Journal of Electroanalytical Chemistry 453 (1998) 107 – 112
Electrochemical gelation of poly(3-hexylthiophene) film
Department of Applied Physics, Fukui Uni6ersity, 3-9-1, Bunkyo, Fukui-shi, 910, Japan
Received 20 January 1998; received in revised form 18 May 1998
When the chemically synthesized poly(3-hexylthiophene) film was oxidized electrochemically in acetonitrile for 1 h, it showed
three times a volume change after being transferred into chloroform solution to yield a permanent gel. The swelling increased asthe potential was more positive. The absorbance at ca 302 nm, attributed to the n y* transition in the conjugated chain,increased with the longer and more positive electrolysis, indicating development of the conjugation. In contrast, the absorbanceat 765 nm, responsible for the electric conduction, decreased with the electrolysis. Both solubility of the polymer in chloroformand the electric conductance also decreased. The volume change was related quantitatively to the solubility with the help ofFlory’s theory of the swelling, in which the swelling ratio is proportional to the fifth power of the volume per crosslinked polymermolecule. FTIR spectra suggested that C – H in the thiophene ring was more highly decomposed with application of the morepositive potential to yield the crosslinking. The electrochemically synthesized film showed variations similar to those of thechemically synthesized film although the variations were less specific. The electrochemical gel was different from the gel initiatedby benzoyl peroxide, which results in crosslinking between the hexyl chains. 1998 Elsevier Science S.A. All rights reserved. Keywords: Crosslinking; Electric conduction; Flory’s theory; FTIR; Gel; Poly(3-hexylthiophene); Solubility
Volume transition of gels occurs as a result of change
in temperature, ionic strength, compositions of sol-
Conducting polymers such as polyaniline are charac-
vents, electric field, and mechanical pressure . It also
terized by the exhibition of electronic conduction in the
responds to electrode processes , in which the phase
doped state [1,2]. Since the conduction is ascribed to
transition is associated with the change of mass transfer
the long extended conjugation on the backbone, chemi-
rate of the electroactive species in the gel [13,14]. Un-
cal modification of the backbone frequently decreases
fortunately, the effect of electrode processes is not so
the conductivity. Thus, modification has often been
specific as variations of temperature or ionic strength,
made to the substituents of the backbone, exemplified
and no remarkable volume change has been reported
by poly(3-alkylthiophene). Various functionalities of
yet, to our knowledge. This is partly because the elec-
poly(3-alkylthiophene) have been used to improve the
tric field relevant to the volume change is restricted to
solubility in various organic solvents , fusibility at
the double layer in the vicinity of the electrode rather
low temperature , crystallinity , conductivity ,
than the gel bulk, partly because the electrochemically
and photosensitivity . In the course of the modifica-
generated species which causes volume change is local-
tion, gelation of poly(3-alkylthiophene) has attracted
ized near the electrode surface , and partly because
attention to the volume change [8 – 10].
the usual electrochemical systems include a high con-centration of ions which makes the change of the ionicstrength insignificant. If a gel made of conducting poly-mers shows a volume change between the conducting
state and the insulating state, the electrochemical
0022-0728/98/$19.00 1998 Elsevier Science S.A. All rights reserved. PII S0022-0728(98)00238-1
J. Li, K. Aoki / Journal of Electroanalytical Chemistry 453 (1998) 107 – 112
switching might cause a rapid volume response ex-
(A). The ITO plate in acetonitrile + 0.1 M TBATFB
tended over the whole polymer. Rapidity is expected
solution was used for a reference for the absorption.
even for films as thick as a few millimeters because the
FTIR spectra were taken with a PROTAGE System
electrochemical switching provides almost homoge-
460 (Nicolet) by mounting a 1 mg sample in a KBr
neous redox states [15 – 23] in spite of the inherently
heterogeneous reaction. The homogeneity is ascribed to
The conductance of the film was measured by casting
the propagation of a conducting zone [15 – 19] and the
the film on the ITO, drying it in air, pressing a copper
rod on the film, and measuring the dc current between
In this report, electrochemical gelation of poly(3-
hexylthiophene) is described in the context of the de-pendence of volume change, solubility, conductance,UV-vis and FTIR spectra on the electrode potential. 3. Results and discussion
This is found in the course of voltammetric variation ofpoly(3-methylthiophene) with the temperature , in
The chemically synthesized film was oxidized at vari-
which an apparent negative activation energy has been
ous potentials (E \0.6 V) for 1 h and then was reduced
at − 0.6 V for 5 min. The film turned dark red. When itwas transferred into chloroform, it was swollen immedi-ately and partly dissolved in the chloroform. The film
kept in the oxidized state did not exhibit any volumechange when transferred into chloroform. Thus reduc-
Three kinds of working electrodes were used: (A) an
tion after the long term oxidation is required for the
indium tin oxide (ITO) plate (9 × 55 mm2); (B) a glass-
swelling. When the film was inserted into ethanol solu-
shielded platinum disk 0.5 mm in diameter; and (C) a
tion, it curled up without any volume change. Even if
platinum plate (40 × 9.2 mm2). The counter electrode
chloroform was removed from the swollen film by
was a platinum coil. The reference electrode was a silver
evaporation, the film remained the same size. The film
transferred back to the acetonitrile solution did not
change in volume. Therefore, the volume change is
trile. All the electrochemical measurements were made
irreversible even with change of solvent; that is, the
in the acetonitrile solution including 0.1 M TBATFB +
electrochemically treated film is a permanent gel. The
molecular sieves at room temperature.
swelling was almost independent of the kind of cation
Two types of poly(3-hexylthiophene) films were pre-
(tetraethylammonium, lithium and tetrabutylammo-
pared either by chemical or electrochemical oxidation.
nium) in solution as well as the kind of electrode
The chemical oxidation of 3-hexylthiophene was made
using anhydrous FeCl in chloroform  to yield a
As a measure of the swelling, we measured the side
dark red polymer. The polymer dissolved in chloroform
length of the rectangular cut film 1 × 1 mm2 through a
was cast on electrodes (A) and (B) to be dried in air.
microscope 30 s after the transfer into chloroform. 95%
Then it was treated electrochemically in 0.1 M
of the swelling was completed within 20 s. Fig. 1 shows
TBATFB + acetonitrile solution or used for voltam-
the dependence of the ratio of the swollen length on the
metric measurements. On the other hand, the electro-
oxidation potential. The ratio was independent of the
chemical preparation was made galvanostatically (3 mA
reduction time if it was over 60 s as well as of the
cm−2) at electrode (B) or (C) for 100 s at 5°C in 0.1 M
reduction potential (from − 0.9 to − 0.1 V) after the
oxidation. Since the swelling is irreversible, each plot in
The film on electrode (C) was rinsed three times with
Fig. 1 was obtained at a new film. The scatter of the
acetonitrile and was then peeled off from the electrode.
data points for different films was within 15%. The
It was cast on electrode (A) by dissolving it slightly in
swelling began at 0.65 V and increased linearly with the
potential. The initiation potential 0.65 V is close to the
In order to make crosslinking chemically in the poly-
peak potential of the cyclic voltammogram of the cast
mer, benzoyl peroxide (BPO) dissolved in chloroform
film ((a) in Fig. 2). Films oxidized for a time less than
was added dropwise to the film. Then the yellow film
5 min did not show the swelling, whereas the swelling
became orange and was swollen. 20 min after the
was saturated at oxidation times longer than 1 h. The
addition, the film was immersed in chloroform for 4 h
current in the cyclic voltammogram decreased with the
to remove the soluble part. Then the polymer became
potential cycle in the domain from 0.65 to 1.0 V,
dark red, and the volume was twice that of the original
indicating that the swelling is associated with electro-
The in situ UV-vis spectrometry and time scan were
We measured the conductance of the dried film after
performed in a one-compartment cell with electrode
electrochemical oxidation for 1 h. Fig. 3 shows the
J. Li, K. Aoki / Journal of Electroanalytical Chemistry 453 (1998) 107 – 112
Fig. 1. Variations of the ratio of the side length of the film (filledcircle) and wavelength of UV spectra around 300 nm (open circle)
Fig. 3. Variation of the conductance of the film with the potential
with the applied potential, E. The chemically synthesized film was
applied for 1 h to the film before the conductance measurement. The
cast on the ITO, to which E was applied in 0.1 M TBATFB + ace-
arrow is the order of the measurement.
tonitrile solution for 1 h, and then reduced at − 0.6 V for 5 min. Forthe measurement of the swollen length, the film was detached from
ascribed to the development and stabilization of the
the ITO, rinsed with acetonitrile three times, cut in a rectangular
conjugation . The variation of the red shift is akin
form (1 × 1 mm2), transferred into chloroform solution, and mountedon a microscope.
to that of l/l , as shown in Fig. 1 although the quanti-
tative comparison of the intensity (l) with the energy(
dependence of the conductance on the applied poten-
u) has no meaning. The similarity indicates a similar
participation in the conjugation. The absorbance in-
tial, which was varied in the positive direction and then
creased not only with the applied potential but also
reversed. The conductance had a maximum at 0.7 V.
with the time, as shown in Fig. 5. The gradual increase
Once a potential more positive than 0.7 V was applied,
and the saturation of the absorbance demonstrate that
the electric conductance was lost and could not be
long time (1 h) electrolysis is crucial to make the gel.
recovered by any potential change. This big hysteresis
The other feature of the potential-dependent spectra is
accords with the irreversible variation of the swelling.
the broad absorption band at ca 770 nm. It has been
Fig. 4 shows UV-vis spectra at various potentials for
assigned to the transition of the bipolaron , which is
electrode (A). With the positive increase in the poten-
thought to be responsible for the electronic conduction
tial, the absorbance at ca 300 nm, which has been
. Fig. 5 also shows the potential dependence of the
assigned to an n y* transition , increased. This is
absorption at 765 nm. The dependence is similar to the
associated with a red shift from 300 nm at 0.6 V to 306
variation of the conductance (Fig. 3). It resembles the
nm at 1.0 V. The increase and the red shift have been
degradation of the film by over-oxidation. The degrada-tion occurs, however, reportedly at potentials greaterthan 1.2 V .
Fig. 2. Cyclic voltammogram (a) of the chemically synthesized andcast film in 0.1 M TBATFB + acetonitrile at the sweep rate 50 mVs−1 at electrode (B). Cyclic voltammograms at 50 mV s−1 of theelectrochemically synthesized films immediately after the synthesis
Fig. 4. UV-vis spectra of the film without oxidation (a), with oxida-
(b), after 0.8 V (c) and 1.0 V (d) oxidation for 20 min.
tion at (b) 0.3; (c) 0.5; (d) 0.7; and (e) 0.9 V. J. Li, K. Aoki / Journal of Electroanalytical Chemistry 453 (1998) 107 – 112
Fig. 5. Variations of absorbances at 302 nm (circle) and 765 nm
Fig. 7. Dependence of (l/l )5, indicating the number of crosslinked
(triangle) with the applied potential. Each point was taken succes-
polymers in the film, on the solubility A/A of the film that was
sively after 5 min potential application.
The higher is the crosslinking level, generally the
insolubility, 1 − A/A , on the oxidation potential,
poorer is the solubility of a polymer. We determined
is the absorbance without the oxidation.
the solubility of the chemically synthesized poly(3-
Films oxidized at E B0.3 V were completely dissolved
hexylthiophene) film in chloroform at various oxidation
in chloroform. The film became insoluble with a posi-
potentials. When the film was oxidized electrochemi-
tive shift of the potential. The film oxidized at E \0.9V
cally and transferred into chloroform solution, it
took on a gel form which was insoluble. Although the
showed actually no solubility. If it was reduced for a
insolubility should be related with the swelling in con-
short time in acetonitrile, it had finite solubility in
junction with crosslinking and molar mass, it does not
chloroform. Thus we oxidized the film at various poten-
have a simple relation with l/l , as shown in Fig. 6.
tials for 1 h in 0.1 M TBATFB + acetonitrile solution,
We consider in more detail the relation between the
reduced it at − 0.5 V for 5 min followed by rinsing it
solubility, the swelling, the crosslinking and the oxida-
with acetonitrile, and immersed it for 3 h in chloro-
tion potential. Intuitively the crosslinking increases the
form, which was then analyzed spectroscopically at 436
insolubility and the swelling ratio unless it occurs so
nm (pp* transition). Fig. 6 shows dependence of the
highly as to block the penetration of solution. Thesolubility decreases generally with an increase in themolar mass. As the molar mass increases by crosslink-ing polymer chains, the amount of uncrosslinked poly-mer decreases. For the simplest case, the solubility maybe proportional to the amount of polymer, 6 , that is
. The relation between the swelling and 6
reminds us of Flory’s theory of the swelling of gels . According to the theory, the one-dimensional swollenlength is given by
where V is the volume of the film before the swelling,
is the interaction parameter between the solvent and
the polymer, and 6 is the volume fraction of the
solvent in the gel. Substitution of A/A
Fig. 6. Dependence of the insolubility of the film to chloroform on
the potential, which was applied to the film for 1 h in acetonitrilesolution. The insolubility is defined as 1 − A/A , where A is the
absorbance of the dissolved polymer in chloroform at 436 nm, and A0
is the absorbance without the electrochemical oxidation. The rightabscissa shows the swollen length which was displayed in Fig. 1. The
We plotted in Fig. 7 values of (l/l )5 against A
solid curve on the plot of 1 − A/A was calculated by combination of
the Nernst equation and the mass balance for E°%=0.67 V and
for the domain of the sufficiently dissolved film, the
J. Li, K. Aoki / Journal of Electroanalytical Chemistry 453 (1998) 107 – 112
plot fell on a straight line through the origin. Theexcellent proportionality may be ascribed to weak de-pendence of and 6 on the oxidation. However, the
present analysis does not always justify Flory’stheory because it includes ambiguity in the parame-ters.
The insolubility caused by the oxidation suggests
electrochemically irreversible crosslinking, which maybe expressed by the electrode reaction: M M+ +e−followed by a chemical chain reaction: nM+ M .
The equilibrium concentration of M+ is representedby the Nernst equation: [M+] = 1/[1 + exp(hF(E−E°%)/RT)], where h is a fractional number of electronsper electroactive site, usually ranging from 0.2 to 0.5[16,23,31,32]. The mass balance leads to [M+] +
Fig. 9. Variation of the FTIR absorbance A (on the left ordinate) at
n[M ] = const. Since M is scarcely soluble, its con-
828 cm−1 for the chemically synthesized films (circle) and the electro-
chemically synthesized films (triangle) which were oxidized at various
potentials. The plots were normalized by the absorption A at 828
1/[1 + exp(hF(E−E°%)/RT)] should have a linear rela-
cm−1 of the film without electrochemical treatment. They agree with
tion with 1 − A/A . Unfortunately, the relation in-
variation of (l/l )5 on the right ordinate (with an increase down-
cludes the unknowns h and E°%. We attempted to find
the linearity by inserting various values of h and E°%. Then the best linearity was obtained with E°%=0.67
symmetric C – H stretching vibration in – CH – .
V and h=0.35. From these values, values of 1−A/
They are ascribed to the hexyl-side chain. Since the
were calculated. The result is shown as a solid
methyl group is not electroactive in the present poten-
curve in Fig. 6. Agreement of the simulation is excel-
tial domain, we regarded the absorbance at 2957
lent, indicating that the insolubility can be explained
cm−1 as a reference for normalizing the other ab-
sorbances. The bands at 3057 and 828 cm−1 have
In order to interpret the macroscopic swelling with
been assigned, respectively, to the stretching vibration
a discussion at the molecular level, we measured
FTIR spectra of the oxidized film. Fig. 8 shows
FTIR spectra of the poly(3-hexylthiophene) chemi-
be ascribed to the 4-position of the thiophene ring.
cally synthesized (A), and electrochemically treated at
The ratio of absorbance A at 828 cm−1 for the film
0.8 (B) and 1.0 V (C) for 1 h. Strong absorption
with potential applied to that for the film before the
bands at 2957, 2918 and 2856 cm−1 have been as-
potential application decreased with the positive po-
signed, respectively, to the asymmetric C – H stretch-
tential shift, as shown in Fig. 9. The variation is very
similar to that of (l/l )5 although the direction of the
variation is opposite. This correspondence indicatesthat the swelling is due to the loss of the aromatic
C – H. If the relation between A
is combined with the proportionality depicted in Fig. 7, it turns out that A
. In other words, the number of the decomposed
thiophene rings has a linear relation with the volumeof a crosslinked polymer, or the decomposition causesthe crosslinking by an equivalent amount. Thus apossible mechanism of the electrochemical oxidationis the oxidation at the sulfur of the thiophene ring,and stabilization by resonance followed by the activa-tion at the 4-position of the thiophene ring, whichcauses coupling with other rings to generate acrosslinked network.
All the discussion above was for crosslinking
by the electrochemical oxidation of the chemically
Fig. 8. FTIR spectra of the chemically prepared films (A), potential-
synthesized film. A question arises on the crosslinking
applied films at 0.8 V (B) and 1.0 V (C) for 1 h. The arrow is at 828cm−1.
of an electrochemically native-born film, because the
J. Li, K. Aoki / Journal of Electroanalytical Chemistry 453 (1998) 107 – 112
synthesized film must include the electrochemical
crosslinking simultaneously. Since the galvanostaticpolymerization started at 3.5 V, the film should be
This work was supported by Grants-in-Aid for Scien-
subjected to the crosslinking. Indeed, FTIR spectra
tific Research (Grant 09237228) from the Ministry of
showed the appearance of the band at 828 cm−1. When
the film was further oxidized for 1 h in a solution of 0.1M TBATFB + acetonitrile, the absorbance at 828 cm−1decreased with the positive shift of the potential, as
shown in Fig. 9. Consequently, the electrochemicallysynthesized film has been partially crosslinked. Cyclic
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