Large deformations and fluorescence response of mechanochromic polyurethane sensors

A mechanochromic composite comprises an excimer-forming fluorescent dye dispersed in a compatible polymeric host. Upon large deformation of the polymeric structure, a shift of the fluorescence emission from the excimer band to the monomer band is observed, due to reorganization of dye aggregates. Mechanochromic elastomers can be tailored to produce a reversible optical response during cyclic deformation. Here, we study the optomechanical properties of mechanochromic elastomers undergoing large uniaxial and biaxial deformations. Through detailed experiments and constitutive modeling, we relate the optical response of the mechanochromic elastomer to the deformation of the polymer network. We confirm that optical response is largely reversible and is modulated by the initial dye concentration in the polymer. We adapt the classical Arruda-Boyce model to elucidate microstructural modifications of the polymer-dye blend. We propose a constitutive hypothesis for the optical behavior of the material, which relates variations of the emission spectrum to surface stretching. This knowledge can aid in the design of strain and stress sensors for applications in mechanics, biomechanics, and life sciences.