Date of Award
Doctor of Philosophy (PhD)
Materials Science & Engineering
This thesis covers three parts of my doctoral research and describes an evolution of intrinsic and extrinsic strategies to manipulate elastic energy and manage stress in functional polymers for various engineering applications. Chapter 2 focuses on tuning bulk and near-surface mechanical properties of hydrogels to foster strong yet reversible adhesion to arbitrary target surfaces, analogous to the naturally observed action of snail epiphragm. At the outset, I synthesize hydrogel precursors and develop conditions for polymerization to tune the material’s intrinsic modulus, especially at the adhesive interface, for sufficient compliance to conformally map rough target surfaces. Whereas intrinsic modulus increases by three orders of magnitude, minimal shrinkage upon dehydration preserves conformal contact with the target surface and stores very little residual stress. This engenders strong shear adhesion courtesy dispersive interactions and mechanical interlocking with a target topography. I design experiments to extract maximal shear adhesion performances and understand underlying adhesion mechanisms and phenomena. Finally, I leverage microscale fabrication techniques to pattern the hydrogel with various surface and bulk geometries for various extrinsic advantages. In Chapter 3, I extend this strategy to other functional polymers with dynamic covalent bonds and shape memory and leverage intrinsic polymer phase transitions to protect topological behavior in mechanical metamaterials. Maxwell lattices can exist in multiple distinct topological states featuring polar elasticity and strongly asymmetric acoustic behavior. However, prior demonstrations of Maxwell lattice-based metamaterials with non-trivial topological mechanical behavior have been limited to either static monoliths or mechanical linkages. I develop a transformable topological mechanical metamaterial (TTMM) made from a shape memory polymer, capable of reversibly exploring its phase space. I propose a kinematic strategy that cascades single uniaxial mechanical inputs at free edge pairs into a biaxial global transformation to reversibly switch between different topological states. I expose the vulnerability of a topologically polarized elastic response to stresses stored during a prior kinematic transformation and proceed to show how intrinsic polymer phase transitions can 'quench' polymer chain mobility and thereby 'cache' these stresses to safeguard a metamaterial’s topological behavior against its kinematic stress history. Finally, in Chapter 4, I pivot towards a predominantly extrinsic strategy of minimizing and managing stress by structuring a functionalized polyelectrolyte polymer into triply periodic minimal surfaces (TPMS) for external-pressure-free high capacity lithium metal anodes. Sheet-based gyroid architectures offer exceptional strength, porosity and surface area per unit volume coupled with excellent manufacturability. Using a highly optimized polymer precursor and μ-scale digital light processing (μ-DLP) additive manufacturing techniques, I create microporous scaffolds that can stabilize and guide lithium metal deposition into dense, dendrite-free morphologies even at high current capacities, without the need for any externally applied pressure.
Christopher Jolly, Jason, "Intrinsically And Extrinsically Modulated Polymer Mechanical Behaviors For Engineered Advantages In Adhesion, Topological Mechanics And Energy Storage" (2022). Publicly Accessible Penn Dissertations. 5370.