Crystalline colloidal array of water voids in hydrogels: Direct evidence for entropic trapping of flexible polymers

We have developed a unique hydrogel material with a monodisperse face-cente red cubic array of spherical water holes. This embedded array of holes Brag g diffracts light due to the refractive index mismatch between the holes an d the interstitial gel medium. Through Bragg diffraction and absorption spe ctral studies, we examined the molecular weight and concentration dependenc e of the partitioning of linear sodium polystyrene sulfonate macromolecules between regions of different spatial constraints, i.e., the array of aqueo us holes, the interstitial gel network, and the reservoir solution. We beli eve that our results are the first experimental verification of the phenome non of equilibrium entropic trapping of flexible macromolecules in a polyme ric gel matrix. This trapping occurs because the flexible macromolecules ma ximize their conformational entropy by partitioning from the gel network ma trix into the embedded water holes. These mesoscopically multiphase periodi c hydrogel materials may be useful for macromolecular separations in electr ophoresis, filtration, and chromatography.