Electrochemically Prepared Films

A limited number of studies have considered the effects of electropolymerization conditions on the mechanical properties of PAn. Kitani and coworkers,50 for example, have shown that it is possible to prepare freestanding films from PAn in the reduced (leucoemeraldine) state. When oxidized to the emeraldine state, the films became brittle. Similar behavior was described in Chapter 3 for PPy and the change in mechanical properties in that case was related to the increased interchain bonding between charged chains resulting in a decrease in toughness. Presumably, a similar explanation applies to PAn. 

The polymerization potential has also been found to influence the mechanical properties of polyaniline PAn/HA emeraldine salt films.50 The most extensible films were formed at a polymerization potential of 0.65 V (versus Ag/Ag+), which displayed an extension to break of around 40%. Preparation of the PAn/HA films at 0.8 V and 1.0 V resulted in more brittle films. It was suggested that degradation of the PAn at polymerization potentials in excess of 0.8 V might explain the poor properties of the 1.0 V film. The difference in behavior of the films prepared at 0.65 V and 0.8 V was attributed to differences in their crosslink density. Unfortunately, 

Most fibers (see Chapter 7) and films of PAn have been prepared from a solution of EB and converted to the emeraldine salt by acid doping. The choice of dopant acid 

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Several studies have shown that electrochemically prepared films have an inhomogeneous nodular (or cauliflower ) structure.125 A number of studies have reported the morphology of PPy films as a function of various processing variables however, little is known of the mechanism of formation of the nodular structure or the effect of this structure on mechanical properties. 

As with polyanilines, polythiophenes can either be prepared directly by electropolymerization, or by casting from solutions (for alkyl-substituted thiophenes). Most interest has focused on the latter because of their improved mechanical properties compared with those of electrochemically prepared films. The factors influencing the mechanical properties of PTh s are reviewed in this section. 

Han et al. [64, 66] reported the synthesis of highly conductive and thermally stable self-doped mercaptopropanesulfonic-acid-substituted polyanilines by the concurrent reduction and substitution reaction between polyaniline and a nucleophile. These reactions were carried out on both electrochemically generated and free standing polyaniline films prepared from emeraldine base dissolved in N-methylpyrrolidinone. The electrochemically prepared films were dedoped with 5 % aqueous NaiCOs to convert them the into the emeraldine base form. The sulfonated polyaniline was prepared by reaction of a polyaniline emeraldine base film with 0.1 M 3-mercapto-l-propanesulfonic acid sodium salt in methanol under nitrogen at room temperature for approximately 14h [66]. A catalytic amount (0.01 M) of acetic acid was reported to accelerate the reaction. The resulting sulfonated polyaniline film was thoroughly rinsed with methanol, followed by 5 % aqueous NaiCOs to remove reactants. 

All report studies of methanol oxidation at polypyrrole supported catalysts have involved the use of electrochemically prepared films of the polymer on a solid electrode. Strike et al. (20), who electrochemically deposited Pt onto such electrodes from a H2PtCl5 solution, reported current densities for methanol oxidation that were enhanced by factors of 10-100 over those at bulk Pt and platinized gold electrodes. Furthermore, a less rapid decay of the current with time... 

Figure 14.16. Infra-red absorption spectrum in an electrochemically prepared film from a-silylated bithiophene. absorptions from SiMe3 (Reprinted with permission from ref. 10).

Figure 14.17. Raman spectra of electrochemically prepared films obtained Irom a-silylated monomers (a) pTh-E2, (b) pTh-E3, (c) pTh-El, A = 5145 A (Reprinted with permission from ref 10).

Poly[l,2-bis(3-alkyl-2-thienyl)ethylene] is obtained by either chemical or electrochemical polymerization of l,2-bis(3-alkyl-2-thienyl)ethylene. The sample morphology of chemically and electrochemically prepared polymers is quite different. A bulk powder is obtained by the chemical route, while homogeneous films are produced by electropolymerization. Chemical synthesis would seem to be more convenient to prepare polymers because the oxidation with FeCl3 gives standard quality polymers in good yield. Electropolymerization is more sensitive to the synthesis parameters. Electrochemically prepared films are more sensitive to photooxidation [147]. 

Polythiophenes usually form disordered layers, in particular if electrochemical deposition is used [15, 202-204]. Preparation of plasma-polymerized polythiophene leads to disordered layers with comparable structure to electrochemically prepared films [205]. 

Switching properties Chemical properties Mechanical properties of polyaniline Electrochemically Prepared Films Solution Cast Films and Fibers Optical properties of polyanilines Circular dichroism spectra... 

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