Urea as a Hydrogen Bond Producer for Fabricating Mechanically Very Strong Hydrogels

Hydrogenbonding plays a very important role in the constructionand stabilization of natural and synthetic polymeric materials. Ureais generally considered and used as a hydrogen bond breaker. No successfulexamples have been provided to show that it can function as a stronghydrogen bond producer to fabricate polymeric materials with excellentmechanical properties. Here, we show that hydrogen-bonded poly(vinylalcohol) (PVA)-urea hydrogels with extraordinary mechanicalproperties can be prepared by drying PVA-urea solutions atan elevated temperature and then swelling in water. The drying processleads to the close contact of PVA chains and urea molecules and henceenables the formation of multiple hydrogen bonds between a urea moleculeand neighboring PVA chains, i.e., urea functions as a bridging molecule.The tensile strength and elastic modulus of the PVA-urea hydrogelsreach 23.8 and 11.28 MPa, respectively, which are much higher thanthose of neat PVA hydrogels with similar water contents. Structuralcharacterizations prove that the added urea is mostly retained inthe hydrogels and strong hydrogen bonding is formed between PVA andurea. The hydrogen bonding and PVA crystallites provide the hydrogelswith strong interactions, and the reversible formation and breakageof hydrogen bonding endow the hydrogels with an efficient energy-dissipatingmechanism, leading to the excellent mechanical properties of the hydrogels.This study overturns the common perception that urea can only be usedas a hydrogen bond breaker rather than a producer, and it providesnew strategies for the preparation of more polymeric materials withexcellent mechanical properties.