(412b) Designing Radical Polymers for Solid-State Electronics and Electrochemical Devices

Boudouris, B. W., Purdue University
Electronically-active polymers continue to impact emerging technology landscapes in myriad device applications. The oft-utilized design paradigm associated with conducting polymers is one rooted in molecules containing large degrees of conjugation along their backbones and the inclusion of chemical dopants that serve to change the oxidation state of the polymers. However, moving from this archetype and towards one with a single-component, charge-neutral macromolecule has significant fundamental and practical benefits, as this type of macromolecular conductor should result in materials that can be designed in a more straightforward fashion and should have longer stability when implemented in devices. In order to address this need, we have designed redox-active macromolecules based on a radical polymer (i.e., macromolecules that are composed of non-conjugated backbones and have pendant groups that contain open-shell entities) design motif. Despite lacking any type of conjugation or crystalline domains, these macromolecules demonstrate high electrical conductivity values.

Specifically, we synthesized poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PTEO) to form a radical polymer with a flexible macromolecular backbone and a relatively low (i.e., near-room temperature) glass transition temperature. In generating a redox-active macromolecule with a flow temperature far-removed from the degradation temperature of the radical group, thermal annealing of the radical polymer thin film resulted in the formation of localized domains with rapid charge exchange reactions. This occurred despite the fact that the polymer thin film remained completely amorphous after the thermal annealing procedure. Because this material obtains local order in the glassy state, a greater than 1,000-fold increase is observed in the electrical conductivity of PTEO relative to all other reports of electrical conductivity in radical polymers. Moreover, the ultimate conductivity of ~20 S m‑1 places this undoped polymer conductor in the same regime as many grades of common commercially-available, chemically-doped conducting polymers. Also, because these materials contain no conjugated bonds, the optical absorption of these materials is weak, making radical polymers excellent transparent conductors. These types of electrical conductivity and optical transparency values open myriad device opportunities for applications that require a transparent conducting electrode that allows for large amounts of photons to pass in to or out of the device (e.g., display technologies). Finally, we demonstrate that PTEO is useful when incorporated into an organic electrochemical transistor (OECT) geometry given the high density of redox-active sites associated with the open-shell macromolecule, and that this device structure is useful in bioelectronic applications. Thus, this work implements a rarely-used macromolecular design paradigm in order to generate a valuable electronically-active and electrochemically-active macromolecule.