Filled elastomers exhibit complex dependence of their viscoelastic modulus, both as a function of temperature and frequency. Otherwise, recent observations on thin polymer films have shown that their glass transition temperature depends on their thickness. Based on these recent results, and on a recent model, we propose that the mechanical behaviour of the filled elastomer is strongly influenced by a gradient of the glass transition temperature in the vicinity of the particles. This allows us to suggest a specific temperature-frequency superposition law for filled rubbers. This law seems to apply very successfully on two systems with different dispersion qualities, revealing the existence of a glass transition temperature gradient in the vicinity of the particles.

The dynamics of the polymer matrix in filled rubbers is modified by the presence of solid particles. We used low-field proton NMR to investigate model filled samples consisting of a dispersion of grafted silica particles into an elastomeric matrix. Exploiting magic-sandwich echo experiments, we were able to determine the fraction of polymer with slower dynamics and to correlate it to the silica specific surface. The presence of immobilized polymer;most probably due to a gradient of glass transition temperature around the solid particles;is detected whether there is a covalent bond between the filler and the matrix or not. Moreover, the fraction of immobilized polymer decreases in similar ways with either an increase of the temperature or the addition of solvent. In the case of covalent bonds between the silica and the polymer, multiple-quantum experiments reveal that the cross-link density of the elastomer matrix is locally increased in the vicinity of the particles. This is an observation that was not made in any conventional filled elastomer system and it can be attributed to the good particle dispersion and the covalent links in our model samples.