The effect of incorporating modified clay into a polyamide 6 (PA6) matrix, on the free volume cavity sizes and the thermal and viscoelastic properties of the resulting nanocomposite, was studied with positron annihilation lifetime spectroscopy, differential scanning calorimetry and dynamic mechanical analysis. At low concentrations of clay the fraction of PA6 crystals melting close to 212 degrees C was increased, while the fraction of the a-form PA6 crystals, melting close to 222 degrees C, was reduced. At higher concentrations of clay, a crystal phase with increased thermal stability emerged. Addition of more than 19 wt% clay caused a reduction of the heat of fusion of PA6. An unexpected reduction of the Delta C-p at the glass transition, contradicting the measured reduction of the heat of fusion, was detected, indicating an altered mobility in the non-crystalline regions. The viscoelastic response of PA6/clay nanocomposites, as compared to unfilled PA6, pointed towards a changed mobility in the non-crystalline regions. At high concentrations of clay (> 19 wt%) an increase of the free volume cavity diameter was observed, indicating a lower chain packing efficiency in the PA6/clay nanocomposites. The increased free volume sizes were present both above and below the glass transition temperature of PA6. (c) 2005 Elsevier Ltd. All rights reserved.