The structure of polymer nanocomposites has important consequences on final properties, like for instance mechanical reinforcement. While the structure of the hard filler phase is usually characterized by electron microscopy and small-angle X-ray scattering, 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. 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.

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.

The conformation of poly(ethyl methacrylate) chains in silica–latex nanocomposites has been studied under zero average contrast conditions (ZAC) using small-angle neutron scattering (SANS). Samples have been prepared by drying colloidal suspensions of silica and polymer nanoparticles (NPs) followed by thermal annealing, 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. On the basis of a statistical analysis, 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. Finally, the pure polymer scattering can be isolated, and the polymer radius of gyration is found to be independent of filler content and NP size.

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.

Small Angle Scattering techniques are regularly used to probe the structure of proteins and proteins assemblies in a solvent. X-rays enable fast and localized measurements thanks to the high flux and the small beam size, but there is a risk of protein degradation that is prevented with neutron scattering. In order to increase the contrast between proteins and the solvent, deuterated solvents are usually used for SANS measurements. Normally, if the contrast is due to scattering density differences between proteins and the solvent, SAXS and SANS profiles are identical. In this talk, we will present the study of gluten protein gels that display radically different SAXS and SANS profiles when the solvent is deuterated. The shape of the high q signal is identical for both measurements, whereas totally different features are measured at low q (figure). The SAXS profile can be successfully described with a model combining a Lorentzian and a q-2 power law [1]. This model was previously used to describe polymeric gels under good solvent conditions and relates the random coil Gaussian chain conformation of proteins and large-scale concentration heterogeneities. For the range of q where a q-2 power law regime is measured for SAXS, SANS profile displays instead a scattered intensity that scales as q-4 and a shoulder at even smaller q from which a characteristic size can be extracted. Measurements with various levels of solvent deuteration were performed to understand these surprising scattering profiles. Quantitative analysis of the contrast with deuteration indicates a progressive H-D exchange of protein chains. Moreover, an increase of the low q q-4 contribution is observed with solvent deuteration. Results were rationalized considering a heterogeneous H-D exchange of protein protons that would be due to local high concentration of proteins where hydrogen would be involved in hydrogen bonds, preventing thus H-D exchange. In particular, the q-4 behavior would originate from the sharp interfaces between protein rich regions which are more hydrogenated and protein diluted regions which are more deuterated. The cross-over between the q-4 and the quasi-plateau at smaller q correspond to a distance of 60 nm, similar to the size of the protein assemblies measured in the dilute regime, reflecting the characteristic size over which H-D exchange can take place.

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