Article pubs.acs.org/IECR
Electrically Conductive Films Made of Pyrrole-Formyl Pyrrole by Straightforward Chemical Copolymerization Yusuke Hoshina and Takaomi Kobayashi* Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata, 940-2188, Japan ABSTRACT: Electrically conductive films were prepared by copolymerization of pyrrole (Py) and 2-formyl pyrrole (FPy) in the presence of trifluoroacetic acid (TFA) used as a catalyst in chloroform. Formation of the copolymer P(Py-co-FPy) films was examined, when the FPy monomer mole fractions was changed. The obtained films showed a metallic greenish black color and had electrical conductivity of 10−4 to 10−1 S/cm when I2 was doped in the copolymer films. As the mole fraction of the FPy increased from 0 to 0.5, the tensile strength of the films increased from 11.1 to 81.3 N/mm2. For analysis of the copolymerization, 1H NMR in deuterated chloroform was measured and the evidence was revealed that the formyl group of FPy was attacked by the acid catalyst before the copolymerization started. furan,30 and N-substituted Py31,32 were prepared by chemical or electrochemical polymerization. Although some reports described advantages of the obtained conductive polymers, their synthetic methods also included numerous steps and reactions facilitated by several instruments. In contrast to these studies, the presently proposed method described a novel approach of PPy film using alternating copolymerization with 2-formyl pyrrole (FPy). This copolymer, P(Py-co-FPy) would be expected to extend the conjugated structure in the polymer backbone through the production of a methine group from FPy. As well as a reaction involving Py and a chemical containing formyl group like benzaldehyde, porphyrin and conjugated aromatic cyclic molecules were normally obtained.33 However, the present work showed copolymerization reaction of Py and FPy when an adequate catalyst was present and the resultant film showing electrical conductivity. There was evidence that a potential approach to produce a new conductive copolymers was described.
1. INTRODUCTION Polypyrrole (PPy) is well-known in several industrial fields as an electrically conductive polymer because of its interesting electrical and optical properties as well as good thermal and environmental stability.1−5 These characteristics have led to its use in numerous applications such as metallization of dielectrics, batteries, anticorrosives, electromagnetic shielding, sensors, and actuators.6−12 However, several shortcomings limit its use in applications. These drawbacks, which are related to their poor physical and mechanical properties as well as insolubility in common solvents used for film preparation,13 have been investigated extensively to improve the processability of conductive polymers. For example, N-substituted or 3-substituted pyrrole polymers have shown better solubility in common solvents.14−17 Although electrochemical polymerization can produce a film with good conductivity,18−20 complex equipment is enquired. In contrast, chemical oxidative polymerization uses a simple apparatus and easy steps to produce conductive polymers such as PPy,21−23 obviating the preparation and use of many instruments. Nevertheless, the obtained PPys in chemical polymerization are generally limited to be powders, and their conductive polymers are difficult for several applications, especially for film formation. Thus, blending scaffold polymers with conductive polymer powders is a useful strategy for their modification to form a film. By using this technique, dispersed conductive polymer powders have been applied for preparation of transparent films.24,25 For preparation of conductive polymers, Salmon et al. reported the preparation of PPy films using sulfuric acid in ethanol.21 However, the resultant films were fragile and unstable because they were strongly oxidized. Consequently, very few investigations have examined single component PPy films prepared by chemical methods. On the other hand, recently, copolymerization including chemical linkage of conductive polymers was applied as an effective method to improve physical and electrochemical properties. Up until now, a few copolymers using chemical polymerization were reported. Py and other conductive units of aniline,26,27 thiophene,28,29 © 2012 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. The pyrrole (Py, Scheme 1a) monomer used was a product of Tokyo Chemical Industry Co. Ltd. Scheme 1. Structures of Py (a) and FPy (b)
(Tokyo, Japan) and distilled under reduced pressure after dehydration using calcium hydride for 24 h. Pyrrole-2carboxaldehyde (2-formyl pyrrole, FPy, Scheme 1b) was Received: Revised: Accepted: Published: 5961
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purchased from Tokyo Chemical Industry Co. Ltd. The acid catalyst and reagents including trifluoroacetic acid (TFA), acetic acid (AA), formic acid (FA), difluoroacetic acid (DFA), hydrochloric acid (HCl), and sulfuric acid (H2SO4) were obtained from Nacalai Tesque Inc. (Tokyo, Japan), and trichloroacetic acid (TCA) and dodecyl benzenesulfonic acid (DBSA) were from Tokyo Chemical Industry Co. Ltd. The chemical dopant, iodine (I2), was a product of Nacalai Tesque Inc. Water was distilled before use. 2.2. Preparation of P(Py-co-FPy). Table 1 presents catalysts used for preparation of the Py-FPy copolymer. For example, Table 1. Polymerization Yield, Tensile Strength, and Electrical Conductivity of P(Py-co-FPy) Synthesized Using Various Catalystsa catalystb AA FA DFA TFA TCA DBSA HCl H2SO4
acid type carboxylic carboxylic carboxylic carboxylic carboxylic sulfonic inorganic inorganic
yieldc (%) 30.3 54.4 78.9 92.2 90.6 114.5 76.0 80.1
tensile strength (N/mm2) 2.3 1.2 64.8 81.3 69.6 2.5 e e
conductivityd (S/cm) 1.6 2.3 2.6 5.5 4.4 4.1 e e
× × × × × ×
Figure 1. Image of P(Py-co-FPy) film prepared with 0.5 of FPy mole fraction.
−3
10 10−3 10−3 10−3 10−3 10−3
Beside CHCl3, several other solvents were used for the copolymerization, as presented in Table 2. In these procedures, Py (200 mg, 3 mmol) and FPy (286 mg, 3 mmol) were Table 2. Polymerization Yield, Tensile Strength and Electrically Conductivity of P(Py-co-FPy) Synthesized with Various Solventa
a
Within CHCl3 or distilled water. Mole ratio of Py/FPy was equal. Catalyst amount was 13 mmol. cResultant polymer films were ground to measure the yield. All of the polymer powders were washed by NaOH and HCl to remove each catalyst. Yield (%) = {amount of P(Py-co-FPy) (g)/amount of Py and FPy (g)} × 100. dConductivity of film was measured after I2 was doped at room temperature. eIn the case of films that were prepared with inorganic acid, the tensile strength and conductivity of the films could not be measured because they were very fragile. b
solvent chloroform diethyl ether benzene methanol ethanol THF acetonitrile
Py (200 mg, 3 mmol) and FPy (286 mg, 3 mmol) were dissolved and stirred in 2 mL of chloroform (CHCl3) in a sample tube (20 mL). Then, the solution containing acid catalyst (13 mmol) and CHCl3 (2 mL) was added to the monomer solution at room temperature. As a result, the solvent color changed immediately from transparent brown to yellowish red. Then, the mixed solution was spin-coated onto the Petri dish at 20 rpm using a spin coater (Opticoat MSA100; Mikasa Co. Ltd., Japan). The polymerization was carried out for about 30 min at room temperature, and then a film was formed in a Petri dish. Figure 1 presents typical images of the resultant thin film of the Py-FPy copolymer using TFA catalyst. The film was a metallic greenish black and insoluble in several solvents. After the polymerization reaction of Py and FPy, the resultant films were ground down and washed well with water and acetone. The resultant copolymer powders were stirred with 3 wt % NaOH aqueous for 30 min to remove acid catalyst and then with 2 N HCl aqueous solution at 30 min to convert them back to acidity. The obtained greenish black powders were dried in a vacuum oven at 60 °C for 12 h. These samples were used for elemental analysis (CHN corder MT-6; Yanaco Analytical Instruments Corp., Japan) and as an application for Fourier transform infrared (FT-IR) spectrometry (Prestige-21; Shimadzu Corp., Japan). For the measurement of electrical conductivity, I2 was doped on the film. Each film was washed with excess water and put in a closed vessel containing a small amount of I2 for chemical doping. Then, the electrical conductivity of the doped film was determined.
solvent type yieldb (%) nonpolar nonpolar nonpolar polar-protic polar-protic polar-aprotic polar-aprotic
92.2 87.5 90.5 82.0 73.1 88.4 91.2
tensile strength (N/mm2) 81.3 27.6 67.0 1.5 3.6 41.5 81.1
conductivityc (S/cm) 5.5 4.5 3.2 4.7 4.2 3.5 4.8
× × × × × × ×
10−3 10−3 10−3 10−3 10−3 10−3 10−3
a
With 3 mmol of TFA. Mole ratio of Py/FPy was equal. bResultant polymer films were ground to measure the yield. All of polymer powders were washed by NaOH and HCl to remove each catalyst. Yield (%) = {amount of P(Py-co-FPy) (g)/amount of Py and FPy (g)} × 100. cConductivity of film was measured after I2 was doped at room temperature.
dissolved in 2 mL of each solvent in a sample tube. Then, the solution of TFA in the selective solvent (2 mL) was added at room temperature. 2.3. Measurements. Characteristics of the resultant Py-FPy copolymers including electrical conductivity, tensile strength, elemental analysis, FT-IR, and UV−visible spectrophotometry were measured. The value of electrical conductivity of each film was obtained using a typical four-point method (Roresta-GP MCP-T610; Mitsubishi Chemical Analytec Co. Ltd., Japan) at room temperature. In order to measure the averaged electrical conductivity of the doped film, the four-point probe was attached to the surface of the film at five different points. The tensile strength of the films was obtained using a load cell (LTS-500N; Minebea Co. Ltd., Japan) with a specimen (2.5 cm × 7.5 cm) of the copolymer film. An elemental analyzer was used to determine the mass percentages of hydrogen (H), carbon (C), and nitrogen (N) of the films. FT-IR spectra of the resultant films were also determined using a FT-IR spectrometer in the transmittance mode. Transmission spectra were obtained using the KBr method. The spectral measurement resolution was 5962
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4 cm−1 for each spectrum. The UV−visible spectra of thin films having 1−10 μm thickness on the quartz glass were measured using a UV−vis-NIR spectrophotometer (V-570; Jasco Corp., Japan) in absorption mode. To investigate the copolymerization process, 1H NMR spectra of monomers in the presence of TFA were measured by a spectrometer (AL-400 NMR JEOL Ltd., Japan), using deuterated chloroform (CDCl3) as the reference solvent.
of the film was 5.5 × 10−3 S/cm. Such comparison suggested that TFA was a good catalyst for preparing copolymer films. Table 2 presents data of the copolymer films prepared in different solvents. Here, TFA was used as a catalyst in methanol and ethanol, and the obtained films showed a low yield and very low tensile strength, meaning fragile nature of the films. However, in CHCl3 solvent, the produced film was flexible without a fragile nature. Therefore, TFA and CHCl3 were selected as the catalyst and solvent, respectively, for further investigation of conductive films. When the polymerization was performed using TFA in CHCl3, the mole fraction of FPy was varied from 0.1 to 0.9. Table 3 presents data for each film produced with the TFA-CHCl3
3. RESULTS AND DISCUSSION 3.1. Copolymerization of Py and FPy. As shown in Tables 1 and 2, several acid catalysts and solvents for the copolymerization were used to prepare the copolymer films of Py and FPy with an equal mole ratio. Different catalysts including AA, FA, DFA, TFA, TCA, DBSA, HCl, and H2SO4 were used for copolymerization. The obtained yields of copolymers were low for the cases of AA and FA. However, when TFA and TCA were used, the polymer yields became higher than 90%. This tendency meant that stronger acid in a series of carboxylic acids had an advantage for producing the copolymer films. In the case of DBSA with a SO3− group, the yield became 114.5% even though the films had been washed well. Figure 2 shows FT-IR spectra of copolymer films prepared
Table 3. Elemental Composition of P(Py-co-FPy) Synthesized from Each Mole Fraction of FPy against Py elemental composition (wt %) mole fraction of FPya
H
C
N
C/N
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 calculated (0.5)b
6.25 5.65 5.38 5.27 5.10 4.92 4.33 4.13 4.08 4.01 3.57
57.57 57.59 57.99 58.35 58.75 59.03 59.45 60.36 60.50 61.01 76.58
13.87 13.84 13.83 13.81 13.78 13.53 13.30 13.28 13.13 13.01 19.85
4.15 4.16 4.19 4.23 4.26 4.36 4.47 4.55 4.61 4.69 3.86
a
Total monomer content in each feed solution was 6 mmol in 2 mL of CHCl3 in the presence of 13 mmol of TFA. bThe values were for the chemical structure of the copolymer in Scheme 1.
system. Here, both monomers were dissolved in 2 mL of CHCl3 and then stirred in a 20 mL sample tube. Then, the amounts of TFA were added with 13 mmol in CHCl3 (2 mL) at room temperature. Without FPy, the color of the Py solution changed from transparent color to a brownish black film, when TFA was added. However, a fragile film was obtained without a metallic greenish black color. When the mole fraction of FPy increased from 0.1 to 0.9, the color of the solution changed from transparent brown to yellowish red and then the formation of the shiny greenish black film was observed. As shown in Figure 3, the values of copolymerization yield increased concomitantly with the increase of FPy feeds until 0.5 of the mole fraction. However, when the mole fraction was increased further from 0.5 to 0.9, the value of the yield decreased gradually. In addition, Table 3 shows elemental analysis for composition of H, C, and N observed in films. The value of the C/N component ratio in the copolymer film increased concomitantly with the increase of the FPy component. This tendency meant that the film contained copolymerization segments of Py and FPy (Scheme 2). Table 3 also contains values of H, C, and N contents in the copolymer shown in the chemical structure of the Scheme 1. The C/N of the calculated value showed higher contents relative to those of the films. This indicated that a small amount of TFA catalyst was present in the copolymer film. In the estimation of the number of the TFA catalysts from the calculated values, it was seemed that the catalyst was present in a half molecule in the chemical structure in the scheme. Also, the increment of carbon contents in the film might result from the increase of the methine group formed by Py and the formyl group of FPy. It was known that the
Figure 2. FT-IR spectra of P(Py-co-FPy) synthesized using different acid catalysts.
using respective catalysts. Two strong peaks of C−H stretching and SO stretching appeared at 2920, 2848, and 1175 cm−1, respectively, when DBSA was used. This indicated that DBSA was still present in the copolymer film, even though the film was well washed. For the cases of HCl and H2SO4, the washing was performed until the water pH was neutral. In the spectrum of the product using HCl catalyst, the peak of CO stretching at 1700 cm−1 indicated that these films were strongly oxidized. In addition, the use of strong inorganic acids such as HCl and H2SO4 weakened the tensile strength of each film. These introduced the fragile nature in the films. Therefore, the tensile strength was unable to be measured. In contrast, the films prepared with TFA had a high tensile strength of more than 81.3 N/mm2. This resulted from the dense structure of the copolymer film prepared in the presence of TFA. The films obtained using AA, FA, and DBSA were also fairly fragile and showed low tensile strength. Table 1 also shows the electrical conductivity of the films prepared using respective catalysts. Electrical conductivity changed in the range of 1.6 × 10−3 to 5.5 × 10−3S/cm when different acidic catalysts were used. However, when using TFA catalyst, the value of the electrical conductivity 5963
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Figure 3. Polymerization yield and tensile strength of P(Py-co-FPy) synthesized from various mole fractions of FPy against Py.
Scheme 2. Expected Structure of P(Py-co-FPy) As a Copolymer
dimerization of Py and FPy was a result according to Scheme 3.34 Here, the formyl group of the FPy first attacked the protonated Py ring because the formyl group was an electron withdrawing group in the presence of Py. Therefore, when only FPy was present in these solutions, no polymerization occurred. To investigate the copolymerization reaction, copolymerization was analyzed with 1H NMR using CDCl3 for Py and FPy in the presence of TFA. The polymerization solvent contained quintuple amounts relative to 13 mmol of TFA in 2 mL of CHCl3. Figure 4 shows 1H NMR spectra against the reaction time at 25 °C, when the mole fraction of Py and FPy was 0.5/0.5. Here, the time 0 meant immediate when the catalyst was added after 1 min passed. The spectra showed that the peak areas of Py and FPy including the formyl group decreased. This was a result from consumption of monomers by the formation of soluble oligomers, which showed NMR signals in the spectra. However, the peak areas for the chemical shift in the range of 8.40−6.70 ppm were observed. This meant that the proton peak of the formyl group (h) was shifted to a high magnetic field with decreasing peak area. This strongly indicated that the formyl group reacted with Py and was then consumed. Peaks of the oligomer appeared at 8.40, 7.95, 7.81, 7.63, 7.56, 7.39, and 6.70 ppm in the spectra measured at 1 min and in others. Generally, a methine proton and pyrrole ring protons of dimer appeared, respectively, at 8 to 6 ppm.35 Here, the peaks of 8.40 and 7.56 ppm might be assigned to the terminal α position of the pyrrole proton. As shown in the
Figure 4. 1H NMR spectra of the reaction solution used 0.5 mole fraction of FPy with TFA.
upper spectrum, the peaks that appeared at 12.85, 11.90, and 11.02 ppm were attributed to the NH group of oligomers of Py and FPy.35,36 In addition, the presence of the two peaks of 2.63 and 2.49 ppm strongly indicated the Py saturated ethylene proton, as shown in Scheme 3. However, the saturated proton almost disappeared at 120 min in the chemical shift region. This meant that the reaction of (1)−(2) occurred in the presence of acid (Scheme 3). It was noteworthy that the spectrum of TFA shifted to a high magnetic field with time, which meant that the TFA was hydrated with generated water in the polymerization. As the reaction time increased, the peaks area of monomers became smaller relative to those of soluble oligomer. These results suggested that consumption of Py and FPy occurred and that finally an insoluble copolymer was formed through soluble oligomers.
Scheme 3. Dimerization Reaction Mechanism between Py and FPy
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3.2. Properties of Py-FPy Copolymer Films. Figure 3 shows that the value of tensile strength of the copolymer films increased from 11.1 to 81.3 N/mm2 with the increase of FPy from 0.1 to 0.5, respectively, until the mole fraction was 0.5. This result indicated that copolymer films prepared at 0.5 mol feed had good mechanical properties. However, with increasing FPy contents from the 0.5 mol fraction, the values decreased. Figure 5 shows FT-IR spectra of the copolymers prepared at
Figure 6. Electrical conductivity of I2 doped P(Py-co-FPy) films prepared by various mole fractions of FPy against Py.
Figure 5. FT-IR spectra of P(Py-co-FPy) synthesized by various mole fractions of FPy against Py.
each mole fraction of FPy. As indicated by (a), N−H stretching vibrations of two types (3380 and 3260 cm−1) were present in the film. The peaks (b) of saturated C−H stretching at 2935 and 2866 cm−1 became very weak when the mole fraction was higher than 0.5. This indicated that the conjugated structure was increased in the polymer backbone, when the mole fraction of FPy increased. In peaks (c) and (d) for CC double bond stretching of the pyrrole ring at 1626 and 1538 cm−1 and CN stretching at 1490 cm−1, these were attributed to the formation of conjugated structure in the copolymer. It was noted that the broad peak (e) observed at 1258 cm−1 was assigned to −CCH− stretching from the methine group. Additionally, the peak (f) of C−H out-of-plane deformation vibration from the methine group and peak (g) of the aromatic C−H out-of-plane deformation vibration appeared at 1008 and 836 cm−1, respectively. Figure 6 shows electrical conductivity of the copolymer film doped with I2. It was particularly interesting that after I2 was doped, the values increased clearly with an increased FPy amount from 10−4 to 10−1 S/cm. Furthermore, to analyze the films, UV−vis spectra of thin films prepared by copolymerization of each Py/FPy component were measured as shown in Figure 7a. The obtained UV−vis spectra presented the characteristic absorption band of the π−π* transition of PPy that appeared in 476, 478, 481, 491, 495, and 498 nm when the FPy mole fraction was 0, 0.1, 0.3, 0.5, 0.7, and 0.9, respectively. This fact indicated that the FPy group was incorporated into the chemical structure of the conjugated polymer chains. At the mole fractions of 0.7 and 0.9, weaker and broader absorption was observed between 700 and 1050 nm, showing that the bipolaron state of PPy was present in the films. Figure 7b implied that the absorption at 300−600 nm was associated with I2 on the film. The UV−vis spectra of the copolymer films showed broadening absorption of the bipolaron state between 900 and 1500 nm for the 0.5 and 0.9 mol fraction samples of FPy. This meant that introduction of the FPy amount in the film increased
Figure 7. −CUV−visible spectra of (a) P(Py-co-FPy) films and (b) I2 doped P(Py-co-FPy) films obtained at various mole fractions of FPy against Py.
the conjugated methine structure in the copolymers. Thereby, this result well corresponded with raising the electrical conductivity.
4. CONCLUSIONS The novel conductive P(Py-co-FPy) films containing copolymerized segments of Py and FPy in the presence of several acid catalysts were prepared. Results showed that the TFA catalyst was useful for good film formation in CHCl3. The copolymer films produced at 0.5 mol fraction showed good tensile strength of about 80 N/mm2. When the Py/FPy feeds changed, the electrical conductivity of the I2 doped copolymer films 5965
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increased clearly from 10−4 to 10−1 S/cm with the increase of the FPy feed. Results of elemental analysis, 1H NMR, FT-IR, and UV−visible spectra proved that methine bonds between Py and FPy were formed in the copolymer. The obtained films showed interesting properties and would be anticipated for use in several applications as a new conductive polymeric material.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +81-258-47-9326. Fax: +81-258-47-9300. E-mail: takaomi@ nagaokaut.ac.jp. Notes
The authors declare no competing financial interest.
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