Structure and Thermodynamics of Neutral and Charged Block Copolymer-Based Materials

Abstract

Next-generation materials are often required to exhibit two (or more) orthogonal properties simultaneously. One example is polymer electrolytes, as both facile ion transport and mechanical robustness are desired. However, these orthogonal properties are hard to achieve in single-component systems, because ion transport usually requires high chain mobility while high chain rigidity or low chain mobility is desired for mechanical stability. One way to overcome this challenge is to develop co-continuous nanostructured materials, such that one domain provides ion transport while the other imparts mechanical robustness. A promising predictable and tunable co-continuous structure is the bicontinuous microemulsion from ternary blends of an AB diblock copolymer and the corresponding A and B homopolymers. However, the structure and thermodynamics of such ternary mixtures are not fully elucidated, even in the limit of neutral ternary blends. Moreover, little is known about ion-containing ternary blends. Therefore, the focus of this thesis work is to understand the fundamental phase behavior of these systems and to ultimately provide insight into the rational design of functional materials. In Chapter 2, we investigate the phase behavior of neutral ternary blends comprising a linear diblock copolymer and the corresponding homopolymers. The impacts of block copolymer compositional asymmetry on ordered, disordered, and macrophase-separated regions of the ternary phase prism are discussed. In Chapter 3, we expand the research to ternary mixtures involving a bottlebrush diblock copolymer and the corresponding linear homopolymers. The overall phase behavior closely resembles that of linear ternary mixtures, except for an unconventional spatial distribution of the homopolymers. Chapters 4 and 5 focus on the self-assembly of charged diblock copolymers, serving as the starting point for the investigation of charged ternary blend phase behavior. Chapter 4 details the phase behavior of a series of symmetric charged diblock copolymers, where the effective interaction parameter was found to increase linearly with the increase in charge fraction. Chapter 5 extends the work to a different model system with a relatively nonpolar charged block. A tilted, “chimney”-like order-disorder transition boundary was observed. However, the composition windows of the ordered phases remain nearly unchanged. Overall, the findings from this thesis research provide valuable insight into the structure and thermodynamics of neutral and charged polymer mixtures, and will inform the rational design of nanostructured polymer electrolytes with tunable structure and properties.