Dynamic Covalent Polymer Microspheres under Supersonic Impact

Polymeric microspheres have recently drawn interest for possible applications in solvent-free cold spray coatings. However, issues of material rebounding, self-adhesion and substrate adhesion remain unsolved. Click chemistry is a powerful platform that uses various building blocks to design dynamic covalent polymer networks with multi-functionalities, including self-healing. It is desirable to understand the responses of dynamic networks to supersonic impact. Here, we fabricate Diels-Alder polymer (DAP) dynamic microspheres composed of furan-attached prepolymers and bismaleimide crosslinkers and explore their in-situ high-strain-rate-induced deformations. The glass transition temperature of the microspheres can be tuned via density of DA bonds, while the solid-to-liquid transition occurs at 120 °C. The angled laser-induced projectile impact test (Θ-LIPIT) is employed to launch DAP-microspheres towards hard substrates at velocities between 150 and 800 m/s and angles from 30 to 90 degrees. The in-situ observation of impact events is carried out by a micrometer- and nanosecond-resolution ultra-high frame rate camera, while low-voltage SEM is used to reveal the deformation morphology. The impact-induced adiabatic heating and stress produced gradients of DA bond densities enabling rigid-to-elastomeric transition observed as elastic response, as well as solid-to-liquid transition observed as radially splashed ejecta of the DAP material.