The extraordinary properties of mechanical metamaterials are created from the geometrical design of their internal structures. As a result, the nature of classical mechanical metamaterials usually would not change and one geometrical design only corresponds to one set of mechanical behaviors. This restricts the application of metamaterials where an adaptive behavior is desirable. In this work, we demonstrate the dynamic and on-demand regulation of mechanical metamaterials through a combination of structural design and multimaterial 3D printing. We design a soft periodic lattice metamaterial containing two distinct deformation modes. One deformation mode is controlled by the zig-zagged topological defects, and the other one is controlled by the thermal activation of the responsive materials imbedded in the lattice. By regulating the deformation mode with ambient temperature, the effective Poisson ratio of the lattice can be intentionally switched between negative values and positive values. The mechanism of thermally tunable deformation is investigated both experimentally and theoretically, resulting in a predictive design space of the metamaterial. We believe our design principle opens up a new route for the exploration of metamaterials exhibiting extraordinary and programmable mechanical behaviors.

This paper investigates the mechanics of hybrid shape memory polymer polystrene (PS) based nanocomposites with skeletal structures of CNFs/MWCNTs formed inside. Experimental results showed an increase of glass transition temperature (T_g) with CNF/MWCNT concentrations instead of a decrease of T_g in nanocomposites filled by spherical particles, and an increase in mechanical properties on both macro- and μm-scales. Compared with CNFs, MWCNTs showed a better mechanical enhancement for PS nanocomposites due to their uniform distribution in the nanocomposites. In nanoindentation tests using the Berkovich tips, indentation size effects and pile-up effects appeared obviously for the nanocomposites, but not for pure PS. Experimental results revealed the enhancement mechanisms of CNFs/MWCNTs related to the secondary structures formed by nanofillers, including two aspects, i.e., filler–polymer interfacial connections and geometrical factors of nanofillers. The filler–polymer interfacial connections were strongly dependent on temperature, thus leading to the opposite changing trend of loss tangent with nanofiller concentrations, respectively, at low and high temperature. The geometrical factors of nanofillers were related to testing scales, further leading to the appearance of pile-up effects for nanocomposites in the nanoindentation tests, in which the size of indents was close to the size of the nanofiller skeleton.