Fatigue of Hydrogels

Abstract

Hydrogels are actively studied and developed in both fundamental understanding and modern applications. Traditional hydrogels are soft and brittle. Hydrogels with toughness and stiffness comparable to natural rubber have been recently developed. However, all hydrogels suffer fatigue, i.e., any material failure under prolonged loads. A hydrogel sample can be uncut or precut with a macroscopic crack. The loading condition on the sample can be static or cyclic. Fatigue is observed in all combinations of sample types and loading conditions. This thesis explores fatigue of hydrogels under both static and cyclic loads. Hydrogels with different molecular structures show various fatigue behaviors, which are discussed in detail. For cyclic fatigue of hydrogels, we study fatigue damage of uncut hydrogels and fatigue fracture of precut hydrogels. We study fatigue fracture of a nearly elastic, covalently crosslinked polyacrylamide hydrogels of various water contents. A polyacrylamide hydrogel is resistant to fatigue damage due to its near-elastic feature, but still suffers fatigue fracture. We also study fatigue of a polyacrylamide-calcium-alginate tough hydrogel with a covalently crosslinked polyacrylamide network and an ionically crosslinked calcium-alginate network. The tough hydrogel suffers both fatigue damage and fatigue fracture due to the continuous breaking of the weak calcium-alginate network and the relatively brittle covalent polyacrylamide network. We then study fatigue of a self-recovery hydrogel containing both covalently crosslinked polyacrylamide and uncrosslinked polyvinyl alcohol. A self-recovery hydrogel is able to recovery its property after many cycles of loads due to the reversible bonds, but is still susceptible to fatigue fracture. With these studies, we find that the threshold for fatigue fracture depends on the covalent network of a hydrogel, but negligibly on any non-covalent interactions in the hydrogel. Above the threshold, the non-covalent interactions slow down the extension of the crack under cyclic loads. For static fatigue of hydrogels, we focus on slow crack of polyacrylamide-calcium-alginate hydrogels. We measure the v-G curves of slow crack of hydrogel samples with the same polyacrylamide network. The v-G curve depends on the crack speed, sample size and amount of calcium ions added in the pre-gel solution. The threshold for slow crack does not depend on the sample thickness. We compare the measured thresholds for both fatigue fracture and slow crack. The threshold for fatigue fracture only depends on the primary network of polyacrylamide, but negligibly on the calcium-alginate toughener. In contrast, the threshold for slow crack depends on both the primary network of polyacrylamide and the solid-like toughener of calcium-alginate. In addition to the fundamental study of fatigue of hydrogels, we present a new design principle of flaw-insensitive hydrogels under both static and cyclic loads. The design utilizes material anisotropy by aligning the polymer chains of the hydrogel in the molecular level to deflect a crack. Upon stretching, an initial flaw deflects, propagates along the loading direction, and peels off the material, leaving the hydrogel flawless again. The designed hydrogel is insensitive to any pre-existing flaw, even under more than ten thousand loading cycles. Finally, we summarize some understanding of fatigue of hydrogels and future challenges for the topic. With the study in this thesis, it is hoped that the theoretical and experimental understanding can facilitate the development of hydrogels that resist both static and cyclic fatigue.