An Introduction to Mechanophores: Utilizing Mechano-Responsive Molecules for Stress Visualization*

Mechanophores (MPs) are a class of stimuli-responsive materials that are of increasing interest to engineers due to their potential applications as stress sensors. A wide array of applications that require continuous structural health monitoring could benefit from MPs including flexible electronics, protective coatings, and polymer matrix composites. This talk will introduce MPs to a broad audience and then share some of our recent work in the calibration and implementation of these molecular damage sensors in a variety of polymeric systems. First, we investigate the role of interfacial strength on the stress field developed in the matrix of a glass/polymer composite. Here, we utilize a mechanically-activated fluorescent dye molecule (MP) to visualize stress gradients around a rigid inclusion upon mechanical deformation. By coupling our experimental observations of mechanophore activation with finite element analysis of the various stress states that develop in the loaded composite, a novel approach to quantitatively calibrate the MP fluorescent activation intensity has been established. We then apply our calibration to several test cases of silica/silicone composites with dramatically different levels of interfacial strength. This mechanophore/mechanical deformation approach enables stress fields to be observed in a powerful new way via fluorescence imaging in a mechanically loaded polymer network. Next, we employ MPs to demonstrate the overlapping stress fields in a polymer matrix as two rigid reinforcement particles are placed in close proximity to one another in a deformed composite. Finally, we employ progressive loading scratch via nanoindentation to highlight the utility of MPs as a defect detection tool in transparent polymer coatings.