Effect of Microstructural Evolution on Macromechanical Properties of Double-Network Hydrogels

Hybrid double-network hydrogels such as polyacrylamide-alginate hydrogels (PAAm-Alg) exhibit excellent mechanical properties due to the synergy between the covalently crosslinked PAAm network and the ionically crosslinked Alg network. However, the relationship between their microstructural evolutions and macromechanical properties remains unclear. In this paper, single-cycle tensile tests at different strains are combined with microstructural observations on longitudinal and transverse planes using SEM (scanning electron microscopy). The SEM images suggest anisotropic stepwise network damage. The results also indicate that residual strain and energy dissipation are associated with Alg-network damage at low strains and PAAm-network damage at high strains. Five stages are further proposed in the deformation process: (I) elastic without network fracture; (II) pre-yielding possibly associated with short Alg strand rupture on the longitudinal plane; (III) yielding possibly associated with long fully stretched Alg strand rupture; (IV) initial hardening involving PAAm network damage on the longitudinal plane; and (V) final hardening involving both network damages on the transverse plane.