Insights into Structural Transformation during Oxidative Stabilization of Pan Nanofibers Reinforced with Carbon Nanotubes and Graphene Nanoplatelets

To elucidate the influence of oxidative stabilization on structural evolution and functional performance, electrospun PAN-based nanofiber mats reinforced with CNTs (1–2 wt %), GNPs (5–10 wt %), and CNT/GNP hybrids containing 1–2 wt % CNTs combined with 3–9 wt % GNPs were systematically investigated. After stabilization, FT-IR and DSC analyses confirmed enhanced cyclization in the presence of nanofillers, reflected by the increase in conjugation index from 0.76 (neat PAN) to 0.90 for CNT/GNP (2/3 wt %) fibers. The DSC results confirm this through the disappearance of the exothermic peak typically associated with the cyclization reaction. Thermogravimetric results showed a significant rise in char yield, reaching up to 70.2% for the 10 wt % GNP mat at 700 °C. Electromagnetic shielding performance was strongly influenced by the porous architecture of the electrospun mats, resulting in predominantly absorptive attenuation behavior. Nanoparticle incorporation, fiber orientation, and thermal treatment directly influenced the dielectric response, leading to higher thickness-averaged specific shielding effectiveness (TASSET). The best performance was achieved for stabilized sPAN/C2 in the orientation parallel to the incident electromagnetic field (0°), reaching εr = 18.90, TASSET = 407.37 dB·cm2·g–1. Under perpendicular alignment (90°), stabilized sPAN/C2/G8 exhibited εr = 8.64 and reached TASSET = 198.48 dB·cm2·g–1, outperforming single-filler systems in this configuration.