Thermogravimetry-Fourier transform infra-red spectroscopy (TG-IR) was used to study the thermal stabilities and thermal degradation mechanism of alpha-carboxy-polycaprolactam prepolymers (PA6), amine-terminated rigid aromatic polyamide prepolymers (PPIA) and their block copolymers (PA6-PPIA-PA6). The results showed that the thermal stability of the block copolymers was improved by the block polycondensation, which eliminates the unstable terminal groups, and the strong hydrogen bond interaction between the amide linkages. (C) 2004 Elsevier B.V. All rights reserved.