Georgia Institute of Technology Materials Science and Engineering

Committee Members:

Prof. John Reynolds, Advisor, CHEM/MSE

Prof. Natalie Stingelin, MSE/ChBE

Prof. Blair Brettman, ChBE/MSE

Prof. Carlos Silva, CHEM/PHYS

"Application-Driven Materials Design Paradigms for Redox-Active, π-Conjugated Polymers"

Abstract:

Among the many applications that capitalize on the electrochemical functionality of π-conjugated polymers, electrochromic devices (ECDs) and organic electrochemical transistors (OECTs) have attracted considerable contemporary interest. The operation of these devices hinges on rapid, reversible electrochemical doping phenomena that effectuate the desired color change (in the case of ECDs) or conductivity modulation (in the case of OECTs). In both cases, such doping processes require simultaneous and coupled transport of electrolyte ions and electrons, the extent of which is intimately tied to a polymer’s chemical composition, complex microstructure, electronic properties, and dynamic mass transfer processes across the polymer-electrolyte interface. Understanding relationships between these factors is critical for informing the development of electroactive polymers and determining viable application spaces. This dissertation aims to explore materials design strategies for tuning the optical and electrochemical properties of active materials in ECDs and OECTs for high-performing, stable electrochemical devices.

First, solution co-processing of dioxythiophene-based copolymers is presented as a straightforward and scalable technique for accessing high-contrast black-to-transmissive electrochromic films with low driving voltages, extended functional lifetimes, and minimal transient chromaticity. This work demonstrates how strategic blend formulation can be leveraged to control the long-term, intermediate, and transient coloration of the achromatic ECDs.

Next, this work seeks to establish a comprehensive understanding regarding the structural factors governing the properties of two distinct classes of aqueous-compatible OECT active materials. A combination of optical, electrochemical, and x-ray techniques are used to probe the redox response, capacitance, solid-state microstructure, and associated device performance of polymers with varying backbone architectures and side chain substitution. In a family of polythiophenes, the length and substitution pattern of the ethylene glycol-based side chain is found to significantly impact the redox stability and capacitance of the active material. Meanwhile, the properties of polar-functionalized poly(3,4-propylenedioxythiophenes) are remarkably less sensitive to side chain length and processing method. These materials show enhanced stability in unbuffered aqueous electrolytes, but they also exhibit markedly less capacitance relative to polythiophene analogues. Furthermore, post-polymerization modification of side chain functionality is presented as a strategy to access hydroxy-functionalized electroactive polymers materials with enhanced redox activity and capacitance in saline relative to glycolated analogues. The results of these studies indicate that underlying structure-property trends in OECT active materials cannot be generalized across material classes.

Finally, this research utilizes a novel application of in situ specular neutron reflectivity (NR) to track actuation and electrolyte uptake by poly(dioxythiophenes) while doping in aqueous media. Contrast-matching methods reveal that electrolyte irreversibly penetrates the polymer film, even prior to application of an electrochemical bias, as indicated by changes in film thickness and neutron scattering length density. Notably, the extent of film swelling and ion distribution in the film depends on the side chain functionality of the parent poly(dioxythiophene). Reflectivity models for extracting thickness and the extent of actuation will be supported by in situ gravimetry and scanning probe techniques. Such work sets a precedent for using NR to study the redox dynamics of electroactive polymers in a more general sense.