Revealing novel insights into the toughening mechanism of double network hydrogels via uniaxial tensile tests

Toughening mechanism of the double network (DN) hydrogel has garnered significant attention in recent years. To interpret this mechanism effectively, studying the fracture toughness of DN gels becomes essential. In DN gels, the apparent fracture toughness is initially decomposed into two components: the dissipated fracture toughness and the intrinsic fracture toughness. However, further decomposition of these two components and understanding the relationships between them remain areas of ongoing investigation. In this study, we propose a fracture model aimed at elucidating the relationships between the components of apparent fracture toughness. By partitioning the area under the uniaxial tensile curve of the DN gel into five regions, we decompose the apparent fracture toughness into four components. The expressions for these components are derived, and we employ four quantities to characterize the toughening mechanism of the DN gel. The proposed fracture model quantitatively demonstrates that the fracture toughness of the DN gel is enhanced through the presence of two interpenetrating networks. Through tearing tests conducted on both as-prepared and swelling gel samples, the proposed fracture model is validated and exhibits good agreement with experimental results. This study introduces a method based on uniaxial tensile tests to interpret the toughening mechanism of DN gel, thereby facilitating the synthesis and design of future soft materials.