The cyclization mechanism of polyacrylonitrile (PAN) in PAN/functionalized carbon nanotube (CNT) composites was examined. The surface functionalization of CNTs was carried out by using diazonium reagents with 4-substituted aniline. The results strongly suggest that the type of functional groups on the CNTs strongly influences the cyclization mechanism of PAN during the stabilization process. The nitrile of PAN in F-Ph-CNT/PAN composite was cyclized through the free radical reaction during thermal stabilization whereas nitrile of PAN in COOH-Ph-CNT/PAN composite underwent cyclization via the ionic reaction due to the acid groups on the surfaces of the CNTs. The fluoro functional groups on the CNTs can act as effective external initiators for nitrile cyclization in homo PAN, in contrast to acid functional groups. Consequently, a lower cyclization temperature (265 degrees C) and enthalpy value (688 J/g) of F-Ph-CNT were shown compared to those of homo PAN. (C) 2012 Elsevier Ltd. All rights reserved.