Superabsorbent Hydrogels That Are Robust and Highly\nStretchable

Abstract

Polymer hydrogels synthesized by\nchemical cross-linking of acrylate\nor acrylamide monomers can absorb more than 100 times their weight\nin water. However, such gels are usually fragile and rupture when\nstretched to moderate strains (∼50%). Many strategies have\nbeen developed to create tougher gels, including double-networking,\nincorporation of nanoparticles as cross-linkers, etc., but these strategies\ntypically retard the water absorbency of the gel. Here, we present\na new approach that gives rise to superabsorbent hydrogels having\nsuperior mechanical properties. The key to our approach is the self-cross-linking\nability of <i><i>N,N</i></i>-dimethylacrylamide\n(DMAA). That is, we conduct a free-radical polymerization of DMAA\n(along with an ionic comonomer such as sodium acrylate) but without\nany multifunctional monomers. A hydrogel still forms due to interchain\ncovalent bonds between the growing linear polymer chains. Gels formed\nby this route can be stretched up to 1350% strain in the unswollen\nstate. The same gels are also superabsorbent and can imbibe up to\n3000 times their weight in water (which is believed to be a record).\nEven in the swollen state, these gels can be stretched up to strains\n∼400% before rupture, which substantially exceeds that of conventional\nsuperabsorbent gels. The superior properties of DMAA-based gels are\nattributed to a more uniform distribution of cross-links within their\nnetworks.