nanodiamonds were reduced 4 times approaching 0.1.
While high loadings of nanodiamonds result in remarkably high hardness and Young’s moduli of the epoxy– nanodiamonds composites, lower concentrations of nanodiamonds can be used to improve the mechanical properties177. The bulk Young’s modulus measured in tensile tests was 25% higher upon addition of 0.5 wt.% as-received nanodiamonds, which also increased the decomposition temperature. However, due to poor nanodiamonds dispersion, the storage modulus of the epoxy composite was significantly reduced, emphasizing the importance of a good dispersion to optimize the mechanical properties of nanodiamonds –polymer composites. A study on the mode I and II fracture toughness of nanodiamonds –epoxy composites have shown that, beyond an improved Young’s modulus and hardness, the mode II fracture toughness of epoxy– nanodiamonds composites with 0.
1 wt.% nanodiamonds is increased. This is because nanodiamonds are thought to inhibit shear deformation, improving fracture toughness195.
For covalent binding to epoxy, pointed to form the most strong ND– polymer interface, nanodiamonds terminated with reactive amino groups was formed by combining ethylenediamine to nanodiamonds –COOH surface via an amide bond (yielding nanodiamonds –CONH(CH2)2NH2, hereafter named nanodiamonds –NH2). The rationale behind using amino terminated ND is similar to molecular curing agents, a reaction of nanodiamonds –NH2 with epoxy resin is predicted to result in a covalently bonded network of nanodiamonds and epoxy molecules. However, to get full advantage of covalent nanodiamonds –polymer interface, it is unfavorably essential to have covalent bonds all the way from nanodiamonds surface to the macromolecules of the matrix. Therefore, first, covalent bonding between the diamine molecules and nanodiamonds particles was confirmed by FTIR, TG, and DSC.
When nanodiamonds –NH2 reacted with the epoxy resin, strong covalent nanodiamonds – the epoxy interface was formed as proof by DSC, which was used to monitor the reaction. As a result, Young’s modulus of a composite containing 3.5 vol.% nanodiamonds –NH2 was enhanced by 60%196. Also, it was found that in order to manufacture ND–NH2–epoxy composites with uniformly dispersed nanodiamonds it is important to keep nanodiamonds –NH2 dispersed in a compatible and inert solvent without drying. Tetrahydrofuran (THF) was chosen for this purpose as it provides a good dispersing medium for nanodiamonds –NH2, dissolves epoxy resin and, according to a previous report, it does not react with components of the