Binding with graphene
The use of an adhesive inspired by mussels can be used to reinforce graphene fibres, as Ellis Davies reports.
Graphene fibres reinforced through a method and substance inspired by mussels can lead to improved material properties, according to researchers at The Korea Advanced Institute of Science and Technology (KAIST). The team used polydopamine, an oxidsed dopamine comprising dihydroxyindole, indoledione and dopamine units, as an infiltrate binder to improve the mechanical and electrical properties of graphene-based liquid crystalline fibres. They think this technology can be applied to various wearable textile-based devices that require high strength and high conductivity.
Dopamine is the adhesive used by mussels to stick themselves to a variety of marine surfaces. It can be seen as the yellow substance around those you may encounter on rocks at a beach. The substance has a chemical structure comprising both alkylamine and catechol functionalities.
A working progress
Researchers first began related work in 2009, when they discovered the graphene oxide liquid crystal aqueous dispersion – the spreading of graphene sheets in a solution. Other studies followed in the fabrication of a fibre, a film and a porous three-dimensional structure. Sang Ouk Kim, Chair Professor of the Multi-Dimensional Directed Nanoscale Assembly Centre Department of Materials Science & Engineering, KAIST, and one of the authors of the study, told Materials World, ‘Among the research based on liquid crystallinity of graphene oxide, low-cost wet-spinning graphene fibre fabrication to replace carbon fibre produced by existing high-cost processes emerged a few years ago. However, to catch up to the high performance of carbon fibre, the structural defects of the graphene alternative, such as internal voids and wrinkles, must be solved.’
Graphene fibre flaws occur because the original two-dimensional structure of graphene flakes and the one-dimensional structure of the fibres are not structurally suited to each other. As a result, they block the effective transfer of charge, which has adverse affects on the mechanical properties and electrical conductivity.
The team developed polydopamine-incorporated fibres to solve the structural problems by increasing the interaction energy between each graphene flake using the adhesive properties of dopamine. This increases the adhesion between graphene layers and controls defects along the fibres. ‘By using this, optimisation of dopamine polymerisation – the creation of polymer chains – conditions on graphene liquid crystal dispersion and subsequent spinning into the fibre structure with a two-step polydopamine infiltration process, solved the problem of inherent defect control of graphene fibres, leading to high strength and high conductivity,’ said Kim.
To fabricate the fibres, researchers used a two-step polydopamine treatment. Firstly, they used the Hummers method – adding potassium permanganate to a solution of graphite, sodium nitrate and sulphuric acid – to prepare a graphene oxide dispersion, after which each flake was coated with polydopamine before being spun into a fibre structure using wet-spinning. The spun fibre was then prepared with dopamine polymerisation to control residual defects, before undergoing high-temperature heat treatment to reduce the graphene oxide to graphene, while converting the polypodamine to a nitrogen-doped graphitic structure.
Improving on the past
Traditional polymeric binders, such as Nafion, enhance the interaction between structures, helping graphene sheets infiltrate voids, resulting in improved mechanical properties. But when it comes to electrical and thermal conductivities, the use of conventional binders causes inevitable deterioration, interrupting the effective transfer of charge among layers. Polydopamine differs because it can act as a binder while also enhancing the mechanical properties of the graphene through its ability to transform into a graphitic structure with heat, increasing electrical conductivity.
The new method also improves on other graphene liquid crystals by optimising the ratio of precursor and polymerisation time. This increases the stability of the dispersion, resulting in the better alignment of graphene oxide flakes. ‘In this study, the optimisation point for this was identified on the basis of experimental data. This particular dispersion was used in fabricating graphene fibres,’ explained Kim.