Rate-dependent fracture behavior of tough polyelectrolyte complex hydrogels from biopolymers

Tough bio-based hydrogels based on the association of physical bonds are promising materials for use in biomedical applications due to their biocompatibility, biomimicry and biodegradability. Understanding the mechanical mechanisms at work during the fracture in such applications is indispensable. Using tough and self-healing bio-based hydrogels attained by the complexation of the negatively charged sodium hyaluronate (HA) and positively charged chitosan as a model system, an investigation of the tensile and fracture behavior over five decades of stretch velocities in physically and chemically crosslinked HA/chitosan hydrogels was systematically performed. In this study, the tensile analysis shows that the both of physically and chemically crosslinked hydrogels at small deformation are viscoelastic at a low stretch velocity, while they exhibit quasi-elastic character at a relatively high stretch velocity. In comparison to the physically and lightly chemically crosslinked HA/chitosan hydrogels exhibiting plastic-like behavior, the highly chemically crosslinked hydrogels demonstrate strong finite chain extensibility before fracture. The fracture analysis shows that the fracture energy of the highly chemically crosslinked hydrogels scales with the crack velocity as a power law relation Gamma infinity V-c(a) (a= 0.15), where the exponent a can be well predicted from the viscoelastic spectrum of the bulk sample. These findings indicate that the viscoelastic energy dissipation around the crack tip dominates the fracture energy of the highly chemically crosslinked hydrogels. On the other hand, the fracture energy of the physically and lightly chemically crosslinked hydrogels is noted to increase with the crack velocity as a complex growth function, which can be effectively predicated by a viscoplastic model. The observed finding shows that the fracture energy of the physically and lightly chemically crosslinked hydrogel is dominated by the unzipping of the polyion complex structure and viscous energy dissipation due to the pulled out chains from the polymer network structure.