Conducting Polyelectrolyte Complexes: Assembly, Structure, and Transport

Decades of progress have yielded a tremendous variety of organic electronics, with great strides in the development of photovoltaics, thermoelectrics and other flexible devices. Ubiquitous in these research areas are films of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT: PSS), a complex of oppositely-charged polyelectrolytes initially suspended in water before film formation. This material has high electronic conductivity and good water processability. Pristine film conductivity is somewhat low, but is dramatically enhanced through simple treatments like ionic liquid addition or shear. Can this enhancement be understood so that further optimization might render PEDOT: PSS commercially viable? PEDOT: PSS is a complicated material, with electrostatic complexation between PEDOT and oppositely-charged PSS, dissociated counterions and an inherent insolubility of PEDOT in water. These characteristics among others muddle the already challenging task of understanding the film formation process. In this doctoral thesis work, the goal is to build on our fundamental understanding of PEDOT: PSS and conducting polyelectrolyte complexes in general. The structural aspects of PEDOT: PSS dispersions are studied upon the addition of four conductivity enhancers: EMIM BF4, NaCl, DMSO and EG. PEDOT: PSS collects into many-chain charged micro-gels that are hundreds of nanometers in scale. An observed sensitivity to ionic strength underscores the dominance of electrostatic forces in PEDOT: PSS solutions. Micro-gels can macroscopically percolate or phase segregate, much like associating polymers. PEDOT: PSS conduction predominatly occurs electronically in films and ionically in solutions. When the four enhancers are introduced, no correlation is found between changes to film conductivity and changes to solution phenomenology. This apparent lack of correlation strengthens the widely-held belief that conductivity enhancement is closely linked to PEDOT ordering. Langevin dynamics simulations show that PEDOT clusters into stacked domains at high polymer concentration and ionic strength, and this clustering can be explained as an interplay between hydrophobic and electrostatic drivers. A new theory of polyelectrolyte complex phase separation is proposed, and it relies on induced dipoles formed from the association of oppositely-charged backbones. It predicts the phase behavior for model systems, but does not apply directly to PEDOT: PSS. Nevertheless, it gives insight into the role of dipoles for complex coacervation