Physiochemical Properties of PANI Thin Films

Introduction

The polymers are the key components of the new fascinating research area owing to their simple preparation and the diversity of their properties. Even though conventional insulating polymers are used in the past 50 years, it did not meet the latest requirements, which led to the development of different conducting materials such as conducting polymer composites, conducting fibers, and conducting films. The polymers are comprehensively used in the manufacturing of insulators and dielectric materials in electrical based industrial organizations due to their insulating and low conducting properties. Nevertheless, some polymers are synthesized with a remarkable ability to conduct electricity. The organic polymers that possess the magnetic, optical and electric properties of a metal are called an intrinsically conducting polymer, and such polymers are the electroactive polymers which possess the properties mentioned above while retaining their structural characteristics (Sreejith, 2004). It is a class of polymer with conjugated double bonds in their backbones, and it demonstrates good electrical conductivity without the use of conductive additives. It becomes highly conductive only in its doped state. The conductivity of the polymers is converted into the metallic state from its insulating state through the doping process. Both n-type (electron-donating) and p-type (electron-accepting) dopants are utilized to induce an insulator to metal transition in electronic polymers (Chiang et al., 1977 & Heeger et al., 1988). It was demonstrated that charges are either added or removed chemically or electrochemically from the polymer chain by introducing acidic or basic solutions during the polymerization or post-processing of the polymer. During this process, the negative (electron) or positive (holes) ions formed are then free to move throughout the polymer chain creating the current.

Polyaniline was first known in 1835 as aniline black, and it is a term that has been used for those products which were obtained by oxidation of aniline. After a few years in 1840, Fritzsche has been studied about the product, which was observed by chemical oxidation of aromatic amine. After that, Letheby (1862) has been discovered that the final product which was obtained by anodic oxidation of aniline at a Pt electrode in aqueous H₂SO₄ solution. Subsequent researchers have achieved the same results with similar observations during the oxidation of aqueous HCl solutions of aniline. For these results, in 1907 Bucherer proposed a phenazine type structure for the synthesization of polymer, which was complicated for understanding. Therefore, a linear arrangement of the quinone-imine linked on para-position has been intended for the product obtained by the chemical oxidation of aniline (Green & Woodhead, 1910). The base of the linear structure was the leucoemeraldine form, which converted into another form such pernigraniline & emeraldine base form shown in Figure 1.

Figure 1.

Different form of PANI

In 2000, three scientists, namely A.J. Heeger, A.G. McDiarmid, and H. Shirakawa were awarded Nobel Prize in the chemistry for their study on intrinsically conducting polymers and obtained the world’s appreciation owing to the unique characteristics, properties and technological applications of these polymers. During the past two decades, these polymers and their composites are the subject of intensive research and development in the academic world and also in the chemical and electronic-based industrial organizations. The use of these composites as new materials has opened an entirely new domain of polymeric applications in industrial and consumer products.

Constitution And Nomenclature

Since the resurgence of polyaniline, there was a lot of confusion regarding its constitution, and later it has been proposed that PANI may be described as a combination of aniline as repeating units.