Polymer/layered silicate nanocomposites, depending on the molecular weight of nylon 6 as the matrix, were prepared through melt blending with a twinscrew extruder to study their morphology and the role of layered silicates (i.e., montmorillonite) on local deformation processes. A thermal analysis of the nanocomposites was performed with differential scanning calorimetry. The dispersion of the layered silicates and nanocomposite morphology were investigated with wide-angle X-ray scattering and transmission electron microscopy. The results showed that the layered-silicate surfaces predominantly induced gamma-form crystals of nylon 6, whereas a-form crystals were nucleated far away from the clay surface. The fraction of y-form crystals and the degree of exfoliation increased with the molecular weight of the nylon 6 matrix. In a high-molecular-weight nanocomposite, a well-defined exfoliated nanomorphology was observed, in which fine nylon 6 lamellae were most likely to be oriented with their planes perpendicular to the injection-molding direction, although to a great extent the layered silicates were arranged on planes parallel to the injection-molding direction. On the contrary, a mixed nanomorphology appeared in a low-molecular-weight nanocomposite, intercalated and exfoliated nanomorphologies coexisting; the layered silicates were randomly dispersed in the nylon 6 matrix, and the nylon 6 lamellae grew in no preferential direction with respect to the layered-silicate surface. The dispersion of the layered silicates and the orientation of the layered silicates as well as the lamellae were directly reflected in the mechanical deformation processes. The results of in situ deformation experiments under a high-voltage electron microscope (1000 kV) indicated that the main deformation mechanism was microvoid formation either within the intercalated tactoids or in the vicinity of layered silicates in the matrix for intercalated and exfoliated nanomorphology systems, respectively. In the exfoliated nanocomposite, that is, the high-molecular-weight nanocomposite, the external applied stresses were much more effectively transferred from the polymer matrix to the layered silicates; furthermore, microvoid formation took place more uniformly and homogeneously throughout the deformed specimen, and thus a well-improved stiffness/ strength/ toughness balance was achieved in comparison with the mixed nanomorphology system (the low-molecular-weight nanocomposite). (C) 2007 Wiley Periodicals, Inc.