Real-time imaging reveals and quantifies heterogeneous molecular damage in double network hydrogels

Double network hydrogels (DNHs) achieve exceptional toughness through sacrificial fracture of a brittle first network embedded in a ductile second network. Despite extensive theoretical and macroscopic studies, direct experimental correlations between molecular bond scission and applied deformation remain scarce, and damage is commonly assumed to follow the macroscopic deformation field. Here, we directly visualize and quantify bond scission in DNHs during deformation by combining in situ confocal laser scanning microscopy (CLSM) with anthracene-based optical force probe (OFP) crosslinkers and a customized uniaxial stretching device. This approach enables spatially resolved mapping of molecular bond rupture in real time while simultaneously probing mechanical response. Surprisingly, we find that bond scission is strongly heterogeneous even under nominally homogeneous macroscopic deformation. Quantitative comparison with a mechanochemical damage model shows that this discrepancy arises from intrinsic network heterogeneity that is not captured by conventional homogeneous-material descriptions. Our results demonstrate that molecular damage in polymer networks cannot be inferred solely from macroscopic deformation fields and highlight the necessity of spatially resolved measurements for understanding the mechanics and fracture of DNHs.