Spatial Analysis of the Damage Zone in Double Network Hydrogel Using the Microelectrode Technique

Double network hydrogels (DN gels) exhibit a high fracture energy. The toughness of DN gels is strongly correlated to the internal fracture of the brittle polymer network near the crack tip in the region known as the damage zone. To gain a detailed understanding of the toughness of DN gels, it is essential to spatially grasp the internal structure of the brittle polymer network in the damage zone. However, traditional methods such as scattering techniques and microscopy have been challenging to achieve this. In this study, we utilized a double network hydrogel composed of a brittle first network of polyelectrolytes and a soft second network of neutral polymer. Through measurements of the electric potential around the damage region formed by tearing fracture, using the microelectrode technique (MET), we evaluated and visualized the spatial distribution of the first network with high spatial resolution. As a result, it was revealed that the damage zone formed spatially when the DN gel extended. Furthermore, in distinct yield zones and preyield zones with varying degrees of damage, different fracture behaviors were observed in the scanning direction of the MET. (1) In the preyield zone: the first network undergoes a local necking phenomenon, causing a separation between the fractured and unfractured parts. (2) In the yield zone: the first network undergoes a global necking phenomenon, resulting in overall fracture. These results demonstrate, from the perspective of polymers structure, that the two-stage energy dissipation in the damage zone contributes to the toughening of the DN gel. This method can be widely applied for spatial structural analysis of polyelectrolyte gels, and it is anticipated to contribute as a novel structural analysis technique.