Electrostatic Effect on the Solution Structure and Dynamics of PEDOT:PSS

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) is a material consisting of two oppositely charged polymers and is available as aqueous dispersions. Various annealing and doping methods dramatically enhance PEDOT:PSS conductivity, rendering it competitive with inorganic conductors like indium tin oxide. Yet a comprehensive understanding of PEDOT:PSS conductivity enhancement remains elusive. To unravel the chief physical interactions at play, we explore aspects of solution-state PEDOT:PSS through light scattering, X-ray scattering, rheological measurement, and the construction of a partial phase diagram. We show these features in neat water and with the addition of several conductivity enhancement agents: dimethyl sulfoxide, ethylene glycol, 1-ethyl-3-methylimidazolium tetrafluoroborate, or sodium chloride. We find that PEDOT:PSS solutions exist as a dispersion of charged, many-chain microgels with structural features and dynamics strongly influenced by electrostatic interactions. Two distinct phase transitions occur with high sensitivity to ionic strength: a coexistence of a dilute and a concentrated PEDOT:PSS phase and a physical gelation. Despite the rich solution properties uncovered here, we find no connection between them and the enhancement of film conductivity.