Solution-Cast Emeraldine Salt

ES films can be prepared directly from solutions when particular counterions are used to impart solubility. The most extensively studied system is the camphorsul- 

More recent studies have shown the consistency of the crystal structure of PAn/ CSA to be variable and highly dependent on the conditions of film preparation. Sara-vanan and coworkers197 have reported a crystallinity as high as 56% in CSA-doped PAn. Djurado and coworkers198 have speculated that the influence of chain branching during polymerization may play a role in the nature of the crystal structure formed. 

Pron and coworkers199 have investigated the crystal structure of other soluble ESs. In these studies, the solubility of the ES was induced using dialkyl ester sulfonate dopants (15), where R = n-pentyl, n-octyl, n-dodecyl, or other alkyl chains  

FIGURE 4.5 Layered stacking of emeraldine salt chains with ra-alkyl diester dopants separating the chains in the fc-direction. 

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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. 

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 ... 

Polyaniline forms a rich set of structures dependent upon the processing sequence and dopant [18,62,127-132]. Generally, doped polyaniline obtained from solution in the doped (conducting salt) form exhibits a local crystalline order of type emeraldine salt-I, ES-I. In contrast, polyaniline obtained by doping powder or films cast as the base form... 

The band structure of fully protonated emeraldine salt was studied previously by semiempirical molecular orbital (MO) calculations and more recently by ab initio calculation [41,42]. A half-filled polaron band formed via the interaction between separate polarons (there are two polarons per tetrameric repeating unit) was proposed to explain the observed optical and electrical properties of fully protonated PANI-ES. The PANI-HCSA films used in these studies were, however, cast from NMP solutions in the form of EB and then doped into the form of ES. The polymer chains in these polymer films, therefore, have the same conformational structure. 

Emeraldine salt films can be prepared directly from solutions when particular counterions are used to impart solubility. The most extensively studied system is the camphorsulfonic acid-doped ES as cast from a m-cresol solution. Lunzy and co-workers have considered the crystal structure of camphorsulfonic acid-doped polyaniline (PAn-CSA) as cast from m-cresol. Although presenting a clear crystal structure with a degree of crystallinity of 25%, the unit cell structure has no resemblance to the structures formed by the emeraldine base. 

Most fibers and films of PAn have been prepared from a solution of emeraldine base and converted to the emeraldine salt by acid doping. The choice of dopant acid has a profound effect on mechanical properties. In fact, MacDiarmid et al. have shown that the mechanical properties depend in a complex way on dopant, casting solvent and polymer molecular weight. Full details of the effects of polymer structure (as influenced by dopant and solvent) on the mechanical properties are yet to be eludicated. 

PANI is commonly prepared by chemical polymerization of aniline using (NH4)2S20s in hydrogen chloride solution (187) and/or by electrochemical polymerization (188). PANI can exist in four different oxidation states leucoemeral-dine (neutral form), pernigraniline (quinoid-like form), emeraldine base (intermediate form), and emeraldine salt (conductive form), as shown in Figure 7. PANI is prepared as the emeraldine salt and then treated with base to yield the emeraldine base form, which is soluble in common organic solvents. After film casting or... 

The chemical preparation of the emeraldine base form of polyaniline and its protonation to the salt form is described in MacDiarmid et al [20]. Materials were prepared both in the powder form and as films cast from solution. The magnetic susceptibility was measured via the Faraday technique [30] and electron spin resonance. Measurement of the temperature and protonation dependence of the magnetic susceptibility provides a direct probe of the development of a nonzero density of states at the Fermi energy and, hence, a metallic ground state [31]. The temperature dependent magnetic susceptibility, of selected samples of emeraldine as a function of... 

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