The dynamics of polymer and filler in simplified industrial silica-styrene-butadiene nanocomposites (silica Zeosil 1165 MP, volume fraction 0 – 21%v) have been studied with broadband dielectric spectroscopy (BDS) and nuclear magnetic resonance (NMR). The fraction of graftable matrix chains was varied from 0 – 100%D3. The introduction of silica nanoparticles is shown to leave the segmental relaxation unaffected, an observation confirmed by the measurement of only a thin (some Angstroms thick) immobilized layer by NMR. The low-frequency measurements are resolved in two distinct dielectric Maxwell-Wagner-Sillars (MWS) processes of different behavior with respect to changes of large-scale silica structures induced by variations of filler fraction and grafting. In particular, depercolation of the silica filler at constant volume fraction with increasing grafting leaves the first MWS-process unaffected, but has direct consequences on rheological reinforcement, and decreases the strength of the second MWS by about a decade. This sensitivity to large-scale reorganizations together with a characterization of local polymer dynamics provides insight over many length- and time-scales into structure and dynamics of nanocomposites, and thus the physical origin of the reinforcement effect. 

The mechanical properties of nanocomposite materials are controlled to a large extent by the filler-filler interactions. Nevertheless, another important contribution - less well understood - is due to the polymer chain-filler interactions. Experimentally, the radius of gyration (Rg) of polymer chains in nanocomposites can be measured by SANS using the zero average contrast conditions (mixing hydrogenated (H) and deuterated (D) chains) in order to match the filler signal and measure the chain form factor. However, many studies display an unexplained polluted SANS signal in the low-q range. In this talk, we will discuss the measurement of polymer chain signal in nanocomposites prepared from the drying of a colloidal dispersion of silica and polymer particles. In such samples, the mixture of H and D chains results from the dissolution of H and D latex beads. This dissolution was followed by SANS in the low q range, as function of thermal annealing (figure 1) and filler content using an original model [1]. Experimental results demonstrate that the dissolution dynamics of polymer chains is significantly slowed down by the presence of silica nanoparticles [1]. Besides, the effect of the filler size on the chain signal in the nanocomposites was studied by a combination of SAXS and SANS measurements and reveal that the filler contribute to the SANS signal in nanocomposites filled with small silica nanoparticles (compared to the latex beads) contrarily to nanocomposites filled with bigger silica particles. We rationalize this observation considering an attractive interaction between polymers and silica particles leading to an inhibited interdiffusion of latex particles in the vicinity of silica particles, and a statistical local assymmetry due to the small number of latex beads defining the environment of small silica particles. Finally, no evolution of the polymer Rg was found in our samples.References[1] Genix et al., Macromolecules (2012), 45, 1663.[2] Banc et al., Macromolecules (2015), in press.

It is often necessary to tailor nanoparticle (NP) interactions and their compatibility with a polymer matrix by grafting organic groups, but the commonly used silanization route offers little versatility, particularly in water. Herein, alumina-coated silica NPs in aqueous sols have been modified for the first time with low molecular-weight phosphonic acids (PAs) bearing organic groups of various hydrophobicities and charges: propyl, pentyl and octyl PAs, and two PAs bearing hydrophilic groups, either a neutraldiethylene glycol (DEPA) or a potentially charged carboxylic acid (CAPA) group. The interactions and aggregation in the sols have been investigated using zeta potential measurements, dynamic light scattering, transmission electron microscopy, and small-angle scattering methods. The surface modification has been studied using FTIR and 31P MAS NMR spectroscopies. Both high grafting density r and high hydrophobicity of the groups on the PAs induced aggregation, whereas suspensions of NPsgrafted by DEPA remained stable up to the highestr . Unexpectedly, CAPA-modified NPs showedaggregation even at low r, suggesting that the carboxylic end group was also grafted to the surface. Surface modification of aqueous sols with PAs allows thus for the grafting of a higher density and a wider variety of organic groups than organosilanes, offering an increased control of the interactions between NPs, which is of interest for designing waterborne nanocomposites.

Dispersion de silice modifiée dans l'eau