Competition between aggregate formation and conformational changes of polyaniline in binary solutions studied by electronic absorption spectroscopy

Aggregation and conformational changes of undoped and protonated doped forms of polyaniline (PANI) in the emeraldine oxidation state were studied in solutions of homogeneous organic solvents and their mixtures with water, using optical absorption spectroscopy. It was found that the conformational changes accompany both an increase of concentration of the undoped PANI in the solution and addition of water to the solution. The first process gives rise to an increase in molar extinction coefficient, while the second to their decrease. Expanded-coil to compact-coil conformational changes in PANI via formation of intra-molecular hydrogen bonds in presence of water in the solutions is proposed. Light scattering experiments support this conclusion. Addition of water to the protonated solution also caused expanded-coil to compact-coil conformational changes in protonated PANI. However, state of the polymer is dependent on the preparation history, i.e., which agent, acid or water, was added first to the solution.