Load-bearing biological tissues, such as muscles, are highly fatigue-resistant, but how the exquisite hierarchical structures of biological tissues contribute to their excellent fatigue resistance is not well understood. In this work, we study antifatigue properties of soft materials with hierarchical structures using polyampholyte hydrogels (PA gels) as a simple model system. PA gels are tough and self-healing, consisting of reversible ionic bonds at the 1-nm scale, a cross-linked polymer network at the 10-nm scale, and bicontinuous hard/soft phase networks at the 100-nm scale. We find that the polymer network at the 10-nm scale determines the threshold of energy release rateG0above which the crack grows, while the bicontinuous phase networks at the 100-nm scale significantly decelerate the crack advance until a transitionGtranfar aboveG0. In situ small-angle X-ray scattering analysis reveals that the hard phase network suppresses the crack advance to show decelerated fatigue fracture, andGtrancorresponds to the rupture of the hard phase network.

Polyampholyte hydrogels (PA gels) are drawing great attention for their excellent mechanical properties including self-healing, high toughness and fatigue resistance.These mechanical performances are found to be attributed to the hierarchical structure of the PA gels, consisting of reversible ionic bonds at the 1 nm scale, permanent polymer network at the 10 nm scale, and bicontinuous phase network at the hundred nm scale. In this work, we systematically studied the phase network formation of these gels, aiming to answer the following three questions: (1) how the phase separation occurs? (2) what determines the phase structure? and (3) is this structure in thermal dynamically equilibrium or not? Our results show that, the phase separation occurs during dialysis of counterions from the gels and it is driven by the Coulombic and hydrophobic 2 interactions. The phase size d0 and the number of aggregated chains in a unit cell of phase structure n scale with the molecular weight of partial chain between permanent effective crosslinking Meff as d0 ~ Meff and n ~ Meff 2 , respectively. Chemical crosslinker and topological entanglement suppress phase separation while hydrophobic interaction favors phase separation. An intrinsic correlation between the polymer density difference () between two phases and d0 is observed,   d0 2 , as a result of the competition between the driving force to induce phase separation and the resistance to suppress the phase separation. The phase separated structure is metastable, which is locally trapped by strong intermolecular interactions.

The effects of temperature on the nonlinear mechanical behaviors of hard-elastic isotactic polypropylene films are systematically studied with in-situ ultrafast synchrotron radiation small-and wide-angle X-ray scattering techniques (SAXS/ WAXS) during uniaxial tensile deformation at temperatures from 30 to 160 °C. Based on the mechanical behaviors and structural evolutions in strain−temperature two-dimensional space, three temperature regions (I, II, and III) are clearly defined with the α relaxation temperature (T α ≈ 80 °C) and the onset of melting temperature (T onset ≈ 135 °C) as boundaries, where different mechanisms dominate the nonlinear deformations after yield. In region I, microstrain in lamellar stacks ε m obtains an accelerated increase after yield and reaches a value significantly larger than corresponding macrostrain ε, during which neither slipping, melting, nor cavitation occurs. We propose stress-induced microphase separation of interlamellar amorphous to be responsible to the hyperelastic behavior in region I. Above T α in region II, due to reduced cohesive strength and enhanced chain mobility, the irreversible reduction of crystallinity and the formation of slender cavities suggest that crystal slipping overwhelms microphase separation and plays the major role in nonlinear deformation, during which chains in lamellar crystals are pulled out and recrystallize into nanofibrillar bridges. In region III above T onset , melting−recrystallization dictates the nonlinear deformation. A schematic roadmap for structural evolution is constructed in strain−temperature space, which may guide the processing of microporous membranes for lithium battery separators as well as other high performance polymer fibers and films.

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.