Space that cannot be accessed is not free: Why volume and dynamics decouple in nanocomposite matrices

We report on our first modeling studies involving nanocomposite systems: polyvinyl acetate (PVAc) and polystyrene (PS), each with silica nanoparticles. In each case, experimental data showed that the segmental relaxation times and glass transition temperatures of the polymer matrix material in the nanocomposite were essentially unchanged relative to those of the neat polymer, however, the specific volume of the former was higher than that of the latter (with temperature fixed). We have demonstrated for many systems that segmental relaxation times at fixed temperature track with density changes, as described using our Cooperative Free Volume (CFV) model. So it was provocative to encounter examples where this connection seemed to break down – but does it, really? This talk will provide the answer, and yield insight as to how nanoparticle presence can affect thermophysical properties, and therefore dynamic response.