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.

Automated summary

Hydrogen bonding is important for the construction and stabilization of polymeric materials. This study shows that hydrogen-bonded poly(vinyl alcohol)-urea hydrogels with excellent mechanical properties can be prepared by drying PVA-urea solutions at an elevated temperature and then swelling in water. The addition of urea enables the formation of multiple hydrogen bonds, resulting in the high strength and modulus of the hydrogels. This discovery challenges the conventional understanding of urea solely as a hydrogen bond breaker and provides new strategies for the fabrication of polymeric materials with enhanced mechanical properties.