Bioinspired noncovalently crosslinked "fuzzy" carbon nanotube bundles with superior toughness and strength

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 inter-tube 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.