Ultra-High Elastic Energy Storage in Metal-Oxide-Infiltrated Hybrid Polymer Nanocomposite

Modulus of resilience, the measure of a material’s ability to store and release elastic strain energy, is critical for realizing advanced mechanical actuation technologies in micro-/nano-electro-mechanical systems. In general, engineering the modulus of resilience is difficult because it requires asymmetrically increasing yield strength and Young’s modulus against their mutual scaling behavior. This task becomes further challenging if it needs to be carried out at the nanometer scale. Here, we demonstrate organic–inorganic hybrid composite nanopillars with one of the highest moduli of resilience per density by utilizing vapor-phase aluminum oxide and zinc oxide infiltration in lithographically patterned negative photoresist SU-8. In situ nanomechanical measurements reveal a metal-like high yield strength (∼500 MPa) with an unusually low, foam-like Young’s modulus (∼7 GPa), a unique pairing that yields ultrahigh modulus of resilience, reaching up to ∼24 MJ/m3 as well as exceptional modulus of resilience per density of ∼13.4 kJ/kg, surpassing those of most engineering materials. We also developed a constitutive model to describe their deformation process as well as to predict the theoretical limit of modulus of resilience. To describe the effects of the interaction between polymer matrix and nanoparticle fillers on mechanical properties of composite materials properly, we introduced a concept called “interphase” in our model. The interphase is a transition region over which the mechanical and physical properties change from the inorganic fillers to the polymer matrix. The interphase parameters like the thickness, the yield strength, and Young’s modulus show the capacity of stress transfer from the polymer matrix to nanofillers and enables us to attain the accurate mechanical properties and to predict the threshold of resilience. Furthermore, the compressive deformation is a dynamic process where the composite nanopillar system changes at each step the strain changes. Thus, besides the “static” state interaction influence, we also need to consider the effect of this “dynamic” change during the deformation process. During the compression process, the increment of density strengthens the composite materials while the buckling propagation produces the time-dependent plastic strain. Thus, by differentiating the deformation process and adding the effects of buckling propagation and densification, we are fully capable to simulate the deformation process of these polymer composite nanopillars. The hybrid polymer nanocomposite features lightweight, ultrahigh tunable modulus of resilience and versatile nanoscale lithographic patternability with potential for application as nanomechanical components which require ultrahigh mechanical resilience and strength such as flexible display or durable three-dimensional structures in polymer-based micro-electro-mechanical-systems (MEMS).