Large deformations and fluorescence response of mechanochromic polyurethane sensors

Abstract

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.

Mechanochromic response of pyrene functionalized nanocomposite hydrogels

“…Optomechanical characterization was performed using a setup similar to the one described in [26]. Slabs of reference dimensions 42 mm × 5 mm × 5 mm were connected through a custom-made set of clamps to a Transducer Techniques 250 g load cell.…”

“…Large deformations of mechanochromic poly(NIPAAm-co-Py-PEGMA) nanocomposite hydrogels were studied under uniaxial tension, using the experimental setup previously described in [26]. Samples containing 1.5 and 3 wt% clay were tested in three different conditions.…”

“…The model in Eq. (1) is calibrated on the experimental dataset using as the fitting parameter [21,26]. Following [38], we assume N = 26.2 and ˛ = 50, which are reference values for crosslinked PNIPAAm networks, 2 while we set v s = 0.1 based on the initial hydrogel composition.…”

“…To gain further insight into hydrogel mechanochromism, we adapt the constitutive hypothesis presented in our previous work on mechanochromic elastomers [26], which assumes a linear relationship between the change in emission and the stretch of the material surface [52]. Specifically, we propose the following relationship for uniaxial stretching of mechanochromic hydrogel slabs:…”

“…To further elucidate their mechanical behavior, we consider a modified form of the Arruda-Boyce model [37] for swollen elastomeric networks, which has already been adopted for the study of PNIPAAm hydrogels [38]. Similarly, we adapt the constitutive hypothesis in [26] to study mechanochromism of polyurethane elastomers and isolate the dependence of the fluorescence emission on key observables for the mechanical deformation. Finally, to demonstrate the possibility of incorporating mechanochromic nanocomposite hydrogels in mechanical sensing systems, we propose the use of hydrogel membranes to sense differential pressures on the order of a few hundred Pascals.…”