PANI Synthesis Using Templates

Self-doped PANI are very interesting due to their unique electrochemical behavior unlike PANI, the self-doped polymer remains in its doped state in near neutral or alkaline media [28]. Fully self-doped PANIs are not easy to synthesize due to the lower reactivity of acid-functionalized anilines. Kim et al. [29, 30] introduced an alternative approach in the template-assisted enzymatic polymerization of aniline. Previously, only polyanionic templates had been used for PANI synthesis. However, acid-functionalized anilines bear a net anionic charge in aqueous solution, and attempts to use SPS as template with carboxyl-functionalized aniline resulted in red-brown colored polymers with no polaron transitions, regardless of the synthetic conditions. The use of polycationic templates, such as those shown in Figure 8.2 allowed the synthesis of linear and electrically conductive PANIs with self-doping ability due to the doping effect of the carboxyl groups present in the polymer backbone. 

Bertino and coworkers fabricated PANI nanofibers using radiolyfic synthesis as a new template-free pathway. PANI nanofibers were formed in aqueous solufions of aniline, ammonium peroxydisulfate, and hydrochloric acid with gamma ray irradiation. Typical PANI nanofibers had fhe diameters of 50-100 nm and the length of 1-3 im [180]. 

The chemical methods for the preparation of nanomaterial could be categorized as either template-directed or template-free. The template synthesis methods commonly used for the production of one-dimensional nanostructured PANI are further subdivided into hard template (physical template) synthesis and soft template (chemical template) synthesis approach according to the solubility of the templates in the reaction media. Non-template routes for the synthesis of one-dimensional nanostructured PANI such as rapid-mixing reaction method, radiolytic synthesis, interfacial polymerization, and sonochemical synthesis have also been reported [56], Other approaches like combined soft and hard template synthesis are also known. An overview of hard-template, soft-template, and template-free procedures are presented in the following paragraphs. 

Figure 8.2 Polycationic templates used during enzymatic synthesis of self-doped PANI. (Reproduced with permission from Kim et al. [29], Copyright (2007) Taylor Francis).

Enzymatic Synthesis of PANI Using Anionic Micelles as Templates... 

Besides polyanions such as SPS, anionic surfactants were able to provide the negative charges needed to template the formation of electrically conductive PANI. Indeed, micelles of sodium dodecylbenzensulfonate (SDBS) were used successfully as templates to induce the formation of linear and electrically conductive PANI [38]. On the other hand, cationic and non-ionic surfactants did not provide the negative charges and low pH environment needed for the synthesis of conducting PANI, as expected. Using a similar approach, dodecyl dipheny-loxide disulfonate (DODD) was used as a bifunctional template for the enzymatic synthesis of PANI [39]. UV-Vis and electrical conductivity studies showed that the electrically-conducting form of PANI was only obtained above the critical micellar concentration (CMC) of DODD. The bifunctional nature of the DODD provided... 

PANI nanowires are ID nanostructures which are very interesting from the nanotechnology point of view, due to their potential use in sensors and nanoscale electronic devices. DNA can be considered as an anionic polyelectrolyte that can behave as a template for PANI enzymatic polymerization in solution [20]. Double stranded DNA can be attached to aminated surfaces by electrostatic self assembly, providing a linear template for further aniline electrostatic assembly. By using this method followed by enzymatic polymerization of the assembled aniline, Ma et al. [69] and Nickels et al. [70] showed that stretched DNA molecules were able to template the enzymatic synthesis of PANI nanowires, according to the mechanism showed in Figure 8.7. 

Solid-State Mechanochemical Synthesis Facile template-free solid-state mechano-chemical synthesis of highly branched PANI-NFs with coralloid tree-like superstrueture, via the oxidative polymerization of aniline hydrochloride with FeCl3 6H2O, has been demonstrated [194]. The synthetic yield ( 8%) was comparable to that of the solution interfacial polymerization method. Solid-phase mechanochemical synthesis of branched PANI-NFs was also achieved by using anhydrous FeCl3 as the oxidant [195]. 

The synthesis of spiral PANl nanostructures (2-D ordered spirals comprised of singlestrand PANI-NFs) by chemical oxidative polymerization using a hydrated surfactant sodium dodecylsulfonate crystallite template was recently described [406]. It was found that a spiral dislocation structure on the surface of a hydrated sodium dodecylsulfonate crystallite was responsible for the growth of the spiral PANl nanoarchitecture. It was revealed that APS has a strong tendency to induce the formation of a spiral dislocation stmcture in hydrated sodium dodecylsulfonate crystallites. A mechanism of adsorption of oligoanilines on the steps of dislocation has been proposed for the growth of PANl spirals. 

It is well known that nanoparticulate colloidal dispersions of PANI in various paints at low concentrations cause tremendous improvements in corrosion protection [504]. PANI-NFs showed similar anticorrosive effects e.g., carbon steel coated with PANI-NFs has better corrosion protection than that with aggregated PANI. Raman spectroscopy analysis indicated that the surface of carbon steel coated with PANI-NFs formed a better passive layer, which is composed of a-ferric oxide and Fc304 [146]. The corrosion resistance performance of soya oil alkyd containing nanostructured PANI composite coatings has recently been studied [447]. An array of Fe nano wires within PANI-NTs was obtained using a two-step template synthesis [316]. This PANI-NT envelope may protect the Fe nanowires against a corrosive atmosphere. 

Other studies in 2000 by Drew et al. reported that it is very difficult to spin fibers of PANI complexed to sulfonated polystyrene (PANFSPS), even when solutions containing sodium chloride and dodecyl benzene sulfonic acid sodium salt were used to lower the surface tension and thereby enhance electrospinning [16,17]. However, PANFSPS nanofibers can be produced by adding a carrier polymer such as PEO, polyacrylonitrile, or polyurethane. Also reported was the use of electrostatically layered sulfonated polystyrene as a template for the surface polymerization of conjugated polymers in their conducting form. Enzymatic synthesis of PANI and a copolymer of pyrrole and PEDOT was done on electrospun nanofiber... 

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