Ultratough artificial nacre based on conjugated cross-linked graphene oxide.

Natural nacre, which consists of almost 95 vol% inorganic content (calcium carbonate) and 5 vol% elastic biopolymers, possesses a unique combination of remarkable strength and toughness, which is attributed to its hierarchical nano-/ microscale structure and precise inorganic–organic interface. Inspired by the intrinsic relationship between the structures and the mechanical properties lying in the natural nacre, different types of nacre-like layered nanocomposites have been fabricated with two-dimensional (2D) inorganic additives, including glass flake, alumina flake, graphene oxide, layered double hydroxides, nanoclay, and flattened double-walled carbon nanotubes. Although great progress has been achieved in tensile mechanical properties, 9] in only very rare cases are artificial layered composites with excellent toughness are obtained. One of the most important causes is the relatively low interfacial strength between interlayers of artificial nacre. Recently, 2D graphene has attracted much research interest owing to its outstanding electrical, thermal, and mechanical properties, and many graphene-based devices have been fabricated, such as bulk composites, one-dimensional fibers, supercapacitors. As the water-soluble derivative of graphene, graphene oxide (GO), with many functional groups on the surface, is one of the best candidates for fabricating artificial nacre, because functional surface groups allow for chemical cross-linking to improve the interfacial strength of the adjacent GO layers. Until now, several methods have been developed to functionalize individual GO sheets and enhance the resultant mechanical properties, including divalent ion (Mg, Ca) modification, polyallylamine or alkylamine functionalization, borate cross-linking, glutaraldehyde (GA) treatment, p–p interactions, and hydrogen bonding. Although the obtained strength and stiffness are significantly enhanced, the modified materials are always accompanied by a reduced ductility or toughness. In a brief, it still remains a great challenge to obtain the ultratough artificial nacre based on the 2D GO sheets. Herein, inspired by the relationship of excellent toughness and hierarchical nano-/microscale structure of the natural nacre, we have developed a novel strategy for fabricating the ultratough artificial nacre based on 2D GO sheets by conjugated cross-linking. Highly p-conjugated long-chain polymers made of 10,12-pentacosadiyn-1-ol (PCDO) monomers are cross-linked with the GO sheets, resulting in a huge displacement upon loading and adsorption of much more fracture energy. The toughness is two times higher than that of the natural nacre. Furthermore, the p-conjugated polymers could add additional benefit to the high electrical conductivity of the chemically reduced GO (rGO). It is expected that this novel type of the ultratough and conductive artificial nacre has great potential in aerospace, flexible supercapacitor electrodes, artificial muscles, and tissue engineering. The typical hierarchical nano-/microscale structure of natural nacre is shown in Figure 1a–c. CaCO3 platelets with a thickness of 200–500 nm are assembled into the layered nanocomposites. The nanoasperities on the surface of the CaCO3 platelets play a key role in the process of dissipating crack energy when loading, through typical toughening mechanisms such as crack deflecting, interlocking, and mineral bridging. Inspired by the relationship of outstanding toughness with the hierarchical nano-/microscale structure of natural nacre, we assembled 2D GO sheets and pconjugated polymer PCDO into the ultratough artificial nacre. The inorganic GO sheets act as the bricks, while the PCDO polymers act as the mortar in the artificial nacre. Thermogravimetric analysis reveals that the weight loading of the PCDO in the composites is about 6.5 wt%, which is comparable to natural nacre (Supporting Information, Figure S1). In contrast to natural nacre, PCDO is not only the organic phase of the artificial nacre, but also the cross-linker of adjacent GO sheets. The long chain of the PCDO causes [*] Prof. Q. F. Cheng, Dr. M. X. Wu, Prof. L. Jiang Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education School of Chemistry and Environment BeiHang University, Beijing 100191 (P. R. China) E-mail: [email protected]