Engineering Carbon Nanotube Nanostructures in Carbon Fiber Reinforced Epoxy Matrix Composites

Incorporating carbon nanotubes (CNTs) into carbon fiber reinforced polymer composites (CFRPs) is challenging because of the need for complicated lab-scale processes and toxic chemical dispersants that makes conventional means of processing less compatible with existing industrial procedures for large-scale applications. Spray coating of engineered CNTs on carbon fiber (CF) fabric substrates can be exploited to effectively integrate the nanostructures in CFRPs allowing them to boost their functionality and tailor the microstructure of composite components. Preparing homogeneous and stable suspensions of CNTs without damaging their intrinsic properties and efficiently transferring the nanostructures on CF surface are essential to enhance the structural performance of CFRPs. In this study, we investigate the scalable fabrication of CNT integrated CFRPs by using a spray coating approach and testing their structural contribution to CFRPs. Cellulose nanocrystals (CNCs) are utilized to create hybrid nanostructures with CNTs (CNC bonded CNT) that enables stabilization of CNTs in nontoxic media, i.e., water, and promotes the scalability of the process. Fundamental molecular interactions between the hybrid nanostructures are exploited to engineer the macroscopic properties of CFRPs. It is shown with both experimental (transmission electron microscopy (TEM) and atomic force microscopy (AFM)) and computational (density functional theory (DFT)) studies that the bonding between CNC and CNT is not merely physical interaction but a stronger primary bonding i.e., covalent bonding. Scanning electron microscopy (SEM) micrographs show hybrid CNC bonded CNTs are homogeneously dispersed on the CF surface. According to <i>in-situ</i> bending test results under the optical microscope, crack propagation is hindered by engineered hybrid nanostructures in the modified CFRP whereas neat CFRP exhibits low crack growth resistance due to the uninterrupted crack propagation in the continuous epoxy matrix. The reason behind this is attributed to the enlarged interfacial span, which facilitates an efficient stress transfer from epoxy to CF, due to the favorable synergy of CNC with CNT. It is concluded that self-assembled hybrid CNC-CNT nanostructures effectively enhance the structural properties of CFRPs due to the synergistic effect of the nanostructures. This strategy provides new possibilities to precisely control the material microstructure and enables the engineering of the bottom-up manufacturing of hybrid nanostructured composites.