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Industrial and model polymer nanocomposites are often formulated with coatingagents to improve polymer-nanoparticle (NP) compatibility. Here thelocalization of silane coating agents in styrene-butadiene nanocomposite isinvestigated through the segmental dynamics of the polymer matrix by broadbanddielectric spectroscopy (BDS), allowing the detection of silanes in the matrixthrough their plasticization effect. This acceleration of dynamics was followedvia the shift of $\tau$max of the $\alpha$-relaxation induced by the presenceof coating agents of different molecular weight and quantity, for differentamounts of incorporated colloidal silica NPs (R $\approx$ 12.5 nm,polydispersity 12%). Any noteworthy contribution of interfacial polymer layerson $\tau$max has been excluded by reference measurements with bare NPs. Ourapproach allowed quantifying the partition between the matrix and the NPinterfaces, and was confirmed independently by calorimetry. As a controlparameter, the silane grafting reaction could be activated or not, which wasconfirmed by the absence (resp. presence) of partitioning with the matrix. Ourmain result is that in the first steps of material formulation, before anygrafting reaction, coating agents both cover the silica surface by adsorptionand mix with the polymer matrix-in particular if the latter has chemicalcompatibility via its functional groups. Silane adsorption was found to becomparable to the grafted amount (1.1 nm-2), and does not increase further,confirming that the plateau of the adsorption isotherm is reached in industrialformulations. These results are hoped to contribute to a better understandingof the surface reactions taking place during complex formulation processes ofnanocomposites, namely the exact amounts at stake, e.g., in industrial mixers.Final material properties are affected both through NP-matrix compatibility andplasticization of the latter by unreacted molecules. 2

Partition of Coating Agents between Nanoparticle Interfaces and the Polymer in Nanocomposites