Effect of mesoscale phase contrast on fatigue-delaying behavior of self-healing hydrogels

Abstract

We investigate the fatigue resistance of chemically cross-linked polyampholyte hydrogels with a hierarchical structure due to phase separation and find that the details of the structure, as characterized by SAXS, control the mechanisms of crack propagation. When gels exhibit a strong phase contrast and a low cross-linking level, the stress singularity around the crack tip is gradually eliminated with increasing fatigue cycles and this suppresses crack growth, beneficial for high fatigue resistance. On the contrary, the stress concentration persists in weakly phase-separated gels, resulting in low fatigue resistance. A material parameter, λtran, is identified, correlated to the onset of non-affine deformation of the mesophase structure in a hydrogel without crack, which governs the slow-to-fast transition in fatigue crack growth. The detailed role played by the mesoscale structure on fatigue resistance provides design principles for developing self-healing, tough, and fatigue-resistant soft materials.

Superstretchable, yet stiff, fatigue-resistant ligament-like elastomers

“…2b ) 9 . Besides the function of Li + -PEGA blocks for eliminating stress concentration and dissipating mechanical energy, strong and dynamic PMMA physical crosslinkers are indispensable for providing interfacial energy to resist the crack formation 32 . The double crosslinkers which differ in strength can achieve hierarchical dissipative ability for improving the toughness and fatigue threshold.…”

“…On the one hand, the ion-dipole interactions provide a dynamic crosslinking to eliminate the stress concentration and dissipate mechanical energy. On the other hand, the PMMA physical crosslinkers will blunt the crack 32 . Moreover, since the loosely packed PMMA nanodomains can be stretched into smaller size, it would provide additional interfacial energy to make the material resistant to multiple cycling tests, which can be validated by scaling theory discussed in the last part.…”

Molecularly Engineered Zwitterionic Hydrogels with High Toughness and Self-Healing Capacity for Soft Electronics Applications

“…Therefore, the toughening mechanism of the molecularly engineered VBIPS gel can be summarized as follows: the incorporated benzene and imidazole groups result in phase separation at room temperature (i.e., the operating temperature) and also strengthen the association strength of the polymer-rich phase. During deformation, the strong phase is supposed to retain the elasticity of the gel, while the weak phase ruptures to dissipate energy. , …”

Mechanochemiluminescent Hydrogels for Real-Time Visualization of Chemical Bond Scission

“…Uncovering the fracture mechanism of hydrogels is essential for determine their long-term performance and offer rational molecular design of advanced hydrogels. However, it remains a great challenge primarily attributable to the following factors: (1) many of the current hydrogels are mechanically weak and brittle; (2) the underlying mechanics are usually very multiscale, 5 including molecular breakage at the nanoscale, local damage occurring at the microscopic scale and stress strain relation at the macroscopic scale. 6…”

“…However, it remains a great challenge primarily attributable to the following factors: (1) many of the current hydrogels are mechanically weak and brittle; (2) the underlying mechanics are usually very multiscale, 5 including molecular breakage at the nanoscale, local damage occurring at the microscopic scale and stress strain relation at the macroscopic scale. 6 To overcome the two limitations, so far, hydrogels that exhibit high resistance to fracture have been developed by using the double network concept, 7,8 sacrificial bond principle, 9 and so on.…”