Superabsorbent Hydrogels That Are Robust and Highly Stretchable

Polymer hydrogels synthesized by chemical cross-linking of acrylate or acrylamide monomers can absorb more than 100 times their weight in water. However, such gels are usually fragile and rupture when stretched to moderate strains (similar to 50%). Many strategies have been developed to create tougher gels, including double-networking, incorporation of nanoparticles as cross-linkers, etc., but these strategies typically retard the water absorbency of the gel. Here, we present a new approach that gives rise to superabsorbent hydrogels having superior mechanical properties. The key to our approach is the self-cross-linking ability of N,N-dimethylacrylamide (DMAA). That is, we conduct a free-radical polymerization of DMAA (along with an ionic comonomer such as sodium acrylate) but without any multifunctional monomers. A hydrogel still forms due to interchain covalent bonds between the growing linear polymer chains. Gels formed by this route can be stretched up to 1350% strain in the unswollen state. The same gels are also superabsorbent and can imbibe up to 3000 times their weight in water (which is believed to be a record). Even in the swollen state, these gels can be stretched up to strains similar to 400% before rupture, which substantially exceeds that of conventional superabsorbent gels. The superior properties of DMAA-based gels are attributed to a more uniform distribution of cross-links within their networks.