Small angle scattering in the Porod region from hydrated paper sheets at varying humidities

The microscopic changes in the structure of paper at different equilibrium moisture contents were examined using the novel technique of contrast variation small angle neutron scattering (SANS). Contrast variation, by appropriate selection of the H2O:D2O ratio, was used to simplify the scattering from hydrated paper to a twophase approximation. The two phases were paper polymers (cellulose, lignin and hemicelluloses) and a phase consisting of water and voids. The twophase approximation is justified by consideration of the chemistry and density of each of the polymer species belonging to paper, and by matching the scattering length density of the sorbed moisture to air. The neutron intensity at the higher scattering angles examined was successfully fitted using the Porod law, which is applicable to scattering from a twophase system at the high scattering angle limit. This limit is related to the specific surface area between the two phases. The limiting scattered intensity of neutrons was found to increase with increasing moisture content for a range of paper materials. It is concluded that the sorption of water increased the limiting intensity because water disrupted polymerpolymer bonding. The Porod law also predicts the slope of the intensity/scattering angle curve at the high scattering angle limit.