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

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.