Using Mechanophores to Study the Mechanical Behavior of Polymers under High-Strain-Rate Impact

The recent advances in microballistic impact testing have enabled the study of the high-strain-rate mechanical response of materials on the microscale. However, these approaches provide limited insight into the mechanical behavior of the material as they cannot experimentally measure in-situ stress nor deformation evolution. To address this limitation, we study the high-strain-rate impact behavior of an anthracene-based mechanophore block copolymer material system using microprojectile rebound experiments. The mechanophores exhibit a fluorescence signal that is commensurate with the applied stress and can thereby act as molecular stress sensors in the test specimen. Importantly, these materials have the requisite temporal and spatial kinetics to enable the measurement of stress evolution during a short-duration (~200 ns), small-scale (~20 μm) impact event with impact velocities ranging from 50 to 500 m/s. Using laser scanning confocal microscopy, we show that the fluorescence information from the impacted sites can be used to quantify the local stresses to help provide insight into the energy absorption mechanisms as a function of impact velocities. This study demonstrates mechanophores as a tool for quantifying impact stresses that can help guide materials design for impact mitigation applications such as armor and spacecraft protection.