Small angle scattering from soft matter—application to complex mixed systems

The advantage of small angle neutron scattering associated with isotopic labelling through deuteration is illustrated in the case of mixed systems, created by associating already well-known systems of characteristic structures; this is also important for applications. Our first mixed system associates charged polymer chains, polyelectrolytes (here polystyrene sulfonate, PSS), with oppositely charged particles, proteins (here lysozyme). Different fractions of deuterated water (D2O) mixed with normal water are used to match the scattering length density of the protein or of the polymer in non-deuterated or deuterated version. First, this allows us to separate the protein and the polymer signal: we can then distinguish a case where the structures of each species alone in water are hardly modified by mixing, except for interconnections yielding a gel, and a case inducing complete change into a structure common to both species, made of aggregated globules. Second, using, for counterions of the polyions, deuterated TetraMethylAmmonium, together with matching both protein and polymer, we establish unambiguously the counterion release into the solvent. Third, matching only a fraction of polymer chains, the other being deuterated, we extrapolate at zero deuterated fraction their form factor and describe the chain conformation inside the complexes. Fourth, we illustrate the possibilities of modelling the signal on a second example of mixed system: a nanocomposite made of silica particles surrounded by polymer dispersed into a deuterated polymer matrix. Chains are then visible in such reinforced polymer system, in particular when it is submitted to elongation: we discuss a possible model for an ideal system, introducing the scattering contribution from deformed chains, another subject studied at LLB. To cite this article: F. Boué et al., C. R. Physique 8 (2007).