The theological and mechanical properties of commercial neat nylon 6 and nylon 6 nanocomposites containing organically-modified montmorillonite (organoclays) produced by either in situ polymerization or melt-blending were investigated. The dynamic and steady shear, capillary and extensional viscosity of the neat nylon 6 and nylon 6 nanocomposite melts were studied, as well as the tensile properties of the solid material. X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated that the organoclays were largely very well exfoliated, although the lateral size scale of the platelets was different for each material. The in situ polymerized nanocomposite exhibited higher melt viscosity and higher tensile ductility than the melt-blended nanocomposite which was related to improved dispersion and polymer-silicate interactions for this material. Scanning electron microscopy confirmed that the nanocomposite failure surfaces showed more evidence of brittle behavior than the failure surfaces of neat nylon 6, and also that agglomerates of organoclay could be seen easily in the fracture surface of the melt-blended nanocomposite, but not to the same degree as in the in situ polymerized nanocomposite. This is in addition to very fine, individually-dispersed silicate laminates that form in each case. (c) 2005 Elsevier Ltd. All rights reserved.