  Fundamental understanding of macroscopic properties of polymer nanocomposites
(PNCs) remains difficult due to the complex interplay of microscopic dynamics
and structure, namely interfacial layer relaxations and three-dimensional
nanoparticle arrangements. The effect of surface modification by alkyl
methoxysilanes at different grafting densities has been studied in PNCs made of
poly(2-vinylpyridine) and spherical 20 nm silica nanoparticles (NPs). The
segmental dynamics has been probed by broadband dielectric spectroscopy, and
the filler structure by small-angle X-ray scattering and reverse Monte Carlo
simulations. By combining the particle configurations with the interfacial
layer properties, it is shown how surface modification tunes the attractive
polymer-particle interactions: bare NPs slow down the polymer interfacial layer
dynamics over a thickness of ca. 5 nm, while grafting screens these
interactions. Our analysis of interparticle spacing and segmental dynamics
provides unprecedented insight into the effect of surface modification on the
main characteristics of PNCs: particle interactions and polymer interfacial
layers.
