DOI: 10.1002/adma.200600249 3D Polymer Microframes That Exploit Length-Scale-Dependent Mechanical Behavior** By Ji-Hyun Jang, Chaitanya K. Ullal, Taeyi Choi, Melburne C. Lemieux, Vladimir V. Tsukruk, and Edwin. L. Thomas* Materials science, especially metallurgy, has long exploited the relationship between microstructural scale, arising from various processing routes, and resultant properties, with in- creasing 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 con- trolled crack propagation. [1–4] Combinations of hard and soft components, control over component size, shape, and arrange- ment, and attention to interfacial strength are widely used to design composite materials with superior properties. Man- made 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., cal- cite) 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-microme- ter-scale composites provide little control over the detailed microstructure of the respective components. [7] For example, recent efforts with polymer–carbon-nanotube composites cur- rently 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 nor- mally only at one length scale. Taking advantage of length- scale-dependent mechanical properties is another strategy used by nature to tailor mechanical behavior. [5] Polymeric ma- terials in particular are also sensitive to the influence of sam- ple dimensions on properties; for example, below a critical film thickness brittle polymers can exhibit increased strain-to- break and improved toughness. [8,9] Lightweight microframe structural materials possessing load-bearing capabilities approaching theoretical limits have recently been constructed from millimeter-sized metallic as- semblies. [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 theoreti- cally analyzed. However, for feature sizes at the millimeter scale, the material’s mechanical properties are not size depen- dent; 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 cap- turing the experimentally observed behavior. Designs that provide conditions such that beams are under compression or tension while avoiding bending are desired. In particular, py- ramidal and octahedral frame structures provide maximum stiffness and strength at a given density. [12] In addition to out- standing specific mechanical properties, the open structure and high surface area (ca. m 2 g –1 ) of microframes add addi- tional functionality, such as the ability to cool by flowing a continuous fluidic phase through the structure. [13] Recent developments in laser interference lithography (IL) demonstrate the ability for fast fabrication of complex poly- meric structures with long-range periodic order. [14] IL can cre- ate connected sub-micrometer-sized elements in complex 2D and 3D networks [15] with micrometer to sub-micrometer spac- ings. Such periodic structures created using IL are being pursued for photonic [14–16] and phononic [17] applications. Peri- odic high-porosity polymeric structures are also attractive candidates for mechanical applications, provided the poly- meric material used for the “skeleton” network of members exhibits appropriate mechanical properties, and the chosen geometry of the 3D structure provides proper mechanical load distribution. Here, we report the use of holographic IL to create a 3D polymer microframe with a four-functional network geometry with sub-micrometer periodicity, low density (ca. 0.3 g cm –3 ), and 200 nm feature size. These large-area, periodic, porous polymer/air structures are fabricated from negative Novolak resin photoresist and due to their length-scale-dependent mechanical behavior, exhibit interesting deformational char- acteristics (e.g., necking of crosslinked struts and their evolu- COMMUNICATIONS Adv. Mater. 2006, 18, 2123–2127 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2123 – [*] Prof. E. L. Thomas, Dr. J-H. Jang, Dr. C. K. Ullal, T. Choi Department of Materials Science and Engineering Institute for Soldier Nanotechnologies Massachusetts Institute of Technology Cambridge, MA 02139 (USA) E-mail: elt@mit.edu M. C. Lemieux,Prof. V. V. Tsukruk Department of Materials Science and Engineering Iowa State University Ames, IA 50011 (USA) [**] We thank G. Groishnyy and S. Kooi for technical assistance, T. Ka- nazawa (JEOL) for some SEM images, and L. Gibson for helpful dis- cussions. This research was supported by the US Army Research Of- fice through the Institute for Soldier Nanotechnologies, under the contract DAAD-19-02D-0002 and AFOSR, grant F496200210205.