Molecular design of a fatigue-resistant and energy-dissipative hydrogel

2022 Summer.Includes bibliographical references.Hydrogels at the most basic iteration are cross-linked polymer networks swollen in water. They show promise in biomedical applications due to their high water content and flexibility. However, intentional design of new hydrogel networks by modification of the choice of polymer, the fabrication of the polymer network, and the choice of cross-link have resulted in hydrogels which have useful properties ranging from fatigue resistance to elasticity to bulk toughness. Of particular interest is a hydrogel which can dissipate energy as a way to resist failure of the polymer network. Unfortunately, many of the design strategies previously used to insert an mechanism for energy dissipation into the hydrogel result in hydrogels which are not elastic or their mechanical properties fatigue throughout multiple cycles of use. Therefore, our goal was to design a hydrogel network that is able to both dissipate energy and be resistant to fatigue of mechanical properties. This design strategy is based on the self-assembly of blends of ABC and ABCBA block polymers, specifically polystyrene-b-polyisoprene-b-poly(ethylene oxide) (PS-PI-PEO, SIO) and polystyrene-b-polyisoprene-b-poly(ethylene oxide)-b-polyisoprene-b-polystyrene (PS-PI-PEO-PI-PS, SIOIS) into a sphere morphology where the A block is spheres of glassy, hydrophobic polystyrene surrounded by the B block of rubbery, hydrophobic polyisoprene as the surface of the sphere. These AB spherical domains sit in a matrix of the C block, poly(ethylene oxide). The spherical domains are tethered together by the SIOIS polymer so that the glassy spheres are evenly-spaced physical crosslinks in the polymer network. The tethered spheres provide the network with elasticity and fatigue resistance while the hydrophobic PI block is accessible to forcibly mix with water as a way to dissipate energy when the hydrogel is strained. This dissertation describes the design, testing, and optimization of a hydrogel where an energy dissipation mechanism was placed directly onto every crosslink of a known elastic and fatigue-resistant network. The possibility of even designing such a network was tested by studying the self-assembly of the SIO polymers into the ABC block polymer sphere morphology. Once, the formation of the sphere morphology in the tethered micelle network was confirmed, the effectiveness of the design strategy of a fatigue-resistant network with an intrinsic energy dissipation mechanism was studied by comparison of the mechanical properties of the SIOIS hydrogel to a similar hydrogel that is fatigue-resistant but does not contain an energy dissipation mechanism. Finally, the design of the SIOIS hydrogel is optimized by studying the effect of changes to the hydrogel processing method and changes to the PS molecular weight on the self-assembly of the energy dissipation PI block and the formation of the tethered micelle network