Polyanilines conformational structures

Hydrogen bond formation as a result of the interaetion of polymer with solvent was found to contribute to changes in the electric properties of polyaniline.Hydrogen bonding causes changes in conformal structure of polymer chains. This increases the electrical conductivity of polyaniline. Water is especially effective in causing such changes but other hy-... 

Ito and co-workers prepared pseudopolyrotaxanes from polyaniline with emeraldine base and /f-cyclodextrin, which were studied by frequency-do-main electric birefringence (FEB) spectroscopy in a solution of N-methyl-2-pyrrolidone (NMP), scanning tunneling microscopy, etc. [120, 121]. The FEB results showed that polyaniline in the solution with cyclodextrin changed its conformation from coil to rod at low temperature below 275 K. Some rod-like structures were observed on a substrate by STM. Thus, the pseu-... 

Figure 8.1 Conformational changes of molecular structure, (a) photoisomerisation of azobenzene, (b) and (c) extension and contraction of polyacetylene and polyaniline, respectively, upon oxidation and reduction...

The fundamentals of electrolytic expansion in polyaniline films have been discussed. Ion insertion and exclusion by electrolytic oxidation and reduction are the primary mechanisms. However, it is also evident that the changes in molecular conformations, arising due to the delocalisation of 7t-electrons and the electrostatic repulsion between the polycations, are other mechanisms operating in a conducting polymer microactuator. By investigating the molecular structure and the higher order structure to optimise the electrolytic expansion, it should be possible to improve the expansion ratio and the force for practical usage. 

Aside from considerations of the polymer form itself, polyaniline can be doped and derivatized in a variety of ways. First, polyaniline can be polymerized in the presence of a variety of acids, which critically influences the resulting electronic properties [1-15]. The particular acid used and polymerization process employed can affect the degree of crystallinity observed [10-15,17-29]. Multiple dopants and substitutions have been achieved in the hope of increasing both the conductivity and solubility of these materials. The derivatives are simple polyanilines functionalized with complex ions such as aryl-SOj, camphorsulfonates, and perfluoroalkyl (and aryl) sulfonates. Dopants vary from simple anions, to oxyanions, to the more typical iodide ions [10-15,17-29,38-44]. The functionalization and/or doping affects, the band population, and the polyaniline chain conformation in turn influence the resulting electronic and structural properties of the polymer. 

Figure 8 shows UV-vis-NIR spectra of thin films of HCSA fully doped polyaniline emeraldine salt that were spin coated on quartz plates from solutions in chloroform and m-cresol, respectively. As discussed previously, different polymer conformations are responsible for these two totally different UV-vis-NlR spectra. Figure 8a indicates a random coil conformation for the polymer chains the three distinctive absorption peaks at 360,440, and 780 nm are consistent with an electronic structure... 

Comparison of chemically and electrochemically prepared PAn films Until recently, it has been generally considered - that PAns prepared by the alternative chemical and electrochemical routes have similar chemical structures, although differences in morphology were noted and there was disagreement in the literature as to which route produces material of the higher molecular weight. From a comparison of the circular dichroism spectra of chemically and electrochemically prepared PAn.(+)-HCSA films (see Chapter 5), we have recently found the first unequivocal evidence that these polyanilines possess different structures/conformations for their polyaniline chains. ... 

Other phenolic compounds have also been shown to cause similar changes in conformation and physical properties of PAn.HCSA salts. Most of these, like m-cresol 1, are highly corrosive and toxic, limiting their desirability for processing and enhancing the electrical conductivity of polyanilines. However, we have recently found that the structurally related molecules thymol 2 and carvacrol 3, which are much less toxic than m-cresol, can also function as effective secondary dopants for PAn.( )-HCSA films and increase the electrical conductivity by up to two orders of magnitude. ... 

Dependence of UV-visible-NIR spectra on chain conformation. In general, conjugated polymers such as polyaniline show a strong coupling between their electronic structure and geometric features such as the polymer chain conformation. A number of theoretical studies have examined the influence of polyaniline chain conformation, and, in particular, the role of phenyl and phenyl/quinoid torsional angles along the chain, on the electronic structure of PAn (and consequently their electronic absorption spectra). These include semi-empirical calculations by Br das et and by de Oliveira et al. ... 

---