Year

1999

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

This thesis describes circular dichroism (CD) and uv-visible-near infrared studies on optically active polyaniline salts doped with (+)- or (-)-camphorsulfonic acid (HCSA), which provide new insights into conformation changes in the polymers caused by the synthetic route, temperature and solvent. The chiroptical properties of optically active PAn.(+)-HCSA films prepared electrochemically were found to be distinctly different to those of films prepared chemically via the acid doping of emeraldine base with (+)- HCSA. The results indicated two different conformations for the polyaniline chains, assigned as "extended coil" and "compact coil", respectively. This is the first time that different conformations have been unequivocally demonstrated for the polymer chains of electrochemically and chemically prepared polyaniline.

A striking thermochromic effect was induced by heating an electrochemically deposited PAn.(+)-HCSA film to 140°C for 10 min. The conformation of the polyaniline chains was shown from CD and uv-visible-near infrared spectral studies to convert from an "extended coil" to a "compact coil" conformation. This change in conformation was accompanied by a decrease in the electrical conductivity of the film. These effects were not reversed upon cooling. Further heating the electrochemically deposited (or chemically cast) optically active PAn.(+)-HCSA films beyond their glass transition temperature resulted in progressive thermal de-doping to form racemic emeraldine base.

The PAn.(+)-HCSA emeraldine salts formed by doping emeraldine base with (+)-HCSA in a range of organic solvents (NMP, DMSO, DMF and chloroform) have been shown to adopt a "compact coil" conformation. The position of the high wavelength, localized polaron absorption band and the corresponding bisignate CD bands underwent a significant red shift, which has been attributed to the slow de-aggregation of the polyaniline chains in solution. In contrast, the dissolution of electrochemically deposited PAn.(+)-HCSA films in these organic solvents or benzyl alcohol resulted in the polymers largely retaining their original solid-state "extended coil" conformation.

The conductivity of chemically cast PAn.(+)-HCSA films was found to increase by one and two orders of magnitude, respectively, after exposure to thymol and carvacrol vapour. This was accompanied by the development of a free carrier tail in the nearinfrared region, which supports the "secondary doping" of the polyaniline chains by both thymol and carvacrol vapours. These new "secondary dopants" have the advantage of being less toxic than the previously studied m-cresol. The alternative doping of emeraldine base with (+)-HCSA using molten thymol or liquid carvacrol as solvent also resulted in the "secondary doping" of the PAn.(+)-HCSA salts formed. Their uv-visiblenear infrared spectra suggested that the polymer adopted a partial "extended coil" conformation. However, their CD spectra indicated that the majority of the polyaniline chains was still in the "compact coil" conformation. Related studies with the dye Thymol Blue showed that while its sulfonate group readily doped emeraldine base dissolved in NMP, DMSO and DMF, the dye did not cause "secondary doping" despite the presence of the thymol group in its structure.

The preparation and chiroptical properties of optically active polyaniline in each of five accessible redox and pH states have been reported for the first time, namely: leucoemeraldine base, emeraldine base, emeraldine salt, pernigraniline base and pernigraniline salt. The new chiral materials were obtained via the chemical reduction or oxidation of optically active PAn.(+)-HCSA or via its de-doping with NH4OH . The chiroptical properties of the optically active polymers so derived depended on the conformation of the precursor emeraldine salt employed, with different behaviour being found with electrochemically deposited ("extended coil") and chemically deposited ("compact coil") PAn.(+)-HCSA precursor films.

This thesis also describes three new synthetic routes to improve the processability of optically active polyanilines, which has been one of the limitations restricting their applications to date.

(i) A simple, new method for the in situ deposition of optically active PAn.(+)-HCSA and PAn.(-)-HCSA films from aqueous solutions of aniline undergoing oxidative polymerization with ammonium persulfate in the presence of (+)- and (-)- camphorsulfonic acid has been developed. The chiroptical properties of these in situ deposited emeraldine salt films indicated that they possess neither a pure "compact coil" nor a pure "extended coil" conformation, but may have an intermediate conformation. De-doping of the in situ deposited films with aqueous 0.1 M NH4OH gave optically active emeraldine base films, which could be re-doped with HC1 vapour or aqueous 1.0 M HC1 to generate optically active PAn.HCl films with retention of polymer configuration.

(ii) Novel chiral polyaniline colloids have been generated via the electrohydrodynamic polymerization of aniline in aqueous (+)- or (-)-HCSA using polystyrene sulfonate (PSS) as a steric stabilizer and co-dopant. The chiroptical properties of these effectively "water soluble", colloidal polyaniline dispersions indicated that the polymer was synthesized largely in the "compact coil" conformation. The optical activity of the colloidal dispersions increased markedly upon standing for 24 hr, indicating that the asymmetric rearrangement of the polyaniline chains induced by the chiral HCSA dopant occurs much more slowly than the initial doping process. Interestingly, the optically active fractions of polyaniline colloids PAn.(+)- HCSA/PSS were remarkably inert to chemical oxidation or reduction and alkaline de-doping. Related studies using polyethylene oxide as an alternative steric stabilizer showed that it was inferior to PSS', with significant over-oxidation of the polyaniline apparent.

(iii)The first reported synthesis of optically active poly(ο-methoxyaniline) has been achieved via the enantioselective polymerization of ο-methoxyaniline from aqueous solutions containing (+)- or (-)-camphorsulfonic acid. The dark green films of the POMA.(+)-HCSA and POMA.(-)-HCSA emeraldine salts exhibited mirror imaged CD bands in the visible region, confirming macroasymmetry of the polymer chains and enantioselectivity in the chiral induction. The steric influence of the methoxy substituent caused the POMA.(+)-HCSA and POMA.(-)-HCSA films to adopt a "compact coil" conformation, in contrast to the "extended coil" conformation for similarly deposited parent PAn.(+)-HCSA salts. Significantly, for processing purposes, these electrochemically prepared POMA.HCSA salts are highly soluble in a range of organic solvents such as DMF, DMSO, NMP,CHCl3 and methanol. They adopt the same conformation in each of these different solvents, but a large red shift in the high wavelength polaron absorption band suggests an increase in the conjugation length of the polymer chain along the solvent series NMP < CHCI3 < DMSO, DMF < methanol.

Deprotonation of the electrochemically deposited POMA.(+)-HCSA film using 1 M NH4OH resulted in the formation of an optically active methoxy-substituted emeraldine base film, which could be re-doped with aqueous 1.0 M HC1 to generate optically active POMA.HC1 films with retention of the main chain chirality of the polymer backbone.

Optically active poly(ο-methoxyaniline) could also be synthesized in solution via the acid doping of methoxy-substituted emeraldine base with (+)-HCS A in DMSO solvent. However, the generation of optical activity in the polymer was substantially slower than for the corresponding parent polyaniline, taking several days to fully develop. The extent of aggregation of the doped POMA.(+)-HCSA salts in solution was highly dependent on the molecular weight of the methoxy-substituted emeraldine base.

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