Elsevier

Synthetic Metals

Volume 158, Issues 21–24, December 2008, Pages 922-926
Synthetic Metals

Polyaniline/silver nanocomposite preparation under extreme or non-classical conditions

https://doi.org/10.1016/j.synthmet.2008.06.021Get rights and content

Abstract

Polyaniline (PANI) and polyaniline/silver nanocomposites have been synthesized by sonochemical and ionizing radiation technique. These composite materials were obtained through sonication and γ irradiation of an aqueous solution of aniline and silver nitrate at room temperature. The suggested mechanisms to explain the formation of these products are based on the fact that both methods produce hydroxyl radical radical dotOH and hydrogen radical radical dotH, which acts as an oxidizing agent for the polymerization process of aniline monomer and as reducing agent for silver ions, respectively. Spectroscopic, X-ray and scanning electron microscope (SEM) measures showed that polyaniline and silver nanoparticles of 40 nm of average diameter are obtained when ultrasonic technique is used whereas silver nanoparticles of 60 nm average, and highly fibrillar polyaniline network with diameter of 60 nm is obtained when γ radiation is used.

Introduction

In the past 20 years, the literature has witnessed a significant increase in the number of articles using what may be called non-traditional or non-classical experimental approach to prepare materials. Ambient or close to ambient conditions have been extended, by orders of magnitude, providing advantages for synthesis, reaction mechanism insight and, in some cases, novel chemistry, which is not accessible under ambient conditions. These special conditions may be called extreme.

Among several chemistry methods that may be included within the categories of extreme or non-classical conditions, some areas have became more evident in the literature, such as microwave chemistry, sonochemistry techniques, supercritical fluid methods, plasma applications and ionization radiation [1].

From the experimental techniques developed to prepare polymeric materials mentioned above, the use of ultrasound radiation [2], [3], [4] turned out to be a very attractive and alternative tool for many researchers. The main advantages of ultrasound waves in the polymerization process are the absence of external chemical initiators and the possibility of bulk polymerization. Propagation of ultrasound waves through a fluid causes the formation of cavitations bubbles [5]. Collapse of these bubbles, described as an adiabatic implosion in the hot-spot theory, is the origin of extreme local conditions: high temperature (5000 K) and high pressure (1000 atm) [5]. Cooling rates obtained after collapse are greater than 1010 K s−1 [6], [7], which leads these experimental conditions to be classified as extreme or non-classical conditions.

This technique has been used in the polymerization of methylmethacrylate [8], and for metal polymer composite material [9], [10]. More recently application has been performed in the conducting polymer and conducting polymer metal composite preparation. However, in the majority of reports, ultrasound has been used as a homogenizing agent in the polymerization process, as in the preparation of conducting polymer colloids [11] microemulsion polymerization assisted by ultrasonic waves [12] and conductive polyaniline/nanocrystalline titanium oxide [13]. In all cases, a conventional oxidant, such as ammonium persulfate was used, together with ultrasound radiation, in the polymerization process.

Concerning now to high energy ionizing radiation, another extreme reaction medium, such as electromagnetic radiation (X-rays and γ-rays), and particles (α-particles, β-particles or electrons, protons and neutrons), we have noticed that few articles related to conducting polymer have been published, and the majority is concerned with the effects induced by the radiation on the conducting polymer properties [14], [15], [16], or with the preparation of metal nanocomposite using γ radiation [17]. In the last case we have noticed that conventional oxidants are used together with γ radiation to start polymer preparation.

Earlier studies in sonochemistry of conducting polymer solution [18] and ionization radiation effects on conducting polymer conductivity [19] developed by our group have shown that when polyaniline dissolved in DMSO interacts with ultrasound radiation, the conducting polymer changes its oxidation states and becomes reduced as a consequence of the radical produced by the homolises of water molecule present in the solvent. On the other hand, when ionization radiation interacts with conducting polymer, the polymer conductivity changes several orders of magnitude and that variation depends on the doped state of the polymer [19]. In this case adsorbed water molecule plays an important role in the interaction process. Since, in both examples above, radicals are produced as a consequence of radiation interaction with the reaction medium, we have decided in this work to use these oxidizing radicals to promote the polymerization of the conducting polymer monomer and to prepare the organic–inorganic nanocomposite, instead to use the conventional polymerization methods found in the literature [20], [21], [22].

Section snippets

Experiment

Aniline (Nuclear) was distilled twice under atmospheric pressure, stored in dark and at low temperature prior to synthesis. Ammonium hydroxide (Merck), isopropylic alcohol (Merck), nitric acid (Merck), silver nitrate (Merck), DMSO (Merck), acetonitrile (Aldrich), sodium chloride (Merck) and all the other reagents were used without further purification. All aqueous solutions were prepared using distilled and deionized water. Stock solution of desired concentration of aniline in nitric acid and

Results and discussion

The effect of the interaction of ultrasound wave with aniline plus nitrate solution can be seen in the absorption spectra as a function of reaction time. At the beginning, solution is colorless. As reaction time increases, solution turns into yellow and, finally, becomes green with subsequent precipitation of the polymer. Fig. 1 shows the absorption bands of the solution as a function of irradiation time. As can be seen, there is only one band at 410 nm after 1 h, and this absorption band

Conclusions

It has been shown that polyaniline/silver composite can be obtained either by ultrasonic waves or by γ radiation ionization interaction with an aqueous aniline nitrate and silver nitrate solution. In both cases, the mechanism proposed to explain the polymerization process is based on the hydroxyl radical radical dotOH produced by the interaction of radiation with water molecule or other radical in solution such as nitrate ion. In the case of γ radiation ionization process, the obtained materials are

Acknowledgments

The authors thank Mr. Francisco Rangel for his assistance on SEM measurements, Marcela Bianca for comments and suggestions to the manuscript, and acknowledge financial support received during the development of this work from REMAN contract No. 550.015/01-9, CNPQ contract No. 305587/2003-0 and No. 473.144/03-4 and RENAMI.

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