Finite strain damage-elastoplasticity in double-network hydrogels

Abstract Finite tensile behavior and fracture toughness of double-network (DN) and triple-network (TN) hydrogels synthesized from 3-sulfopropyl acrylate potassium salt (SAPS) and acrylamide (AAm) are discussed. The mechanical behavior of pseudo-semi-interpenetrating polymer network (pseudo-SIPN), pseudo-interpenetrating polymer network (pseudo-IPN) and TN hydrogels prepared from the two types of DN networks are considered. In general, the mechanical behavior of tough hydrogels is dependent upon the crosslink densities of the first and second networks in DN and TN hydrogels and the covalent connectivity of the second or third network to the previous network architecture. Pseudo-SIPN hydrogels, where no crosslinking agent is used to make the second network, exhibit necking for tensile deformations. When a very low concentration of crosslinking agent is used in the second polymerization step (pseudo-IPN), necking is suppressed and strain hardening occurs. Increasing the crosslink density of the second network produces more pronounced strain hardening, but also embrittlement of the hydrogel. For tough hydrogels where the first network is very brittle, i.e., very high crosslink density, it is difficult to prevent necking. The formation of a third, loosely crosslinked network within pseudo-SIPN and pseudo-IPN hydrogels that exhibit necking prevents necking and produces strain hardening. Loading – unloading tensile experiments conducted on DN hydrogels indicated that regardless of the stretch ratio used (high strains above or low strains below the yield point), the sample exhibited plastic flow and a residual strain. The value of the residual strain was insensitive to the magnitude of the strain at which the load was removed.