Internal Fracture in Tough Double Network Hydrogels Revealed by Various Modes of Stretching

The cyclic stretching measurements in various geometries including uniaxial, planar, unequal and equal biaxial extension, reveal the distinctive features of the internal fracture in the double network (DN) hydrogels. The modulus reduction, dissipated energy (D), dissipation factor (Δ, the ratio of dissipated energy to input strain energy) in each loading-unloading cycle are evaluated as a function of the imposed maximum elongation (λ_i,m) in i-direction (i=x,y) in each cycle. The modulus reduction and Δ depend on the stretching mode when compared at the same λ_x,m, but each of them exhibits a universal relation independently of the stretching mode when the magnitude of left Cauchy-Green deformation tensor is used as a variable. In contrast, Δ in filled elastomers shows the corresponding universal relation when the first strain invariant is used as a variable (Mai et al., Soft Matter 13, 1966–1977, 2017). The difference in governing variable indicates that the influence of the cross-effect of strains (λ_iλ _j; i,j =x,y,z and i ≠ j) on the dissipation factor is pronounced in the DN gels whereas it is minimal in the filled elastomers. (Mai et al., Macromolecules 51, 5245–5257, 2018)