Titration behavior and spectral transitions of water-soluble polythiophenecarboxylic acids

Water-soluble poly(3-thiophene acetic acid) (P3TAA) and its copolymers with 3-n-methylthiophene or with 3-n-octadecylthiophene were synthesized. The s olution properties of these water-soluble polythiophene carboxylic acids we re studied by potentiometric titration, viscosity measurements, and UV-visi ble spectroscopy. We have found that the Henderson-Hasselbach plot of the t itration curve showed an inflection of the slope around pH = 5-6. The reduc ed viscosity and electronic spectra of the P3TAA aqueous solution also show ed an abrupt change at this pH region. These results are quite different fr om that of a common flexible polyelectrolyte, such as poly(acrylic acid) (P AA) and strongly suggest that the pH-induced abrupt conformational changes occurred from the aggregated state to the ex-tended state of the polymer ma in chain. The observed results are explained in terms of the rigid hydropho bic main chain and the nature of the conjugated polymer.