Polyaniline physical structures

J. M. Cinder, A. J. Epstein, Role of ring torsion angle in polyaniline-electronic structure and defect states, Physical Review B 1990, 41,10674. 

In considering the potential applications of electroactive polymers, the question always arises as to their stability. The deterioration of a physical property such as conductivity can be easily measured, but the chemical processes underlying it are not as easy to be revealed. In order to understand them, XPS has been used to follow the structural changes which occur in the polymer chain and the counter-ions of the doped polymer. The following sections present some XPS findings on the degradation of electroactive polymers, such as polyacetylene, polypyrrole, polythiophene and polyaniline, in the undoped and doped states. 

Some conjugated polymers, such as polythiophene and polyaniline were synthesized already in the last century [8a,b], It is not surprising that, for example, polyaniline has played a major role in research directed toward synthetic metals because it possesses a relatively stable conducting state and it can be easily prepared by oxidation of aniline, even in laboratories without pronounced synthetic expertise (see section 2.6). It is often overlooked, however, that a representation of, for example, polypyrrole or polyaniline by the idealized structures 1 and 2 does not adequately describe reality, since various structural defects can occur (chart 1). Further, there is not just one polypyrrole, instead each sample made by electrochemical oxidation must be considered as a unique sample, the character of which depends intimately on the conditions of the experiment, such as the nature of the counterion or the current density applied (see section 2.5). Therefore, one would not at all argue against a practical synthesis, if the emphasis is on the active physical function and the commercial value of a material, even if this synthesis is quick and dirty . Care must be exercised, however, to reliably define the molecular structure before one proceeds to develop structure-property relationships and to define characteristic electronic features, such as effective conjugation length or polaron width. 

In this section we will present the models used to describe relaxation phenomena in polymers or other materials. Then, we will present the special case of conducting polymers and the physical origin of the relaxation will be outlined. We will then propose a model able to explain the microwave properties of polyaniline samples. Experimental results concerning the effect of structural parameters of conducting polymers will be given at the end of this section. 

This model allows us to describe the evolution of the polyaniline radioelectrical properties for different doping levels. What is interesting in this approach is that physical parameters are taken into account. Moreover, Structural characteristics, such as coherence length, can be recovered. Finally, the calculations we carried out confirm that charge carriers are localised over some benzene rings, the three-dimensional aspect of the transport phenomenon occurs after only about 10 periods. 

The combination of the properties of nano-TiO and polyaniline enables to solve successfully the problems of the chemistry, physics and electronics. Specific electronic structures of the nano-TiO (as the n-type semiconductor) and polyaniline (as the electron s conductor in majority of the cases and as a p-type semiconductor under certain conditions) give the possibility to design the systems for different applications. For example, today such materials are equipped in the photocatalytic conversions of the different pollutants especially [7 4. The modification of the surface of TiO particles by polyanilines layers raises the catalytic activity of titanium (IV) oxide [5, 79]. Composite materials, which have integrated properties of 5-doped nano-TiO and polyaniline layers can be effective in the photo-catalytic processes especially. 

Conjugated polymers can now claim a considerable and uninterrupted degree of attraction over a period of several decades [1]. In the initial years, research into the synthesis of the first representatives of the new substance class of conjugated polymers (polyacetylene, poly(para-phenylene), poly(parfl-phenylenevinylene), polythiophene, polypyrrole, polyaniline) was important. The resulting polymers were characterized in most cases by their insolubility and infusibility, properties that considerably hindered their structural characterization and their processing. The majority of such compounds possessed no fully defined structure and the physical properties were influenced by structural defects. Moreover, it was often difficult to distinguish between neutral molecules and the "doped" species resulting firom oxidation or reduction. 

K. G. Neoh, E. T. Kang, K. L. Tan, Structural study of polyaniline films in reprotonation/deprotonation cycles, Journal of Physical Chemistry 1991, 95, 10151. 

N. S. Sariciftci, H. Kuzmany, H. Neugebauer, A. Neckel, Structural and electronic transitions in polyaniline - a Fourier transform infrared spectroscopic study, Journal of Chemical Physics 1990, 92, 4530. 

An alternative to in situ polymerization involves direct intercalation of macromolecules into layered structures. Silicates are most often used. The insertion of polymer molecules into layered host lattices is of interest from different points of view. First, this insertion process leads to the construction of organic-inorganic polylayered composites. Second, the intercalation physical chemistry by itself and the role intercalation plays in the gain of electronic conductivity are of interest. This becomes important in the construction of reversible electrodes " or when improving the physicomechanical properties of nylon-layered silicate nanocomposites, hybrid epoxide clay composites," and nanomaterials based on hectorite and polyaniline, polythiophene or polypyrrole. ... 

However, by whichever method polyaniline is synthesized, the basic structure does not vary greatly, but the secondary (substituents) and tertiary (dopants) aspects play such a large role in controlling overall physical and chemical behaviour. This is also true for almost all organic material and hence this chapter deals with the effect of aromatic organic sulphonic acid on polyaniline. 

For example, the relationship between pseudocapacitance and heat treatment temperature of nickel oxide was investigated using XRD, x-ray absorption spectroscopy, and CV. These techniques can provide important information about structure arrangements and the electrochemical properties of nickel oxide at various heat treatment temperatures. Similar studies were performed on composites (graphene-polyaniline, Mn02-mesoporous carbon) [30,31]. XRD can reveal material structures revealing the relationship of electrochemical properties and the effects of certain chemical or physical alterations. 

Electroactive polyaniline films were synthesized by the catalysis of biUru-bin oxidase (BOD, a copper-containing oxidoreductase). The polymerization of aniline was carried out on the surface of a sohd matrix such as glass sUde, plastic plate, or platinum electrode to form homogeneous films [33]. The BOD was immobilized on the surface by physical absorption. The optimum pH was around 5.5. Some aniline derivatives such as p-aminophenol and p-phenylenediamine were good substrates for BOD. Structural analysis suggested the BOD synthesized polyanihne possessed partially 1,2-substititued structures. Cyclic voltammetric studies demonstrated that the PANl films were electrochemically reversible in redox properties, but differed from that of chemically or electrochemically synthesized PANl. The difference was attributed to the partial 1,2-substitution. Laccases are known to oxidize phenolic compounds in nature in the presence of oxygen and are capable in polyaniline synthesis in vitro [34-36]. 

In the quest for improved performance, better pro-cessibility, and novel applications (1], a wealth of newer compounds with specific chemical architectures have been synthesized. The three most common approaches that have been implemented are greater main-chain flexibility (e.g., polyaniline [2]), side-chain substitution (e.g.. poly(3-alkylthiophenes) [3J), and fabrication from a soluble precursor polymer (e.g., poly-(/ -xylene-a-dimeth-ylsulfonium chloride) to yield poly(/ -phenyleneviny-lene) [4]). All of these modifications can produce materials with a range of structural forms and, in some instances, striking new physical behavior. 

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