Titration behavior and spectral transitions of water-soluble polythiophene carboxylic acids

Water-soluble poly(3-thiophene acetic acid) (P3TAA) and its copolymers with 3-n-methylthiophene or with 3-n-octadecylthiophene were synthesized. The solution properties of these water-soluble polythiophene carboxylic acids were studied by potentiometric titration, viscosity measurements, and UV-visible spectroscopy. We have found that the Henderson-Hasselbach plot of the titration curve showed an inflection of the slope around pH = 5-6. The reduced viscosity and electronic spectra of the P3TAA aqueous solution also showed an abrupt change at this pH region. These results are quite different from that of a common flexible polyelectrolyte, such as poly(acrylic acid) (PAA) and strongly suggest that the pH-induced abrupt conformational changes occurred from the aggregated state to the extended state of the polymer main chain. The observed results are explained in terms of the rigid hydrophobic main chain and the nature of the conjugated polymer.