Small angle neutron scattering study to determine the structure of high strength hydrogels.

Hydrogels are swollen polymer networks containing more than 90{\%} water. Most hydrogels, however, are mechanically too weak to be used as load bearing devices. Gong \textit{et al.} have overcome this problem by synthesizing hydrogels with a double network (DN) structure. Modifying the polyelectrolyte network structure by polymerization of high molecular weight uncharged polymer \textit{in situ}, resulted in orders of magnitude increase in their load bearing ability. Despite 90{\%} water, these tough gels exhibit a fracture stress of 170 kg/cm$^2$, similar to that of articular cartilage found in the bone-joints of human body. In this work, we determined the structure of DN-gels using small angle neutron scattering. Structural origins for high toughness found in DN-gels were then examined by comparing the structure of DN-gels with that of pure polyelectrolyte network and polyacrylamide solution.