  We introduced silica nanoparticles into poly ($N$,$N$-dimethylacrylamide)
gel, (PDAM-NC gel), and poly (acrylamide) gel (PAM-NC gel), and carried out
SAXS measurements under uniaxial elongation. It is well known that PDAM chains
are strongly adsorbed onto silica nanoparticles while PAM chains are not.
Interestingly, we found from SAXS measurements that scattering profiles depend
on the polymer-nanoparticle interaction. A four-spot pattern was observed in
the 2D structure factors of PDAM-NC gel, which was assigned to a movement of
the nanoparticles affinely under elongation. However, unexpectedly, we observed
sharp peaks in the 2D structure factors of PAM-NC gel in the parallel direction
to the stretching. These peaks appeared in much lower-$q$ region than the
theoretical prediction of affine deformation of nanoparticles. We conjecture
that these peaks do not correspond to the correlation of individual particles
but to the correlation of high cross-linking regions because the interaction
between PAM chains and silica nanoparticles is relatively weak and the
displacements of silica nanoparticles are sensitive to spatial cross-linking
inhomogeneity.
