Journal of Electroanalytical Chemistry 625 (2009) 75–81
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry
journal homepage: www.elsevier.com/locate/jelechem
Electrochemomechanical properties of aniline/2-methoxyaniline copolymers
Walter Armada, Guillermo Contrafatti, Leonardo Lizarraga, Estela María Andrade, Fernando Victor Molina 1,*
INQUIMAE and Departamento de Química Inorgánica, Analítica y Química Física, Facultad de Ciencias Exactas y Naturales, UBA. Ciudad Universitaria, Pabellón II,
C1428EHA Buenos Aires, Argentina
a r t i c l e
i n f o
Article history:
Received 14 August 2008
Received in revised form 6 October 2008
Accepted 8 October 2008
Available online 17 October 2008
Keywords:
Conducting polymer
Cyclic voltammetry
Volume changes
Swelling
Polymer/anion interactions
a b s t r a c t
Copolymers of aniline (ANI) and 2-methoxyaniline (MOA) in variable ratios, as well as the corresponding
homopolymers, were electrosynthesized and characterized by elemental analysis, IR spectroscopy, and
direct laser desorption ionization. The resulting copolymer composition obtained from the analysis was
correlated with the feed composition through the Mayo–Lewis equation, confirming the higher reactivity
of MOA. The electrochemomechanical behaviour was studied through direct microscopical observation of
the volume changes caused by redox switching in sulphuric and perchloric acid media. It was found that
the freshly electropolymerized films had, for the same voltammetric charge, smaller volume as the MOA/
ANI ratio increased. The volume changes observed during the first cycle where consequently strongly
dependent on the polymer composition, but also on the electrolyte anion. The stationary cycle changes
showed less dependence on both. The behaviour reveals different anion/polymer interactions as the
copolymer composition is changed. From the perspective of mechanical applications the ANI–MOA
copolymers, for most of the composition range, did not show marked differences compared with poly(aniline), thus being materials with similar characteristics to this homopolymer but being more processable.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
Aryl amine polymers such as poly(aniline) (PANI) and derivatives have received great attention since its rediscovery by Shirakawa et al. [1]. One of the main reasons is its wide range of potential
applications, including presently fields such as actuators [2,3],
electrochromic and photovoltaic devices [4–6], secondary batteries
[7] fuel cells [8], supercapacitors [9,10], sensors [11–16], nanomaterials [17,18], photocatalysts [19] and corrosion protection [20]
among many other ones [21–23].
Despite the fact that PANI can be synthesized quite easily, there
are difficulties in practical applications arising from its poor processability due, among other reasons, to the limited number of
appropriate solvents available. To overcome that, polymers of derivatives such as 2-methylaniline [24–27], 2-ethylaniline [28] or 2methoxyaniline (MOA) [29–33] have been investigated, and also
copolymers or composites [34] of aniline (ANI) and other suitable
monomers [35–37]. A copolymer which is gaining increasing attention is poly(aniline-co-2-methoxyaniline) (PAcoM) [38–43]. Poly(2methoxyaniline) (PMOA) has better solubility [38,40,41] and
(references therein) and hence better processability but a lower conductivity, thus PAcoM of diverse compositions have been proposed
as a way of combining the properties of the two homopolymers.
* Corresponding author. Fax: +54 11 4576 3341.
E-mail address: [email protected] (F. V. Molina).
1
ISE member.
0022-0728/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jelechem.2008.10.004
Wang et al. [38] synthesized PAcoM in several monomer ratios as
well as the homoplymers, finding highly increased solubility of
the copolymers. Ram et al. [39] chemically synthesized PAcoM with
different aniline molar feed fraction, fa, values, and determined the
product composition through 1H NMR, observing an increased molar fraction of MOA, (1Fa), in the polymer relative to the feed, due to
the higher reactivity of this monomer. Motheo et al. [40] synthesized PAcoM electrochemically and determined the polymer
composition by elemental analysis, finding also higher reactivity
of MOA. Mokreva et al. [41] obtained PAcoM chemically using various polymerization techniques, reporting higher solubility for
copolymers compared with PANI. Özyilmaz et al. presented the
application of electrochemically prepared PAcoM for the protection
of steel [42] and brass [43] from sodium oxalate/toluenesulphonic
acid solutions, reporting good protection capability in both cases.
The electrochemistry of aryl amine polymers has been intensively studied in the past years, mainly the insulator/conductor
transition (redox switching) observed during the oxidation from
the fully reduced form leucoemeraldine (LE) to the oxidized one
emeraldine (EM) [21,23,44–47]. An interesting feature of conducting polymers are the dimensional changes that undergo during the
redox switching (or other redox processes), known as electrochemomechanical properties [48], leading to actuator applications.
They have been quite extensively studied in the case of polypyrrol
[49–51]. In the case of PANI, the dimensional changes in the oxidation of LE to EM in PANI have been observed by several authors
[52–59], and consequently it has been proposed the use of PANI
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W. Armada et al. / Journal of Electroanalytical Chemistry 625 (2009) 75–81
for actuators [60,61]. Volume and length changes have been generally attributed to proton/anion exchange (i.e., doping/dedoping)
[48,51,55,56,62,63]. Electrochemical quartz crystal microbalance
(EQCM) studies [64,65] indicate, however, that water movement
exists and can play a significant role in the process.
The study of electrochemomechanical properties of ANI–MOA
copolymers will bring insight into alternative materials to overcome the poor processability of ANI, and would be also useful because it could aid to gain insight into the electrochemical switching
process. In this paper, we present a study of the electrochemomechanical deformations of relatively thick polymer films caused by
switching between LE and EM forms in H2SO4 and HClO4 media.
The volume changes were measured with the aid of an image capture and processing system. PAcoM with different monomer ratios
were studied, as well as the corresponding homopolymers PANI
and PMOA. All these were electropolymerized and characterized
by elemental analysis, FTIR spectroscopy and direct laser desorption ionization (LDI-TOF).
2. Materials and methods
2.1. Polymerization
ANI/MOA copolymers were electrochemically synthesized from
solutions having different molar ratios aniline/2-metoxianiline,
varying from 100:1 to 3:1, with a total monomer concentration
of 0.5 M. PANI and PMOA homopolymers were also obtained from
the respective monomer solutions. Synthesis solutions were prepared in acid media, provided by the mix of 1.5 M sulphuric + 1.5 M perchloric acids. A standard three-electrode cell with
separate compartments for each electrode was employed to electrosynthesize polymers for cyclic voltammetry, FTIR, elemental
analysis and LDI-TOF. For the volume change experiments a different three-electrode cell, suitable for microscopic observation, was
used as described before [57–59]. The reference electrode was always a reversible hydrogen electrode (RHE) and the counter electrode was a platinum foil. In all cases, potentials reported belong
to the standard hydrogen electrode scale. A Teq HP (S. Sobral, Buenos Aires, Argentina) potentiostat/galvanostat under computer
control and a PAR model 273 potentiostat/galvanostat also under
computer control were employed. The working electrode consisted
of a platinum disk inserted into a PTFE holder. The disk diameter
was 0.5 mm for microscopy experiments, and was inserted from
the cell side so as to be laterally observed on the microscope
[54]; for characterization purposes a 10 mm disk or a 5 cm2 Pt foil
was employed; for elemental analysis and LDI-TOF measurements
the polymers were mechanically removed and processed. Electrodes were polished with alumina, washed in an ultrasonic bath
and rinsed carefully.
In all cases, polymer electrosynthesis was done by potential cycling at a scan rate of 0.05 Vs1 between 0.1 and 1.0 V. The process
was monitored through the total cathodic charge, QC, (between 0.1
and 1.0 V). The synthesis was stopped when QC = 3.5 C cm2 for
films used for microscopic observations, and at QC = 2 mC cm2
for FTIR studies.
The reagents and N-methylpyrrolidinone (NMP) were all analytical grade; ANI and MOA were vacuum distilled under reducing
conditions shortly before use; all other were used as received. Ultra
pure water from a Millipore MilliQ system was used throughout.
2.2. Samples treatment
Different treatments were performed on polymer samples after
the synthesis, according to the technique to be applied. Elemental
analysis, LDI-TOF and FTIR measurements were performed on
polymer samples that were firstly rinsed in a dilute solution of
NH3 and hydroxylamine in order to obtain the reduced base form
of the polymers, and were dried in air at 45 °C. Polymer films were
mechanically removed from the electrode for LDI-TOF and elemental analysis. FTIR measurements and microscopic observations
were directly performed on the polymer films over the electrode.
2.3. Elemental analysis
The elemental analysis of C, H and N was performed in a Carlo
Erba EA 1108 instrument with a thermal conductivity detector. The
sample combustion was carried on a tube reactor where polymer
samples were quantitatively transformed into CO2, H2O and N2.
Gas chromatography with a porapac column of variable length
was used for separating the gas mixture. The standard compound
employed to calibrate the method was sulfanilamide.
2.4. FTIR
The IR spectra were recorded ex situ on thin films (2 mC cm2)
on the Pt disks using the diffuse reflectance technique in a Nicolet
510P FTIR spectrophotometer, equipped with a Spectra Tech Baseline Diffuse Reflectance accessory. See Andrade et al. [26,66] for
further details.
2.5. LDI-TOF
Measurements were performed using a Bruker Daltonics OmniFLEX UV-MALDI-TOF mass spectrometer equipped with a pulsed
Nitrogen laser (kem = 337 nm) and tuneable pulsed-delayed extraction (PDE). The experiments were conducted with an accelerating
voltage of 19 kV and the voltage gradient for ion extraction was
1.7 kV. The sample was irradiated just above the threshold laser
power for obtaining ions. Usually, 60 spectra were accumulated.
All samples were measured in linear mode in the positive ion
mode. Sample preparation consisted of dissolving 1 mg of each
polymer in 200 lL of NMP. The mixtures obtained were well stirred (using a vortex) in order to completely dissolve polymers.
Then, 0.5 lL of these solutions were placed on different sample
probe spots and the solvent was evaporated at atmosphere pressure and room temperature. This operation was repeated three
times in order to deposit an appreciable amount of sample in each
spot. Mass spectra were performed with external mass calibration
using b-cyclodextrin as standard and nor-harmane as matrix. Norharmane was also used as an additional standard for calibration.
Polymer measurements were performed in LDI-TOF conditions
without assistant matrices because their use (eg. dihydroxybenzoic
acid) worsened mass spectra resolution, mainly lowering m/z ratios [67].
2.6. Microscopic observations
These experiments were conducted in a cell with top and bottom optical windows, with the working electrode inserted sideways allowing side view microscopical observations of polymer
films [57]. The cell was placed on the stage of a Leica DM-RX
microscope, equipped with a CCD camera coupled to an image
acquisition board, in turn inserted in the computer controlling
the potentiostat. The software set-up allowed synchronization between electrochemical potential scan and image capture within
0.1 s. Images were processed for length and area measurements
using Scion Image software. After polymerization, the working
electrode was extracted from the cell, carefully rinsed with high
purity water and reinserted. The cell was refilled with fresh electrolyte solutions, either 1 M H2SO4 or 1 M HClO4. Each measurement was performed in a new film. Firstly, voltammetric cycling
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W. Armada et al. / Journal of Electroanalytical Chemistry 625 (2009) 75–81
3. Results and discussion
3.1. Characterization
3.1.1. Elemental analysis
The resulting mass contents of C and N were employed to compute the aniline molar fraction, Fa, in the different copolymers.
First, the atomic ratios C/N (rC/N) were computed for the copolymers through mass percentages results; then, assuming that
copolymers presented only aniline and 2-metoxianiline monomers
in their structures and their rC/N values were a linear combination
of rC/N values for PANI and PMOA (6 and 7), Fa values were straightforwardly obtained as Fa = 7 rC/N. Fig. 1 shows Fa as function of
the aniline molar fraction in the feed (synthesis solution), fa. The
results reported by Ram et al. [39] for 1H NMR measurements on
chemically synthesized PAcoM are also plotted (closed squares)
showing excellent agreement. Composition of copolymers obtained by a chain mechanism is described by the so-called
Mayo–Lewis equation [69]
r 1 fa þ fa ð1 fa Þ
r1 fa2 þ 2f a ð1 fa Þ þ r 2 ð1 fa Þ2
0.6
0.4
0.2
0.0
0.0
0.2
0.4
ð2Þ
where r1 and r2, are kinetic constant ratios of propagation steps for
ANI and OMA, known as reactivity ratios. The continuous line in
Fig. 1 is the fitting of experimental data to Eq. (2), resulting in
the parameters values: r1 = 0.07 ± 0.04 and r2 = 10 ± 6. These values
indicate that MOA is more reactive than ANI in the copolymerization process, as is expected due to the lower OMA oxidation potential [32]. According to Mayo et al. [70], the product r1r2 = 0.7 would
indicate a nearly ‘‘ideal” copolymer with some alternating tendency. In the case of aniline-co-2-ethylaniline and similar copolymers it has been reported the formation of a block copolymer [28].
However, as pointed out by Motheo et al. [40], the criteria used to
analyze the copolymer composition in terms of the reactivity ratios
was developed for chemically prepared polymers, thus its
application can be considered speculative when applied to
0.6
0.8
1.0
fa
ð1Þ
Image analysis was performed with Scion Image [68] software
by pixel count in order to obtain the value of polymer area using
standard image processing operations and an appropriate calibration. The later was attained by taking images of a reference scale;
the spatial resolution was equal or less than 2 lm; the main source
of error was the exact determination of the dark zone in the images.
The accuracy of the area was estimated to be better than 0.1%.
Fa ¼
0.8
Fig. 1. Aniline molar fraction in the copolymers (Fa) as a function of aniline molar
fraction in the feed (fa). Open squares, experimental results of this work; closed
squares, 1H NMR results of Ref. [37]; continuous line, fitting to the Lewis–Mayo
equation.
electrochemically prepared polymers. In the following, the properties are analyzed in terms of the Fa values obtained from (2).
3.1.2. FTIR spectra
The spectra obtained are in general agreement with that it has
been reported for PANI [71,72] and PMOA [32]. The IR spectra of
aryl amine polymers have been discussed in the literature
[26,71,72] and (references therein) so that here the focus is in
the differences arising from copolymer composition. Fig. 2 shows
the FTIR spectra in the region 800–1450 cm1 of different electrochemically synthesized copolymers. The band at 1260 cm1, which
is attributed to the Ph–O–C stretching [32], is marked in the figure
because it is indicative of the presence of OMA in the copolymer.
Thus, this band is intense in PMOA spectrum (Fig. 2e) and does
e
A (arbitrary units)
DV
DA 1:5
1
¼ 1þ
V
A
1.0
Fa
was performed between 0.10 and 0.65 V at 0.010 V s1 up to a stationary profile, in order to stabilize the polymer film in the fresh
electrolyte solution. Afterwards, the potential was held at the
cathodic limit for 60 min in order to completely reduce and relax
the polymer [57]. After the holding time, the image of the fully reduced and relaxed polymer film was captured and, using standard
image processing techniques, yielded the initial area value A0. At
this point, cyclic voltammetry measurements were started with
simultaneous recording of video images, and this recording was
extended for six full cycles in order to reach a stationary response.
The subsequent images yielded the corresponding area values AE,
at different potential values and at regular intervals. The relative
changes in the observed area DA/A = (AE A0)/A0 were computed
from the images captured, and were used to determine the relative
volume changes of the polymer film upon oxidation. As the film
expansion was approximately isotropic [59], the relative volume
changes are obtained as
d
c
b
a
1260 cm-1
1400
1300
1200
1100
1000
900
800
/ cm-1
Fig. 2. FTIR spectra of polymers with the following ANI molar fractions: (a)
fa = Fa = 1.0; (b) fa = 0.91, Fa = 0.46; (c) fa = 0.83, Fa = 0.3; (d) fa = 0.75, Fa = 0.22; and
(e) fa = Fa = 0.0.
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W. Armada et al. / Journal of Electroanalytical Chemistry 625 (2009) 75–81
not appear in PANI spectrum (Fig. 2a). As expected from the results
of Fig. 1, when the copolymer is synthesized from a solution with
only 9% of OMA its spectrum (Fig. 2b) presents an intense band
in 1260 cm1. This is in qualitative agreement with the value of
Fa = 0.47 predicted by Eq. (2). A second band at 1035 cm1, which
has been attributed to C–O–C symmetric stretching [40], shows a
similar dependence with the OMA polymer contents. Thus, the IR
spectra features are consistent with the composition analysis
results.
3.1.3. LDI-TOF mass spectra
Mass spectrometry of macromolecules has had a great advance
with the use of soft-ionization techniques such as MALDI [73,74];
however, in the case of polyaniline it is generally found that no
molecular peak can be observed, only oligomers produced by fragmentation [67]. Due to this fact, the use of matrix assisted desorption is of little help and in fact, it has been found preferable to use
direct laser desorption [75], so that in this study LDI-TOF was employed. Fig. 3 presents the different LDI-TOF mass spectra for PANI,
copolymers and PMOA. Spectra are shown in the region of 850–
1150 m/z only for illustrative reasons. The signal/noise ratio was
good near to 2600 m/z (not shown) where it was also possible to
distinguish between peaks and background noise. The polymer
patterns were similar along the whole spectrum. Fig. 3a and e correspond to the mass spectra of PANI and PMOA, respectively. PANI
mass spectrum shows a sequence of main peaks (with the highest
intensities) surrounded by peripheral peaks of lower intensity;
Fig. 3a shows three of these main peaks. PMOA mass spectrum presents some similarity with PANI showing a sequence of main signals surrounded by small signals; Fig. 3e shows two of these
principal signals [67,76]. Main peaks in PANI mass spectrum were
separated by 91 m/z while in PMOA mass spectrum they were separated in 121 m/z. These differences agree with the molecular
masses of the respective monomers. So, main peaks can be associated with polymer chains having different number of monomers
(different lengths). The difference between any main peak and its
peripheral peaks was approximately 15 m/z for both spectra. This
value is coincident with the molar mass of the NH entity. That
means that peripheral peaks correspond to polymer fragments
with the same number of monomers as the main peaks but with
different numbers of terminal NH2 groups [67,76]. Fig. 3b–d correspond to the mass spectra of PAcoM obtained from feed solutions
having Fa of 0.46, 0.30 and 0.22, respectively. These mass spectra,
in contrast to Fig. 3a and e, did not present a clear sequence of
main and peripheral peaks. These mass spectra showed a sequence
of similar intensity peaks separated in 15 m/z. These differences
may be related to the presence or absence of NH2 groups in the
chains extremes as in the cases already described (Fig. 3a and e);
and they may be related to the different amounts of each monomer
in the polymer chains as the molar mass difference of the two
monomers is 30 uma. These features indicate the effective presence of a copolymer with a distribution of ANI and OMA
monomers.
3.1.4. Cyclic voltammetry
Fig. 4a shows the cyclic voltammograms for the thick polymer
films obtained of PANI, POMA and several copolymers. Fig. 4a presents the first cycle after holding the potential 60 min at the cathodic limit, whereas Fig. 4b shows the stationary cycles (after three
cycles there are very small changes in the voltammetry profile,
see Fig. 6b and d below). The general features are similar to those
found in the literature [40,77] for thinner films. The peak potential
shifts anodically as Fa decreases whereas the peak width is lower
for the copolymers than the homopolymers.
3.2. Volume changes
Fig. 5 shows images of polymer films obtained in their completely reduced and relaxed state (after holding the potential
60 min. at 0.1 V) in 1 M H2SO4. These films had a thickness between
150 and 300 lm and a maximum diameter of 750–800 lm. It is
noticeable that copolymer films obtained from solutions with higher aniline molar fraction presented higher thickness. This result to-
e
Intensity (arbitrary units)
d
c
b
a
900
950
1000
1050
1100
1150
m/z
Fig. 3. LDI mass spectra of polymers with the following ANI molar fractions: (a)
fa = Fa = 1.0; (b) fa = 0.91, Fa = 0.46; (c) fa = 0.83, Fa = 0.30; (d) fa = 0.75, Fa = 0.22; and
(e) fa = Fa = 0.0.
Fig. 4. Cyclic voltammograms of the (co)polymer films in 1.0 M H2SO4. (a) First
cycle after holding the potential at the cathodic limit for 60 min and (b) Stationary
cycle.
W. Armada et al. / Journal of Electroanalytical Chemistry 625 (2009) 75–81
79
Fig. 5. Microscopy images of polymers synthesized with different ANI/OMA ratios, in the fully reduced and relaxed state in 1 M H2SO4 media. (a) fa = Fa = 1.0; (b) fa = 0.99,
Fa = 0.88; (c) fa = 0.98, Fa = 0.78; (d) fa = 0.96, Fa = 0.65; (e) fa = 0.94, Fa = 0.56; (f) fa = 0.91, Fa = 0.46; (g) fa = 0.89, Fa = 0.41; (h) fa = 0.86, Fa = 0.35; (i) fa = 0.80, Fa = 0.26 and (k)
fa = Fa = 0.0.
gether with the fact that all films were electropolymerized up to the
same charge, are indicative that copolymer films with higher values
of Fa present less dense morphologies.
Fig. 6a shows the volume changes for a copolymer film with
Fa = 0.88, in 1 M H2SO4. Fig. 6b presents the corresponding cyclic
voltammogram during volume change measurements. In both,
the solid line is the curve obtained in the first cycle, the dashed line
is the curve for the last (stationary) cycle and the dotted lines are
the intermediate cycles. These results are consistent with previous
studies on PANI films [59]. In the first cycle, there is no volume
change up to E 0.4 V. After this point, the volume increases steadily reaching a relative volume change at the anodic limit (first cycle), DV/Van,1, of around 6.4%. As observed before [57,59], the
volume increase starts near the voltammetric peak (that is, when
a considerable amount of charge has passed). Then, in the cathodic
scan, the volume decreases showing hysteresis, and at the cathodic
limit it did not reach the initial value, resulting in a net relative volume increase, DV/Vnet, of about 0.08%. In the following cycles, the
point of initial volume increase moves down to E 0.38 V following the change in the voltammetric peak potential. These cycles
present small variations in the profile of the volume changes compared with the first cycle. A slight increment of hysteresis and a
diminution in the response of the film is observed. The volume
change at the anodic limit in the stationary cycle, DV/Van,s, resulted
of 5.4%, and the volume returns to the same value at the cathodic
limit. Fig. 6c and d show the same experiment conducted in 1 M
HClO4. Here, during the cathodic scan of the first cycle the volume
goes below the initial value reaching a minimum and then increases slightly reaching the cathodic limit with a negative DV/Vnet
value of about 0.5%. In subsequent cycles there is an additional
decrease relative to the initial value, but there is no net change.
Table 1 presents the values of DV/Van,1 and DV/Van,s for the different copolymer films studied in 1 M H2SO4 and 1 M HClO4 media.
The values of DV/Van,1 do not show important variations in both
media except for PMOA in H2SO4, which has the highest value.
The values of DV/Van,s in H2SO4 are almost constant whereas the
corresponding values in HClO4 decrease slightly from PANI to
PMOA for the different copolymer films. Fig. 7 shows the results
of DV/Vnet in 1 M H2SO4 and 1 M HClO4 media as a function of
the aniline molar fraction in the polymer, Fa. It is observed that
for Fa P 0.5 the net volume changes are almost constant in both
media (with a small difference between sulphuric and perchloric
acids), whereas below that value DV/Vnet varies in opposite directions depending on the anion, increasing with OMA contents for
H2SO4 medium and decreasing in the case of HClO4.
As it has been discussed previously [57–59], volume changes in
aryl amine polymers are a result of backbone changes upon redox
switching (changes in the geometry around N atoms), affected by
Fig. 6. (a) Relative volume changes for a copolymer film synthesized from a solution with a ratio fa = 0.99, Fa = 0.88, in 1 M H2SO4 media under cyclic voltammetry
(10 mV s1), after holding for 60 min the potential at the cathodic limit. (b) the corresponding current/potential voltammograms. Continuous line: first cycle; dotted line:
intermediate cycles; dashed line: last cycle. (c) and (d), the same in 1 M HClO4.
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W. Armada et al. / Journal of Electroanalytical Chemistry 625 (2009) 75–81
Table 1
Relative volume changes of PAcoM copolymer films in 1 M H2SO4 and 1 M HClO4.
Fa
1 M H2SO4
1.0
0.88
0.78
0.65
0.55
0.35
0.26
0.0
1 M HClO4
DV/Van,1
DV/Van,s
DV/Van,1
DV/Van,s
0.070
0.064
0.071
0.062
0.077
0.051
0.075
0.102
0.062
0.054
0.059
0.054
0.068
0.037
0.064
0.064
0.056
0.065
0.058
0.049
0.063
0.045
0.048
0.064
0.060
0.055
0.062
0.045
0.063
0.053
0.049
0.030
Fig. 7. Values of DV/Vnet as function of Fa in the copolymer films. Squares, in 1 M
H2SO4 media; triangle, in 1 M HClO4.
anion–polymer interaction; this is at variance with other conducting polymers such as polypyrrole, where ionic ingress/egress are
the driving force for the electrochemomechanical properties
[51,56]. Briefly, the LE–EM redox switching reactions can be written [57–59]
½—B—NH—B—NH—B—NH—B—NHn ðHþ Þ4aLE n
½—B—NH—B—NH—B—N B NHn ðHþ Þð3aLE þaPE Þn
þ ne þ nð1 þ aLE aPE ÞHþ
ð3Þ
½—B—NH—B—NH—B—N —B—NHn ðHþ Þð3aLE þaPE Þn
½—B—NH—B—NH—B—N@Q @Nn ðHþ Þð2aLE þ2aEM Þn
þ ne þ nð1 þ aLE þ aPE 2aEM ÞHþ
ð4Þ
where NH- represents amine groups, B benzenoid rings, Q quinoid
rings and B–N–B stands for the radical cation (polaron) protoemeraldine state; aLE, aPE, and aEM stand for the fraction of protonated
amine, radical cation and imine N atoms, respectively. At acidic
pH it is expected that quinoid N atoms will be fully protonated
whereas the other N types will, in principle, have a < 1. Of course,
anions will be also present in the internal medium in order to satisfy electroneutrality. Reactions (3) and (4) will liberate protons and
thus require ionic transport between the internal and external media. Hillman and Mohamoud [65], described a mechanism involving
proton exit in the first part of the LE–EM conversion and anion entry
in the latter part, with water ingress over all the oxidation process.
The volume changes are interpreted as follows [59]. During the first
part of the anodic scan reaction (3) is the main process, which does
not cause significant changes in the backbone conformation (i.e.,
bond angles around N atoms); protons are expulsed and anions
and water ingress to the film, with a net result of proton expulsion;
volume changes are small. Afterwards, reaction (4) starts, with
changes in the bond angles which lead to a more rigid structure
of the polymer chains; ingress of anions and water and egress of
protons are also produced. Almost all volume increase is produced
here, which is thus attributed to the polymer conformation changes.
The anion effect is related to anion–polymer interactions, with
HSO
4 being strongly interacting and ClO4 relatively weakly interacting [57,59]. Strongly interacting anions alter the conformation
of the polymer chain, whereas weakly interacting anions (ClO4 )
are relatively neutral towards polymer conformation.
Returning to the results of Fig. 7, for Fa > 0.5 with ANI predominating in the copolymer the behaviour is similar to PANI, with no
dependence on the copolymer composition, whereas in the opposite case the increasing OMA contents causes a dependence on
both composition and anion of DV/Vnet. The importance of the an
ion is clearly seen; as both HSO
4 and ClO4 have similar radii (0.221
and 0.225 nm respectively) size effects should be excluded and
interactions taken into account. The anion influence on the electropolymerization and electrochemical response of aryl amine
polymers have been subject of study since several years ago [78–
82], however the exact nature of such influence is not yet well
established. As pointed out elsewhere [79] it is considered that
in the polymerization perchlorate promotes a compact polymer
structure whereas bisulphate promotes an open one, which can
be related to the relative polymerization rates and, in turn to the
hydration enthalpies, both being higher for the HSO
4 ion [78].
Due to the almost equal size and charge of these anions, and their
very similar acidity [57], this difference should be caused by increased solvent association with HSO
4 as a result of hydrogen
bonding. Similarly, it can be expected that this anion would show
more interaction with the polymer than ClO4 due to hydrogen
bonding with the amine/imine groups; however, to the best of
our knowledge, there is no experimental or theoretical evidence
to confirm or reject that. It should be also noted that the influence
of these anions on aniline nucleation and polymerization rate
[79,81], attributed to ion pairing with the protonated monomer,
is also consistent with a lower hydration of the ClO4 anion.
Considering the case of POMA, there is a small net volume de
crease with ClO4 ; this is similar to PANI, where the volume decreases during the cathodic scan below the initial value and
afterwards increases slightly without recovering the initial value
(Fig. 6c). This net volume decrease was attributed [59] to the slow
relaxation of the polymer backbone upon reduction, preventing the
return of the film to its original value in the time scale of the potential cycling; the small dependence on copolymer composition con
firms that ClO4 has weak interactions with the polymer and
consequently little influence on the volume changes. In the presence of HSO
4 , POMA shows a relatively high net volume increase
in the first cycle, but a very similar value of DV/Van,s compared with
PANI; moreover, as observed in Fig. 5, POMA and copolymer films
with Fa < 0.5 have also more compact morphologies when fully reduced and relaxed. These facts indicate that, upon oxidation of the
freshly polymerized film, the ingress of HSO
4 ions and their interaction with the oxidized polymer causes a structure opening which
is not recovered in the cathodic scan, and even is not fully recovered with subsequent holding of the potential at the cathodic limit
(results not shown). In going from POMA to copolymers with increased ANI contents, the influence of OMA diminishes, vanishing
at Fa 0.5. As discussed above, both the compact initial morphology of POMA (and OMA predominating copolymers) and the opening in the presence of HSO
4 can be attributed to increased
hydrogen bonding involving the O atoms of the methoxy groups.
Considering potential electrochemomechanical applications of
these polymers, it is found that for a wide range of copolymer composition the behaviour of PAcoM is similar to PANI, yet having better processability due to its enhanced solubility [38,40,41] as the
OMA contents increases.
W. Armada et al. / Journal of Electroanalytical Chemistry 625 (2009) 75–81
Acknowledgements
The authors gratefully acknowledge financial support from the
Universidad de Buenos Aires (UBACYT X105), the Consejo Nacional
de Investigaciones Científicas y Técnicas (CONICET, PIP 5216/2005)
and the Agencia Nacional de Promoción Científica y Tecnológica
(grant No. 06-12467). F.V. M. is a member of the Carrera del Investigador Científico of CONICET.
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