Graphene nanoplatelets (GNP) and carbon nanotubes (CNT) are used to enhance electrical and mechanical properties of epoxy-based nanocomposites. Despite the evidence of synergetic effects in the hybrid GNP-CNT-epoxy system, there is still a lack of studies that focus on the influence of different dispersion methods on the final properties of these ternary systems. In the present work, direct and indirect ultrasonication methods were used to prepare single- and hybrid-filled GNP-CNT-epoxy nanocomposites, varying the amplitude and time of sonication in order to investigate their effect on electrical and thermomechanical properties. Impedance spectroscopy was combined with rheology and electron microscopy to show that high-power direct sonication tends to degrade electrical conductivity in GNP-CNT-epoxy nanocomposites due to damage caused in the nanoparticles. CNT-filled samples were mostly benefitted by low-power direct sonication, achieving an electrical conductivity of 1.3 × 10⁻³ S·m⁻¹ at 0.25 wt.% loading, while indirect sonication was not able to properly disperse the CNTs and led to a conductivity of 1.6 ± 1.3 × 10⁻⁵. Conversely, specimens filled with 2.5 wt. % of GNP and processed by indirect sonication displayed an electrical conductivity that is up to 4 orders of magnitude higher than when processed by direct sonication, achieving 5.6 × 10⁻⁷ S·m⁻¹. The introduction of GNP flakes improved the dispersion state and conductivity in hybrid specimens processed by indirect sonication, but at the same time impaired these properties for high-power direct sonication. It is argued that this contradictory effect is caused by a selective localization of shorter CNTs onto GNPs due to strong π-π interactions when direct sonication is used. Dynamic mechanical analysis showed that the addition of nanofillers improved epoxy’s storage modulus by up to 84%, but this property is mostly insensitive to the different processing parameters. Decrease in crosslinking degree and presence of residual solvent confirmed by Fourier-transform infrared spectroscopy, however, diminished the glass transition temperature of the nanocomposites by up to 40% when compared to the neat resin due to plasticization effects.

The aviation industry relies on lightweight carbon fiber-reinforced polymers (CFRP) for fuel efficiency, which necessitates lightning strike protection (LSP) and electromagnetic shielding due to their electrical insulating characteristics. Traditional metallic meshes used for LSP are heavy and corrosion-prone, prompting the exploration of alternatives. This research showcases CFRP nanocomposites with enhanced LSP properties through the incorporation of graphene nanoplatelets (GNPs) and carbon nanotubes (CNTs). While the enhanced conductivity in the nanofilled epoxy matrix did not impact the overall conductivity of CFRP panels, a significant damage reduction was observed after simulated lightning strike tests. Similar approaches in the literature have also noted this discrepancy, but no attempts to reconcile it have been made. This work provides a framework to explain the damage reduction mechanism while accounting for the modest conductivity improvements in the nanoreinforced CFRPs. Additionally, a simple, nondestructive method to assess surface resin degradation after a lightning strike test is proposed, based on the fluorescence of diphenyl ketones. The discussion is supported by electrical conductivity measurements, damage pattern evaluation using the proposed UV-illumination method, ATR-FTIR, and scanning electron microscopy analysis pre- and postlightning strike simulation.

Lightweight polymer nanocomposites with enhanced electrical and mechanical performance are attractive candidates for fuel-efficient aeronautical applications. This study investigates the incorporation and selective localization of graphene nanoplatelets (GNP), carbon nanotubes (CNT), and carbon black (CB) in epoxy and epoxy–poly(ether imide) (PEI) semi-interpenetrating polymer networks (semi-IPNs). Although the semi-IPN architecture promoted the expected selective localization of the nanofillers within the continuous PEI-rich phase, the anticipated double-percolation effect was not observed. We demonstrate that the main factor driving the loss of nanoparticle connectivity was the presence of epoxy-rich droplets in the PEI-rich phase, which disrupted long-range conductive pathways. The influence of these droplets depended strongly on the nanofiller type, with changes in droplet morphology correlating with increased percolation thresholds and reduced filler connectivity. These results demonstrate that selective localization and phase cocontinuity alone are insufficient to guarantee improved electrical percolation, which requires a favorable mesoscopic morphology. Likewise, impact resistance and fracture mechanisms were also dependent on droplet morphology. A formulation containing 15 PHR of PEI (IPN15) and 2.5 wt % GNP achieved an optimal balance of properties, exhibiting electrical conductivity comparable to that of the conventional epoxy/GNP nanocomposite while providing more than a 250% increase in impact strength. Overall, the findings demonstrate the critical role of mesoscopic morphology in governing the electrical performance of multiphase nanocomposites and highlight both the limitations and potential of epoxy–PEI semi-IPNs as a platform for the development of advanced multifunctional materials.