The effects of processing history and annealing treatment on the thermal property and polymorphic Structure of nylon-6/clay nanocomposites (NCNs) have been investigated. The nanocomposites, including intercalated and exfoliated ones, were prepared by extruding nylon-6 (N6) with sodium montmorillonite (Na-MMT) or organo-montmorillonite (OMMT), respectively. DSC analysis revealed multiple melting endotherms in either the extruded or the injection-molded N6 and NCNs samples. It has been observed that a small exothermic peak around 195 degrees C just before the lower melting peak in the skin regions. We demonstrated that this sub-T-m transition was directly related to the processing-induced shear stresses. WAXD analysis was further performed to characterize the polymorphic structure of injection-molded N6 and NCNs before and after annealing. Annealing at a temperature (80 degrees C) above the T-g of N6 resulted in increase of the absolute content of gamma-form in the skin regions and of the relative content of gamma-form in the core regions of NCNs. In particular, annealing only leaded to increase the fraction of gamma-form in the exfoliated N6/OMMT nanocomposites, which might be related to a confining effect of MMT platelet on the polymer chains mobility. (c) 2005 Elsevier Ltd. All rights reserved.