Emeraldine films

Polyaniline was first prepared at the turn of the century. Several oxidation states are known. The conductivity and the color of the material vaiy progressively with oxidation. Only one form, however, known as the emeraldine salt, is truly conducting. The material can be prepared readily by either electrochemical or chemical oxidation of aniline in aqueous acid media. Common oxidants, such as ammonium peroxydisulfate, can be used. Flexible emeraldine films can be cast from solutions of A methylpyrrolidone and made conductive by protonic doping. This is done by dipping the films in acid or exposing them to acid vapors. The process results in protonation of the imine nitrogen atoms ... 

The significant change in the morphological and physical properties of emeraldine films observed on... 

As cast emeraldine films (obtained from a solution of emeraldine base in A-methyl pyrrolidone NMP) are diamagnetic due to the absence of a charge carrier, unlike this film on doping shows paramagnetism. The total magnetic susceptibility of this film on doping shows paramagnetism and this is due to temperature-independent Pauli susceptibility and temperature-dependent Curie susceptibility. 

Figure 5-19. N(ls) XPS core level spectra of emeraldine base adsorbed on ITO. The top most spectrum corresponds to ultra-thin Him (in the mono layer regime) while the bottom spectrum corresponds to thick film.

Zengin et al. [41] characterized a polyaniline (PANI)/MWNT composite. The FTIR spectra of the composite film show benzoid and quinoid ring vibrations at 1500cm-1 and 1600 cm-1, respectively, which indicate the presence of emeraldine salt (ES) of polyaniline. A weak broad band near 3400 cm-1 is assigned to the N—H stretching mode. The strong band at 1150cm-1 is characteristic of PANI conductivity. The FTIR spectrum of PANI/MWNT composite in the ES form exhibits several clear differences from the spectrum of neat ES PANI (1) the composite spectrum shows an inverse... 

Results and Discussion. The 2-ethyl polyaniline concentration in the silica gel film was determined by constructing a Beer s law calibration curve from solutions of known concentration. Assuming an average molecular weight of 5000, the 2-Et PANi concentration in the silica gel was found to be 9.6 x 10 4 M. The refractive indices of CS2 and 2-Et PANi SiC>2 were estimated to be 1.6 and 1.4 at 1.06 im, respectively. The emeraldine base doped silica gel was found to have low losses due to scatter, and exhibited good transparency at 1.06 im. Spectrophotometric measurements at 1.06 fim yielded absorption coefficients of 0.1 cm-1 (> 99% T over 1 mm pathlength) for the CS2 reference and 4 cm 1 (96% T over 1 mm pathlength) for the 2-Et PANi doped silica film. 

Polyaniline in the emeraldine base state doped with di(butoxyethoxyethyl) ester of sulphosuccinic acid had high film conductivity and an elongation at break of 195%. This high flexibility is particularly needed for elastomer coatings to impart elasticity on conductive materials. 

TABLE 1. Effect of selected dopants on the film conductivity and elongation at break for polyaniline (emeraldine base). 

PAni has a very complex structure and doping behaviour, see Fig. 9.6, and the spectra are sensitive to the polymer morphology, the level of oxidation and degree of protonation. This accounts for the considerable variation in tire spectra that have appeared in the literature. The effects are illustrated in Fig. 9.33 for various forms of the protonated salt. These spectra refer to dried films, electrochemically prepared at different electrode potentials, and subject to oxidation by exposure to air. This variation in preparation conditions means that the degrees of oxidation and protonation are not well defined, as evidenced by the pronounced differences in the spectra of the emeraldine prepared at the... 

This alternative approach has also been successfully employed to produce optically active polyanilines. The use of optically active dopant anions such as (H-) - or ( - ) - camphorsulfonate (CSA ) [37-39], (-i-)-or( —) - tartrate [40] and related chiral anions induces macroasymmetry in to the polyaniline chains. We [41] and others [42] have recently shown that films of optically active polyaniline salts such as PAn( -1- )-HCSA, or the optically active emeraldine base (EB) derived from them, exhibit chiral discrimination towards chiral chemicals such as the enantiomers of CSA and amino acids. 

The UV-Vis spectrum of nanostructured PDMA-PSS film shows a strong peak in the region around 800 nm due to the polaron —> band transition for emeraldine salt form of... 

Polyanilines (Scheme 36) are conjugated polymers whose it electrons are delocalized over the whole molecule. They are important conducting polymers that also act as semiconductors, in a similar manner to inorganic semiconductors121 m. They are made by chemical or electrochemical (anodic) oxidation of aniline. The product, a poor textile colorant, dates from the 1860s, and is still known by the name given at that time, emeraldine. In the electrochemical process, it is possible to produce thin films directly on conductive substrates. Polyanilines have been used in photoelectrochemical devices124-126. 

The most popular method of processing is based on limited evidence that the reduced or de-doped form is soluble. It is widely applied to PANi based on extensive experimental studies by MacDiarmid and his group. In brief, a solution of the de-doped and neutral form of the CP (the emeraldine base) in an organic solvent is used to coat a substrate and the cast film is chemically doped after drying. 

A significant development in PAn chemistry was our report26,27 of the first optically active PAn s, prepared through the electrochemical polymerization of aniline in the presence of the chiral dopant acids (+)- or (-)-10-camphorsulfonic acid (HCSA). Films of the emeraldine salts PAn/(+)- HCSA and PAn/(-)-HCSA can be... 

A remarkable recent example of the influence of electropolymerization temperature on the properties of PAn products is observed in the potentiostatic polymerization of aniline in the presence of chiral (+)-HCSA to give optically active PAn/(+)-HCSA films. The CD spectra of the polymers electrodeposited at < 25°C were inverted compared to the spectra of analogous emeraldine salts deposited at > 40°C, indicating an inversion of the preferred helical hand for PAn chains.38 The observations may be rationalized in terms of a temperature-induced interconversion between two initially (kinetically) formed diastereomeric PAn products. 

The mechanical properties of PAn differ considerably between the electrochemi-cally prepared polymer and that produced from solvent casting. As described earlier, electropolymerized emeraldine salts are highly porous and, consequently, have low mechanical strength. Freestanding films may be prepared electrochemically, but their poor mechanical properties limit their usefulness. In contrast, the polymers made from solution are much less porous and are widely used as freestanding films and fibers. The effect of polymer structures and morphology on PAn mechanical properties are described in the following text. 

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... 

It was noted by MacDiarmid and Epstein as early as 1989 that PAn salts may also be deposited as films on a variety of substrates by immersing the substrate in the polymerization mixture.29 In fact, during standard chemical polymerization, one often observes the deposition of a thin, extremely adherent green emeraldine salt (ES) film on the walls of glass reaction vessels, as well as the bulk precipitation of PAn/HA powder. By judicious manipulation of the polymerization conditions such as reagent concentrations/ratios and modification of the substrate surface, one can maximize the surface deposition as opposed to polymer precipitation.30 This phenomenon has been developed into a widely useful in situ polymerization technique for the preparation of PAn films on a variety of insulating surfaces such as glass and plastics, as well as on fibers and fabrics. 

HCl-doped to the emeraldine salt. Similar increases in elastic modulus would be expected to result from the mechanical drawing, because the process causes considerable alignment of the polymer chains in the draw direction. In one study, drawn EB films gave a room temperature modulus of 12 GPa,84 which is considerably higher than is typical of unoriented polymers. Such mechanical orientation also increases the crystallinity84 and conductivity of the PAn films and fibers. 

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