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Controlling Mechanical Properties of Well-Defined Polymer Networks

Polymer networks are one of the most versatile and highly studied material class that revolutionized many aspects of life. Connecting the final network properties to the molecular parameters of its building blocks remains a major research thrust. Recent advances in network synthesis techniques allowed for accurate predictions of elastic modulus in model networks. Tew Group has developed highly efficient, thiol-norbornene networks with controllable mechanical properties. Chapter 2 focuses on modifying the gel fracture energy predicted by Lake-Thomas theory by accounting for loop defects. This study allowed for a priori estimates of gel fracture energy by combining theory, experiments, and simulations. Chapter 3 focuses on cavitation rheology techniques that allow for low- and high-strain characterization of model networks. Particularly, cavitation to fracture transition was observed both in needle-induced cavitation (NIC) and laser-induced cavitation (LIC). This transition was proven to be well-controlled by changing the elastofracture length of end-linked networks. These results are expected to inform the behavior of soft tissues and materials when subjected to high strain-rate deformation, which is particularly important in traumatic brain injury. With the increasing need for recyclable materials, covalent polymer networks certainly present a challenge as the bonds between polymer chains are permanent. However, including transient bonds in the network is expected to yield materials that possess the same qualities of a covalent polymer network but can now be reprocessed at the end of their lifecycle. Chapter 4 demonstrates the first example of in-situ crosslinked metal-ligand networks. Their stress relaxation time was characterized by stress relaxation experiments via indentation to overcome the limitations of rheological frequency sweeps. Interestingly, relaxation time was dependent on crosslink density only when the interaction strength was in the intermediate regime. Chapter 5 discusses incorporating of poly(dimethyl siloxane) (PDMS) side chains into the reconfigurable network platform from Chapter 4 to yield metal-ligand bottlebrush elastomers. Addition of PDMS allowed for flexible networks at room temperature that can be characterized by tensile tests. Increasing the crosslink density in these systems led to higher stress at break unlike conventional networks. Advancements in this system will offer next-generation materials such as super-soft and self-healing sensors and actuators.
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