PVC/CaCO3 polymer nanocomposites of differing compositions were produced using a two-roll mill and compression molding. In all formulations, 0.6 phr of titanate was incorporated to assist dispersion during processing. The morphology was observed using transmission electron microscopy, and the static and dynamic mechanical and fracture properties were determined. Fracture toughness examination was performed according to strain energy release test method. The presence of nanometer-sized CaCO3 particles led to a slight decrease in the tensile strength but improved the impact energy absorption, storage modulus, and fracture toughness. The use of titanate coupling agent softened the polymer matrix and reduced the matrix's modulus. Fracture surface examinations by scanning electron microscopy showed that the coupling agent improved particlematrix bonding and inhibited void formation around the particles. Finite element analysis suggested that the improved particlematrix bonding reduced the matrix's plasticity around the particles, which decreased the toughening efficiency of the composites. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013