Multicolor Mechanochromic Polymer Blends That Can Distinguish between Tensile-Stress States

Mechanochromic polymers can be used to visualize mechanical stimuli applied to materials. However, many of these polymers exhibit single-color mechanochromism, i.e., the polymer changes from its original color to another, thus potentially limiting the range of applications. Here, a versatile and readily accessible strategy for creating multicolor mechanochromic polymer blends that can detect whether a material is currently under stress or has already experienced stress is presented. The polymer blends are prepared by blending a segmented polyurethane and a polycaprolactone, each embedded with a radical-type mechanochromophore. These polymers appear either blue, pink, or green by stretching depending on the mechanochromophore employed. The introduction of different mechanochromophores into each of the segmented polyurethane and polycaprolactone polymers with different chain mobilities and the subsequent blending of these polymers affords a mechanochromic polymer blend that can be used to visually distinguish via a color change whether it is experiencing stress or has recently experienced stress. The colors observed under stress and after stress can be tuned as easily as mixing paint based on the combination of the mechanochromophores ("rainbow mechanochromism"). The strategy developed in this study can be expected to significantly advance the research of mechanochromic polymer materials.