Swelling and cross-linking density effects on the structure of partially ionized gels

We report on small angle neutron scattering experiments performed on weakly ionized gels at various cross-link densities or swelling ratios and on the corresponding solutions. The excess of scattering intensity observed in the gels compared to the equivalent solutions is attributed to frozen fluctuations of concentration associated with more densely cross-linked regions. The variation of the excess scattering intensity with cross-linking degree or swelling ratio supports the picture of a rather compact structure for these regions in contrast with the large branched overlapping structure suggested for other gels. The presence of electrostatic interactions during the gelation reaction reduces noticeably the extent and the amplitude of the frozen fluctuations.