The Effect of Network Formation on the Mechanical Properties of 1D:2D Nano:Nano Composites

Mixtures of 1D carbon nanotubes and 2D nanosheets are important in electrochemical applications where the nanosheets are the active material, while the nanotubes provide electrical conductivity and mechanical reinforcement. While the conductivity of such nano:nano composites has been studied, the mechanical properties have not. Here, we report a detailed study of the structural, electrical, and mechanical properties of composites of MoS2 nanosheets mixed with carbon nanotubes at various volume fractions, phi. Microscopic analysis reveals the nanotube network to evolve from a loosely connected structure for phi < 1% to a strongly entangled continuous structure for phi > 1%. Significantly, while the network consists of low (similar to 200) aspect-ratio bundles for phi < 1 vol %, above this value, entangled ropes predominate, with the aspect ratio rising sharply with increasing phi, reaching similar to 4700 for the 6.4 vol % sample. While this transition does not affect the electrical properties, it has a significant effect on the mechanical properties. Below phi = 1%, both the modulus and failure strain follow short-fiber composite behavior with the modulus increasing as per the rule of mixtures and failure strain falling with phi. However, above phi = 1%, both parameters increase with phi, consistent with continuous-fiber network behavior. The tensile strength also transitions at 1 vol % from a regime limited by the matrixfiber interfacial strength (phi < 1 vol %) to one limited by the strength of the ropes (phi > 1 vol %). Similarly, the toughness is constant at low volume fraction but increases strongly for phi > 1 vol %, consistent with a model based on percolation theory.