Composites play an important role as structural materials in a range of engineering fi elds due to their potential to combine the best mechanical properties of their constituents. In biology, composites are ubiquitous and exhibit fascinating and precise architectures at fi ne length scales; bone, hexactinellid sponges and nacreous abalone shells are prime examples. Here, typical biological composite topologies are emulated with multi-material 3D printing at micrometer resolution. From base materials that are brittle and exhibit catastrophic failure, synthetic composites are created with superior fracture mechanical properties exhibiting deformation and fracture mechanisms reminiscent of mineralized biological composites. This complementary computational model predictions of fracture mechanisms and trends in mechanical properties are in good agreement with the experimental fi ndings. The reported fi ndings confi rm that specifi c topological arrangements of soft and stiff phases as a design mechanism enhances the mechanical behavior in composites. This study demonstrates 3D printing as a means to create fracture resistant composites. Moreover, these results indicate that one can use computer models to design composite materials to exhibit tailored fracture properties and then use 3D printing to synthesize materials with such mechanical performance.
The mechanical properties of spider silks drive interest as sources of new materials. However, there remains a lot to learn regarding the relationships between sequence, structure, and mechanical properties. In order to predict the types of sequence–functional relationships, synthesis–characterization–computation are integrated using recombinant spider silk‐like block copolymers. Two designs are studied, both with origins from the spider Nephila clavipes. These proteins are studied both experimentally and in silico to understand the relationships between sequence chemistry, processing, structure, and materials function. Films formed from the two proteins are thoroughly characterized. In parallel, molecular modeling is used to assess the propensity of the two sequences to form β‐sheets or crystalline structures. The results demonstrate that the modeling predicts the structural differences between the two silk‐like polymers and these features can also be related to differences in functional outcomes. With this example of relating sequence design (hydrophobic–hydrophilic domains), experiment (genetic design and synthesis), processing (film and fiber formation) and modeling (predictions of crystallinity), synergy among these methods is demonstrated for predictable material outcomes. This approach offers a robust discovery path when looking towards next generation approaches to targeted materials outcomes.
Carbon nanotubes (CNTs) constitute a prominent example of structural nanomaterials, with many potential applications that could take advantage of their unique mechanical properties. Utilizing the inherent strength of CNTs at larger length-scales is, however, hindered by the inherently weak intertube bonding interactions, allowing slippage of nanotubes within a bundle before large macroscopic stresses are reached. Many lamellar biological materials crosslink stiff fibrous components via the introduction of a soft binding matrix to achieve a combination of high strength and toughness, as seen in cellulosic wood, silk, or collagenous bone fibrils. Here we present atomistic-based multi-scale simulation studies of bundles of carbon nanotubes with the inclusion of a binding polymer (polyethylene chains with functional end groups) to demonstrate the control of mechanical properties via variations of polymer structure, content and fiber geometry. A hierarchical approach (coarse-grain molecular modelling) is implemented to develop a framework that can successfully integrate atomistic theory and simulations with material synthesis and physical experimentation, and facilitate the investigation of such novel bioinspired structural materials. Using two types of nanomechanical tests, we explore the effects of crosslink length and concentration on the ultimate tensile stress and modulus of toughness of a carbon nanotube bundle. We demonstrate that the ultimate tensile stress can be increased four-fold, and the modulus of toughness five-fold, over an uncrosslinked bundle with the inclusion of 1.5 nm long crosslinking polymer at 17 wt% concentration, providing the structural basis for a fibre material that combines high levels of stress at high levels of toughness. These noncovalently crosslinked carbon nanotube bundles exhibit residual strengths after initiation of failure that depend on the crosslink length, and are similar to plastically sheared wood cells. Our work demonstrates the implementation of a wood-inspired carbon nanotube based fibre material with superior mechanical properties.
The production of carbon nanotube (CNT) yarns possessing high strength and toughness remains a major challenge due to the intrinsically weak interactions between "bare" CNTs. To this end, nanomechanical shear experiments between functionalized bundles of CNTs are combined with multiscale simulations to reveal the mechanistic and quantitative role of nanotube surface functionalization on CNT-CNT interactions. Notably, the in situ chemical vapor deposition (CVD) functionalization of CNT bundles by poly(methyl methacrylate) (PMMA)like oligomers is found to enhance the shear strength of bundle junctions by about an order of magnitude compared with "bare" van der Waals interactions between pristine CNTs. Through multiscale simulations, the enhancement of the shear strength can be attributed to an interlocking mechanism of polymer chains in the bundles, dominated by van der Waals interactions, and stretching and alignment of chains during shearing. Unlike covalent bonds, such synergistic weak interactions can re-form upon failure, resulting in strong, yet robust fi bers. This work establishes the signifi cance of engineered weak interactions with appropriate structural distribution to design CNT yarns with high strength and toughness, similar to the design paradigm found in many biological materials.
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