The pressure-volume-temperature (PVT) dependence of commercial polymers (viz. polys-caprolactam, PA-6, polystyrene, PS, polypropylene, PP, and epoxy), and of nanocomposites based on these resins, was studied in the molten state. Incorporation of 2-wt% organoclay decreases the specific volume by about 1.0%. The Simha-Somcynsky (S-S) lattice-hole theory provided excellent description of the PVT behavior. Congruence between the theory and experiment yielded an explicit measure of the free volume. It has been found that in polymeric nanocomposites (PNC) the reduction of the free volume, Delta h, correlated with the degree of clay dispersion (e.g. expressed as the interlayer spacing, d(001)), or more precisely with total clay surface area accessible to macromolecules. The free volume reduction originates in adsorption and solidification of macromolecules on the high-energy clay surface - the phenomenon observed in the Surface Force Analyzer. The solidification offers consistent explanation for several reported puzzling behaviors of PNC.