Small-angle neutron scattering measurements are described for poly(dimethyl
siloxane) ablutions and gels containing fumed silica filler, swollen in to
luene. Contrast variation with solvent deuteration is used, first to determ
ine the density of the silica, and then to distinguish the three different
partial structure factors of the filled polymer samples. The concentration
of the polymer solution immediately surrounding the filler particles is fou
nd to be enhanced over a distance range of about 20 Angstrom, and then depl
eted beyond this range. The distribution of the filler particles is describ
ed by a stretched exponential with an exponent n = 1/3. Comparison of the p
artial structure factors of the silica and the polymer indicates that less
than 10% of the free surface of the filler particles is in contact with pol
ymer.