We have developed a unique hydrogel material with a monodisperse face-centered cubic array of spherical water holes. This embedded array of holes Bragg diffracts light due to the refractive index mismatch between the holes and the interstitial gel medium. Through Bragg diffraction and absorption spectral studies, we examined the molecular weight and concentration dependence of the partitioning of linear sodium polystyrene sulfonate macromolecules between regions of different spatial constraints, i.e., the array of aqueous holes, the interstitial gel network, and the reservoir solution. We believe that our results are the first experimental verification of the phenomenon of equilibrium entropic trapping of flexible macromolecules in a polymeric gel matrix. This trapping occurs because the flexible macromolecules maximize their conformational entropy by partitioning from the gel network matrix into the embedded water holes. These mesoscopically multiphase periodic hydrogel materials may be useful for macromolecular separations in electrophoresis, filtration, and chromatography.