Dramatically enhancing mechanical properties of hydrogels by drying reactive polymers at elevated temperatures to introduce strong physical and chemical crosslinks

Mechanically strong and tough hydrogels have a wide range of practical applications, and many preparation methods have been developed in recent two decades. However, facile and convenient methods for preparing hydrogels with excellent mechanical properties are still lacking. In this work, we report a strategy for preparing hydrogels with very high mechanical strengths by utilizing the strong physical and chemical crosslinks formed by drying reactive polymers at elevated temperatures. The obtained polyvinyl alcohol-polyacrylic acid (PVA-PAA) hydrogels show very high tensile strengths and elastic moduli up to 28.9 MPa and 26.6 MPa, which are several tens and hundreds of times those of PVA-PAA hydrogels prepared with freezing-thawing method, respectively. And the mechanical properties of the hydrogels can be adjusted by controlling drying temperature and PAA content. The crosslinking nature of the hydrogels are studied and proved with swelling and rheology measurements as well as structural characterizations. The drying at elevated temperatures enables the occurrence of esterification reaction and promotes the crystallization of PVA chains and the formation of stronger hydrogen bonds between PVA and PAA chains. This work provides a simple and facile method to prepare high-strength hydrogels for wide potential applications.