Nylon 6 nanocomposites were prepared by the in situ polymerization of F-caprolactam with ultrasonically dispersed organically modified montmorillonite clay (Cloisite 3013(F)). Dispersions of the clay platelets with concentrations in the range 1-5 wt % in the monomer were characterized using rheological measurements. All mixtures exhibited shear-thinning, signifying that the clay particles were dispersed as platelets and forming a "house of cards" structure. Samples with Cloisite concentrations above 2 wt % showed a drop in viscosity between the initial shearing and repeated shearing, indicative of shearing breaking down the initial "house of cards" structures formed on sonication. DMTA measurements of the samples showed an increase in the P-relaxation temperature with increasing clay concentration. The bending modulus, at temperatures below T,,, showed an increase with increasing clay concentration up to 4 wt %. X-ray diffraction measurements showed that all nylon 6/Cloisite 30B samples were exfoliated apart from the 5 wt %, which showed that some intercalated material was present. The nylon crystallized into the a-crystalline phase, which is the most thermodynamically stable form. Preference for this form may be a consequence of the long time associated with the postcondensation step in the synthesis or the influence of the platelets on the nucleation step of the crystal growth. DSC measurements showed a retardation of the crystallization rate of nanocomposite samples when compared with that of pure nylon 6, due to the exfoliated clay platelets hindering chain movement. This behavior is different from that observed for the melt-mixed nylon 6/clay nanocomposites, which show an enhancement in the crystallization rate. (C) 2008 Wiley Periodicals, Inc.