Morphological Constraints on Double Percolation and Toughening in Nanocarbon-Filled Epoxy–Poly(ether imide) IPN Nanocomposites

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.