Multiscale Design, Fabrication, and Mechanical Analysis of Structural Hierarchies in Functional Materials

Citation

Lee, Seola (2025) Multiscale Design, Fabrication, and Mechanical Analysis of Structural Hierarchies in Functional Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1ee0-bc98. https://resolver.caltech.edu/CaltechTHESIS:03312025-221040469

Abstract

Hierarchical structuring has emerged as a powerful strategy in functional material design to enhance mechanical performance and impart functional properties across multiple scales. In architected materials, leveraging tessellated multiscale geometrical features enables unconventional properties such as ultra-low density and high energy absorption. Similarly, in functional polymers, rational design of molecular chemistries and polymer microstructures allows for tunable mechanical properties and stimuli-responsive behaviors. However, a substantial knowledge gap persists in understanding how multiscale interactions connect to determine the macroscale performance of these materials. This gap arises from challenges in scalable fabrication, multiscale characterization, and limited mechanistic insight from theory and simulations. To address these challenges, this thesis presents a comprehensive approach that integrates scalable fabrication, multiscale characterization, and theoretical modeling to develop hierarchical materials with tunable functionalities. Specifically, we (1) demonstrate scalable fabrication of hierarchical materials using additive manufacturing, (2) investigate the bulk mechanical responses by tuning the smallest level in hierarchical design, and (3) perform multiscale studies to bridge the gap between unit-level interactions and macroscale performance.

In the first study, we explore the role of structural hierarchies in architected polymeric materials for enhanced energy dissipation. Using metasurface-based holographic lithography, we fabricate nano-architected polymeric sheets and demonstrate how geometrical parameters for unit cell design such as relative density and beam aspect ratio influence stiffness, energy dissipation, and deformation modes. These findings highlight the significance of hierarchical structuring in enhancing mechanical performance and establish design principles for scalable manufacturing. In the second study, we focus on dynamic polymers and examine how dynamic crosslinking at the molecular level influences macroscale material responses. A single-step stereolithography approach is developed to tune molecular-level controls in the material. Through multiscale modeling and experimental characterizations, we reveal how dynamic bonding mechanisms govern stiffness, stretchability, and fracture energy. The results underscore the significance of multiscale interactions in tuning mechanical behavior and suggest a pathway for designing materials with programmable responses. In the final study, we build on these insights by integrating molecular-level controls with nonlinear structural responses such as buckling and shape transformations. The central premise is that molecular interactions dictate local responsiveness, while structural geometries can amplify or suppress these responses through localized deformation or stress redistribution. As a demonstration, we explore how tailored viscoelasticity and controlled instabilities can determine the buckling mode of a structural beam. This synergistic interplay highlights the potential of the materials requiring programmed reconfigurability, shape morphing, and stimuli-responsive properties.

The findings presented in this thesis offer a robust framework for bridging molecular-level design with macro-scale performance through scalable fabrication and characterization strategies. By expanding the design space of material-level behavior, this work lays the groundwork for developing next-generation materials with enhanced functionality, adaptability, and intelligence for applications such as soft robotics, healthcare, and sustainable materials.

Thesis Files