Study on the Construction of Dopamine/Poly(ethyleneimine)/Aminoated Carbon Nanotube Multilayer Films on Aramid Fiber Surfaces to Improve the Mechanical Properties of Aramid Fibers/Epoxy Composites

To improve the mechanical properties of aramid fiber (AF) reinforced epoxy resin (EP) composites without damaging the strength of the AF body, in this paper, poly(ethyleneimine) (PEI) and aminoated carbon nanotubes (NH2-CNTs) were successfully deposited on the AF surface layer by layer using poly(dopamine) (PDA) as the initial layer. The modified aramid fibers PDA-AF, PEI-PDA-AF, and NH2-CNTs-PEI-PDA-AF were prepared. The microstructure and chemical composition of the AF surface at different modification stages were systematically characterized. The interfacial properties, mechanical properties, and strengthening mechanism of AF surface-modified composites were studied. The results showed that with the successful deposition of PDA, PEI, and NH2-CNTs layer by layer, the interfacial properties and mechanical properties of the composites gradually improved. Among them, NH2-CNTs-PEI-PDA-AF showed the best strengthening effect. Compared with the unmodified aramid fiber (R-AF), the monofilament tensile strength of NH2-CNTs-PEI-PDA-AF increased by 8.1%, the contact angle with EP decreased by 21.9%, and the interface energy and adhesion energy increased by 115 and 21.4%, respectively. Compared with R-AF/EP, the interlaminar shear strength (ILSS), bending strength, and tensile strength of NH2-CNTs-PEI-PDA-AF/EP were increased by 75, 44.5, and 14.9%, respectively. The significant improvement of the interface properties and mechanical properties between NH2-CNTs-PEI-PDA-AF and EP can be attributed to the introduction of a large number of amino active groups in the NH2-CNTs-PEI-PDA coating layer on the AF surface, which strengthens the chemical-bond cooperation between the AF and EP matrix. At the same time, a large number of NH2-CNTs deposited on the surface effectively increased the surface roughness of AF, improved the mechanical meshing between the AF and EP matrix, and then improved the contact angle, surface energy, and interface bonding strength between the AF and EP matrix. Moreover, a large number of NH2-CNTs on the surface of AF also modified and enhanced the EP in the interface region, which could make the load more effectively transfer from the resin to the fiber, so that AF could carry the load more uniformly, significantly improving the mechanical properties of NH2-CNTs-PEI-PDA-AF/EP.