Stretchy and strong polyurethane–urea supramolecular (PUUS) hydrogels with various stimulus-responsive behaviours: the effect of chain-extenders

The development of an easy synthetic strategy combined with straightforward tailoring of physical properties and functionalities, such that optimal performance can be targeted for various applications, still remains challenging. Previously, we reported the construction of thermo- and water-responsive strong and tough supramolecular hydrogels based on the cooperatively enhancing effect between H-bonding and hydrophobic forces. Recently, the strategy was greatly simplified to a one-pot two-step approach. In this work, by simply changing the chain extenders used in the second step, different kinds of functional units are easily introduced into the hard segments in the main chain of the copolymers to endow the resultant strong and tough supramolecular hydrogels with various stimulus-responsive properties. A dynamic covalent bond (disulphide or imine bond, e.g. –S–S– or –CN–) in the main chain provides the resulting hydrogels with reduction or pH responsive degradation (gel-to-sol transition) on demand behaviors, respectively; the azobenzene unit endows the yielding hydrogels with UV-Vis controlled stiffness; the pyridine group containing supramolecular hydrogel shows metal ion responsive mechanical and fluorescent behavior. These results provide progress toward addressing the challenges of achieving structural and synthetic simplicity married with sophisticated functionalities.