Thermomechanical characterization of a shape memory polymer based self-repairing syntactic foam

While the current self-healing approaches such as micro-capsules, hollow fibers, thermally reversible covalent bonds, ionomers, incorporation of thermoplastic particles, etc., are very effective in self-healing micro-length scale damage, self-healing of structural scale or macro-length scale damage remains one of the grand challenges facing the self-healing community. We believe that self-healing of structural damage may need multiple steps, at least two steps: close then heal (CTH), similar to the biological healing of wounds in the skin. In a previous study [1], it has been proven that the confined shape recovery functionality of a shape memory polymer (SMP) based syntactic foam can be utilized to repair structural damage such as impact damage repeatedly, efficiently, and almost autonomously. The purpose of this study is to investigate the effect of various design parameters on the closing efficiencies of both the pure SMP and the SMP based syntactic foam. A systematic test program is implemented, including glass transition temperature (Tg) determination by dynamic mechanical analysis (DMA), isothermal compressive constitutive behavior at various temperatures, and stress-controlled uniaxial compression programming and shape recovery. During thermomechanical cycle testing, two stress levels are utilized for programming and three confinement conditions (fully confined, partially confined, and free) are investigated for shape recovery. It is found that the programming stress is restored under confined recovery conditions, which helps in fully closing the crack; the foam shifts the Tg higher and increases the stiffness at temperatures above the Tg; higher programming stresses lead to slightly higher shape fixity but lower shape recovery in free recovery cases; a higher programming stress also results in a higher peak stress for confined recovery conditions; while the peak stress recovered is controlled by thermal stress, the final stress recovered is controlled by the programming stress, which is stored and recovered using an entropic mechanism. This study lays a solid foundation for using shape memory polymer based composites to self-repair macro-length scale damage.