1. Polymers: from insulators to conductors
Synthetic polymers or plastic materials, whose history extends back 150 years, penetrated the majority of the industrial fields having numerous technical, medical, or scientific applications.
The high impact and success of these materials are typically associated with a wide range of characteristics/properties. For instance, they possess excellent combination of physical and chemical properties such as high mechanical strength, toughness/impact resistance, flexibility, malleability, and low density, as well as good resistance to corrosive chemicals (solvents, acids, or bases), which are frequently incompatible with metals. They are also easy to synthesize and process, showing rapid applicability to the industry at low costs.
In general, plastics are associated with good electrical resistivity and are commonly employed as insulation for electrical cables. Recently (last 50 years), polymers have been associated with conducting or semiconducting applications, which confirm the proficiency to synthesize polymers with intrinsic conduction.
In 1977, the first semiconducting polymer has been reported, namely doped polyacetylene, whose electrical conductivity was comparable to that of metals [1, 2]. The impact of this discovery did not pass unnoticed. It has been recognized by the 2000s Nobel Prize for Chemistry for the three chemists (Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa) who contributed to its development.
Starting from this discovery, the interest for the polymeric structures of “synthetic metals” class has shown a vertiginous increase. In the last 50 years, the aim of many worldwide studies has been that of confining in one material the facile processing of polymers with the semiconducting or conducting properties, specifics to classical metals and semiconductors, everything carried out at a low price and environmentally friendly.
If the first-generation intrinsic conducting polymers show low processability due to low solubility and instability in the environment, the latest generation is easily processed in powders, flexible thin films, nanowires and nanoparticles, being now soluble in a variety of solvents and have a high stability in the environment. Electrical conductivities of intrinsic conducting polymers span from insulators (<10−10 S/cm) to metals (5 × 105 S/cm).
Although it was discovered more than 150 years ago, only recently the polyaniline has gained an increased interest among the scientific community, once the discovery of intrinsic conduction properties in polymers, and highlighting the possibility of polymer synthesis with high electrical conductivity.
The facile synthesis, moisture stability, and the simple doping/dedoping chemistry are characteristics which allow polyaniline to distinguish among all other conducting polymers. If systematic research has made possible the development of simple synthesis methods for polyaniline, additional efforts should be devoted to the complete understanding of the complex polymerization mechanism and its oxidation nature. Having rich chemistry and wide applicability, polyaniline is one of the extensively investigated polymers.
2. Synthesis of polyaniline
Usually, polyaniline is synthesized by either chemical [3, 4] or electrochemical [5, 6] oxidation of the aniline monomer in acidic solution, and the aqueous medium is preferred. Although chemical and electrochemical synthesis methods of polyaniline are simple, needing only some elements of fine chemistry, the polymerization mechanism is much more complex .
In addition to the large variety of chemical and electrochemical methods of synthesis, polyaniline has been also synthesized by unconventional methods such as plasma polymerizations , vapor-phase deposition polymerizations (VDP) technique , inverse emulsion polymerization , photochemical polymerization , enzymatic polymerization , and autocatalysis polymerizations .
The synthesis of polyaniline by chemical, electrochemical, or other nonconventional methods is closely related to the desired final application. For instance, the electrochemical method is preferred when the final application requires highly ordered thin films. On the other hand, plasma polymerization is the method of choice in the case of conformal, nanometer-thick films with high substrate adhesion which do not allow the use of solvents during the synthesis process.
An important feature of polyaniline is that the structural unit contains two entities, one with benzene structures and the other with the quinoid structures, having a different ratio.
As presented in Figure 1, molecular structure of polyaniline consists of monomer units assembled from reduced (x) and oxidized (1 − x) units, where n represents the polymerization degree. Depending upon the oxidation state, polyaniline is found in one of the following forms:
Insulating leucoemeraldine form where the polyaniline is completely reduced and x = 1
Pernigraniline, the fully oxidized form of polyaniline (x = 0) consisting solely of imine linkage
Emeraldine, the “half-oxidized” form (x = 0.5), in its protonated form considered to be the most useful form of polyaniline due to its high stability and the capacity to be easily converted to the salt structure by doping, the form with the highest electrical conductivity. On the contrary, both leucoemeraldine and pernigraniline obtained by emeraldine’s reduction and oxidation, respectively (Figure 2), show low electrical conductivity even in their doped form.
While leucoemeraldine structure consists of the benzenoid rings linked together by the amine groups (–NH–), the pernigraniline consists of benzenoid and quinoid rings bonded together by the imine moieties (–N=). Due to the basicity of the nitrogen-containing groups (imine and/or amine) in the backbone of the polyaniline, it can easily undergo doping in an acid/base process.
As shown in Figure 2, there are three forms of polyaniline salt, corresponding to the doped form of polyaniline, which can be obtained by the protonation of polyaniline base. Protonation and deprotonation cause essential changes in both electrical conductivity and absorption properties leading to color changes of the polymer.
Based on MacDiarmid’s and Heeger’s studies, reversible conversion between the emeraldine salt and emeraldine base represents a unique example of doping related to the protonation rather than adding or removing electrons to or from the polymer network. Unlike other heterocyclic conducting polymers (e.g., polypyrrole), the presence of –NH groups in the backbone of the polymer allows them to react with both protons (H+) and metallic cations (e.g., Li+, Zn2+, Ni2+ etc.), enabling thus both p- and n-type doping states.
In addition, polyaniline possesses remarkable electrochromic properties, having the capacity to reversibly change the conductivity, the color, and the oxidation potential depending upon the surrounding conditions, such as pH of the medium and/or the presence of oxidizing and reducing agents.
3. Applications of polyaniline
Polyaniline is one of the most investigated and useful conjugated polymer with a dynamic evolution of applications (Figure 3). Assisted by both oxidation and protonation processes, it is the only conducting polymer whose electronic structure can be controlled in a reversible manner. In addition to the classical redox doping, polyaniline’s unique electrochemical component offers also the suitable chemistry for an easy doping/dedoping in the presence of “protonic acids” by which the number of electrons remains unchanged.
Polyaniline presents an intrinsic conductivity and a remarkable ability to undergo reversible changes from conducting form to semiconducting form and to a dielectric form. Last but not the least, polyaniline is easy to synthesize and shows a high stability in the environment.
Although many significant technological advances have been undertaken since their discovery regarding polyaniline’s synthesis and processing, the fact that it cannot compete with traditional metals in the electrical applications such as electrical or transmission cables is generally accepted.
One of the first applications of polyaniline was in the structure of light batteries. Batteries have been developed with respectable performance using polyaniline as positive electrode (cathode) and lithium-aluminum alloys as negative electrode (anode) [14, 15]. Polyaniline also has been used in the field of biomedical applications; on this line, biosensors have been developed starting with immobilization of the enzymes on a conducting polyaniline matrix . The color change property associated with polyaniline in various oxidation/reduction states has found potential uses in electrochromic devices, smart window [17, 18]. Electrical charges and conformations of the multiple oxidation states also make the polyaniline highly promising for applications such as supercapacitors or actuators [19, 20]. Polyaniline with its distinct responses to doping processes (acid/base) makes it an ideal option for applications in chemical vapor or liquid sensor structures [21, 22].
The different colors of different forms of polyaniline, doping/dedoping response, and the charges and conformations of the multiple oxidation/reduction states make the polyaniline material highly promising for applications in many other fields: antistatic , electromagnetic interference shielding , anticorrosive coatings , toxic metal recovery , fuel cells , and microbial fuel cells .
As an overview, polyaniline has gone a long way from a pure scientific laboratory curiosity to a material that can find its final destination in a wide range of commercial products.
This first chapter, “Introductory Chapter,” aims to make a first and very brief introduction to the much wider field of polyanilines. Intrinsically conducting polymers and their synthesis, as well as their properties and applications, have been studied for a long time. Among them, polyaniline is a conducting polymer with incredible promises.
This book, “Polyaniline: From Synthesis to Practical Applications,” explores some intriguing aspects of polyanilines—methods of synthesis, properties, applications, as well as some remaining challenges in developing applications using polyanilines.