Surface modification of filler nanoparticles in suspension and incorporation in nanocomposites
People involved : C. Schmitt-Pauly (PhD), J. Alauzun (ICGM), H. Mutin (ICGM), D. Musino (PhD), E. Chauveau (AI), A.C. Genix (MdC), J.Oberdisse (DR)

Our research activities in the field of surface modification of filler nanoparticles with small molecules in solvents have been funded by Labex ChemiSyst (Montpellier), in particular through a PhD-grant.
Colloidal dispersions are largely used in the industry, from paints to cosmetics. Even though these systems have been widely investigated and are now well understood, they are still under study, and the key point remains the understanding and control of the interactions with the dispersion medium and between the dispersed objects. Mastering these interactions leads to a control of their state of aggregation. This can be achieved in different ways, for example by modifying electrostatic interactions, or by modifying the surface of the objects by adsorption of organic molecules or macromolecules[PCCP 19173, 2015]. A large variety of oxide nanoparticles is conveniently synthesized by hydrolysis in aqueous medium. In many cases, aggregation can be avoided even in the absence of surfactants by playing with the electrostatic repulsions between the particles, allowing low-cost and easily scalable production of concentrated nanoparticle sols in water. In the case of NPs, the use of colloidal sols is of high interest, as it avoids the use of dried NPs, which is controversial due to safety concerns about the toxicity of nanoparticles. Consequently, the development of surface modification methods directly in the colloidal dispersion is needed.
After having shown that the aggregation properties in aqueous suspensions can be tuned via the surface modification with alkylphosphonic acids of various degrees of hydrophobicity [PCCP 19173, 2015], we have incorporated the surface-modified nanoparticles in a polymer matrix following a matrix route[Polymer 138, 2016]. The aggregation properties of these filler particles have been studied by scattering techniques, namely using the correlation hole analysis providing an estimate of the local aggregate density, and successfully correlated with interfacial properties.

An original protocol of simultaneous surface modification and transfer from aqueous to organic phases of anatase TiO2 nanoparticles (NPs) using alkylphosphonic acids (PAs) is studied [Langmuir 10966, 2015]. The influence of the solvent, of the nature and concentration of the PA, of the size, concentration, and aggregation state of the TiO2 NPs was investigated. Complete transfer was observed for linear alkyl chains (5, 8, 12 and 18 C atoms), even at very high sol concentrations. After transfer, the grafted NPs were characterized by 31P solid-state MAS-NMR. The dispersion state of NPs before and after phase transfer was monitored by dynamic light scattering (DLS). Small-angle neutron scattering (SANS) was used to characterize the structure of PA-grafted NPs in the organic solvent. Using a quantitative core-shell model cross-checked under different contrast conditions, it is found that the primary particles making up the NPs are homogeneously grafted with a solvated PA-layer. The nanometric thickness of the latter is shown to increase with the length of the linear carbon chain of the PA, independently of the size of the primary TiO2 NP. Interestingly, a reversible temperature-dependent aggregation was evidenced visually for C18PA, and confirmed by DLS and SANS : heating up the sample induces the break-up of aggregates, which reassemble upon cooling. Finally, in the case of NPs agglomerated by playing with the pH or the salt concentration of the sols, the phase transfer with PA is capable of redispersing the agglomerates. This new and highly versatile method of NP surface modification with PAs and simultaneous transfer is thus well suited for obtaining well-dispersed grafted NPs. In future work, these surface modified nanoparticles will be incorporated in a polymer matrix following the latex route, and their aggregation properties studied by small angle scattering.
Last but not least, a similar protocol of grafting silane molecules in suspension on model silica nanoparticles of well-controlled size (as opposed to the simplified industrial system) is currently developed in the framework of a PhD. These surface modified nanoparticles are then incorporated in the industrially relevant styrene-butadient matrix, and their state of aggregation as well as the dynamical properties of the system studied, using small-angle scattering and BDS, respectively.

• 1) Surface modification of alumina-coated silica nanoparticles in aqueous sols with phosphonic acids and impact on nanoparticle interactions, C. Schmitt-Pauly, A.-C. Genix, J. Alauzun, M. Sztucki, J. Oberdisse, H. Mutin, Physical Chemistry Chemical Physics 2015, 17, 19173-19182
• 2) Simultaneous phase transfer and surface modification of TiO2 nanoparticles using alkylphosphonic acids : optimization and structure of the organosols, Céline Schmitt Pauly, Anne-Caroline Genix, Johan G. Alauzun, Gilles Guerrero, Marie-Sousai Appavou, Javier Pérez, Julian Oberdisse, P. Hubert Mutin, Langmuir 2015, 31(40), 10966-10974.
• 3) Structure of alumina-silica nanoparticles grafted with alkylphosphonic acids in poly(ethylacrylate) nanocomposites, Céline Schmitt-Pauly, Anne-Caroline Genix, Johan G. Alauzun, Jacques Jestin, Michael Sztucki, P. Hubert Mutin, Julian Oberdisse, Polymer 2016, 97, 138-146