Localized yielding around crack tips of double-network gelsa

Double-network (DN) gels, a type of interpenetrating polymer network (IPN) consisting of rigid and flexible polymer components, exhibit two outstanding mechanical behaviors: yielding deformation of the entire specimen in tensile tests and quite high fracture energy in tearing tests. In this study, atomic force microscope (AFM) measurements were conducted on DN gels to determine the local Young's moduli immediately below the fracture surfaces E-f and below the usual molded surfaces E-m, and compare the local modulus with bulk Young's moduli measured before and after the yielding deformation, denoted as E-h and E-s, respectively. E-m and E-h are around 0.1 MPa; E-f and E-s, around 0.01 MPa, one order lower than the former two moduli. The order relation indicates that yielding deformation occurred locally around the crack tip of the DN gel during fracture. This supports the basic assumption of phenomenological models recently proposed to explain high fracture energy of DN gels.