Comparative Mechanochromic Performance of Perylene Diimide-Doped Polyurethanes: Blending vs Bonding

The research on mechanochromic polymers encompasses the use of aggregachromic dyes loaded into various polymeric matrices. However, in recent years, the covalent incorporation of these dyes into polymeric chains has gained momentum in the direction of boosting the sensitivity in mechanochromic materials. This work compares the mechanochromism of a thermoplastic polyurethane containing a perylene diimide probe, either bonded to an “ad hoc” synthesized polyurethane or simply blended. The goal was to determine how the dye concentration and the two incorporation approaches affect the mechanochromic performance. The results show that bonding the chromophore to the polymeric chains lowers its tendency to aggregate, thus leading to a less perceivable color change than that of dispersed dye systems under uniaxial tensile deformation. Conversely, loading–unloading cycles and stress relaxation experiments provide evidence that the covalent incorporation of the dye provides a higher sensitivity with an improved correspondence between the mechanical and optical behavior.