3D Polymer Microframes That Exploit Length‐Scale‐Dependent Mechanical Behavior

Materials science, especially metallurgy, has long exploited the relationship between microstructural scale, arising from various processing routes, and resultant properties, with increasing emphasis on the benefits of ultrafine-scale structures. Critical elements for the design of lightweight materials for mechanical applications include a means for stiffening and strengthening as well as providing energy absorption and controlled crack propagation. [1–4] Combinations of hard and soft components, control over component size, shape, and arrangement, and attention to interfacial strength are widely used to design composite materials with superior properties. Manmade ordered composites are typically assembled via machine or hand lay up and generally rely on macroscopic components. On the other hand, load-bearing structures in nature are much more complex, often using a combination of hard (e.g., calcite) and soft (e.g., proteins) components and importantly, feature elegant self-organized hierarchical designs extending from the nanometer to the millimeter scale. [5] Moreover, nature has evolved certain structures into nearly optimized mechanical designs. [6] Most current man-made sub-micrometer-scale composites provide little control over the detailed microstructure of the respective components. [7] For example, recent efforts with polymer–carbon-nanotube composites currently lack the processing ability to pattern the components at various length scales to create optimally designed materials. Patterning using self-assembly is one means to mimic nature but at present most ordered structures occur more by chance than by purposeful design and the pattern is achieved normally only at one length scale. Taking advantage of lengthscale-dependent mechanical properties is another strategy used by nature to tailor mechanical behavior. [5] Polymeric materials in particular are also sensitive to the influence of sample dimensions on properties; for example, below a critical film thickness brittle polymers can exhibit increased strain-tobreak and improved toughness. [8,9] Lightweight microframe structural materials possessing load-bearing capabilities approaching theoretical limits have recently been constructed from millimeter-sized metallic assemblies. [10] Their open architecture provides a density well below 10% of that of the corresponding bulk materials. [11] Several structures have been fabricated, tested, and theoretically analyzed. However, for feature sizes at the millimeter scale, the material’s mechanical properties are not size dependent; indeed, theoretical models that rely only on inputting a constitutive equation based on bulk material behavior along with the particular truss geometry do a very good job of capturing the experimentally observed behavior. Designs that provide conditions such that beams are under compression or tension while avoiding bending are desired. In particular, pyramidal and octahedral frame structures provide maximum stiffness and strength at a given density. [12] In addition to outstanding specific mechanical properties, the open structure and high surface area (ca. m 2 g –1 ) of microframes add addi