Fracture of Complex Hydrogels: Dynamic and Microstructural Effects

Abstract

Hydrogels are a unique class of soft materials composed of three-dimensional polymer networks swollen with large amounts of water. This hybrid solid–liquid composition enables hydrogels with tissue-like mechanical properties, permeability to small molecules, and exceptional designability. By adjusting the combination of solvent, polymer, and network topology, hydrogels can be tailored into functional materials that meet diverse application requirements. These properties make hydrogels highly attractive for a wide range of applications, particularly in biomedical engineering, soft robotics, and bioelectronics. Understanding the mechanical behavior of hydrogels is essential for guiding material design, evaluating failure criteria, and ensuring reliable performance in practical applications. It also provides valuable inspiration for the development of other soft polymeric materials such as elastomers and thermoplastics. In this thesis, we investigate the mechanical behavior of hydrogels, with a primary focus on their fracture properties. The complex conditions near the crack tip make the fracture process highly sensitive and show interesting phenomena when the chain topology, external stimuli, or loading conditions are altered at the crack tip. A fundamental feature of hydrogels, and many other polymers, is viscoelasticity, which exhibits both elastic and viscous behavior depending on the timescale of deformation. Viscoelasticity originates from the molecular architecture of polymer networks. The flexibility of covalent C–C bonds allows for segmental motion, while interchain interactions dissipate energy and enable stress relaxation, with recovery to some degree upon unloading. In swollen systems like hydrogels, polymer–solvent interactions further influence chain mobility and relaxation dynamics by facilitating or hindering molecular motion. Additionally, chain entanglements act as transient constraints on motion, contributing to energy dissipation and delayed elastic recovery. Together, these factors result in the complex, time-dependent mechanical response that defines viscoelastic materials. While viscoelasticity is important for global deformation, its role becomes even more critical when considering how cracks initiate and propagate. In particular, the fracture behaviors of hydrogels are fundamentally shaped by their time-dependent mechanical response, which governs how energy is dissipated near the crack tip and how the material resists crack propagation. In these systems, energy dissipation around the crack tip is not only governed by intrinsic material toughness but also by the rate of deformation and solvent dynamics. A faster loading rate can decrease the apparent fracture toughness by decreasing energy dissipation through viscoelastic relaxation mechanisms. Similarly, the solvent viscosity in hydrogels modulates chain mobility and effective chain length, with higher solvent viscosity leading to much decreased resistance to crack propagation. In certain conditions, though hydrogels can show nearly perfect elasticity, their fracture can still show nonelastic process due to the complex and non-uniform crack tip zone. One striking phenomenon observed under these conditions is crack branching, where rapid deformation results in the formation of multiple crack paths. This behavior underscores the complex interplay between material structure, loading conditions, and environmental factors in defining the fracture response of viscoelastic materials. In addition to passive viscoelastic effects, polymer networks can be engineered to actively respond to mechanical stress through mechanochemically triggered reactions. One such example is the incorporation of disulfide bonds (-S–S-), which can undergo dynamic exchange reactions under triggers like UV light and free radicals. When materials containing these bonds undergo crack initiation, localized stress near the crack tip can be relaxed when the dynamic reaction is activated, effectively redistributing stress and limiting crack growth, as the network is able to reorganize and reform in response to mechanical damage. This mechanochemical coupling introduces a powerful design strategy: leveraging molecular reactivity not only to enhance material toughness but also to enable the material to adaptive, self-protective behavior under extreme conditions. Building on the concept of responsive materials, we introduce a triggerable crosslinking strategy using ferric citrate to strengthen natural polymer matrices. In this work, a complex crosslinker, ferric citrate, is applied to coordinate with chitosan, forming a robust film. The resulting ferric citrate-crosslinked chitosan film exhibits significantly improved mechanical strength, enhanced acid resistance, and recyclability. This system demonstrates a promising approach for the development of sustainable and functional biopolymer materials, where the crosslinking is not only reversible but also tunable based on environmental pH. By integrating a metal–ligand coordination chemistry with sodium citrate, we achieve a recyclable and chemically resistant material platform suitable for applications in packaging, filtration, or biomedical devices. To further enhance the mechanical performance of hydrogels, we developed a method for fabricating hierarchically structured fibrous hydrogels using Wet Rotary Jet Spinning (WRJS). This scalable and rapid technique enables the production of continuous microfibers with controlled alignment and diameter. Taking polyvinyl alcohol (PVA) as a model system, we fabricated a fibrous PVA hydrogel with superior mechanical strength and flaw tolerance. The fibers, with diameters below 10 µm, were rapidly produced in large quantities by spinning PVA solution into a coagulation bath, followed by a salting-out process that stabilizes the fibrous structures. Inspired by the mechanical architecture of spider silk and other natural hierarchical structures, the resulting hydrogel combines high extensibility, strength, and crack resistance, attributed to the aligned, hierarchical fibrous network that dissipates energy effectively under stress. This hierarchical design offers a compelling route to engineering tough, scalable hydrogels with structural features mimicking natural load-bearing tissues, like tendons. This dissertation presents a comprehensive investigation into the fracture mechanics of hydrogels, with particular emphasis on the complex mechanical behaviors near crack tips. Through a combination of experimental studies, material design, and theoretical insights, we explore how viscoelasticity, dynamic crosslinking, and hierarchical structure influence crack propagation and energy dissipation. The findings contribute to a deeper understanding of time-dependent fracture in soft materials and provide new strategies for designing tough, responsive, and multifunctional hydrogel systems suitable for a broad range of applications.