Diminishing Interfacial Effects with Decreasing Nanoparticle Size in Polymer-Nanoparticle Composites.

Using molecular simulations on model polymer nanocomposites at fixed filler loading, we show that interfacial polymer dynamics are affected less with decreasing nanoparticle (NP) size. However, the glass transition temperature T_{g} changes substantially more for an extremely small NP. The reason for this apparent contradiction is that the mean NP spacing decreases with decreasing particle size. Thus, all polymers are effectively interfacial for sufficiently small NPs, resulting in relatively large T_{g} shifts, even though the interfacial effects are smaller. For larger NPs, interfacial relaxations are substantially slower than the matrix for favorable NP-polymer interactions. The minority "bound" polymer dynamically decouples from the polymer matrix, and we only find small changes in T_{g} relative to that of the bulk polymer for large NPs. These results are used to organize a large body of relevant experimental data, and we propose an apparent universal dependence on the ratio of the face-to-face distance between the NPs and the chain radius of gyration.