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Effect of volume ratio of acetonitrile to water on the morphology and property of polypyrrole prepared by chemical oxidation method.


Because of the good properties of easy preparation, high electronic conductivity, and excellent stability (1-6). the conducting polymer polypyrrole (PPY) has attracted more and more attentions since its first preparation by the group of Kanazawa with electrochemical oxidation polymerization method (1). As far as the synthesis of PPY is concerned, there are two methods. electrochemical Oxidation polymerization and chemical oxidation polymerization, were widely applied. According to the available reports about PPY's preparation, a series of factors such as solvent (7-11), temperature (12), (13). monomer concentration (14), and doping ion (12), (15) have been proposed to have strong effects on PPY's property. For example. it is now well known that water has a marked effect on PPY's preparation if it's synthesized in water and acetonitrile mixed solvent (7), (8), (16-19). and this effect was concluded as "water effect-by some researchers (19 -24). More and more people are trying to investigate this" water effect-and its influence on the morphology and property of PPY. However. their efforts are mainly focused on PPY's electrochemical preparation. In this article, we will try to reveal the "water effect" in PPY's chemical preparation. Therefore. we designed a set of control experiments with various volume ratios of acetonitrile to water (VRAW), to study the continuous change of the morphology and properties of PPY prepared by chemical oxidation polymerization.



The chemicals such as pyrrole, acetonitrile, and ferric chloride were in analytical grade, commercially available and used without further purification or treatment. The water used in the experiments was distilled.

Polypvrrole's Preparalion

In the mixed solvent of water and acetonitrife, PPY was prepared by chemical oxidation method by using ferric chloride as oxidant. In a typical procedure, 2.8 m.1 pyrrole was dissolved in the mixed solvent of 40 ml acetonitrile and 340 ml water. Then the mixture was moved to a 500 ml three-necked round bottom flask with ice bath and magnetic stirring. Until the mixture reduced to 5 [degrees]C, a solution of [FeC1.sub.3] (1 mol [1.sup.-1], 20 ml) was dropwise added (there was 360 ml water in the mixed solvent at this time, and VRAW is 1:9). The polymerization reaction was carried out at 0-5[degrees] C with magnetic stirring for 10 h. Dark PPY was filtered and repeatedly washed with ethanol and water until the pH of the filtrate is 7, then it was dried at 50 [degrees]C for 48 h. The product was noted as N 1. By controlling VRAW of the mixed solvent was 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, and 7:3, a series of PPY products were obtained and marked as NI, N2, N3, N4, N5, N6, and N7, respectively.

Characterization and Measurement

The FTIR spectra of N1-N7 in the range of 4000- 400 [cm.sup.-1] were recorded on a Nicolet 5700 spectrophotometer, by the KBr pellet technique. The micrographs of N1-N7 were got at a JSM-5900LV scanning electron microscopy (SEM). The product weight of N1-N7 was measured by a JA2003A Electronic Balance with an accuracy of I mg. The four-probe electrical conductivities of N1-N4 were tested on an SZ85 digital multimeter employing the standard conventional technique (the products can not be pressed into disks when VRAW exceeds 40%, and can not be measured by the four-probe resistance).


FTIR Spectroscopy of Products

Figure 1 shows the FTIR spectra of N1-N7. Take NI as an example, the broad bond at 3200-3500 [cm.sup.-1] is attributed to N--H and C--H stretching vibrations, the bonds at 1541 and 1456 [cm.sup.-1] are assigned to C=C and C--stretching vibrations of PPY, and the peaks at 1038 and 1300 [cm.sup.-1] which can be attributed to C--H deformation and C--N stretching vibrations (25). The above analysis suggests that polypyrrole is prepared in our experiments.

Effect of VRAW on the Morphology of PPY

Figure 2a-g orderly show the SEM images of N1- N7. As shown in Fig. 2a-d, PPY with particle morphology is prepared, and the particles' agglomeration is obvious in the as-prepared products. Moreover, the particle size of the obtained PPY continues to decrease as VRAW in the reaction mixture increases in the range of 1:9 to 4:6. The average particle size of N1-N4 is 0.65, 0.59, 0.45, and 0.27 [micro] m, respectively. However, when VRAW reaches 5:5, the morphology of PPY is changed from particle to film, as shown in Fig. 2e. Furthermore, through the comparison of Fig. 2e-g, it's easy to find that the PPY film turns to be smooth with the further increase of VRAW in the range of 5:5 to 7:3. As our analysis about the above phenomenon, the following explanations are concluded. Potypyrrole is formed by the radical cation mechanism (21) and grown up by a "nucleation-and-growth" mechanism (26). When pyrrole is dissolved in the mixed solvent with a VRAW less than 5:5, spherical acetonitrile micelles with pyrrole in them will be formed (as simulated in the Fig. 3a and c), which results in the generating of particle-shaped PPY as shown in Fig. 2a-d or simulated in Fig. 3b and d. With the increase of VRAW in the scope of 1:9 to 4:6, more and smaller micelles will be formed in the reaction mixture and less pyrrole will be dissolved in each micelle, so the particle size of finally obtained PPY decreases. When VRAW is equal to or more than 5:5, the micelles formed in the reaction mixture are changed from spherical structure to double layer structure (as Fig. 3e shows) because of the high content of acetonitrile, which results in the forming of film-shaped PPY as shown in Fig. 2e-g or simulated in Fig. 31 As VRAW increase in the range of 5:5 to 7:3, more tight double layer micelle will be formed, which contributes to getting smooth PPY film.

Effect of VRAW on the Yield and Conductivity of PPY

To comprehensively study the effect of VRAW on the yield and conductivity of PPY, another PPY product (marked as NO) was prepared in pure water without acetonitrile (the VRAW was marked as NO), and its yield and conductivity were also measured.

Figure 4 is the curve of the yield of PPY vs. VRAW in the reaction mixture. As the curve shown, the yield of N 1 is higher than that of NO That's mainly because pyrrole's polymerization area changes from water-acetonitrile interface to acetonitrile micelle when acetonitrile is added to the reaction mixture, which is beneficial to the conversion of pyrrole monomer and the doping of chlorine ion for iron chloride is water and acetonitrile soluble. On the other hand. as far as PPY prepared in the mixture solvent of acetonitrile and water, its yield decreases with the increase of VRAW in the range of 1:9 to 7:3. This result indicates that acetonitrile also prevents from synthesizing of polypyrrole as it provides polymerization area for pyrrole monomer through forming micelles. Because the dielectric constant of acetonitrile is 3:7 and smaller than that of water (which is 80). VRAW's increase will reduce the dielectric constant of the reaction mixture, enlarge the coulombic repulsion between the radical cations and weaken the radical-radical coupling (21), (27), (28), which finally result in the decrease of PPY's yield.

Figure 5 shows the conductivities of N0-N4. As the comparison of NO and N 1, it indicates that PPY's conductivity is increased when acetonitrile is added to the reaction mixture. According to the above analysis, acetonitrile's addition is beneficial to chlorine ion doping. which will improve the conductivity of PPY. For PPY products prepared as acetonitrile was added. it's clear that their conductivity decreases with the increase of VRAW in the scope of 1:9 to 4:6. It can be inferred that more--CN groups attack the radical cation because of their strong pro-nuclear property if more acetonitrile was used in the reaction mixture, which shorten the life span of radical cation and finally results in the reducing of PPY's polymerization degree and conductivity (29). What's more. because of pyrrole and its oligomers can be dissolved in acetonitrile, the presence of acetonitrile into the polymer matrix leads to a more expanded distribution of chains or particles, which increases the "electron hopping distance" (30). The VRAW's increase will lead to the increase of the distance. so "electron hopping" becomes more difficult and the conductivity decreases with the increase of VRAW. So, based on the above two aspects of reasons, the conductivity decreases with the increase of VRAW in the scope of 1:9 to 4:6.


Polypyrrole is synthesized by chemical oxidation method in the mixed solvent of water and acetonitrile. When VRAW is between 1:9 and 4:6, particle-shaped PPY is prepared and its diameter decreases with the increase of VRAW: when VRAW is between 5:5 and 7:3. film-shaped PPY is obtained. The change of PPY's morphology is attributed to the structure change of acetonitrile micelle from spherical structure to double layered structure. Compared with the PPY synthesized in pure water without acetonitrile, which prepared at a VRAW of 1:9 shows higher yield and conductivity: however, the Further increase of VRAW results in the decrease of the yield and conductivity.

Correspondence to: Shuchun Hu: e-mail:

Contract grant sponsor: Fundamental Research Funds for the Central Universities: contract grant number: SWJTUO9CX051. Contract grant sponsor: Ph.D. Programs Foundation (for new teachers) of Ministry of Education of China; contract grant number: 20070613017. Contract grant sponsor: Fundamental Science Foundation of Southwest Jiaotong University: contract grant number: 2008B18. Contract grant sponsor: National Undergraduates Innovating Experimentation Project: contract grant number: 101061301.

DOI 10.1002/pen.23101

Published online in Wiley Online library (

[c] 2012 Society of Plastic Engineers


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Zhenhao Huang, Shuchun Hu, Nan Zhang, Xiaolang Chen, Dan Chen, than Jin, Xian Jian

Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Department of Polymer Materials, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China

DOI 10.1002/pen.23101
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Author:Huang, Zhenhao; Hu, Shuchun; Zhang, Nan; Chen, Xiaolang; Chen, Dan; Jin, Qian; Jian, Xian
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Jul 1, 2012
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