Hierarchically Structured Deformation-Sensing Mechanochromic Pigments

Mechanochromic materials alter their color in response to mechanical force and are equally useful for fundamental studies and practical applications. Several approaches have been used to render polymers mechanochromic, but they generally suffer from limitations in sensing range, capacity to provide quantitative information, and their capability to enable broad and simple implementation. Here, is it reported that these problems can be overcome by combining photonic structures, which alter their reflection upon deformation, with covalent mechanophores, whose absorption changes upon mechanically induced bond scission, in hierarchically structured mechanochromic pigments. This is achieved by synthesizing microspheres consisting of an elastic polymer with spiropyran-based cross-links and non-close-packed silica nanoparticles. A strain of less than 1% can be detected in a shift of the reflection band from the photonic structure, while the onset strain for the conversion of the spiropyran into fluorescent merocyanine ranges from 30 to 70%, creating a broad strain detection range. The two responses can be tailored, e.g. via the content and size of the silica nanoparticles or the cross-link density of the polymer. The mechano-sensing photonic pigments are demonstrated to be readily incorporated into different polymeric materials of interest and quantitatively probe spatially heterogeneous deformations over a large strain range.