Hierarchically Structured Nanocomposites via a "Systems Materials Science" Approach

CONSPECTUS: Nanocomposite materials can achieve desirable characteristics otherwise unavailable to single component systems, making them attractive platforms to precisely modulate a material's mechanical, electromagnetic, thermal, and optical properties. Because these properties are often dependent on the organization of constituent materials just as much as their relative composition, intentionally programming composite properties requires hierarchical structural control across many length scales. However, the fundamental forces governing the atomic, molecular, nanoscale, microscale, and macroscale composition and structure of a material are all interlinked, and thus must be manipulated simultaneously to properly create ideal designer materials. This fundamental interdependency indicates the need for a "systems materials science" approach to rational nanocomposite design. Much like the fields of "systems biology" and "systems chemistry", a "systems materials science" approach would emphasize emergent connections arising from complex networks of interactions between individual components. In the context of materials synthesis, a systems-level approach would need to consider how structural changes across multiple length scales (chemical bonding, nano- and microstructural evolution, macroscopic geometry) influence one another during all steps of material synthesis and processing. In this account, we highlight our recent work exploring pathways to "systems materials science" inspired design via the development of versatile, programmable, and scalable nanocomposite building blocks. Our group has established a suite of polymer-grafted nanoparticle designs that are inherently composite architectures, containing rigid inorganic cores with dynamic polymer ligand brushes. These building blocks provide molecular and nanoscale handles to dictate particle assembly into higher-order structures by exploiting biomolecular recognition, supramolecular chemistry, nanoparticle synthesis, and an array of different processing conditions. Moreover, they also enable systems-level approaches to material design, as they provide a means of using nano- to macroscale modifications to material structure as a means of altering molecular to nanoscale behaviors. We outline the advancements that have guided our thinking about composite synthesis, underscore key design motifs, and detail how feedback and feedforward mechanisms can govern structure formation at multiple length scales. The contents of this account are organized by length scale, starting with an examination of molecular interactions capable of guiding assembly.This section considers the trade-offs between precision and scalability, culminating in a discussion of strategies which provide a balance of programmability, compositional versatility, and accessibility. We proceed to describe the thermodynamic principles of building block assembly, showing how the resulting nanostructures can be dictated via both composition and assembly environment. Further, we connect molecular and nanoscale design considerations to higher order mesoscale structures (with principal dimensions ranging from hundreds of nanometers to hundreds of microns). We discuss the kinetic factors controlling longrange ordering and the additional variables they overlay on design and assembly techniques. Following this, we discuss macroscale structural features where we emphasize the difficulty in processing and manipulating these materials while maintaining or programmably modulating their nano- and microscale order. We end with an examination of relevant application areas for these hierarchical composites, the implications of recent advances, and key challenges for future research. From this work, we conclude that "systems materials science" will be a critical guiding philosophy to advancing nanocomposite design and development.