Numerous ongoing efforts strive to exploit the full potential of graphene—the most fashionable current material in materials science—and rely on its exceptional tensile strength for various structural applications. However, this two-dimensional carbon structure still faces challenging scalability and mass production roadblocks. More importantly, bulk composites cannot seamlessly integrate graphene into their structure without sacrificing many of the beneficial properties of this material. Graphene oxide (GO), which includes various epoxy and hydroxyl groups, is easier to mass-produce and combine with other materials. However, stacked GO sheets are held together by intermolecular hydrogen bonds of adjacent oxygen-containing groups. On their own, without additional reinforcement, these networks stand up poorly to shear stresses and cannot generate composites that are both strong and tough.
In order to solve this challenge, researchers from the Laboratory for Atomistic and Molecular Mechanics at the Massachusetts Institute of Technology turned to mussels for inspiration. They found that the feet of these mollusks contain adhesive proteins with a structure that closely resembles polydopamine (PDA), which is a dopamine molecule that is polymerized under alkaline conditions. The research team, led by Markus J. Buehler, chemically bonded this material with graphene oxide layers and used a combination of experimental and computational approaches to describe the structure and properties of the resulting composites. The team published their results in a recent issue of Nano Futures (doi:10.1088/2399-1984/aa6aed).
Buehler says, “We were excited about the opportunities that arise when combining distinct biological material platforms into a new system, such as done here by taking advantage of the great adhesion properties of mussel threads, the intriguing layered geometry of nacre, combined with graphene oxide to realize a synthetic analog of the layered minerals. These concepts allowed us to construct a de novo designer material that offers exceptional mechanical properties combined with other useful traits.”
The research group assembled the composites into stacks that were modeled after the structure of nacre shells. Much like their biological counterpart, the materials drew their superior strength from the interwoven polymer. Density functional theory calculations showed that, instead of oxygen-to-oxygen functional group bonding, the polymer found reactive epoxy groups and formed carbon-to-carbon bonds, which are significantly stronger. Since the reaction simultaneously defunctionalized graphene oxide, the resulting graphene (with a 130 GPa tensile strength) was much stronger than its oxidized version (63 GPa tensile strength).
The chemical bonds were the first important element of the structure. The hydrophilic nature of the material intercalated different amounts of water in between the layers, and the researchers relied on x-ray diffraction and molecular dynamics simulations to assess the effects of this process on layer-to-layer stacking. The “dry” GO-PDA composites featured large interlayer spacings, and small amounts of water that intercalated between the sheets without pushing them apart. In contrast, water molecules used strong hydrogen bonding to bridge adjacent laminates, which pulled them together and shrunk the composite.
(a) Scanning electron microscope images of the graphene oxide-polydopamine (GO-PDA) material; (b) a snapshot of a molecular dynamics simulation for GO and GO-PDA with different water content. Credit: Chun-Teh Chen and Shengjie Ling.
Intercalated water molecules, and the resulting additional bonding reinforcement, yielded greater strength and toughness of the resulting GO-PDA composites. The researchers subjected these materials to different air moisture content levels, and an increase in relative humidity from 33% to 75% increased the toughness by 123%. The strength of the resulting composites, which had reached 170 MPa, exceeded the level of their natural nacre counterparts.
Bioinspired nanomaterials inherit designs that are the product of millions of years of evolution, and the resulting properties offer numerous improved capabilities. The work by Buehler’s group shows that composite structures and laboratory-engineering materials, such as graphene composites, stand to benefit by drawing inspiration from living organisms. This work also shows the benefits of a combined approach that relies on both computational simulations and experimental results in an effort to design high-performing, commercially viable materials and composites.
