Contrast matching gone wrong? Nanocomposites seen by SANS
Résumé
The structure of polymer nanocomposites has important consequences on final properties, like e.g.
mechanical reinforcement. The structure of the hard filler phase is usually characterized by electron
microscopy and small-angle X-ray scattering (SAXS), and we will review recent examples of filler
structure in model and industrial nanocomposites. The chain conformation can only be
measured by small-angle neutron scattering (SANS) with isotopic substitution. Over the past 15 years,
considerable efforts have been dedicated to such measurements. Results appear to converge toward
Gaussian statistics of chains, unperturbed by the presence of filler NPs, and exceptions seem to be due
to system-dependent effects. In nearly all studies, however, unexplained pollution of the scattering
data in the low-q range 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 (ZAC) 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 masses (17 and 100 kg/mol). By mixing appropriate amounts of
hydrogenated and deuterated polymer, chain scattering contrast is introduced, and in principle silica
scattering is suppressed. However, the SANS scattering intensities displayed distinct deviations at low
angles. Comparison of the SANS and SAXS intensities proves that the measurement of chain structure
by SANS is perturbed by unwanted silica contributions, i.e., the silica is ‘mysteriously’ visible. A
quantitative model rationalizing these contributions for the first time is proposed. It is based on a
statistical H/D-mismatch in the local environment of the silica NPs. Such a mismatch leads to the
existence of a nanometer-thick polymer shell surrounding silica NPs, which carries H/D contrast and
thereby produce an undesired scattering contribution. These shells are presumably maintained by the
reduced mobility of polymer close to interfaces or attractive polymer-silica interactions. The effect of
unwanted filler scattering is NP size-dependent − it is pronounced only for smaller beads. This is
probably why ZAC turned out to be so difficult with polymer nanocomposites.