Mechanisms Dictating Relaxation and Fracture in Physically Associated and Ionically Crosslinked Triblock Copolymer Gels.

摘要

Understanding molecular mechanisms which dictate the macroscopic properties of a material is crucial for engineering solutions to enhance mechanical performance. For synthetic polymer gels, this understanding is of particular importance in that these materials exhibit drastically reduced strengths and toughnesses in comparison to naturally occurring, biological gels. Currently for synthetic polymer networks, the gold standard for obtaining high toughness is through combining two disparate networks into a single functional material. These materials, referred to as double network gels, demonstrate both high strength and toughness but have no fatigue resistance as energy dissipation comes at the expense of breaking covalent bonds. In this work, the structure and performance of double network gels is replicated using non-covalently associated networks.;The first of these networks is composed of symmetric, amphiphilic triblock copolymers which provide a facile, self-assembled route toward the formation of a physically crosslinked polymer hydrogel. Due to a well-defined network architecture and homogeneity of crosslinks, these materials are capable of withstanding much larger strains than chemically crosslinked materials of comparable stiffness. In the Shull research group, thermoreversible acrylic block copolymer gels have been widely studied, making these materials excellent candidates for use as model networks that lack chemical crosslinks. The first half of this dissertation examines the synthesis of these materials, mechanisms determining gel relaxation, and methods for establishing homogeneous aqueous networks through thermoreversible assembly and vapor phase solvent exchange.;Upon immersion into solutions of divalent cations, these networks form metallic complexes that serve as dynamic, pseudo-covalent linkages which replace the second, tightly crosslinked network in a double network structure. The second half of this dissertation explores the mechanical response of these ionically crosslinked networks. These materials witness an increase in both stiffness and toughness by 2 to 3 orders of magnitude depending on pH, solution concentration, and cation identity. Additionally, due to the non-covalent character of the network linkages, these gels exhibit a degree of recoverable energy dissipation not seen in conventional double network gels.