Title: Electrostatic Interactions in Solutions of Charged Polymers: Conjugated Polyelectrolytes and Block Polyampholytes

Advisors: Glenn Fredrickson and Rachel Segalman

Abstract: 

Leveraging the electrostatic self-assembly of charge-containing polymers has the potential to be a powerful tool for developing nanostructured functional materials. However, these polymers are impacted by a multitude of competing effects on their thermodynamics and structure in solution: charge—charge interactions and excluded volume or solvent quality effects, which depend on not only the overall polymer properties (e.g., chain length, overall charge density) and solvent properties (e.g., dielectric constant) but also the specifics of polymer chain connectivity, sequence, shape, and stiffness. In this development, we have focused on two classes of charge-containing polymers as case studies for expanding the fundamental understanding and applications for these functional materials: π-conjugated polyelectrolytes (CPEs) and block polyampholytes. 

In solutions of CPEs, the interplay between chain stiffness, aromatic (i.e., liquid crystalline or π-π interactions), and charge correlations, is dominated by electrostatic interactions and results in network formation at low concentration. These CPE soft-solids are thermoresponsive, allowing for processing at moderate temperatures, and remain semiconducting, yielding mixed electronic and ionic conductivity. This dominance of electrostatic interactions is then utilized to develop design rules for alternatives to PEDOT:PSS based on the polyelectrolyte complexation of a CPE with an oppositely charged polyelectrolyte. The polyelectrolyte complexes—particularly those resulting from coacervation, a liquid-liquid phase separation—are dense in the electroactive material and enhance the photophysical properties.

In solutions of block polyampholytes, design rules are determined for predicting the self-coacervation phase behavior as a function of sequence of the polyampholyte and character of the small ions. The length of like-charged blocks is shown to have a strong effect on the accessible chain conformations and propensity to phase separate as the block length impinges on the electrostatic correlation length. Small ions only weakly affect the structure and thermodynamics of nearly charge neutral block polyampholytes; as the charge imbalance on the block polyampholyte increases, however, the liquid-liquid phase separation is suppressed at low salt content as "tadpole" configurations are stabilized in dilute solution.