  We investigated the surface structure of hydrogels of
poly(N,N-dimethylacrylamide) (PDMA) hydrogels synthesized and cross-linked
simultaneously by redox free radical polymerization. We demonstrate the
existence of a less cross-linked layer at the surface of the gel at least at
two different length scales characterized by shear rheology and by neutron
reflectivity, suggesting the existence of a gradient in cross-linking. The
composition of the layer is shown to depend on the degree of hydrophobicity of
the mold surface and is weaker for more hydrophobic molds. While the
macroscopic tests proved the existence of a relatively thick under-cross-linked
layer, we also demonstrated by neutron reflectivity that the gel surface at the
submicrometric scale (500 nm) was also affected by the surface treatment of the
mold. These results should have important implications for the measurement of
macroscopic surface properties of these hydrogels such as friction or adhesion.
