Low-energy impact testing on epoxy composites reinforced with DNA-functionalized carbon nanotubes

The discovery of the exceptional electrical, thermal and mechanical properties of carbon nanotubes (CNTs) has initiated many investigations into the possibility to embed them in polymer matrices with the main objective of creating advanced multifunctional materials. This possibility is particularly relevant in the aerospace field, where the use of such materials can satisfy the lightweight requirement by replacing complex and heavier subsystems of a spacecraft. For these reasons, CNTs are largely investigated as reinforcements for epoxy matrix, which is the predominant polymer for structural applications in spacecraft and aircraft vehicles. Despite their high elastic modulus and tensile strength, the enhancing of the mechanical properties of epoxy resins reinforced with CNT, either single-walled or multi-walled, are largely unpredictable due to their strong dependence on the manufacturing process. In particular, the mechanical performance of epoxy nanocomposites is greatly affected by the dispersion efficiency of the nanotubes inside the polymer matrix. To improve their dispersion in epoxy resins, CNTs are commonly functionalized following covalent strategies. Chemical treatments, by which functional groups are attached to the nanotube walls, make the CNTs more compatible with the matrix improving the homogenization of the dispersion. However, surface modifications cause a degradation of the overall properties of the nanotubes, so that the benefits added by the more homogenous dispersion are in fact lost in the final composite material. In previous work, we demonstrated the possibility to obtain a homogeneous dispersion of multi-walled carbon nanotubes (MWCNTs) in an epoxy resin using a non-covalent functionalization based on DNA wrapping. The advantage of this approach is that MWCNTs maintain the intact features of pristine nanotubes. Here, we study the effects of the use of such functionalization on the fracture toughness of the composite material. In addition, we investigate the low impact behavior using a dropped weight test, in order to assess on a whole the impact resistance of such materials. For the tests we used an experimental set-up consisting of an in-house drop tower system capable to simulate low velocity impact conditions with energy of 9.8 J. The performance evaluation is linked to the observation of the samples behavior during the test, followed by microscopy inspection of the fracture surfaces.