Processing‐Driven Trade‐Offs in Thermal and Mechanical Performance of Polyetherimide‐Carbon Nanotube‐Expanded Graphite Hybrid Composites

ABSTRACT This study examines the influence of processing strategy on the thermal and mechanical properties of polyetherimide (PEI)‐carbon nanotube‐expanded graphite composites. Two melt compounding approaches, single‐step (SS) and two‐step (TS) were evaluated at identical filler loadings (15 wt% EG, 0.5 wt% CNT, plus additives). Both approaches enhanced the in‐plane thermal conductivity relative to neat PEI ( k 0 = 0.22 W/m K). The SS approach achieved 5.75 ± 0.5 W/m K, approximately 20% higher than TS (4.8 ± 0.5 W/m K), attributed to greater graphite exfoliation and improved formation of conductive pathways. However, TS better preserved mechanical properties. These findings highlight the trade‐off between thermal performance and mechanical strength, underscoring the importance of processing strategies for optimizing multifunctional PEI hybrid composites.