UC Santa Barbara

Semiconducting conjugated polymers (CPs) are an important class of organic electronic materials due to their mechanical flexibility, molecular design versatility, and compatibility with inexpensive and scalable processing methods. However, the limited solubility of CPs in most solvents and their melt intractability remain one of the key challenges in harnessing their optoelectronic performance. In particular, these processing limits hinder the fabrication of CPs into thick films and bulk structures that are required in a wide range of applications, such as actuators, bioelectronic scaffolds, or thermoelectric modules.

Complexation between two oppositely charged polyelectrolytes offers unique opportunities for solvent-lean processing of semiconducting polymers, facilitated by the formation of a polymer-dense fluid phase known as the coacervate. Moreover, the electrostatic interactions in these systems provide additional handles for controlling the material’s structure and properties. Despite extensive investigation on coacervate of two insulating polyelectrolytes, studies focusing on the coacervate where at least one of the components is conjugated are sparse. The processability of the conjugated coacervate and the solid-state properties of the resulting polymer complex are still not well understood.

This thesis investigates how electrostatic interactions can be leveraged to design advanced semiconducting polymeric materials by developing fundamental understanding on the structure – processing – property relationships of charged conjugated polymer complexes. First, we demonstrate how electrostatic attraction between a conjugated polyelectrolyte (CPE) and an oppositely charged insulating polymeric ionic liquid can be utilized to formulate a conjugated coacervate with notably high polymer loading. We show that this coacervate can be easily blade coated to fabricate ?m-thick films, a task that is otherwise challenging to achieve with conventional solution casting of semiconducting polymers. We then look into the solid-state structure of charged conjugated polymer complexes and examine how electrostatic interactions can be leveraged to control the self-assembly of this material. We find that manipulation of electrostatic parameters, including polymer charge fraction and counterion concentration, can adjust the morphology of these polymer complexes from a homogeneously mixed state to a weakly structured state in which the local ordering arises from backbone-immiscibility-induced segregation. Subsequently, we elucidate how this structural evolution influences the local order and interconnectivity of the CPE chains within these complexes. Our findings demonstrate that the structural disorders along the CPE backbone is alleviated in strongly mixed complexes. Charge transport on the other hand is improved in all morphologies, indicating the enhancement in the long-range connectivity of the CPE upon complexation. These studies suggest electrostatic interactions as an effective handle for controlling the structure and properties of charged conjugated polymer complexes to obtain targeted optoelectronic performance. Finally, we utilize the attractive electrostatic interactions to effectively combine conjugated and bottlebrush polyelectrolytes. The resulting material is a phase-compatible complex that is soft, stretchable, elastic, and conductive. This study emphasizes ionic complexation as an exciting pathway for engineering multifunctional polymeric materials.