Polyanilines polymerization

Keywords Carbon nanotubes, carbon nanofibers, fullerene, graphene, nanocomposites, polyacetylene, polyaniline, polymerization, polypyrrole... 

B. D. Malhotra, S. Ghosh, R. Chandra, Polyaniline polymeric acid composite, a novel conducting rubber, Journal of Applied Polymer Science 1990, 40, 1049. 

Using this technique, a variety of conductive polymers has been generated, such as polypyrrole, polythiophene, polyaniline, polyphenylene oxide pyrrole, and polyaniline/polymeric acid composite. 

Synthesis and characterization of polyaniline. Polymerization of aniline was carried out according to a general method (Figure 1) described by Y. Cao et. al (10) but with some changes of experimental parameters. 

Fig. 16.3 Transmission electron micrographs of thin sections of a polycarbonate membrane during the template synthesis of polyaniline. Polymerization time was 30 min (A) and 6 h (B). Note thickness of polyaniline layer on pore walls in (A) vs. (B).

By integrating CNTs with PANi nanofibers, high density and high-suiface areas are possible that can lead to improvements in the conductivity and the development of electronic devices with superior properties [78]. The composites of PANi-CNT can be synthesized by various methods such as electrochemical processing, surfactant free aqueous polymerization, micelle-CNT hybrid template directed synthesis, inverse emulsion pathways, interfacial polymerization, plasma polymerization, in-situ and ex-situ polymerization. The details of these methods have been discussed by Oueiny et al. [8]. In-situ polymerization is one of the most important methods developed so far to integrate CNTs and polyaniline. Polymerization methods include stirring, static placement, sonication and emulsion polymerization [78]. 

Polyaniline (PANI) can be formed by electrochemical oxidation of aniline in aqueous acid, or by polymerization of aniline using an aqueous solution of ammonium thiosulfate and hydrochloric acid. This polymer is finding increasing use as a "transparent electrode" in semiconducting devices. To improve processibiHty, a large number of substituted polyanilines have been prepared. The sulfonated form of PANI is water soluble, and can be prepared by treatment of PANI with fuming sulfuric acid (31). A variety of other soluble substituted AJ-alkylsulfonic acid self-doped derivatives have been synthesized that possess moderate conductivity and allow facile preparation of spincoated thin films (32). 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Recently developed catalyst systems make it possible to construct carbon (.v/>2 )-catbon (sp3) bonds and carbon-nitrogen bonds under mild conditions (Scheme 9.14).19,20 These new developments have also been incorporated into step-growth polymerization. Kanbara et al. reported the synthesis of polyanilines and related polymers in 1996 (Scheme 9.15).21 Wang and Wu reported the synthesis of polyketones in 1999 (Scheme 9.16).22... 

Ideal electrochemical polymerization was considered to give ideal linear and conjugated polymeric chains. The real situation is that films electrogenerated from the basic monomers are insoluble and infusible. Only polyaniline films are partially soluble in some solvents. 

Most 2,5-unsubstituted pyrroles and thiophenes, and most anilines can be polymerized by electrochemical oxidation. For pyrroles, acetonitrile,54 or aqueous55 electrolyte solutions are normally used, while the polymerization of thiophenes is performed almost exclusively in nonaqueous solvents such as acetonitrile, propylene carbonate, and benzonitrile. 0 Polyanilines are generally prepared from a solution of aniline in aqueous acid.21 Platinum or carbon electrodes have been used in most work, although indium-tin oxide is routinely used for spectroelectrochemical experiments, and many other electrode materials have also been employed.20,21... 

Polymerization at constant current is most convenient for controlling the thickness of the deposited film. Charges of ca. 0.3, 0.2, and 0.08 C cm-2 are required to produce 1 fim of polypyrrole,59 poly(3-methylthio-phene)60 (no data are available for polythiophene), and polyaniline 43 respectively. Although these values can reasonably be used to estimate the thicknesses of most electrochemically formed conducting polymer films, it should be noted that they have considerable (ca. 30%) uncertainties. For each polymer, the relationship between charge and film thickness can... 

In the case of polyaniline (see Fig. 26.4), the situation is more complicated, since polymerization occurs across the basic nitrogen atom, the electron state of which depends on pH. Thus, doping is possible not only by oxidation but also by a pH change conduction of the material rises by 9 to 10 orders of magnitude between pH 5 and 1 ( 1M aqueous HCl). 

Although much less so than pyrrole polymers, indole polymers are beginning to be synthesized and studied as new materials. Electropolymerized films of indole-5-carboxylic acid are well-suited for the fabrication of micro pH sensors and they have been used to measure ascorbate and NADH levels. The three novel pyrroloindoles shown have been electrochemically polymerized, and the polymeric pyrrolocarbazole has similar physical properties to polyaniline. 

The use of conjugated polymer as membranes to separate various liquid mixtures has been reported in the literature [19,20], From those, polyaniline (PANi) is one of the most interesting and studied conjugated polymers. Polyaniline is usually prepared by direct oxidative polymerization of aniline in the presence of a chemical oxidant, or by electrochemical polymerization on different electrode materials [21,22], The possible interconversions between different oxidation states and protonated and depronated states [23], figure 4, make this material remarkable for different purposes. Under most conditions, PANi... 

Feng W, Bai XD, Lian YQ, Liang J, Wang XG, Yoshino K (2003). Well-aligned polyaniline/car-bon-nanotube composite films grown by in-situ aniline polymerization. Carbon 41 1551-1557. 

Y. F. Huang, C. W. Lin, Facile synthesis and morphology control of graphene oxide/polyaniline nanocomposites via in-situ polymerization process, Polymer, vol. 53, pp. 2574-2582, 2012. 

Apart from the insulating polymeric matrices, conductive polymers such as polypyrrole and polyaniline have been used as nanocomposite electrodes by chemical or electrochemical polymerization [13, 17, 116, 117]. Such materials provide high conductivity and stability. However, the use of insulating polymers can be more advantageous than the conductive polymers when employed in cyclic voltammetry. 

The polymer resulting from oxidation of 3,5-dimethyl aniline with palladium was also studied by transmission electron microscopy (Mallick et al. 2005). As it turned out, the polymer was formed in nanofibers. During oxidative polymerization, palladium ions were reduced and formed palladium metal. The generated metal was uniformly dispersed between the polymer nanofibers as nanoparticles of 2 mm size. So, Mallick et al. (2005) achieved a polymer- metal intimate composite material. This work should be juxtaposed to an observation by Newman and Blanchard (2006) that reaction between 4-aminophenol and hydrogen tetrachloroaurate leads to polyaniline (bearing hydroxyl groups) and metallic gold as nanoparticles. Such metal nanoparticles can well be of importance in the field of sensors, catalysis, and electronics with improved performance. 

Polyacetylene has good inert atmospheric thermal stability but oxidizes easily in the presence of air. The doped samples are even more susceptible to air. Polyacetylene films have a lustrous, silvery appearance and some flexibility. Other polymers have been found to be conductive. These include poly(p-phenylene) prepared by the Freidel-Crafts polymerization of benzene, polythiophene and derivatives, PPV, polypyrrole, and polyaniline. The first polymers commercialized as conductive polymers were polypyrrole and polythiophene because of their greater stability to air and the ability to directly produce these polymers in a doped form. While their conductivities (often on the order of 10" S/m) are lower than that of polyacetylene, this is sufficient for many applications. 

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