  Both the dispersion state of nanoparticles (NPs) within polymer
nanocomposites (PNCs) and the dynamical state of the polymer altered by the
presence of the NP/polymer interfaces have a strong impact on the macroscopic
properties of PNCs. In particular, mechanical properties are strongly affected
by percolation of hard phases, which may be NP networks, dynamically modified
polymer regions, or combinations of both. In this article, the impact on
dispersion and dynamics of surface modification of the NPs by short
monomethoxysilanes with eight carbons in the alkyl part (C8) is studied. As a
function of grafting density and particle content, polymer dynamics is followed
by broadband dielectric spectroscopy and analyzed by an interfacial layer
model, whereas the particle dispersion is investigated by small-angle X-ray
scattering and analyzed by reverse Monte Carlo simulations. NP dispersions are
found to be destabilized only at the highest grafting. The interfacial layer
formalism allows the clear identification of the volume fraction of interfacial
polymer, with its characteristic time. The strongest dynamical slow-down in the
polymer is found for unmodified NPs, while grafting weakens this effect
progressively. The combination of all three techniques enables a unique
measurement of the true thickness of the interfacial layer, which is ca. 5 nm.
Finally, the comparison between longer (C18) and shorter (C8) grafts provides
unprecedented insight into the efficacy and tunability of surface modification.
It is shown that C8-grafting allows for a more progressive tuning, which goes
beyond a pure mass effect.
