Damage cross-effect and anisotropy in tough double network hydrogels revealed by biaxial stretching

Anisotropy of strain-induced internal damage in tough double network (DN) hydrogels is characterized by a sequence of two tensile experiments. Firstly, the virgin DN gels are subjected to a single biaxial loading–unloading cycle using various combinations of the two maximum strains λx,m and λy,m in the x- and y-directions (λx,m ≥ λy,m). Secondly, the rectangular subsamples, which are cut out from the unloaded specimens so that the long axis can have an angle (θ) relative to the larger pre-strain (x-)axis, are stretched uniaxially along the long axis. Directional internal damage caused by various types of pre-stretching is evaluated by comparing the loading curves of the virgin gels and the subsamples with various θ. The modulus reduction (ΔEθ) and strain-energy reduction (Dθ) are characterized as functions of λx,m, λy,m and θ. The anisotropy of damage increases with the anisotropy of imposed pre-strain field as well as λx,m, which is also observed in the anisotropic re-swelling behavior of the subsamples. The damage and the extensibility of the subsamples with θ = 0° increase with λy,m, and the damage of the subsamples with θ = 90° significantly increases with λx,m. These results reveal the presence of a pronounced damage cross-effect: a finite portion of the chain fractures in the first brittle network in one direction is caused by loading in the other orthogonal direction. This feature is in contrast to the very modest damage cross-effect in the silica reinforced elastomers, which show apparently similar stress-softening behavior but with a different origin. The strong damage cross-effect is a key feature of the internal fracture mechanism of the tough DN gels.