Enhancing the fracture toughness of hierarchical composites through amino‒functionalised carbon nanotube webs

The introduction of carbon nanotubes (CNTs) in structural fibre-reinforced polymers, to imbue the composite with multifunctional properties (e.g. enhancing electrical/thermal conductivity, structural health monitoring), has received much attention in recent years. Maintaining, and preferably enhancing, the structural integrity of the composite is imperative. Consequently, strong interfacial bonding between the CNTs and the polymer matrix is sought. If the sought multifunctionality is dependent on specific CNT alignment or orientation, achieved through fragile CNT assemblies, gas‒phase chemical functionalisation of the CNT assembly is a viable approach in order to chemically modify the CNT surface without damaging the CNT assembly. This study reports on the gas‒phase amino‒functionalisation of CNT webs (CNTw) and further explores its influence on the in situ electrical conductivity. The placement of an ethylenediamine-functionalised multilayer CNTw (0.2 g m−2) between CF plies resulted in a 13% enhancement in the interlaminar Mode I fracture toughness, while providing an electrical conductivity of 103 S m−1 in the direction of the CNTs within the interleaved CNTw. The effectiveness of the amino‒functionalised CNTw in enhancing the mechanical properties of an epoxy composite is related to an epoxy opening reaction, as demonstrated by Differential Scanning Calorimetry (DSC). Raman and X-ray photoelectron spectroscopies are used to confirm that gas‒phase amino‒functionalisation does not damage the graphene-based structure and its structural dependent properties.