Renowned for its honeycomb lattice structure of carbon atoms, graphene is hailed as a material of excellence because of its superior electric conductivity and mechanical properties. These traits are crucial in the development of malleable electronics, cutting-edge energy storage systems, and composite materials tailored for the aerospace sector. However, fashioning enduring and stretchable films from graphene has presented challenges. In a recent issue of Angewandte Chemie, scholars have introduced an innovative way by interconnecting graphene nanolayers using rotatable linking structures, aiming to gloss over past constraints.
Traditionally, the remarkable attributes of minuscule graphene nanolayers reduce upon compositing into foils due to the weak van der Waals forces—chiefly hydrogen bonds—that hold them together. Efforts to enhance the mechanical robustness of graphene foils by fostering stronger interactions have met with limited triumph, leaving significant potential for enhancement in the elasticity and resilience of these materials.
Revolutionary Techniques for Cross-linking
Spearheaded by Xuzhou Yan at Shanghai Jiao Tong University (China), research teams adopted a novel strategy: they used mechanically interlocked molecules to interlink graphene nanolayers. These molecules consist of subunits that, while not bonded chemically, remain inseparably interwoven in space. The group selected rotaxanes as their linking molecules. Rotaxanes comprise a “wheel” (a macrocyclic molecule) which is “strung” onto an “axle” (a molecular thread), secured by bulky endgroups to keep the wheel from disengaging. The team designed the axle endowed with an ionic section (ammonium) to position the wheel precisely.
The researchers affixed a molecular “anchor” (OH group) to both the wheel and axle using a linking agent. They initially oxidized graphene to obtain graphene oxide, which bears several oxygen-rich functional groups on the surface of the graphene sheet. These include carboxyl groups, adept at bonding to the OH groups (through esterification). Following this chemical binding, the wheel and axle effectively interlink the sheets, after which the graphene oxide is restored to graphene.
Once tensile force is applied to these films, it must overcome the binding forces between the wheel and the axle’s ammonium segment, resulting in enhanced tensile strength. Subjected to extreme tension, the axle is eventually pulled through the wheel until it comes into contact with the cap. This traversal extends the rotaxane bridges, enabling the layers to glide past one another, thus notably enhancing the film’s extension capabilities.
Electrodes crafted from this graphene-rotaxane composition can endure stretching up to 20% and handle repeated bending without damage, while maintaining their high electrical conductivity. Fracture only occurs when stretched beyond 23%. These new foils exhibited significantly augmented strength compared to foils lacking rotaxane (247.3 vs 74.8 MPa), and were found to be more pliable (23.6 vs 10.2%) and resilient (23.9 vs 4.0 MJ/m3). Additionally, the team fabricated a rudimentary “clamping device” outfitted with mechanical joints activated by the innovative foils.
The research received funding from the National Natural Science Foundation of China, the Shanghai Natural Science Foundation, the Shuguang Program supported by the Shanghai Education Development Foundation and Shanghai Municipal Education Commission, and the Starry Night Science Fund of Zhejiang University’s Shanghai Institute for Advanced Study.
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