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

Abstract

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 styrenebutadiene 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 τmax of the α-relaxation induced by the presence of coating agents of different molecular weight and quantity, for different amounts of incorporated colloidal silica NPs (R ≈ 12.5 nm, polydispersity 12%). Any noteworthy contribution of interfacial polymer layers on τmax 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 matrixin 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.

Tailoring Interfacial Properties in Polymer–Silica Nanocomposites via Surface Modification: An Atomistic Simulation Study

“…Specifically, we considered two common alkylsilane agents with different alkyl groups (depicted by the number of carbon atoms n a ), along with two silane grafting densities, σ A = 0.5 and 1.0 nm −2 , which are typical in practice. 32,37,53,54 We show that the monolayer coverage of silanes is not achieved for the considered slab systems. Silane grafting effectively tunes the PI polymer density profile near the substrate.…”

“…To discern the influence of silane molecular structure and surface grafting density, we focused on slab systems with planar silica substrates, representing the large particle limit, to avoid the interplay of surface curvature effects , with chain packing. Specifically, we considered two common alkylsilane agents with different alkyl groups (depicted by the number of carbon atoms n a ), along with two silane grafting densities, σ A = 0.5 and 1.0 nm –2 , which are typical in practice. ,,, We show that the monolayer coverage of silanes is not achieved for the considered slab systems. Silane grafting effectively tunes the PI polymer density profile near the substrate.…”

Microstructure and Segmental Dynamics of Industrially Relevant Polymer Nanocomposites

“…We have seen in section 4.3.1 that additives to the polymer matrix possibly modify their dynamical properties, and that this can be probed by BDS. In a recent approach, we have made use of this effect in order to study the partition of silane coating molecules between the silica filler and the SB polymer matrix in a model PNC [80]. The idea behind this is that coating agents are part of the standard formulations of industrial PNCs, but it is difficult to know if they really go only to the filler interface.…”

“…As a final result, we have determined the adsorption isotherm of the coating agent onto the filler within the PNC (not shown, [80]). The isotherm tells us that the maximum adsorbable amount is reached for the lowest silane concentration used in formulations, which also corresponds to industrial standards.…”