We wish to establish the relationship between free volume and molecular mobility in a series of PDMS-nanoparticle composites. The size of the free volume has already been determined using positron annihilation spectroscopy. It was found that there is less effect from covalently bound nanoparticles as compared to a simple physical dispersion. Although the free volume generally correlates well with the properties (such as glass transition temperature) the effects of the covalent binding may lead to pinning of the holes and a reduction in mobility. We therefore wish to measure the molecular mobility using QENS on the same samples.