  A study of the reinforcement effect of a soft polymer matrix by hard
nanometric filler particles is presented. In the main part of this article, the
structure of the silica filler in the matrix is studied by Small Angle Neutron
Scattering (SANS), and stress-strain isotherms are measured to characterize the
rheological properties of the composites. Our analysis allows us to quantify
the degree of aggregation of the silica in the matrix, which is studied as a
function of pH (4-10), silica volume fraction (3-15%) and silica bead size
(average radius 78 A and 96 A). Rheological properties of the samples are
represented in terms of the strain-dependent reinforcement factor, which
highlights the contribution of the filler. Combining the structural information
with a quantitative analysis of the reinforcement factor, the aggregate size
and compacity (10%-40%) as a function of volume fraction and pH can be deduced.
In a second, more explorative study, the grafting of polymer chains on
nanosilica beads for future reinforcement applications is followed by SANS. The
structure of the silica and the polymer are measured separately by contrast
variation, using deuterated material. The aggregation of the silica beads in
solution is found to decrease during polymerization, reaching a rather low
final aggregation number (less than ten).
