PANI-HCSA

More recently, it was found that when a functionalized protonic acid (for example, camphorsulfonic acid, HCSA) was used as the dopant, the resulting complex of emeraldine salt was soluble (in the conductive form) in common nonpolar or moderately polar organic solvents [21]. The conductivities of PANI-HCSA films... 

FIGURE 3 UV-vis-NIR spectra of PANI-HCSA (fully doped, doping level = 100%) solutions in (a) chloroform and (b) m-cresol. 

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. 

B. Conformations of PANI-HCSA with Different Doping Levels... 

FIGURE 6 UV-vis-NIR spectra of PANI-HCSA in m-cresol with different doping levels (a) 20%, (b) 100%, and (c) 200%. 

The conformation of PANl-HCSA in a solution is essentially kept when the solvent is evaporated in casting films. As a result, thin films of PANI-HCSA cast from solvents such as chloroform have a random coil conformation and a shorter conjugation length, whereas films cast from solvents such as m-cresol have a more expanded conformation and longer conjugation length. Polymer chains with a more expanded conformation usually result in a more crystalline structure in forming solid films. 

A. Thin Films of PANI-HCSA Cast from Different Solvents... 

Because the absorption at 2600 nm is characteristic of a more expanded conformational structure and the absorption at 780 nm is characteristic of a random coil conformation, the ratio of the absorbances at these two wavelengths can be used to estimate the relative ratio of these two conformational structures. As shown in Fig. 10, there exists a monotonically increasing relationship between the conductivity and the ratio of the intensity of the free carrier tail to the intensity of the localized polaron for free standing films of PANI-HCSA. This structure-property relationship serves as a criterion in choosing the appropriate solvent to process a conductive polymer into the highly conductive form. In particular, the simplicity of the UV-vis-NIR spectroscopic technique makes this method even more attractive for practical applications,... 

FIGURE 10 Relationship between bulk conductivities of freestanding films of PANI-HCSA (fully doped) and the intensities of the free carrier tail of the corresponding solutions. 

In addition to the enormous changes observed in the UV-vis-NIR spectra, other measurements are consistent with an increase in conjugation length. For example, the surface resistance of the film was reduced fi om —200 to 2 kft/square and the film became insoluble even in boiling chloroform after exposure to m-cresol vapor. These observations implied that the crystallinity of the film was increased after exposure to m-cresol vapor. Similar results were also obtained when a thin film of PANI-HCSA spun from a solution in NMP (—2% w/w) was exposed to m-cresol vapor at room temperature in a similar way (Fig. 1 lb). It was also found that other solvents such as /7-cresoI, 2-chlorophenol, 2-fluorophenol, and 3-ethylphenyl had the same function as m-cresol in this process. 

Structure development in blends of poly aniline doped with camphor sulfonic acid (PANI-HCSA) and PA was studied by Hopkins and co-workers (206) using SAXS and SANS. At 3 vol% PANI loading concentration, salt domains with characteristic length of 22 nm are expected to be present in the blend with PA12. This differs from the blend of PAG where a fractal geometry was foimd. For higher salt concentrations, no simple structural model was found. 

Films of optically active polyaniline salts such as PAni(+)-HCSA, or the optically active EB derived from them, have recently been shown - to exhibit discrimination toward chiral compounds such as the enantiomers of CSA and amino acids. The studies suggest that these novel chiral materials may indeed have potential for the separaration of enantiomeric chemicals, such as chiral drugs. 

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