨ah¨asalo, A. Pivrikas, A. Ivaska, Synth. Met. 156 (2006) 549. [26] K. Chiba, T. Ohsaka, N. Oyama, J. Electroan. Chem. Int. Electrochem. 217 (1987) 239. [27] K. Singh, U. Mishra, Ind. J. Chem. 37A (1998) 613. [28] A.D. Borkar, M.C. Gupta, S.S. Umare, Polym. Plast. Technol. Eng. 40 (2001) 225. [29] G.-L. Yuan, N. Kuramoto, Macromolecules 36 (2003) 7939. [30] X.-G. Li, H.-J. Zhou, M.-R. Huang, J. Pol. Sci. 42 (2004) 6109. [31] M.R. Nabid, A.A. Entezami, Polym. Adv. Technol. 16 (2005) 305. [32] X.-G. Li, H.-J. Zhou, M.-R. Huang, Polymer 46 (2005) 1523. [33] S. Quillard, G. Louarn, K. Berrada, S. Lefrant, K.A. Coplin, S.W. Jessen, A.J. Epstein, Nonlinear Opt. 10 (1995) 253.
983
[34] A. Malinauskas, R. Holze, Electrochim. Acta 43 (1998) 521. [35] G. Niaura, R. Maˇzeikien˙e, A. Malinauskas, Synth. Met. 145 (2004) 105. [36] M.C. Bernard, A. Hugot-Le Goff, S. Joiret, H. Arkoub, B. Sa¨ıdani, Electrochim. Acta 50 (2005) 1615. [37] T. Lindfors, A. Ivaska, J. Electroanal. Chem. 580 (2005) 320. [38] T. Lindfors, C. Kvarnstr¨om, A. Ivaska, J. Eletroanal. Chem. 518 (2002) 131. [39] Y. Wei, W.W. Focke, G.E. Wneck, A. Ray, A.G. MacDiarmid, J. Phys. Chem. 93 (1989) 495. [40] J.E. Pereira da Silva, D.L.A. de Faria, S.I. C´orboda de Torresi, M.L.A. Temperini, Macromolecules 33 (2000) 3077–3083. [41] Lj.D. Arsov, W. Plieth, G. Koßmehl, J. Solid State Electrochem. 2 (1998) 355. [42] C. Liu, J. Zhang, G. Shi, F. Chen, J. Appl. Polym. Sci. 92 (2004) 171. [43] G. Socrates, Infrared and Raman characteristic group frequencies, in: Tables and Charts, third ed., John Wiley & Sons, 2001, p. 109. [44] K. Berrada, S. Quillard, G. Louarn, S. Lefrant, Synth. Met. 69 (1995) 201. [45] M. Bartonek, N.S. Sariciftci, H. Kuzmany, Synth. Met. 36 (1990) 83. [46] Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada, T. Nakajima, T. Kawagoe, Macromolecules 21 (1988) 1297. [47] A. Hugot-Le Goff, M.C. Bernard, Synth. Met. 60 (1993) 115. [48] S. Quillard, G. Louarn, J.P. Buisson, S. Lefrant, J. Masters, A.G. MacDiarmid, Synth. Met. 49–50 (1992) 525. [49] J. Laska, Synth. Met. 129 (2002) 229. [50] S. Quillard, K. Berrada, G. Louarn, S. Lefrant, M. Lapkowski, A. Pron, New J. Chem. 19 (1995) 365.
Synthetic Metals 157 (2007) 974–983
Characterization of poly(N-alkylanilines) by Raman spectroscopy Maija Blomquist a,b , Tom Lindfors a,∗ , Ari Ivaska a b
a Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Abo ˚ Akademi University, Biskopsgatan 8, 20500 Turku/Abo, ˚ Finland The Finnish National Graduate School in Nanoscience (NGS-NANO), Nanoscience Center, P.O. Box 35, 40014 University of Jyv¨askyl¨a, Finland
Received 26 January 2007; received in revised form 9 August 2007; accepted 3 October 2007 Available online 19 November 2007
Abstract Thin films of poly(N-alkylaniline) were synthesized in acidic aqueous solution and in mixtures of aqueous and organic solvents. The polymer films (alkyl = methyl, ethyl, propyl and butyl) were characterized by Raman spectroscopy with the excitation wavelengths of 514.5, 632.8 and 780 nm. The main Raman bands have been characterized for the leucoemeraldine, emeraldine and pernigraniline oxidation states between −0.2 and 0.8 V (vs. Ag|AgCl). This fundamental study shows that the structure of the half-oxidized emeraldine form contains quinoid units, which supports the commonly accepted oxidation and reduction scheme of poly(N-alkylanilines). © 2007 Elsevier B.V. All rights reserved. Keywords: Poly(N-alkylanilines); Electropolymerization; Raman and UV–vis spectroscopy; Cyclic voltammetry
1. Introduction Poly(N-alkylanilines) (PNANIs) are interesting materials because the addition of side groups to polyaniline (PANI) backbone results in improved solubility and processability [1,2]. PNANIs have despite of this been studied much less than PANI. Poly(N-methylaniline) (PNMA) has so far gained most attention among PNANIs. In earlier works, both electrochemically and chemically synthesized PNMA, poly(N-ethylaniline) (PNEA), poly(N-propylaniline) (PNPA) and poly(N-butylaniline) (PNBA) have been reported [1,3–32]. The electrochemistry of PNANIs is generally agreed to follow Scheme 1, which shows the leucoemeraldine (LE), emeraldine (E) and pernigraniline (PN) oxidation states of PNANIs [12,15]. The emeraldine form is the only electrically conducting form of the PNANIs. In the literature, PNANIs has not been systematically characterized with Raman spectroscopy. Only handful of papers, to the best of our knowledge, deals with Raman spectroscopy of substituted polyanilines. Quillard et al. compared Raman spectra of PANI and poly(o-toluidine) [33]. Also photoinduced spectra of poly(o-ethylaniline) and PANI were compared
∗
Corresponding author. Tel.: +358 2 2154422; fax: +358 2 2154479. E-mail address: [email protected] (T. Lindfors).
0379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2007.10.001
in their work. The substituted polymers have a change in symmetry compared with PANI, which leads to new peaks in the spectra in comparison to the PANI spectrum. Malinauskas et al. have characterized PNMA and a copolymer of N-methylaniline and N(3-sulphopropyl)-aniline by Raman spectroscopy [34]. Kilmartin and Wright reported a Raman study of four substituted polyanilines: PNMA, poly(m-methylaniline), poly(o-methoxyaniline) and poly(o-ethoxyaniline) [15]. The spectra showed a recognizable PANI pattern. Raman spectra of the conducting form of ring-sulphonated polyaniline (SPAN) and PANI were compared by Niaura et al. [35]. Some of the Raman bands shifted to higher frequencies in the Raman spectra of SPAN compared to PANI, which indicate structural differences between these two polymers. Structural and electronic characteristics of sulphonated polyanilines and poly(o-methoxyanilines) were compared with Raman spectroscopy by Bernard et al. [36]. They concluded that the substituent could modify the polaronic nature to become more random. Furthermore, Wei et al. have studied the Raman spectra of PNMA electropolymerized in an organic solvent [8]. In this paper, PNMA, PNEA, PNPA and PNBA films were polymerized in acidic aqueous solutions and characterized by Raman spectroscopy. The PNMA and PNBA were for comparison also polymerized in presence of 10% (v/v) acetonitrile (ACN) or dimethyl sulfoxide (DMSO). After the polymerization, the polymer films were systematically studied in the
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with 0.3 m Al2 O3 powder and rinsed with deionized water prior to the film preparation. The potential was controlled with an Autolab (PGSTAT 20 or PGSTAT 100) potentiostat. Prior to all measurements the polymerization and characterization solutions were purged with nitrogen for 20 min. The solutions were also blanketed with nitrogen before they were transferred to the electrochemical cell and during the experiments. After the polymerization and prior to the Raman measurements, the films were conditioned overnight in the cell filled with 1.0 M HClO4 . 2.3. Raman spectroscopy
Scheme 1. The redox mechanism of poly(N-alkylanilines) showing the leucoemeraldine (LE), emeraldine (E) and pernigraniline (PN) forms. X and A− are the alkyl chain and the mobile anion, respectively.
potential range of the leucoemeraldine, emeraldine and pernigraniline transitions. The Raman spectra were measured with the excitation wavelengths of 514.5, 632.8 and 780 nm. This paper is a continuation of our earlier work [25] on PNMA and PNBA films synthesized in mixtures of aqueous and organic solvents. One of the main objectives of this work is to study how the length of the N-substituted alkyl chain affects the Raman spectra of PNANIs. 2. Experimental 2.1. Chemicals N-Methylaniline (NMA) was obtained from Fluka, Nethylaniline (NEA) and N-butylaniline (NBA) from Aldrich, and N-propylaniline (NPA) was obtained from TCI Europe nv. ACN and DMSO were obtained from Riedel-de Ha¨en. All chemicals were used as received. 2.2. Electropolymerization in a flow cell The electropolymerization was performed in a flow cell described in our earlier work [37]. The electropolymerization of 0.1 M NMA, NEA, NPA or NBA was done in the flow cell in 1.0 M HClO4 or in a solvent mixture containing 10% (v/v) organic solvent by cycling the potential between −0.2 and 0.85 V (50 cycles; 50 mV/s). The polymer films formed on the GC disc working electrode (A = 0.07 cm2 ) were characterized by cyclic voltammetry in the flow cell in aqueous monomer free solutions of 1.0 M HClO4 (10 cycles; 50 mV/s). The polymerization and characterization solutions passed continuously through the cell with a flow rate of approximately 0.3 ml/min. The solutions were transferred to the cell with a peristaltic pump. A Pt wire served as the counter electrode and Ag|AgCl wire as the reference electrode. The GC disk electrode was always polished
The flow cell including the working electrode was filled with 1.0 M HClO4 and placed in a 90◦ angle relative to the incoming laser beam and the scattering was therefore collected at a 180◦ configuration. The Raman measurements were done with the 514.5 nm (LaserPhysics, Ar ion laser), 632.8 nm (Renishaw, HeNe laser) and 780 nm (Renishaw, NIR diode laser) excitation wavelengths for PNMA and PNBA films polymerized in aqueous solution. The 780 nm laser was used for the PNEA and PNPA films, and for the PNMA and PNBA films, which were polymerized in the mixtures of aqueous and organic solvents. The Raman spectra were measured with potential intervals of 0.1 V from −0.2 to 0.8 V by stepwise increasing the potential. The films were first reduced at −0.2 V for 15 min before the Raman spectrum was recorded at this potential. Thereafter, the potential was always kept 6 min at the other potentials before the spectra were measured. The laser light was switched on just prior to the Raman measurements in order to avoid unnecessary irradiation of films. All measurements were conducted with a Renishaw Raman imaging microscope (with WireTM v1.3 Raman software) connected to a Leica DMLM microscope. The spectrometer was always calibrated against a Si-standard (520.0 cm−1 ) before starting the Raman measurements. The Raman measurements were conducted with 50% of the maximum laser power in order to avoid degradation of the polymer film. The full power of all laser wavelengths was ∼20 mW, but after passing through the Raman and Leica microscope, the power of the laser beams were reduced approximately to 4 mW (514.5 nm), 1.5 mW (632.8 nm) and 5 mW (780 nm). The detector time used was 30 s (632.8 and 780 nm) and 2 × 30 s (514.5 nm). 3. Results and discussion 3.1. Polymerization of poly(N-alkylanilines) in the flow cell The cyclic voltammograms (CVs) recorded during polymerization of NMA, NEA, NPA and NBA are shown in Fig. 1. The redox peak currents observed during polymerization of PNPA and PNBA are lower than for PNMA and PNEA. This was also observed in our earlier studies [25,38]. The decrease in the redox peak currents of PNPA and PNBA are probably due to the increase of the alkyl chain length, which influences the polymerization process resulting in the formation of less polymeric material on the electrode surface. The longer alkyl chains increase the ring torsion and lower the conductivity of the formed film.
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Fig. 1. Polymerization of 0.1 M (a) NMA, (b) NEA, (c) NPA, (d) NBA in an aqueous solution of 1.0 M HClO4 solution. Every 10th cycle is shown and the 50th cycle (last cycle) is indicated with 50; v = 50 mV/s.
The CVs recorded during characterization of the PNMA, PNEA, PNPA and PNBA films are shown in Fig. 2a. For PNMA the LE/E and the E/PN transitions can be observed at Ep,a1 = 305 mV/Ep,c1 ≈ 265 mV and Ep,a2 = 499 mV/Ep,c2 = 455 mV, respectively. For PNEA the same transitions are located at Ep,a = 521 mV/Ep,c = 452 mV, for PNPA at Ep,a = 646 mV/Ep,c = 492 mV, and for PNBA at Ep,a = 708 mV/Ep,c ≈ 290 mV. The one broad oxidation peak for all the other PNANIs expect for PNMA are due to overlapping of the LE/E and E/PN transitions. The shift
of the LE/E transition to higher potentials and the E/PN transition to lower potentials with an increase of the alkyl chain length is in good accordance with other studies [12,39]. The potential intervals of the LE/E and E/PN transitions observed in the CVs of the PNMA and PNBA films (Fig. 1, curves 1 and 4) are in good accordance with the potential intervals of the same transitions, which were previously observed in electrochemical in situ UV–vis measurements of these films [25]. The UV–vis spectra also indicate that the laser excitation wavelengths used in this study enhance the vibrations
Fig. 2. Characterization of the PNANI films in 1.0 M HClO4 (5th cycle; v = 50 mV/s). (a) (1) PNMA, (2) PNEA, (3) PNPA, (4) PNBA; (b) PNMA films prepared in 1 M HClO4 containing 10% (v/v) (1) ACN, (2) DMSO and PNBA films prepared with 10% (v/v) (3) ACN and (4) DMSO.
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Fig. 3. Raman spectra of (A) PNMA and (B) PNBA film. Excitation wavelength: 514.5 nm. The potentials applied are indicated with (a) −0.2 V, (b) 0 V, (c) 0.2 V, (d) 0.4 V, (e) 0.6 V and (f) 0.8 V. The spectra were measured with 0.1 V intervals.
originating from the quinoid structures in the following order: 780 nm > 632.8 nm > 514.5 nm. The CVs of PNMA and PNBA polymerized in aqueous 1.0 M HClO4 in presence of 10% ACN or 10% DMSO are shown in Fig. 2b. For PNMA prepared in a mixed solvent containing 10% ACN, the LE/E and the E/PN transitions can be observed at Ep,a1 ≈ 440 mV/Ep,c1 ≈ 270 mV and Ep,a2 = 573 mV/Ep,c2 = 466 mV, respectively. For PNMA prepared in a solution containing 10% DMSO, the anodic LE/E and the E/PN transitions can be observed at Ep,a1 = 294 mV/Ep,c1 ≈ 250 mV and Ep,a2 = 536 mV/Ep,c = 565 mV. A third oxidation peak is observed at ∼540 mV, which is probably due to degradation products of PNMA. PNBA films polymerized in presence of 10% (v/v) ACN or DMSO have only one broad oxidation and reduction peak. The overlapping LE/E and E/PN transitions can be observed for the PNBA (10% ACN) film at Ep,a = 668 mV/Ep,c ≈ 375 mV and for the PNBA (10% DMSO) film at Ep,a = 705 mV/Ep,c = 477 mV, respectively. The organic solvents, ACN and DMSO, were chosen for this study on basis of the results from our previous work [25]. It was found that the addition of ACN into the polymerization solution increased the oxidation and reduction currents of PNMA. On the other hand, the addition of DMSO decreased the redox currents. The films polymerized in mixtures of aqueous and organic solvents are hereafter referred to as PNMA (10% ACN), PNMA (10% DMSO), PNBA (10% ACN) and PNBA (10% DMSO). Films polymerized in acidic aqueous solutions are notified as PNMA, PNEA, PNPA and PNBA.
3.2. Raman measurement The Raman spectra were measured from −0.2 to 0.8 V by increasing the potential stepwise with 0.1 V. The UV–vis spectra, which were reported earlier by us [25], show that the 632.8 and 780 nm laser excitation wavelengths mostly enhance vibrational modes of the quinoid units. The vibrations related to the nitrogen atoms are expected to be mainly affected by the Nsubstitution compared to unsubstituted PANI [15]. According to our earlier reported UV–vis spectra of PNMA [25], the vibrational modes of the quinoid units are only slightly enhanced by the 514.5 nm laser compared to the benzenoid units. In the case of PNBA, the 514.5 nm laser is expected to enhance rather equally vibrations originating from both quinoid and benzenoid units [33]. In this study, the main vibrational bands in the Raman spectra of poly(N-alkylanilines) are interpreted. 3.2.1. Excitation wavelength of 514.5 nm 3.2.1.1. Poly(N-methylaniline). The Raman spectra of PNMA and PNBA measured with the 514.5 nm laser excitation wavelength are shown in Fig. 3. The spectra have been separated from each other for the sake of clarity. The assignments of the main vibrational bands of the Raman spectra are given in Table 1 for PNMA and in Table 2 for PNBA. The Raman band indicated with an asterisk in Figs. 4–6 originates from HClO4 . All references to the Raman bands, which were interpreted in this study, are given in Table 1. The PNMA spectrum measured at −0.2 V (Fig. 3A, spectra a) shows strong vibrational bands at 1618, 1358 and 1185 cm−1 .
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Table 1 Assignments of the Raman bands of PNMA obtained with the laser excitation wavelengths of 514.5, 632.8 and 780 nm
The benzenoid and quinoid units are denoted by B and Q, respectively. The electrically conducting emeraldine form is abbreviated as E. a Shoulder.
At this potential, the PNMA film is in the fully reduced LE form. The bands at 1618 and 1185 cm−1 are assigned to C C stretching and C H in-plane bending of benzenoid units, respectively (Table 1). The band at 1618 cm−1 shifts to 1625 as the potential is increased and more quinoid structures are formed. A weak shoulder to the peak at 1618 cm−1 appears in the Raman spectra at ∼1590 cm−1 when the potential is increased to approximately 0.2 and 0.3 V. This peak is assigned to the C C ring stretching vibrations of the quinoid structures in the emeraldine form of PNMA. The appearance of the quinoid vibrations in the Raman spectra is in good accordance with the CV of PNMA (Fig. 2a, curve 1), which shows that the LE to E transition takes place approximately between 0.1 and 0.4 V. The potential interval of the E to PN transition is approximately at 0.45–0.6 V. When the potential is increased to 0.8 V, the C C ring stretching band at
∼1590 cm−1 can clearly be distinguished in the Raman spectra since the number of quinoid units in the PNMA film is increased. Also the CH CH and C N stretching bands of the quinoid units at 1410 and 1440 cm−1 , respectively, can clearly be distinguished in the spectra. The C C ring stretching band of the quinoid units at ∼1515 cm−1 becomes also slightly more visible in the Raman spectra at higher potentials. The quite sharp and narrow Raman band at ∼1185 cm−1 (Fig. 3A, spectra a) is assigned to C H inplane bending of the benzenoid units, which are the dominant species in the polymer structure at −0.2 V. This band becomes broader at 0.2–0.3 V and splits into two overlapping bands at 0.8 V, which are related to the C H in-plane bending of the benzenoid (1193 cm−1 ) and quinoid (1175 cm−1 ) units. This is in good accordance with the formation of quinoid structures
Table 2 Assignments of the Raman bands of PNBA obtained with the laser excitation wavelengths of 514.5, 632.8 and 780 nm
The references for the bands are listed in Table 1. The benzenoid and quinoid units are denoted by B and Q, respectively. The electrically conducting emeraldine form is abbreviated as E. a Shoulder.
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Fig. 4. Raman spectra of (A) PNMA and (B) PNBA film. Excitation wavelength: 632.8 nm. The peak originating from HClO4 is marked with an asterisk. The potentials applied are indicated with (a) −0.2 V, (b) 0 V, (c) 0.2 V, (d) 0.4 V, (e) 0.6 V and (f) 0.8 V. The spectra were measured with 0.1 V intervals.
Fig. 5. Raman spectra of (A) PNMA and (B) PNBA film. Excitation wavelength: 780 nm. The peak originating from HClO4 is marked with an asterisk. The potentials applied are indicated with (a) −0.2 V, (b) 0 V, (c) 0.2 V, (d) 0.4 V, (e) 0.6 V and (f) 0.8 V. The spectra were measured with 0.1 V intervals.
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Fig. 6. Raman spectra of (A) PNEA and (B) PNPA film. Excitation wavelength: 780 nm. The peak originating from HClO4 is marked with an asterisk. The potentials applied are indicated with (a) −0.2 V, (b) 0 V, (c) 0.2 V, (d) 0.4 V, (e) 0.6 V and (f) 0.8 V. The spectra were measured with 0.1 V intervals.
at higher oxidation potentials, which results in the decrease in the intensity of the benzenoid bands. It is expected from the UV–vis spectra of PNMA [25] that the 514.5 nm laser only slightly enhances the vibrations originating from the quinoid units. The excitation wavelength of 514.5 nm should therefore be more suitable than the 632.8 and 780 nm lasers to follow the vibrations of benzenoid units at different oxidation potentials. It should be noted, that a shoulder to the band at 1618 cm−1 can be clearly seen at ∼1640 cm−1 in the Raman spectra measured at −0.2 V. It almost completely disappears from the spectra of the conducting E form at 0.2 and 0.3 V, but grows again in intensity in the potential range of 0.4–0.6 V. A new band at 1681 cm−1 appears in the spectra at E > 0.6 V, which has been previously related to cyclic N-containing species formed by crosslinking [40]. This band was also observed by Arsov et al. [41] and Liu et al. [42], when PANI was studied with the 514.5 nm laser excitation wavelength. It is therefore possible that the band at ∼1640 cm−1 is also related to crosslinked structures and shifts to higher wavenumbers at E > 0.6 V. The semiquinone radical units (CN•+ ) are the only species, which give rise to vibrational bands of PANI in the wavenumber region of 1300–1400 cm−1 . It is expected that the CN-vibrations are mostly affected by the N-substitution in poly(N-alkylanilines). Surprisingly, a strong Raman band at 1358 cm−1 is observed in the spectrum measured at −0.2 V (Fig. 3, spectrum a). The CV (Fig. 1, curve 1) and UV–vis spectra [25] indicate that the PNMA film is mainly in the fully reduced LE form at this potential. The UV–vis spectra indicate, however, that the vibrations in quinoid structures are slightly enhanced
by the 514.5 nm laser, which could be one possible reason for the strong Raman band at ∼1358 cm−1 [25]. However, the full interpretation of this Raman band may be more complex. A slight shift of the band at 1358 cm−1 to higher wavenumbers (1370–1375 cm−1 ) is observed in the potential interval of the LE to E conversion (0.2–0.4 V). Simultaneously, a new Raman band at ∼1335 cm−1 appears as a shoulder to the ∼1370 cm−1 band (0.4 V) and is clearly seen in the spectrum at higher potentials. It seems that mainly one type of CN bond dominates in the reduced LE form, when the PNMA backbone consists only of benzenoid units. The conversion of the LE to the E form is connected with a shift of the 1358 cm−1 band to 1370 cm−1 (0.4 V) and simultaneous appearance of the 1335 cm−1 band. The intensity of the band at ∼1370 cm−1 has its maximum at ∼0.3 V and decreases at higher potentials due to formation of the electrically non-conducting PN form. It is therefore characteristic of the LE to E conversion of PNMA that the 1358 cm−1 band splits into two bands at 0.2–0.4 V. At this potential, the PNMA backbone consists probably of two different types of CN bonds, which appear at different wavenumbers in the Raman spectra. One possible reason for the vibrational band at ∼1335 cm−1 could be dealkylation of the PNMA backbone resulting in the formation of semiquinone type radical structures (CN•+ ), which are usually observed in PANI at 1330–1335 cm−1 . The Raman bands of PNMA in the wavenumber region of 1300–1400 cm−1 are assigned to the conducting E form, but it should be stressed that the exact interpretation of these bands is still unclear. The bands in the fingerprint region (not shown in Fig. 3) keep their intensity constant in the whole potential range of −0.2 to 0.8 V.
M. Blomquist et al. / Synthetic Metals 157 (2007) 974–983
3.2.1.2. Poly(N-butylaniline). The Raman spectra of the reduced LE form of the PNBA film show principally the same vibrational features as the PNMA film (Fig. 3B, spectra a). However, CH CH and C N stretching vibrational bands at 1415 and 1440 cm−1 , respectively, indicate that quinoid structures are present in the polymer structure at −0.2 V. The CN vibrational bands at ∼1335 and 1362 cm−1 , which are assigned to the conducting E form, also confirm the presence of small fractions of quinoid structures in the PNBA film at −0.2 V. Two new quinoid bands at ∼1184 and 1589 cm−1 appear in the Raman spectra as the PNBA film is transformed from the LE to E form at approximately 0.4–0.5 V. The potential interval of the LE to E transition is in good accordance with the CV (Fig. 2a, curve 4) and UV–vis spectra [25] of PNBA. For PANI, the Raman band at ∼1250 cm−1 is usually assigned to semiquinone radical cations. The presence of this band together with the band at ∼1290 cm−1 may possibly indicate a partial dealkylation of the butyl groups from the polymer backbone. This results in PANI type segments included in the PNBA film. The N-substituted alkyl groups (aliphatic primary amines) should give rise to vibrational bands at 760–795 and 810–850 cm−1 [43]. The intensities of all vibrational bands in these wavenumber regions are, however, very weak. No specific vibrational bands can therefore be assigned to primary aliphatic amines. It should be noted, that the Raman bands of the Nsubstituted alkyl groups should not be influenced to any greater extent by the oxidation state of the polymer films (Scheme 1). For both PNMA and PNBA, the transition from the LE to E form is accompanied by the formation of quinoid structures, which supports the validity of the commonly accepted oxidation and reduction mechanism of poly(N-alkylanilines) (Scheme 1). 3.2.2. Excitation wavelength of 632.8 and 780 nm 3.2.2.1. Poly(N-methylaniline). The Raman spectra of PNMA measured with the 632.8 and 780 nm laser excitation wavelength are shown in Figs. 4A and 5A. The vibrational bands of the Raman spectra are listed in Table 1. Previously reported UV–vis spectra of PNMA show, that both laser excitation wavelengths enhance vibrations originating from quinoid units [25]. The PNMA spectrum at −0.2 V has a broad band at 1572 cm−1 (632.8 nm) and 1570 cm−1 (780 nm), which is mainly due to C C ring stretching of quinoid units. The intensity of this band increases significantly at higher potentials and shifts to 1579 cm−1 (632.8 nm) and 1581 cm−1 (780 nm) at 0.8 V. At lower potentials, the C C stretching band is probably overlapping with the C C ring stretching band of the benzenoid units. A clear shoulder at ∼1606 cm−1 (632.8 nm) and ∼1600 cm−1 (780 nm), which belongs to the C C stretching band, becomes visible in the Raman spectra at ∼0.3 V. At low potentials (<0.2 V), only a very weak band due to the C H in-plane bending of quinoid units is observed at ∼1170 cm−1 (−0.2 V) for the 780 nm laser. On the other hand, for the 632.8 nm laser, this band becomes visible in the Raman spectra and starts to grow in intensity in the potential interval of 0.1–0.4 V, where the LE to E transition takes place. A broad band is observed at 0.8 V, consisting of both C H in-plane bending vibrations of both quinoid (1155 cm−1 ) and benzenoid
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units (1190 cm−1 ). The 780 nm laser shows only one rather narrow band at ∼1169 cm−1 , which shows that the vibrations of the quinoid units are mostly enhanced with this laser excitation wavelength. Simultaneously with the growth of the C H in-plane bending vibrations, a vibrational band of the C N stretching at ∼1470 cm−1 (632.8 nm) appears in the Raman spectra in the potential interval of the LE to E transition. This band is already present at −0.2 V (∼1480 cm−1 ) in the Raman spectra measured with the 780 nm laser excitation wavelength (Fig. 5A, spectrum a). In the potential interval of 0.1–0.4 V, the Raman bands at 1310 cm−1 (632.8 nm) and 1298 cm−1 (780 nm) are shifted to 1345 cm−1 . This shift is associated with the LE to E transition. At higher potentials this band is shifted further to 1355 cm−1 . A very weak Raman band at 1250 cm−1 is also observed at E ≥ 0.4 V (Fig. 5A). In PANI, this band is associated with the conducting semiquinone radical cation structure. A Raman band at ∼780 cm−1 , which is assigned to the ring in-plane deformation of quinoid units, grows also in intensity at E ≥ 0.3 V. 3.2.2.2. Poly(N-butylaniline). The Raman spectra of PNBA measured with the 632.8 and 780 nm laser show basically the same features as the PNMA spectra (Figs. 4B and 5B). The LE to E transition takes place approximately between 0.3 and 0.5 V. In this potential interval, the Raman bands at ∼1325 cm−1 (632.8 nm) and ∼1380 cm−1 (780 nm) are shifted to ∼1385 cm−1 (632.8 nm) and ∼1373 cm−1 (780 nm), respectively. Simultaneously, a weak shoulder to these bands appears in the spectra at ∼1335 cm−1 and becomes more pronounced at higher potentials. As with the 514.5 nm laser excitation wavelength, the vibrational band at ∼1335 cm−1 may possibly be assigned to CN vibrations of dealkylated polyaniline type segments in the PNBA backbone. The C N stretching vibration at ∼1480 cm−1 (Fig. 5B) indicate that quinoid structures are present in the PNBA film at −0.2 V. 3.2.2.3. Poly(N-ethylaniline) and poly(N-propylaniline). The Raman spectra of PNEA and PNPA were studied only with the 780 nm laser excitation wavelength (Fig. 6 and Table 3). The spectra of PNEA and PNPA are very similar to the Raman spectra of PNBA (Fig. 5B). It seems that an alkyl group longer than ethyl has no bigger influence on the Raman spectra. All substituted poly(N-alkylanilines) with ethyl, propyl and butyl substituents are expected to be more easily dealkylated than PNMA. This is reflected in the CN vibrational band at ∼1335 cm−1 . According to the Raman spectra of PNEA and PNPA, the LE to E transition takes place approximately between 0.1 and 0.4 V, which is in a quite good accordance with the CVs of PNEA and PNPA (Fig. 2a). At 0.7 V (PNEA) and 0.8 V (PNPA), the PN form should be the dominant oxidation state within the PNEA and PNPA films. 3.2.2.4. PNMA and PNBA polymerized in mixed solvents. The Raman bands of PNMA and PNBA films, which were polymerized in mixed aqueous solvents containing 10% ACN or 10% DMSO, were studied with the 780 nm laser excitation
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M. Blomquist et al. / Synthetic Metals 157 (2007) 974–983
Table 3 Assignments of the Raman bands of PNEA and PNPA Wavenumber (cm−1 )
Assignments
PNEA
PNPA
−0.2 V
0.4 V
0.8 V
−0.2 V
0.4 V
∼1605 1570 ∼1485 ∼1373 ∼1293 – ∼1160 ∼775
∼1605 1570 ∼1475 1377 ∼1325 ∼1229 ∼1170 ∼780
∼1605 1577 1488 1379 ∼1331 ∼1247 1179 ∼782
– 1572 ∼1478 ∼1375 ∼1310 – ∼1175 ∼785
– 1570 ∼1480 ∼1374 – ∼1250 1170 ∼780
0.8 V ∼1605 1576 1489 ∼1380 ∼1330 1258 1180 780
C C C C
C ring stretching (B) C ring stretching (Q) N stretching (Q) N stretching of the E form
C N stretching of the E form C H in-plane bending (Q) Ring in-plane deformation (Q)
The Raman measurements were conducted with the 780 nm laser excitation wavelength. The references for the bands are listed in Table 1. The benzenoid and quinoid units are denoted by B and Q, respectively. The electrically conducting emeraldine form is abbreviated as E.
wavelength. The Raman spectra of the PNMA (10% ACN) and PNMA (10% DMSO) films show the same characteristic features as the PNMA film (Fig. 5A) and are therefore not shown here. Compared to the PNMA spectrum, an additional CN vibrational band was observed at ∼1252 cm−1 for both PNMA (10% ACN) and PNMA (10% DMSO) films. The Raman spectra of PNMA (10% DMSO) show a broad band at ∼1284 cm−1 in the potential interval of −0.2 to 0.1 V. As for PNMA, this band shifts to ∼1355 cm−1 in the potential interval of the LE to E transition. A similar behavior was observed for PMNA (10% ACN). The close similarity of the Raman spectra of PNMA, PNMA (10% ACN) and PNMA (10% DMSO) indicate only small differences in the chemical structure of the polymer backbone of these materials. The same conclusion was made for the PNBA films in this study due to the great similarity of the Raman spectra of PNBA, PNBA (10% ACN) and PNBA (10% DMSO).
The Raman spectra of PNEA and PNPA films are very similar to PNBA indicating that the alkyl substituents longer than ethyl have only a minor influence on the Raman spectra of these materials. The spectra of PNMA films polymerized in a mixture of aqueous and organic solvent indicate also that the chemical structure of these films are quite similar in comparison to the PNMA film, which was polymerized in pure aqueous solution of HClO4 . Acknowledgements ˚ Akademi ProThis work is part of the activities of the Abo cess Chemistry Centre within the Finnish centre of Excellence Program (2000–2011) sponsored by the Academy of Finland. Maija Blomquist gratefully acknowledges the financial support of Svenska Tekniska Vetenskapsakademien and the Finnish National Graduate School of Nanoscience.
4. Conclusions References A fundamental Raman characterization of four different poly(N-alkylanilines) was conducted with the laser excitation wavelengths of 514.5, 632.8 and 780 nm. In contrast to the conducting emeraldine salt form of PANI, the results obtained in this study show that the half-oxidized conducting emeraldine form of PNMA, PNEA, PNPA and PNBA consists of both benzenoid and quinoid structures. This supports the validity of the commonly used oxidation and reduction scheme of PNANIs (Scheme 1). In their reduced state at −0.2 V, all PNANIs studied showed usually one CN vibrational band in the Raman spectra in the wavenumber region of 1300–1400 cm−1 . A shift of this Raman band was observed in the potential interval of the transition from the fully reduced non-conducting leucoemeraldine form to the electrically conducting emeraldine form. This transition is often, especially in PNBA, accompanied with the appearance of a CN vibrational band at ∼1335 cm−1 , which may indicate the presence of polyaniline type segments in the polymer structure. The formation of these segments may possibly be due to partial dealkylation of the N-substituted alkyl groups.
[1] J.W. Chevalier, J.Y. Bergeron, L.H. Dao, Macromolecules 25 (1992) 3325. [2] R. Holze, in: H.S. Nalwa (Ed.), Advanced Functional Molecules and Polymers, vol. 2, Gordon and Breach Science Publishers, 2001, pp. 171–221 (Chapter 7). [3] N. Comisso, S. Dahlio, G. Mengoli, R. Salmaso, S. Zecchin, G. Zotti, J. Electroanal. Chem. 255 (1988) 97. [4] T. Lindfors, A. Ivaska, J. Electroanal. Chem. 535 (2002) 65. [5] A. Malinauskas, R. Holze, Ber. Bunsenges. Phys. Chem. 101 (1997) 1859. [6] A. Malinauskas, R. Holze, Electrochim. Acta 44 (1999) 2613. [7] C. Sivakumar, A. Gopalan, T. Vasuvedan, T.-C. Wen, Synth. Met. 126 (2002) 123. [8] D. Wei, T. Lindfors, C. Kvarnstr¨om, L. Kronberg, R. Sj¨oholm, A. Ivaska, J. Electroanal. Chem. 575 (2005) 19. [9] A. Watanabe, K. Mori, A. Iwabuchi, Y. Iwasaki, Y. Nakamura, Macromolecules 22 (1989) 3521. [10] J. Langer, Synth. Met. 35 (1990) 295. [11] L.H. Dao, J.Y. Bergeron, J.W. Chevalier, M.T. Nguyen, R. Paynter, Synth. Met. 41–43 (1991) 655. [12] C. Barbero, M.C. Miras, O. Haas, R. K¨otz, J. Electroanal. Chem. 310 (1991) 437. [13] J. Yano, M. Kokura, K. Ogura, J. Appl. Electrochem. 24 (1994) 1164. [14] A. Lian, S. Besner, L.H. Dao, Synth. Met. 74 (1995) 21. [15] P.A. Kilmartin, G.A. Wright, Synth. Met. 104 (1999) 145.
M. Blomquist et al. / Synthetic Metals 157 (2007) 974–983 [16] A.A. Athawale, B. Deore, M. Vedpathak, S.K. Kulkarni, J. Appl. Polym. Sci. 74 (1999) 1286. [17] G.A. Planes, M.C. Miras, C. Barbero, Polym. Int. 51 (2002) 429. [18] R. Siwakumar, R. Saraswathi, J. Power Sources 104 (2002) 226. [19] R. Siwakumar, R. Saraswathi, Synth. Met. 138 (2003) 381. [20] J. Yano, Y. Ota, A. Kitani, Mater. Lett. 58 (2004) 1934. [21] A. Falcou, A. Duchˆene, P. Hourquebie, D. Marsacq, A. Balland-Longeau, Synth. Met. 149 (2005) 115. [22] A. Lian, S. Besner, N.E. Cherid, J.Y. Bergeron, L.H. Dao, Proc. Polym. Mater.: Sci. Eng. 70 (1994) 445. [23] G.-E. Hwang, K.-Y. Wu, M.-Y. Hua, H.-T. Lee, S.-A. Chen, Synth. Met. 92 (1998) 39. [24] P. Syed Abthagir, R. Saraswathi, S. Sivakolunthu, Thermochim. Acta 411 (2004) 109. [25] M. Blomquist, T. Lindfors, L. V¨ah¨asalo, A. Pivrikas, A. Ivaska, Synth. Met. 156 (2006) 549. [26] K. Chiba, T. Ohsaka, N. Oyama, J. Electroan. Chem. Int. Electrochem. 217 (1987) 239. [27] K. Singh, U. Mishra, Ind. J. Chem. 37A (1998) 613. [28] A.D. Borkar, M.C. Gupta, S.S. Umare, Polym. Plast. Technol. Eng. 40 (2001) 225. [29] G.-L. Yuan, N. Kuramoto, Macromolecules 36 (2003) 7939. [30] X.-G. Li, H.-J. Zhou, M.-R. Huang, J. Pol. Sci. 42 (2004) 6109. [31] M.R. Nabid, A.A. Entezami, Polym. Adv. Technol. 16 (2005) 305. [32] X.-G. Li, H.-J. Zhou, M.-R. Huang, Polymer 46 (2005) 1523. [33] S. Quillard, G. Louarn, K. Berrada, S. Lefrant, K.A. Coplin, S.W. Jessen, A.J. Epstein, Nonlinear Opt. 10 (1995) 253.
983
[34] A. Malinauskas, R. Holze, Electrochim. Acta 43 (1998) 521. [35] G. Niaura, R. Maˇzeikien˙e, A. Malinauskas, Synth. Met. 145 (2004) 105. [36] M.C. Bernard, A. Hugot-Le Goff, S. Joiret, H. Arkoub, B. Sa¨ıdani, Electrochim. Acta 50 (2005) 1615. [37] T. Lindfors, A. Ivaska, J. Electroanal. Chem. 580 (2005) 320. [38] T. Lindfors, C. Kvarnstr¨om, A. Ivaska, J. Eletroanal. Chem. 518 (2002) 131. [39] Y. Wei, W.W. Focke, G.E. Wneck, A. Ray, A.G. MacDiarmid, J. Phys. Chem. 93 (1989) 495. [40] J.E. Pereira da Silva, D.L.A. de Faria, S.I. C´orboda de Torresi, M.L.A. Temperini, Macromolecules 33 (2000) 3077–3083. [41] Lj.D. Arsov, W. Plieth, G. Koßmehl, J. Solid State Electrochem. 2 (1998) 355. [42] C. Liu, J. Zhang, G. Shi, F. Chen, J. Appl. Polym. Sci. 92 (2004) 171. [43] G. Socrates, Infrared and Raman characteristic group frequencies, in: Tables and Charts, third ed., John Wiley & Sons, 2001, p. 109. [44] K. Berrada, S. Quillard, G. Louarn, S. Lefrant, Synth. Met. 69 (1995) 201. [45] M. Bartonek, N.S. Sariciftci, H. Kuzmany, Synth. Met. 36 (1990) 83. [46] Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada, T. Nakajima, T. Kawagoe, Macromolecules 21 (1988) 1297. [47] A. Hugot-Le Goff, M.C. Bernard, Synth. Met. 60 (1993) 115. [48] S. Quillard, G. Louarn, J.P. Buisson, S. Lefrant, J. Masters, A.G. MacDiarmid, Synth. Met. 49–50 (1992) 525. [49] J. Laska, Synth. Met. 129 (2002) 229. [50] S. Quillard, K. Berrada, G. Louarn, S. Lefrant, M. Lapkowski, A. Pron, New J. Chem. 19 (1995) 365.