Relation of synthesis and fatigue property in elastic soft materials

Fatigue has long been studied for many materials, but many aspects are not well understood. Our recent study of the distinct roles of crosslinks and entanglements in the synthesis-property relation of a polymer network under monotonic load leads to a fundamental question: how do crosslinks and entanglements affect the synthesis-property relation of a polymer network under cyclic load? Here we study the relation of synthesis and fatigue property of elastic soft materials without precut cracks. We prepare polyacrylamide hydrogels by free radical polymerization as a model system, and swell the hydrogels in water to equilibrium or to a certain amount of polymer content. The synthesis parameters include the crosslinker-to-monomer molar ratio and the water-to-monomer molar ratio in the precursor, as well as the polymer content in the hydrogel. Three series of hydrogels are prepared. For each hydrogel, the stress-stretch curve under cyclic stretch of various amplitudes and the number of cycles to rupture are measured, giving four properties: fatigue life, endurance stretch, endurance stress, and endurance work. When the crosslinker-to-monomer molar ratio in the precursor is high, the degree of network imperfection on average is low. When the water-to-monomer molar ratio in the precursor is low, the number of entanglements per polymer segment on average is large. We show that crosslinks decrease the susceptibility to fatigue and entanglements increase the endurance stress. By contrast, both crosslinks and entanglements negligibly affect the endurance stretch and the endurance work.