Progress in preparation, processing and applications of polyaniline

Abstract

The present paper reviews the synthesis, processing and applications of polyaniline (PANI). The paper includes the advantages of the intrinsically conducting polymers (ICPs) over the other conducting polymers and the superiority of PANI among other ICPs. Details are provided of the different methods used for the synthesis of PANI along with a number of special methods used to obtain a nanostructured PANI. A detailed discussion on the mechanism of electrical conduction in PANI and the factors those influence the conductivity of PANI is also included. A discussion on the problems of effective utilization of PANI and the methods adopted to overcome these problems is also provided. Finally, the possible applications of PANI are discussed.

Introduction

With the discovery in 1960 of intrinsically conducting polymers (ICPs), an attractive subject of research was initiated because of the interesting properties and numerous application possibilities of ICPs. It was expected that ICPs would find their potential applications in multidisciplinary areas such as electrical, electronics, thermoelectric, electrochemical, electromagnetic, electromechanical, electro-luminescence, electro-rheological, chemical, membrane, and sensors [1], [2], [3], [4]. However, many of the potential uses for ICPs have yet to be explored because of a number of obstacles that need to be overcome. Among the available ICPs, polyaniline (PANI) is found to be the most promising because of its ease of synthesis, low cost monomer, tunable properties, and better stability compared to other ICPs. Hence, the authors of the present paper have carried out extensive studies on the synthesis, characterization and application of PANI and its composites [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. However, the main problem associated with the effective utilization of all ICPs including PANI is inherent in their lower level of conductivity compared to metal, and their infusibility and poor solubility in all available solvents [21], [22]. However, the solubility of some ICPs can be improved through doping with a suitable dopant or modifying the starting monomer [6], [11]. Therefore, there is ample scope for modifying the conductivity and processability of PANI through the selection of a suitable dopant and suitable level of doping and also by controlling its structure during synthesis [5], [6]. Another avenue for the successful utilization of PANI is through blending it with a commercially available polymer that has good processability and mechanical properties. Polymer composites containing PANI, where it is used as conducting filler in other polymers (matrixes), have received much attention because of the combination of improved processability and fairly good mechanical properties coupled with good conductivity. Consequently, there is a wider scope for the practical applications of such composites [15], [16], [17], [18], [19], [20], [21], [22], [23]. Few problems are encountered during the preparation of flexible composites using PANI particles as conducting filler in an insulating rubber matrix. In a rubber compound, PANI is mixed with rubber along with other ingredients such as curatives, anti-degradants and colorants, and is finally vulcanized to achieve the required product in its final shape. However, dopants (usually acids) those are used in PANI adversely affect the process of vulcanization, accomplished either through sulfur or peroxide [24]. One solution to avoid this problem is to use thermoplastic elastomer (TPE) as a matrix polymer, which does not require conventional vulcanization. Another way of solving this problem is to accomplish curing through irradiation [16], [17], [18], [19], [25]. The irradiation process is also found to modify the properties of PANI [10]. To use and exploit the promising properties of ICPs, it is important to not only achieve a high degree of conductivity but also make them suitable for processing either alone or in a matrix of a suitable host polymer. In recent years, most of the research on PANI and other ICPs has mainly focused on these two aspects.

Generally, there are four primary types of electrically active polymer system with different degrees of conductivity [26]. The first and the most widely used conducting polymeric systems are the composites in which an insulating polymer matrix is filled with a particulate or fibrous conductive fillers such as a carbon or a metal to impart high conductivity. Applications for such composites are wide spread, these are used for interconnections, printed circuit boards, encapsulations, die attach, heat sinks, conducting adhesives, electromagnetic interference (EMI) shielding, electrostatic discharge (ESD), and aerospace engineering [26].

The second group of polymers is known as ionically conducting polymers. Here, the origin of electrical conductivity is a result of the movement of ions present in the system. An example of such a polymer is polyethylene oxide, in which lithium ions are mobile. These types of polymers have application in the battery industry [26].

The third group of polymers is known as redox polymers. The system contains immobilized redox centers (electroactive centers). However, these centers are not necessarily in contact with one another, but can conduct charge by electron transfer from one center to another through the well known “hopping” mechanism. During conduction, electrons tunnel from one redox center to another through an insulating barrier. The systems need to have a large number of redox centers to increase the probability of such tunneling [26].

The fourth group of conducting system is conjugated polymers. These polymers consist of alternating single and double bonds, creating an extended π-network. The movement of electrons within this π-framework is the source of conductivity. However, dopant is required to increase the level of conductivity for this type of polymers.

The conductivity of a filled conductive system depends on the relative concentration of filler and matrix, type of polymer, polymer viscosity, dispersion and distribution of the filler in the polymer matrix [27], [28], [29], [30]. A higher loading of the spherical metal fillers is required to achieve a percolation threshold [31]. A relatively lower loading is required for structured fillers (high aggregating tendency) such as carbon black compared to isolated spherical fillers [32]. Instead of particulate carbon black if short carbon fiber or carbon nanotubes are used, higher conductivity can be achieved by the addition of a relatively lower level of conductive filler in the polymer matrix [33], [34], [35]. There are several limitations of using metal fillers in insulating polymer matrixes. For example, they impart heavy weight, poor surface finish, poor mechanical properties, and easy oxidative degradation to the end product [36], [37]. Composites made of carbon black are not suitable in a clean environment and sensitive areas because of the possibility of contamination. A colored product is also not possible if carbon black is used as conducting filler [36]. Because of the high opacity of metal or carbon filled composites, such composites cannot be used for optoelectronic applications.

ICPs are inherently conducting in nature due to the presence of a conjugated π electron system in their structure. ICPs have a low energy optical transition, low ionization potential and a high electron affinity [1]. A high level of conductivity (near metallic) can be achieved in ICPs through oxidation–reduction as well as doping with a suitable dopant [1], [6]. The first ICP to be discovered was polyacetylene, synthesized by Shirakawa Louis et al. [38]. Shirakawa et al. found that the conductivity of polyacetylene could be increased by several orders of magnitude through chemical doping and in reality it can be converted from an insulator to a metal like conductor. Following the study on polyacetylene, other polymers such as polypyrrole (PPY), polythiophene, PANI, poly(p-phenylenevinylene), and poly(p-phenylene), as well as their derivatives, have been synthesized and reported as a new group of polymers known as ICPs. The structures of a few intrinsically ICPs are shown in Fig. 1 [39]. The conductivity of a number of ICPs relative to copper and liquid mercury is presented in Fig. 2 and their conductivity, stability and processability are presented in Table 1 [40]. The conductivity of doped polyacetylene is comparable with that of metallic copper but its stability and processability are very poor compared to normal polymer even with respect to those shown in Fig. 1. The conductivity of polyphenylene is quite high but from the environmental stability point of view it is poor. Conversely, the conductivity of polypyrrole, polythiophene and PANI is comparatively less but these polymers have better stability and processability compared to those of polyacetylene and polyphenylene.

The thermal stability of PANI is superior to other ICPs. The processability and conductivity of PANI are also fairly good. From economic point of view, PANI is significantly superior to other ICPs because the aniline monomer is less expensive than other monomers used for ICP. The synthesis of PANI is very simple, properties can be tuned easily, and it has numerous application possibilities [41]. All these factors contribute to PANI being superior to other ICPs.