Structural investigations of a neutralized polyelectrolyte gel and an associating neutral hydrogel

Small angle neutron scattering (SANS) measurements are used to differentiate the local organization of the polymer chains in two different classes of hydrogel. In neutral polyvinyl alcohol gels, hydrogen bonding gives rise to long range structural perturbations that are superimposed on the underlying chemically cross-linked network, thus producing excess scattering at small values of the scattering vector q. This secondary superstructure causes an increase in the elastic modulus. The intensity scattered by the thermal fluctuations in these gels can be described by an Ornstein-Zernike lineshape and is consistent with the osmotic modulus deduced from macroscopic osmotic and mechanical observations. Strongly charged polyacrylate hydrogels, however, display a qualitatively different scattering response. At low q a power law behavior is observed characteristic of a fractal surface. At intermediate q another component of osmotic origin is visible, which varies as q(-1), which indicates that the presence of divalent cations favors linear alignment of the network chains. Acceptable agreement is found between the estimate of the thermal fluctuations deduced from SANS and the results derived from independent osmotic observations. © 2005 Elsevier Ltd. All rights reserved.