Pii: s0022-0728(98)00238-

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 Abstract
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 1. Introduction
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 [11]. It also terized by the exhibition of electronic conduction in the responds to electrode processes [12], 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 [3], fusibility at the double layer in the vicinity of the electrode rather low temperature [4], crystallinity [5], conductivity [6], than the gel bulk, partly because the electrochemically and photosensitivity [7]. 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 [14], 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 [24], 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 2. Experimental
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 [25] 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 [26]. 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 [27], 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 [28]. Fig. 5 also shows the potential dependence of the assigned to an n “y* transition [26], 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 [29].
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 (p“p* 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 [30].
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 – [33].
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 Acknowledgements
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 References
shown in Fig. 9. Consequently, the electrochemicallysynthesized film has been partially crosslinked. Cyclic [1] A.F. Diaz, J.F. Rubinson, H.B. Mark, Advances in polymer voltammograms (b,c,d in Fig. 2) of the electrochemi- science 84, Electronic Applications, Springer-Verlag, Berlin,1988, pp. 114 – 131.
cally synthesized film lost the electrochemical activity [2] G.P. Evans, Advances in Electrochemical Science and Engineer- gradually with a longer oxidation time. This is consis- ing, vol. 1, in: H. Gerischer, C.W. Tobias (eds), VCH, New tent with the loss of conductance in Fig. 3.
Crosslinking is usually a result of radical formation [3] K. Yoshino, S. Nakajima, R. Sugimoto, Jpn. J. Appl. Phys. 26 by benzoyl peroxide (BPO). We compared FTIR spec- [4] M. Sato, S. Tanaka, K. Kaeriyama, J. Chem. Soc. Commun.
tra of the electrochemically oxidized gel with those of the crosslinked polymer by BPO [3 – 8]. When the BPO- [5] K. Yoshino, Y. Manda, H. Takahashi, et al., J. Appl. Phys. 68 crosslinked film was transferred into ethanol, it shrank into half the volume. Therefore it is a kind of gel. It [6] U. Barsch, F. Beck, Electrochim. Acta 41 (1996) 1761.
[7] L.M. Abrantes, J.P. Correia, Electrochim. Acta 41 (1996) 1747.
showed the same absorbance at 828 cm−1 as the film [8] K. Yoshino, K. Nakao, R. Sugimoto, Jpn. Al. Appl. Phys. 27 without the electrochemical oxidation. Thus the aro- C – H is retained in the BPO-crosslinked film [9] S. Morita, S. Shakuda, R. Sugimoto, K. Yoshino, Synth. Met.
whereas it disappeared owing to the gelation in the electrochemical oxidation. The absorbances at 2918 and [10] S. Morita, S. Shakuda, T. Sugimoto, K. Yoshino, Jpn. J. Appl.
2856 cm−1 were 15% less than those for the film [11] T. Tanaka, Sci. Am. 244 (1981) 124.
without the electrochemical oxidation. Consequently, [12] M. Irie, T. Tanaka, Polym. Prepr. Jpn. 40 (1991) 461.
the crosslinking occurs in the hexyl chain. This is quite [13] T. Tatsuma, K. Takada, H. Matsui, N. Oyama, Macromolecules different from the gelation by the electrochemical [14] K. Takada, T. Haseba, T. Tatsuma, N. Oyama, Anal. Chem. 67 [15] K. Aoki, T. Aramoto, Y. Hoshino, J. Electroanal. Chem. 340 4. Conclusion
[16] K. Aoki, M Kawase, J. Electroanal. Chem. 377 (1994) 125.
[17] J.C. LaCroix, A.F. Diaz, Makromol. Chem, Macromol. Symp. 8 The electrochemical oxidation of poly(3-hexylthio- [18] Y. Tezuka, S. Ohyama, T. Ishii, K. Aoki, Bull. Chem. Soc. Jpn.
phene) films caused crosslinking to yield a permanent gel, which was swollen in chloroform. Further, it [19] K. Aoki, Y. Tezuka, J. Electroanal. Chem. 267 (1989) 55.
caused a red shift of the n “y* transition, a decrease in [20] K. Aoki, J. Cao, Y. Hoshino, Electrochim. Acta 38 (1993) 1711.
the electric conductance, a decrease in solubility in [21] Y. Tezuka, T. Kimura, T. Ishii, K. Aoki, J. Electroanal. Chem.
chloroform in the short term reduced state, and a [22] K. Aoki, J. Cao, Y. Hoshino, Electrochim. Acta 39 (1994) 2291.
decrease in the amount of C – H group. The variation [23] K. Aoki, T. Edo, J. Cao, Electrochim. Acta 43 (1997) 285.
of the solubility was related to the oxidation through [24] K. Aoki, J. Li, J. Electroanal. Chem. 441 (1998) 161.
the reaction nM + ne− “M . It was also correlated [25] R. Sugimoto, S. Takeda, H.B. Gu, K. Yoshino, Chem. Express with the volume change through Flory’s theory. The [26] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Iden- long term oxidation decomposed a 4-position of the tification of Organic Compounds, 5th ed, Wiley, New York, thiophene ring to cause the red-shift of the lone pair at the sulfur of the thiophene ring. Thus it blocks the [27] D.L. Ellis, M.R. Zakin, L.S. Bernstein, M.F. Rubuer, Polym.
resonance on the original polythiophene chains, and hence both conductance and electrochemical activity of [28] J. Yue, Z.H. Wang, K.R. Cromack, A.J. Epsein, A.G. MacDi- armid, J. Am. Chem. Soc. 113 (1991) 2665.
voltammetry decreased with the oxidation. Thus, most [29] J. Roncali, Chem. Rev. 92 (1992) 711.
observations can be explained consistently.
[30] P.J. Flory, Principles of Polymer Chemistry, Cornell, New York, The electrochemical crosslinking is different from the crosslinking by BPO in that the conjugation by poly- [31] K. Kalaji, L. Nyholm, L.M. Peter, J. Electroanal. Chem. 325 thiophene chains is destroyed. Therefore it is unsuitable [32] D.E. Stilwell, S.M. Park, J. Electrochem. Soc. 136 (1989) 427.
to produce an electronic conducting gel. However, it [33] E.W. Tasai, S. Basak, J.P. Ruiz, J.R. Reynolds, K. Rajeshaw, J.
generates a gel with a large volume change.

Source: http://asura.apphy.u-fukui.ac.jp/~aoki/publication/PDF/JEC453_107.pdf