  Industrial and model polymer nanocomposites are often formulated with coating
agents to improve polymer-nanoparticle (NP) compatibility. Here the
localization of silane coating agents in styrene-butadiene nanocomposite is
investigated through the segmental dynamics of the polymer matrix by broadband
dielectric spectroscopy (BDS), allowing the detection of silanes in the matrix
through their plasticization effect. This acceleration of dynamics was followed
via the shift of $\tau$max of the $\alpha$-relaxation induced by the presence
of coating agents of different molecular weight and quantity, for different
amounts of incorporated colloidal silica NPs (R $\approx$ 12.5 nm,
polydispersity 12%). Any noteworthy contribution of interfacial polymer layers
on $\tau$max has been excluded by reference measurements with bare NPs. Our
approach allowed quantifying the partition between the matrix and the NP
interfaces, and was confirmed independently by calorimetry. As a control
parameter, the silane grafting reaction could be activated or not, which was
confirmed by the absence (resp. presence) of partitioning with the matrix. Our
main result is that in the first steps of material formulation, before any
grafting reaction, coating agents both cover the silica surface by adsorption
and mix with the polymer matrix-in particular if the latter has chemical
compatibility via its functional groups. Silane adsorption was found to be
comparable to the grafted amount (1.1 nm-2), and does not increase further,
confirming that the plateau of the adsorption isotherm is reached in industrial
formulations. These results are hoped to contribute to a better understanding
of the surface reactions taking place during complex formulation processes of
nanocomposites, namely the exact amounts at stake, e.g., in industrial mixers.
Final material properties are affected both through NP-matrix compatibility and
plasticization of the latter by unreacted molecules. 2
