Self-doping effects on the morphology, electrochemical and conductivity properties of self-assembled polyanilines

doi:10.1016/j.tsf.2008.06.079

Thin Solid Films

Volume 517, Issue 2, 28 November 2008, Pages 500-505

https://doi.org/10.1016/j.tsf.2008.06.079 Get rights and content

Abstract

Copolymerizations of a self-doping monomer (o-aminobenzenesulfonic acid, SAN) and aniline (AN) in different molar ratios via the self-assembly process were conducted to prepare self-doping polyanilines (SPANIs). The morphology of SPANIs can be changed from microspheres and nanotube to coral reef like structures by simply increasing the molar ratio of the monomer (AN) to the dopant (SAN). The relationship among the AN to SAN molar ratio, the morphology, UV–Vis absorption behaviors, thermal stability, electrochemical behavior, crystalline density, and conductivity of self-doped PANIs are investigated. The nanostructures were strongly dependent on the AN to SAN mole ratio. UV–Vis absorption behaviors changed with increasing degrees of self-doping, which correspond to the various SPANI redox states. Moreover, the decomposition temperature was also relative to the degree of self-doping. A higher degree of self-doping led to higher SPANI degradation temperatures. Oxidation and reduction current peaks occur at more positive potentials for SPANIs with a higher degree of self-doping. High crystalline density led to the high room temperature conductivity of the SPANIs, which was attributed to the interaction between the polymer chains.

Introduction

Conducting polymers have recently received much attention due to their long conjugation length, metallic conductivity, and their promise for applications in molecular wires, nano-electronics, optoelectronic devices, and biomedical devices [1], [2], [3]. Researchers have extensively studied nanostructure conducting polymers, including polypyrrole [4], [5], polyalkyl-thiophene [6], [7], and polyaniline (PANI) [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. PANI has been studied more than the other two conducting polymers because it is relatively cheap, easy to prepare, exhibits unique chemical and physical properties, and has good thermal and environmental stabilities [14], [15]. The conductivity and physical properties of polymers are affected by the PANI nanostructure. Several studies have synthesized PANIs with various nanostructures, such as microporous structures, hollow microspheres, and nanotubes [8], [9], [10], [21]. Researchers have used different organic acids as dopants for preparing PANI with various nanostructures [8], [9], [10]. On the other hand, synthesizing PANI in the presence of protonic acid as a self-doping monomer, such as 2-aminonaphthalene-4,8-disulfonic acid, and o-aminobenzenesulfonic acid (SAN), has also attracted a great deal of interest in recent years [16], [17], [18], [19], [20], [21]. In comparison with the external acid doped PANI method [8], [9], [10], a self-doping monomer does not need to be removed after PANI polymerization because the dopant also plays the roles of surfactant and comonomer of PANI [16], [17], [18], [19], [20], [21].

The nanostructure and morphology of conducting polymers play important roles in determining material properties and their potential applications in technology. Conducting polymers with hollow spheres hold promise for application in the medicine and catalyst fields [8]. Nanotube and nanowire structures are favorable for application in optoelectronic nano-devices [8], [9], [10], [21]. External acid doped PANIs with nanotube structure prepared by the self-assembly process have been widely studied by several research groups [8], [9], [10]. Zhang et al. reported that PANIs with nanostructures varying from hollow spheres to nanotubes could be produced by simply changing the dopant to monomer molar ratio [8]. Zhang et al. also investigated the hydrogen bonding effect on self-assembled PANI nanostructures [9]. A third study by Zhang et al. reported that highly crystalline PANI nanotubes could be produced by dicarboxylic acid doping [10]. More recently, sulfonated PANIs with different degrees of sulfonation (or doping) were produced by the self-assembly process in the presence of SAN as a self-doping monomer [21]. Yang et al. discussed the nanofiber structure formation mechanism of self-doped PANI (SPANI) [21].

According to previous studies, most self-doping sulfonated PANIs were produced by the electrochemical synthesis method [17], [19], [20]. These studies reported on the electrochemical properties and conductivity of sulfonated PANIs. However, sulfonated PANIs morphologies have not been taken before [17], [19], [20]. Researchers have also investigated the copolymerizations of a self-doping monomer SAN and aniline (AN) in different mole ratios via self-assembly process as a method for preparing nanotube structure SPANIs [21]. This paper studies in detail the synthetic condition effects on nanotube diameter, such as the AN/SAN molar ratio, monomer concentration, oxidant concentration, reaction temperature and time. Nevertheless, the discussion of AN to SAN mole ratio effects on nanostructures only ranged from 0.5 to 4 M. This paper does not examine the influence of the degree of sulfonation or doping on thermal stability, electrochemical properties, and crystalline density. Therefore, the preparation of self-doping PANIs with different nanostructures was conducted by changing the AN/SAN molar ratio from 1 to 8. The doping degrees of SPANIs were obtained using an elemental analyzer. The relationship between the molar ratio of AN to SAN, the morphology, UV–Vis absorption behaviors, thermal stability, electrochemical property, crystalline density, and conductivity of the self-doping PANIs are all discussed.

Section snippets

Synthesis of self-doped polyaniline

As Scheme 1 indicates, the SPANIs were synthesized by the self-assembly method [21]. The reaction conditions of various SPANIs are summarizes in Table 1. For example, AN (0.083 M) and SAN (0.083 M) in different molar ratios (1:1) were stirred and dissolved in distilled water under 4 °C. An oxidant ammonium peroxydisulfate (APS; 0.4 M) aqueous solution was then added to the AN-SAN mixture solution and stirred for 3 min. SPANI polymerization occurred under a stationary condition under 4 °C for

Chemical structure characterization of SPANIs

To prove whether or not the SPANIs were produced by copolymerization, FTIR spectroscopy was used to determine the presence of –SO3⁻ groups attached to the aromatic rings. Fig. 1 shows the FT-IR spectra of SPANIs SP1–SP4. The characteristic peaks around 1580 and 1480 cm^− 1 correspond to the stretching of the benzenoid and quinoid units in SPANIs, respectively [22]. The absorption peaks at 1300 and 1246 cm^− 1 represent the C–H stretching vibration with aromatic conjugation [23]. The peak at 1040 cm⁻

Conclusion

The sulfonation degree effect on the morphology, UV–Vis absorption, thermal stability, electrochemical behavior, crystalline density, and conductivity property of self-assembled polyaniline was investigated. The morphology of self-doped PANI changed from nanoclusters and nanotubes to a coral reef like nanostructure by increasing the AN to SAN molar ratio in the copolymerization reaction. The coral reef like nanostructure has never before been observed in the SPANIs. The thermal stability,

Acknowledgments

The authors thank the National Science Council of Taiwan, ROC, for financial support (Grant NSC 96-2221-E-224-030). Helpful assistance in the measurement of electrochemical and conductivity properties from Prof. C. W. Lin is also acknowledged.

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Self-doping effects on the morphology, electrochemical and conductivity properties of self-assembled polyanilines

Abstract

Introduction

Section snippets

Synthesis of self-doped polyaniline

Chemical structure characterization of SPANIs

Conclusion

Acknowledgments

Synth. Met.

Synth. Met.

Synth. Met.

Synth. Met.

Electrochim. Acta

Polymer

Synth. Met.

Synth. Met.

Synth. Met.

Polym. Adv. Tech.

J. Chem. Soc., Chem. Commun.

Analyst

Aust. J. Chem.

J. Chem. Soc., Chem. Commun.

Polym. Commun.

Adv. Funct. Mater.