Super strength nanocomposites owe a debt to mighty molluscs

10 May 2017

A mother of pearl shell

Mother-of-pearl and mussel threads could hold the key to developing graphene-based nanocomposite materials with enhanced properties.

Researchers from the Massachusetts Institute of Technology, USA, employed the structure of mother of pearl (also known as nacre), and the extreme adhesiveness of mussel threads to create a graphene oxide-polydopamine (GO-PDA) composite with improved electrical conductivity and tensile strength.

They report their results today in the new journal Nano Futures.

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.

“However, most engineering materials sacrifice strength for toughness. Biological materials often do not face this tradeoff, thanks to their hierarchical structures and multi-functional abilities optimized 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 per cent calcite minerals and with 5 per cent 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 multiscale 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 said: “These GO–PDA papers have remarkable properties, with strength up to 170 megapascals, toughness up to 5.6 megajoules 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.”