materials research | Graphene and nanofi llers
Haydale’s Martin Williams showed these SEM/TEM images of a graphene nanoplatelet (left) and few-layer graphene (right)
mechanical properties deteriorated at higher loadings. Tensile testing showed glassy polymeric behaviour in
composites containing both GO and bwGO. There was an increased elastic modulus for both and higher tensile strength only with the addition of GO; however, there was also increased brittleness at higher loadings for both, due to the formation of agglomerates, leading to a deterioration of the interface. Overall, Valles found superior mechanical behaviour
for GO over bwGO. This suggests that the presence of OD in the GO provides a stronger interface with the PMMA matrix, she concluded. However, both outper- formed unreinforced PMMA and there is clear potential for either or both in PMMA composites. Toby Sainsbury of the National Physical Laboratory (NPL) implicitly took issue with some of this. The NPL has worked extensively on the characterisation of
graphene and other 2D nanomaterials using a huge variety of common and specialised analytical tech- niques in a project that has occupied 25 people. He regards GO as “not a great material – it looks ragged”. Sainsbury was more in tune with Martin Williams, technical manager at Haydale, in describing the surface functionalisation of nanomaterials as “absolutely key to application”. This holds true for graphene, GO and the various boron nitride nanomaterials that also show huge promise and which have similar structures to and equally rich chemistry as graphene.
“We are working on determining functionality, the percentage graphene content needed, where the functional groups are, and where these can be added to polymers, oligomers and reactive groups to create different surface effects,” Sainsbury said.
Williams contrasted the expectations for nanocar-
bons – graphene and CNTs – against the reality. They are actually inert materials, with low surface energy and highly agglomerated. In short, they need function- alisation to realise their potential, not least in terms of dispersion in the polymer matrix. Haydale, a specialist in the fi eld, has carried out
studies of the plasma functionalisation of nanocarbons, variously using oxygen, acid vapour and ammonia plasma. This, he said, is a non-damaging method compared to acid functionalisation, causes no environ- mental damage and is robust, variable and controllable. Used with nanomaterials, plasma functionalisation
improves chemical compatibility in terms of chemical bonding and wetting behaviour. It contributes to deagglomeration via improved dispersion and aids in etching by removing contamination or increasing crystallinity. All this, Williams said, has led to greatly increased performance at low loadings in carbon fi bre-reinforced composites. “Nanocarbons provide multi-functional improve- ments, so you need to be clear about what you want to achieve before you start,” said Williams. As work between Haydale and Swansea University with thermo- plastic composites has shown, there may be a balance between the improvement and deterioration of compet- ing properties, such as strength, stiffness, processabil- ity or barrier properties. The choice of matrix may be governed by how much
The thermal conductivity of PA 6,6 is increased through the addition of 45 wt. % boron nitride and further increased by replacing just 0.1 wt.% of the boron nitride with Carbodeon’s NanoDiamonds
32 COMPOUNDING WORLD | February 2014
fi ller it can tolerate; polycarbonate does not take to fi llers, while polyethylene does. Similarly the choice of nanocarbon may be governed by its morphology, surface area and fi ll levels. Compatibility between the matrix and nanocarbon is always crucial, hence the need for functionalisation. In the same vein, Gavin Farmer of Carbodeon observed that it is necessary to replace “material
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