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Preparation and characterization of PPy doped with different anionic surfactants.

INTRODUCTION

Polypyrrole (PPy) is one of the most studied conducting polymers because of its good electrical conductivity, environmental stability, and relative ease of synthesis. PPy can be prepared by electrochemical (1-3) or chemical (4), (5) oxidation in various organic solvents and aqueous media. In a chemical oxidative polymerization of pyrrole monomer many oxidants have been used, such as ferric chloride (4), (6), ferric perchlorate (7), ammonium peroxy-disulfate (8), (9), and others. The structural formula of PPy is given as Scheme 1 in which A-indicates the anionic ion coming from oxidants or additives. The preparation conditions and various additives introduced into reaction mixture decide the properties of obtained conducting polymer (10), (11).

The formation process of PPy by chemical oxidation has been studied during the polymerization of pyrrole in the presence of anionic surfactants, such as sodium dodecyl sulfate (SDS) and sodiumdodecylbenzenesulfonate (SDBS) (12-15). The addition of surfactants accelerated the polymerization and the conductivity about 40 S cm- I was reached with PPy prepared in the presence of sodium n-alkyl sulfate 1161. Later Omastova et al. (17) synthesized PPy from solution containing anionic surfactant and phenol derivates containing electron-withdrawing group. The surfactants can affect the preparation of PPy in three fundamental ways: (1) anionic surfactants may form an ionic bond with the PPy polycation; (2) the hydrophobic part of the surfactant molecules may adsorb on the produced conducting polymer; and (3) the surfactant micelles, if present, may affect the distribution of the reactants between the micellar and aqueous phase, the pyrrole monomer dissolved preferentially into the micelles assembly because of its hydrophobic nature, thus altering the locus and the course of pyrrole polymerization (14), (15). The first and the second possibilities are addressed in obvious paper which decided the conductivity of the PPy to a certain extent and the third may be relative to the morphology of the prepared conducting polymer. In this article, PPy was synthesized in various anionic surfactant solutions with ferric chloride as oxidant. An interesting regulation of the molar ratio of the oxidant and the monomer was firstly discovered which was closely related to the conductivity of the PPy when the molar ratio of the anionic surfactant and the monomer was 0.3. The possible reasons were proposed to explain the new phenomenon which may be helpful to improve the conducting characteristic of the PPy.

EXPERIMENTAL

Reagents

Pyrrole (Py) was purified by distillation under reduced pressure and stored in a refrigerator at about 2[degrees]C before use. The oxidants, ferric chloride (Fe[Cl.sub.3]*5[H.sub.2]O; Fluka), and the surfactants: /3-naphthalene sulfonic acid ([beta]-NSA; Aldrich), SDBS (Fluka), SDS (Aldrich), were used as received. Water was distilled before use.

Chemical Preparation of Polypyrrole

An oxidant, 15.87 mmol (4.29 g) anhydrous FeC13*5H20 was dissolved in 20 ml distilled water and stirred for I hr, then dropwise added into 14.4 mmol (1.0 ml) freshly distilled pyrrole. The polymerization was carried out for 12 h at room temperature with moderate stirring. The precipitated PPy was filtered off and washed three times with 1000 ml distilled water. The black PPy powder was dried in a vacuum drier at 60[degrees]C for 8 hr.

Chemical Preparation of Polypyrrole With Stufactants

The 4.32 mmol surfactant was dissolved in 70 ml distilled water with magnetic stirring bar and 14.4 mmol (1.0 ml) freshly distilled Py was then inserted dropwise into the surfactant solution with magnetic stirring for 1 hr. The 15.87 mmol of [FeCl.sub.3]*5[H.sub.2]O dissolved in 20 ml of distilled water was mixed dropwise in reaction vessel. The reaction time and the treatment of the product were the same as above.

Measurements

The morphology of the product was directly observed with scanning electron microscopy (SEM) (FEIco-Holland, JSM-6700F). An X-ray diffraction (XRD) pattern was taken with a Shimadzu XRD 6000 instrument at a 10[degrees]/min scanning speed from 10[degrees] to 80[degrees]. Fourier transform infrared spectroscopy (FTIR) spectra of the samples were obtained with a Shimadzu FTIR-8400s spectrophotometer in the 4000-500 [cm.sup.-1] range, the sample was impressed into KBr pellets. Energy dispersive spectroscopy (EDS) results were tested by Vario ELIII elemental analysis system GmbH. Germany. Two-probe technique X-ray photoelectron spectroscopy (XPS) measurements were made on an ESCALAB 250 spectrometer with an MgK[alpha] X-ray source (1253.6 eV photons). Four-point probes resistivity measurement system was received from Guangzhou and the electrical conductivity of composites was measured using four-point probes technique at room temperature.

RESULTS AND DISCUSSION

Figure 1 shows comparison of the morphology of PPy prepared in the presence of anionic surfactant. PPy-[beta]-NSA is lump and striped while PPy-SDS and PPy-SDBS are globular particles and PPy-SDBS particles look more compact. SEM study showed that the presence of the anionic surfactant in polymerization mixture strongly influenced the morphology of chemically prepared PPy.

An interesting regulation could be observed from Fig. 2 that the conductivity of the prepared PPy increased at first and then decreased with the increase of the n (n was the mole ratio of [FeCl.sub.3]/Py). Although the anionic surfactant was different, the value of conductivity reached its maximum at the same n when the number of n is 1.1. The mole ratio of [FeCl.sub.3]/Py may affect the molecular weight and the length of the conjugated chain of the prepared PPy which decided the conducting characteristic together to some extent (16). So the most appropriate molar ratio was 1.1 when [FeCl.sub.3] was used as oxidant during the in situ polymerization of PPy in order to improve the conductivity of the polymer. At the same time, the maximum of conductivity changed when the anionic surfactant was different, which implied that anionic ion of the surfactant entered the PPy chains as counter anions and influenced the conductivity of the polymer. The conductivity of the obtained PPy was related with the structure of the anionic surfactant, so surfactant acted as both soft template and dopant during the synthetic process.

The results of elemental analysis of PPy prepared in the presence of the anionic surfactants are comparable as listed in Table 1. In the samples synthesized in the presence of [beta]-NSA. SDS, and SDBS, there is a fraction of C higher compared to results of elemental analysis of PPy-Cl, but the content of N is lower. This is caused by the presence of aliphatic or aromatic chains of surfactants in PPy. As demonstrated in Scheme 1, the abbreviated structure of PPy is [([C.sub.4][H.sub.3]N).sup.x+]x[A.sup.-]1 in which [A.sup.-] is the doped anionic ion. The value of .v is 0.33 and the doped anionic ion is [Cl.sup.-] when no surfactant entered into the solution according to the literature (16), (17). The doped anionic ions are [Cl.sup.-] and corresponding surfactant anion when PPy is prepared in surfactant solution. The dopant of surfactant anion plays important role which improves the conductivity of PPy greatly as shown in Table 1. The ratio of S/M indicates the number of the surfactant anionic ions which enter into the PPy chain as counter anions. The conductivity increased gradually with more surfactant ions entering into the PPy chains which was reflected by the number of S/M (17), (18).

TABLE 1. Element analysis of obtained PPy doped with
different anionic surfactants (mass percent).

PPy-
turfuctunt       [Type.sup.a]              C             H

PPy-[Cl.sup.b]   -                     56.54         3.914

                               [62.48.sup.c]  [3.93.sup.c]

PPy-/[beta]-NSA  A                     60.81         3.829

PPy-SDS          A                     61.54         7.145

PPy-SDBS         A                     65.53         7.424

PPy-
turfuctunt                   N      S      O     S/M    [sigma] (S
                                                      [cm.sup.-1])

PPy-[Cl.sup.b]           16.40      -      -                2.55 x
                                                       [10.sup.-4]

                 [18.22.sup.c]

PPy-/[beta]-NSA          11.96  6.556  14.41  0.0315         33.33

PPy-SDS                  9.421  6.432  13.30  0.0233         25.00

PPy-SDBS                 7.257  6.765  10.09  0.0194          4.74

Calculated value. SIM, the ratio of S to the molecular weight
of the corresponding surfactant.


Because the conductivity depended strongly upon the species of surfactants that were used, the MR spectra of the PPy synthesized with surfactant were measured to detect the presence of the surfactant. Figure 3 shows the FTIR spectra of PPy prepared in the presence of different anionic surfactants. The characteristic PPy peaks are located at 1550 and 1445 [cm.sup.-1] due to the pyrrole ring stretching and the conjugated C--N stretching mode, respectively. The peaks at 1294 and 1031 [cm.sup.-1] are related to the in-plane vibrations of C--H, 1174 [cm.sup.-1] is assigned to the C--N stretching mode, and 900 cm- I due to the N--H in-plane deformation vibrations (l1-13). The peak at 1094 cm- I may occur in the PPy--CI samples, accompanying with higher conductivity arose by the mode of in-plane defon-nation vibration of NH on protonated nitrogen in the polymer chains (17). The peak at 962 [cm.sup.-1] reveals the C--C out of plane ring deformation vibration and the peak at 672 [cm.sup.-1] corresponds to the stretching vibration of the [SO.sub.3.sup.-] group which indicates the surfactant anionic ion entering the PPy chains as dopant (19), (20).

PPy is a highly rigid polymer because of its linear structure and less flexible chain folding to induce crystalline domain. However, in the presence of organic surfactants, the dopant-PPy undergoes various interactions, which tends to organize the polymer chains in three-dimensional highly ordered fashions (21). The XRD spectra of the doped PPy samples are shown in Fig. 4. The peak at around 2[theta] = 8-10[degrees] represents the enhancement of the structural order of counterions in the polymer chains, indicating the possibility of higher conductivity (22). Another three weak diffraction peaks at 2[theta] = 36[degrees], 43[degrees], 54[degrees] in curves a and c are more obvious than curve b. The main peaks at approximately 20 = 20-27[degrees] are believed to occur because of the interplanar van der Waals arrangement of the pyrrole--pyrrole rings in PPy chains and doped amorphous pyrrole-counterion or inter-counterion interactions, respectively. In curve b, diffraction peak at 2[theta] = 22.0[degrees] is sharp and strong and this peak is shifted to 25.7[degrees] in curve c and the degree was about 27[degrees] in curve a. The shift may be due to the difference in the doping level and related to the Py--Py and Py-counterions. While changing sulfonic acid from SDS, SDBS to [beta]-NSA, the peak at 2[theta] = 22[degrees]shifted toward higher angle (from 2[theta] = 21.6[degrees] to 27.0[degrees]) which implied the decrease of d-spacing due to the orientation of benzene ring in SDBS and [beta]-NSA compared with SDS (23), (24). This decrease of d-spacing provides another convincing evidence that the surfactant anions enter into the conducting polymer; meanwhile, the doping level and the degree of the structural order of counterions in the polymer chains influence the conductivity of the obtained products together as listed in Table I.

XPS studies have been carried out that the oxidized PPy complexes exhibit a major N Is core-level spectral component at a binding energy (BE) of about 399.7 eV, characteristic of the pyrrolylium nitrogens (--NH--structure), and a high BE tail attributable to the positively charged nitrogens (--N+H--structure) whose BE is shifted by about +1.2 eV suggesting the presence of surfactant anion in a more positive environment (24), (25). Figure 5 shows the different NI s binding energy of different samples, implying the oxidized state of the charged nitrogens in the PPy. The N Is core-level spectrum, with a lower BE about 399.75 eV is observed for the PPy-[beta]-NSA complex (Fig. 5a) and a higher BE about 400.78 Ev for positively charged nitrogen after oxidation. The corresponding BE are 399.78 eV and 400.95 eV, 399.98 eV and 401.16 eV for PPy-SDS and PPy-SDBS, respectively (Fig. 5b and c). As a result of increase in the number of positively charged nitrogens in the polymer chains which indicated by the area ratio of higher BE to the lower BE as listed in Table 2 would be conducive to the formation of polarons and bi-polarons (26). The higher area ratio may hint the more surfactant anions entering the polymer chains and the better conducting characteristic. Therefore, PPy-[beta]-NSA complex with the area ratio about 1.12 exhibits the highest conductivity compared with the other two samples. So, the dopant level and the binding state of the charged nitrogens with the surfactant anions decide the conductivity of the obtained polymer together.

TABLE 2. XPS data of N Is of different samples.

PPy-surfactant   Area     Center (eV)

PPy-[beta]-NSA   8615.9        399.75
                 9618.2        400.78
PPy-SDS          5312.0        399.78
                 5210.6        400.95
PPy-SDBS         9221.0        399.98
                 9027.8        401.16


CONCLUSIONS

Doped PPy with excellent conducting characteristic had been successfully prepared by a facile method of in situ chemical oxidation in various anionic surfactant solvents. An interesting regulation of the molar ratio of the oxidant to the monomer was firstly discovered when the molar ratio of the anionic surfactant to the monomer was 0.3. The value of conductivity reached its maximum when the molar ratio of the oxidant to the monomer was 1.1 although the doped surfactants are different. Relative testing results show that the PPy samples exhibit different morphology and structure. The doping level, the degree of the structural order of counterions in the polymer chains, and the binding state of the charged nitrogens with the surfactant anions are decided by the surfactant anions with [FeCl.sub.3] as oxidant. Further studies are developed to find out the conducting mechanism of the polymers in order to modify their conductivity and expand the application of the conducting polymers.

Correspondence to: Mei Li; e-mail: I imei@spu.edu.cn

Contract grant sponsor: College Scientific Plan Fund of Shandong Education Department; contract grant number: JIOLD23; contract grant sponsor: Doctoral Startup Foundation of Shandong Institute of Light Industry; contract grant number: 12042826.

DOI 10.1002/pen.23538

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2013 Society of Plastics Engineers

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Mei Li, (1), (2), (3) Wenguang Li, (1) Jun Liu, (1) Jinshui Yao (1), (2), (3)

(1) School of Materials Science and Engineering, Shandong Polytechnic University, Jinan 250353, People's Republic of China

(2) Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass and Functional Ceramics, Jinan 250353, People's Republic of China

(3) Key Laboratory of Amorphous and Polycrystalline Materials in Shandong Polytechnic University, Jinan 250353, People's Republic of China
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Author:Li, Mei; Li, Wenguang; Liu, Jun; Yao, Jinshui
Publication:Polymer Engineering and Science
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Geographic Code:9CHIN
Date:Nov 1, 2013
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