Mighty molluscs aid super strength nanocomposites

Lead author Professor Markus Buehler said: “Composites are widely used in the design of tunable materials for specific properties such as light weight and high flexibility, as well as high strength and toughness.”

He continued: “However, most engineering materials sacrifice strength for toughness. Biological materials often do not face this trade-off, thanks to their hierarchical structures and multi-functional abilities optimised over millions of years of evolution. They offer an exciting new paradigm to advance our materials design ability, especially when we can combine distinct material platforms that exist separately in nature, but we as engineers can combine in novel ways.”

Nacre is a natural material that offers good guidelines for creating high performance composites, being made up of 95% calcite minerals and with five percent biopolymer proteins, and having a toughness around 3,000 times higher than its base material (i.e., the brittle calcite minerals) alone.

Co-author Dr Chun-Teh Chen said: “Our study combined nacre-inspired graphene oxide (GO) sheets, with polydopamine (PDA) as an intermediate material. PDA is a mussel thread-inspired material, light in weight and with an extremely robust adhesive ability, making it very well suited to being an intermediate material in a nanocomposite.”

Using multi-scale modelling methods including density functional theory (DFT) calculations and molecular dynamics simulations, the team designed a series of nacre-like GO–PDA papers, which they then produced by experimental synthesis.

Dr Chen continued: “These GO–PDA papers have remarkable properties, with strength up to 170MPa, toughness up to 5.6MJ per cubic metre, and shrinking strain up to 0.6%. Our simulated pulling tests show that the enhanced mechanical strength of GO–PDA papers mainly comes from the additional non-covalent interactions provided by PDA, instead of from covalent cross-links between adjacent GO–PDA sheets suggested in other studies.

“This could indicate an opportunity to further increase the mechanical properties of GO–PDA papers, if a large number of covalent cross-links can form to connect GO–PDA sheets together. We believe the high mechanical properties and shrinking ability of GO–PDA papers make the material a good candidate for creating humidity driven self-folding devices.”