There are two phases in a polymer system: a solid phase occupied by polymer chains and a space phase named as free volume. In Figs. 4 and 6, the temperature dependence of density of PET/silica composites shown a smaller gradient than that of the pure polymer system, which means that there is less free volume provided for the movement of the polymer chains. From a microscopic viewpoint, the exit of the silica enhances the polymer cross-linkage and decreases the free volume of the polymer chains.