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

Hydrogen bonding plays a very important role in the construction and stabilization of natural and synthetic polymeric materials. Urea is generally considered and used as a hydrogen bond breaker. No successful examples have been provided to show that it can function as a strong hydrogen bond producer to fabricate polymeric materials with excellent mechanical properties. Here, we show that hydrogen-bonded poly(vinyl alcohol) (PVA)–urea hydrogels with extraordinary mechanical properties can be prepared by drying PVA–urea solutions at an elevated temperature and then swelling in water. The drying process leads to the close contact of PVA chains and urea molecules and hence enables the formation of multiple hydrogen bonds between a urea molecule and neighboring PVA chains, i.e., urea functions as a bridging molecule. The tensile strength and elastic modulus of the PVA–urea hydrogels reach 23.8 and 11.28 MPa, respectively, which are much higher than those of neat PVA hydrogels with similar water contents. Structural characterizations prove that the added urea is mostly retained in the hydrogels and strong hydrogen bonding is formed between PVA and urea. The hydrogen bonding and PVA crystallites provide the hydrogels with strong interactions, and the reversible formation and breakage of hydrogen bonding endow the hydrogels with an efficient energy-dissipating mechanism, leading to the excellent mechanical properties of the hydrogels. This study overturns the common perception that urea can only be used as a hydrogen bond breaker rather than a producer, and it provides new strategies for the preparation of more polymeric materials with excellent mechanical properties.