Silicone‐Based Polyurethane for Visual Damage Sensing: The Critical Role of Chemical Bonding in Mechanochromic Soft Materials

Mechanochromic soft materials that visually indicate mechanical damage are highly desirable for nondestructive testing. However, achieving effective stress transfer in low-modulus silicone-based matrices remains challenging, especially when mechanophores are merely physically blended. In this study, we report a covalently integrated spiropyran (SP2) into a silicone-based polyurethane (SiPU) network, enabling distinct mechanochromic response under tensile deformation. In contrast, its physically blended counterpart (SP1-SiPU) exhibits no color change. Through rational design of the polyurethane hard/soft segments, we obtained a SiPU matrix with balanced modulus and toughness, facilitating efficient force transduction to the covalently bonded SP2. The resulting material demonstrates a strain-dependent color transition from pale yellow to light purple at 400% strain, with UV-vis spectroscopy confirming the SP-to-MC transformation. This work highlights the essential role of chemical bonding in enabling mechanochromism in soft materials and provides a new design paradigm for real-time damage visualization in silicone-based elastomers.