POLYANILINE DISPERSIONS .2. UV-VIS ABSORPTION-SPECTRA

Optical absorption spectra of polyaniline dispersions in aqueous media have been investigated. Two absorption bands located at 340 and 610 nm are found for blue polyaniline base in ammonia solutions. The spectra of alkalized dispersions are close to those observed for polyaniline base in N-methylpyrrolidone solution (maxima at 332 and 638 nm), but the position of the maxima depends on the nature of the alkali. In the acidic region, three distinct absorption bands of the green protonated polyaniline are observed at 350, 430 and 810 nm. The spectroscopic response to change of pH is fast but additional slow changes occur and may extend for hours when close to neutral media. The absorption coefficient of polyaniline in 1 M hydrochloric acid for a local maximum between 380 and 420 nm is estimated as epsilon = 31 500 +/- 1700 cm2 per gram of polyaniline hydrochloride. Spectroscopy is used to follow up the dispersion polymerization of aniline. After an induction period, a fast polymerization takes place and the originally colourless solution turns blue. A subsequent change of colour to green indicates termination of polymerization.