Résumé
The structure of polymer nanocomposites has important consequences on final properties, like for instance mechanical reinforcement. The structure of the hard filler phase is usually characterized by electron microscopy and small-angle X-ray scattering, and we will review recent examples of filler structure in model [1] or industrial nanocomposites [2]. The chain conformation can only be measured by small-angle neutron scattering (SANS). Continuous efforts over the past 15 years have produced a body of sometimes contradictory, because system-dependent, results, see e.g. [3]. In virtually all studies, however, a mismatch ruining the polymer form factor at low-angles has been observed, in spite of careful contrast-matching. In this study, the conformation of polymer chains in silica-latex-nanocomposites has been studied under zero-average contrast conditions using SANS. Samples have been prepared by drying colloidal suspensions of silica and polymer nanoparticles (NPs) for two different silica NPs (radius of 5 and 15 nm) and two chain molecular weights (17 and 100 kg/mol). By appropriate mixing of hydrogenated and deuterated polymer, chain scattering contrast is introduced, and in principle silica scattering suppressed. The silica structure consisting mostly of small fractal aggregates is characterized by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) on the same samples. The measurement of the chain structure by SANS, however, is perturbed by unwanted silica contributions, as often reported in the literature. Here, the contribution of contrast-matched silica is evidenced as a function of system parameters, namely chain mass, silica size, and volume fraction, and a model rationalizing these contributions for the first time is proposed. Via a statistical model, a nanometer-thick polymer shell surrounding silica NPs is shown to create contrast, which is presumably maintained by the reduced mobility of polymer close to interfaces or attractive polymer-silica interactions. This shell is proven to be quantitatively important only for the smallest silica NPs. As a consequence, the pure polymer scattering can be isolated, and the polymer radius of gyration is found to be independent of filler content and NP size [4].[1] A. Banc et al, Macromolecules 2014, 47 (9), 3219–3230 [2] G. Baeza et al, Soft Matter 2014, 10, 6686-6695.[3] A.C. Genix and J. Oberdisse, Current Opinion in Colloid and Interface Science, 2015, in press. [4] A. Banc et al, Macromolecules 2015, 48, 6596−6605.