Unraveling the ultrahigh modulus of resilience of Core-Shell SU-8 nanocomposite nanopillars fabricated by vapor-phase infiltration

Modulus of resilience, the maximum strain energy density that can be stored in an elastically deformed solid, is an important mechanical property for developing artificial muscles in robotics, soft electronics panels, and micro-/nano-electromechanical actuators. In this study, core–shell SU-8 nanocomposites were fabricated via vapor-phase infiltration of nanoscale amorphous aluminum oxides into SU-8 nanopillars and performed transmission electron microscopy, nanomechanical testing, analytical modeling, and atomistic simulations to gain a fundamental insight into the ultrahigh modulus of resilience much higher than that of most high-strength materials. This study shows that the ultrahigh modulus of resilience results from: the low aspect ratio of amorphous aluminum oxide nano-particulates; the particulate size thicker than the free volume size; and the thin aluminum oxide interconnecting links within nano-particulates. These unique microstructural features produce the unusual combination of low specific Young's modulus (E), 4 MPa/(kg/m³), and high specific yield strength (σ_y), 0.2 MPa/(kg/m³), leading to the specific modulus of resilience, 5.21 ± 0.39 kJ/kg (σ_y²/(2E)) about ten times higher than materials with the similar yield strength. This study demonstrates that vapor-phase infiltration is an excellent fabrication method to produce a polymer nanocomposite that can absorb and release a large amount of elastic strain energy.